m.tech presentation(design, growth & fabrication of solar cell)

Design, Growth & Fabrication of InxGa1-xN (0 ≤ x ≤ 0.25) Based Solar Cell

Dissertation by

Rajkumar Sahu

Advisor: Mr. Sonachand AdhikariCo-advisor: Dr. Sanjay Tiwari

CSIR-Central Electronics Engineering Research Institute, Pilani , RajasthanSchool of Studies in Electronics & Photonics , Pt. Ravishankar Shukla University, Raipur

May 22, 2014

Rajkumar [email protected]


May 22, 2014Slide 2

Outline1. Motivation & background

• Photovoltaic (PV), High-efficiency, InGaN.

2. Objectives• Status, Research Objective, Approach

3. Theory and modeling• Design, Silvaco-Atlas, Optimization

4. Experimental• Growth & Fabrication

5. Conclusion


May 22, 2014Slide 3

Motivation for PV: Population growth

•Energy Demand -


May 22, 2014Slide 4

Motivation for PV: Energy demand

World marketed energy consumption, 1990 - 2035.(source: Based on data from U.S. Energy Information Administration, 2011)


May 22, 2014Slide 5

Motivation for PV: Greenhouse Gases, Acid rain


May 22, 2014Slide 6

Motivation for PV: Global warming

(Source:-IPCC, 31st march 2014)


May 22, 2014Slide 7

Motivation for PV: The Sun

ONE SOLUTION COMES UP EVERY MORNING!

SOLUTION


May 22, 2014Slide 8

What is Photovoltaics (PV)?

Photovoltaics is the DIRECT method……of converting SUNLIGHT into ELECTRICITY…

…using a device known as SOLAR CELL.


May 22, 2014Slide 9

Operation of a solar cell• Working principle of Solar Cell based on Photovoltaic effect.• Photovoltaic effect is generation of Electric power from light.• Single junction solar cell is simply PN junction under illumination of light.

• Operating diode in fourth quadrant generates power.


May 22, 2014Slide 10

Advantages of PVGreen Technology

No combustion/emission, radioactivity, disposal High public acceptance

Infinite ResourceFuel, semiconductor

Flexibility and convenienceGrid connected, stand-alone, modularQuick installation, integration

High-quality output power



Generations of PV1st Generation…

Bulk Silicon, single junctionMature technology, 93% market shareLimitation: Efficiency ~25%, Si

2nd Generation…Thin films decrease material costsLimitation: Low efficiencies, stability

3rd Generation…High efficiency Lower costHigh output power density



Efficiency limit in single junction solar cell

Efficiency - 25%Loss mechanisms in a single junction solar cell.

Transmission of low energy photons ~23%.Thermalization of high energy photons ~29%.Junction/Contacts ~14%.Recombination due to material quality ~5%.Other: Curve factor Loss, Shading, Reflection ~5%.



High efficiency approaches: Tandem solar cell

Solar cells with decreasing band gaps are stacked with greatest band gap on the top.High energy photons are absorbed by top layers decreasing thermalization losses.Low energy photons are transmitted to lower band gap layers.

Concept of a tandem solar cell.



High efficiency approaches: Quantum-well solar cellProposed by Keith Barnham’s group in 1990.

Multi-Quantum-Well (MQW) system is added to the i-region of a p-i-n solar cell.

Quantum Wells (QW) can absorb photons with energy less than that of the bulk material.

↑Absorption → ↑Current → ↑Efficiency

Concept of quantum well solar cell



III-Nitride material system

Wide direct-band gapWide direct-band gap High absorptionRadiation hardness High carrier velocities Piezoelectric polarization



III-Nitride material system - Challenges1. Substrate mismatch with GaN

Substrate Latticemismatch

Thermal expansion

Sapphire 16% -34%SiC 3% +2%ZnO 2% -14%

Si 17% +100%

2. Material Quality

High dislocation density -1010cm2

Low lifetimes and diffusion lengths

3. P-GaN

P-type dopingOhmic contact



Overview: InGaN solar cell researchLawrence Berkeley National Lab

Proposed full spectrum InGaN solar cells.InGaN/Si tandem (modeling).

