Influence of Microstructure on the Electronic Transport Behavior of Microcrystalline Silicon Films

Sanjay K. Ram Dept. of Physics,

Indian Institute of Technology Kanpur, INDIA

OutlineCh. I: Introduction

Ch. II: Experimental Details

Ch. III: Structural Investigation

Ch. IV: Electrical Transport Properties 1: Dark conductivity

Ch. V: Electrical Transport Properties 2: Photoconductivity

Ch. VI: Numerical Modeling of Steady State

Photoconductivity in µc-Si:H

Ch. VII: Summary and Conclusions

Chapter-I

INTRODUCTION

crystalline structure crystallites in a-Si random networkLong-range order Medium-range order Short-range order

Role of Si thin films in large area microelectronics

Amorphous silicon (a-Si:H)Advantages:

Possibility of low temperature plasma depositionPlays a dominant role in the application of solar cells and TFTsGood photosensitivityWide band gap

Issues:Low carrier mobility (μn~1 cm2/V-s & μp~10-3 cm2/V-sMetastabilityPoor doping efficiency

Thin film Poly SiAdvantages:

Solid phase crystallization/LPCVDGrain sizes of 10 nm to 1 μm are commonVery high carrier mobilityGreater stability under electric field and light-induced stressGood for TFTsHigh doping efficiency

Issues:High temperature depositionBoundaries are not passivated

Why μc-Si:H thin films ??

Promising material for large area electronicsPossibility of low temperature depositionGood carrier mobilityGreater stability under electric field and light-induced stressGood doping efficiencyBoundaries are passivated

Further development requires proper understanding of carrier transport properties correlative with film microstructure

Three main length scales for disorder:Local disorder: µc-Si:H contains a disordered amorphous phase Nanometrical disorder: nanocrystals consist of small crystalline (c-Si) grains of

random orientation and a few tens of nanometres size. Micrometrical disorder: conglomerates are formed by a multitude of nanocrystals and

generally acquire a pencil-like shape or inverted pyramid type shape.

1. Complex microstructure

Why is a comprehensive description of optoelectronic properties of µc-Si:H difficult ???

Film growth

voids

substrate

grains grain boundaries

columnar boundaries

conglomerate crystallites

surfaceroughness

µc-Si:H is not a unique material.Electronic transport can be studied or understood after a proper structural characterization of the material.The quantitative analysis of microstructure of µc-Si:H is difficult and often ambiguous.Tools at different length scales required.Electrical transport properties are influenced by the constituent phases.The correlation between microstructure and electrical properties is unexplored.

Issues

2. Non-availability of a complete DOS map of μc-Si:H system

Difference between DOS map of c-Si and amorphous Silicon (a-Si:H)

Issues

Smaller grains a-Si like properties

Large grains c-Si like properties

There is no unique effective DOS profile that can satisfy the whole range of materials included under the common name of microcrystalline Si, or explain all the transport processes.

Need for TOP Gate TFT

Smooth Top layer of the filmBigger sizes of crystallite at the Top layerInverted pyramid shaped columnar crystallites are preferable

Need for BOTTOM Gate TFT

Crystallization should start at the beginning of the growthTo reduce the amorphous incubation layer at the bottom glass interface

Desired μc-Si:H material in TFTs (Staggered type)

Approach In this work, we have studied the microstructure of

µc-Si:H films having varying degrees of crystallinity and tried to identify the role of different deposition parameters on film microstructure and morphology.

We have studied the optoelectronic properties of such well characterized films and attempted to correlate these properties to the film microstructure.

Lastly, we have carried out an extensive numerical modeling study of phototransport properties of μc-Si:H system to understand the experimental findings.

Our Results

Fully Crystallized plasma deposited μc-Si:H can be

deposited and carrier transport in such films is different.

Films with different microstructures lead to different

effective density of states map that can be used to

parameterize the electrical transport behavior.

