My Thesis I investigated the microstructure of a wide variety of nano and microcrystalline Si (μc-Si:H) films produced under different growth conditions using different characterization probes (spectroscopic ellipsometry, Raman spectroscopy, atomic force microscopy and X-ray diffraction) at different stages of film growth. In microstructural studies, I applied a novel modeling method for deconvolution of Raman spectra of the μc-Si:H films and elucidated schematic growth models for the SiF4 based single phase μc-Si:H material. I carried out studies on the optoelectronic properties of these microstructurally different films using dark and photo- conductivity as functions of several discerning parameters. The results of these studies led me to expound a novel way of classifying the wide range of materials into three types based on microstructural attributes and correlative optoelectronic properties. My electrical transport studies have uncovered some new aspects of the carrier conduction routes and mechanisms in the single phase μc-Si:H material. I have proposed the complete effective distributions of density of states (DOS) applicable to this wide microstructural range of μc-Si:H material based on the results of experimental and numerical simulation studies of the phototransport properties of the material.

1. Influence of Microstructure on
the Electronic Transport Behavior
of Microcrystalline Silicon Films
Sanjay K. Ram
Dept. of Physics,
Indian Institute of Technology Kanpur, INDIA

2. Outline
Ch. 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

3. Chapter-I
INTRODUCTION

4. crystalline structure crystallites in a-Si random network
Long-range order Medium-range order Short-range order

5. Role of Si thin films in large area microelectronics
Thin film Poly Si
Amorphous silicon (a-Si:H)
Advantages:
Advantages:
Solid phase crystallization/LPCVD
Possibility of low temperature
Grain sizes of 10 nm to 1 μm are
plasma deposition
common
Plays a dominant role in the
Very high carrier mobility
application of solar cells and TFTs
Greater stability under electric field
Good photosensitivity
and light-induced stress
Wide band gap
Good for TFTs
Issues:
High doping efficiency
Low carrier mobility (μn~1 cm2/V-s
Issues:
& μp~10-3 cm2/V-s
Metastability High temperature deposition
Poor doping efficiency Boundaries are not passivated

6. Why μc-Si:H thin films ??
Promising material for large area electronics
Possibility of low temperature deposition
Good carrier mobility
Greater stability under electric field and light-induced
stress
Good doping efficiency
Boundaries are passivated
Further development requires proper
understanding of carrier transport properties
correlative with film microstructure

7. Why is a comprehensive description of
optoelectronic properties of µc-Si:H difficult ???
1. Complex microstructure
columnar boundaries
grains grain boundaries
conglomerate crystallites
surface
roughness
voids
Film
growth
substrate
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.

8. Issues
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.

9. 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)

10. 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.

11. Desired μc-Si:H material in TFTs
(Staggered type)
Need for BOTTOM Gate TFT
Need for TOP Gate TFT
Smooth Top layer of the film Crystallization should start at the
beginning of the growth
Bigger sizes of crystallite at the
Top layer To reduce the amorphous
incubation layer at the bottom
Inverted pyramid shaped
glass interface
columnar crystallites are
preferable

12. 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.

13. 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.

14. Chapter-II
EXPERIMENTAL DETAILS

15. Sample Preparation
PECVD
RF
Parallel-plate glow discharge
HH
H Si H H
N H H
H H
plasma deposition system
H
Si N Si N Si N
μc-Si:H
Substrate: Corning 1773
film
High purity feed gases: Silane flow ratio
SiF4 , Ar & H2 (R)= SiF4/H2
R=1/1 R=1/5 R=1/10
Rf frequency 13.56 MHz
Ts=200 oC
Thickness series

16. Film characterization
Electrical Properties
Structural Properties
σd(T) measurement
15K≤T ≤ 450K
X-ray Diffraction
σPh(T,∅) measurement
15K≤T ≤ 325K
Raman Scattering
CPM measurement
In-situ Spectroscopic Ellipsometry
TRMC
Atomic Force Microscopy

17. Chapter-III
STRUCTURAL INVESTIGATION

18. Spectroscopic Ellipsometry Study
45
F0E31
40
Fit
35
a-Si:H
30 Top Layer (3.1 nm)
c-Si Fcf = 15 %, Fcl= 62 %, Fv = 23 %, Fa=0 %
25 Upper Middle Layer (864 nm)
<ε2>
20 Fcf = 9.8 %, Fcl = 90.2 %, Fv = 0 %, Fa=0 %
15 Lower Middle Layer (311 nm)
Fcf = 86.2 %, Fcl= 0 %, Fv= 4.5 %, Fa=9.3 %
10
5 Bottom Interface Layer (27 nm)
Fcf = 0%, Fcl = 0 %, Fv = 25 %, Fa= 75 %
0
-5
-10
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
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.

