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

Outline

• Introduction: motivation

• Experimental Details

• Microstructural Characterization

– Spectroscopic ellipsometry

– Atomic force microscopy

– X-ray diffraction

– Bifacial Raman spectroscopy

• Conclusions

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

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

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

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

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

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

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

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

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)

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

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

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

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

• 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

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)

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

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

• 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

Thanks