Chapter-II

EXPERIMENTAL DETAILS

Sample Preparation

R=1/1 R=1/5 R=1/10

Substrate: Corning 1773

High purity feed gases:SiF4 , Ar & H2

Rf frequency 13.56 MHz

Silane flow ratio (R)= SiF4/H2

Thickness seriesTs=200 oC

μc-Si:Hfilm

R F

HSi SiNSi N

HSiH

HHN

N

H H

HHH

P E C V DR F

HSi SiNSi N

HSiH

HHN

N

H H

HHH

P E C V D

Parallel-plate glow discharge plasma deposition system

Film characterization

Structural Properties Electrical Properties

X-ray Diffraction

Raman Scattering

In-situ Spectroscopic Ellipsometry

Atomic Force Microscopy

σd(T) measurement15K≤T ≤ 450K

σPh(T,∅) measurement15K≤T ≤ 325K

CPM measurement

TRMC

Chapter-III

STRUCTURAL INVESTIGATION

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0-10

-505

1015202530354045

F0E31 Fit a-Si:H c-Si

<ε2>

Energy (eV)Measured <ε2> spectrum for the µc-Si:H sample #E31 [deposition condition: R(SiF4/ H2)= 1/1, Ar flow = 25 sccm, TS = 200 °C, thickness = 1200 nm]. Peaks at about 3.5 and 4.2 eV are observed.

Top Layer (3.1 nm)Fcf = 15 %, Fcl= 62 %, Fv = 23 %, Fa=0 %

Upper Middle Layer (864 nm)Fcf = 9.8 %, Fcl = 90.2 %, Fv = 0 %, Fa=0 %

Lower Middle Layer (311 nm)Fcf = 86.2 %, Fcl= 0 %, Fv= 4.5 %, Fa=9.3 %

Bottom Interface Layer (27 nm)Fcf = 0%, Fcl = 0 %, Fv = 25 %, Fa= 75 %

Spectroscopic Ellipsometry Study

Bifacial Raman StudyA deconvolution model that includes crystallite size distribution was

employed for analysis of Raman data.

400 425 450 475 500 525 5500.0

0.3

0.6

0.9

1.2 glass side exp. data of F0E31 cd1 cd2 a fit with - cd1cd2a

Inte

nsity

(arb

. uni

t)

Raman Shift (cm-1)450 475 500 525 550

0.0

0.3

0.6

0.9

1.2 film side exp. data of F0E31 cd1 cd2 fit with - cd1cd2

Raman Shift (cm-1)

Inte

nsity

(arb

. uni

t)

collection

excitation

film

glassglassfilm

excitation

collectionSmall grain (cd1) Large grain (cd2) a-Si:H

Size (nm)[σ (nm)]

XC1(%)

Size (nm)[σ (nm)]

XC2(%) Xa (%)

Film side cd1+cd2 6.1, [1.68] 20 72.7, [0] 80 0

Glass side cd1+cd2+a 6.6, [1.13] 8.4 97.7, [4.7] 52.4 39.2

Sample #E31 (1200 nm,

R=1/1)

Fitting Model

sample #B04 (thickness = 950 nm, R=1/10, roughness (σrms) = 5.26 nm)

(b)

(c)

(a)

0 100 200 300 4000.00

0.05

0.10

0.15

0.20

0.25

Freq

uenc

y (a

rb. u

nit)

Conglomerate surface grain size (nm)

B04 (t =950 nm; R=1/10)

Surface Morphology by AFM

Types of samples studiedFixed deposition parameters

Plasma Power (W) 20

RF frequency (νrf) (MHz) 13.56

Total Pressure (Torr) 1

SiF4 flow rate (sccm) 1

Ar flow rate (sccm) 25

R=SiF4/H2 = 1/1

R=SiF4/H2 = 1/5

R=SiF4/H2 = 1/10

Set-A (thickness is ~ 50 nm)

Set-B (thickness is ~ 400 nm)

Set-C (thickness is ~ 950 nm)

TS series R=1/5 TS: 100 - 350°C

R:1/1 to 1/20

R seriesTS=200°C

Thickness :50 nm to 1200 nm

Thickness seriesTS=200°C

2 3 4 5-10

0

10

20

30

Energy (eV)