19. Bifacial Raman Study
A deconvolution model that includes crystallite size distribution was
employed for analysis of Raman data.
1.2 1.2 glass side exp. data of F0E31
film side exp. data of F0E31
cd1
cd1
cd2
cd2
Intensity (arb. unit)
a
Intensity (arb. unit)
fit with - cd1cd2 fit with - cd1cd2a
0.9 0.9
0.6 0.6
0.3 0.3
0.0
0.0
400 425 450 475 500 525 550
450 475 500 525 550
-1 -1
Raman Shift (cm ) Raman Shift (cm )
collection collection
Small grain (cd1) Large grain (cd2) a-Si:H
Sample #E31
Fitting
(1200 nm, Size (nm) XC1 Size (nm) XC2
Model excitation
Xa (%)
R=1/1) [σ (nm)] [σ (nm)]
(%) (%)
excitation
cd1+cd2 6.1, [1.68] 20 72.7, [0] 80 0
Film side
film
glass
cd1+cd2+a 6.6, [1.13] 8.4 97.7, [4.7] 52.4 39.2
Glass side
glass
film

20. Surface Morphology by AFM
(b)
0.25
(a) B04 (t =950 nm; R=1/10)
0.20
Frequency (arb. unit)
0.15
(c)
0.10
0.05
0.00
0 100 200 300 400
Conglomerate surface grain size (nm)
sample #B04 (thickness = 950 nm, R=1/10, roughness (σrms) = 5.26 nm)

21. Types of samples studied
Fixed deposition parameters
Plasma Power (W) 20
13.56
RF frequency (νrf) (MHz)
Total Pressure (Torr) 1
SiF4 flow rate (sccm) 1
Ar flow rate (sccm) 25
R=SiF4/H2 = 1/1
Thickness series Thickness :
R=SiF4/H2 = 1/5
50 nm to 1200 nm
TS=200°C
R=SiF4/H2 = 1/10
Set-A (thickness is ~ 50 nm)
R series R:
Set-B (thickness is ~ 400 nm)
1/1 to 1/20
TS=200°C
Set-C (thickness is ~ 950 nm)
TS series R=1/5 TS: 100 - 350°C

22. Effect of Film Growth
E31 (R=1/1, t=1200nm)
30 Growth time
30 min
(c)
(b)
60 min
190 min
20 225 min
230 min
< ε2 >
10
0
(a) (d)
-10
2 3 4 5
Energy (eV)
Film side
R (SiF4 / H2) = 1/10
0.25
Intensity (arb. unit)
thickness ---->
thickness series of R =1/10
B04 (t=950 nm)
B04 (t=950 nm)
0.20
Frequency (arb. unit)
B11 (t=390 nm)
B22 (t=170 nm)
B23 (t=590 nm)
F152 (t=52 nm)
0.15
D281 (t=422 nm)
B11 (t=390 nm)
0.10
B22 (t=170 nm)
0.05
F152 (t=52 nm)
0.00
450 475 500 525 550 0 100 200 300 400
-1
Raman Shift (cm ) Conglomerate surface grain size (nm)

23. 30 F151 (R=1/1, t=62 nm)
Effect of R (SiF4/H2)
F152 (R=1/10, t=55 nm)
H2 dilution
F16 (R=1/20, t=58 nm)
25
20
SE: The film of higher value of R shows more void
< ε2 >
15
fraction at the top layer, indicating more rough
10
surface compared to the films of lower value of R.
5
X-ray: Films deposited at highest R=SiF4/H2 flow
0
2.5 3.0 3.5 4.0 4.5 5.0
ratio 1/1 shows a preferred orientation of (400).
Energy (eV)
While films deposited at R=1/5 shows a preferred
4500
(111) (400)
4000
(220)
orientation in (220) direction.
(311)
1/1
3500
1.2 µm
3000
AFM: Films are rougher for higher values of R.
Intensity (a.u.)
2500
1/5 1.1 µm
Average grain size increases with the increase of R.
2000
1500
1000
1/10 0.95 µm
500
0
0.30
20 30 40 50 60 70
H2 dilution ----->
Cu Kα 2θ (degrees) F16 (t=58 nm; R=1/20)
F152 (t=55 nm; R=1/10)
R =1/1 R =1/10 0.25
R =1/20 F151 (t=62 nm; R=1/1)
Frequency (arb. unit)
0.20
(t ~ 55 nm)
Set-A
0.15
0.10
0.05
0.00
0 40 80 120 160
Conglomerate surface grain size (nm)

