study of intrinsic and doped silicon-carbon films by pecvd ......films or buffer layers to improve...

Study of Intrinsic and Doped Silicon-Carbon Films by PECVD of Medium and High Vacuum

for Solar Cells

by:

M. Sc. William Wenceslao Hernández Montero

A thesis submitted in partial fulfillment of the requirements for the degree of:

DOCTOR OF SCIENCES IN ELECTRONICS

At the:

Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE)

February 2017 Tonantzintla, Puebla

Advisor: Dr. Carlos Zúñiga Islas

Principal Research Scientist Electronics Department, INAOE

©INAOE, 2017 All rights reserved

The author hereby grants to INAOE permission to reproduce and to distribute copies of this

thesis document in whole or in part

i

Abstract

Solar cells are the main elements for the generation of electrical energy from solar

radiation. Crystalline silicon (c-Si) wafers and hydrogenated amorphous silicon films (a-

Si:H) are mature technologies to fabricate silicon-based solar cells. Next-generation

photovoltaics requires that materials and structures combine a high-efficiency at low-costs.

Silicon could continue in this field by optimizing the structure of photovoltaic devices and

improving the properties of materials. Tandem solar cells of a-Si:H/µc-Si:H/µc-Si:H and

heterojunctions of a-Si:H/c-Si have demonstrated enhancement of efficiency compared to

current market solar cells. On the other hand, carbon is incorporated in p-type doped silicon

films or buffer layers to improve the performance of thin-film solar cells. Thus, silicon and

carbon are key materials for photovoltaics.

The synthesis of silicon-based films by Plasma-Enhanced Chemical Vapor

Deposition (PECVD) is convenient; because various forms can be obtained, for instance:

amorphous, microcrystalline, nanocrystalline, quasi-epitaxial. Moreover, many factors can

be tuned such as frequency, power, temperature, pressure, precursor gases, e.g., silane

(SiH4) or hydrogen (H2). From the experimental point of view, these factors can be varied

and the films can be analyzed. However, the number of experiments increases

exponentially. The two-level factorial design of experiments (DOE) was implemented to

find the main effects for the synthesis of silicon-based films with optimal properties for

solar cells; saving time and resources. The number of experiments for intrinsic films was 8;

pressure, SiH4, and H2 were the factors at two levels. Since these parameters allow a

morphological control of the films. For the study of doped silicon films, diborane (B2H6)

and phosphine (PH3) gases were incorporated. Gas methane (CH4) was used to alloy with

carbon the intrinsic and doped silicon films. The factorial DOE was implemented again for

the study of doped and alloyed films; low and high values for B2H6, PH3, and CH4 were set.

The structural and optoelectronic properties of the intrinsic and doped silicon-

carbon films were studied. These films were synthesized by PECVD at frequencies of 110

kHz and 13.56 MHz. The reactor of low-frequency (LF) works at 110 kHz and medium

vacuum (MV) of 10-3 Torr. On the other hand, the reactor at radio-frequency (RF) of 13.56

MHz reaches a high vacuum (HV) of 10-7 Torr. The influence of vacuum level, leak rate,

gas partial pressure and gas flows was analyzed. Results revealed that the intrinsic silicon

ii

films obtained by RF and LF showed an optimal photosensitivity of 104 to be used as

absorbing layers, despite the medium vacuum of LF-PECVD. In both reactors, the best

photosensitivity was obtained in the experiment HSiP-011 (at a low level of H2, and high

levels of SiH4 and pressure). Various morphologies were obtained by HV/RF-PECVD such

as amorphous, nanocrystalline, subwavelength structured. The intrinsic silicon-carbon films

at a high level of CH4 showed a low photosensitivity, which implies that these films are not

suitable to be used as absorbing layers. However, these films showed down-conversion

luminescence. Thus, these layers are suitable as photoluminescent and anti-reflective

coatings due to their combination of refractive index, optical gap, and conversion of UV to

visible radiation. For doping experiments, the deposition parameters of the intrinsic films

with the best photosensitivity were chosen. An activation energy of 0.31 eV for p-type

layers and 0.35 eV for n-type layers was obtained by MV/LF-PECVD. High levels of B2H6,

PH3, and SiH4 were required for efficient doping along with low levels of CH4 and H2.

Finally, p-i-n structures were fabricated by MV/LF-PECVD by using Corning glass

coated with transparent conductive oxide (TCO), and titanium as back-electrode. The p-, i-,

and n-type layers with the best properties were used. However, some structures showed

adherence problems to the TCO substrates, and short-circuits were determined by current-

voltage measurements. Therefore, plasma cleaning of TCO substrates and chamber

passivation were analyzed. Plasma cleaning with H2 reduces the TCO surface; samples with

this treatment showed delamination. Plasma cleaning with O2 oxidizes the TCO surface; no

delamination was observed. Plasma cleaning with Ar created short-circuits in the p-i-n

structures due to defects. Thus, the MV/LF-PECVD reactor was pre-cleaned in Ar and H2

plasmas for 15 min to avoid the formation of pinholes and defects. Later, a silicon film was

pre-deposited using the conditions of experiment HSiP-011 to passivate the chamber.

The obtained parameters of the p-i-n solar cells fabricated by MV/LF-PECVD were:

VOC=0.7 V, JSC=4 mA/cm2, FF=0.5, and efficiency η=1.3%. This efficiency is comparable

to η=1.1% of the first p-i-n a-Si:H solar cell reported in the literature. The low current

density could be attributed to the lack of light confinement techniques such as texturing of

TCO, the lack of a back reflector made of TCO/Ag layers, and the high sheet resistance of

the electrodes (TCO and titanium) of 10 Ω/. Further strategies for efficiency enhancement

are required, along with the thickness optimization of the p-, i-, and n-type layers.

iii

Resumen

Las celdas solares son el principal elemento para la generación de energía eléctrica a

partir de la radiación solar. El silicio cristalino (c-Si) y las películas de silicio amorfo

hidrogenado (a-Si:H) son tecnologías de gran desarrollo para la fabricación de celdas

solares. La próxima generación de celdas solares requiere de materiales y estructuras que

combinen una alta eficiencia a bajo costo. El silicio podría continuar vigente, mejorando

sus propiedades y optimizando los dispositivos. Celdas solares tipo tándem de a-Si:H/µc-

Si:H/µc-Si:H y hetero-uniones de a-Si:H/c-Si han demostrado una mejora en la eficiencia.

Además, el carbono se incorpora a películas tipo-p y buffer para mejorar las características

de las celdas solares. Por tanto, el silicio y el carbono son claves para la energía solar.

La síntesis de películas de a-Si:H por depósito químico en fase vapor asistido por

plasma (PECVD, por sus siglas en inglés) es conveniente, ya que pueden obtenerse diversas

morfologías: amorfa, microcristalina, nanocristalina, quasi-epitaxial. Además, pueden

ajustarse varios factores, tales como: frecuencia, potencia, presión, gases precursores p.ej.

silano (SiH4) o hidrogeno (H2). Desde el punto de vista experimental, cualquiera de estos

factores se puede variar y analizar su respuesta. Sin embargo, el número de experimentos se

incrementa exponencialmente. El diseño de experimentos factoriales (DOE, por sus siglas

en inglés) se implementó para determinar los principales efectos en la síntesis de películas

basadas en silicio; y así optimizar tiempo y recursos. El número de experimentos para las

películas intrínsecas fue de 8, usando tres factores: la presión y los flujos de SiH4 y H2. Ya

que estos parámetros permiten un control estructural de las películas. Para el estudio de las

películas dopadas, se incorporaron los gases diborano (B2H6) y fosfina (PH3). El gas

metano (CH4) se utilizó para la aleación con carbono. El DOE se implementó para el

estudio de películas dopadas y aleadas; usando dos niveles para los gases B2H6, PH3 y CH4.

Se estudiaron las propiedades optoelectrónicas y estructurales de las películas

intrínsecas y dopadas de silicio-carbono. Las películas se obtuvieron por PECVD a

frecuencias de 110 kHz y 13.56 MHz. El reactor de baja frecuencia (LF) de 110 kHz

alcanza un vacío medio de ~10-3 Torr. Por otro lado, el reactor de radio frecuencia (RF) de

13.56 MHz alcanza un alto vacio de ~10-7 Torr. Se analizó la influencia del nivel de vacío,

la tasa de fuga, la presión parcial y el flujo de los gases. Los resultados de las películas de

silicio intrínsecas obtenidas por RF y LF mostraron una fotosensibilidad óptima de 104, a

iv

pesar del vacío medio del reactor LF-PECVD. En ambos reactores se obtuvo la mejor

fotosensibilidad en el experimento a bajo nivel de H2, alto nivel de SiH4 y alto nivel de

presión (muestra HSiP-011). Distintas morfologías se obtuvieron por RF-PECVD, tales

como: amorfa, nano-cristalina, submicro-estructurada. Las películas intrínsecas de silicio-

carbono a un alto flujo de CH4 tuvieron baja fotosensibilidad; por lo que no son adecuadas

como capas absorbentes. Sin embargo estas películas mostraron fotoluminiscencia; y se

podrían aplicar como capas antireflejantes y fotoluminiscentes debido a su combinación de

índice de refracción, banda óptica y conversión de radiación UV a visible. Para los

experimentos de dopado se usaron las condiciones de depósito de la película con la mejor

fotosensibilidad. Se obtuvo una energía de activación de 0.31 eV para películas tipo-p y de

0.35 eV para películas tipo-n depositadas por LF-PECVD. Para un dopado eficiente se

requirió de un alto flujo de B2H6 o PH3, un alto flujo de SiH4, y un bajo flujo de CH4 y H2.

Finalmente se fabricaron estructuras p-i-n por LF-PECVD sobre vidrio Corning

cubierto con TCO, utilizando titanio como electrodo posterior. Las películas tipo p, i, y n

con las mejores características fueron utilizadas. Sin embargo, algunas estructuras tuvieron

problemas de adherencia a los substratos de TCO; además se observaron cortos circuitos en

las estructuras mediante mediciones de corriente-voltaje. Por lo tanto, se analizó la limpieza

por plasma de los substratos de TCO y la pasivación del reactor. La limpieza con plasma de

H2 reduce la superficie del TCO; estas muestras presentaron delaminación. La limpieza con

plasma de O2 oxida la superficie del TCO, con este tratamiento no se observó

delaminación. La limpieza con plasma de Ar originó cortos circuitos en las estructuras p-i-n

debidos a defectos. Para evitar la formación de polvos y defectos, el reactor se limpió en

plasma de Ar y H2 durante 15 minutos cada uno. Enseguida, se depositó una película de

silicio para pasivar la cámara, usando las mismas condiciones del experimento HSiP-011.

Los parámetros obtenidos de las estructuras p-i-n fabricadas por LF-PECVD son:

VOC=0.7 V, Jsc=4 mA/cm2, FF=0.5, y η=1.3%. Esta eficiencia es comparable a la eficiencia

η=1.1% de la primer celda p-i-n reportada en la literatura. La baja densidad de corriente se

podría atribuir a la falta de técnicas para el confinamiento de la luz, tales como texturizar la

superficie del TCO, a la falta de un reflector de TCO/Ag, o a la alta resistencia de cuadro de

los electrodos (TCO y titanio) de 10 Ω/ . Por lo que se requieren estrategias adicionales

para mejorar la eficiencia; y la optimización del espesor de las películas tipo p, i, y n.

v

Agradecimientos

A mi asesor de tesis, el Dr. Carlos Zúñiga Islas por su confianza y apoyo durante el

desarrollo de este trabajo.

A los doctores del INAOE: Claudia Reyes, Mario Moreno, Wilfrido Calleja, Alfonso

Torres, Javier De la Hidalga, Alonso Corona, Edmundo Gutiérrez, e Ignacio Zaldívar.

Al personal técnico del laboratorio de Microelectrónica de INAOE: Adrian Itzmoyotl,

Leticia Tecuapetla, Armando Hernández, Víctor Acá, Pablo Alarcón, Ignacio Juárez y Tere.

Al personal del laboratorio de colorimetría de INAOE: Juana Medina y Guadalupe Flores.

Al Dr. Enrique Quiroga González y al Dr. José Soto Manríquez del Instituto de Física Luis

Rivera Terrazas de la BUAP.

A Laura Elvira Serrano De la Rosa del Instituto de Física Luis Rivera Terrazas de la BUAP

por las facilidades otorgadas en las mediciones de espectroscopia Raman.

A investigadores y personal administrativo de la coordinación de electrónica de INAOE.

Al INAOE y todo su personal.

Y al Consejo Nacional de Ciencia y Tecnología (CONACYT) por la beca otorgada como

estudiante de Doctorado No. 224193.

Dedicado a mi amada familia,

Mis padres: Aurora Montero y Armando Hernández

Mis hermanos: Franco, Andrea, Rosy y Vero

Mis sobrinos: Valeria, Julián, Ximena y Arlette

Y a mi amada Sandy

vii

Content

Abstract ................................................................................................................................... i

Resumen ............................................................................................................................... iii

Agradecimientos .................................................................................................................... v

Content ................................................................................................................................ vii

List of acronyms ................................................................................................................... ix

List of symbols ...................................................................................................................... xi

1. Introduction ................................................................................................................... 1

1.1. Solar spectra and solar cells .................................................................................... 1

1.2. State of the art ......................................................................................................... 3

1.3. Motivation ............................................................................................................... 5

1.4. Objectives ............................................................................................................... 7

1.5. Thesis overview ...................................................................................................... 8

2. Silicon-based films for solar cells ................................................................................ 9

2.1. Intrinsic silicon films: amorphous and crystalline forms ...................................... 10

2.2. Doping of silicon films ......................................................................................... 12

2.3. Alloying of silicon films ....................................................................................... 14

2.4. Influence of gases: flows vs. partial pressure ....................................................... 14

3. Experimental techniques and methodology ............................................................. 17

3.1. Plasma-Enhanced Chemical Vapor Deposition (PECVD) ................................... 17

3.1.1. Medium-vacuum and low-frequency PECVD (MV/LF-PECVD) ............... 19

3.1.2. High-vacuum and radio-frequency PECVD (HV/RF-PECVD) ................... 20

3.1.3. Vacuum level and leak rate ........................................................................... 21

3.2. Design of experiments (DOE) .............................................................................. 22

3.2.1. Intrinsic and doped layers ............................................................................. 22

3.2.2. Structure of p-i-n solar cells .......................................................................... 24

3.3. Preparation of samples .......................................................................................... 26

viii

3.4. Films characterization .......................................................................................... 27

3.4.1. Structural characteristics .............................................................................. 28

3.4.2. Optical properties ......................................................................................... 30

3.4.3. Electrical properties ...................................................................................... 32

3.4.4. Optoelectronic properties ............................................................................. 34

3.5. Solar cell parameters ............................................................................................ 36

4. Results and discussion ................................................................................................ 39

4.1. Preliminary OFAT experiments ........................................................................... 40

4.1.1. Intrinsic silicon-carbon films by MV/LF-PECVD ....................................... 40

4.1.2. Intrinsic silicon-carbon films by HV/RF-PECVD ....................................... 41

4.2. DOE for intrinsic layers ....................................................................................... 46

4.2.1. Intrinsic silicon films by HV/RF-PECVD ................................................... 46

4.2.2. Intrinsic silicon films by MV/LF-PECVD ................................................... 53

4.3. Improvement of intrinsic layers by MV/LF-PECVD ........................................... 58

4.4. DOE for doped layers ........................................................................................... 60

4.4.1. Doped silicon films with B2H6 and PH3 by MV/LF-PECVD ...................... 60

4.4.2. Doped silicon-carbon films with B2H6 by MV/LF-PECVD ........................ 62

4.5. Preliminary p-i-n structures for solar cells by MV/LF-PECVD .......................... 65

4.6. Improvement of doped layers by MV/LF-PECVD .............................................. 67

4.7. Improvement of p-i-n structures by MV/LF-PECVD .......................................... 70

5. Conclusions ................................................................................................................. 77

Publications .......................................................................................................................... 81

Appendix A. Status, characteristics and deposition of films by MV/LF-PECVD ............... 83

List of tables ......................................................................................................................... 88

List of figures ....................................................................................................................... 89

List of equations ................................................................................................................... 91

References ............................................................................................................................ 94

ix

List of acronyms

PECVD Plasma-Enhanced Chemical Vapor Deposition

MV Medium Vacuum

HV High Vacuum

LF Low Frequency

RF Radio Frequency

VHF Very High Frequency

DOE Design of Experiments

OFAT One Factor at a Time

Si Silicon

Ge Germanium

C Carbon

B Boron

P Phosphorous

p- P-type doped silicon

i- Intrinsic silicon

n- N-type doped silicon

TCO Transparent Conductive Oxide

ITO Indium Tin Oxide

FTO Fluorinated Tin Oxide

ARC Anti Reflective Coating

PL Photoluminescence

Al Aluminum

Ti Titanium

a- amorphous

nc- nanocrystalline

µc- microcrystalline

sw- subwavelength structured

pm- polymorphous

SiC Silicon-carbon alloy

x

SiGe Silicon-germanium alloy

sccm standard cubic centimeters per minute

ppm parts per million

FM Flow Meter

IR Infrared

NIR Near Infrared

MIR Mid Infrared

FIR Far Infrared

FTIR Fourier-Transform Infrared spectroscopy

LSM Low Stretching Mode

MSM Medium Stretching Mode

HSM High Stretching Mode

TO Transversal Optic Raman mode

AFM Atomic Force Microscopy

SEM Scanning Electron Microscopy

UV-Vis Ultra Violet – Visible spectrum

AM 1.5 Air mass coefficient 1.5

PV Photovoltaics

SiH4 Silane

CH4 Methane

B2H6 Diborane

PH3 Phosphine

H2 Hydrogen

Ar Argon

HIT Heterojunction with Intrinsic Thin layer

DASH Dopant-free ASymmetric Heterocontact

ESL Electron Selective Layer

HSL Hole Selective Layer

RIE Reactive Ion Etching

cps Counts per second

xi

List of symbols

α Absorption coefficient cm-1

λ Wavelength nm

k Wavenumber cm-1

Q Gas flow sccm

XC Carbon content in gas phase %

XSi Silane content in gas phase %

Rd Dilution ratio %

Cg Dopant concentration in gas phase %

Cs Dopant concentration in solid phase %

ηd Doping efficiency %

QL Leak rate mbar L/s

p Pressure Torr

pp Partial pressure Torr

NE Number of experiments

Eg Optical gap eV

n Refractive index

R Reflectance

T Transmittance

A Absorbance

σd Dark conductivity Ω-1cm-1

σph Photoconductivity Ω-1cm-1

Sph Photosensitivity

µτ Mobility-lifetime product cm2/V

n0 Electron concentration cm-3

p0 Hole concentration cm-3

µ Mobility cm2/V s

G Generation rate cm-3 s-1

Φ Photon flux Photons/cm2 s

Eα Activation energy eV

xii

EF Fermi energy eV

EC Conduction band eV

EV Valence band eV

I Current A

V Voltage V

P Power W

T Temperature °C

f Frequency Hz

t time s

tf Film thickness nm

Vd Deposition rate Å/s

η Efficiency %

VOC Open-circuit voltage V

ISC Short-circuit current A

FF Fill factor %

J Current density A/cm2

Vbi Built-in potential V

rs Series resistance Ω-cm2

rsh Shunt resistance Ω-cm2

A Area cm2

R* Microstructure parameter %

XR Raman crystallinity %

dR nanocrystal size nm

S Siemens 1/Ω

R Resistance Ω

σcs Capture cross-section cm2

Nd Density of defects cm-3

kB Boltzmann constant eV/K

q Elementary charge C

Iλ Intensity of laser mW/cm2

IAM1.5 Intensity of AM 1.5 spectrum mW/cm2

Chapter 1. Introduction

1

1. Introduction

The generation of renewable and free energy is a fundamental task in order to

diminish the disturbance of nature due to the energetic needs of humans. Renewable energy

contributed in 2015 almost to 24% (5830 TWh) of all electricity generation in the world,

versus depleting energy with 76% approximately, which dominates the market [1]. To date,

installed global renewable energy is hydropower with 58%, solar photovoltaics and

concentrated solar power with 13%, biomass with 6% and geothermal with 1%. The current

top countries with renewable electricity capacity are China, United States, Germany, Japan

and India. Government, private sector and scientific community in México should address

these issues in order to satisfy the future energetic requirements to become an auto

sustainable and independent country.

A solar cell is the key element for the generation of electrical energy from solar

radiation, this area is also known as photovoltaics. The first generation of silicon solar cells

required crystalline wafers of ~300 µm in thickness to fabricate p-n junctions. The second

generation introduced hydrogenated amorphous silicon films (a-Si:H) as an intrinsic layer

with ~300 nm in thickness, as well as boron- and phosphorous-doped films used as p- or n-

type layers in p-i-n junctions [2]. Silicon has been a key element for electronics, and carbon

is the most versatile element included in a diversity of alloys, with various allotropes.

These elements are abundant in our environment. In particular, the silicon-carbon alloy has

found diverse technological applications to enhance the operation range of electronic

devices; such as power electronics, devices working at high temperature and high

frequency [3], as well as in solar cells as buffer layers or p-type window layers.

Hydrogenated amorphous silicon (a-Si:H) and its alloy with carbon (a-SiC:H) are

used typically in silicon-based thin-film solar cells as intrinsic and doped layers,

respectively [4]. These materials are synthesized by plasma-enhanced chemical vapor

deposition (PECVD). Plasma deposition allows obtaining a material with suitable

properties for applications in large-area electronics [5].

1.1. Solar spectra and solar cells

Solar spectrum covers Ultra-Violet (UV), visible (Vis) and near-infrared (NIR)

radiation, as can be seen in Figure 1.1(a). Thus, the absorption coefficient of materials used

Study of intrinsic and doped silicon-carbon films by PECVD of medium and high vacuum for solar cells

2

for solar cells should be high in the emission range of the solar spectrum. The efficiency of

solar cells is compared through the standard air mass coefficient (AM 1.5), which is an

artificial version of the solar spectrum. Shockley-Queisser limit [6] states that the

maximum efficiency is 32.2% for a single-junction solar cell of 1.1 eV of band gap [6], [7].

Recently, multi-junction solar cells with concentrator have overcome this limit [8]. Even

though the energy of incident photons in solar cells must be higher than the optical gap,

only the photons close to the optical gap contribute to generating electron-hole pairs. For

this reason, the concept of multi-junction solar cell has been proposed to improve the

efficiency of solar cells. Therefore, the optical gap of absorbing layers must match the UV,

visible or near-infrared region.

Tailoring of the optical gap, Eg, to specific energies for the intrinsic layer in thin

film solar cells is fundamental. Several values for Eg have been proposed to cover the entire

solar spectrum: 0.7, 0.95, 1.12, 1.43, 1.84, and 2.4 eV [9]. Accordingly to the number of

layers, films with different Eg are required for optimal operation. For instance, the optical

gap should be Eg=1.1 eV for a single junction solar cell. For a double junction solar cell,

Eg=0.98 eV and Eg=1.87 eV are ideal [2]. Micromorph solar cells satisfy this requirement

because their structure is a top junction of a-Si:H with a thickness of 200-300 nm and

Eg=1.75 eV; the bottom junction is made of μc-Si:H with a thickness of 1-2 μm and Eg=1.1

eV [10], [11].

The basic operating principle of a c-Si solar cell is depicted in Figure 1.1(b). The

conversion process takes place in a p-n junction fabricated of a boron-doped p-type c-Si

wafer and an n-type doped region, which can be formed by diffusion or implantation of

dopant atoms, such as phosphorous.

Figure 1.1. (a) Solar radiation spectrum [12]. (b) Operating principle of a c-Si solar cell.

(a) (b)


3

The operation principle of c-Si solar cells is as follows: electron-hole pairs are

generated by incident photons of energy higher than the optical gap of the absorbing

material. Low-energy photons pass through the solar cell without interaction with the

material. Electrons are separated from p to n direction and holes are separated from n to p

direction. These charge carriers are sweep to an external load by the built-in electric field

created in the depletion region of the p-n junction.

1.2. State of the art

Photovoltaics field began with the first observation of the photovoltaic effect by

Alexandre Edmond Becquerel in 1839, via an electrode in a conductive solution exposed to

light. To date, the theoretical limit of efficiency for single-junction solar cells (Shockley-

Queisser limit) of 32.2% has been overtaken. For non-concentrated photovoltaics, Boeing-

Spectrolab has achieved an efficiency of 38.8% in a 5-junction (5J) solar cell [13], [14]. For

concentrated photovoltaics (CPV), the world record is 46% of efficiency in a 4-junction

solar cell of III-V compound semiconductors [14].

Figure 1.2 shows the efficiency comparison of current technologies in the market.

Multicrystalline and crystalline silicon wafer technologies are the main materials for the

photovoltaic market, covering 68% and 24% of total production in 2015, respectively [15].

Thin film solar cell technologies based on a-Si:H, cadmium telluride (CdTe), and copper-

indium-gallium-selenide (CIGS) are also present in the global market with 8% of the total

production [1], [15]. Nevertheless, several materials have been investigated for solar cells

such as gallium arsenide, quantum dot, perovskite, dye-sensitized, organic materials [14].

Figure 1.2. Efficiency comparison of solar cell technologies: best lab cells vs. best modules [15].


4

Thin-film technologies are the basis for the second generation photovoltaics. p-i-n

a-Si:H solar cells consist of three regions: p-type, intrinsic and n-type layers. p-i-n

structures typically use metals and transparent conductive oxides as electrodes [4]. The

intrinsic layer acts as the space charge region, contrasting to the conventional c-Si solar

cells. However, thin-film solar cells exhibit photo-degradation due to light soaking. This

light-induced degradation is a major disadvantage of a-Si:H solar cells. This is known as

the Staebler-Wronski effect [16]. The drop in efficiency may be in the range of 15-30% [4].

Thin-film solar cells based on a-Si:H began in 1976 with an efficiency of 1.1% in an area

of 3.5 cm2 [17]. Recently in 2015, H. Sai et al. reported a 13.6%-efficient triple-junction

silicon solar cell made of a-Si:H/µc-Si:H/µc-Si:H with a honeycomb texturing and using

silicon oxide films as tunnel-junction recombination layers [18]. Figure 1.3(a) shows the

structure that currently holds the world record for a-Si:H thin-film solar cells.

HIT (hetero-junction with intrinsic thin layer) solar cells combine the low cost of

doped and intrinsic layers of a-Si:H technology with the high efficiency of c-Si solar cells;

this kind of cell reached an efficiency of 24.7% [19]. A record efficiency of 26.3% for HIT

solar cells was reported by Kaneka [14]. Figure 1.3(b) shows the structure of HIT solar

cells. DASH (dopant-free asymmetric heterocontact) solar cells have replaced doped layers

with another kind of materials called hole-selective layer (HSL), e.g. molybdenum oxide

(MoO); and electron-selective layer (ESL), e.g. lithium fluoride (LiF). a-Si:H films are

deposited between MoO and LiF as passivation layers. DASH solar cells have reached an

efficiency of 19%. These cells are also known as dopant-free silicon solar cells [20].

Figure 1.3. Solar cell architectures based on a-Si:H films: (a) a-Si:H solar cell based on thin-films [18], and

(b) heterojunction with intrinsic thin layer (HIT) solar cell [19]. Efficiency and costs for the three generations

of solar cell technologies: (I) Si wafers, (II) thin films and (III) advanced materials [2].

(a)

(b)

(c)


5

Third generation photovoltaics has been proposed in addition to the first and second

generations. The thermodynamic limit restricts the maximum conversion efficiency of

sunlight to 73.7% [2], [10]. This limiting efficiency is higher than the limit of conventional

cells given by the Shockley-Queisser limit of 32.2%. It is mainly due to the loss of

absorbed photons that are converted to heat. Figure 1.3(c) shows the efficiency versus

cost/area and cost/watt for the three generations of photovoltaics. The prospect is that

advanced films will drive to high efficiencies at a low cost per watt and cost per area. The

relation cost/watt and cost/area could be improved in combination with the use of handy

materials, simple processes and low-cost substrates.

Many physical approaches have been proposed to improve the absorbing material in

a solar cell [2], [21], [22] such as:

• Multiple energy threshold processes: which have been implemented in tandem cells,

multiple quantum well solar cells and devices that use the impurity photovoltaic

effect.

• Multiple electron-hole pairs: by using the high energy photons above the band gap.

Other quantum multiplication is the creation of two lower energy photons from a

single high energy photon, or an electron plus a lower energy photon.

• Hot carrier effects: via the collection of photoexcited carriers that must be collected

before they get the chance to cool down to ambient temperature.

• Thermal techniques: where the energy can be extracted from the heated absorber,

then being converted to electricity by solar thermal electric, thermionic,

thermoelectric, thermophotovoltaics, thermophotonics, etc.

1.3. Motivation

Silicon dominates current solar cell technologies, either in the form of c-Si wafers

or in the form of silicon films. HIT and DASH solar cells are outstanding examples of the

improvement in costs, processes, and efficiency using thin-films and wafers of silicon.

According to trends, next-generation solar cells will continue being fabricated from silicon;

maybe through hybrid structures and materials. Thus, silicon-based films are a key material

for photovoltaics. On the other hand, the synthesis of silicon films by PECVD is suitable,

because various forms of silicon can be obtained such as amorphous, nanocrystalline,


6

polymorphous, quasi-epitaxial, etc. However, the understanding of materials obtained by

PECVD is limited to experimental studies and empirical evidence. Besides, an undesirable

variation of the films due to the chamber characteristics or changes made in the reactor

could be expected. Indeed, the conditions for deposition of films in one reactor not

necessarily produce films with the same characteristics in another reactor, conditioning the

reproducibility of results. Then, there are different issues in the field of solar cells and

materials obtained by PECVD.

Microelectronics lab at INAOE possesses excellent instruments and equipment for

the fabrication and characterization of integrated circuits with a 10 µm design rule.

