Microcrystalline silicon is a heterogenous material. We show that different effective DOS distribution can be possible for micro-structurally different μc--Si:H thin films

1. Study Of Anomalous Behavior Of
Steady State Photoconductivity
In
Highly Crystallized Undoped
Microcrystalline Si Films
Sanjay K. Ram
Dept. of Physics,
Indian Institute of Technology Kanpur, INDIA

2. Outline
Motivation
Sample preparation & structural characterization
Steady state photoconductivity (SSPC)
measurements
Qualitative analysis
Numerical simulation of SSPC
Conclusion

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

4. ISSUES
Why is comprehensive description of its opto-electronic
properties difficult ???
Complex microstructure & inhomogeneity
in the growth direction
columnar boundaries grain
grains
conglomerate crystallites boundaries
surface
roughness
voids
Film
growth
substrate

5. ISSUES
Non-availability of complete density of
state (DOS) map of µc-Si:H system
Difference between Density of States (DOS)
map of c-Si and amorphous Silicon (a-Si:H)

6. ISSUES
Electrical transport ???
Is it dominated by crystalline phase ???
or
By interfacial regions between crystallites or grains???
A large number of studies claim that electronic
transport in μc-Si:H films is analogous to that
observed in a-Si:H films
GOAL
To study the opto-electronic properties of well
characterized μc-Si:H films
Identify the role of microstructure in determining
the electrical transport behavior

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

8. Film characterization
Electrical Properties
Structural Properties
σd(T) measurement
15K≤T ≤ 450K
Xray Diffraction
σPh(T,∅) measurement
15K≤T ≤ 325K
Raman Scattering
CPM measurement
In-situ Spectroscopy Ellipsometry
Hall effect
TRMC
Atomic Force Microscopy

9. Raman Scattering
3.2 Layer side
R (SiF4 / H2) = 1/10
μc-Si (X, SiF4)
μc-Si Std. 2.8
Intensity (arb. unit)
FB04GF
t=950 nm
2.4
t=590 nm FB23GF
2.0
t=422 nm
Intensity (a.u.)
F281GF
1.6
t=390 nm FB11GF
1.2
t=170 nm FB22GF
0.8
0.4 t=52 nm F152GB
450 475 500 525 550
-1
Raman Shift (cm )
c-Si
400 450 500 550 600 Effect of thickness variation
-1
Raman shift (cm )

10. Spectroscopy ellipsometery study
B23 (R=10, t=590 nm)
45 B11 (R=10, t=390 nm)
25
F0E31
40 B22 (R=10, t=170 nm)
F152(R=10, t=55 nm)
Fit
35 F16 (R=20, t=35 nm)
a-Si:H 20
30
c-Si
25
<ε2>
< ε2 >
20 15
15
10
10
5
0
5
-5
-10
0
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
2.5 3.0 3.5 4.0 4.5 5.0
Energy (eV) Energy (eV)
Fig. Measured <ε2> spectrum for the µc-Si:H
samples. The sample name, thickness and
its 1/R value are shown in the graph.

11. X-ray diffraction study
XRD study to see the effect
of R (H2/SiF4)
4500
Film deposited at SiF4/H2
(111) (400)
4000
(220)
flow ratio 1/1 shows a preferred
(311)
1/1
3500
orientation of (400).
1.2 µm
3000
Intensity (a.u.)
film deposited at SiF4/H2
2500
R
1/5 1.1 µm
flow ratio of 1/5 shows a
2000
preferred orientation in (220)
1500
direction.
1000
1/10 0.95 µm
500
These results demonstrate the
0
20 30 40 50 60 70
effectiveness of using fluorine
Cu Kα 2θ (degrees)
based precursors in controlling
the orientation of
polycrystalline films on
insulating glass substrates.

12. Structural Findings
Random Orientation More Void fraction
R =1/10
Individual grains are bigger
(220) orientation
R =1/5
(400) orientation Tightly packed Smooth top layer
R =1/1
Good crystallinity at bottom interface

13. Classification from coplanar electrical
transport point of view
TYPE-A
More amorphous tissue
Small grains
Thickness
(50-250 nm)
TYPE-B
Moderate amorphous tissue
Thickness
Small grains
(300-600 nm)
TYPE-C
Tightly packed columnar crystals
Less amorphous tissue
Thickness
(900-1200 nm)
Big grains

14. We have measured temperature and light intensity
dependent steady state photoconductivity (SSPC)
for the samples of different microstructure
SSPC Process
absorption of photons recombination of
transport of
and generation of free excess free electrons
mobile carriers
electron-hole pairs and holes through
recombination centers

15. In a disordered material:
σph (T, φ)=e[μn(n-n0) + μp(p-p0)]
γ
σ ph ∝ GL
Light Intensity dependence:
where, GL = φ (1-R)[1-exp(-αd)]/d
Significance of γ
γ is a measure of characteristic width of tail states nearer to Ef
According to the Rose model:
the exponentially distributed tail state shows: γ = kTC/(kT+kTC)
In amorphous semiconductor 0.5<γ <1.0
γ=0.5 bimolecular recombination kinetics
γ=1 monomolecular recombination

