Evidence Of Bimodal Crystallite Size Distribution In Microcrystalline Silicon Films
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Transcript of Evidence Of Bimodal Crystallite Size Distribution In Microcrystalline Silicon Films
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Evidence of Bimodal Crystallite Size Distribution in µc-Si:H Films
Sanjay K. Ram1,2, Md. Nazrul Islam3, Satyendra Kumar2
and P. Roca i Cabarrocas1
1LPICM (UMR 7647 du CNRS ), Ecole Polytechnique, France2Dept. of Physics, I.I.T. Kanpur, India
3QAED-SRG, Space Application Centre (ISRO), Ahmedabad – 380015, India
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Outline
• Introduction: motivation
• Experimental Details
• Microstructural Characterization
– Spectroscopic ellipsometry
– Atomic force microscopy
– X-ray diffraction
– Bifacial Raman spectroscopy
• Conclusions
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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.
Film growth
voids
substrate
grains grain boundaries
columnar boundaries
conglomerate crystallites
surfaceroughness
Complex microstructure of μc-Si:H
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Motivation
• Need for proper microstructural characterization• Different microstructural tools: different length
scales• Influence on carrier transport
– Film morphology– compositional variation in constituent crystallites – crystallite size distribution (CSD)
• Elucidation of CSD in single phase µc-Si:H as studied by different microstructural tools
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Sample preparation
Parallel-plate glow discharge plasma deposition system
R=1/1 R=1/5 R=1/10
Substrate: Corning 1773
High purity feed gases:SiF4 , Ar & H2
Rf frequency 13.56 MHz
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
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Film characterization
Structural Properties Electrical Properties
Xray Diffraction
Raman Scattering
Spectroscopy Ellipsometry
Atomic Force Microscopy
σd(T) measurement15K≤T ≤ 450K
σPh(T,∅) measurement15K≤T ≤ 325K
CPM measurement
Hall effect
TRMC
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2 3 4 5-5
0
5
10
15
20
25
30
d=390 nm
d=55 nm
d=170 nmd=590 nm
d=950 nm
E2 (4.2 eV)E1 (3.4 eV)
Energy (eV)
< ε 2 >
2 3 4 5-10
01020304050
Spectroscopic Ellipsometry : measured imaginary part of the pseudo-dielectric function <ε2> spectra
c-Sipc-Si-l
μ c-Si:H(d = 950 nm)
a-Sipc-Si-f
E2 (4.2 eV)E1 (3.4 eV)
Energy (eV)<
ε 2 >(a)
* Reference c-Si in BEMA model : LPCVD polysilicon with large (pc-Si-l) and fine (pc-Si-f) grains
thickness series of R=1/10
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Analyses of SE data: schematic view for two films
(initial and final growth stages)
TSL (7.9 nm)Fcf = 32.3 %, Fcl = 0.6 %,
Fv = 67.1%, Fa =0 %
BL (48.2 nm)Fcf = 88.4 %, Fcl = 0 %, Fv = 10.1 %, Fa = 1.5 %
d =
950
nm
TSL (8.3 nm)Fcf = 73.6 %, Fcl = 0 %,
Fv = 26.4 %, Fa =0 %
MBL (918.9 nm)Fcf = 50.4%, Fcl = 40.8%,
Fv=8.8 %, Fa=0%
BIL (27.7 nm)Fcf = 0 %, Fcl = 0 %,
Fv = 35.6 %, Fa =64.4 %
d =
55 n
m
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20 30 40 50 60 70
Cu Kα 2θ (degrees)
(400)
(311)(220)
(111)
Inte
nsity
(arb
.uni
t)
68.0 68.5 69.0 69.5 70.0
Exp. XRD peak (400) Total Fit Peak 1 (22.4 nm) Peak 2 (9 nm)
2θ (degree)
Inte
nsity
(arb
. uni
t)
26 27 28 29 30 31 32 33
Exp. XRD peak (111) Total Fit Peak 1 (14.8 nm) Peak 2 (4.8 nm)
2θ (degree)
Inte
nsity
(arb
. uni
t)
45 46 47 48 49 502θ (degree)
Inte
nsity
(arb
. uni
t)
Exp. XRD peak (220) Total Fit (11.4 nm)
55 56 57 582θ (degree)
Inte
nsity
(arb
. uni
t)
Exp. XRD peak (311) Total Fit Peak 1 (48 nm) Peak 2 (11.4 nm)
thickness ~ 1 µm
X-ray diffraction analysis
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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)
0 100 200 300 400
Freq
uenc
y (a
rb. u
nit)
Conglomerate surface grain size (nm)
d = 55 nm
d = 180 nm
d = 390 nm
d = 590 nm
d = 950 nm
σrms= 2.1 nm + 0.2 nm
σrms= 7 nm + 0.1 nm
σrms= 4.3 nm + 0.4 nm
σrms= 3.3 nm + 0.1 nm
σrms= 4 nm + 0.3 nm
thickness series of R=1/10
Surface morphology by AFM
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0 100 200 300 4000.0
0.1
0.2
(d)
Freq
uenc
y (a
rb. u
nit)
Surface grain size (nm)
46 47 48 49 50
Inte
nsity
(arb
. uni
t)
2θ (degree)
Exp. XRD peak (220) Total Fit Peak 1 Peak 2
Surface Morphologyby AFM
Presence of Size Distribution
X-ray diffraction
20 30 40 50 60 70
(400)(311)
(220)
(111)
Cu Kα 2θ (degrees)
Inte
nsity
(arb
. uni
t)
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Bifacial Raman Study
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
collection
Small 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
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Deconvolution of Raman Spectroscopy Data
• Conventionally: RS profiles are deconvoluted assuming:– a single mean crystallite size – a peak assigned to grain boundary material– an amorphous phase is included to account for the
asymmetric tail• Samples in our study:
– No a-Si:H phase– Presence of two (mean) sizes of crystallites
• Previous efforts to include CSD in fitting of Raman Data– To achieve a more accurate mathematical fitting of the
asymmetry observed in the RS profile as a result of CSD
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Incorporation of CSD in Raman AnalysisAccording to our model, Φ(L) representing the CSD of an
ensemble of spherical crystallites, total Raman intensity profile for the whole ensemble of nanocrystallites becomes:
(1)
For a normal CSD, Φ(L) is given as:
(2)
where the mean crystallite size L0 and the standard deviation σ are the characteristics of the CSD.