Fabrication processes include oxidation, metallization, lithography, standard cleaning, dry

and wet etching, thin films deposition, etc. PECVD reactors are used for the deposition of

silicon-based films. PECVD reactor from Applied Materials Reinberg model AMP3300 is

operated at a low-frequency (LF) of 110 kHz. It allows the deposition on substrates of

large-area; the diameter of the single chamber reactor is 66 cm. Then, various substrates

with the standard size of 15.4×15.4 cm2 of solar cells can be easily loaded in the chamber.

However, the medium vacuum level (MV) of this MV/LF-PECVD reactor could be a

disadvantage. On the other hand, a modern cluster tool from MVSystems consists of a

radio-frequency (RF) PECVD with three independent chambers to deposit p-, i-, and n-type

layers. The main advantages are that this reactor operates at the standard RF of 13.56 MHz,

it reaches a high vacuum level (HV), and that each layer can be deposited in a separated

chamber. However, only one standard substrate can be loaded in this HV/RF-PECVD

reactor.

Therefore, it is fundamental the investigation of intrinsic and doped silicon and

silicon-carbon films using the reactors available in microelectronics lab at INAOE. These

kinds of layers have demonstrated their importance in thin-film solar cells as well as in

high-efficiency designs such as HIT or DASH solar cells. PECVD reactors must be enabled

and characterized in order to obtain films with optimal properties to be used in silicon-

based solar cells. It is expected that this thesis will contribute to:

• Find the optimal conditions to deposit photosensitive silicon films by using SiH4

and H2, to be applied as intrinsic layers (i) in solar cells.


7

• The incorporation of carbon into intrinsic silicon films could be beneficial for a

larger optical gap to absorb energies in the range of 1.8<Eg(eV)<2.4. Enabling

silicon-carbon films (a-SiC:H) to be used as intrinsic layers in multi-junction solar

cells.

• Find the optimal conditions to deposit doped films by using B2H6 and PH3, to be

applied as p- and n-type layers, respectively.

• The incorporation of carbon in p-type boron-doped a-Si:H films is studied because

these films could act as a buffer or p-type doped layers. Besides, the effect of

carbon in n-type phosphorous-doped a-Si:H layers is studied.

• Diminish the Staebler-Wronski effect by the inclusion of nanocrystals in the layers

to stabilize the performance of the silicon-based films against light-induced

degradation. Because high-efficiency conversion approaches are based on the

properties of nanosized semiconductor particles or nanocrystals.

• Find the optimal conditions to fabricate silicon-based solar cells on low-cost

substrates by means of MV/LF-PECVD reactor from INAOE, because the vacuum

level and leak rate are critical parameters to deposit films with enough quality for

device applications.

The main problem is obtaining a solar cell based on silicon films with optimal

performance and that it can be reproducible. Then, this study will contribute to improving

deposition process, finding optimal procedures, and the selection of adequate conditions to

fabricate stable, functional and reproducible silicon-based solar cells.

1.4. Objectives

Synthesize, characterize and study intrinsic and doped silicon and silicon-carbon

films for solar cell applications. This work is focused on the study of intrinsic and doped

layers based on silicon films. The goal is obtaining films with optimal properties for solar

cells. Particular objectives are:

• Synthesis of intrinsic silicon and silicon-carbon films with high photosensitivity to

be used as absorbing layers by MV/LF-PECVD or HV/RF-PECVD.


8

• Synthesis of doped silicon and silicon-carbon films with high conductivity and low

activation energy to be used as p-type and n-type layers.

• Characterization of the structural, optical and electrical properties of the intrinsic

and doped silicon and silicon-carbon films for applications in solar cells.

• Improvement of the intrinsic and doped silicon and silicon-carbon films; including

strategies to overcome the medium vacuum level of the MV/LF-PECVD reactor.

• Fabrication of the solar cells using low-cost substrates: Corning 2947 glass or

flexible substrates.

• Evaluate the impact of the intrinsic and doped silicon and silicon-carbon films in

solar cells through p-i-n structures.

1.5. Thesis overview

Introductory section in chapter 1 includes historical background, state of the art,

motivation, and objectives of this thesis. Chapter 2 provides to the reader fundamental

concepts in the field of silicon-based films as well as their use in solar cells as intrinsic and

doped layers. Chapter 3 describes the methodology and experimental techniques used in

this work for the synthesis of the doped and intrinsic silicon-based films by PECVD, and

the characterization of their structural, optical and electrical properties. In chapter 4 are

reported the measurements that were performed in the intrinsic and doped layers along with

the study and analysis of the films and solar cells that were obtained in this work.

Discussions about the films and solar cells obtained by MV/LF-PECVD are included, along

with the improvement of the films as well as the p-i-n structures by using the two-level

factorial design of experiments. Finally, conclusions and main contributions of this work

are addressed in chapter 5; future work is pointed out.

Chapter 2. Silicon-based films for solar cells

9

2. Silicon-based films for solar cells

The characteristics of structures and materials for solar cells are very important for

achieving an efficient photovoltaic device. In this regard, variations in the structure of

silicon films have resulted in microcrystalline silicon (μc-Si:H) [10], [23] nanocrystalline

silicon (nc-Si:H) [24], polymorphous silicon (pm-Si:H) [25], quasi-epitaxial silicon (qEpi-

Si) [26], [27], subwavelength structured silicon (sw-Si:H) [28], etc.

Optical gap of intrinsic layers can be varied from 1.8 eV for a-Si:H to 1.1 eV for μc-

Si:H and 2.1 eV for pm-Si:H films [29]. This allows the fabrication of tandem solar cells,

based on the concept of spectrum splitting. Silicon films deposited at the transition region

of μc-Si:H and a-Si:H are composed of nanocrystalline inclusions in an amorphous matrix

[30], [31]. This morphology corresponds to a reduction in the Staebler-Wronski effect,

which is an undesired metastable state where the efficiency of a-Si:H solar cells decreases

due to light soaking [16], [32]. On the other hand, the synthesis of silicon films by plasma

deposition is convenient, because many factors can be tuned, such as temperature, pressure,

power, frequency region (DC, LF, RF, VHF, MW), and precursor gases [4], [5], [22], [29].

Photosensitivity indicates the ability of a material to generate electron-hole pairs

from incident photons that are absorbed. An optimal range for photosensitivity is 103-105;

intrinsic silicon films meet this requirement [29]. Photosensitivity is the ratio between

photoconductivity and dark conductivity. Thus, high photoconductivity and low dark

conductivity are ideal for intrinsic layers. Besides, high dark conductivity and low

activation energy are required for doped films. The main requirements for doped and

intrinsic layers to be considered for solar cell applications are listed in Table 2.1 [29], [33].

In the next chapter, each property will be discussed in detail and its role in solar cells. Table 2.1. Requirements of intrinsic silicon films [29] and doped layers [33] for solar cell applications.

Property Intrinsic p-type n-type

Dark conductivity, σd (Ω-1cm-1) < 10-10 10-6 10-3 Photoconductivity, σph (Ω-1cm-1) > 10-5 Photosensitivity, Sph 103-105 Activation energy, Eα (eV) 0.8-0.9 0.3-0.45 0.15-0.3 Optical gap, Eg (eV) < 1.8 2 1.75 Absorption coefficient, α (cm-1) > 104 Defect density, Nd (cm-3) < 1016 Mobility-lifetime product, µτ (cm2/V) > 10-7 Hydrogen content, CH (at.%) 10 Microstructure parameter, R* < 0.1


10

Figure 2.1(a) shows the basic structure of a single junction p-i-n solar cell. The band

diagram for this junction is shown in Figure 2.1(b). The main layers are p-type, intrinsic,

and n-type. In superstrate configuration light enters from p-type layer. For this reason,

carbon is incorporated into p-type layers to increase the optical gap from 1.75 to 2 eV. The

doped layers are very thin, in the range of 10-20 nm. Ideally, the doped layers allow a good

ohmic contact between the internal layers and the external electrodes. These layers create

an electric field across the intrinsic a-Si:H layer to collect the photo-generated carriers from

the absorbing layer. The strength of the electric field depends on the thickness of the

intrinsic layer and the doping efficiency of doped layers.

Figure 2.1. (a) Single junction p-i-n solar cell of a-Si:H [29], (b) band diagram of a p-i-n solar cell [33].

Therefore, a high electrical conductivity is required for p- and n-type doped layers

in order to establish low resistance contacts with the electrodes [19], [33]. Front electrode is

made of a material called transparent conductive oxide (TCO), which is conductive with a

low sheet resistance of 1 Ω/, and transparent with a transmittance of ~90% [29]. Light

trapping strategies in this electrode include texturization or the inclusion of scattering

structures in order to increase the absorption in the intrinsic layer due to light confinement.

Back electrodes can be composed of a TCO layer with aluminum (Al) or silver (Ag). This

combination acts as an efficient back reflector of radiation [29]. Additionally, light

enhancement techniques have improved the performance of thin-film solar cells by

plasmonic structures [34], [35].

2.1. Intrinsic silicon films: amorphous and crystalline forms

The structure of plasma deposited materials depends strongly on the preparation

parameters. According to the literature, the main parameters for controlling the structure of

(a) (b)


11

silicon films are pressure, residence time [25], [36], gas flows of SiH4 and H2 [30], [31].

The effect of the chamber pressure on the morphology of silicon films is shown in Figure

2.2(a). Figure 2.2(b) shows the influence of dilution ratio on the structure of silicon films.

Dilution ratio, Rd, is defined as the ratio of precursor gases: hydrogen, H2, and silane, SiH4:

2

4d

HRSiH

= (2.1)

Besides the definition of dilution ratio, the proportion of SiH4 and H2 is also defined

by the silane concentration [30]. This relation is given by:

4

4 2Si

SiHXSiH H

=+

(2.2)

The condition for nanocrystalline silicon is between the region of a-Si:H and µc-

Si:H varying the dilution ratio, as can be seen in Figure 2.2(b). On the other hand, the

transition between different morphologies of silicon (amorphous, microcrystalline,

nanocrystalline, clusters, and powders) can be controlled by increasing the pressure of the

reactor, as can be seen in Figure 2.2(a). Radio-frequency (RF) plasma of 13.56 MHz was

used for the synthesis and study of those films. Moreover, RF-PECVD technique is the

standard for the fabrication of a-Si:H solar cells.

Figure 2.2. (a) Pressure changes for the synthesis of microcrystals (1), nanocrystals (2), clusters (3) and

powders (4) [37]. (b) Structure of plasma deposited silicon films varying the dilution ratio, Rd [31].

Nevertheless, μc-Si:H films can be synthesized at very high frequencies (VHF).

VHF of 70 MHz was introduced by the Group at Université de Neuchâtel [4]. The band gap

of 1.1 eV for µc-Si:H films instead of 1.8 eV for a-Si:H films is beneficial for absorbing the

NIR region of the solar spectrum. The advantage of high frequencies is the high deposition

rate (~10 Å/s), compared to deposition rate by RF (1-3 Å/s). The quality of films by VHF is

(a) (b)


12

not affected, because initial efficiencies up to 10% have been reported for μc-Si:H based

solar cells [38], [39]. Record efficiency of 11.8% was reported for a µc-Si:H solar cell

versus record efficiency of 10.2% for an a-Si:H solar cell [14]. Record initial efficiency of a

“micromorph” tandem cell of a-Si:H/µc-Si:H is 14.7% [39].

On the other hand, pm-Si:H films are constituted of nanocrystals embedded in an

amorphous matrix. pm-Si:H deposition is obtained through the plasma synthesized

nanocrystals, contrasting to amorphous silicon which grows from SiHx radicals. This form

of silicon has been studied by Pere Roca i Cabarrocas and his group since the first report of

pm-Si:H [40]. The main properties of pm-Si:H are a low defect density and a higher

resistance to light soaking than a-Si:H; the optical gap of pm-Si:H is higher than a-Si:H

[25], [36], [37], [41]. Initial efficiencies of ~9% have been reported for pm-Si:H solar cells.

However, solar cell delamination has been observed. In order to overcome surface

delamination, the typical stack sequence of superstrate (p-i-n) was inverted to substrate

sequence (n-i-p) to obtain stable pm-Si:H based solar cells [42].

2.2. Doping of silicon films

Doping of a-Si:H films with boron or phosphorous was studied by Le Comber and

Spear [43]. They observed that conductivity of intrinsic a-Si:H could be increased by

several orders of magnitude by mixing phosphine (PH3) or diborane (B2H6) with silane

(SiH4). Activation energy of intrinsic a-Si:H is decreased from 0.8 eV to 0.15 eV for n-type

phosphorous doped a-Si:H, and 0.3 eV for p-type boron doped a-Si:H. Room temperature

dark conductivity can be varied by a factor of 108 [29], [44]. Figure 2.3(a) shows the room

temperature conductivity of doped a-Si:H as a function of the fraction of PH3/SiH4 and

B2H6/SiH4. Figure 2.3(b) shows the doping efficiency as a function of the dopant gas

concentration. A low dopant gas concentration leads to a high doping efficiency. However,

the doping efficiency of silicon films is very low, compared to c-Si wafers, which is full.

The doping efficiency is inversely proportional to the square root of the dopant

concentration in gas phase. This dependence applies to the concentration in gas phase rather

than the dopant concentration in solid phase [44]. The doping efficiency, ηd, is defined as

the fraction of dopant atoms which are active dopants [5] (with four-fold coordination):

4 4 3 4/ ( ) /d N N N N Nη = + = (2.3)


13

Where N3 and N4 are the densities of three-fold (inactive dopant) and four-fold

(active dopant) states; N is the total dopant concentration in solid phase.

Figure 2.3. (a) Room temperature conductivity of doped a-Si:H as a function of doping gases: PH3 and B2H6

[29]. (b) Doping efficiency as a function of dopant gas concentration [44].

The dopant concentration in gas phase for n- and p-type layers is given by:

3 3 4( ) / ( )gC n PH PH SiH= + (2.4)

2 6 2 6 4( ) / ( )gC p B H B H S iH= + (2.5)

Where, Cg(n) and Cg(p) are the gas phase concentration of n-type and p-type doped

layers, respectively. SiH4 is the gas silane, B2H6 is the gas diborane, and PH3 is the gas

phosphine. The dopant concentration in terms of solid phase [5], can be defined by:

( ) / ( )sC n P P Si= + (2.6)

( ) / ( )sC p B B Si= + (2.7) Where Cs(n) and Cs(p) are the concentration in solid phase of phosphorous, P, and

boron, B, respectively; Si is the silicon content in solid phase. The model of continuous

random network allows atoms to take their preferred coordination. In this model, the

optimum number of covalent bonds, Z, for an atom with N valence electrons is:

8 (for 4), and (for 4)Z N N Z N N= − ≥ = < (2.8)

Where N is the number of valence electrons. Equation 2.8 is known as the ‘8-N

rule’ [45]. The low efficiency of doped films is attributed to most atoms take Z=3.

The defect density of silicon films increases with the square root of dopant

concentration in gas phase (see Figure 2.3(b)); the equation is given by: 19 1/2 33 10 cmd gN C −= × (2.9)

(a) (b)


14

This dependence is only valid for the dopant concentration in gas phase, Cg(n) or

Cg(p), not that of dopant concentration in solid phase (Cs(n) or Cs(p)). Therefore, when

dopants are added (B or P), defects are created.

2.3. Alloying of silicon films

Carbon is an exceptional material that gives rise to outstanding properties, due to

their several allotropes such as diamond-like carbon, amorphous carbon, nanocrystalline

carbon, graphite, graphene [46], [47]. These forms are of great importance in solar cells

because layers based on carbon are beneficial. Boron-doped a-SiC:H films are commonly

used as p-type window layers. These films exhibit similar properties as p-type doped a-

Si:H, but when carbon is added the optical gap increases. The optical gap can be tuned by

varying the carbon content in gas phase, XC, defined as:

4

4 4C

CHXCH SiH

=+

(2.10)

Where CH4 is the methane flow and SiH4 is the silane flow. The optical gap of a-

SiC:H films can be adjusted between 1.7 and 2.1 eV [4]. Besides, a-SiC:H films are applied

in solar cells as buffer layers at the p/i interface. Buffer layers should inhibit the chemical

diffusion of boron from p-type into the intrinsic layer; and electrical diffusion of the photo-

generated electrons from intrinsic into the p-type layer [29]. Unfortunately, the

incorporation of carbon is detrimental for the optoelectronic properties of intrinsic a-SiC:H

films [4], [29]. Thus, a-SiC:H films are not suitable as absorbing layers. For this reason,

solar cells are fabricated with intrinsic a-Si:H or μc-Si:H films. On the other hand, pm-

SiC:H films obtained by RF-PECVD have exhibited a strong electroluminescence and

photoluminescence for optoelectronic applications [37], [48]. Besides, doped µc-SiC:H

films prepared by VHF-PECVD have shown an optical gap between 1.8-2.1 eV and a

conductivity between 0.001-1 S/cm; properties of the films prepared by VHF-PECVD

(40.86 MHz) were superior to films at 13.56 MHz [49].

2.4. Influence of gases: flows vs. partial pressure

The properties of intrinsic and doped films are strongly influenced by the gases and

their relative concentration as was discussed previously. Gas flows are expressed in units of


15

sccm (standard cubic centimeters per minute). Standard ccm mean that a given gas at the

atmospheric pressure of 1 atm, and temperature of 0 °C fills 1 cm3 of volume every minute.

Flow meters are designed for a specific gas. Conversion factors are required if a

different gas is used. For instance, 103 sccm of O2 in a flow meter for N2; using the factor

K=0.9926 [50], the real flow is O2=N2×K=103×0.9926=992.6 sccm. Oxford Instruments

provides other solution for SiH4 diluted at 5% in N2 [51]. For a flow of 100 sccm of this

mixture, the percentage that corresponds to SiH4 and N2 is 5 sccm and 95 sccm,

respectively. Dopant gases (B2H6 or PH3) are typically diluted in H2. The question is what

percentage of flow corresponds to B2H6 and H2. The real flow of B2H6 can be estimated

using a flow meter for N2 or B2H6 [50]. But it is complicated when a flow meter different to

N2 is used, because conversion factors for few gases are available [50], [52].

Nevertheless, gas flows can be monitored through the gas partial pressure exerted in

a chamber, when a given flow is set for deposition. Also, the prediction of solid phase

properties from gas phase is desirable. It has been observed a correlation between the

concentration in solid phase and gas phase with the optical properties of refractive index,

optical gap [53], conductivity, and gas partial pressure [54] of SiGe films. Figure 2.4(a)

shows the dependence of germanium content and refractive index. A linear behavior is

observed between refractive index and atomic fraction of germanium in solid phase. Figure

2.4(b) shows the correlation between partial pressure and gas flows of GeH4 and SiH4.

Figure 2.4. (a) Refractive index of intrinsic a-SiGe:H films as a function of germanium content in solid phase

[53]. (b) Dependence of gas flows and partial pressure measured in the LF-PECVD reactor [54].

Properties of materials can be tailored from the gas phase by properly alloying and

doping. For instance, the sample 849 at XGe=0.60 (gas phase) showed Xat=0.52 and

(a) (b)


16

Xw=0.29 (solid phase) [53]. This sample was prepared with 100 sccm of SiH4 flow and 150

sccm of GeH4 flow, these gas flows produced the same gas partial pressure of 126 mTorr

[54]. This observation is important because gas partial pressure is better than gas flows to

estimate the solid phase properties of alloyed films. Dalton’s law of partial pressure states

that in a mixture of non-reacting gases, the total pressure is equal to the sum of the partial

pressures of the individual gases. The fraction of the individual gas, xi, in the mixture, is:

i i ii

total total total

p V nxp V n

= ≡ ≡ (2.11)

Where, pi is the partial pressure of the individual gas and ptotal is the sum of all

partial pressures in the mixture [54], [55]. This equation is valid for mole, n, and volume,

V, fraction. Equation 2.2 for silane content is analogous to Dalton’s law.

Equation 2.11 is valid for doping gases. Low gas concentrations are measured in

units of parts per million (ppm). Doping gases (B2H6 and PH3) are expressed in these units

[43]. Gas tanks of PECVD reactors for B2H6 and PH3 are balanced at 1% in H2 (104 ppm).

For instance, 1 ppm means that there is 1 molecule of certain gas in a mixture of 1 million

of air molecules. The volume fraction is the most common value used for ppm units; it is

defined as the ratio between the volume of a constituent, Vi, and the total volume VT:

6( .) 10 i

total

Vppm volV

= (2.12)

Equation 2.12 is equivalent to Dalton’s law; also units of vol./vol., p/p, or mol/mol

used for gas concentrations do not change over temperature and pressure [56]. From the

solid phase point of view, the fraction content in solid phase of an element (j) in an alloy (i-

j) can be expressed by means of weight fraction, Xw, or atomic fraction, Xat [53]; and using

the atomic weights (Ar) of elements (i) and (j), Ar(i) and Ar(j), respectively:

( ) ( )( )( ) ( ) ( ) ( )

w rat

w r w r

X j A iX jX j A i X i A j

=+

(2.13)

( ) ( )( )( ) ( ) ( ) ( )

at rw

at r at r

X j A jX jX j A j X i A i

=+

(2.14)

Xw and Xat are proportional to Ar of the elements in the alloy. Then, a correlation is

observed between Equations 2.13, 2.14, and Dalton’s law. Therefore, it is important to take

into account the flows and partial pressure for each gas. Because silicon films will be

alloyed with carbon and they will be doped with boron and phosphorus.

Chapter 3. Experimental techniques and methodology

17

3. Experimental techniques and methodology

In this section is described the methodology to obtain the intrinsic and doped

silicon-carbon films, along with the characterization techniques and analysis tools. In order

to determine their main structural, optical, and electronic properties required for solar cells.

Strategies for the synthesis of silicon-carbon films are discussed from the point of view of

the factorial design of experiments. The fabrication of p-i-n structures is proposed, avoiding

the use of lithographic techniques. This simplifies the fabrication process since thin-film

solar cells are large-area devices (~1 cm2) of a very low thickness (~10-9 m).

The study of intrinsic and doped silicon-carbon films is fundamental for solar cell

applications because these films will be used as p-, i- and n-type layers. The main

properties to be analyzed are absorption characteristics, optical gap, activation energy, dark

conductivity, photoconductivity, photosensitivity, mobility-lifetime product, hydrogen

content, morphology, and deposition rate. Figure 3.1 shows the steps to study the intrinsic

and doped silicon-carbon films, as well as the p-i-n junctions.

Figure 3.1. Steps sequence for the study of the intrinsic and doped silicon-carbon films and p-i-n structures.

3.1. Plasma-Enhanced Chemical Vapor Deposition (PECVD)

Plasma Enhanced Chemical Vapor Deposition (PECVD) is based on electron

impact and dissociation of process gases, e.g., SiH4. The plasma is created inside of a

controlled volume. Two electrodes ionize the gas at low pressure (between 0.1–3 Torr).

Plasma contains positive, negative and neutral species (radicals). The powered electrode is

called cathode and the grounded electrode is called anode, where the substrate is placed for

deposition. Positive ions and radicals reach the substrate and undergo surface reactions

during deposition. Radicals are considered as precursors for the growth of a-Si:H and µc-

Si:H films. According to the literature, SiH3 radicals are the main precursors of device

quality films [29], [57]. Some models have been developed to explain the complex process

of deposition and etching of materials by plasma [58]. Further details about modeling and

plasma physics for thin film deposition are available in the literature [58], [59], [60].


18

PECVD technique allows the deposition at low temperatures of ~300°C. This kind

of plasmas can be produced by electric fields of direct current (DC) or alternating current

(AC). Typical excitation frequencies for AC are low-frequency (LF) of 100 kHz, radio-

frequency (RF) of 13.56 MHz, very high-frequency (VHF) of ~70 MHz, and microwaves

(MW) of 2.45 GHz [59].

Figure 3.2(a) shows the dependence of electron energy and electron density as a

function of excitation frequency. By increasing the excitation frequency, the average

electron energy decreases and the electron density increases. A high electron concentration

in the plasma increases the beneficial radical density (atomic hydrogen and film growth

precursor SiHx). These radicals provide selective etching of disordered amorphous phase

(reducing defects and voids) and fast growth of crystalline grains [4]. Figure 3.2(b) shows

the behavior of ion energy, E(eV), and ion flux, dϕ/dE, as a function of frequency. Ions

determine the final film quality. At high frequencies, the peak of ion energy is reduced and

the ion flux is increased. The high ion flux and low energy lead to a softer but intensified

ion bombardment. This may be a reason why plasmas favor microcrystalline growth when

excitation frequency increases [61].

Figure 3.2. Effect of frequency on (a) density and energy of electrons [4], (b) energy and flux of ions [61].

Therefore, the increment of the deposition rate of silicon films as the plasma

frequency increases is correlated with the behavior of electron energy and electron density

as well as the ion energy and ion flux. At 13.56 MHz, ions do not respond to the RF field.

At 100 kHz, ions can respond and provide ion bombardment of the growing film [51].

Then, the frequency is a key factor for the synthesis of silicon films by PECVD. Because in

this dissertation, films obtained by RF of 13.56 MHz and LF of 110 kHz are studied.

(a) (b)


19

3.1.1. Medium-vacuum and low-frequency PECVD (MV/LF-PECVD)

Figure 3.3 shows the schematic diagram of the MV/LF-PECVD reactor Reinberg

model AMP3300. The single chamber reactor has a parallel electrode configuration of 66

cm of diameter and inter-electrode spacing of 6 cm. Power and ground wires are connected

to top and down electrodes, respectively. Samples are loaded on the bottom electrode. The

substrate temperature is controlled by a three-zone heater (inner, center and outer). Gases

for intrinsic layers include CH4, SiH4, and GeH4. Doping gases are B2H6 and PH3, and the

dilution gas is H2. Argon is used for opening the chamber and to purge and fill the lines of

all gases when the reactor is not in use. Each gas line has its own flow meter (FM) to set the

gas flows. Pressure for process mode is set at the desired value; pressure in the chamber is

monitored through a sensor. Pressure controller sends a signal to the valve at the gas outlet

to hold the set point pressure selected in the controller. Roots and mechanical pumps are

adequate for deposition mode. However, these pumps only reach a vacuum level of 10-3

Torr in vacuum mode, which is in the range of medium vacuum. Turbomolecular pumps

reach a high vacuum [59]. A turbomolecular pump would be suitable in order to improve

the vacuum level.

Figure 3.3. Schematic diagram of the MV/LF-PECVD reactor.

Mechanical

Pump

SiH4

GeH4

Roots

Pump

CH4

PH3

B2H6

H2

Pressure controller

Temperature controller

Power and frequency controller

Valve

Ar Argon purge

Line of gases

FM FM

FM FM

FM FM

FM

Samples

+ –

Main Valve


20

3.1.2. High-vacuum and radio-frequency PECVD (HV/RF-PECVD)

The schematic diagram of the multi-chamber HV/RF-PECVD from MVSystems is

shown in Figure 3.4; p, i, and n layers can be deposited separately in this cluster tool. The

main characteristics of the chambers are:

• PL2 is used for p-type doped layers. Gases: SiH4, H2, CH4, and B2H6.

• PL4 is used for intrinsic layers. Gases: SiH4, GeH4, and H2.

• PL7 is used for n-type doped layers. Gases: SiH4, H2, GeH4, and PH3.

The area of each chamber is 25×25 cm2 with a height of 13 cm. Samples are loaded

in the substrate holder, which is placed at the top electrode to prevent that powders and dust

generated in plasma fall on the samples, creating defective layers with pinholes. The

maximum area for substrates is 15.6×15.6 cm2, with an inter-electrode separation of 1.5 cm

for the PL2 chamber, and 1.9 cm for PL4 and PL7 chambers. Gases for intrinsic layers

include CH4, SiH4, and GeH4. Doping gases are B2H6 and PH3, and the dilution gas is H2.

N2 gas is available for maintenance of pumps. The operation of the cluster tool can be

manual, semi-automatic, and fully automatic. The turbomolecular pump backed by a

rotatory pump reaches a vacuum of 10-7 Torr, which is in the range of high vacuum [59].

However, a roots pump would be suitable for deposition mode.

Figure 3.4. Schematic diagram of the HV/RF-PECVD reactor.

Rotatory

Pump

Turbomolecular

Pump

Pressure controller

Temperature controller

Power and frequency controller

Valve

Lines of gases

Samples

Gases for chamber PL2 (p+)

Gases for chamber PL2 (p+) PL4 (i)

PL7 (n+)

Gases for chamber PL7 (n+)

– +

SiH4

GeH4

CH4

PH3

B2H6

H2

FM FM

FM FM

FM FM


21

3.1.3. Vacuum level and leak rate

Pumping systems are usually composed of a turbomolecular and a mechanical pump

to reach a high vacuum. A separated process pump is used for removing gases and other

by-products generated during deposition, e.g. roots pump [4]. Hence, the pumps of MV/LF-

PECVD would be complemented by the vacuum system of HV/RF-PECVD to reach a high

vacuum; and vice versa, the pumps of the HV/RF-PECVD would be complemented by the

vacuum system of MV/LF-PECVD, which is optimal for deposition mode.

MV/LF-PECVD reactor was manufactured in the decade of 70’s [59]; it is of a

relatively old technology but it is a functional reactor. Films synthesized by LF tend to be

amorphous; films with crystalline inclusions are less probable. On the other hand, MV/RF-

PECVD is a new reactor. In theory, all morphologies can be synthesized. Besides, a high

vacuum is desired for reducing the incorporation of impurities in the films. However, the

high investment of sophisticated equipment and maintenance could affect the cost/W and

cost/area of solar cells. Contaminants for device-quality intrinsic silicon films should be

below of the following limits: O < 1019 cm-3, C < 1018 cm-3, and N < 1017 cm-3 [62]. When

impurities exceed these limits, fill factor of the intrinsic layers decreases due to a reduced

lifetime of the photo-generated carriers [4]. Moreover, the leaks in PECVD reactors should

be low enough that the required operating pressure in the vacuum container is not affected.