< ε 2 >

E31 (R=1/1, t=1200nm)Growth time

30 min 60 min 190 min 225 min 230 min

450 475 500 525 550

Film sideR (SiF4 / H2) = 1/10

B04 (t=950 nm)

B23 (t=590 nm)

D281 (t=422 nm)

B11 (t=390 nm)

B22 (t=170 nm)

F152 (t=52 nm)

Inte

nsity

(arb

. uni

t)

Raman Shift (cm-1)0 100 200 300 400

0.00

0.05

0.10

0.15

0.20

0.25thickness series of R =1/10

thickness ---->

Conglomerate surface grain size (nm)

Freq

uenc

y (a

rb. u

nit) B04 (t=950 nm)

B11 (t=390 nm) B22 (t=170 nm) F152 (t=52 nm)

(c)

(a) (d)

(b)

Effect of Film Growth

2.5 3.0 3.5 4.0 4.5 5.00

5

10

15

20

25

30H2 dilution

Energy (eV)

< ε 2 >

F151 (R=1/1, t=62 nm) F152 (R=1/10, t=55 nm) F16 (R=1/20, t=58 nm)

20 30 40 50 60 700

500

1000

1500

2000

2500

3000

3500

4000

4500

1/1

1/5

1/10

1.2 µm

1.1 µm

0.95 µm

(400)

(311)(220)

(111)

Cu Kα 2θ (degrees)

Inte

nsity

(a.u

.)

0 40 80 120 1600.00

0.05

0.10

0.15

0.20

0.25

0.30 H2 dilution ----->

Conglomerate surface grain size (nm)

Freq

uenc

y (a

rb. u

nit)

F16 (t=58 nm; R=1/20) F152 (t=55 nm; R=1/10) F151 (t=62 nm; R=1/1)

Set-A

(t ~

55 n

m)

R =1/1 R =1/10 R =1/20

SE: The film of higher value of R shows more void fraction at the top layer, indicating more rough surface compared to the films of lower value of R.X-ray: Films deposited at highest R=SiF4/H2 flow ratio 1/1 shows a preferred orientation of (400). While films deposited at R=1/5 shows a preferred orientation in (220) direction.AFM: Films are rougher for higher values of R. Average grain size increases with the increase of R.

Effect of R (SiF4/H2)