24. Spectroscopic Ellipsometry Raman Scattering and AFM
AFM: σrms = 0.9 nm
Top Layer (0.98 nm)
Fcf = 33 %, Fcl = 0 %, Fv = 67 %, Fa =0 %
Outcome &validation
RS from front side
Bulk Layer (59.6 nm)
Set-A XC1 = 35 %, Xa = 65 %
Fcf = 73 %, Fcl = 0 %, Fv = 6 %, Fa = 21 %
of analytical approach
RS from glass side
XC1= 26.8 %, Xa= 73.2 %
AFM: σrms = 4.16 nm
Top Layer (4.2 nm)
Fcf = 43 %, Fcl = 32 %, Fv = 25 %, Fa =0 %
Middle Bulk Layer (424 nm) RS from front side
Characterization probes
XC1 = 35 %, XC2= 65 %, Xa = 0 %
Fcf = 58.7 %, Fcl= 37.6 %, Fv=3.7 %,
Set-B
Fa=0%
operating at different
RS from glass side
Bottom Interface Layer (22 nm)
XC1 = 17 %, Xa = 83 %
Fcf = 0 %, Fcl= 0 %, Fv = 9.4 %, Fa =90.6 %
length scales leads to a
comprehensive picture of
AFM: σrms = 5.2 nm
Top Layer (5.1 nm)
Fcf = 33 %, Fcl = 43 %, Fv = 24 %, Fa =0 %
film microstructures.
RS from front side
Middle Bulk Layer (888 nm)
XC1= 34 %, XC2= 66 %, Xa= 0 %
Fcf = 51 %, Fcl = 45 %, Fv = 3 %, Fa =0 %
RS from glass side
Bottom Interface Layer (33 nm)
A large number of μc-Si:H
XC1 = 13.5 %, XC2 = 45.5 %, Xa = 41 %
Fcf = 0 %, Fcl = 0 %, Fv = 32 %, Fa =68 %
Set-C
films can be classified into
Top Layer (3.1 nm)
Fcf = 15 %, Fcl = 62 %, Fv = 23 %, Fa=0 %
RS from front side
three different class of
XC1 = 20 %, XC2= 80 %, Xa= 0 %
Upper Middle Layer (864 nm)
Fcf = 9.8 %, Fcl = 90.2 %, Fv = 0 %, Fa=0 %
microstructures.
Lower Middle Layer (311 nm)
Fcf = 86.2 %, Fcl= 0 %, Fv= 4.5 %,
Fa=9.3 % RS from glass side
Bottom Interface Layer (27 nm) XC1 = 8.4 %, XC2 = 52.4 %, Xa = 39.2 %
Fcf = 0%, Fcl = 0 %, Fv = 25 %, Fa= 75 %

25. Types of film growth
100
R=1/10
FV %
80
FCf %
Fraction (%)
FCl %
60
(a)
Random Orientation
40
R =1/10
20
More Void fraction
0
Individual grains are bigger
0 200 400 600 800 1000 1200
Bulk Layer Thickness (nm)
100
80
Fraction (%)
R=1/5
60
FV %
(b)
(220) orientation
FCf %
R =1/5
40 FCl %
20
0
0 200 400 600 800 1000 1200
Bulk Layer Thickness (nm)
100
(400) orientation
80
R =1/1
Tightly packed
Fraction (%)
60 R=1/1
FV % (c)
Smooth top layer
40 FCf %
FCl %
20
Good crystallinity at bottom interface
0
0 200 400 600 800 1000 1200
Bulk Layer Thickness (nm)

26. Roughness Analysis and its correlation with film growth
R=1/10
Roughness by AFM, σrms(nm) 7 R=1/5
R=1/1
6
5
10
4
Roughness by SE, σSE(nm)
Roughness by AFM, σrms(nm)
6 average thickness ~ 55 nm,
3 SiF4 = 1 sccm,
Ar =25 sccm,
8
o
Ts = 200 C)
4
2
2
6
1 0
0 5 10 15 20
H2 dilution
0
0 200 400 600 800 1000 1200 4
Film thickness (nm)
10 2
Roughness by SE, σSE(nm)
σSE= 0.85 σrms + 0.3nm
8 0
0 2 4 6 8 10
Roughness by AFM, σrms(nm)
6
4
R=1/10
guide line for R=1/10
2 R=1/5
guide line for R=1/5
R=1/1
guide line for R=1/1
0
0 200 400 600 800 1000 1200
Thickness (nm)