The leak rate, QL(mbar L/s), can be determined quantitatively by:

LpQ Vt

∆=

∆ (3.1)

Where Δp and Δt are the change of pressure in mbar, and the interval of time in

second for that change, when pumping is off. V is the volume in liters. As a thumb rule, an

equipment is considered leaky if QL>10-4 mbar L/s and tight if QL<10-4 mbar L/s [63].

Leak rate for MV/LF-PECVD reactor was estimated after 1 hour in vacuum mode:

V=18 L, Δt=180 s, Δp=9-6 mTorr=3×10-3 Torr ≡ 4×10-3 mbar. Then, QL=4×10-4 mbar L/s.

This leak rate is comparable to the thumb rule limit.

Leak rate for HV/RF-PECVD reactor was estimated by means of: V=8.125 L,

Δt=20 s, Δp=2.5-0.2×10-7 Torr=2.3×10-7 Torr ≡ 3×10-7 mbar. Then, QL=1.2×10-7 mbar L/s,

which is optimal. Experiments will demonstrate if the MV/LF-PECVD reactor is functional

to obtain device-quality silicon-based films.


22

3.2. Design of experiments (DOE)

One factor at a time (OFAT) is varied in typical experiments. For instance, the

frequency is varied in a given range using multiple steps (see Figure 3.2). These studies are

important to determine the effect of one factor. PECVD is a multifactorial system, then it

becomes difficult obtaining a general view of causes and effects based on single OFAT

experiments. Design of experiments (DOE) is the design that aims to explain the variation

of information under conditions that are hypothesized to reflect the variation [64]. DOE is

an important tool because PECVD reactors have various parameters which modify strongly

the properties of thin-films. Typically, the parameters are set from empirical observation

and experience. Unfortunately, there are no standard recipes to synthesize silicon films with

the desired properties that work for all reactors. Thus, it is convenient the implementation

of DOE and the analysis of parameters involved in plasma deposition.

From the literature review, the most influential parameters for PECVD are:

• Primary parameters (strong dependence): pressure, gases, and frequency.

• Secondary parameters (low dependence): electrical power and temperature.

From another point of view, the plasma parameters can be viewed as:

• Thermodynamic parameters: temperature, pressure, chamber volume, and gases.

• Electromagnetic parameters: electrical power, frequency, and bias voltage.

3.2.1. Intrinsic and doped layers

Considering all the parameters involved in the synthesis of the doped and intrinsic

silicon-carbon films, it is interesting to calculate the total number of experiments. For

example, using a sampling of 3 steps of the full-scale range (low, medium, and high levels),

and using three factors (pressure, SiH4 flow, and H2 flow); it yields 27 experiments (33

combinations). Therefore, it is inferred that the number of experiments, NE, is given by: k

EN S= (3.2)

Where S is the sampling and k is the number of factors. In the case of the PECVD

system, using a sampling of ten steps (10 values in the full range of each factor), with 6

factors (power, temperature, pressure, SiH4 flow, CH4 flow, and H2 flow). Substituting

these parameters in Equation 3.2, the number of combinations results as high as Sk = 106.

This number of experiments is impractical for implementation.


23

An attractive option is the implementation of the two-level factorial DOE, which

could be convenient in order to diminish the number of experiments required to find the

main effects [64]. Only two levels (high and low) are required. The number of experiments

is reduced substantially to Sk = 2k. The aim of these experiments is the exploration of the

ranges in which PECVD works. Table 3.1 shows the operation ranges of MV/LF- and

HV/RF-PECVD. The low and high levels could be selected according to experimental and

practical issues. For instance, a value at 10% of maximum could be suitable for the low

level, and a value close to 50% of maximum is ideal to avoid thermal stress,

electromagnetic overloads or waste of gases in the reactors, etc. Table 3.1. Operation ranges for MV/LF- and HV/RF-PECVD reactors [65], [66].

Factor MV/LF-PECVD HV/RF-PECVD Minimum Maximum Minimum Maximum

Silane, SiH4 (sccm) 0 100 0 100 Methane, CH4 (sccm) 0 500 0 200 Hydrogen, H2 (sccm) 0 1000 0 500 Diborane, B2H6 (sccm) 0 1000 0 20 Phosphine, PH3 (sccm) 0 1000 0 20 Pressure, p (Torr) 10-3 10 10-7 1.9 Power, P (Watt) 0 1000 0 100 Temperature, T (°C) 25 350 50 600

Minitab software was used as a tool in order to obtain the main effects that have an

influence on the properties and characteristics of the films. The Pareto principle states that

for many events, the 80% of the effects come from 20% of the causes (80/20 principle).

The Pareto chart of the effects is an important graph. As an example, Figure 3.5(a) shows

the Pareto graph that was obtained from the analysis of RMS roughness of silicon films. In

this graph is observed that the main effect is the factor “A” because it showed a significant

effect on the roughness of the films. The alpha parameter “α” is set at 0.05 for all analysis,

in order to discriminate the main effects and interactions from null effects. The cube plot

with the low and high levels that take each factor is shown in Figure 3.5(b). Only 8

experiments are required for the synthesis of intrinsic silicon films, varying H2 flow (A)

pressure (B), and SiH4 flow (C) at low and high levels.

The next step is alloying and doping of the silicon films. From DOE for intrinsic

silicon films (DOE i-Si), one experiment will be chosen in order to set the deposition

conditions for carbon alloying (DOE i-SiC). DOE p-Si and DOE n-Si consist on adding

dopant gases, i.e., study the effect of doping with B2H6 and PH3. Finally, experiments of


24

alloying with carbon the doped silicon films are carried out. DOE n-SiC and DOE p-SiC

deal with the study of carbon incorporation in the doped layers.

Figure 3.5. Statistical tools: (a) Pareto chart of the effects and (b) cube plot of factors (A, B, and C).

Table 3.2 summarizes the number of experiments for each DOE, which are

substantial but practical for implementation. All factors will be fixed once that a film of

high photosensitivity is obtained from experiments of intrinsic silicon films (DOE i-Si);

since it implies a low defect density. The criterion for the successive experiments is adding

one factor (CH4, B2H6 or PH3) in combination with the main effect determined from DOE

i-Si. Then, 2 factors at 2 levels will be implemented for DOE of doping and alloying, it

results in 4 experiments (22). The maximum number of experiments is 28, covering doped

and intrinsic layers, as can be seen in Table 3.2. Table 3.2. Summary of DOE.

Design of Experiments (DOE)

Number of experiments Type of layer Material Dopant

DOE i-Si 8 Intrinsic Silicon - DOE i-SiC 4 Intrinsic Silicon-carbon - DOE n-Si 4 n-type Silicon Phosphorous DOE p-Si 4 p-type Silicon Boron DOE n-SiC 4 n-type Silicon-carbon Phosphorous DOE p-SiC 4 p-type Silicon-carbon Boron

3.2.2. Structure of p-i-n solar cells

Solar cells are large-area devices; the size of modules is 1×1.6 m2. The standard

area of a lab solar cell for research is 1×1 cm2. The thickness of the films should be about

200 nm for intrinsic layers and 20 nm for doped layers. The fabrication process for

commercial thin-film solar cells includes several steps, such as Corning glass preparation,

TCO coating, laser scribing of TCO, deposition of intrinsic and doped layers, laser scribing

(a) (b)


25

of semiconductor layers, metallization, and laser scribing of metals [4], [29]. Figure 3.6

depicts the geometry, materials, and dimensions of thin-film solar cells connected in series.

Figure 3.6. Structure of thin-film solar cells in superstrate configuration connected in series [4].

Figure 3.7(a) shows the proposed fabrication steps for a p-i-n structure. It is based

on the use of TCO coated Corning glass; deposition of p, i and n layers; and metallization

using a shadow mask. Special care to the separation of electrodes must be taken in order to

reduce parasitic series resistance because in Figure 3.6 is observed that the active cell is 1

cm of length with an inactive separation of 0.05 cm due to the three steps of laser scribing.

The active area of the solar cell was delimited using shadow masks of different areas (0.08,

0.16, 0.32, 0.5, and 1 cm2). A separation of 0.15 cm between electrodes for TCO and n-type

layer was selected. Figure 3.7 (b) and (c) show the geometry of the masks, which were

prepared using flexible acetate. The geometry was made by an image processor; later the

geometry was printed and cut. Titanium is deposited in the white and gray area to define

the contact to the n-type layer and TCO, respectively. The blue area is covered during

evaporation to avoid the deposition of metals in this region.

Figure 3.7. (a) Fabrication steps of p-i-n solar cells in superstrate configuration. Designed shadow masks of

area: (b) 0.08, 0.16, 0.32; (c) 0.5 and 1 cm2; the area of the masks is 1”×1”.

(a) (c)

(b) Contact to TCO 1.00 0.50

Contact to TCO 0.32 0.08, 0.16


26

3.3. Preparation of samples

The silicon-based films are deposited on Corning 2947 glass of 1”×3”, c-Si wafers

of 6” of diameter, and flexible Kapton polyimide to study their properties. In order to

handle the substrates during the fabrication process, Corning glass and c-Si are cut into

small pieces of 1”×1.5” using tip scriber. The cleaning procedure is carried at the beginning

of the process to remove contaminants. Corning glass substrate is cleaned by using the

steps 1 to 3. c-Si wafers are cleaned using the 5 steps. The cleaning procedure is as follows:

1. 10 minutes in trichloroethylene by ultrasonic cleaning.

2. 10 minutes in acetone by ultrasonic cleaning.

3. De-ionized water rinse (3 times).

4. 10 seconds dip in 7:1 buffer solution (H2O:HF).

5. De-ionized water rinse (3 times).

Corning glass is dried in spinner (5 minutes). c-Si wafers are dried by N2 gas flow.

Finally, substrates are stored in a Petri dish.

Flexible polyimide substrates are cleaned 2 times for 10 minutes in isopropyl

alcohol (IPA) by ultrasonic cleaning and dried by laminar flow. TCO coated Corning glass

is cleaned with acetone for 10 min by ultrasonic cleaning. Afterwards, these substrates are

cleaned with IPA for 10 minutes [67]. Finally, they are dried by laminar flow and stored in

a Petri dish.

After cleaning, stripes of titanium are deposited on polyimide substrates and

Corning 2947 glass by e-gun evaporation using a shadow mask to form the metallic

electrodes. Table 3.3 shows the list of substrates along with their characteristics. Table 3.3. Characteristics of substrates.

Substrate Thickness, t

Area, A

Resistivity, ρ (Ω-cm) Comments

Corning 2947 glass 1.0 mm 1” × 3” 8.69 × 1010 Size of 1” × 1.5”

Polyimide 50 µm 11” × 8.5” 1 × 1017 Size of 1” × 1”

<100> c-Si wafer “low-ρ” 620 µm 6” of diameter 16.74 (4-probes) Size of 1” × 1.5”

Metallic stripes (by e-gun evaporation) 300 nm Width = 2 mm

Length = 1.5 cm Ti = 3.17×10-4

Stripes spacing: Width1=2 mm Width2=4 mm Width3=8 mm

TCO: Indium tin oxide (ITO) Fluorinated tin oxide (FTO)

200 nm 1” × 1” Rsh=10 Ω/, Transmittance=85%

TECHINSTRO [67] ITO: TIX001 FTO: TIXS001


27

3.4. Films characterization

The characterization program includes structural, optical, electrical and

optoelectronic measurements, which involves the excitation and detection of some kind of

energy (e.g., voltage, current, temperature, UV-Vis or IR radiation). Table 3.4 summarizes

the characterization techniques, optimal substrates, properties and characteristics that are

obtained, along with the equipment and instruments. OriginPro software was used to

analyze all data acquired from measurements; tools such as fitting, peak analyzer, batch

processing, graphing, and statistics were used [68]. Measurements are grouped as follows:

• Structural: thickness, deposition rate, FTIR and Raman spectra.

• Optical: photoluminescence, optical gap, and absorption spectra.

• Electrical: sheet resistance, conductivity, and activation energy.

• Optoelectronic: photoconductivity, photosensitivity, mobility-lifetime product, and

solar cell parameters under darkness and AM 1.5 illumination. Table 3.4. Characterization techniques [69].

Technique Substrate Properties and characteristics

Equipment and instruments

Profilometry Corning glass Thickness Deposition rate

Stylus profilometer KLA Tencor P-7 (Scan length: 1000 µm, scan speed: 100 µm/sec., sampling rate: 200 Hz)

Fourier Transform Infrared Spectroscopy (FTIR)

c-Si p-type Bondings: Si-H, C-H, Si-C Hydrogen content Microstructure parameter

Spectrometer BRUKER Vector 22 (scan range: 350-4000 cm-1 with MIR lamp, resolution: 1 cm-1)

Raman Corning glass Crystallinity Horiba Jobin Yvon HR800, He-Ne laser of 632.8 nm, CCD detector

Photoluminescence Corning glass or c-Si

Emission spectra Size of nanocrystals

Horiba Jobin Yvon Spectrofluorometer FluoroMax 3

Transmittance UV-Vis Corning glass

Optical properties: n, k Optical gap Thickness

Perkin-Elmer lambda-3B Spectrophotometer UV-Vis (scan range: 190-900 nm, res. 1 nm)

Current voltage, I(V)

Polyimide or Corning glass with stripes

Resistivity Conductivity Keithley’s 6517A electrometer

Current voltage with temperature, I(V,T)

Polyimide or Corning glass with stripes Activation energy

-Temp. controller 331 Lake shore -Keithley’s 6517A electrometer -PC, LabVIEW, GPIB 488.2 interface -Cryogenic station Janis res. corp.

Photosensitivity Polyimide with stripes Dark conductivity Photoconductivity

Solar simulator ORIEL Sol 2A Class ABA and electrometer 6517A

Solar cell parameters p-i-n structures VOC, ISC, FF, efficiency -Solar simulator ORIEL Sol 2A, ABA -Electrometer Keithley 6517A -Source Meter Unit Keithley 2401

Mobility-lifetime Polyimide with stripes Mobility-lifetime product Defect density Laser of 632.7 nm and 2 mW

Microscopy (SEM, AFM, Optical) Various Visual information about

morphology and structure -Hitachi SEM SU3500 -Easy scan DFM


28

3.4.1. Structural characteristics

A. Profilometry

Film thickness, tf(nm), and deposition rate, Vd(Å/s), were determined by stylus

profilometry, using a KLA-7 Tencor profilometer. A step is generated in the films by

plasma etching with SF6 or CF4 gases. The setup parameters of measurements are a scan

length of 1000 µm, a scan speed of 100 µm/s, and a sampling rate of 200 Hz.

B. Fourier-Transform Infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy was used to determine the

absorbance spectrum by means of a BRUKER spectrometer model Vector 22. The setup

parameters for measurements are as follows: a resolution of 2 cm-1, a scan time of 30, the

wavelength range from 350 to 4000 cm-1, 5 minutes for stabilization of chamber

environment in an inert ambient (N2 flow of 80 sccm). Additional data treatment of

baseline was carried out for all IR spectra. In order to process the raw data, the analysis is

focused on stretching and wagging modes of the kind C-H, Si-H and Si-C. The Gaussian

peaks were fitted, determining the amplitude, standard deviation, position, and area under

the Gaussian distribution. The concentration of hydrogen bonds of Si-H and C-H (NSi-H,C-H)

given in cm-3 is determined by using the expression:

,2.303 cte G

Si H C Hf p

A SNt k− − =

⋅ (3.3)

Where SG is the area under the Gaussian given in cm-1, kp is the central position of

the peak given in cm-1, Acte is a constant that depends on the vibration modes, and tf is the

thickness of the film in cm [70]. Table 3.5 lists the position of the characteristic bonds of

Si-H, Si-C and C-H, the position of low-dimensional bonds (C3-C7, Si3-Si7) [71], the

position of contaminants (CO2, H2O, SiO2), and the values of the constant Acte.

In order to estimate the hydrogen content, CH (at.%), it is required to know the

atomic density of the material, ρ. The density of silicon is 5 × 1022 at./cm3; for carbon films

it varies, because only amorphous carbon (a-C:H) or tetrahedral amorphous carbon (ta-C:H)

could contain hydrogen. Then, 1.6 g/cm3 for a-C:H and 2.4 g/cm3 for ta-C:H are used.

Hence, the density of carbon is 9.7×1022 at./cm3. The vibration mode at 640 cm-1 is

responsible for all vibrations. Thus, it is used to estimate the percentage of hydrogen


29

content into the a-Si:H films [70], because it is bonded with hydrogen in all possible

configurations. The expression to determine the hydrogen content is:

, 100 [ .%]Si C HH

NC at

ρ−= ⋅ (3.4)

The microstructure parameter, R*, is determined from the analysis of the stretching

Si-H vibration mode, located at 2000 cm-1 by:

* HSM

LSM HSM

IRI I

=+

(3.5)

Where ILSM is the integrated absorption of the low stretching mode (LSM) located at

2000 cm-1, and IHSM is the integrated absorption of high stretching mode (HSM) located at

2090 cm-1 [29]. The same region could exhibit a peak located at 2030 cm-1 known as

medium stretching mode (MSM) which is attributed to pm-Si:H films formation [29], [36]. Table 3.5. Position, kp, and kind of bonding for the SiC films [70], [71]. Acte is the constant for H2 analysis.

Position kp (cm-1)

Bonding of the silicon-carbon films

Factor Acte (cm-2)

Position kp (cm-1)

Bonding of oxygen contaminants

640-650 SiHn wagging 2.1×1019 666, 2360 CO2 (weather) 760-800 SiC stretching 1600, 3600 H2O (weather) 845 SiH2 rocking 450 SiO2 rocking 880-900 SiH2 scissors 800 SiO2 bending 950-960 Si-CHn wagging and rocking 1080 SiO2 stretching 980-1000 CH wagging 1×1021 k (cm-1) Bonding of few atoms 1200-1500 CHn=2,3, Si(CH3), C(CH3) bend. 486, 542; (470,525) C3; (Si3) 1200-2200 C-C(1200) C=C(1660) C≡C(2200) 352, 396; (430,470) C4; (Si4) 2000 SiH stretching 9×1019 530, 542; (233) C5; (Si5) 2080 SiH2 stretching 2.2×1020 480, 564; (404,458) C6; (Si3) 2140 SiHn stretching (porous silicon) 2855-2955 –C–Hn sp3 hybridized 7×1020 355-582; (420) C7-11; (Si7) 2975-3085 =C–Hn sp2 hybridized 424,437; (1809, 2090) C13; (C14) 3300 ≡C–H sp1 hybridized 468, 646, 1362, 2096 C16

C. Raman spectroscopy

Raman spectroscopy is a powerful technique since it provides information that is

specific to the chemical bonds and symmetry of molecules. This information allows the

identification of molecules and their structure (amorphous, crystalline, nanocrystalline).

Each peak in Raman spectra corresponds to a specific vibration of a given chemical bond.

The Raman shift of contaminants is 1335 cm-1 for CO2 (it is 2349 and 667 cm-1 in FTIR);

1595, 3657, 3756 cm-1 for H2O (by Raman and FTIR).


30

Raman spectrum was obtained using an integrated micro-Raman system. The

system includes a micro-spectrometer HORIBA Jobin Yvon HR800, an OLYMPUS BX41

microscope, and a thermoelectrically cooled CCD detector, an optical microscope (10, 50,

and 100X), and a video camera for focusing. A laser of 632.8 nm (He-Ne) is used as the

excitation source. The system allows a maximum spatial resolution of 6 µm and spectral

resolution of 0.5 cm-1. A c-Si sample is measured as a standard reference, a sharp peak at

520 cm-1 is calibrated, which corresponds to the transverse optical mode of c-Si.

Raman spectrum of carbons, such as diamond peak is located at 1332 cm-1, single

crystal graphite (labeled G) at 1580 cm-1. Disordered graphite has a second mode around

1350 cm-1 (labeled D for disorder). Raman spectrum of most disordered carbons is

dominated by these two G and D modes of graphite, despite the carbons do not have

particular graphitic ordering [46], [47].

The analysis by Raman spectroscopy of silicon supports the presence of crystalline

regions. Phonon modes can be identified, which correspond to transverse acoustic (TA) at

150 cm-1, longitudinal acoustic (LA) at 300 cm-1, longitudinal optic (LO) at 380 cm-1, and

transverse optic (TO) mode centered between 480 and 520 cm-1. Contributions of TA, LA,

and LO modes in Raman spectra are associated with nc-Si:H films [72]. The TO band was

fitted and analyzed using three Gaussian peaks to estimate the Raman crystallinity, XR. It

can be determined by means of:

505 520

480 505 520

( )( )R

I IXI I I

+=

+ + (3.6)

Where I480, I505, and I520 are the integrated intensities for amorphous (480 cm-1) and

crystalline phases (505–520 cm-1). The crystallite size, dR, was estimated using the formula:

22522R

TO

d πω

=−

(3.7)

Where ωTO is the position of the fitted peak of TO mode [23], [73].

3.4.2. Optical properties

A. Transmittance

Transmittance spectrum was measured by a Perkin-Elmer lambda-3B

spectrophotometer in the UV-Vis region (190-900 nm). A resolution of 1 nm and a scan


31

rate of 120 nm/min. were used. Transmittance for all the samples was measured on the

films deposited on Corning 2947 glass substrates. Optical properties of refractive index, n,

extinction coefficient, k, and thickness were estimated directly with PUMA software [74].

The renormalization procedure was performed referencing the transmittance measurements

to the Corning glass substrate in order to improve the estimation of optical constants and

thickness with PUMA software [53]. The extinction coefficient was converted to

absorption coefficient by α=4πk/λ, where α is the absorption coefficient given in cm-1 and λ

the wavelength of photons in cm. The optical gap was determined by Tauc’s method [5],

[75] in the strong absorption region (α > 104 cm-1) using: 1/2 1/2( ( ) ) ( )ga h h B h Eν ν ν⋅ = − (3.8)

Where hν is the photon energy, B is a factor for fitting, and Eg is the optical gap.

B. Photoluminescence

Photoluminescence (PL) is the light emission from a material after the absorption of

electromagnetic radiation. Measurements of PL spectra can be performed in two

configurations: emission and excitation. PL at visible wavelength was observed first from

porous silicon. Since that time, the origin of PL was not clear: localized defect states or

quantum confinement effects. Visible PL was explained by Canham as follows: when the

structure size becomes similar to the size of the Bohr radius (~5 nm), the photo-generated

electron-hole pairs (excitons) are trapped into a quantum well. The solution of the

Schrödinger equation gives discrete energy levels. The energy levels are inversely

proportional to the square of the width of the well, i.e., the smaller the size of the

nanocrystal or nanoparticle, the higher the energy. Photoluminescence emission energy is a

function of the nanocrystal size, which is represented by an inverse power law: 39.1

0 73.3)( −+= ncPL dEdE (3.9)

Where E0 is the band gap of bulk silicon (1.12 eV) and dnc is the nanocrystal

diameter [76]. The nature of photoluminescence is explained commonly by the model of

quantum confinement. However, the spatial confinement, the radiative surface states, the

interface states, the radiative and non-radiative defects must be taken into account to

explain the origin of PL [69].


32

Photoluminescence spectrum was measured by a Spectrofluorometer Horiba Jobin

Yvon FluoroMax 3. Setup parameters were as follows: scan speed of 160 nm/s, scan range

of 370 to 1000 nm, resolution of 1 nm, and integration time of 0.25 s. The samples

deposited on Corning 2947 glass substrates were used to measure the PL spectra of

emission, using an excitation wavelength of 330 nm (3.75 eV). This energy is higher than

the optical gap of silicon-carbon films (1.8<Eg(eV)<2.4).

3.4.3. Electrical properties

A. Current-voltage measurements, I(V)

Ohm’s law states that the electrical current, I, between two points is directly

proportional to the potential difference, V, across the two points, it is given by:

( ) VI VR

= (3.10)

Where R is the resistance in Ohm; resistivity or conductivity can be determined

from this measurement using geometrical factors. In the case of two planar electrodes, the

conductivity is a function of the separation length, L, the thickness of the film, tf, the width

of the electrodes, W, and the measured resistance [69]. Figure 3.8 depicts the structure for

the analysis of conductivity, which can be determined by means of:

1

f

LR t W

σ = (3.11)

Figure 3.8. Structure for conductivity analysis.

Methodology for reduction of electrical noise requires the initial measurement of an

I(V) curve without sample; verifying that a current below of ~1×10-12 A is obtained, for a

voltage sweep from 0 to 100 V. The setup for I(V) measurements are: initial voltage=100

V, final voltage=0 V, step voltage=1 V, delay time=1 s, and sweep rate of 1 V/s to ensure

stable measurements. In this study I(V) measurements are applied to the analysis of:

• Conductivity and sheet resistance.

σ

L

W tf

Metallic stripes Thin-film


33

• Arrhenius plot by I(V,T) measurements for activation energy determination.

• Photoconductivity and solar cell parameters.

B. Four-point probes

This technique is very useful to measure the resistivity of TCOs, metals, etc. The

method is based on four probes arranged spatially in a collinear array. According to the

method, a current is applied to the external probes and a voltage is measured in the internal

probes [69]. The source-meter 2400 Keithley instrument was used to supply the current and

measure the voltage. Depending on the kind of sample, the current is adjusted to fit the

voltage in a proper range for the instrument. Current is set at 4.532 mA for c-Si wafers of

~10 Ω -cm of resistivity. It is increased for metals to 45.32 mA. For high resistivity

samples, it is decreased to 4.532 µA. The resistivity can be obtained by:

4.532 fV tI

ρ = (3.12)

Where ρ is the resistivity, given in Ω-cm, tf is the thickness of the film in cm, V is

the measured voltage, and I is the applied current. Sheet resistance, Rsh, is defined by:

Rsh=ρ/tf [69].

C. Current-voltage measurements with temperature, I(V,T)

At high temperature, electronic transport in doped and intrinsic a-Si:H films leads to

a dependence between conductivity and temperature [44]:

0( ) exp( / )a BT E k Tσ σ= − (3.13)

This expression is known as Arrhenius temperature dependence. Where kB is the

Boltzmann constant (kB =8.617×10-5 eV/K); Eα is the activation energy, which is defined as

the separation of the Fermi energy, EF, from the mobility edge, EC, as Eα = EC – EF. The

pre-exponential factor, σ0, is correlated with the activation energy by:

0 00 exp( / )a MNE Eσ σ= (3.14)

Where σ00 is a constant and EMN is called the Meyer-Neldel characteristic energy. Eα

can be changed in a-Si:H by impurity doping (n-type and p-type). Typical values for a-Si:H

are: EMN=67 meV and σ00=1 S/cm. However, Eα can be affected by illumination due to the


34

photodegradation, resulting: EMN=43 meV and σ00=0.3 S/cm. Values of σ00=10-6 S/cm for

µc-Si:H have been determined [44].

The measurement of voltage and temperature dependent current, I(V,T), was carried

out to estimate the activation energy. The setup system consists of an electrometer Keithley

6517A for voltage sweep, a cryogenic station Janis res. corp. to hold the sample, and a

temperature controller 331 Lake shore for temperature sweep from 300 to 430 K in steps of

10 K. The cryogenic station must be in a vacuum environment; the vacuum pressure for all

measurements was 60×10-3 Torr to avoid heat losses and degradation of the sample due to

the temperature stress. A delay time of 6 minutes was set for the temperature sweep in

order to reach thermal equilibrium between the sample and the substrate holder.

3.4.4. Optoelectronic properties

A. Photoconductivity and photosensitivity

Dark conductivity of a semiconductor is given by σd=q(n0µn+p0µp), where n0 and p0

are the density of electrons and holes in thermal equilibrium, while µn and µp are the

electron and hole mobility, respectively. In the case of a-Si:H, µn and µp are very low

(~1 cm2/V). Photoconductivity is the result of photon absorption in a semiconductor;

photon energy should be higher than the band gap of the semiconductor material. The

absorbed photons create an excess of electron-hole pairs, Δn and Δp. As a consequence, the

densities of electrons and holes increase above their equilibrium values (n0 and p0), i.e.,

n=n0+Δn and p=p0+Δp [77]. Photoconductivity is defined as the net change in electrical

conductivity under illumination and is expressed by:

0 0( ) (( ) ( ) )ph n p n pq n p q n n p pσ µ µ µ µ= ∆ + ∆ = − + − (3.15)

Where Δn and Δp are the excess of electron and hole densities, respectively. In a

degenerate semiconductor, Δn and Δp are even lower than p0 and n0; and the effect of

incident photons can be considered as a small perturbation. However, in an insulator or a

non-degenerate semiconductor, Δn and Δp can be comparable to or larger than n0 and p0

[29], [58]. The photoconductivity, σph, is determined from dark conductivity, σd, and

conductivity under illumination, σ, using the expression:

dph σσσ −= (3.16)


35

Photosensitivity, Sph, is defined as the ratio of photoconductivity and dark

conductivity. An optimal photosensitivity is in the range of 103-105 [29]. It is given by:

ph dph

d d

Sσ σ σσ σ

−= = (3.17)

Photosensitivity of the films can be determined from current-voltage measurements,

I(V), under AM 1.5 illumination and darkness conditions. The objective is the study of the

photosensitivity of the material integrated over the whole solar spectrum by means of the

analysis of dark conductivity and photoconductivity.

B. Mobility-lifetime product

The dominant carrier transport in p-i-n structures is by drift, not by diffusion like in

c-Si solar cells based on p-n junctions. Mobility-lifetime product, µτ, is the parameter that

includes generation, transport, and recombination processes of majority carriers in a-Si:H

films. For device-quality a-Si:H films, the mobility-lifetime product should be ~1×10-7

cm2/V [29]. The mobility-lifetime product can be estimated by the Equation 3.18 [78]:

ph

qGσ

µτ = (3.18)

Where σph is the photoconductivity due to the laser exposure, q is the elemental

charge, and G is the generation rate. G is given by:

(1 )(1 exp( ))f

f

R tG

tα− − −

= Φ (3.19)

Where R is the reflectance, tf is the film thickness, α is the absorption coefficient

and Φ is the photon flux [29]. In order to obtain a uniform carrier generation within the

thickness, a monochromatic beam of ~600 nm is typically used for a-Si:H films, because

the absorption coefficient is small at such wavelengths. Hence, a standard monochromatic

red laser of 632.7 nm of wavelength and 2 mW of power was used. The relation between

the intensity of radiation and photon flux is given by the equation:

1.24I qλ λ= Φ × (3.20)

Where Iλ(W/m2) is the intensity of radiation, Φ(Joule) is the photon flux, λ(µm) is

the laser wavelength, and q is the elementary charge (q=1.609×10-19 C) [79].