16. Experimental Results
[σph(φ , T)] of sample #B22 of Type-A
-5
10
-5
-5 10
10
σph (Ω cm )
-1
−1 -6
10
-6
10
σph (Ω cm )
σph (Ω cm )
-1
-6
-1
10
3 4 5 6 7 -7
10
-1
1000 / T (K )
−1
−1
-7
10
-8
σd 10
-8
10 310 K
275 K
-9
10
17 250 K
1.2 x 10 (100%)
225 K
16
8.4 x 10 (75.4%)
-9
10 175 K
16
7.6 x 10 (65%) 125 K
16
-10
5.5 x 10 (49%) 80 K
10
16 50 K
2.0 x 10 (15%)
30 K
-10 15
1.6 x 10 (1.25%)
10 14 15 16 17
10 10 10 10
0 10 20 30 40 50
2
-1
Intensity F (photons/cm . sec)
1000 / T (K )
σph(T) vs φ
σph(φ ) vs T
Note: σPh (T) shows thermal quenching
(TQ) with an onset at ~ 225K

17. Experimental Results
[σph(φ , T)] of sample #B23 of Type-B
-4
10 2
-5
Φ ( photons/cm -sec )
10
14
1x10
16
1x10
-6
10
σph (Ω cm )
16
5x10
-1
-7
σph (Ω cm )
10
17
10
-1
−1
-8
−1
10 324 K
300 K
-9
σd
10
275 K
250 K
225 K
200 K
175 K
-10 153 K
10 128 K
-11 101 K
10 72 K
60 K
50 K
25 K
-12
10 12 13 14 15 16 17
10 10 10 10 10 10
4 8 12 16 20 2
Φ (Photons/cm -sec)
-1
1000 / T (K )
σph(T) vs φ
σph(φ ) vs T
Note: σPh (T) shows NO TQ

18. Experimental Results
[σph(φ , T)] of sample #F06 of Type-C
2
Φ ( photons/cm -sec )10-3
-4
10
17
15K
1x10
20K
16
-4 8x10
10
σph (Ω cm )
-4
-1
10 30K
16
2x10
40K
−1
15
7x10
σph (Ω cm )
50K
-1
-5
-6
10
σph (Ω cm )
15
10
2x10 60K
-1
-6
10 14
70K
6x10
80K
14
1x10 -6
−1
σd 10 90K
3 4 5 6
−1
100K
-1
1000 / T (K )
-8 -8
10 10 150K
200K
250K
300K
-10
10 -10
10
-12
10
0 10 20 30 40 50 14 15 16 17
10 10 10 10
-1 2
1000 / T (K ) Φ (photons/cm . sec)
σph(T) vs φ
σph(φ ) vs T
Note: σPh (T) shows TQ with an onset at 225 K

19. Comparison of phototransport properties of all
the three types of samples
Findings:
TQ and 0.5<γ <1 : as
1.0
found in Type-A:
0.8
NO TQ and 0.5<γ <1 : as
0.6
found in Type-B:
γ
0.4
γ
TQ and value
B22
F06
0.2 B23
approaches to a lowest
value of 0.14 at 225 K: as
0 10 20 30 40 50 60 70
-1
found in Type-C:
1000/T (K )
temperature dependencies of
light intensity exponent (γ)

20. DISCUSSION
Qualitative analysis
Causes of TQ :
The transformation of the recombination traffic
from VBT states to DB
The asymmetry in band tails in the gap.
Low value of defect densities or increasing n-type
doping level may shift the onset of TQ to higher T.
Causes of sublinear behavior of γ (<0.5)
The saturation of recombination centers
The shift of EF towards band edges in doped
material.

21. 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
Symmetric band tails
Usually observed in typical µc-Si:H
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 to be responsible for such behavior

22. Qualitative analysis
Phototransport properties of Type-C (TQ and γ<0.5)
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

23. Qualitative analysis
Possible explanation for sublinear behavior of γ (<0.5) in Type-C
In Type-C material, EF is found to be very close to Ec (EC-EF ~
0.34 eV)
δn ≈ n0 then Rose model
In doped a-Si:H when kTc << kTv
doesn’t hold (by C. Main ….)
γ=T/Tv for low excitation
γ= Tc/Tv at high excitation
According to Polycrystalline Si model two different VBT is
also possible;
A sharper shallow tail near the edge-> originating from grain boundary
defects
A less steeper deeper tail associated with the defects in columnar
boundary regions.
Capture cross section for the deeper tail is smaller than the shallower
one.