27. Summary of Structural Studies
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”.

28. Chapter-IV
Electrical Transport Properties-I:
Dark conductivity
Above room temperature (300 – 450 K)
Below room temperature (15 – 300 K)

29. Above room temperature (300-450K) dark
conductivity (σd) measurement
Effect of film thickness on electrical properties
R ( = SiF4/H2) =1/10 R (= SiF4/H2) =1/1
-3 -3
10 10
-4
10
-4
-1
10
σd (Ω.cm)
-1
-5
σd (Ω.cm)
10
-5
10 -6
10
-6 -7
B04 (t=950 nm, Ea=0.33 eV)
10 10
B23 (t=590 nm, Ea=0.44 eV)
E31 (t=1200 nm, Ea=0.2 eV))
B11 (t=390 nm, Ea=0.44 eV)
F06 (t=920 nm, Ea=0.15 eV))
-8
10
B22 (t=170 nm, Ea=0.54 eV)
-7 E30 (t=450 nm, Ea=0.55 eV))
10 B21 (t=150 nm, Ea=0.54 eV)
F05 (t=180 nm, Ea=0.57 eV))
F152 (t=55 nm, Ea=0.54 eV)
-9 F151 (t=62 nm, Ea=0.58 eV))
10
Fit
Fit
2.0 2.5 3.0 3.5 2.0 2.5 3.0 3.5
-1 -1
1000/T (K ) 1000/T (K )
In thermally activated process dark
electrical conductivity (σd) of
disordered materials is given as:
σd=σo e –Ea / kT

30. σd (R=1/10)
-3
σd (R=1/5)
10
Classification from coplanar
σd (R=1/1)
σd (R=1/5, TS )
electrical transport point of view
-1
σd (Ω.cm)
-5
10
-7
10
High density of inter-
grain & inter-columnar
-9
10
0 200 400 600 800 1000 1200
Thickness (nm)
boundaries
0.7 TYPE-A
Zone-1
Small grains
Zone-3
Zone-2
Thickness (50-250 nm)
0.6
0.5
Ea (eV)
Marked variation in
0.4
morphology & moderate
0.3
Ea (R=1/10)
0.2
disordered phase in
Ea (R=1/5)
Ea (R=1/1)
TYPE-B
Ea (R=1/5, TS)
0.1
columnar boundary
Thickness (300-600 nm)
0 200 400 600 800 1000 1200
Mixed grains
Thickness (nm)
Percentage of Large Grains (FCl %)
100
FCl % (R= 1/10)
FCl % (R= 1/5)
80
Tightly packed
FCl % (R= 1/1)
columnar crystals
60
Less amorphous tissue
40
large grains
TYPE-C
20
Thickness (900-1200 nm)
0
0 200 400 600 800 1000 1200
Bulk Layer Thickness (nm)

31. Activation Energy, Ea
W = [ 2 E g / 3 + kTln ( N c / n)]N s / n
qVd qVd
Qd
Vd = [2 Eg / 3 + kTln( N c / n)]2 N s2 q /(2nε s )
EC W W
QS NS
EF
Energy band diagram at the grain boundaries •In Type-C samples-- material
becomes relatively defect free (less
Type-C Type-B Type-A
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
The Grain Boundary Trapping (GBT) approximately the energy difference
Model by Lecomber et al between the edges of the transport
[J. Non-Cryst. Solids, 59-60, 795 (1983) ]
bands and Ef

32. The significance of σ0
Correlation between σ0 and Ea
In Type-A and Type-B materials
According to Meyer-Neldel Rule (MNR)
such correlation leads to
Exp. data of type- A & B samples
4 Fit
10
σ0=σ00 eGEa
σ0 (Ω cm )
-1
3
10
where G or EMN (1/G) and σ00 are
−1
MNR parameters
2
10
−1 -1
σ00 = 0.014(Ω cm )
1 -1
10 G = 19.7 eV
4 MNR
10
EMN= 51 meV anti MNR
0.3 0.4 0.5 0.6 0.7 3
10
Ea(eV)
σ0 (Ω cm )
2
-1
In Type-C materials 10
2
10 1
−1
10
−1 -1
σ00 = 86.8 (Ω cm )
-1
G = - 44.6 eV
1
10 0
10
EMN= - 22.5 meV
σ0 (Ω cm )
-1
Anti MNR in type-C samples
0
MNR in type-A & B samples
10 -1
10
−1
MNR in a-Si:H
Anti MNR in doped μc-Si:H
-1
-2
10
10
Exp. data of type-C samples
-2
10 Fit
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
data of doped μc-Si:H (Lucovsky et al)
Ea(eV)
0.00 0.05 0.10 0.15 0.20 0.25
Ea(eV)