( ) ( ) ( )dLLILLI ,,, '0 ωσω ∫ Φ=
( ) ( )⎥⎥⎦
⎤
⎢⎢⎣
⎡ −−=Φ 2
20
2 2exp
2
1σπσ
LLL
•Islam & Kumar, Appl. Phys. Lett. 78 (2001) 715.
•Ram et al Thin Solid Films 515 (2007) 7619
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By putting Eq.(1) into Eq.(2) and then integrating the results over the crystallite sizes L, and by restricting the dispersion parameter σto be less than L0/3 one gets the modified Raman intensity profile as:
(3)
where the parameter ,
which incorporates the distribution broadening parameter σ into the Raman intensity profile.
( )( ) ( )
( ){ } ( )20
2
220
22
0 2
2exp
,,Γ+−
⎭⎬⎫
⎩⎨⎧
−
∝q
qfLqqqf
LIωω
ασω
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛+=
ασ 22
11 qqf
•Islam & Kumar, Appl. Phys. Lett. 78 (2001) 715.
•Ram et al Thin Solid Films 515 (2007) 7619
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• In the absence of an explicit amorphous hump, the asymmetry in the Raman lineshape of RS profiles, seen as a low energy tail, is attributed to the distribution of smaller sized crystallites
• Incorporation of a bimodal CSD in the deconvolution of RS profiles:– avoids the overestimation of amorphous content while
fitting the low frequency tail
– Avoids the inaccuracies in the estimation of the total crystalline volume fraction in the fully crystalline µc-Si:H material.
• RS(F) data bimodal CSD • RS(G) data bimodal CSD + an amorphous phase
RS Data Deconvolution : Our Modelinclusion of crystallite size distribution
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400 450 500 550
fit model "cd+a"
fit model "cd+a"
a
a
a
fit model "cd1+cd2+a"
cd
cd
cd2
cd1
fit model "cd1+cd2"
cd2
cd1
d = 55 nm, RS(G)
d = 55 nm, RS(F)
d = 950 nm, RS(G)
d = 950 nm, RS(F)
Inte
nsity
(arb
. uni
t)
Raman shift (cm-1)
RS analysis
* deconvolution of RS profiles using a bimodal size distribution of large crystallite grains (LG ~70–80nm) and small crystallite grains (SG ~6–8nm)
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200 400 600 800 1000 12000
20
40
60
80
100 (a)
Film Thickness (nm)
F cf ,
F cl ,
F v (%)
by S
E
Fcf Fcl Fv
200 400 600 800 10000
20
40
60
80
100(b)
Xa, X
c1, X
c2 (%
) by
RS
Film Thickness (nm)
Xc1 (%) Xc2 (%) Xa (%)
200 400 600 800 1000 12000
20
40
60
80
100 (a)
Film Thickness (nm)
F cf ,
F cl ,
F v (%)
by S
E
Fcf Fcl Fv
200 400 600 800 10000
20
40
60
80
100(b)
Xa, X
c1, X
c2 (%
) by
RS
Film Thickness (nm)
Xc1 (%) Xc2 (%) Xa (%)
Fractional composition of films: Qualitative agreement between RS and SE studies
Samples belong to thickness series of R=1/10
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1
2
3
4
5
6
Top surface layer (c)
Rou
ghne
ss b
y SE
, σSE
(nm
)
0
20
40
60
80
100 (b)Bulk Layer
Frac
tiona
l com
posi
tions
by
SE (%
) Fcf
Fcl
Fv
Fa
200 400 600 800 1000 12000
20
40
60
80
100
Film thickness (nm)
(a)Interface Layer
Fa
Fv
0
20
40
60
80
100 (a)RS(F)
Xc1
Xc2
Xa
0 200 400 600 800 1000 12000
20
40
60
80
100 (b) Xc1
Xc2
Xa
RS(G)
Film Thickness (nm)
Frac
tiona
l com
posi
tions
by
RS
(%)
thickness series of R=1/1
Summary of variation in fractional compositions and roughness with film growth
Samples belong to thickness series of R=1/1
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• Microstructural characterization studies on plasma deposited highly crystalline µc-Si:H films to explore the distribution in the crystallite sizes
• SE two types of crystallites having two distinct sizes • XRD two mean sizes of crystallites• Surface morphological images size distribution• Deconvolution of experimentally observed RS profiles
using a bimodal size distribution of crystallites • In Raman spectra of single-phase µc-Si:H material:
appearance of a strong and longer low-frequency tail, without any distinguishable amorphous hump, can be due to the presence of size distribution in nanocrystallites, instead of a contribution from disordered or amorphous phase.
Conclusions
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