Cornell UniversityMaterial growth.

University of HoustonSimulation, material characterization.

Novel Semiconductor Material Lab, ChinaFabricated 2.7-2.8 eV InGaN p-n solar cells 0.43 VOC , FF 57%.



RESEARCH OBJECTIVES

Objectives & approach.



Research objectives

Develop an accurate modeling tool for III-nitride solar cells.

Optimize MOCVD epitaxial growth of InGaN Eg 2.51 eV

Design & fabricate InGaN solar cells Eg 2.51 eV

Understand loss mechanisms in solar cellsMaterial quality, fabrication issues

Develop robust & efficient fabrication scheme



ModelingSilvaco-Atlas

PC1D, etc.

Fabricationn & p contact, Current

spreading layer

CharacterizationI-V, TLM

2.4 – 2.9 eVInGaN solar cell

GrowthMOCVD(In-situ

Characterization: GaN Growth)

Research approach



THEORY

Preliminary design and modeling.



Modeling of solar cells: Silvaco-AtlasDevice parameter files

Structure, Region, Electrodes, Doping, etc.

Material filesModel, Contact, Interface, Indium(%)

OpticalRefractive index, Absorpotion

Silvaco-AtlasSolar cell simulation program

Simulate two and three-dimensional semiconductor devices.

OutputGraphical (Tony plot)

I-V, band diagram, electron & hole conc., mobility etc.



Modeling of solar cells: Silvaco-AtlasStep 1: Preliminary modification Step 2: Advance modification

Material filesModels, Contact, Interface,

Indium(%)

OpticalRefractive index, Absorption

Polarization



Primary design: p-i-n solar cell

p-region ~ 100 nmMaximize absorption in i-region.Provide charge to junction.

n-region ~ 2 µmHole diffusion length ~ 2 µm.

Test material i-region. GaN/InGaN p-i-n solar cell.



Optimization : p-region

ThicknessOn increasing the p-GaN thickness, generated charge carriers are not separated out instead they start recombining in p-region which results in decrease in the Jsc as shown in fig. (a).

DopingWe also investigated the effect of doping by taking different doping concentration. Results shows that Jsc first increases and then decreases with increase in doping concentration, as shown in fig. (a).

ab

c d



Preliminary design: i-region thicknessIt can be observed that Jsc increases with increasing i-layer thickness. Since i-layer is low bandgap semiconductor compare to p-GaN, it can absorb the photons of some lesser energy than p-GaN.

There is no significant change in the Fill Factor(FF). However, FF starts to decrease as we increase the thickness because series resistance of the cell also increases with increasing thickness of i-layer



p-i-n structure with varying Indium Composition

Jsc of the double hetero-junction GaN/InGaN solar cell increases with increase in indium composition till 20%, which contributes to increase in efficiency but beyond this composition Jsc decreases sharply as shown in Figure.



Experimental

Growth and Fabrication.



Preliminary InGaN growthEpitaxy: Emcore MOCVD D-125 rotating disk reactor with short jar configuration.

Material investigated: InGaN: [In] 0 – 25%.

Growth variables:Film thickness: 20 – 100 nmTemperature: 640 – 800°CTMIn: 30 – 250 SCCMTMGa: 15 – 150 SCCM



MOCVD growth overview



MOCVD growth of GaN In-situ Characterization

Simulated reflectance profile for GaN growth with extinction

coefficient of 0.001

Simulated reflectance profile for GaN growth with extinction

coefficient of 0.153



0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000 13000 14000

0

2000

4000

6000

8000

10000

12000

14000

16000

Ref

lect

ion

Time (Sec)

p-i-n Solar Cell

MOCVD growth of GaN template

low temperature nucleation layer growth at 550 oC high temperature GaN growth at 1060 oC, lateral growth, and surface roughening which induce a lightly drop in the reflectance intensityisland coalescence which the amplitude and intensity of oscillations increases, qusi-2D GaN growth (500torr)qusi-2D GaN growth