33. Below room temperature (15-300K) dark conductivity (σd) measurement
-2 -2
10 10
E31
E31
E31
-3 E31
E25
10 E25
-4
10 E25
F06
E25
F06
σd ( Ω cm )
-1
B04
F06
B04 F06
B23
-4 -4
B23 B04
D26
10
-6
10
σd ( Ω cm )
B04
10
σd ( Ω cm )
−1 B11
D26
σd ( Ω cm )
-1
-5 B23
-1
B22
B23
10
-1
B11
B22 B11
-8
B22
10 Fits B22
−1
−1
-6 -6
−1
Fits
10 10
-7 -10
10
10 5 10 15 20 25
-1
1000/T (K )
-8 -8
-9
10 10
10
-11 -10 -10
10 10 10
10 20 30 40 50 25 30 35 40 6 8 10 12 14
-1 -1/4 -1/4
1000/T (K ) 100*T (K ) -1/2 -1/2
100*T (K )
T–½ dependence of σd(T) :
tunneling of carriers between
neighboring conducting crystals ~
granular metals?
× ES hopping --unrealistically
large Coulomb gap .
σd(T):
T–¼ dependence
Diffusional model gives
reasonable hopping parameter
values.
× Mott’s percolation-- unphysical
parameters.

34. 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

35. Chapter-V
Electrical Transport Properties-II:
Photoconductivity

36. Steady State Photoconductivity (SSPC)
γ
Light Intensity Dependence: σ ph ∝ GL
In a disordered material:
σph (T, φ)=e[μn(n-n0) + μp(p-p0)] where, GL = φ (1-R)[1-exp(-αd)]/d
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.

37. Steady State Photoconductivity:
Experimental Results
1.0
Light intensity exponent (γ)
σph (Ω cm )
-5
-1
-5
10 B22
10
−1
-6
10
σph (Ω cm )
0.8
-1
-6
10
3 4 5 6 7
-7 -1 1.0
−1
1000 / T (K )
10 B22
σd
0.8
-8
0.6
10 2
Φ (photons/cm sec)
γ
0.6
17
1.2 x 10
16
8.4 x 10
-9
10 16
7.6 x 10 0.4
5 10 15 20
16
5.5 x 10 -1
1000/T (K )
16
2.0 x 10
0.4
-10 15
1.6 x 10
10 0 10 20 30 40 50 60 70
10 20 30 40 50 -1
1000/T (K )
-1
1000 / T (K )
Type-A (#B22, t= 170 nm)
0.5 < γ < 1 with TQ effect

38. -4
10
1.0
Light intensity exponent (γ)
Φ ( photons/cm -sec )
2
B23
14
1x10
16
1x10
-6 16
10 5x10
17
10
σph (Ω cm )
0.8
-1
−1
-8
10
σd 1.0
0.6 0.8
γ
-10
10 0.6
B23
0.4
5 10 15 20
-1
1000/T (K )
0.4
-12
10
10 20 30 40 50 60 70
5 10 15 20
-1
-1
1000/T (K )
1000 / T (K )
Type-B (#B23, t=590 nm)
0.5 < γ < 1 with No TQ effect

39. -3
2
Φ ( photons/cm -sec ) 10
1.0
Light intensity exponent (γ)
17
1x10
F06
16
σph (Ω cm )
8x10
-1
-4 -4
10 10
16
2x10
−1 0.8
15
7x10
-5
15
2x10 10
14
σph (Ω cm )
-6 6x10
10
-1
0.6
14
1x10 -6
10
3 4 5 6 1.0
−1
Light intensity exponent (γ)
-1
1000 / T (K ) F06
-8 0.8
10 0.4
σd 0.6
0.4
0.2
-10 0.2
10
0.0
5 10 15 20
-1
1000 / T (K )
0.0
-12
10 10 20 30 40 50 60 70
10 20 30 40 50
-1
1000 / T (K )
-1
1000 / T (K )
Type-C (#F06, t= 920 nm)
0.15 < γ < 1 with TQ effect

40. Photoconductivity Exponent:
Applicability of Rose Model
density of states (arb. unit) DOS of μc-Si:H (Type-B)
21 21
DOS of μc-Si:H (Type-A)
10 10
MPC-DOS of coplanar μc-Si:H (ICRS =0.5)
**
[Ref. ]
!
MPC-DOS of HWCVD μc-Si:H [Ref. ]
19 19
!
MPC-DOS of SPC μc-Si:H [Ref. ]
10 10
!!
TOF-DOS of μc-Si:H [Ref. ]
*
SSPC-DOS of μc-Si:H [Ref. ]
17 17
10 10
15 15
10 10
13 13
10 10
0.0 0.2 0.4 0.6
EC- E (eV)
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.