Set-C

Spectroscopic Ellipsometry Raman Scattering and AFM

RS from front sideXC1 = 35 %, Xa = 65 %

RS from glass sideXC1= 26.8 %, Xa= 73.2 %

AFM: σrms = 0.9 nmTop Layer (0.98 nm)Fcf = 33 %, Fcl = 0 %, Fv = 67 %, Fa =0 %

Bulk Layer (59.6 nm)Fcf = 73 %, Fcl = 0 %, Fv = 6 %, Fa = 21 %

RS from front sideXC1 = 20 %, XC2= 80 %, Xa= 0 %

RS from glass sideXC1 = 8.4 %, XC2 = 52.4 %, Xa = 39.2 %

Top Layer (3.1 nm)Fcf = 15 %, Fcl = 62 %, Fv = 23 %, Fa=0 %

Upper Middle Layer (864 nm)Fcf = 9.8 %, Fcl = 90.2 %, Fv = 0 %, Fa=0 %

Lower Middle Layer (311 nm)Fcf = 86.2 %, Fcl= 0 %, Fv= 4.5 %, Fa=9.3 %

Bottom Interface Layer (27 nm)Fcf = 0%, Fcl = 0 %, Fv = 25 %, Fa= 75 %

RS from front sideXC1 = 35 %, XC2= 65 %, Xa = 0 %

RS from glass sideXC1 = 17 %, Xa = 83 %

AFM: σrms = 4.16 nmTop Layer (4.2 nm)Fcf = 43 %, Fcl = 32 %, Fv = 25 %, Fa =0 %

Middle Bulk Layer (424 nm)Fcf = 58.7 %, Fcl= 37.6 %, Fv=3.7 %,

Fa=0%

Bottom Interface Layer (22 nm)Fcf = 0 %, Fcl= 0 %, Fv = 9.4 %, Fa =90.6 %

RS from front sideXC1= 34 %, XC2= 66 %, Xa= 0 %

RS from glass sideXC1 = 13.5 %, XC2 = 45.5 %, Xa = 41 %

AFM: σrms = 5.2 nmTop Layer (5.1 nm)Fcf = 33 %, Fcl = 43 %, Fv = 24 %, Fa =0 %

Middle Bulk Layer (888 nm)Fcf = 51 %, Fcl = 45 %, Fv = 3 %, Fa =0 %

Bottom Interface Layer (33 nm)Fcf = 0 %, Fcl = 0 %, Fv = 32 %, Fa =68 %

Set-B

Set-A

Characterization probes operating at different length scales leads to a comprehensive picture of film microstructures.

A large number of μc-Si:H films can be classified into three different class of microstructures.

Outcome &validation of analytical approach

0 200 400 600 800 1000 1200

0

20

40

60

80

100

R=1/1

Frac

tion

(%)

Bulk Layer Thickness (nm)

FV % FCf % FCl %

0 200 400 600 800 1000 1200

0

20

40

60

80

100R=1/10

Frac

tion

(%)

Bulk Layer Thickness (nm)

FV % FCf % FCl %

0 200 400 600 800 1000 1200

0

20

40

60

80

100

R=1/5

Frac

tion

(%)

Bulk Layer Thickness (nm)

FV % FCf % FCl %

(a)

(c)

(b)

R =

1/10

Random Orientation

Individual grains are bigger

More Void fraction

R =

1/1

R =

1/5 (220) orientation

(400) orientation

Smooth top layer

Tightly packed

Good crystallinity at bottom interface

Types of film growth

0 200 400 600 800 1000 12000

2

4

6

8

10

Thickness (nm)

Rou

ghne

ss b

y SE

, σSE

(nm

)

R=1/10 guide line for R=1/10 R=1/5 guide line for R=1/5 R=1/1 guide line for R=1/1

0 2 4 6 8 100

2

4

6

8

10

σSE= 0.85 σrms + 0.3nm

Rou

ghne

ss b

y SE

, σSE

(nm

)

Roughness by AFM, σrms(nm)

0 200 400 600 800 1000 12000

1

2

3

4

5

6

7

0 5 10 15 200

2

4

6

R

ough

ness

by

AFM

, σrm

s(nm

)

Film thickness (nm)

R=1/10 R=1/5 R=1/1

average thickness ~ 55 nm,SiF4 = 1 sccm, Ar =25 sccm, Ts = 200 oC)

Ro

ughn

ess

by A

FM, σ

rms(n

m)

H2 dilution

Roughness Analysis and its correlation with film growth

Fully crystallized microcrystalline silicon films having big grains have been deposited using standard 13.56 MHz PECVD at low substrate temperatures.

Effective control of film orientation has been demonstrated by varying the SiF4 : H2 flow ratios in the feed gas.

Tailing and asymmetry in the Raman spectrum on lower wave numbers need not be a contribution from amorphous silicon tissue, rather may indicate the contribution from smaller nanocrystallites.

The roughness analysis by two different methods, SE and AFM shows no ambiguity in their results and are in good agreement with each other.

“Surface roughness is an external mirror of the internal bulk processes”.

Summary of Structural Studies

Electrical Transport Properties-I: Dark conductivity

Above room temperature (300 – 450 K)Below room temperature (15 – 300 K)

Chapter-IV

Above room temperature (300-450K) dark conductivity (σd) measurement

Effect of film thickness on electrical properties

2.0 2.5 3.0 3.5

10-7

10-6

10-5

10-4

10-3R ( = SiF4/H2) =1/10

σ d (Ω

.cm

)-1

1000/T (K -1)

B04 (t=950 nm, Ea=0.33 eV) B23 (t=590 nm, Ea=0.44 eV) B11 (t=390 nm, Ea=0.44 eV) B22 (t=170 nm, Ea=0.54 eV) B21 (t=150 nm, Ea=0.54 eV) F152 (t=55 nm, Ea=0.54 eV) Fit