36

3.5. Solar cell parameters

The air mass coefficient, AM 1.5, is used to characterize the performance of solar

cells under standardized conditions. A solar simulator is a light source of spectral content

AM 1.5, and 100 mW/cm2 of intensity. The p-i-n solar cells were characterized by means

of a solar simulator model Oriel Sol 2A Class ABA, and a source meter unit Keithley 2401

with a LabVIEW interface. The main solar cell parameters are defined as follows:

Open-circuit voltage, VOC, is the voltage at which no current flows through the

external circuit, it depends on thermal voltage, illumination and saturation current:

0

ln 1B LOC

k T IVq I

= +

(3.21)

Short-circuit current, ISC, is the current that flows through the external circuit when

the electrodes are short-circuited:

LSC II = (3.22)

Fill-factor, FF, is the ratio of maximum obtainable power, to the product of the

open-circuit voltage and the short-circuit current:

MAX MAX MAX

OC SC OC SC

P I VFFV I V I

= = (3.23)

Efficiency, η, is the ratio of energy output from the solar cell to energy input:

IN

OCSC

IN

MAX

PFFVI

PP

==η (3.24)

From the I(V) curve can be determined directly VOC and ISC, but IMAX and VMAX are

determined by a Power-Voltage plot, finding the combination of IMAX and VMAX where the

power is maximum. In order to determine the efficiency, it is required the knowledge of

power input, PIN. It can be calculated using the intensity of AM 1.5 spectrum, IAM1.5=100

mW/cm2, and the active area, A, of the solar cell in cm2, by means of:

1.5IN AMP I A= (3.25)

Figure 3.9(a) shows the equivalent circuit of a solar cell, it behaves as a current

source under illumination, denoted with IL. However, many loss mechanisms inherent to

the structure and materials are present. These losses avoid 100% of the conversion

efficiency of solar energy into electrical energy. Loss processes in a standard single-

junction solar cell include lattice thermalisation loss, junction loss, contact loss,


37

recombination loss, and loss of low energy photons below the band gap [2]. Losses can be

modeled through a series resistance, rs, and a shunt resistance, rsh (shown in Figure 3.9(a)).

Figure 3.9(b) shows the current voltage, I(V), characteristics under illumination and

dark conditions. The main parameters that characterize a solar cell are depicted in the

figure, such as VOC, ISC, FF, and η [4], [8]. The I(V) equations for dark and illumination

conditions are shown in the figure. In dark conditions, the I(V) curve shows the typical

rectifying behavior of a p-n junction, but under illumination conditions the current displays

a shifting down behavior. Photogenerated current yields a region in the fourth quadrant of

the I(V) curve, i.e., the p-n junction generates power.

Figure 3.9. (a) Equivalent circuit for solar cells, and (b) current voltage, I(V), characteristics of a solar cell,

illustrating their main parameters under illumination and dark conditions.

Typical parameters of solar cells based on a-Si:H are: VOC=0.58 V, JSC=11 mA/cm2,

FF=40%, and η=1.10% for the first a-Si:H p-i-n solar cell [17]; versus VOC=0.89 V,

JSC=16.3 mA/cm2, FF=69%, and η=10.2% for the world record a-Si:H p-i-n solar cell [14].

Additionally, equations for the modeling of solar cells are listed. Equation 3.26 is

the ideal model without parasitic resistances (rs and rsh). This model is illustrated in Figure

3.9(b). Equation 3.27 includes rs and rsh. Equation 3.28 includes a recombination term for

the intrinsic layer [80], which diminishes the current under illumination, IL.

[ ]0( ) exp( / ) 1B LI V I qV nk T I= − − (3.26)

[ ]0( ) exp( ( ) / ) 1 ss B L

sh

V IrI V I q V Ir nk T Ir−

= − − + − (3.27)

[ ] ( )

2

0( ) exp( ( ) / ) 1( )fs

s B L Lsh bi s

tV IrI V I q V Ir nk T I Ir V V Irµτ−

= − − + − +− −

(3.28)

(a) (b)


38

The I(V) model shown in Equation 3.28 was developed for thin-film solar cells of a-

Si:H [80]. In this model is observed the importance of film thickness and mobility-lifetime

product. Hence, it is essential to address the influence of both parameters on the solar cell

performance. According to the bibliography, an optimal thickness for the intrinsic layers is

between 200–500 nm. Besides a high mobility-lifetime product is desirable. In this sense,

Figure 3.10 shows the dependence of the short-circuit current density of p-i-n solar cells as

a function of defect density and thickness of the intrinsic layer [44].

Figure 3.10. Short-circuit density, JSC, plotted as a function of the defect density, Nd, for p-i-n solar cells of

different thickness (220, 443 and 680 nm) [44].

If defect density of intrinsic films is high, JSC is low. And vice versa, when defect

density is low, JSC increases. Three regions can be distinguished in the Figure 3.10. For

Nd>1016 cm-3, a low thickness is appropriate to obtain a JSC in the range of 1-18 mA/cm2.

When Nd is close to 1016 cm-3, a JSC of 19 mA/cm2 is observed; being negligible the effect

of thickness. Finally, when Nd is 1015 cm-3, a thicker film of 680 nm is ideal to obtain a

high JSC of 23 mA/cm2.

Defect density, Nd, is inversely proportional to the mobility-lifetime product.

Assuming ballistic motion of carriers and drift mobility [5], Nd can be estimated by:

16

Ed

B C

aNk T q µτ σ

= ⋅⋅

(3.29)

Where σC is the capture cross-section, its value is ~10-15 cm2, kBT/q is the thermal

voltage (25.9 mV), aE is the scattering mean free path of carriers (~5 Å) [5]. Thus, the

defect density can be obtained as a function of the mobility-lifetime product and constants.

Chapter 4. Results and discussion

39

4. Results and discussion

In this chapter is presented the study of intrinsic and doped silicon and silicon-

carbon films, as wells as their application in thin-film solar cells. The motivation of this

dissertation is to investigate if solar cells based on silicon films can be obtained at the

facilities of INAOE. Initial OFAT experiments of intrinsic SiC films obtained by MV/LF-

PECVD showed a low photosensitivity. However, as will be seen later, intrinsic silicon

films with a high photosensitivity were obtained, despite the medium vacuum level and the

significant leak rate. On the other hand, intrinsic SiC films were synthesized by HV/RF-

PECVD through simple OFAT experiments. These films were used as standard references

because HV/RF-PECVD reactor reaches a high vacuum and an excellent leak rate.

In the initial experiments, CH4 flow was varied to study its effect on the

optoelectronic properties of the films. However, the two-level factorial DOE was an

efficient tool for planning the experimental work. Pressure and gases were selected as

factors. Firstly, the intrinsic silicon films were deposited by using H2 and SiH4. Afterwards,

B2H6 and PH3 were incorporated for the synthesis of the doped films. Finally, CH4 was

incorporated to alloy the intrinsic and doped films. Films with properties as doped, buffer,

absorbing, and photoluminescent layers were obtained by MV/LF-PECVD. Amorphous,

nano-crystalline, and subwavelength structured films were obtained by HV/RF-PECVD.

The optoelectronic and structural properties of the intrinsic and doped films were

analyzed. Photosensitivity, activation energy, and conductivity were the properties of high

priority to select the p-, i-, and n-type layers. Finally, p-i-n solar cells were fabricated by

MV/LF-PECVD, using Corning glass coated with FTO as front-electrode and titanium as

back-electrode. The effect of reactor passivation and plasma cleaning of TCO substrates

with O2, Ar, and H2 were analyzed. These surface treatments were fundamental to obtain

stable, functional, and reproducible p-i-n solar cells. This chapter is organized as follows:

1. Preliminary OFAT experiments of intrinsic films.

2. Two-level factorial DOE for intrinsic and doped films.

3. Improvement experiments for intrinsic and doped films.

4. Preliminary experiments of p-i-n solar cells.

5. Improvement experiments for p-i-n solar cells.


40

4.1. Preliminary OFAT experiments

4.1.1. Intrinsic silicon-carbon films by MV/LF-PECVD

In these experiments is studied the effect of carbon content in intrinsic silicon films,

using H2 and Ar as dilution gases by MV/LF-PECVD. SiH4 diluted at 10% in H2 and CH4

were the precursor gases. CH4 was varied from 0 to 250 sccm. Fixed parameters were as

follows: p=300 W, f=110 kHz, p=0.6 Torr, T=160 ºC, SiH4=250 sccm, H2=2750 sccm, and

Ar=2000 sccm. Corning 2947 glass and p-type <100> c-Si wafers were used as substrates.

Table 4.1 shows the parameters of XC and Rd. The thickness measured by profilometry is

listed in Table 4.1. Samples are labeled according to their carbon content in gas phase.

Appendix A describes the procedures for deposition of the thin-films by MV/LF-PECVD. Table 4.1. Gas flows, thickness and deposition rate of the SiC films obtained by MV/LF-PECVD.

Process Sample CH4 (sccm)

SiH4 (sccm)

Time, t (min) XC Rd Thickness,

tf (nm) Deposition rate, Vd (Å/s)

1276 C0-H 0 250 30 0 119 70.49 0.39 1277 C0.55-H 30 250 30 0.55 55 64.6 0.35 1278 C0.77-H 85 250 30 0.77 27 61.5 0.34 1279 C0.91-H 250 250 30 0.91 11 62.4 0.34 1280 C1.00-H 250 0 30 1.00 11 Undetected - 1281 C0-Ar 0 250 60 0 89 58.2 0.16 1282 C0.55-Ar 30 250 60 0.55 40 124.2 0.34 1283 C0.77-Ar 85 250 60 0.77 20 200.5 0.55 1284 C0.91-Ar 250 250 60 0.91 8 74.7 0.20 1285 C1.00-Ar 250 0 60 1.00 8 62.6 0.17

The results of conductivity and photosensitivity are shown in Figure 4.1(a) and (b),

respectively. Photoconductivity and dark conductivity were similar (~10-7 S/cm) as can be

observed in the Figure 4.1(a). All samples showed a low photosensitivity as can be seen in

Figure 4.1(b). Hence, these samples are not suitable as absorbing layers (i-type layers).

Figure 4.1. (a) Conductivity, and (b) photosensitivity of the SiC films by MV/LF-PECVD.

0.0 0.2 0.4 0.6 0.8 1.010-8

10-7

10-6

10-5

σph (H) σph (Ar)

Phot

ocon

duct

ivity

, σph

(Ω−c

m)-1

Carbon content in gas phase, XC

10-8

10-7

10-6

10-5

σd (H) σd (Ar) Dark conductivity, σ

d (Ω−cm

) -1

0.0 0.2 0.4 0.6 0.8 1.00

2

4

6

8

10

12

Sph (H) Sph (Ar)

Phot

osen

sitivi

ty, S

ph

Carbon content in gas phase, XC(a) (b)


41

Dark conductivity of intrinsic a-Si:H is ~10-9 S/cm, and typical current is ~1 pA

using a bias of 100 V. Apparently, these films are µc-Si:H (~10-6 S/cm). However, titanium

stripes were deposited directly on Corning 2947, and the film was deposited later on it.

This configuration is not suitable due to the high parasitic conductivity of Corning 2947.

Because a current of 1 nA was measured by using a bias of 100 V. Thus, stripes deposited

directly on Corning 2947 disturb the real I(V) measurements. One solution is the deposition

of stripes directly on Corning 1737 or flexible polyimide, and later the film. Because the

conductivity of these substrates (10-15 S/cm) is lower than the conductivity of a-Si:H films.

Optical gap, refractive index and extinction coefficient are shown in Figure 4.2(a)

and (b). Optical gap of the SiC films is in the range of 1.8 and 2.4 eV, these values are

comparable to the Eg reported in the literature. Refractive index at 633 nm decreases as the

carbon content in gas phase increases. It is in the range of 1.7-3.5. Extinction coefficient

showed random values, tendencies were not observed for this property.

Figure 4.2. (a) Optical gap from Tauc method and at α=104 cm-1; (b) refractive index, n, and extinction

coefficient, k, at 633 nm of the SiC films deposited by MV/LF-PECVD.

Hence, MV/LF-PECVD reactor requires of analysis to determine the causes of the

low photosensitivity of the SiC films, because these films are inappropriate to be used as

absorbing layers in solar cells. The vacuum level was 70 mTorr and QL=10-3 mbar L/s

(QL≤10-4 mbar L/s is optimal); also the deposition parameters require optimization.

4.1.2. Intrinsic silicon-carbon films by HV/RF-PECVD

Intrinsic silicon films alloyed with carbon are studied in these experiments; the

films were deposited by HV/RF-PECVD at optimal leak rate. One factor at a time was

0.0 0.2 0.4 0.6 0.8 1.0

1.8

2.0

2.2

2.4

3.5

E04 (H) E04 (Ar) E (H) -Tauc E (Ar) -Tauc

Opt

ical g

ap, E

g (eV

)

Carbon content in gas phase, Xc0.0 0.2 0.4 0.6 0.8 1.0

1

2

3

4

Refra

ctive

inde

x, n

(@ 6

33 n

m)

Carbon content in gas phase, Xc

n(H) n(Ar)

0.00

0.02

0.04

0.06

0.08

k(H) k(Ar)

Extinction coefficient, k (@ 633 nm

)

(a) (b)


42

varied, in this case, the CH4 flow. Gas flows, pressure, power, temperature, and time were

fixed. The obtained properties are correlated to the carbon content in gas phase.

CH4 was varied in the range of 1 to 60 sccm. Fixed parameters were as follows: P=3

W, f=13.56 MHz, p=0.7 Torr, SiH4=30 sccm, H2=sccm, and deposition time of 2700 s.

Corning 1737 glass and p-type <100> c-Si wafers were used as substrates. Table 4.2 lists

the samples, gas flows, XC and Rd parameters, along with the thickness and deposition rate

determined by profilometry. These films were not diluted with H2. However, SiH4 gas is

diluted at 10% in H2. Table 4.2. Conditions of deposition and thickness of the intrinsic SiC films deposited by HV/RF-PECVD.

Process Sample CH4 (sccm)

SiH4 (sccm)

H2 (sccm) XC Rd Thickness,

tf (nm) Dep. rate, Vd (Å/s)

22A C95 60 30 0 0.95 0.42 246 0.911 21A C87 20 30 0 0.87 1.17 258 0.716 21B C77 10 30 0 0.77 2.07 256 0.948 21C C50 3 30 0 0.50 4.50 278 1.029 21D C25 1 30 0 0.25 6.75 220 0.814

Figure 4.3(a) shows the FTIR spectra of the intrinsic SiC films from 500-3300 cm-1.

Figure 4.3(b) shows the Raman spectra of the intrinsic SiC films from 100 to 2500 cm-1.

Figure 4.3. (a) FTIR spectra and (b) Raman spectra of the SiC films prepared by HV/RF-PECVD.

The characteristic peaks in the IR spectra of the SiC films are in good agreement

with the position and vibration modes reported in the literature (see Table 3.5). However,

peaks at the region of ~1600 cm-1 could be attributed to the formation of small clusters (C3-

C6, Si3-Si6) or C=C bondings (sp2 hybridization). Raman spectra of all films showed the

characteristic peak at 480 cm-1 attributed to the amorphous structure. Peaks at 505 cm-1

attributed to grain boundaries, or at 520 cm-1 attributed to nc-Si:H were negligible.

500 750 1000 1250 1500 1750 2000 2250 30000,00

0,05

0,10

2140

, Si-H

n

2965

, =C-

H n sp2 h

ybry

dize

d 28

60, -

C-H n s

p3 hyb

rydi

zed

1750

, C2S

i

1470

, C2H

6 15

90, C

=C

1360

, C-H

3 ben

d12

55, C

-C 10

30, C

-H, C

6H12

900,

Si-H

scis

sor

850,

Si-H

2 roc

k

645,

Si-H

wag

771,

Si-C

stre

tch

2080

, Si-H

2 stre

tch

2000

, Si-H

stre

tch

C95 C87 C77 C50 C25

Abso

rban

ce, A

Wavenumber, k (cm-1)500 1000 1500 2000 2500

0.00.20.40.60.81.01.21.41.61.82.0

C95 C87 C77 C50 C25

Ram

an in

tens

ity, I

(A.U

.)

Raman shift, (cm-1)

a-Si:H

(a) (b)


43

Figure 4.4(a) shows the transmittance spectra of the SiC films, and Figure 4.4(b)

shows the PL spectra of the SiC films. The sample C95 exhibits a strong PL at 580 nm

(dnc=2.13 nm), and the sample C25 shows a low PL at 416 nm (dnc=1.60 nm). The strong

PL observed for the sample C95 can be attributed to the presence of nanocrystalline

inclusions. In fact, PL in intrinsic SiC films is attributed to radiative recombination of

electron-hole pairs of the sp2 bonded clusters in sp3 bonded amorphous matrix [81].

Figure 4.4. (a) Transmittance and (b) photoluminescence of the SiC films deposited by HV/RF-PECVD.

Figure 4.5(a) and (b) show the refractive index and absorption coefficient spectra,

respectively. The refractive index is consistent with the values reported in the literature; it

decreases as the carbon content increases.

Figure 4.5. (a) Refractive index and (b) absorption spectra estimated by means of PUMA software.

The optical gap at 104 cm-1 was determined from Figure 4.5(b), Eg increases as the

carbon content increases. Figure 4.6(a) shows the behavior of refractive index, optical gap

and mobility-lifetime product as a function of the carbon content in gas phase. The

0 100 200 300 400 500 600 700 800 900-0.10.00.10.20.30.40.50.60.70.80.91.0

C95 C87 C77 C50 C25

Tran

smitt

ance

, T

Wavelength, λ (nm)400 500 600 700 800 900 1000

0

1x104

2x104

3x104

C95 C87 C77 C50 C25416 nm

Excitation: λ = 330 nm

Phot

olum

ines

cenc

e, P

L (c

ps)

Wavelength, λ (nm)

580 nm

300 400 500 600 700 800 9001.52.02.53.03.54.04.55.05.56.0

C95 C87 C77 C50 C25

Refre

activ

e in

dex,

n

Wavelength, λ (nm)1.5 2.0 2.5 3.0 3.5 4.0101

102

103

104

105

106

107

C95 C87 C77 C50 C25Ab

sorp

tion

coef

ficie

nt, α

(cm

-1)

Photon energy, hν (eV)

(a) (b)

(a) (b)


44

mobility-lifetime product showed optimal values for photovoltaic applications (µτ >10-7

cm2/V). Sample C87 was the exception because µτ was one order of magnitude lower.

Figure 4.6(b) shows the results of conductivity and photosensitivity of the intrinsic SiC

films. As can be seen in Figure 4.6(b), photosensitivity decreases as the carbon content

increases. Samples C25 and C50 showed the best photosensitivity (Sph>104), which is

suitable for solar cell applications. However, the incorporation of carbon in high

concentrations deteriorates the photosensitivity of the intrinsic SiC films.

Figure 4.6. (a) Refractive index, optical gap, mobility-lifetime product, (b) conductivity and photosensitivity

of the SiC films prepared by HV/RF-PECVD.

Silicon nanocrystals embedded in dielectrics such as SiO2, a-SiN, and a-SiC are

promising for the third generation photovoltaics [22]. In an ideal closed system at thermal

equilibrium, emitted and absorbed fluxes must be equal according to energy conservation

principle. The Kirchhoff’s law states that absorptance, α, and emittance, ε, are equal: ε=α

[82]. Moreover, Eli Yablonovitch states that a great solar cell should be a great light

source, e.g., solar cells and LEDs based on GaAs [83]. Besides, p-i-n structures fabricated

with nanocrystalline SiC films are used as electroluminescent devices [48], [84]. However,

they are not used in solar cells, because in luminescent devices the embedded nanocrystals

are high-quality radiative recombination centers for the photogenerated carriers in the

amorphous matrix. Therefore, the sample C95 is not suitable to be used as the absorbing

layer in a solar cell due to its low photosensitivity despite of its high photoluminescence.

Nevertheless, the sample C95 showed a high down-conversion photoluminescence,

i.e., absorbed photons at UV region are reemitted to the visible region. This sample also

meets the requirements of optical gap and refractive index to work as an anti-reflective

0 10 20 30 40 50 60 70 80 90 100

2.0

2.5

3.0

3.5

4.0

4.5

Refractive index, n Optical gap, Eg

Mobility-lifetime, µτ

Carbon content in gas phase, XC (%)

Refra

ctive

inde

x, n

; Opt

ical g

ap, E

g (eV

)

10-9

10-8

10-7

10-6

Mob

ility-

lifetim

e pr

oduc

t, cm

2 / V

0 20 40 60 80 10010-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Dark conductivity Photo conductivity

Photosensitivity

Carbon content in gas phase, XC (%)

Cond

uctiv

ity, σ

(Ω−c

m)-1

10-1

100

101

102

103

104

105

106

Phot

osen

sitivi

ty, S

ph

(a) (b)


45

coating (ARC). Hence, this sample is adequate to be used as a photoluminescent and ARC

on c-Si solar cells. In combination with a TCO layer, superstrate and substrate

configurations are possible using this kind of ARC in p-i-n solar cells.

ARCs made of Si3N4, SiO2, ITO and ZnO are commonly used in solar cells.

Reflectance, R, at the air-silicon interface is approximated by R=(n-1)2/(n+1)2, where n is

3.5 for silicon; then R=30%. The combination of refractive index and thickness of the ARC

is chosen to minimize the reflection at ~483 nm of wavelength (peak of solar spectrum).

Formulae from Novotny [85] and Wolf [86] were used for the calculation of reflectance.

The expression for modeling the reflectance in terms of complex reflection coefficients

(r12, r23), thickness, tf, and propagation wave vector, kz2 is given by: 2 2

12 23 12 23 22

12 23 12 23 2

2 cos(2 )1 ( ) 2 cos(2 )

z f

z f

r r r r k tR

r r r r k t+ +

=+ +

(4.1)

Figure 4.7(a) shows the normal incidence reflectance obtained from Equation 4.1,

varying the thickness of the ARC. Reflectance for bare silicon was R=0.3. Interference

fringes are observed. A reflectance of R=0 is found at 490 nm for a thickness of tSiC=60

nm, which is optimal for anti-reflective and photoluminescent coatings based on SiC films.

Refractive index of SiC layer and bulk c-Si were used to estimate the reflectance: nSiC=2

and nc-Si=3.5, respectively. Figure 4.7(b) shows the SiC samples deposited on Corning

1737 glass illuminated with natural light and UV radiation of λ=252 nm. The sample C95

shows a strong luminescence under UV radiation. Also, Corning glass emits visible

radiation under UV exposition, as can be observed at the regions without deposition.

Figure 4.7. (a) Design of anti-reflective coatings using the obtained properties of the SiC films. (b) Samples

of SiC deposited on Corning 1737 under visible and UV radiation.

200 300 400 500 600 700 800 9000,0

0,1

0,2

0,3

0,4 Si ARC 20 nm ARC 60 nm ARC 120 nm

Refle

ctan

ce, R

Wavelength, λ (nm)

C25 C50 C77 C87 C95

C25 C50 C77 C87 C95

(a) (b)

Visible illumination

UV illumination


46

4.2. DOE for intrinsic layers

4.2.1. Intrinsic silicon films by HV/RF-PECVD

In the previous section, single OFAT experiments were carried out by MV/LF- and

HV/RF-PECVD, in which tendencies were sketched and correlated to the CH4 flow.

However, description of each parameter was poorly discussed about the parameters that

were selected, such as power, temperature, gas flows, and pressure of the chamber. In these

experiments is intended to find optimal values for these parameters, where films with

suitable properties for photovoltaics can be synthesized. The two-level factorial DOE was

implemented in order to find the main effects and optimize the experimental work.

Pressure, SiH4, and H2 were varied at two-levels because these factors are very influential

in the film properties. These experiments correspond to DOE i-Si from Table 3.2.

The intrinsic silicon films were synthesized by HV/RF-PECVD. Fixed parameters

were as follows: substrate temperature of 150 °C, power of 30 W, and frequency of 13.56

MHz; glow discharge was maintained for 30 minutes. Corning 2947 glass, flexible

polyimide sheets, and p-type <100> c-Si wafers were used as substrates. For electrical

characterization, titanium stripes were deposited on polyamide substrates by e-gun. H2 was

the dilution gas and SiH4 diluted at 10% in H2 was the precursor gas. Low and high values

for pressure, SiH4, and H2 flows were established. The cube plot with the low and high

values that take each factor is shown in Figure 4.8(a). Figure 4.8(b) shows the thickness of

the films, tf, as a function of the dilution ratio. Rd was set at 0.5, 2.5, 5 and 25. The samples

are labeled according to their levels, e.g., HSiP-000 means that all factors are at a low

level. Structural and optoelectronic properties were analyzed.

Figure 4.8. (a) Cube plot of factors and (b) thickness of the silicon films as a function of dilution ratio.

1.3

0.7

100

10 25050

Pressure

Silane flow

Hydrogen flow

Cube Plot

(sccm)

(sccm)

(Torr)

(a) (b)


47

Figure 4.9(a) shows the absorbance spectra obtained by FTIR spectroscopy.

Medium stretching mode (MSM) located at 2030 cm-1 is attributed to nanocrystalline

surfaces in pm-Si:H films. The Gaussian peak at 2016 cm-1 can be seen as a combination of

LSM and MSM. In the IR spectra were observed the contributions of LSM, MSM and

HSM. Figure 4.9(b) shows the Raman spectra of the films. The region at 500-520 cm-1 is

correlated to the presence of nanocrystals. The samples at a low level of H2 (Rd=0.5 and

Rd=5) showed these features. The contribution of MSM in IR spectra confirmed the

formation of nanocrystals. Figure 4.10 shows the AFM images of the silicon films.

Figure 4.9. (a) Absorbance and (b) Raman spectra of the silicon films prepared by HV/RF-PECVD.

Figure 4.10. AFM images of the surface of the intrinsic silicon films deposited by HV/RF-PECVD.

400 600 800 2000 2200

0.00

0.02

0.04

0.06

0.08

0.10

0.12 HSiP-000 HSiP-001 HSiP-010 HSiP-011 HSiP-100 HSiP-101 HSiP-110 HSiP-111

Abso

rban

ce, A

Wavenumber, k (cm-1)

300 350 400 450 500 550 600 650 700

1000

2000

3000

4000

5000 HSiP-100 HSiP-101 HSiP-110 HSiP-111

HSiP-000 HSiP-001 HSiP-010 HSiP-011

Ram

an in

tens

ity, I

(a.u

.)

Raman shift, (cm-1)

480500

520

(a) (b)


48

Significant variations are observed in the scanned area (3×3 µm2). RMS roughness,

particle size and density were determined by Scanning Probe Image Processor (SPIP).

Subwavelength structures of ~300 nm of size were determined for the samples at a low

level of H2 (Rd=0.5 and Rd=5). It is known that roughness increases approximately to 15

nm in µc-Si:H films. In this sense, as-deposited silicon films with a roughness in the range

of 20-36 nm were obtained without any pre- or post-deposition treatments.

Optical gap was estimated from transmittance measurements, shown in Figure

4.11(a). The low transmittance in the weak absorption region of samples HSiP-010 and

HSiP-011 is correlated with the formation of subwavelength structures that scatter the

incident radiation. Dark conductivity and photoconductivity are shown in Figure 4.11(b).

Photosensitivity indicates the degree of transduction from optical to electrical power of the

incident photons that are absorbed in the films. A high vacuum level is optimal to achieve

good electronic properties because it diminishes the concentration of oxygen and impurities

in the chamber. The vacuum pressure in the HV/RF-PECVD was 10-7 Torr. However, the

gas flows were critical for obtaining a high photosensitivity. Even with an optimal vacuum

level and an excellent leak rate, photosensitivity was not optimal for all experiments. The

best photosensitivity was obtained in the samples that showed subwavelength structures.

Figure 4.11. (a) UV-Vis transmittance. (b) Dark and AM 1.5 conductivity of the intrinsic silicon films

deposited by HV/RF-PECVD.

Table 4.3 lists all the structural, optical, and electrical properties that were analyzed

for the eight samples whose factors of SiH4, H2 and pressure were varied. All of them are

in good agreement with the typical values that are known for a-Si:H or nc-Si:H films. The

main effects are listed in the last column. The graphs of each property were difficult to

200 300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8


Tran

smita

nce,

T

Wavelength, λ (nm)000 001 010 011 100 101 110 111

10-10

10-9

10-8

10-7

10-6

10-5

10-4

Dark conductivity Photo conductivity

DOE 2-level HSiP

Dark

con

duct

ivitiy

, σd (

Ω−c

m)-1

10-10

10-9

10-8

10-7

10-6

10-5

10-4

AM1.

5 p

hoto

cond

uctiv

itiy, σ

ph(Ω

−cm

)-1

(a) (b)


49

interpret due to the lack of tendencies, e.g., in the graph of photoconductivity and dark

conductivity, as shown in Figure 4.11(b). Here we have three factors varying one at once,

with responses that varied orders of magnitude in some cases. For samples at a low level of

H2 (Rd=0.5 and Rd=5), two groups of samples resulted in similar characteristics, being the

SiH4 flow the key parameter. At a high level of SiH4, films with enhanced photosensitivity

were obtained due to the presence of random subwavelength structures that makes the film

opaque, scattering the light and enhancing the absorption. These films are appropriate as

absorbing layers in solar cells. On the other hand, at a low level of SiH4, films that include

crystalline regions were obtained. These films showed a low photosensitivity and a high

dark conductivity which is suitable for p- or n-type layers. Table 4.3. Properties and main effects of the intrinsic silicon films prepared by HV/RF-PECVD.