24. Numerical Simulation
Motivation
Experimental results cannot discern the states where the recombination
actually occurs
S-R-H mechanism and Simmons-Tylor Statistics are extensively used to
understand recombination mechanism in steady state process
EC
R9 R10
CBT R15 R4
R3
R1
R2
GL
R16
U R13 R14
R6
R7 R8 R5
VBT R11 R12
EV
DB 0
VBT CBT
DB + DB -
Schematic illustration of DOS in amorphous semiconductor and
representation of electron (solid lines) and hole transitions (dotted lines)

25. 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
EC
QCT = ∫ N CT (E )FCT (E )dE = QCT (n, p )
EV
EC
QVT = ∫ NVT (E )[1 − FVT (E )]dE = QVT (n, p )
EV
( )
QDB (n, p ) − QDB (n0 , p0 ) = N DB FDB + 2 FDB − FDB − 2 FDB
− −
0 00
Recombination equation
S n n + S CT p '
CT
GL = U CT + U VT + U DB
FCT (E ) =
p
( ) ( )
S n n + n ' + S CT p + p '
CT
p
[( )]dE
)(
EC
U DB = ∫ N DB (E ) n FDB S n + FDB S n − FDBε n + FDBε n
+ + −−
0 0 0 0
EV
S n n ' + S VT p
VT
FVT (E ) =
p
( ) ( )
S n n + n ' + S VT p + p '
VT
p

26. ⎡ − ((Ec − E ) − Etc1 )⎤ ⎤
⎡
⎢ ⎥⎥
exp ⎢
⎡ ( Ec − E ) ⎤ ⎢ ⎣ ⎦⎥
kTc 2
N ct1 = N ct1 × exp ⎢− ×
0
⎥
CBT
⎡ − ((Ec − E ) − Etc1 ) ⎤ ⎥
kTc1 ⎦ ⎢
⎣
⎢1 + exp ⎢ ⎥⎥
⎢ ⎦⎥
⎣ kTc 2
⎣ ⎦
⎡ (− (E − E v ) + Etv1 ) ⎤ ⎤
⎡
⎢ ⎥⎥
exp ⎢
⎡ (E − E v ) ⎤ ⎢ ⎣ ⎦⎥
kTv 2
×
= N vt1 × exp ⎢−
0
VBT
⎥
N vt1
⎡ (− (E − E v ) + Etv1 ) ⎤ ⎥
kTv1 ⎦ ⎢
⎣ ⎢1 + exp ⎢ ⎥⎥
⎢ ⎦⎥
⎣ kTv 2
⎣ ⎦
⎡ (E − EDB )2 ⎤
ND
N DB ( E ) =
DB exp ⎢ ⎥
(2π ) W ⎣ 2W 2 ⎦
1/ 2

27. 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-Tylor 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

28. Simulation results for Type-C material (ex. #F06)
21
10
μn = 10 cm2V-1s-1
-4 19 -3 -1
CBT G=10 cm sec
10
VBT1 20 -3 -1
G=10 cm sec
19
10 21 -3 -1
G=10 cm sec
μp = 0.5 cm2V-1s-1
DOS (cm eV )
σph (Ω cm )
-1
-1
-3
-5
-1
10
17
10
EC- EF=0.34 eV
15
10 VBT2 -6
10
DB
13
10
0.0 0.3 0.6 0.9 1.2 1.5 1.8 4 6 8 10
EV -1
EC
(E-EV) eV 1000/T (K )
0.6 Recombination rates (cm sec )
-1
19
10
γ
-3
Uct1
17
10
0.5 Uvt1
Uvt2
UDB
15
10
γ
0.4 13
10
11
10
0.3 100 150 200 250 300
4 6 8 10
T (K)
-1
1000/T (K )

29. Simulation results for Type-B material (#B23)
21
10
μn = 10 cm2V-1s-1
-5
CBT1 10
VBT1
19
DOS (cm eV )
10
-1
μp = 0.5 cm2V-1s-1
σph (Ω cm )
-1
EC- EF=0.42 eV -6
-3
10
17
10
-1
-7
10
CBT2
15
10 18 -3 -1
G=10 cm sec
DB 19 -3 -1
G=10 cm sec
VBT2 20 -3 -1
G=10 cm sec
21 -3 -1
G=10 cm sec
-8
13
10
10
4 6 8 10
0.0 0.3 0.6 0.9 1.2 1.5 1.8
-1
(E-EV) eV EC
EV 1000/T (K )
0.9 Recombination rates (cm sec )
-1
19
10
γ
Uct1
-3
Uct2
0.8 17
10 Uvt1
Uvt2
UDB
15
0.7 10
γ
13
10
0.6
11
10
0.5 100 150 200 250 300
4 8 12 16 20
T (K)
-1
1000/T (K )

30. Summary
The qualitative as well as quantitative analysis of the
study of our phototransport properties of undoped µc-
Si:H thin films are in good agreement
Micro-structural differences leads to totally different
phototransport behavior.
The recombination rate of deeper valence band tail is
higher in percolated grains than in unpercolated grains
We propose different effective DOS distribution for
micro-structurally different μc-Si:H thin films