In thermally activated process dark electrical conductivity (σd) of disordered materials is given as:

σd=σo e –Ea / kT

2.0 2.5 3.0 3.510-9

10-8

10-7

10-6

10-5

10-4

10-3

R (= SiF4/H2) =1/1

σ d (Ω

.cm

)-1

1000/T (K -1)

E31 (t=1200 nm, Ea=0.2 eV)) F06 (t=920 nm, Ea=0.15 eV)) E30 (t=450 nm, Ea=0.55 eV)) F05 (t=180 nm, Ea=0.57 eV)) F151 (t=62 nm, Ea=0.58 eV)) Fit

0 200 400 600 800 1000 12000

20

40

60

80

100

Perc

enta

ge o

f Lar

ge G

rain

s (F

Cl %

)

Bulk Layer Thickness (nm)

FCl % (R= 1/10) FCl % (R= 1/5) FCl % (R= 1/1)

0 200 400 600 800 1000 120010-9

10-7

10-5

10-3

σ d (

Ω.c

m)-1

Thickness (nm)

σd (R=1/10) σd (R=1/5) σd (R=1/1) σd (R=1/5, TS )

0 200 400 600 800 1000 1200

0.1

0.2

0.3

0.4

0.5

0.6

0.7Zone-3Zone-2Zone-1

Thickness (nm)

E a (eV

)

Ea (R=1/10) Ea (R=1/5) Ea (R=1/1) Ea (R=1/5, TS)

Classification from coplanar electrical transport point of view

TYPE-AThickness (50-250 nm)

TYPE-BThickness (300-600 nm)

TYPE-CThickness (900-1200 nm)

High density of inter-grain & inter-columnar boundaries

Small grains

Marked variation in morphology & moderate disordered phase in columnar boundary

Mixed grains

Tightly packed columnar crystals

Less amorphous tissuelarge grains

The Grain Boundary Trapping (GBT) Model by Lecomber et al

[J. Non-Cryst. Solids, 59-60, 795 (1983) ]

•In Type-C samples-- material becomes relatively defect free (less traps at interface) with large grains (more free carriers)-- depletion width decreases --- Ea represents GB barrier height.

•In Type-A samples-- depletion layers extend towards the center of crystallite--- Ea will represent approximately the energy difference between the edges of the transport bands and Ef

Activation Energy, Ea

EC

qVd qVd

EF

NS

W WQS

Qd

nNnNkTlnEW /)]/(3/2[ scg +=

)2/()]/(3/2[ s2s

2cgd εnqNnNkTlnEV +=

Energy band diagram at the grain boundaries

Type-AType-BType-C

According to Meyer-Neldel Rule (MNR) such correlation leads to

σ0=σ00 eGEa

where G or EMN (1/G) and σ00 are MNR parameters

The significance of σ0

Correlation between σ0 and EaIn Type-A and Type-B materials

In Type-C materials

0.3 0.4 0.5 0.6 0.7

101

102

103

104

σ00

= 0.014(Ω−1cm-1)G = 19.7 eV -1

EMN= 51 meV

Ea(eV)

σ 0 (Ω

−1cm

-1 )

Exp. data of type- A & B samples Fit

0.00 0.05 0.10 0.15 0.20 0.25

10-2

10-1

100

101

102

σ00

= 86.8 (Ω−1cm-1)G = - 44.6 eV -1

EMN= - 22.5 meV

Ea(eV)

σ 0 (Ω−1

cm-1 )

Exp. data of type-C samples Fit data of doped μc-Si:H (Lucovsky et al) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

10-2

10-1

100

101

102

103

104

Anti MNR in type-C samples MNR in type-A & B samples MNR in a-Si:H Anti MNR in doped μc-Si:H

MNRanti MNR

σ 0 (Ω−1

cm-1 )

Ea(eV)

Below room temperature (15-300K) dark conductivity (σd) measurement

T–½ dependence of σd(T) : tunneling of carriers between neighboring conducting crystals ~ granular metals? × ES hopping --unrealistically large Coulomb gap .