Sample HSiP-XXX Main Effects Property 000 001 010 011 100 101 110 111

Thickness, tf (nm) 170 203 425 1056 32 54 259 406 Si Deposition rate, Vd (Å/s) 0.947 1.130 2.361 5.866 0.181 0.300 1.442 2.256 Si RMS roughness, Sq (nm) 26.72 19.86 24.94 36.61 2.43 1.25 2.52 0.7 H Particle density (µm-2) 2.77 1.33 5.77 3.44 0 0 0 0 H Particle size (nm) 330.5 259.3 252.0 331.6 0 0 0 0 H Abs. coeff., α×105 (cm-1) @UV 2.61 2.21 1.30 0.70 10.0 7.77 1.87 1.24 Si Abs. coeff., α×103 (cm-1) @NIR 3.70 13.4 10.8 9.84 13.9 96.1 2.09 0.37 - Optical gap, Eg (eV) 2.05 2.06 1.69 1.62 1.79 2.16 1.83 1.77 Si Dark conductivity, σd×10-9 (S/cm) 160 44 1.5 0.6 36 17 0.5 9.9 Si Photosensitivity, Sph×103 0.018 0.54 5.7 11 0.001 0.056 8.1 0.54 Si Mobility-lifetime, µτ×10-7 (cm2/V) 0.24 0.16 0.54 1.10 0.001 0.034 1.37 0.47 Si Defect density, Nd×1015 (cm-3) 13.2 19.3 5.92 3.15 3857 94.5 2.34 6.88 Si Raman crystallinity, XR (%) 53.0 38.0 5.0 1.0 0.0 0.0 9.0 7.0 H-Si Hydrogen content, CH (%) 19.92 21.05 25.84 19.41 23.36 10.68 23.95 22.77 - Microstructure parameter, R* 0.65 0.37 0.03 0.12 0.95 0.91 0.03 0.12 Si Si-H mode position (cm-1) 2093 2016 2002 2018 2090 2100 2022 2006 Si

Silicon form (x-Si) nc/sw nc/sw sw nc/sw a a nc a

The two-level factorial DOE and the analysis of main effects are important tools,

which provided fundamental information to take decisions in this experimental work. The

SiH4 flow was the main effect in these experiments. Ordering the main effects in

descendant form, the SiH4 flow had an impact on 10 properties, followed by H2 flow with 3

properties, and finally an interaction factor of hydrogen-silane (H-Si). The blank spaces

indicate that none factor had an effect. The silicon form was inferred from the structural

characterization. AFM provided information about subwavelength structured silicon films

and Raman crystallinity along with the Si-H stretching mode by FTIR revealed bonding

characteristics at the atomic scale, which could be amorphous or nanocrystalline.


50

Light absorption in structures with wavelength scale periodicity enhances when the

wavelength in the material, λ, is comparable to the periodicity, L. In order to maximize the

absorption, the periodicity should be smaller than the wavelength range of interest

(0.6<L/λ<1) to obtain an enhancement factor, F, in the range of 4n2-12n2 approximately

[87], where n is the refractive index of the medium. For a large periodicity, L>>λ, the

absorption factor approaches to 4n2 (Yablonovitch limit). However, the random periodicity

of the subwavelength structured films limits the enhancement factor to Yablonovitch limit.

Additional characterization was carried out for the samples at a low level of H2

because these films exhibited optimal properties to be used in thin-film solar cells.

Depositions on all substrates of all experiments were optimal. For instance, Figure 4.12(a)

shows the substrates used for the sample HSiP-000. At first glance, no problems of

adherence or anomalous characteristics were observed. However, the exception was the

sample HSiP-011, shown in Figure 4.12(b). Deposition on Corning glass resulted in full

coverage. But the same deposition on the c-Si wafer was self-lifted from the wafer surface

a few hours after the samples were unloaded from the chamber. No cracks were induced in

this 1 μm-thick silicon membrane. Problems of adherence were observed for the stripes

evaporated on Corning samples, but flawless electrodes were obtained in polyimide

samples [28].

(a) Sample HSiP-000 (b) Sample HSiP-011

Figure 4.12. (a) Sample HSiP-000 and (b) Sample HSiP-011 deposited on Corning 2947 glass, c-Si and

polyimide substrates.

Figure 4.13 shows the scanning electron microscopy images for the samples

deposited on polyimide. A scale of 5 µm was used for the measurements. The diameter of

the subwavelength structures, dSW, and the surface coverage, CSW, were determined using

IMAGE J software. The mean dSW and its standard deviation are indicated in Figure 4.13


51

along with the surface coverage. The presence of subwavelength structures in combination

with random wavelength-scale periodicity could enhance the absorption in a particular

region of the solar spectrum for λ~dSW and λ~L.

Figure 4.13. Scanning electron micrographs of the silicon samples at low-hydrogen dilution with

subwavelength structures.

Finally, light-induced degradation was determined. The silicon films were exposed

to AM 1.5 illumination. Exposure time was 480 min, with interruptions of AM 1.5

illumination every 120 min. The sampling time to determine the AM 1.5 conductivity was

15 min. The evolution of the dark and AM 1.5 conductivity as a function of time is shown

in Figure 4.14. Dark conductivity was measured at the beginning and the end of each

interruption of AM 1.5 illumination. It showed an approximate variation of 1 order of

magnitude when it was measured before and after light soaking. During light soaking, the

samples were fan-cooled at a temperature of 33 °C in order to avoid light-induced heating.

Interruptions in the illumination at 120 and 360 min showed that the AM 1.5 conductivity

did not change significantly. After the interruption at 240 min, samples were annealed at a

temperature of 160 °C for 2 h under a vacuum level of ~10-3 Torr. Although they were

tested simultaneously under the same conditions, different behaviors were observed for the

various samples. Samples HSiP-010 and HSiP-011 exhibited the Staebler–Wronski effect.

Because after annealing, the AM 1.5 conductivity was recovered. Degradation of the AM


52

1.5 conductivity of these films was observed again between 240 and 480 min. An inverse

Staebler–Wronski effect was observed for samples HSiP-000 and HSiP-001 because unlike

the case of those characterized by light-induced degradation, AM 1.5 conductivity of these

samples was increased. Sample HSiP-000 showed an increase of one order of magnitude in

AM 1.5 conductivity (over the course of 480 min), as can be seen in Figure 4.14.

Figure 4.14. Conductivity as a function of time under an illumination of 1 sun. AM 1.5 illumination was

interrupted every 120 min. In the second interruption, the films were annealed at 160 °C for 2 h in vacuum.

Table 4.3 showed that samples at a low dilution of H2 exhibited the best properties.

In these experiments, it was shown that silicon films with subwavelength structures can be

grown during plasma deposition, just by properly setting the gas flows. The stability of

subwavelength structured films deposited on flexible substrates (without roughness) could

then enable their use in large-area and light-weight photovoltaic applications [28]. On the

other hand, the 1-µm thick silicon membrane requires additional studies to prove their use

as an intrinsic (absorbing) layer. Several challenges must be addressed, for instance, the

deposition of p- and n-type layers, or HSL and ESL along with the deposition of TCOs or

metals to set the contacts; avoiding cracks in the membrane.

0 60 120 180 240 300 360 420 480

10-7

10-6

10-5

10-4 HSiP-010 HSiP-011

HSiP-000 HSiP-001

Interruption inthe illumination

AM 1

.5 c

ondu

ctivi

ty, σ

ph (Ω

-cm

)-1

Time, t (minutes)

AnnealingInterruption inthe illumination t=2 hours

T=160°C

10-1010-910-810-710-610-510-410-310-210-1100101

Dark

con

duct

ivity

, σd (

Ω-c

m)-1


53

4.2.2. Intrinsic silicon films by MV/LF-PECVD

Previous experiments of intrinsic silicon films were performed in the MVSystem

cluster tool, by applying the two-level factorial DOE. The vacuum pressure of the HV/RF-

PECVD reactor (10-7 Torr) is better than the vacuum pressure of the MV/LF-PECVD

reactor (10-3 Torr). However, turbomolecular pumps of HV/RF-PECVD reactor are used

for vacuum and deposition mode. Hence, these pumps are affected by the powders

generated during deposition mode. For this reason, the HV/RF-PECVD reactor is used to

obtain a-Si:H films; and only a specific set of parameters are allowed. Thus, alloying and

doping experiments in the HV/RF-PECVD reactor were discarded due to these issues.

Therefore, it was decided the improvement of deposition conditions of the MV/LF-

PECVD reactor to obtain films with suitable properties for solar cells. The aim of these

experiments is the synthesis of silicon films with high photosensitivity. Gas partial pressure

concepts were applied to improve its performance. Because preliminary experiments

(Section 4.1.1) were not successful. Appendix A describes the methodology for optimal

deposition in this reactor. The two-level factorial DOE and the analysis of the main effects

are applied. Again, pressure, SiH4 flow, and H2 flow are the factors under study. Low and

high values for each factor are proposed, which were matched to the operating ranges of

the MV/LF-PECVD reactor. These experiments correspond to DOE i-Si from Table 3.2.

Fixed parameters were as follows: temperature of 250°C, power of 500 W glow

discharge was maintained for 30 minutes. Corning 2947 glass and p-type <100> c-Si

wafers were used as substrates. For electrical characterization, titanium stripes were

deposited on flexible polyamide sheets by e-gun evaporation using shadow masks. SiH4 at

100% was the precursor gas and H2 the dilution gas. Table 4.4 shows the results of

profilometry measurements, along with the dilution ratio and the silane content. Table 4.4. Deposition parameters and thickness of the intrinsic silicon films obtained by MV/LF-PECVD.

Sample H2 (sccm)

SiH4 (sccm)

Pressure (Torr)

Dilution ratio, Rd XSi

Thickness, tf (nm)

Deposition rate, Vd (Å/s)

HSiP-000 80 20 0.6 4 0.2 105 0.583 HSiP-001 80 20 1.6 4 0.2 120 0.722 HSiP-010 80 80 0.6 1 0.5 240 1.333 HSiP-011 80 80 1.6 1 0.5 250 1.388 HSiP-100 320 20 0.6 16 0.058 110 0.611 HSiP-101 320 20 1.6 16 0.058 125 0.666 HSiP-110 320 80 0.6 4 0.2 285 1.583 HSiP-111 320 80 1.6 4 0.2 295 1.633


54

Optical, structural and electrical properties of the silicon films were analyzed. The

samples were labeled according to the low or high values that take each factor. For

instance, sample HSiP-011 indicates that in the experiment H2 is low, SiH4 is high and

pressure is high. Deposition rate of the films increases as the SiH4 flow increases, as can be

observed in Figure 4.15(a). When SiH4 level is high (80 sccm), deposition rate is in the

range of 1.3-1.6 Å/s. Deposition rate of films at low-SiH4 flow (20 sccm) is 0.5-0.7 Å/s.

Figure 4.15. (a) Deposition rate of the silicon films, (b) influence of dilution ratio on deposition rate.

Besides, in Figure 4.15(b) is observed that four samples have the same dilution ratio

(Rd=4). But the deposition rate shows a significant variation comparing the samples at the

same pressure, which implies different properties of the films. Despite the silicon films

have an identical XSi or Rd; when flows are scaled, the deposition rate is different.

Figure 4.16(a) shows the FTIR spectra and Figure 4.16(b) shows the Raman

spectra. These measurements revealed that structure of all silicon films is amorphous.

Figure 4.16. (a) FTIR and (b) Raman spectra of the intrinsic silicon films prepared by MV/LF-PECVD.

000 001 010 011 100 101 110 1110.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Deposition rate

Depo

sitio

n ra

te, V

d (A

/s)

Sample HSiP-XXX0 2 4 6 8 10 12 14 16 18 20 22 24 26

0.6

0.8

1.0

1.2

1.4

1.6

1.8

H2=80, SiH4=80

H2=80, SiH4=20

H2=320, SiH4=80

H2=320, SiH4=20

Pressure=0.6 Torr Pressure=1.6 Torr

Depo

sitio

n ra

te, V

d (Å

/s)

Dilution ratio, R

500 1000 1500 20000

500

1000

1500

2000

HSiP-000 HSiP-001 HSiP-010 HSiP-011 HSiP-100 HSiP-101 HSiP-110 HSiP-111

Abso

rptio

n co

effic

ient

, α (c

m-1)

Wavenumber, k (cm-1)100 200 300 400 500 600 700 800 9000

200400600800

10001200140016001800 HSiP-000

HSiP-001 HSiP-010 HSiP-011 HSiP-100 HSiP-101 HSiP-110 HSiP-111

Ram

an in

tens

ity, (

A.U.

)

Raman shift, (cm-1)

(a) (b)

(a) (b)


55

Figure 4.17(a) shows the hydrogen content obtained from the fitting with Gaussian

functions the wagging mode located at 640 cm-1. It is in the range of 22-38%. Figure

4.17(b) shows the results of microstructure parameter and the position of the Si-H

stretching mode located at the region of 2000-2030 cm-1. The microstructure parameter,

R*, was determined from the analysis of the stretching vibration modes of the FTIR

spectra. Contributions of LSM, MSM, and HSM were observed in these samples.

Figure 4.17. (a) Results of hydrogen content; (b) stretching mode and microstructure parameter analysis.

Figure 4.18 shows the AFM measurements. RMS roughness, Sq, was determined by

Scanning Probe Image Processor (SPIP). The roughness of the silicon films was in the

range of 0.5–2 nm (in the scanned area 3×3 µm2). These values are typical for a-Si:H films. HSiP-000 HSiP-001 HSiP-010 HSiP-011

HSiP-100 HSiP-101 HSiP-110 HSiP-111

Figure 4.18. AFM images of the intrinsic silicon films deposited by MV/LF-PECVD.

000 001 010 011 100 101 110 11120

25

30

35

40

Hydr

ogen

con

tent

, CH (

%)

Sample, HSiP-XXX000 001 010 011 100 101 110 111

1995

2000

2005

2010

2015

2020

Si-H stretch Microstructure parameter

Sample, HSiP-XXX

Si-H

stre

tch

posit

ion,

k (c

m-1)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Micr

ostru

ctur

e pa

ram

eter

(a) (b)


56

Figure 4.19(a) shows the transmittance spectra. The Tauc plot is shown in Figure

4.19(b) to estimate the optical gap of the silicon films; Eg was in the range of 1.7-2 eV. In

this characterization, the Beer-Lambert law was applied to convert from transmittance to

absorption coefficient. Because when PUMA software is used, the absorption spectrum is

distorted in the strong-absorption region.

Figure 4.19. (a) Transmittance and (b) Tauc plot of the intrinsic silicon films obtained by MV/LF-PECVD.

Finally, the optoelectronic characterization is presented. Figure 4.20(a) shows the

results of conductivity and photosensitivity measurements. Sample HSiP-011 exhibited an

optimal photosensitivity of ~104, due to the combination of high photoconductivity and low

dark conductivity. The samples at a high level of H2 showed the worst photosensitivity.

Figure 4.20(b) shows the results of mobility-lifetime product characterization. Mobility-

lifetime product was in the range of 10-8-10-7 cm2/V, which is optimal.

Figure 4.20. (a) Conductivity and photosensitivity of the silicon films prepared by MV/LF-PECVD, (b)

results of mobility-lifetime characterization.

200 300 400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8


Tran

smita

nce,

T

Wavelength, λ (nm)1 2 3 4 5 6 7

0

500

1000

1500

(α⋅h

ν)1/

2 , (eV

cm-1)1/

2

HSiP-000 HSiP-001 HSiP-010 HSiP-011 HSiP-100 HSiP-101 HSiP-110 HSiP-111

Photon energy, hν (eV)

000 001 010 011 100 101 110 11110-10

10-9

10-8

10-7

10-6

10-5

10-4

dark conductivity photo conductivity

Cond

uctiv

ity, σ

(Ω−c

m)-1

Sample, HSiP-XXX

10-1

100

101

102

103

104

105

Photosensitivity

Phot

osen

sitivi

ty

000 001 010 011 100 101 110 1110,10,20,30,40,50,60,70,80,91,0

Mob

ility-

lifetim

e pr

oduc

t, µτ

x10

-7 (c

m2 /V

)

Sample, HSiP-XXX

µτ

(a) (b)

(a) (b)


57

In order to achieve good electronic properties, a high vacuum level is desirable

because it diminishes the concentration of contaminants in the reactor. However, the

methodology for optimal deposition and the varied factors in this DOE were critical for

obtaining a high photosensitivity despite the medium vacuum level of 10-3 Torr.

The I(V) curves were measured on the films deposited on polyimide Kapton

substrates with titanium stripes. Depending on the assignation of these films in a solar cell

as the p-type, n-type or intrinsic layer; each film must be chosen accurately according to its

electrical and optical properties. The best film to be used as an intrinsic layer is the sample

HSiP-011 because it showed the highest photosensitivity.

Minitab software was used to obtain the main effects that have an influence on the

properties of the films. Table 4.5 shows the analysis of results using the software Minitab

to determine the main effect for each property. From the table can be concluded that the

SiH4 flow was the main effect for the following properties: thickness, deposition rate,

microstructure parameter, the position of the Si-H stretching mode, absorption coefficient

at the UV region, mobility-lifetime product and defect density. Table 4.5. Properties and main effects of the intrinsic silicon films deposited by MV/LF-PECVD.

Sample HSiP-XXX Main Effect Property 000 001 010 011 100 101 110 111

Thickness, tf (nm) 105 130 240 250 110 120 285 295 Si Deposition rate, Vd (Å/s) 0.583 0.722 1.333 1.388 0.611 0.666 1.583 1.638 Si RMS roughness, Sq (nm) 1.913 0.376 2.148 1.028 0.644 0.507 0.339 1.975 - Hydrogen content, CH (%) 29.34 22.22 31.97 31.88 34.68 23.37 28.88 38.36 - Microstructure parameter, R* 0.87 0.92 0.29 0.59 0.89 0.87 0.21 0.56 Si Si-H mode position (cm-1) 2001 2002 2012 2006 1995 2000 2017 2012 Si Abs. coeff., α×105(cm-1) @UV 3.9 3.4 2.0 1.8 3.7 3.4 1.7 1.5 Si Optical gap, Eg (eV) 1.94 2.01 1.72 1.86 1.79 2.03 1.73 1.88 - Mobility-lifetime, µτ×10-7 (cm2/V) 0.73 0.53 0.15 0.17 0.47 0.73 0.14 0.16 Si Defect density, Nd×1015 (cm-3) 4.4 6.0 21 18 6.7 4.3 22 19 Si Dark conductivity, σd×10-9 (S/cm) 0.60 0.20 1.1 0.2 68 1.00 7900 8600 H-Si Photoconductivity, σph×10-6 (S/cm) 0.18 0.06 1.8 2.2 2.0 0.13 6.3 2.2 - Photosensitivity, Sph×103 0.29 0.31 1.6 12 0.03 0.13 8e-4 2e-4 -

Silicon form (x-Si) a a a a a a a a

In fact, similarly to previous DOE by HV/RF-PECVD, the SiH4 flow was the main

effect in these experiments. Ordering the main effects in descendant form, the SiH4 flow

had an impact on 7 properties and an interaction factor of Hydrogen-Silane had an

influence on dark conductivity. The blank spaces indicate that none factor had an effect.


58

4.3. Improvement of intrinsic layers by MV/LF-PECVD

Experiments for improvement and reproducibility were carried out before of doping

and alloying experiments. The aim of this study is the optimization of the sample HSiP-

011. Only the photoconductivity and dark conductivity were determined. The comparison

of previous and new results is shown in Table 4.6. Table 4.6. Experiments of intrinsic silicon films for improvement and reproducibility of sample HSiP-011.

Property Reference (HSiP-011)

H2=80 sccm SiH4=80 sccm p=1.6 Torr T=250 ºC



Dark conductivity, σd (S/cm) 0.2×10-9 0.14×10-9 8.5×10-9 0.3×10-9 Photoconductivity, σph (S/cm) 2.2×10-6 4.3×10-6 1.7×10-7 1.7×10-5 Photosensitivity, Sph 1.2×104 3×104 20 5.6×104

Result Similar Bad Optimal

Conductivity and photosensitivity between the reference (HSiP-011) and a new

process were similar, as can be observed in Table 4.6. Low temperatures enable the use of

flexible substrates and low thermal stress of the films. For this reason, an experiment at the

temperature of 150 °C was performed. However, a low photosensitivity of 20 was

obtained. Perhaps the low photosensitivity of preliminary experiments for intrinsic SiC

films (section 4.1.1) can be attributed to the temperature of 160 °C. Therefore, the

temperature was set again to T=250°C in the temperature controller. Finally, previous

results of the factorial DOE revealed that a high flow of SiH4 is optimal. Thus, the flows of

SiH4 and H2 were incremented from 80 to 100 sccm, in order to keep the proportion of

gases. Results were encouraging because of the photoconductivity increases. It was

reflected in an increment of photosensitivity from 1.2×104 to 5.6×104.

Taking into account the new conditions of the experiment HSiP-011 (100 sccm for

H2 and SiH4), the optimization of intrinsic SiC films as photo-luminescent and anti-

reflective coatings was performed. Two-level values were set for CH4 and SiH4 flows.

These experiments correspond to DOE i-SiC from Table 3.2. Table 4.7 shows the gas flows

and the properties obtained from the electrical and optical characterization of these films. Table 4.7. Deposition conditions of the intrinsic silicon-carbon films by MV/LF-PECVD.

Process Sample SiH4 (sccm)

CH4 (sccm)

XC (%) Rd tf

(nm) Eg

(eV) σd×10-9

(S/cm) σph×10-9

(S/cm) Sph

P1386 Si1,C1 100 500 83 0.16 515.5 2.03 0.08 1.72 21 P1387 Si1,C0 100 100 50 0.50 216.0 1.54 0.40 7.78 19 P1388 Si0,C1 20 500 96 0.19 325.2 2.23 0.19 4.87 25 P1389 Si0,C0 20 100 83 0.83 175.5 2.10 0.23 5.39 23


59

Fixed parameters were as follows: p=1.6 Torr, P=500 W, T=250 ºC, H2=100 sccm,

and deposition time of 30 min. Figure 4.21(a) and (b) show the FTIR and Raman spectra of

the SiC films, respectively. Photoluminescence and transmittance spectra are shown in

Figure 4.22(a) and (b), respectively. Incorporation of carbon decreases the photosensitivity

of these films to be used as absorbing layers. However, the sample Si0,C1 showed high

phosphorescence as can be seen in Figure 4.21(b). Also, Figure 4.22(a) shows a high PL.

Further optimization of these films could enhance the PL.

Figure 4.21. (a) FTIR and (b) Raman spectra of the intrinsic SiC films deposited by MV/LF-PECVD.

Figure 4.22. (a) Photoluminescence and (b) transmittance of the intrinsic SiC films by MV/LF-PECVD.

PL of films obtained by HV/RF-PECVD (2×104 cps) was higher than the PL of the

films obtained by MV/LF-PECVD (5×103 cps). This difference of one order of magnitude

could be attributed to the frequency of deposition (LF vs. RF). Moreover, the sample C95

showed peaks in FTIR spectrum at the region of 1000-1600 cm-1 (section 4.1.2), which are

responsible for PL in SiC films [81]. However, the FTIR spectrum of SiC films prepared

by MV/LF-PECVD did not show these features.

500 1000 1500 2000 2500 30000.000.020.040.060.080.100.120.140.160.18

2900

, -C-

Hn s

p3 h

ybrid

ized

2100

, Si-H

2 st

retc

h20

00, S

i-H s

tretc

hing

1000

, C-H

wag

ging

780,

Si-C

640,

Si-H

wag

ging

P1386 Si1,C1 P1387 Si1,C0 P1388 Si0,C1 P1389 Si0,C0

Abso

rban

ce, A

Wavenumer, k (cm-1)0 500 1000 1500 2000 2500

0

1000

2000

3000

4000

5000

6000

7000 P1386 Si1,C1 P1387 Si1,C0 P1388 Si0,C1 P1389 Si0,C0

Ram

an in

tens

ity, (

A.U.

)

Raman shift, (cm-1)

400 500 600 700 800 900 10000

5000

10000

15000

20000 P1386 Si1,C1 P1387 Si1,C0 P1388 Si0,C1 P1389 Si0,C0

Phot

olum

ines

cenc

e, P

L (A

.U.)

Wavelength, λ (nm)200 300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

Tran

smita

nce,

T

Wavelength, λ (nm)

P1386 Si1,C1 P1387 Si1,C0 P1388 Si0,C1 P1389 Si0,C0

(a) (b)

(a) (b)


60

4.4. DOE for doped layers

Previous experiments for intrinsic films varying pressure, H2 and SiH4 flows were

successful. Films with high photosensitivity were obtained by HV/RF- and MV/LF-

PECVD in the experiment HSiP-011. Equation 2.1 for dilution ratio, Equation 2.2 for

silane content, and Equation 2.10 for estimation of carbon content in gas phase have been

applied. In the successive DOE for doping, PH3 and B2H6 gases are incorporated as factors.

The conditions to obtain the intrinsic layer that showed the best electronic properties were

applied (sample HSiP-011) since it implies a low defect density. In order to estimate the

concentration of dopants in the gas phase, Equations 2.4 and 2.5 were modified to include

the terms that correspond to H2 and CH4 flows, by means of:

3

3 4 2 4

( )gPHC n

PH SiH H CH=

+ + + (4.2)

2 6

2 6 4 2 4

( )gB HC p

B H SiH H CH=

+ + + (4.3)

4.4.1. Doped silicon films with B2H6 and PH3 by MV/LF-PECVD

These experiments correspond to DOE n-Si and DOE p-Si from Table 3.2. PH3 and

B2H6 gases were selected for variation. Two-level values for PH3 and B2H6 were set at 200

sccm (low level) and 1000 sccm (high level). Fixed parameters were as follows: SiH4=100

sccm, H2=100 sccm, P=500 W, p=1.6 Torr, and deposition time of 3 min 45 s. Table 4.8

shows the deposition parameters and the main results obtained from characterization. Table 4.8. DOE for doped silicon films varying dopant gases (PH3 and B2H6), and their main properties.

Parameter Sample (Process)

Si:B1 (P1371)

Si:B0 (P1376)

Si:P1 (P1399)

Si:P0 (P1400)

Synt

hesi

s

Type p p n n H2 (sccm) 100 100 100 100 SiH4 (sccm) 100 100 100 100 PH3 (sccm) - - 1000 200 B2H6 (sccm) 1000 200 - - Dilution ratio, Rd 11 3 11 3 Dopant concentration, Cg 0.0083 0.005 0.0083 0.005

Cha

ract

eriz

atio

n Thickness, tf (nm) 30.4 19.7 26.4 26.5 Activation energy, Eα (eV) @T↑ 0.36 0.43 0.50 0.50 Activation energy, Eα (eV) @T↓ 0.39 0.43 0.45 0.40 Optical gap, Eg (eV) 0.48 0.76 0.63 0.63 Dark conductivity, σd×10-6 (S/cm) 1.87 29.4 0.12 1.09 Photoconductivity, σph×10-6 (S/cm) 4.67 127.8 3.30 24.0 Photosensitivity, Sph 2.4 4.34 27.5 22


61

The best σd for p-type doped layers was 29.4×10-6 S/cm (sample Si:B0). The best σd

for n-type doped layers was 1.09×10-6 S/cm (sample Si:P0). The best Eα for p-type doped

layers was 0.36 eV (sample Si:B1). The best Eα for n-type doped layers was 0.40 eV

(sample Si:P0) after sweeping of temperature from 30 to 160 °C (T up). Further

optimization is required to achieve Eα=0.3 eV in n-type doped layers. Besides, all samples

exhibited a low photosensitivity.

A low flow of B2H6 or PH3 is optimal to obtain high dark conductivity and low

activation energy in n- and p-type doped layers. This result is consistent with the

observations reported for doping of silicon films (Section 2.2) where a low concentration of

dopant atoms in gas phase yields to a high doping efficiency.

FTIR and Raman spectra are shown in Figure 4.23(a) and (b), respectively. The low

absorbance of spectra was attributed to the low thickness of the films (~20 nm). However,

the sample Si:B1 shows a high peak of absorbance at 2525 cm-1 in the FTIR spectrum.

Raman spectroscopy confirms that the films have an amorphous structure. It is observed

also that TO and TA modes of Raman spectrum showed a similar intensity.

Figure 4.23. (a) FTIR and (b) Raman spectra of the p- and n-type doped silicon films.

Photoluminescence and transmittance spectra are shown in Figure 4.24(a) and (b),

respectively. The sample at a high level of boron showed a high PL in the visible region.

The low optical gap could be influenced by the thickness. Because optical gap was

determined by Tauc’s method and Equation 3.8, in which the absorption coefficient

spectrum is used; and full absorption is not achieved due to the low thickness.

500 1000 1500 2000 2500

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

2525

, B2H

6670,

Si-H

wag

2090

, Si-H

2 st

retc

hing

2000

, Si-H

stre

tchi

ng

P1371 Si:B1 P1376 Si:B0 P1399 Si:P1 P1400 Si:P0

Abso

rban

ce, A

Wavenumber, k (cm-1)100 200 300 400 500 600 700 800 900 10000

100

200

300

400

500

380,

LO

mod

e

150,

TA

mod

e

480,

TO

mod

e


Ram

an in

tens

ity, (

A.U.

)

Raman shift, (cm-1)

a-Si:H

300,

LA

mod

e

(a) (b)


62

Figure 4.24. (a) Photoluminescence and (b) transmittance spectra of the p- and n-type doped silicon films.

Figure 4.25(a) and (b) show the Arrhenius plot of the p- and n-type doped layers,

respectively. It is concluded that a low flow of dopants (200 sccm) is optimal for p- or n-

type doped layers because high σd and low Eα were obtained in samples Si:B0 and Si:P0.