41. QUALITATIVE ANALYSIS
Phototransport properties of Type-A (TQ and 0.5< γ<1)
This type of behavior is usually observed in typical a-Si:H
Rose 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-B
Usually 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 material
Rose 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

42. Chapter-VI
Numerical Modeling of Steady
State Photoconductivity in µc-Si:H

43. Motivation
Experimental results cannot discern the states where the
recombination actually occurs
S-R-H mechanism and Simmons-Taylor Statistics are extensively
used to understand recombination mechanism in steady state
process
EC
R9 R10
CBT R15 R4
R3
R1
R2
R16
GL
DB U R13 R14
R6
R7 R8 R5
VBT R11 R12
EV
DB 0
VBT CBT
DB + DB -
Schematics of different recombination processes taking place
within the gap of a disordered material.

44. Charge neutrality equation
[n − n0 ] − [ p − p0 ] + [QCT (n, p ) − QCT (n0 , p0 )] − [QVT (n, p ) − QVT (n0 , p0 )] + N DB (FDB + 2 FDB − FDB − 2 FDB ) = 0
− −
0 00
GL = U CT + U VT + U DB
Recombination equation
Steps in Numerical Simulation
DOS distribution is first assumed
Guess values of n and p are given
Charge 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 applied
Newton-Raphson method for finding roots of n and p
Simpson’s method for numerical integration
n and p are obtained
We calculated σph (T, φ)=e[μn(n-n0) + μp(p-p0)]
The corresponding γ values are obtained as in experimental
case

45. Simulated Steady State Photoconductivity Results
Type-A
21 21
10 10
Effective DOS (cm eV )
VBT1
CBT1
-1
19 19
10 10
-3
EC- EF=0.46 eV
17 17
10 10
DB
15 15
10 10
VBT2 CBT2
13 13
10 10
0.0 0.3 0.6 0.9 1.2 1.5 1.8
EC
EV (E-EV) eV
Light intensity exponent (γ)
1.0
-6 20 -3 -1
10 G=10 cm sec
19 -3 -1
G=10 cm sec
18 -3 -1
G=10 cm sec
-7
10
σph (Ω cm )
0.8
-1
17 -3 -1
G=10 cm sec
-8
10
-1
-9
10 0.6
-10
10
0.4
-11
10 5 10 15 20
5 10 15 20
-1 -1
1000/T (K ) 1000/T (K )

46. Type-B
21 21
10 10
Effective DOS (cm eV )
CBT1
VBT1
-1
19 19
10 10
-3
EC- EF=0.42 eV
17 17
10 10
15 15
10 10
DB CBT2
VBT2
13 13
10 10
0.0 0.3 0.6 0.9 1.2 1.5 1.8
EC
EV (E-EV) eV
1.0
Light intensity exponent (γ)
21 -3 -1
G = 10 cm sec
20 -3 -1
G = 10 cm sec
-5 19 -3 -1
10 G = 10 cm sec
0.8
σph (Ω cm )
-1
-1
-6
10
0.6
-7
10
0.4
5 10 15 20 5 10 15 20
-1 -1
1000/T (K ) 1000/T (K )

47. Type-C
21 21
10 10
CBT1
Effective DOS (cm eV )
VBT1
-1
19 19
10 10
-3
EC- EF=0.34 eV
17 17
10 10
DB
15 15
VBT2
10 10
CBT2
13 13
10 10
0.0 0.3 0.6 0.9 1.2 1.5 1.8
EV EC
(E-EV) eV
1.0
Light intensity exponent, γ
21 -3 -1
G=10 cm sec
20 -3 -1
0.8
G=10 cm sec
-4
10 19 -3 -1
G=10 cm sec
σph (Ω cm )
-1
0.6
-5
-1
10
0.4
-6
10
0.2
-7
0.0
10
5 10 15 20
5 10 15 20
-1 -1
1000/T (K ) 1000/T (K )