T–¼ dependence σd(T): Diffusional model gives reasonable hopping parameter values. × Mott’s percolation-- unphysical parameters.

10 20 30 40 5010-11

10-9

10-7

10-5

10-3

5 10 15 20 2510-10

10-8

10-6

10-4

σ d (

Ω −

1 cm-1

)

1000/T (K -1)

E31 E25 F06 B04 B23 D26 B11 B22

σ d (

Ω −

1 cm-1

)

1000/T (K -1)

E31 E25 F06 B04 B23 D26 B11 B22

25 30 35 4010-10

10-8

10-6

10-4

10-2

σ d ( Ω

−1 cm

-1 )

100*T -1/4 (K -1/4)

E31 E25 F06 B04 B23 B22 Fits

6 8 10 12 1410-10

10-8

10-6

10-4

10-2

100*T -1/2 (K -1/2)

σ d ( Ω

−1 cm

-1 )

E31 E25 F06 B04 B23 B11 B22 Fits

Summary of Dark Electrical Transport Studies

Thermally activated carrier transport is found in above room temperature (300-450 K).

Significant correlation between the observed electrical properties (σd and Ea) of the films with their microstructural properties is established.

Classification of μc-Si:H films based on microstructural attributes that are well correlated to electrical transport properties

The change in Ea with the film thickness is directly related to the density of localized states at the Fermi level in the grain boundary.

The dependence of conductivity prefactor on the activation energy of type-A and type-B μc-Si:H films follows Meyer Neldel rule.

Statistical shift of Fermi level as an origin of MNR in our samples.

The grain boundary trapping model also supports the shift of Fermi level in changing the microstructure of the film.

However, type-C μc-Si:H films show a signature of anti MNR

Chapter-V

Electrical Transport Properties-II: Photoconductivity

Steady State Photoconductivity (SSPC)

What is γ ?γ is a measure of characteristic width of tail states nearer to Ef

Rose’s Model: γ = kTc/(kT+kTc)In amorphous semiconductor 0.5<γ <1.0

γ=0.5 => bimolecular recombination kinetics

γ=1 => monomolecular recombination.

γσ Lph G∝In a disordered material: σph (T, φ)=e[μn(n-n0) + μp(p-p0)]

Light Intensity Dependence: where, GL = φ (1-R)[1-exp(-αd)]/d

10 20 30 40 5010-10

10-9

10-8

10-7

10-6

10-5

3 4 5 6 710-6

10-5

σd

1000 / T (K -1)

σ ph (Ω

−1cm

-1)

Φ (photons/cm2sec) 1.2 x 1017

8.4 x 1016 7.6 x 1016 5.5 x 1016 2.0 x 1016 1.6 x 1015

1000 / T (K -1)

σ ph (Ω

−1cm

-1)

0 10 20 30 40 50 60 700.4

0.6

0.8

1.0

5 10 15 200.4

0.6

0.8

1.0

Ligh

t int

ensi

ty e

xpon

ent (

γ)1000/T (K -1)

B22

γ

1000/T (K -1)

B22

Type-A (#B22, t= 170 nm)0.5 < γ < 1 with TQ effect

Steady State Photoconductivity:Experimental Results

5 10 15 2010-12

10-10

10-8

10-6

10-4

σd

1000 / T (K -1)

σ ph (Ω

−1cm

-1)

Φ ( photons/cm2-sec ) 1x1014

1x1016

5x1016

1017

10 20 30 40 50 60 700.4

0.6

0.8

1.0

5 10 15 200.4

0.6

0.8

1.0

1000/T (K -1)

Ligh

t int

ensi

ty e

xpon

ent (

γ) B23

1000/T (K -1)

γ

B23

Type-B (#B23, t=590 nm)0.5 < γ < 1 with No TQ effect

10 20 30 40 5010-12

10-10

10-8

10-6

10-4

3 4 5 610-6

10-5

10-4

10-3

1000 / T (K -1)

σ ph (Ω

−1cm

-1)

σd

Φ ( photons/cm2-sec ) 1x1017

8x1016

2x1016

7x1015

2x1015

6x1014

1x1014

σ ph (Ω

−1cm

-1)

1000 / T (K -1)

10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

F06

Ligh

t int

ensi

ty e

xpon

ent (

γ)1000 / T (K-1)

F06

Ligh

t int

ensi

ty e

xpon

ent (

γ)

1000 / T (K-1)

Type-C (#F06, t= 920 nm)0.15 < γ < 1 with TQ effect

DOS distribution obtained for SSPC measurement of type-A and B µc-Si:H are plotted along with DOS profiles of µc-Si:H suggested in literature from other experimental techniques.