Figure 4.25. Activation energy of the (a) p-type and (b) n-type doped silicon films.

4.4.2. Doped silicon-carbon films with B2H6 by MV/LF-PECVD

P-type doped silicon films alloyed with carbon are studied in this DOE. These

experiments correspond to DOE p-SiC from Table 3.2. Fixed parameters were: SiH4=100

sccm, H2=100 sccm, and deposition time of 3 min 45 s. Two-level values were: 100 and

500 sccm for CH4; 200 and 1000 sccm for B2H6. Table 4.9 shows the deposition

parameters and the main results obtained from characterization. The sample Si:C0,B1

(P1379) showed the best dark conductivity (5.73×10-6 S/cm). Then, a high flow of B2H6 is

convenient, when carbon is incorporated in p-type doped layers. Also, its activation energy

300 400 500 600 700 800 900 10000

50001000015000200002500030000350004000045000


Phot

olum

ines

cenc

e, P

L (A

.U.)

Wavelength, λ (nm)200 300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0 P1371 Si:B1 P1376 Si:B0 P1399 Si:P1 P1400 Si:P0

Tran

smita

nce,

T

Wavelength, λ (nm)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.410-7

10-6

10-5

10-4

10-3

10-2

Si:B0

Ea=0.36 eV (1371 Tup) Ea=0.39 eV (1371 Tdown) Ea=0.43 eV (1376 Tup) Ea=0.43 eV (1376 Tdown)

Cond

uctiv

ity, σ

(Ω−c

m)-1

Temperature-1 1/T (1000/K)

p-type silicon layers

Si:B1

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.410-7

10-6

10-5

10-4

10-3

10-2

Si:P0 n-type silicon layers

Cond

uctiv

ity, σ

(Ω−c

m)-1



Si:P1

(a) (b)

(a) (b)


63

was optimal (~0.36 eV). On the other hand, the sample Si:C1,B0 (P1393) showed the

lowest Eα of 0.10 eV and a high optical gap of 2.22 eV despite its low thickness of 17.9

nm. But the low conductivity (1.3×10-9 S/cm) is not useful for doped layers. However, this

sample could be applied as a buffer layer. Indeed, resistive layers enhance VOC in different

types of solar cells, high lifetimes and low carrier mobilities are ideal for buffer layers [88]. Table 4.9. DOE for doped silicon-carbon films varying CH4 and B2H6, and their main properties.

Parameter Sample (Process)

Si:C0,B1 (P1379)

Si:C1,B1 (P1380)

Si:C0,B0 (P1392)

Si:C1,B0 (P1393)

Synt

hesi

s

Type p p p p H2 (sccm) 100 100 100 100 SiH4 (sccm) 100 100 100 100 CH4 (sccm) 100 500 100 500 B2H6 (sccm) 1000 1000 200 200 Carbon content, XC (%) 50 83 50 83 Dilution ratio, Rd 5.5 1.8 1.5 0.5 Dopant concentration, Cg 0.0077 0.0058 0.004 0.002

Cha

ract

eriz

atio

n Thickness, tf (nm) 16.3 19.8 22.8 17.9 Activation energy, Eα (eV) @T↑ 0.36 0.37 0.48 0.10 Activation energy, Eα (eV) @T↓ 0.36 0.35 0.38 0.10 Optical gap, Eg (eV) 1.47 1.41 0.93 2.22 Dark conductivity, σd×10-6 (S/cm) 5.73 2.13 0.44 0.0013 Photoconductivity, σph×10-6 (S/cm) 12.9 7.01 1.49 0.0043 Photosensitivity, Sph 2.25 3.29 3.38 3.30

Figure 4.26(a) and (b) show the FTIR and Raman spectra of the p-type doped SiC

films, respectively. The intensity of absorbance spectrum was low, thus the characteristic

modes were affected by the noise due to the low thickness of the samples. On the other

hand, Raman spectroscopy only allowed the observation of silicon modes. The peaks

attributed to TA, LO and TO modes are shown in the Figure 4.26(b).

Figure 4.26. (a) FTIR and (b) Raman spectra of the silicon-carbon films doped with boron (p-type).

500 1000 1500 2000

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

980,

C-H

770,

Si-C

stre

tchi

ng

670,

Si-H

wag

P1379 Si:C0,B1 P1380 Si:C1,B1 P1392 Si:C0,B0 P1393 Si:C1,B0

Abso

rban

ce, A

Wavenumber, k (cm-1)

2090

, Si-H

2 st

retc

hing

100 200 300 400 500 600 700 800 900 1000

0

100

200

300

400

500

600

380,

LO

mod

e

480,

TO

mod

e

150,

TA

mod

e


Ram

an in

tens

ity, (

A.U.

)

Raman shift, (cm-1)(a) (b)


64

Figure 4.27 (a) and (b) show the PL and transmittance of the p-type SiC films,

respectively. A high PL was measured for the sample Si:C1,B1 (P1380). The transmittance

of the sample Si:C1,B0 (P1393) showed a different behavior compared to the other films.

Figure 4.27. (a) Photoluminescence and (b) transmittance spectra of the p-type SiC films doped with B2H6.

Figure 4.28 (a) and (b) show the Arrhenius plot of the p-type SiC films. A

significant variation of Eα is observed when B2H6 is set at a low level, and CH4 is varied at

two-levels. In particular, the sample Si:C1:B0 showed the lowest variation in conductivity

despite sweeping of temperature from 300 to 430 K, as a result, a low Eα was obtained.

Figure 4.28. Activation energy of the p-type SiC films: (a) at high level of B and (b) low level of B.

Boron-doped a-SiC:H films and boron-doped nc-SiC:H films have been applied in

n-i-p and p-i-n solar cells as p-type doped layers. By using these films, the highest VOC was

obtained [4], [29]. Besides, the inclusion of intrinsic a-SiC:H and a-Si:H films as buffer

layers at the p/i interface have contributed to maximizing the values of VOC [4], [89]. For

these reasons, boron-doped SiC films were studied in this section.

300 400 500 600 700 800 900 10000

50001000015000200002500030000350004000045000


Phot

olum

ines

cenc

e, P

L (A

.U.)

Wavelength, λ (nm)200 300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0


Tran

smita

nce,

TWavelength, λ (nm)

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.410-9

10-8

10-7

10-6

10-5

10-4

10-3

Si:C1,B1

p-type silicon-carbon layers

Cond

uctiv

ity, σ

(Ω−c

m)-1



Si:C0,B1

2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,410-9

10-8

10-7

10-6

10-5

10-4

10-3

Si:C1,B0

Si:C0,B0

p-type silicon-carbon layers

Cond

uctiv

ity, σ

(Ω−c

m)-1


Ea=0.48 eV (1392 Tup) Ea=0.10 eV (1393 Tup)

(a) (b)

(a) (b)


65

4.5. Preliminary p-i-n structures for solar cells by MV/LF-PECVD

According to the results obtained from previous experiments, the intrinsic and

doped layers with the best properties were chosen. The selection criteria for doped layers

are high dark conductivity and low activation energy. The selection criteria for intrinsic

layers are high photosensitivity and low dark conductivity. Two p-i-n structures were

fabricated. The first p-i-n junction was an entire silicon structure; for the second p-i-n

junction only the p-type a-Si:H layer was changed to a-SiC:H,B. Corning glass coated with

ITO of 20 nm and Rsh of 80 Ω/ was used. Fixed parameters were as follows: T=250 °C,

P=500 W, and p=1.6 Torr. The gas flows for deposition of the intrinsic and doped layers

are listed in Table 4.10. The sample P1395 was deposited using the conditions of

experiment HSiP-011 after improvement (SiH4 and H2 of 100 sccm). This process was used

as a reference for the deposition of intrinsic layers due to its optimal photosensitivity. Table 4.10. Conditions for deposition of the intrinsic and doped layers by MV/LF-PECVD.

Structure Layer Process CH4 (sccm)

SiH4 (sccm)

H2 (sccm)

B2H6 (sccm)

PH3 (sccm)

tf (nm)

t (min:s)

p-i-n 1415

(p) a-Si:B P1376 0 100 100 200 - 15 3:00 (i) a-Si P1395 0 100 100 - - 222 30:00 (n) a-Si:P P1400 0 100 100 - 200 20 3:00

p-i-n 1416

(p) a-SiC:B P1379 100 100 100 1000 - 16 3:45 (i) a-Si P1395 0 100 100 - - 222 30:00 (n) a-Si:P P1400 0 100 100 - 200 20 3:00

After plasma deposition, titanium electrodes were evaporated on ITO and n-type a-

Si:H,P layers by means of e-gun using a shadow mask to form the contacts to the anode

(ITO) and cathode (n-type a-Si:H layer). Figure 4.29 shows the image of the fabricated

structures. Samples were cleaned using the procedure described in section 3.3 “Preparation

of samples”. In order to clean the surface of ITO coated glass, an additional surface

treatment in H2 plasma was performed in the MV/LF-PECVD reactor before of deposition

of the p-, i-, and n-type layers. Conditions for H2 plasma treatment were as follows: P=500

W, p=1.6 Torr, and H2=100 sccm for 60 seconds.

Figure 4.29. p-i-n solar cells fabricated by MV/LF-PECVD with ITO and titanium electrodes.


66

I(V) measurements under darkness and AM 1.5 illumination were carried out after

the fabrication of the p-i-n structures. Figure 4.30 (a) and (b) show the results of the I(V)

measurements in darkness and AM 1.5 illumination, respectively.

Figure 4.30. I(V) of p-i-n structures 1415 and 1416: (a) in darkness and (b) under AM 1.5 illumination.

The I(V) measurements of the p-i-n junctions in darkness conditions showed the

typical rectifying behavior of diodes. Under AM 1.5 illumination the I(V) curves showed

the behavior like a solar cell. Table 4.11 lists the solar cell parameters obtained from

characterization of the fabricated p-i-n junctions. Table 4.11. Results of solar cell parameters of the preliminary p-i-n junctions.

Structure Area (cm2)

ISC (mA)

JSC (mA/cm2)

VOC (V) FF η

(%) p-i-n 1415 0.32 0.60 1.87 0.85 0.36 0.57

p-i-n 1416 0.32 0.62 1.93 0.76 0.30 0.44

The low current density and hence the low efficiency could be attributed to the high

sheet resistance of ITO (80 Ω/); also the thickness of ITO was 20 nm, which results in a

reflectance of approximately 30% (see Figure 4.7(a)). The measured Rsh of titanium was 6

Ω/. Therefore, electrical and optical losses were critical for the optimal performance of

the fabricated p-i-n solar cells.

The standard procedure of deposition by MV/LF-PECVD was applied, along with

the strategy “H2 flow as a gas carrier” (see Appendix A) during the fabrication of these

preliminary p-i-n solar cells. Probably, the level of contaminants was critical or out of

limits. In particular, crossed contamination could have affected the quality of the i-type

layer due to the p-type layer deposited previously (boron and carbon contamination).

-2 -1 0 1 2

0

1

2

3

4

5 Dark I(V) p-i-n 1415 Dark I(V) p-i-n 1416 Dark log(I(V)) p-i-n 1415 Dark log(I(V)) p-i-n 1416

Voltage, V (V)

Curre

nt, I

(mA)

10-8

10-7

10-6

10-5

10-4

10-3

10-2

Curre

nt, l

og(I)

(A)

-2 -1 0 1 2

-4

-2

0

2

4

6

8

10 J(V) p-i-n 1415 J(V) p-i-n 1416 P(V) p-i-n 1415 P(V) p-i-n 1416

Voltage, V(V)Cu

rrent

den

sity,

J (m

A/cm

2 )

0

2

4

6

Powe

r, P

(mW

)

(a) (b)


67

4.6. Improvement of doped layers by MV/LF-PECVD

Experiments to improve the properties of the doped layers were performed. Before

these experiments by MV/LF-PECVD, a leak in the line of H2 was detected and solved

(refer to Appendix A). Hence, the films obtained from OFAT experiments and DOE for

intrinsic and doped layers could have contained contaminants. For this reason, H2 and

doping gases (PH3 and B2H6) were selected as factors to investigate if the properties are

improved after the implementation of the strategies described in Appendix A, or the

properties remain without significant changes. Two-level values for H2, PH3, and B2H6

were set at 50 and 500 sccm. Because samples Si:B0 and Si:P0 (at B2H6=PH3=200 sccm)

showed optimal σd and Eα. Fixed parameters were: SiH4=100 sccm, H2=100 sccm, CH4=0

sccm, P=500 W, T=250 °C, and deposition time of 3 min.

Table 4.12 shows the deposition parameters and properties of dark conductivity at

room temperature, activation energy, and thickness for the n-type doped layers. Table 4.12. DOE for n-type doped layers varying H2 and PH3 at low and high levels; and results of activation

energy, dark conductivity, and thickness.

Process Sample Type SiH4 (sccm)

PH3 (sccm)

H2 (sccm)

Eα @T↑ (eV)

Eα @T↓ (eV)

σd×10-6 (S/cm)

tf (nm)

P1461 PH-11 n 100 500 500 0.59 0.36 0.75 20.8 P1462 PH-10 n 100 500 50 0.59 0.37 1.31 24.1 P1463 PH-01 n 100 50 500 1.02 0.46 0.09 8.5 P1464 PH-00 n 100 50 50 1.29 0.35 0.03 17.6

Sample PH-10 showed the best dark conductivity and optimal activation energy of

0.37 eV, after temperature annealing at 430 K. Comparing the best film reported in section

4.3.2 (Sample Si:P0 of process P1400) with sample PH-10 (P1462); activation energy

decreased from 0.4 to 0.37 eV and dark conductivity increased from 1.09×10-6 to 1.31×10-6

S/cm.

Experiments for p-type silicon films are listed in Table 4.13. Results of activation

energy, dark conductivity at room temperature, and thickness are included. Table 4.13. DOE for p-type doped layers varying H2 and B2H6 at low and high levels; and results of

activation energy, dark conductivity, and thickness.

Process Sample Type SiH4 (sccm)

B2H6 (sccm)

H2 (sccm)

Eα @T↑ (eV)

Eα @T↓ (eV)

σd×10-6 (S/cm)

tf (nm)

P1466 BH-11 p 100 500 500 0.37 0.32 18.7 16.9 P1467 BH-10 p 100 500 50 0.37 0.31 33.0 7.0 P1468 BH-01 p 100 50 500 0.46 0.39 37.8 29.7 P1469 BH-00 p 100 50 50 0.46 0.38 0.16 26.1


68

Sample BH-10 showed the best dark conductivity and the best activation energy.

Comparing the best film reported in section 4.3.2 (Sample Si:B0 of process P1376) with

sample BH-10 (P1467), the activation energy decreased from 0.43 to 0.31 eV and dark

conductivity increased from 29.4 to 33.0×10-6 S/cm. For n- and p-type doped layers, it was

observed that a low flow of H2 and a high flow of dopant gases (B2H6 or PH3) are optimal

to improve the activation energy and dark conductivity.

Reference flows were taken from DOE for doped layers, i.e., SiH4=100 sccm,

H2=100 sccm, B2H6=200 sccm, and PH3=200 sccm. Then, a lower and a higher value than

200 sccm were selected for H2, B2H6, and PH3, i.e., 50 and 500 sccm, respectively.

According to results, the tendency is minimizing the H2 flow and increasing the flows for

B2H6 and PH3. However, the incorporation of H2 is for safety reasons, because it acts as a

gas carrier to avoid abrupt pressure changes during incorporation or evacuation of

dangerous gases (SiH4, CH4). For this reason, H2 gas was included in all experiments.

Therefore, the synthesis conditions of samples PH-10 (P1462) and BH-10 (P1467)

were selected. Experiments for alloying these films with carbon at two-levels were carried

out. These experiments correspond to DOE p-SiC and DOE n-SiC from Table 3.2. The

only factor that was varied in these experiments is the CH4 flow at low (100 sccm) and

high (500 sccm) levels. Thus, n- and p-type doped SiC films were studied. Table 4.14

shows the obtained properties for these films. Table 4.14. DOE for n- and p-type doped silicon-carbon films varying the CH4 flow at two-levels.

Process Sample Type CH4 (sccm)

PH3/B2H6 (sccm)

H2 (sccm)

Eα @T↑ (eV)

Eα @T↓ (eV)

σd (S/cm)

tf (nm)

P1470 C0:BH-10 p 100 B2H6=500 50 0.39 0.31 36.7×10-6 10.3 P1471 C1:BH-10 p 500 B2H6=500 50 0.44 0.37 0.01×10-6 25.4 P1474 C0:PH-10 n 100 PH3=500 50 0.53 0.40 0.41×10-9 33.7 P1475 C1:PH-10 n 500 PH3=500 50 0.40 0.39 0.03×10-9 27.7

From the results observed in Table 4.14, it is concluded that carbon incorporation

deteriorates the properties of n-type doped SiC films (samples C0:PH-10 and C1:PH-10)

because the activation energy increases and dark conductivity decreases for both levels of

CH4. In the case of p-type layers, the properties were improved because the sample C0:BH-

10 showed that dark conductivity increased from 33×10-6 to 36.7×10-6 S/cm meanwhile

activation energy remained unchanged in 0.31 eV. Hence, a low CH4 flow for p-type doped

layers is beneficial in combination with low H2, high SiH4, and high B2H6 flows.


69

In order to replicate the films that were used for the fabrication of the preliminary

solar cells (p-i-n 1415 and 1416), processes P1376, P1379, and P1400 were repeated again.

In this phase of experimental work, all gas lines do not have leaks because they were

monitored and resulted optimal. The procedure of H2 gas as a carrier was applied (refer to

Appendix A), which reduces the contamination of the MV/LF-PECVD reactor. Fixed

parameters were as follows: SiH4=100 sccm, H2=100 sccm, P=500 W, T=250 °C, and

deposition time of 3 min. Table 4.15 shows the comparative results of these samples. Table 4.15. Reproducibility of processes P1376, P1379, and P1400.

Process CH4

(sccm) B2H6 (sccm)

PH3 (sccm)

Previous results New results Type Eα @T↑

(eV) Eα @T↓ (eV)

σd×10-6 (S/cm)

Eα @T↑ (eV)

Eα @T↓ (eV)

σd×10-6 (S/cm)

P1376 p - 130 - 0.43 0.43 29.4 0.41 0.35 11.3 P1379 p 100 1000 - 0.36 0.36 5.73 0.35 0.31 31.4 P1400 n - - 200 0.50 0.40 1.09 0.55 0.35 4.39

After the optimization of leaks and vacuum level, a significant improvement of the

film properties was observed, when comparing the results of previous experiments with the

new experiments. Sample P1376 showed that activation energy decreased from 0.43 to

0.35 eV. However, dark conductivity decreased from 29.4×10-6 to 11.3×10-6 S/cm; whereas

samples P1379 and P1400 exhibited a significant improvement in activation energy and

dark conductivity. Experiment P1376 shows a B2H6 flow of 130 sccm rather than 200

sccm. Because flow meter was replaced due to a failure, and the flow of 130 sccm was

used in the new flow meter, because with this flow the partial pressure was equivalent to

use a flow of 200 sccm in the damaged flow meter (See Appendix A).

Deposition parameters of the intrinsic films with the best photosensitivity were

chosen for doping experiments. Because a high photosensitivity implies a low defect

density and a high mobility-lifetime product, this was beneficial due to the creation of more

defects when doping. Thus, there were good structural characteristics initially.

On the other hand, the thickness of films influences strongly the estimation of

optical, electrical, structural properties. In particular, dark conductivity and activation

energy are dependent on thickness. Indeed, by decreasing the film thickness (from 3.8 to

0.1 µm), dark conductivity decreases (from 6×10-4 to 40×10-6 S/cm) and activation energy

increases (from 0.21 to 0.4 eV) [90]. Typically, doped layers are studied with a similar

thickness as intrinsic layers [5], [43]. However, one of the objectives was the study of

doped layers with a thickness comparable to that used in solar cells (~15 nm).


70

4.7. Improvement of p-i-n structures by MV/LF-PECVD

Previous sections showed optimal results. Global trends for optimal deposition by

MV/LF-PECVD were: a decrease in H2 and CH4 flows, and an increase in B2H6 or PH3

flows for the doped layers; as well as an increase in pressure and SiH4 flow for the intrinsic

layers. Besides, I(V) measurements of the preliminary p-i-n structures showed the typical

diode-like behavior in darkness, and as solar cell under AM 1.5 illumination. These results

were encouraging since functional solar cells were fabricated. However, reproducibility and

optimization of efficiency have not yet been explored. This section addresses these issues.

The layers with the best properties were selected. Solar cells were characterized

initially and after a thermal treatment of 160 °C in vacuum for 1 hour; because activation

energy of the doped layers decreased after temperature variation from 300 to 430 K. Also,

interfaces between layers are improved by annealing. Plasma cleaning of ITO and FTO

substrates and the effect of reactor passivation with an intrinsic silicon layer were analyzed.

A. Experiments 1472 and 1473

Two p-i-n structures were fabricated in experiments 1472 and 1473. The first p-i-n

junction was a complete silicon structure; for the second p-i-n junction only the p-type a-

Si:H layer was changed to B-doped a-SiC:H. Commercial Corning glass coated with ITO

and FTO of 180 nm of thickness and Rsh of 10 Ω/ were used as substrates. These new

substrates are electrically and optically better than substrates used in preliminary

experiments. Fixed parameters for all layers were: T=250°C, P=500 W, and p=1.6 Torr.

The gas flows for deposition by MV/LF-PECVD of the doped and intrinsic layers

are listed in Table 4.16 and Table 4.17. Before of deposition, the ITO and FTO substrates

were treated in O2 plasma to avoid the delamination of the p-i-n layers from the surface of

ITO and FTO (delamination was observed in preliminary experiments 1415 and 1416).

After plasma deposition, titanium electrodes were evaporated on TCOs and n-type layer by

means of e-gun using a shadow mask to form the area of the electrodes of 0.5 cm2.

p-i-n structure 1472 (OSiTi)

Deposition of passivation layer by MV/LF-PECVD:

• Pressure of 8 mTorr after 60 min in vacuum.

• P=500 W, T=250°C, p=2.4 Torr, SiH4=100 sccm, and H2=100 sccm for 25 min.


71

Plasma treatment (micro RIE): P=200 W and O2 pressure of 200 mTorr for 5 min. Table 4.16. Deposition conditions of p-i-n structure 1472 (OSiTi).

Layer Material H2 (sccm)

SiH4 (sccm)

B2H6/PH3 (sccm)

CH4 (sccm)

p (Torr)

t (min:s)

tf (nm)

p a-Si:B 50 100 500(B2H6) - 1.6 2:30 10 i a-Si 100 100 - - 2.4 30:00 120 n a-Si:P 100 100 200(PH3) - 1.6 2:30 20

p-i-n structure 1473 (OSiCTi)

Deposition of passivation layer by MV/LF-PECVD: None.

Plasma treatment (micro-RIE): P=200 W and O2 pressure of 200 mTorr for 5 min. Table 4.17. Deposition conditions of p-i-n structure 1473 (OSiCTi).


SiH4 (sccm)

B2H6/PH3 (sccm)

CH4 (sccm)

p (Torr)

t (min:s)

tf (nm)

p a-SiC:B 100 100 1000(B2H6) 100 1.6 2:30 10 i a-Si 100 100 - - 1.6 30:00 220 n a-Si:P 100 100 200(PH3) - 1.6 2:30 20

I(V) measurements under AM 1.5 illumination of the p-i-n structures 1472 and 1473

are shown in Figure 4.31(a). I(V) curves, shown in Figure 4.31(b), were measured after

thermal treatment at a temperature of 160 °C for 1 hour in MV/LF-PECVD reactor.

Figure 4.31. I(V) under AM 1.5 illumination of p-i-n structures 1472 (OSiTi) and 1473 (OSiCTi): (a) initial

and (b) after thermal treatment at 250°C for 1 hour.

Table 4.18 shows the solar cell parameters obtained from the I(V) measurements of

the p-i-n structures 1472 (OSiTi) and 1473 (OSiCTi). Sample OSiTi deposited on FTO

showed the best efficiency after thermal treatment. Sample OSiCTi deposited on FTO

showed a similar efficiency before and after thermal treatment. Comparing the efficiency of

preliminary p-i-n structures with the efficiency of samples 1472 and 1473, it was improved

from 0.57 to 1%; this result is remarkable.

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-10-8-6-4-202468

10

AM15 FTO OSiCTi AM15 FTO OSiTi AM15 ITO OSiCTi AM15 ITO OSiTi

Curre

nt d

ensit

y, J

(mA/

cm2 )

Voltage, V(V)-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-10-8-6-4-202468

10

Curre

nt d

ensit

y, J

(mA/

cm2 )

Voltage, V(V)

AM15 FTO OSiCTi AM15 FTO OSiTi AM15 ITO OSiCTi AM15 ITO OSiTi

(a) (b)


72

Table 4.18. Results of efficiency for the p-i-n structures 1472 (OSiTi) and 1473 (OSiCTi).

Sample Initial After thermal treatment Voc (V) Jsc (mA/cm2) FF η (%) Voc (V) Jsc (mA/cm2) FF η (%)

FTO/OSiCTi 0.84 3.06 0.35 0.92 0.79 3.36 0.34 0.91 FTO/OSiTi 0.56 3.64 0.38 0.78 0.57 4.16 0.45 1.08 ITO/OSiCTi 0.80 2.82 0.36 0.82 0.81 3.14 0.38 0.98 ITO/OSiTi 0.51 2.96 0.40 0.60 0.56 3.42 0.43 0.82

B. Experiments 1479 and 1480

In these experiments is varied the pressure during deposition (1.6 and 2.4 Torr),

since one of the tendencies in intrinsic layers is increasing the pressure. Also, passivation

and plasma treatment were varied. The gas flows for deposition of the doped and intrinsic

layers are listed in Table 4.19 and Table 4.20. A plasma treatment in argon on the ITO and

FTO substrates was performed to investigate if delamination of the p-i-n structures could be

avoided. After deposition by MV/LF-PECVD, titanium/silver electrodes were deposited by

sputtering using a shadow mask to form the area of the electrodes of 0.5 cm2.

p-i-n structure 1479 (Si1.6-Ti/Ag)


• Pressure of 10 mTorr after 60 min in vacuum mode.


Plasma treatment (MV/LF-PECVD): P=300 W, p=300 mTorr, and Ar=100 sccm for 5 min. Table 4.19. Deposition conditions of p-i-n structure 1479 (Si1.6-Ti/Ag).


SiH4 (sccm)

B2H6/PH3 (sccm)

CH4 (sccm)

p (Torr)

t (min:s)

tf (nm)

p a-SiC:B 100 100 1000 (B2H6) 100 1.6 2:30 10 i a-Si 100 100 - - 1.6 45 330 n a-Si:P 50 100 500 (PH3) - 1.6 2:30 20

p-i-n structure 1480 (Si2.4-Ti/Ag)




Plasma treatment (MV/LF-PECVD): P=300 W, p=300 mTorr, and Ar=100 sccm for 5 min. Table 4.20. Deposition conditions of p-i-n structure 1480 (Si2.4-Ti/Ag).


SiH4 (sccm)

B2H6/PH3 (sccm)

CH4 (sccm)

p (Torr)

t (min:s)

tf (nm)

p a-SiC:B 100 100 1000(B2H6) 100 2.4 5:00 10 i a-Si 100 100 - - 2.4 60 240 n a-Si:P 50 100 500(PH3) - 2.4 5:00 20


73

I(V) measurements under AM 1.5 illumination for the p-i-n structures 1479 and

1480 are shown in Figure 4.32(a). The I(V) curves after thermal treatment at a temperature

of 160 °C for 1 hour in MV/LF-PECVD reactor are shown in Figure 4.32(b).

Figure 4.32. I(V) under AM 1.5 illumination of p-i-n structures 1479 and 1480: (a) initial and (b) after thermal

treatment at 250°C for 1 hour.

Table 4.21 lists the solar cell parameters that were determined from the I(V) curves

of the p-i-n structures 1479 and 1480. A bad result was obtained for the p-i-n structure

Si1.6-Ti/Ag (sample 1479). Because the I(V) curve did not show a diode-like behavior.

Pressure for passivation layer was 2.4 Torr, but pressure for p-i-n structures was 1.6 Torr.

This mismatching between layers created dust and voids that short-circuited metallic

electrodes and TCOs. The sample ITO/Si2.4-Ti/Ag showed an efficiency of 1.23%;

however, it was reduced after thermal treatment. Table 4.21. Results of efficiency for the p-i-n structures 1479 and 1480.

Sample Initial After thermal treatment Voc (V) Jsc (mA/cm2) FF η (%) Voc (V) Jsc (mA/cm2) FF η (%)

FTO/Si1.6-Ti/Ag* 0.01 1.78 0.09 0.001 0.03 3.34 0.23 0.03 ITO/Si1.6-Ti/Ag* 0.18 2.62 0.23 0.11 0.01 0.94 0.02 0.000 FTO/Si2.4-Ti/Ag 0.75 2.80 0.38 0.80 0.76 2.58 0.40 0.80 ITO/Si2.4-Ti/Ag 0.78 3.76 0.42 1.23 0.73 3.20 0.41 0.96

*The I(V) measurements failed due to a mismatch between the passivation layer and a subsequent layer

C. Experiments 1490 and 1491

Optimization of efficiency was achieved but reproducibility failed in some samples

due to passivation of the reactor. Besides, plasma treatments modify the surface of

materials; plasma cleaning with O2 is better than H2 for ITO substrates, which improves the

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-10-8-6-4-202468

10

Curre

nt d

ensit

y, J

(mA/

cm2 )

Voltage, V(V)

FTO Si1.6 Ti/Ag ITO Si1.6 Ti/Ag FTO Si2.4 Ti/Ag ITO Si2.4 Ti/Ag

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0-10-8-6-4-202468

10

FTO Si1.6 Ti/Ag ITO Si1.6 Ti/Ag FTO Si2.4 Ti/Ag ITO Si2.4 Ti/Ag

Curre

nt d

ensit

y, J

(mA/

cm2 )

Voltage, V(V)(a) (b)


74

performance of devices [91]; it depends on the substrate and materials to be deposited.

