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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.
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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!