0.0 0.2 0.4 0.61013

1015

1017

1019

1021

1013

1015

1017

1019

1021

EC- E (eV)

dens

ity o

f sta

tes

(arb

. uni

t) DOS of μc-Si:H (Type-B) DOS of μc-Si:H (Type-A) MPC-DOS of coplanar μc-Si:H (ICRS =0.5)

[Ref.**] MPC-DOS of HWCVD μc-Si:H [Ref.!] MPC-DOS of SPC μc-Si:H [Ref.!] TOF-DOS of μc-Si:H [Ref.!!] SSPC-DOS of μc-Si:H [Ref.*]

Photoconductivity Exponent: Applicability of Rose Model

QUALITATIVE ANALYSIS

Phototransport properties of Type-A (TQ and 0.5< γ<1)This type of behavior is usually observed in typical a-Si:HRose model works and width of CBT is deduced (kTc ~ 30 meV )

Possible explanation for “No TQ and 0.5< γ<1 “ as found in Type-BUsually observed in typical µc-Si:H

Symmetric band tails

Rose model works and width of CBT is deduced (kTc~25-28 meV)

According to Balberg et al (Phys. Rev. B 69, 2004, 035203): a Gaussian type VBT responsible for such behavior

Possible explanations for TQ behavior in Type-C materialRose model does not hold for Type-C material

DBs unlikely to cause TQ

Possibilities of asymmetric band tail states in this type of material

lower DOS near the CB edge, i.e. a steeper CBT than VBT (supported by defect pool model)

The CPM measurement supports the fact kTC<<kTV

Chapter-VI

Numerical Modeling of Steady State Photoconductivity in µc-Si:H

MotivationExperimental results cannot discern the states where the recombination actually occursS-R-H mechanism and Simmons-Taylor Statistics are extensively used to understand recombination mechanism in steady state process

R1R2

R3

R4

R5

R6

R7 R8

R12

R9R15

R13

R10

R14

VBT CBTDB + DB 0 DB -

R11

R16GL

EV

EC

U

VBT

CBT

DB

Schematics of different recombination processes taking place within the gap of a disordered material.

[ ] [ ] ( ) ( )[ ] ( ) ( )[ ] ( ) 022,,,, 000000000 =−−++−−−+−−− −−

DBDBDBDBDBVTVTCTCT FFFFNpnQpnQpnQpnQppnn

DBVTCTL UUUG ++=

Charge neutrality equation

Recombination equation

Steps in Numerical SimulationDOS distribution is first assumedGuess values of n and p are givenCharge neutrality equation & recombination rates equation are simultaneously solved for a fixed value of T and GL

S-R-H mechanism and Simmons-Taylor Statistics are appliedNewton-Raphson method for finding roots of n and pSimpson’s method for numerical integrationn and p are obtainedWe calculated σph (T, φ)=e[μn(n-n0) + μp(p-p0)]The corresponding γ values are obtained as in experimental case

0.0 0.3 0.6 0.9 1.2 1.5 1.81013

1015

1017

1019

1021

1013

1015

1017

1019

1021

DB

EC- EF=0.46 eV

CBT2

CBT1

VBT2

VBT1

Effe

ctiv

e D

OS

(cm

-3eV

-1 )

EV EC(E-EV) eV

5 10 15 2010-11

10-10

10-9

10-8

10-7

10-6

G=1020 cm-3sec-1

G=1019 cm-3sec-1

G=1018 cm-3sec-1

G=1017 cm-3sec-1

1000/T (K -1)