Thus, the following observations were taken into account for the last experiments:

• A cleaning procedure of the MV/LF-PECVD reactor is convenient before of passivation

(using IPA or acetone) because different materials are deposited in this reactor.

• Handling of TCO substrates with tweezers or gloves creates a surface with defects at

the area in contact, mainly at the edge. Thus, devices with short-circuits are very likely.

• O2 plasma treatment helps to oxidize the surface of TCO, promoting a good adhesion of

films. Contrary to H2 plasma that reduces the TCO surface. Also, organic contaminants

are removed from the TCO surface by plasma cleaning with O2.

• Pre-deposition (passivation layer) is required before the main layer, in order to cover

the chamber with a film of similar characteristics as the film of subsequent deposition

(main layer). A good matching between layers is optimal to avoid the detachment of

material from the inner walls of the reactor, causing powders, defects, and voids.

• Samples deposited on FTO showed null problems of adherence, compared with samples

deposited on ITO that exhibited fewer delamination after plasma cleaning with O2.

• A buffer layer or an intermediate procedure is required at the p/i interface in order to

reduce the crossed contamination of the intrinsic layer. The gas replacement concept

was applied before of deposition of p- and i-type layers (refer to Appendix A).

The efficiency of solar cells deposited by a large-area single-chamber VHF-PECVD

reactor has been optimized by adding deionized H2O at the p/i interface during few minutes

in a process called “water vapor flush” [92]. Contamination of the intrinsic layer with boron

is reduced in this way. Unfortunately, this kind of processes is beyond the scope of this

work. Finally, the conditions for deposition of the last p-i-n structures are listed in Table

4.22 and Table 4.23. A treatment in O2 plasma was carried out for the FTO coated Corning

glass to avoid the delamination of the p-i-n structures. After plasma deposition by MV/LF-

PECVD, titanium electrodes were evaporated by e-gun using a shadow mask to form the

area of electrodes (0.32, 0.5, and 1 cm2).

p-i-n structure 1490 (pSi-nSi)

Plasma treatment of MV/LF-PECVD reactor:

• H2 plasma: H2=100 sccm and p=1.6 Torr for 15 min.

• Ar plasma: Ar=100 sccm and p=1.6 Torr for 15 min.


75




Plasma treatment (micro-RIE): P=300 W and O2 pressure of 300 mTorr for 5 min. Table 4.22. Deposition conditions of p-i-n structure 1490 (pSi-nSi).


SiH4 (sccm)

B2H6/PH3 (sccm)

CH4/GeH4 (sccm)

p (Torr)

t (min:s)

tf (nm)

p a-Si:B 50 100 500(B2H6) - 1.6 2:30 10 i a-Si 100 100 - - 1.6 30 220 n a-Si:P 50 100 500(PH3) - 1.6 2:30 20

p-i-n structure 1491 (pSiC-nSiGe)

Plasma cleaning of MV/LF-PECVD reactor:

• H2 plasma: H2=100 sccm and p=1.6 Torr for 15 min.

• Ar plasma: Ar=100 sccm and p=1.6 Torr for 15 min.




Plasma treatment (micro-RIE): P=300 W and O2 pressure of 300 mTorr for 5 min. Table 4.23. Deposition conditions of p-i-n structure 1491 (pSiC-nSiGe).


SiH4 (sccm)

B2H6/PH3 (sccm)

CH4/GeH4 (sccm)

p (Torr)

t (min:s)

tf (nm)

p a-SiC:B 100 100 1000(B2H6) 100(CH4) 1.6 2:30 10 i a-Si 100 100 - - 1.6 30 220 n a-SiGe:P 50 100 500(PH3) 20(GeH4) 1.6 2:00 20

A tendency is observed for the solar cell parameters, as the active area decreases the

efficiency increases, as can be seen in Table 4.24. However, no significant changes are

observed before and after thermal treatment. I(V) measurements under AM 1.5 illumination

of the p-i-n structures 1490 and 1491 are shown in Figure 4.33 (a) and (b). Table 4.24. Solar cell parameters of the p-i-n structures 1490 and 1491 with FTO and titanium electrodes.

Sample Area (cm2)

Initial After thermal treatment Voc (V) Jsc (mA/cm2) FF η (%) Voc (V) Jsc (mA/cm2) FF η (%)

pSi-nSi 0.32 0.67 4.03 0.50 1.36 0.67 3.96 0.50 1.33 pSi-nSi 0.50 0.69 3.62 0.48 1.19 0.66 3.90 0.47 1.22 pSi-nSi 1.00 0.69 3.16 0.46 1.01 0.68 3.44 0.45 1.06 pSiC-nSiGe 0.32 0.74 3.43 0.37 0.94 0.68 3.65 0.38 0.91 pSiC-nSiGe* 0.50 - - - - - - - - pSiC-nSiGe 1.00 0.76 2.62 0.36 0.73 0.71 2.94 0.36 0.76

*The I(V) measurement failed due to a defective region attributed to bad cleaning and/or by handling, otherwise all I(V) measurements for sample pSiC-nSiGe would have failed.


76


treatment at 250°C for 1 hour.

The efficiency of the p-i-n solar cells was improved from 0.57% to 1.36%. Besides,

the conditions of deposition by MV/LF-PECVD were improved. The sheet resistance of

TCO was reduced from 80 to 10 Ω /. Nevertheless, different strategies can be tested to

improve the efficiency. In this sense, light confinement in the absorbing layer could be

achieved by using a luminescent and anti-reflective SiC film that acts also as a cladding

layer. A texturized surface of the solar cells could enhance the light absorption; sample

HSiP-011 by HV/RF-PECVD could be suitable due to its subwavelength structures. In

addition, the thickness of TCO, p-, i-, and n-type layers requires optimization.

Finally, Figure 4.34(a) shows defects of sample 1479 on ITO, created by Ar plasma

treatment and mismatching between passivation and main layers. Figure 4.34(b) shows the

sample 1491 on FTO, which exhibited good efficiency and stability without defects.

Figure 4.34. p-i-n structures deposited by MV/LF-PECVD: (a) Sample Si1.6-Ti/Ag (1479) exhibited defects,

(b) sample pSiC-nSiGe (1491) showed good performance and stability.

In general, unsuccessful results in I(V) measurements were attributed to:

• Substrates with defects due to bad cleaning or handling with tweezers or gloves.

• Defects created during plasma cleaning or plasma deposition.

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10

pSi-nSi/032 pSi-nSi/050 pSi-nSi/100 pSiC-nSiGe/032 pSiC-nSiGe/050 pSiC-nSiGe/100

Curre

nt d

ensit

y, J

(mA/

cm2 )

Voltage, V (V)

Electrodes:-FTO -Titanium

i-layer: silicon

-2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0-10-8-6-4-202468

10

Electrodes:-FTO -Titanium

i-layer: siliconCu

rrent

den

sity,

J (m

A/cm

2 )

Voltage, V (V)

pSi-nSi/032 pSi-nSi/050 pSi-nSi/100 pSiC-nSiGe/032 pSiC-nSiGe/050 pSiC-nSiGe/100

(a) (b)

(a) (b)

Chapter 5. Conclusions

77

5. Conclusions

This dissertation began with the literature review in order to choose the adequate

factors for variation in the PECVD reactors. Afterwards, the analysis tools to study the

properties of the films were selected. Later, the films with optimal properties were used to

fabricate p-i-n solar cells; concluding with the evaluation of their performance. All efforts

were oriented to obtain functional p-i-n solar cells based on silicon films. For this reason,

the most critical and important properties of the films were analyzed and improved.

In the first design of experiments for intrinsic films, photosensitivity was optimized

by increasing the flow of SiH4 and increasing the pressure of deposition for MV/LF-

PECVD and HV/RF-PECVD reactors. The best photosensitivity was obtained in the

experiment HSiP-011 (at a low level of H2, and high levels of SiH4 and pressure). Various

morphologies were obtained by HV/RF-PECVD such as amorphous, nanocrystalline,

subwavelength structured. Alloying of silicon films with carbon was studied; the films at

low CH4 flow showed optimal photosensitivity and null photoluminescence. Reciprocally,

the films at high CH4 flow exhibited high photoluminescence and null photosensitivity.

Later, doped silicon and silicon-carbon films were studied by MV/LF-PECVD. The

activation energy was 0.31 eV for p-type layers and 0.35 eV for n-type layers; a dark

conductivity of ~10-5 S/cm was obtained. A high flow of B2H6 or PH3 was required in

combination with a high SiH4 flow, a low CH4 flow, and a low H2 flow for efficient doping.

The layers with the best properties were used to fabricate p-i-n structures. First p-i-n

solar cells were characterized obtaining a low efficiency of η=0.57%. External factors that

caused the low efficiency could be attributed to parasitic series resistances of electrodes.

The sheet resistance of ITO was 80 Ω/, which is higher than the optimal of 1 Ω/. The

sheet resistance of titanium was 6 Ω/. Typically, the standard contact with the n-type layer

is made of Al/Ag layers, with an intermediate layer of a TCO. Then, titanium and TCO

could have affected the electrical and optical performance. Optical losses were attributed

mainly to reflections at the silicon/ITO interface due to the thickness of 20 nm. Internal

factors for the low efficiency could be attributed to the doped and intrinsic layers that

required additional improvements.

The study of structures made of various layers is not comparable to the study of a

single layer. It is difficult to deal with structures of various layers, overall when layers of


78

different compositions are sequentially deposited. Crossed contamination is very likely,

which deteriorates the quality of films; also detachment of layers is latent due to the

variation of morphologies between substrate and film. Experiments for improvement of the

intrinsic and doped layers were carried out, solving the main issues observed during the

experimental work. The improvements were made in the plasma reactor solving leaks and

by means of strategies to improve the medium vacuum. Besides, the pressure and gas flows

during deposition were key factors for obtaining films with optimal properties for solar cell

applications.

The vacuum level and leak rate of MV/LF-PECVD reactor were critical to improve

the properties of the intrinsic and doped films. In this thesis is demonstrated that highly

photosensitive intrinsic layers and highly conductive doped layers with low activation

energy can be synthesized by MV/LF-PECVD. The obtained parameters of the fabricated

p-i-n solar cells were: VOC=0.78 V, Jsc=4 mA/cm2, FF=0.5, and efficiency η=1.3%. The

low JSC could be attributed to the lack of light trapping techniques such as texturing of

TCO, the lack of a back reflector made of TCO/Ag layers, and the high sheet resistance of

the electrodes. The low FF could be attributed to the crossed contamination of the intrinsic

layer with carbon or boron; hence, the p/i interface requires a buffer layer or an additional

treatment.

Through this study, it was demonstrated that solar cells based on silicon films can

be obtained by MV/LF-PECVD. Moreover, functional, reproducible, and stable p-i-n solar

cells were obtained. Nevertheless, more studies on doped and intrinsic layers are required

to improve the performance of p-i-n solar cells. It is evident that the state-of-the-art

efficiency was not overcome. However, some percent of something is better than all

percent of nothing. The main conclusions are:

• The synthesis of silicon-based films by implementing the factorial design of

experiments was proposed, in order to diminish the number of experiments and

optimize the experimental work.

• Structural, optical, electrical, and optoelectronic properties of the intrinsic and doped

silicon and silicon-carbon films were analyzed.

• The intrinsic silicon films prepared by HV/RF-PECVD showed excellent optoelectronic

properties, resulting in an optimal photosensitivity of ~104.

Chapter 5. Conclusions

79

• Photosensitivity of the intrinsic silicon films obtained by MV/LF-PECVD was similar

to the photosensitivity obtained by HV/RF-PECVD.

• Nanocrystalline and subwavelength structured silicon films were synthesized by

HV/RF-PECVD.

• Intrinsic silicon-carbon films prepared at high CH4 content in gas phase are suitable to

be used as anti-reflective and photoluminescent coatings.

• The properties of the doped and intrinsic layers were analyzed and improved for solar

cell applications. Functional solar cells were obtained by using these films.

• p-i-n solar cells were fabricated on low-cost ITO and FTO substrates (front electrodes);

titanium was deposited as back electrode.

• Plasma cleaning of ITO and FTO substrates with O2 allowed obtaining stable p-i-n solar

cells because delamination was not observed using this treatment.

• Cleaning and passivation of the reactor with a film of similar characteristics as the main

layer contributed to avoiding defects on the p-i-n solar cells.

• Reproducibility was achieved by implementing systematic procedures during the

preparation of samples.

• Leaks in MV/LF-PECVD reactor were solved; strategies of “H2 as a gas carrier” and

“atmosphere to H2 gas replacement” were proposed to overcome the medium vacuum.

• Gas partial pressure concept was applied, since there is a correlation between solid and

gas phase composition of the films prepared by PECVD.

• Plasma processing is a powerful technique, deposition and cleaning by plasma were

fundamental techniques for the realization of this thesis.

• MV/LF-PECVD reactor allowed the synthesis of high-quality silicon-based films under

different deposition conditions. In addition, various substrates of different size fit very

well, e.g., standard letter size of 8.5×11 in.


80

Future work

A lot of work is still pending; evidently, solar cells can be purchased online or with

some provider. However, the development of science and technology of solar cells is more

important. Certainly, this work could have taken other directions, i.e., a single change in

one deposition parameter could have led to other results; maybe better or maybe worse. All

efforts were oriented to optimize the properties of intrinsic and doped films to achieve

functional, reproducible, and stable p-i-n solar cells. Nevertheless, future work is necessary

to pursue the generation of clean and free energy through silicon-based solar cells. In this

sense, the following topics are proposed:

• Thickness optimization of the p-, i- and n-type layers.

• Implementation of the “water vapor flush” procedure or equivalent to avoid crossed

contamination in large-area single-chamber reactors.

• Study of buffer layers based on intrinsic or doped silicon-carbon films.

• Non-expired gases (mainly for B, P, and Si) are required for new experiments.

• A new tank of CH4 gas is necessary at 100% of purity rather than 99.97%.

• Implementation of the doped and intrinsic layers in HIT, DASH or hybrid structures.

• Incorporation of a power source that operates at RF or VHF for the synthesis of

intrinsic and doped silicon-based films of various morphologies.

• Incorporation of a turbo molecular pump to achieve a high vacuum level.

• Enabling of a system for TCO deposition on large-area substrates.

• Implementation of a laser scriber system to delimit accurately the area of electrodes.

• Work on novel structures and materials that incorporate silicon-based films.

• Maintain the MV/LF-PECVD reactor in optimal conditions (periodic maintenance,

cleaning by sandblasting the surface exposed to depositions, replacement of pump oil

periodically, etc).

81

Publications

Congress

1. William W. Hernández-Montero

2.

, Carlos Zúñiga-Islas, Adrián Itzmoyotl-Toxqui,

Claudia Reyes-Betanzo, Joel Molina-Reyes, Mario Moreno-Moreno, Wilfrido Calleja-

Arriaga, and Alfonso Torres-Jácome, “Optical and electrical properties of silicon-

carbon thin films deposited by RF-PECVD for photovoltaic applications,” XXIII

International Materials Research Congress, Cancun, México, August 17-21, 2014.

William W. Hernández-Montero

3.


Armando Hernández, Joel Molina-Reyes, Mario Moreno-Moreno, Wilfrido Calleja-

Arriaga, and Pedro Rosales-Quintero, “Fabrication and characterization of silicon-

carbon thin film solar cells by PECVD,” XXIII International Materials Research

Congress, Cancún, México, August 17-21, 2014.


4.

, Carlos Zúñiga-Islas, Javier De la Hidalga-Wade,

Wilfrido Calleja-Arriaga, Adrián Itzmoyotl-Toxqui, “Subwavelength structured silicon

films deposited by RF-PECVD for photovoltaics,” VIII International Conference on

Surfaces Materials and Vacuum. Puebla, México, September 21-25, 2015.



“Down-conversion photoluminescence of silicon-carbon films for antireflective

coatings in solar cells,” XXV International Materials Research Congress, Cancún,

México, August 14-19, 2016.


82

Journals and proceedings


DOI:

, Carlos Zúñiga-Islas, Francisco J. De la Hidalga-

Wade, Wilfrido Calleja-Arriaga, Adrián Itzmoyotl-Toxqui, “Synthesis of nano-

meso/structured silicon films by plasma deposition” in MRS Proceedings, 1817

(Materials Research Society, 2016), imrc2015abs226.

https://doi.org/10.1557/opl.2016.51


DOI:

, Carlos Zúñiga-Islas, “Influence of gas partial pressure

on the optoelectronic and structural properties of Si, SiC and SiGe films by PECVD,”

in Optical Nanostructures and Advanced Materials for Photovoltaics, (Optical Society

of America, 2016), JW4A.43.

https://doi.org/10.1364/FTS.2016.JW4A.43


DOI:

, Carlos Zúñiga-Islas, Adrián Itzmoyotl-Toxqui, Joel

Molina-Reyes, and Laura E. Serrano-De la Rosa, “Influence of SiH4 and pressure on

PECVD preparation of silicon films with subwavelength structures,” Journal of

Vacuum Science and Technology B, Vol. 35, Issue 1, 011204 (2017).

http://dx.doi.org/10.1116/1.4973303


, Carlos Zúñiga-Islas, Claudia Reyes-Betanzo, Adrian

Itzmoyotol-Toxqui, Victor Aca-Aca, “Two-level factorial experiments for intrinsic and

doped silicon-carbon films by MV/LF-PECVD,” Thin Solid Films, (submitted).

https://doi.org/10.1557/opl.2016.51

https://doi.org/10.1364/FTS.2016.JW4A.43

http://dx.doi.org/10.1116/1.4973303

83

Appendix A. Status, characteristics and deposition of films by MV/LF-PECVD

Standard procedures for the synthesis of films by LF-PECVD can be found in the

Doctoral thesis of Carlos Zúñiga [65] or in the User’s Manual of the reactor Reinberg

AMP3300 from Applied Materials. However, there are differences in the experimental

setup. Because gas lines have been modified, gas tanks are different, flow meters are

different, and the most important change is the modification of the vacuum system.

Perhaps the most critical modification is that turbomolecular pump was removed

due to their limited functionality; a high vacuum was not produced any longer, and only it

reached a vacuum pressure of ~70 mTorr when preliminary experiments were carried out.

This vacuum level was improved to 3 mTorr, after addressing some issues:

• Maintenance of roots and mechanical pumps (change of oil, cleaning of pump parts).

• Maintenance of the damaged o-ring from butterfly valve that controls the pressure in

the chamber during deposition. This problem was detected when an experiment at 1.6

Torr was in progress. But, when SiH4 and H2 flows were added at 80 sccm. Set point

pressure only reached 1.4 Torr, instead of 1.6 Torr. After solving this problem, some

testing experiments at 2.4 Torr were carried out successfully.

• Repair of leaks in H2 gas line; when leaks were measured, the base pressure of 6 mTorr

increased to 11 mTorr (this measurement is performed from the output of H2 tank to the

chamber. Thus, all possible leaks in pipes, connections, valves, flow meters, etc., can be

detected). Approximately, 5 mTorr was added according to Dalton’s law of partial

pressures, which implies environment contamination. Also when pumps were off, this

pressure of 11 mTorr increased very quickly at a rate of ~2 mTorr/s. This rate is similar

to the addition of gases to reach the setpoint pressure in deposition mode (a flow of

contaminants of ~100 sccm). The leak was found after the pressure reducing valve.

• The pressure at the output of each gas tank was set at 25 psi in the pressure regulator

(25 psi=1.7 atm=1297 Torr) to ensure a positive pressure in case of leaks.

The behavior of gas partial pressure and gas flows for each gas of MV/LF-PECVD

reactor are shown in Figures A.1, A.2, and A.3. These graphs could be useful for the

replication of experiments reported in this thesis because correlate the pressure exerted by

each gas in vacuum mode of the reactor. The gas flows were reported, however, if a flow

meter is changed, it is very likely that partial pressure at that flow will be modified. Figure


84

A.1, A.2 and A.3 show the dependence of gas partial pressure as a function of gas flows

(B2H6, PH3, CH4, SiH4, and H2). For comparison, Ar was used as a reference gas.

Figure A.1 Partial pressure as a function of gas flows for H2, B2H6, PH3, CH4 and SiH4.

Figure A.2 Partial pressure vs. gas flow for (left) SiH4 and (right) CH4. Argon was used as a reference gas.

Figure A.3 Partial pressure vs. gas flow for (left) B2H6 and (right) PH3. Argon was used as a reference gas.

0 200 400 600 800 10000

50

100

150

200

250

300

350 H2 B2H6 PH3 CH4 SiH4

Parti

al p

ress

ure

of g

ases

(mTo

rr)

Flow of gases (sccm)

0 100 200 300 400 5000

20

40

60

80

100 SiH4 Ar in SiH4 line

Gas

par

tial p

ress

ure

of S

iH4 (

mTo

rr)

Gas flow of SiH4 (sccm)0 100 200 300 400 500

0

20

40

60

80

100 CH4 Ar in CH4 line

Gas

par

tial p

ress

ure

of C

H 4 (m

Torr)

Gas flow of CH4 (sccm)

0 250 500 750 10000

50

100

150

200

250

300

350 B2H6 Ar in B2H6 line

Gas

par

tial p

ress

ure

of B

2H6 (

mTo

rr)

Gas flow of B2H6 (sccm)0 250 500 750 1000

0

50

100

150

200

250

300

350 PH3 Ar in PH3 line

Gas

par

tial p

ress

ure

of P

H 3 (m

Torr)

Gas flow of PH3 (sccm)

85

Table A.1 lists the main characteristics for each gas (flow meters, ranges, purity, etc.) Table A.1 Characteristics of gases used for deposition of films by LF-PECVD.

Gas Max. flow (sccm)

Flow meter (Brand) Purity Expired

Silane, SiH4 100 N2 (Fathom) 100% Yes Methane, CH4 500 N2 (Fathom) 99.97% Yes Germane, GeH4 500 N2 (Fathom) 100% - Hydrogen, H2 1000 SiH4 (Fathom) 100% No Diborane, B2H6 1000 NH3(Fathom) 1% (99% of H2) Yes Phosphine, PH3 1000 N2O (Fathom) 1% (99% of H2) Yes Argon, Ar 2000 N2O, (Omega) 100% No

Standard methodology for deposition of silicon films

1. Switch on all the systems in the reactor (controllers for temperature, pressure,

power, frequency, flows, vacuum). Open the chamber with Ar and load the samples.

2. Set the reactor in vacuum mode, and wait 2 hours at least in order to reach the

vacuum pressure (~10-3 Torr) and equilibrium of temperature (set in the controller).

3. Measure gas leak rate, if it is QL~10-4 mbar L/s or lower begin the experiments. In

another case, wait for 1 hour and repeat leak rate measurement or check the reactor.

4. Set the desired pressure in the controller and open the process valve.

5. Put the flow meters to 0 sccm and add flows slowly to gas lines in order to avoid

abrupt changes of partial pressure because explosions are latent (all gases are highly

inflammable, explosive, and/or toxic).

6. Wait until the set pressure is reached. For instance, 2 minutes are sufficient to

change from vacuum pressure of 6 mTorr to 1.6 Torr.

7. Switch on the power and frequency controller.

8. Set the desired power, 2 seconds are sufficient to change the power from 0 to 500

Watt. (Frequency is fixed at 110 kHz and never is changed).

9. Start chronometer and wait until the deposition time has finished. (Typical times for

intrinsic layers are 30-60 minutes and 2-4 minutes for doped layers).

10. Close the process valve and wait until vacuum pressure is reached to open the

chamber with argon and unload the samples.

11. When all experiments have finished, purge all gas lines with argon for security and

maintenance reasons.


86

Strategies for reduction of defects in layered structures and vacuum improvement

The standard procedure is followed; only intermediate steps are modified:

• Passivation

The first modification is before step 1. Passivation of the chamber with the same conditions

as the main experiment is required. This step is important to avoid the formation of

powders and to cover the chamber with a layer of similar characteristics as the main

experiment to inhibit crossed contamination and addition of impurities from the previous

deposition in the reactor. Probably the formation of defects (voids, pinholes) is diminished

due to a good matching between the main layer and passivation layer.

• H2 flow as a gas carrier

Step 4 from standard methodology can be modified by using this step. Because the low

partial pressure in vacuum mode is 6 mTorr, it means that there are 6 mTorr of atmospheric

gases in the reactor. In order to purify the chamber with H2, a flow of 100 sccm was set

before deposition. This flow is maintained for a few minutes (5-10 min) to replace

environment gases in the chamber with H2.

• Atmosphere to H2 gas replacement

In a similar way as H2 flow as a gas carrier, this option can be performed instead of step 4.

The low partial pressure in vacuum mode is 6 mTorr. The idea is replacing the atmospheric

gases at low pressure by adding H2 at high pressure. According to Dalton’s law of partial

pressure, using a pressure of 1.6 Torr for H2, and assuming a vacuum pressure of 6 mTorr:

0.006 0.0037 0.37%1.6 0.006

ii

total

pxp

= = = =+

Then, if H2 is evacuated and the vacuum process begins again, it means that at the

low pressure of 6 mTorr, only 0.37% of this pressure (0.0222 mTorr) corresponds to

impurities from the environment. Repeating this procedure again:

50.0000222 1.38 10 0.0013%1.6 0.0059778 0.0000222

ii

total

pxp

−= = = × =+ +

After two repetitions of the procedure “atmosphere to H2 gas replacement” the

impurities of the environment were reduced to 0.0013%. Hence, the vacuum level does not

matter. The important factor is that all leaks must satisfy the thumb rule of QL~10-4 mbar

L/s or lower. These factors are very critical to obtain films of enough quality.

87

Finally, some testing experiments were performed to demonstrate the capacities of

deposition by MV/LF-PECVD. A variety of substrates were cleaned and later they were

loaded in the chamber for deposition of a silicon-carbon film. Figure A.4 shows the various

types of large-area substrates, in which solar cells could be fabricated.

Figure A.4. Substrates for deposition by MV/LF-PECVD: teflon, flexible acetate, glass, polyimide Kapton,

stainless steel, aluminum foil, Corning 2947 glass.

After deposition, the chamber was opened to evaluate the quality of deposition. It

was successful due to the good procedures of cleaning and deposition, as can be seen in

Figure A.5(a). However, samples deposited on flexible substrates suffered from bending.

Figure A.5(b) shows the comparison of flexible substrates after deposition. In particular the

worst option to be used in flexible solar cells is the sample deposited on polyimide Kapton

of 50 µm, since it was completely rolled. Nevertheless, deposition on low-cost flexible

acetate was stable and optimal to be applied in low-cost flexible solar cells.

Figure A.5. Deposition by MV/LF-PECVD on various substrates: (a) flexible and rigid substrates, (b) bending

of flexible substrates after rigid substrates were unloaded.