MOCVD growth of GaN template

(c) low temperature nucleation layer growth at 550 oC (d) temperature ramp and morphology transformation(e) high temperature GaN growth at 1060 oC, lateral growth, and surface roughening which induce a lightly drop in the reflectance intensity(f) island coalescence which the amplitude and intensity of oscillations increases, qusi-2D GaN growth (500 torr)(g) qusi-2D GaN growth



Baseline solar cell fabrication

p-type Contact Formation

Current Spreading Layer

n-type Contact Formation

Mesa Etchingp-GaN

i-InGaN

n-GaN

u-GaN buffer layer

Sapphire

InGaN/GaN Epi layer




3535

P-type Contact Formation



Mesa Etchingp-GaN

i-InGaN

n-GaN

u-GaN buffer layer

Sapphire

InGaN/GaN Epi layer







Mesa Etchingp-GaN

i-InGaN

n-GaN

u-GaN buffer layer

Sapphire

InGaN/GaN Epi layer

Ti/Al/Ni/Au







Mesa Etchingp-GaN

i-InGaN

n-GaN

u-GaN buffer layer

Sapphire

InGaN/GaN Epi layer Ni/Au

Ti/Al/Ni/Au



p-GaN

MQW active layer

n-GaN

u-GaN buffer layer

Sapphire

Ni/Au

Ti/Al/Ni/Au


FINAL DEVICE

Mesa etching

n-type contact formation

Current spreading layer

CONTACTING

SCHEMS



n-contact resistance measurement

Fig. n-contact resistance measurement

-30 -20 -10 0 10 20 30 40 500

5

10

15

20

25

30

35

Mean Linear Fit of Sheet1 Resistance

Res

ista

nce

()

Gap (m)

y-Intercept = 10.46Slope = 0.42

x-Intercept = -24.98

c = 6.53x10-5 cm2

Contact Res. = 5.23 Sheet Res. = 41.87 sq.



ConclusionIndium Gallium Nitride is a semiconductor material with potential to be used in photovoltaic devices.

Established InGaN as a high-efficiency photovoltaic material.

p-i-n double hetero junction structure is optimized with conventional structure and also effect of indium variation is observed on characteristic parameters.



ReferencesNeff, H., Semchinova, O., Lima, A., Filimonov, A., Holzhueter, G.,“Photovoltaic properties and technological aspects of In1-xGaxN/Si, Ge(0 ＜ x ＜ 0.6) heterojunction solar cells,” Sol. Energy Mater. Sol. Cells 90, 982-997(2006).Jani, O.et al., “Design and characterization of GaN/InGaN solar cells,” Appl. Phys. Lett. 91, 132117 (2007).Shih-Wei Feng et al., “Theoretical simulations of the effects of the indium content, thickness, and defect density of the i -layer on the performance of p - i - n InGaN single homojunction solar cells ” Appl. Phys. Lett. 108, 093118 (2010).Iulian Gherasoiu et al., “Photovoltaic action from InxGa1-xN p-n junctions with x > 0.2 grown on silicon ” Phys. Status Solidi C 8, No. 7–8, 2466–2468 (2011).



AcknowledgmentI am thankful to the Director, CSIR-CEERI, Pilani for giving me opportunity to work in this laboratory.

I am thankful to my supervisor Mr. Sonachand Adhikari.

I am also thankful to Dr. C. Dhanvantri (Group Leader-ODG), Dr. S. Pal and Dr. Sumitra Singh for constant encouragement in this work. I also thank all ODG members for their support.

I am thankful to training in-charge Mr. Vinod K. Verma.


May 22, 2014Slide 2Thank You

m.tech presentation(design, growth & fabrication of solar cell)

Documents