σ ph (Ω

-1cm

-1)

5 10 15 200.4

0.6

0.8

1.0

1000/T (K -1)

Ligh

t int

ensi

ty e

xpon

ent (

γ)

Type-ASimulated Steady State Photoconductivity Results

0.0 0.3 0.6 0.9 1.2 1.5 1.81013

1015

1017

1019

1021

1013

1015

1017

1019

1021

DB

EC- EF=0.42 eV

CBT2

CBT1

VBT2

VBT1

Effe

ctiv

e D

OS

(cm

-3eV

-1 )

EV EC(E-EV) eV

5 10 15 20

10-7

10-6

10-5

G = 1021 cm-3sec-1

G = 1020 cm-3sec-1

G = 1019 cm-3sec-1

1000/T (K -1)

σ ph (Ω

-1cm

-1)

5 10 15 200.4

0.6

0.8

1.0

Li

ght i

nten

sity

exp

onen

t (γ)

1000/T (K -1)

Type-B

0.0 0.3 0.6 0.9 1.2 1.5 1.81013

1015

1017

1019

1021

1013

1015

1017

1019

1021

CBT2Effe

ctiv

e D

OS

(cm

-3eV

-1 )

EC- EF=0.34 eV

DB

CBT1

VBT2

VBT1

ECEV

(E-EV) eV

5 10 15 2010-7

10-6

10-5

10-4

G=1021 cm-3sec-1

G=1020 cm-3sec-1

G=1019 cm-3sec-1

1000/T (K -1)

σ ph (Ω

-1cm

-1)

5 10 15 200.0

0.2

0.4

0.6

0.8

1.0

1000/T (K -1)

Ligh

t int

ensi

ty e

xpon

ent,

γ

Type-C

50 100 150 200 250 300 350106

109

1012

1015

1018

T (K)

Rec

ombi

natio

n ra

tes

(cm

-3se

c-1)

UVBT1

UVBT2

UDB UCBT1

UCBT2

50 100 150 200 250 3001011

1014

1017

1020

T (K)

Rec

ombi

natio

n ra

tes

(cm

-3se

c-1)

UVBT1

UVBT2

UDB UCBT1

UCBT2

50 100 150 200 250 300107

1010

1013

1016

1019

T (K)

Rec

ombi

natio

n ra

tes

(cm

-3se

c-1)

UVBT1

UVBT2

UDB UCBT1

UCBT2

Summary of Phototransport Studies

Morphological & Microstructural differences in the different types of such

µc-Si:H material leads to totally different phototransport behavior.

The results obtained by numerical modeling are found to be in good agreement

with the experimental findings, esp. TQ, its onset in the photoconductivity and

the γ values.

The effective DOS distribution we have proposed differs for

different microstructures of μc-Si:H thin films and successfully

explains the different phototransport properties in the light of

their microstructural properties as well.

Summary and Conclusions

Multi-pronged approach of characterization techniques used to provide a complete, quantitative and unambiguous µc-Si:H microstructural picture

The µc-Si:H films produced by the PECVD of SiF4 based precursor are highly crystalline, even at substrate temperatures as low as ~100°C

Achievement of preferential crystalline orientation

Presence of CSD supported by RS, SE and AFM analysis

A linear correlation is observed between the rms roughness measured by AFM and the top surface layer measured by SE

Microstructural characterization shows three types of microstructures that leads to distinct electrical transport behavior

Simultaneous observation of MNR and anti-MNR in undoped µc-Si:H thin films

Low temperature dark conductivity evinces a T-1/2 dependence supporting tunneling mechanism, and a T-1/4 dependence compatible with diffusional model.

Different types of µc-Si:H films exhibit different phototransport behaviors, explained on the basis of Rose model in types A and B, whereas type-C material exhibits anomalous behavior, explained using an effective DOS consisting of two VBT slopes.

Numerical modeling of phototransport properties is found to giveresults corroborative with the experimental results. We have proposed complete effective density of states distributions for different types of µc-Si:H thin films having different microstructures.

Thank you!