(a) (b)

Teflon Corning glass

Flexible acetate Aluminum foil

Thick glass Stainless steel

Polyimide


88

List of tables

Table 2.1. Requirements of intrinsic silicon films [29] and doped layers [33] for solar cell applications. ......... 9

Table 3.1. Operation ranges for MV/LF- and HV/RF-PECVD reactors [65], [66]. .......................................... 23

Table 3.2. Summary of DOE. ............................................................................................................................ 24

Table 3.3. Characteristics of substrates. ............................................................................................................ 26

Table 3.4. Characterization techniques [69]. ..................................................................................................... 27

Table 3.5. Position, kp, and kind of bonding for the SiC films [70], [71]. Acte is the constant for H2 analysis. 29

Table 4.1. Gas flows, thickness and deposition rate of the SiC films obtained by MV/LF-PECVD. ............... 40

Table 4.2. Conditions of deposition and thickness of the intrinsic SiC films deposited by HV/RF-PECVD. .. 42

Table 4.3. Properties and main effects of the intrinsic silicon films prepared by HV/RF-PECVD. ................. 49

Table 4.4. Deposition parameters and thickness of the intrinsic silicon films obtained by MV/LF-PECVD. .. 53

Table 4.5. Properties and main effects of the intrinsic silicon films deposited by MV/LF-PECVD. ................ 57

Table 4.6. Experiments of intrinsic silicon films for improvement and reproducibility of sample HSiP-011. . 58

Table 4.7. Deposition conditions of the intrinsic silicon-carbon films by MV/LF-PECVD. ............................ 58

Table 4.8. DOE for doped silicon films varying dopant gases (PH3 and B2H6), and their main properties. ..... 60

Table 4.9. DOE for doped silicon-carbon films varying CH4 and B2H6, and their main properties. ................. 63

Table 4.10. Conditions for deposition of the intrinsic and doped layers by MV/LF-PECVD. ......................... 65

Table 4.11. Results of solar cell parameters of the preliminary p-i-n junctions. ............................................... 66

Table 4.12. DOE for n-type doped layers varying H2 and PH3 at low and high levels; and results of activation

energy, dark conductivity, and thickness. ......................................................................................................... 67

Table 4.13. DOE for p-type doped layers varying H2 and B2H6 at low and high levels; and results of activation

energy, dark conductivity, and thickness. ......................................................................................................... 67

Table 4.14. DOE for n- and p-type doped silicon-carbon films varying the CH4 flow at two-levels. ............... 68

Table 4.15. Reproducibility of processes P1376, P1379, and P1400. ............................................................... 69

Table 4.16. Deposition conditions of p-i-n structure 1472 (OSiTi). ................................................................. 71

Table 4.17. Deposition conditions of p-i-n structure 1473 (OSiCTi). ............................................................... 71

Table 4.18. Results of efficiency for the p-i-n structures 1472 (OSiTi) and 1473 (OSiCTi). ........................... 72

Table 4.19. Deposition conditions of p-i-n structure 1479 (Si1.6-Ti/Ag). ........................................................ 72

Table 4.20. Deposition conditions of p-i-n structure 1480 (Si2.4-Ti/Ag). ........................................................ 72

Table 4.21. Results of efficiency for the p-i-n structures 1479 and 1480. ........................................................ 73

Table 4.22. Deposition conditions of p-i-n structure 1490 (pSi-nSi). ............................................................... 75

Table 4.23. Deposition conditions of p-i-n structure 1491 (pSiC-nSiGe). ........................................................ 75

Table 4.24. Solar cell parameters of the p-i-n structures 1490 and 1491 with FTO and titanium electrodes. ... 75

89

List of figures

Figure 1.1. (a) Solar radiation spectrum [12]. (b) Operating principle of a c-Si solar cell. ................................ 2

Figure 1.2. Efficiency comparison of solar cell technologies: best lab cells vs. best modules [15]. .................. 3

Figure 1.3. Solar cell architectures based on a-Si:H films: (a) a-Si:H solar cell based on thin-films [18], and

(b) heterojunction with intrinsic thin layer (HIT) solar cell [19]. Efficiency and costs for the three generations

of solar cell technologies: (I) Si wafers, (II) thin films and (III) advanced materials [2]. .................................. 4

Figure 2.1. (a) Single junction p-i-n solar cell of a-Si:H [29], (b) band diagram of a p-i-n solar cell [33]. ...... 10

Figure 2.2. (a) Pressure changes for the synthesis of microcrystals (1), nanocrystals (2), clusters (3) and

powders (4) [37]. (b) Structure of plasma deposited silicon films varying the dilution ratio, Rd [31]. ............. 11

Figure 2.3. (a) Room temperature conductivity of doped a-Si:H as a function of doping gases: PH3 and B2H6

[29]. (b) Doping efficiency as a function of dopant gas concentration [44]. .................................................... 13

Figure 2.4. (a) Refractive index of intrinsic a-SiGe:H films as a function of germanium content in solid phase

[53]. (b) Dependence of gas flows and partial pressure measured in the LF-PECVD reactor [54]. ................. 15

Figure 3.1. Steps sequence for the study of the intrinsic and doped silicon-carbon films and p-i-n structures. 17

Figure 3.2. Effect of frequency on (a) density and energy of electrons [4], (b) energy and flux of ions [61]. . 18

Figure 3.3. Schematic diagram of the MV/LF-PECVD reactor. ....................................................................... 19

Figure 3.4. Schematic diagram of the HV/RF-PECVD reactor. ....................................................................... 20

Figure 3.5. Statistical tools: (a) Pareto chart of the effects and (b) cube plot of factors (A, B, and C). ............ 24

Figure 3.6. Structure of thin-film solar cells in superstrate configuration connected in series [4]. .................. 25

Figure 3.7. (a) Fabrication steps of p-i-n solar cells in superstrate configuration. Designed shadow masks of

area: (b) 0.08, 0.16, 0.32; (c) 0.5 and 1 cm2; the area of the masks is 1”×1”. ................................................... 25

Figure 3.8. Structure for conductivity analysis. ................................................................................................ 32

Figure 3.9. (a) Equivalent circuit for solar cells, and (b) current voltage, I(V), characteristics of a solar cell,

illustrating their main parameters under illumination and dark conditions. ...................................................... 37

Figure 3.10. Short-circuit density, JSC, plotted as a function of the defect density, Nd, for p-i-n solar cells of

different thickness (220, 443 and 680 nm) [44]. ............................................................................................... 38

Figure 4.1. (a) Conductivity, and (b) photosensitivity of the SiC films by MV/LF-PECVD. .......................... 40

Figure 4.2. (a) Optical gap from Tauc method and at α=104 cm-1; (b) refractive index, n, and extinction

coefficient, k, at 633 nm of the SiC films deposited by MV/LF-PECVD. ........................................................ 41

Figure 4.3. (a) FTIR spectra and (b) Raman spectra of the SiC films prepared by HV/RF-PECVD. ............... 42

Figure 4.4. (a) Transmittance and (b) photoluminescence of the SiC films deposited by HV/RF-PECVD. .... 43

Figure 4.5. (a) Refractive index and (b) absorption spectra estimated by means of PUMA software. ............. 43

Figure 4.6. (a) Refractive index, optical gap, mobility-lifetime product, (b) conductivity and photosensitivity

of the SiC films prepared by HV/RF-PECVD. ................................................................................................. 44

Figure 4.7. (a) Design of anti-reflective coatings using the obtained properties of the SiC films. (b) Samples of

SiC deposited on Corning 1737 under visible and UV radiation. ..................................................................... 45


90

Figure 4.8. (a) Cube plot of factors and (b) thickness of the silicon films as a function of dilution ratio. ........ 46

Figure 4.9. (a) Absorbance and (b) Raman spectra of the silicon films prepared by HV/RF-PECVD. ............ 47

Figure 4.10. AFM images of the surface of the intrinsic silicon films deposited by HV/RF-PECVD. ............. 47

Figure 4.11. (a) UV-Vis transmittance. (b) Dark and AM 1.5 conductivity of the intrinsic silicon films

deposited by HV/RF-PECVD. .......................................................................................................................... 48

Figure 4.12. (a) Sample HSiP-000 and (b) Sample HSiP-011 deposited on Corning 2947 glass, c-Si and

polyimide substrates. ......................................................................................................................................... 50

Figure 4.13. Scanning electron micrographs of the silicon samples at low-hydrogen dilution with

subwavelength structures. ................................................................................................................................. 51

Figure 4.14. Conductivity as a function of time under an illumination of 1 sun. AM 1.5 illumination was

interrupted every 120 min. In the second interruption, the films were annealed at 160 °C for 2 h in vacuum. 52

Figure 4.15. (a) Deposition rate of the silicon films, (b) influence of dilution ratio on deposition rate. ........... 54

Figure 4.16. (a) FTIR and (b) Raman spectra of the intrinsic silicon films prepared by MV/LF-PECVD. ...... 54

Figure 4.17. (a) Results of hydrogen content; (b) stretching mode and microstructure parameter analysis. .... 55

Figure 4.18. AFM images of the intrinsic silicon films deposited by MV/LF-PECVD. ................................... 55

Figure 4.19. (a) Transmittance and (b) Tauc plot of the intrinsic silicon films obtained by MV/LF-PECVD. . 56

Figure 4.20. (a) Conductivity and photosensitivity of the silicon films prepared by MV/LF-PECVD, (b)

results of mobility-lifetime characterization. .................................................................................................... 56

Figure 4.21. (a) FTIR and (b) Raman spectra of the intrinsic SiC films deposited by MV/LF-PECVD. ......... 59

Figure 4.22. (a) Photoluminescence and (b) transmittance of the intrinsic SiC films by MV/LF-PECVD. ...... 59

Figure 4.23. (a) FTIR and (b) Raman spectra of the p- and n-type doped silicon films. ................................... 61

Figure 4.24. (a) Photoluminescence and (b) transmittance spectra of the p- and n-type doped silicon films. .. 62

Figure 4.25. Activation energy of the (a) p-type and (b) n-type doped silicon films. ....................................... 62

Figure 4.26. (a) FTIR and (b) Raman spectra of the silicon-carbon films doped with boron (p-type). ............. 63

Figure 4.27. (a) Photoluminescence and (b) transmittance spectra of the p-type SiC films doped with B2H6. . 64

Figure 4.28. Activation energy of the p-type SiC films: (a) at high level of B and (b) low level of B. ............ 64

Figure 4.29. p-i-n solar cells fabricated by MV/LF-PECVD with ITO and titanium electrodes. ..................... 65

Figure 4.30. I(V) of p-i-n structures 1415 and 1416: (a) in darkness and (b) under AM 1.5 illumination. ....... 66

Figure 4.31. I(V) under AM 1.5 illumination of p-i-n structures 1472 (OSiTi) and 1473 (OSiCTi): (a) initial

and (b) after thermal treatment at 250°C for 1 hour. ......................................................................................... 71


treatment at 250°C for 1 hour. .......................................................................................................................... 73


treatment at 250°C for 1 hour. .......................................................................................................................... 76

Figure 4.34. p-i-n structures deposited by MV/LF-PECVD: (a) Sample Si1.6-Ti/Ag (1479) exhibited defects,

(b) sample pSiC-nSiGe (1491) showed good performance and stability. ......................................................... 76

91

List of equations

2

4d

HRSiH

= (2.1) .......................................................................................................... 11

4

4 2

SiHSCSiH H

=+

(2.2) ............................................................................................. 11

4 4 3 4/ ( ) /d N N N N Nη = + = (2.3) ................................................................................. 12

3 3 4( ) / ( )gC n PH PH SiH= + (2.4) ................................................................................. 13

2 6 2 6 4( ) / ( )gC p B H B H S iH= + (2.5) ................................................................................ 13

( ) / ( )sC n P P Si= + (2.6) ................................................................................................ 13

( ) / ( )sC p B B Si= + (2.7) ............................................................................................... 13

8 (for 4), and (for 4)Z N N Z N N= − ≥ = < (2.8) ........................................................ 1319 1/2 33 10 cmd gN C −= × (2.9) ............................................................................................. 13

4

4 4C

CHXCH SiH

=+

(2.10) ................................................................................................. 14

i i ii

total total total

p V nxp V n

= ≡ ≡ (2.11) ...................................................................................... 16

6( .) 10 i

total

Vppm volV

= (2.12) ........................................................................................... 16

( ) ( )( )( ) ( ) ( ) ( )

w rat

w r w r

X j A iX jX j A i X i A j

=+

(2.13) ...................................................................... 16

( ) ( )( )( ) ( ) ( ) ( )

at rw

at r at r

X j A jX jX j A j X i A i

=+

(2.14) ...................................................................... 16

LpQ Vt

∆=

∆ (3.1) .......................................................................................................... 21

kEN S= (3.2) ............................................................................................................... 22

,2.303 cte G

Si H C Hf p

A SNt k− − =

⋅ (3.3) ........................................................................................ 28

, 100 [ .%]Si C HH

NC at

ρ−= ⋅ (3.4) ......................................................................................... 29


92

* HSM

LSM HSM

IRI I

=+

(3.5) .................................................................................................. 29

505 520

480 505 520

( )( )R

I IXI I I

+=

+ + (3.6) .......................................................................................... 30

22522R

TO

d πω

=−

(3.7) ............................................................................................. 30

1/2 1/2( ( ) ) ( )ga h h B h Eν ν ν⋅ = − (3.8) ................................................................................. 31

39.10 73.3)( −+= ncPL dEdE (3.9) ........................................................................................ 31

( ) VI VR

= (3.10) ............................................................................................................ 32

1

f

LR t W

σ = (3.11) ........................................................................................................ 32

4.532 fV tI

ρ = (3.12) .................................................................................................... 33

0( ) exp( / )a BT E k Tσ σ= − (3.13) ....................................................................................... 33

0 00 exp( / )a MNE Eσ σ= (3.14) ........................................................................................... 33

0 0( ) (( ) ( ) )ph n p n pq n p q n n p pσ µ µ µ µ= ∆ + ∆ = − + − (3.15) ............................................ 34

dph σσσ −= (3.16) ................................................................................................... 34

ph dph

d d

Sσ σ σσ σ

−= = (3.17) ............................................................................................. 35

ph

qGσ

µτ = (3.18) ........................................................................................................... 35

(1 )(1 exp( ))f

f

R tG

tα− − −

= Φ (3.19) .............................................................................. 35

1.24I qλ λ= Φ × (3.20) ................................................................................................ 35

0

ln 1B LOC

k T IVq I

= +

(3.21) ........................................................................................... 36

LSC II = (3.22) ............................................................................................................ 36

93

MAX MAX MAX

OC SC OC SC

P I VFFV I V I

= = (3.23) ....................................................................................... 36

IN

OCSC

IN

MAX

PFFVI

PP

==η (3.24) ............................................................................................ 36

1.5IN AMP I A= (3.25) ..................................................................................................... 36

[ ]0( ) exp( / ) 1B LI V I qV nk T I= − − (3.26) .................................................................... 37

[ ]0( ) exp( ( ) / ) 1 ss B L

sh

V IrI V I q V Ir nk T Ir−

= − − + − (3.27) ............................................ 37

[ ] ( )

2

0( ) exp( ( ) / ) 1( )fs

s B L Lsh bi s

tV IrI V I q V Ir nk T I Ir V V Irµτ−

= − − + − +− −

(3.28) .......... 37

16

Ed

B C

aNk T q µτ σ

= ⋅⋅

(3.29) ......................................................................................... 38

2 212 23 12 23 2

212 23 12 23 2

2 cos(2 )1 ( ) 2 cos(2 )

z f

z f

r r r r k tR

r r r r k t+ +

=+ +

(4.1) ........................................................................ 45

3

3 4 2 4

( )gPHC n

PH SiH H CH=

+ + + (4.2) ............................................................................. 60

2 6

2 6 4 2 4

( )gB HC p

B H SiH H CH=

+ + + (4.3) ............................................................................. 60


94

References

1. 2015 Renewable Energy Data Book, http://www.nrel.gov/docs/fy17osti/66591.pdf.

2. M. A. Green, Third Generation Photovoltaics, (Springer, New York, 2006).

3. Z. Chuan Feng and J. H. Zhao, Silicon Carbide: Materials, Processing and Devices,

(Taylor and Francis, New York, 2004).

4. A. Schiff, S. Hegedus, and X. Deng, “Amorphous silicon-based solar cells,” in

Handbook of Photovoltaic Science and Engineering, edited by A. Luque and S.

Hegedus (Wiley, Chichester, 2011).

5. R. A. Street, Hydrogenated Amorphous Silicon, (Cambridge University, 1991).

6. W. Shockley and H. J. Queisser, "Detailed balance limit of efficiency of p-n junction

solar cells," Journal of Applied Physics, Vol. 32, No. 3 (1961).

7. S. Rühle, "Tabulated values of the Shockley-Queisser limit for single junction solar

cells," Solar Energy, Vol. 130, 139-147 (2016).

8. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, "Solar cell efficiency

tables (version 47)," Progress in Photovoltaics: Research and Applications, 24, 3-11

(2016).

9. E. L. Wolfe, Quantum nanoelectronics, (Wiley, 2009).

10. J. Meier, S. Dubail, R. Flückiger, D. Fischer, H. Keppner, A. Shah, "Intrinsic

microcrystalline silicon (µc-Si:H) - a promising new thin film solar cell material,"

IEEE Frist World Conferenece on Photovoltaic Energy Conversion, IEEE (1994).

11. A. Shah, J. Meier, E. Vallat-Sauvain, C. Droz, U. Kroll, N. Wyrsh, J. Guillet, U. Graf,

"Microcrystalline silicon and 'micromorph' tandem solar cells," Thin Solid Films, 403-

404, 179-187 (2002).

12. A. Kitai, Principles of solar cells, leds and diodes, (Wiley, 2011), pp. 166.

13. P. T. Chiu, D. L. Law, R. L. Woo, S. Singer, D. Bhusari, W. D. Hong, A. Zakaria, J. C.

Boisvert, S. Mesropian, R. R. King, N. H. Karam, “35.8% space and 38.8% terrestrial 5

J direct bonded cells,” Proc. 40th IEEE Photovoltaic Specialist Conference (2014).

14. M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, D. H. Levi, A. W. Y.

Ho-Baillie, "Solar cell efficiency tables (version 49)," Progress in Photovoltaics:

Research and Applications, 25, 3-13 (2017).

95

15. Photovoltaics report, Fraunhofer ISE: https://www.ise.fraunhofer.de/de/downloads/pdf-

files/aktuelles/photovoltaics-report-in-englischer-sprache.pdf (2016).

16. D. L. Staebler and C. R. Wronski, “Reversible conductivity changes in

discharge‐produced amorphous Si,” Applied Physics Letters, 31, 292 (1977).

17. D. E. Carlson, C. R. Wronski, "Amorphous silicon solar cells," Applied Physics

Letters, Vol. 28, No. 11 (1976).

18. H. Sai, T. Matsui, T. Koida, K. Matsubara, M. Kondo, S. Sugiyama, H. Katayama,

"Triple-junction thin-film silicon solar cell fabricated on periodically textured substrate

with a stabilized efficiency of 13.6%," App. Phys Lett., 106, 213902 (2015).

19. M. Taguchi, A. Yano, S. Tohoda, K. Matsuyama, Y. Nakamura, T. Nishiwaki, K.

Fujita, E. Maruyama, "24.7 record efficiency HIT solar cell on thin silicon wafer,"

IEEE Journal of Photovoltaics, Vol. 4, No. 1 (2014).

20. J. Bullock, M. Hettick, J. Geissbüle. A. J. Ong, T. Allen, C. M. Sutter-Fella, T. Chen,

H. Ota, E. W. Schaler, S. Wolf, C. Ballif, A. Cuevas, A. Javey, "Efficient silicon solar

cells with dopant-free asymmetric heterocontacts," Nature Energy,15031 (2016).

21. M. D. Archer, A. J. Nozik, Nanostructured and photoelectrochemical system for solar

photon conversion, (Imperial College Press, London, 2008).

22. S. Pizzini, Advanced Silicon Materials for Photovoltaic Applications, (Wiley,

Chichester, 2012).

23. J. Meier, R. Flückiger, H. Keppner, A. Shah, "Complete microcrystalline p-i-n solar

cell - Crystalline or amorphous cell behavior?," Applied Physics Letters, Vol. 65, No. 7

(1994).

24. Y. He, C. Yin, G. Cheng, L. Wang, X. Liu, G. Y. Hu, "The structure and properties of

nanosize crystalline silicon films," Journal of Applied Physics, Vol. 75, No. 2 (1994).

25. P. Roca i Cabarrocas, A. Fontcuberta i Morral, S. Lebib, Y Poissant, "Plasma

production of nanocrystalline silicon particles and polymorphous silicon thin films for

large-area electronic devices," Vol. 74, No. 3 (2002).

26. M. Farrockh-Baroughi, S. Sivoththaman, "A novel silicon photovoltaic cell using a

low-temperature quasi-epitaxial emitter," IEEE Electron Device Letters, Vol. 28 , No. 7

(2007).


96

27. M. Labrune, M. Moreno, P. Roca i Cabarrocas, "Ultra-shallow junctions formed by

quasi-epitaxial growth of boron and phosphorous-doped silicon films at 175 °C by rf-

PECVD," Thin solid films, 518, 2528 (2010).

28. W. W. Hernández-Montero, C. Zúñiga-Islas, A. Itzmoyotl-Toxqui, J. Molina-Reyes, L.

E. Serrano-De la Rosa, "Influence of SiH4 and pressure on PECVD preparation of

silicon films with subwavelength structures," JVST B, Vol. 35, No. 1, 011204 (2017).

29. M. Zeman, “Advanced amorphous silicon solar cells technologies,” in Thin Film Solar

Cells Fabrication Characterization and Applications, edited by J. Poortmans and V.

Arkhipov (Wiley, Chichester, 2006).

30. B. Strahm, A. A. Howling, L. Sansonnens, Ch. Hollenstein, “Plasma silane

concentration as a determining factor for the transition from amorphous to

microcrystalline silicon in SiH4/H2 discharges,” Plasma Sources Sci. Technol., Vol.

16, 80 (2007).

31. R. W. Collins and A. S. Ferlauto, “Advances in plasma-enhanced chemical vapor

deposition of silicon films at low temperatures,” Current Opinion in Solid State and

Materials Science, 6(5) (2002).

32. K. H. Kim, E. V. Johnson, and P. Roca i Cabarrocas, “Irreversible light-induced

degradation and stabilization of hydrogenated polymorphous silicon solar cells,” Sol.

Energy Mater. Sol. Cells 105, 208 (2012).

33. J. Cárabe, J. J. Gandia, "Thin film solar cells," Opto-electronics review, Vol. 12, No. 1

(2004) .

34. C. Bauer, H. Giessen, "Light harvesting enhancement in solar cells with

quasicrystalline plasmonic structures," Optics Express, Vol. 21, No. S3 (2013).

35. H. N. Tran, V. H. Nguyen, B. H. Nguyen, D. L. Vu, "Light trapping and plasmonic

enhancement in silicon, dye-sensitized and titania solar cells," Advances in Natural

Sciences: Nanoscience and Nanotechnology, 7, 013001 (2016).

36. K. H. Kim, "Hydrogenated polymorphous silicon: establishing the link between

hydrogen microstructure and irreversible solar cell kinetics during light soaking," Ph.D.

thesis (Ecole Polytechnique ParisTech, 2012).

37. V. Suendo, "Low temperature plasma synthesis of silicon nanocrystals for photonic

application," Ph.D. thesis, (Ecole Polytechnique ParisTech, 2012).

97

38. A. Shah et al., "VHF PECVD" Solar Energy Materials and Solar Cells, 78, 469-491

(2003).

39. E. Vallat-Sauvain, A. Shah, J. Bailat, "Advances in Microcrystalline Silicon Solar Cell

Technologies," in Thin Film Solar Cells Fabrication Characterization and Applications,

edited by J. Poortmans and V. Arkhipov (Wiley, Chichester, 2006).

40. P. Roca i Cabarrocas, “Plasma Deposition of Silicon Clusters: A Way to Produce

Silicon Thin Films with Medium-Range Order,” 1998, MRS Spring Meeting, MRS

Proceedings Vol. 507 (1998).

41. P. Roca i Cabarrocas, A. Fontcuberta i Morral, Y. Poissant, “Growth and

optoelectronic properties of polymorphous silicon thin films,” Thin Solid Films, 403-

404 (2002).

42. K. H. Kim, S. Kasouit, E. V. Johnson, P. Roca i Cabarrocas,"Substrate vs superstrate

configuration for stable thin film silicon solar cells," Solar Energy Materials and Solar

Cells, 119, 124-128 (2013).

43. P. G. Le Comber, W. E. Spear, "Electronic properties of doped amorphous Si and Ge,"

AIP Conferece Proceedings, Vol. 31, 284 (1976).

44. J. Singh, K. Shimakawa, Advances in amorphous semiconductors, (Taylor & Francis,

London, 2003), Chap. 7-8.

45. N. F. Mott, "Conduction in non-crystalline materials. 3. Localized states in a

pseudogap and near extremities of conduction and valence bands," Philos. Mag., 19,

835 (1969).

46. J. Robertson, "Diamond-like amorphous carbon," Materials Science and Engineering,

R 37, 129-281 (2002).

47. P. K. Chu, L. Li, “Characterization of amorphous and nanocrystalline carbon films,”

Material Chemistry and Physics, 96, 253-277 (2006).

48. V. Suendo, G. Patriarche, P. Roca i Cabarrocas, “Luminescence of polymorphous

silicon carbon alloys,” Optical Materials, 27 (2005), 953-957.

49. T. Toyama, Y. Nakano, T. Ichihara, H. Okamoto, "p- and n-type microcrystalline Si(1-

X)C(X) fabricated by plasma CVD with 40.68-MHz excitation source," Journal of

Non-Crystalline Solids, 338-340(2004).


98

50. User's guide Digital Mass Flow Meters FMA4000,

https://www.omega.com/manuals/manualpdf/M4651.pdf.

51. https://nanolab.berkeley.edu/process_manual/chap6/6.20PECVD.pdf.

52. User's Guide Mass Flow Meters FMA A2100-A2400,

http://www.omega.com/Manuals/manualpdf/M2842.pdf.

53. W. W. Hernández, I. E. Zaldívar, C. Zúñiga, A. Torres, C. Reyes, A. Itzmoyotl,

“Optical and compositional properties of amorphous silicon-germanium films by

plasma processing for integrated photonics,” Opt. Mat. Express, Vol. 2, No.4 (2012).

54. William W. Hernández-Montero, Carlos Zúñiga-Islas, “Influence of gas partial

pressure on the optoelectronic and structural properties of Si, SiC and SiGe films by

PECVD,” in Optical Nanostructures and Advanced Materials for Photovoltaics, OSA

(2016).

55. https://en.wikipedia.org/wiki/Partial_pressure.

56. http://www.analyticexpert.com/2015/10/understanding-units-of-gas-concentration/.

57. A. Gallagher, "Neutral radical deposition from silane discharges," Journal of Applied

Physics, 63 (7), 2406-2413 (1988).

58. S. Kasap, P. Capper, Springer Handbook of Electronic and Photonic Materials,

(Springer Science & Bussines Media, 2006), Chap. 26.

59. S. M. Rossnagel, J. J. Cuomo, W. D. Westwood, Handbook of plasma processing

technology (Noyes Publcations, New Jersey, 1990).

60. M. A. Lieberman, A. J. Lichtenberg, Principles of plasma discharges and materials

processing (John Wiley & Sons, 2005).

61. M. Heintze and R. Zedlitz, "New diagnostic aspects of high rate a-Si:H deposition in a

VHF plasma" J. Non-Cryst. Sol. 198 (1996) 1038.

62. T. Kinoshita, M. Isomura, Y. Hishikawa, S. Tsuda, "Influence of oxygen and nitrogen

in the intrinsic layer of a-Si:H solar cells," Japanese journal of applied physics, 35(7R),

3819 (1996).

63. W. Umrath, Fundamentals of vacuum technology, (1998),

https://www3.nd.edu/~nsl/Lectures/urls/LEYBOLD_FUNDAMENTALS.pdf .

64. Douglas C. Montgomery, Design and Analysis of Experiments, (Wiley, 2001), 5th Ed.

99

65. C. Zúñiga, "Películas de carbon con baja permitividad depositadas por PECVD a baja

frecuencia para aplicaciones como aislante entre metales," Ph.D. thesis (Instituto

Nacional de Astrofísica, Óptica y Electrónica, 2005).

66. Manual PECVD “Cluster-Tool MVS”.

67. https://www.techinstro.com/.

68. OriginLab User's Guide, Help and Tutorials: http://originlab.com/doc/.

69. D. Schroder, Semiconductor material and device characterization, (Wiley, 2006).

70. V. P. Tolstoy, I. V. Chernyshova, and V. A. Skryshevsky, Handbook of Infrared

Spectroscopy of Ultrathin films, (Wiley, New Jersey, 2003).

71. NIST Database: http://webbook.nist.gov/chemistry/vib-ser.html.

72. W. Wei, G. Xu, J. Wang, T. Wang, “Raman spectra of intrinsic and doped

hydrogenated nanocrystalline silicon films,” Vacuum, 81 (2007).

73. J. Sancho-Parramon, D. Gracin, M. Modreanu, A. Gajović, "Optical spectroscopy

study of nc-Si-based p-i-n solar cells," Solar Energy Materials and Solar Cells, 93

(2009).

74. R. Swanepoel, “Determination of the thickness and optical constants of amorphous

silicon,” J. Phys. E 16(12), 1214–1222 (1983).

75. J. Singh, Optical Properties of Condensed Matter and Applications (Wiley, 2006).

76. Lorenzo Pavesi and Rasit Turan, Silicon nanocrystals: Fundamentals, Synthesis and

Applications, (Wiley, Weinheim, 2010).

77. S. M. Sze, Physics of semiconductor devices, (Wiley, New Jersey, 2007).

78. N. Beck, N. Wyrsch, Ch. Hof, and A. Shah, “Mobility lifetime product –A tool for

correlating a-Si:H film properties and solar cell performance,” J. App. Phys., 79(12)

(1996).

79. www.pveducation.org.

80. J. Merten, J. M. Asensi, C. Voz, A. V. Shah, R. Platz, and J. Andreu, “Improved

equivalent circuit and Analytical Model for Amorphous Silicon Solar Cells,” IEEE

Transactions on Electron Devices, Vol. 45, No. 2 (1998).

81. Sk. F. Ahmed, D. Banerjee, M. K. Mitra, K. K. Chattopadhyay, "Visible

photoluminescence from silicon-incorporated diamond like carbon films synthesized

via diect current PECVD technique," Journal of luminescence, 131, 2352-2358 (2011).


100

82. Michael Bass, et al., Handbook of Optics, Volume II: Design, Fabrication and Testing,

Sources and Detectors, Radiometry and Photometry, 3rd Ed., (OSA, 2009).

83. O. D. Miller, E. Yablonovitch, S. R. Kurtz, “Strong Internal and External

Luminescence as Solar Cells Approach the Shockley-Queisser Limit,” IEEE Journal of

Photovoltaics, Vol. 2, No. 3, (2012).

84. J. Wang, V. Suendo, A. Abramov, L. Yu, P. Roca i Cabarrocas, “Strongly enhanced

tunable photoluminescence in polymorphous silicon-carbon thin films excitation-

transfer mechanism,” Appl. Phys. Lett. 97, 221113 (2010).

85. Lukas Novotny and Bert Hecht, Principles of Nano-Optics, (Cambridge University

Press, Cambridge, 2006).

86. Max Born and Emil Wolf, Principles of optics, (Cambridge University Press, 1999).

87. Z. Yu, A. Raman, and S. Fan, “Fundamental limit of nanophotonic light trapping in

solar cells,” Proc. of the National Academy of Sciences, Vol. 107, No. 41, (2010).

88. B. von Roedem and G.H. Bauer, "Material requirements for buffer layers used to

obtain solar cells with high open circuit voltages," MRS Spring meeting (1999).

89. S. De Wolfe, A. Descoeudres, Z. C. Holman, C. Balif, "High-efficiency silicon

heterojunction solar cells: a review," Green, Vol. 2, Issue 1 (2012).

90. D. G. Ast and M. H. Brodsky, "Thickness and temperature dependence of the

conductivity of phosphorus-doped hydrogenated amorphous silicon," Philosophical

Magazine B, Vol. 41, Issue 3 (1980).

91. C. C. Wu, C. I. Wu, J. C. Sturm, and A. Kahn, "Surface modification of indium tin

oxide by plasma treatment: An effective method to improve the efficiency, brightness,

and reliability of organic light emitting devices," Appl. Phys. Lett., 70, 1348 (1997).

92. U. Kroll, C. Bucher, S. Benagli, I. Schönbächler, J. Meier, A. Shah, J. Ballutaud, A.

Howling, "High efficiency p-i-n a-Si:H solar cells with low boron cross-contamination

prepared in a large-area single-chamber PECVD reactor," Thin Solid Films,451 (2004).

study of intrinsic and doped silicon-carbon films by pecvd ......films or buffer layers to improve...

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