Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped...

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Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon Sanjay K. Ram Sanjay K. Ram Dept. of Physics, I.I.T. Kanpur, India & LPICM (UMR 7647 du CNRS ), Ecole Polytechnique, France

Transcript of Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped...

Page 1: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline

Undoped Microcrystalline Silicon

Sanjay K. RamSanjay K. Ram

Dept. of Physics, I.I.T. Kanpur, India&

LPICM (UMR 7647 du CNRS ), Ecole Polytechnique, France

Page 2: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Motivation: study of μc-Si:H thin films

• Promising material for large area electronics

– Good carrier mobility

– Greater stability

– Low temperature deposition

• Electrical transport properties : important for

device applications

Page 3: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

How is film microstructure related to transport properties in µc-Si:H???

Optimization of μc-Si:H device applications: Issues

Complex microstructure of μc-Si:H

Film growth

voids

substrate

grains grain boundaries

columnar boundaries

conglomerate crystallitessurface

roughness

Page 4: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

• To study the optoelectronic properties of well characterized μc-Si:H films

• Identify the role of microstructure in determining the electrical transport behavior

Objectives

Page 5: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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

Page 6: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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

Page 7: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Microstructural Properties

Page 8: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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•Ram et al, Thin Solid Films 515 (2007) 7619.

•Ram et al, Thin Solid Films (2008) in print.

Page 9: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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

Fcf = small grains

Fcl = large grains

Fv = voids

Fa = amorphous phase

Page 10: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

20 30 40 50 60 70

Cu Kα 2θ (degrees)

(400)

(311)(220)

(111)

Inte

nsity

(arb

.uni

t)

X-ray diffraction

thickness ~ 1 µm

Page 11: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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)

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)

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)

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

Page 12: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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•Ram et al, Thin Solid Films 515 (2007) 7619.

•Ram et al, Thin Solid Films (2008) in print.

Surface morphology by AFM

Page 13: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

R=1/1, thickness = 1200 nm

400 425 450 475 500 525 550

(a) Expt. data (glass side)

Inte

nsity

(arb

. uni

t)

Raman Shift (cm-1)

(b) Expt. data (film side)

Bifacial Raman Spectroscopy

collection

excitation

film

glass

glassfilm

excitation

collection

Page 14: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Deconvolution of Raman Spectroscopy Data

• Microstructure of our samples: – No a-Si:H phase– Presence of two (mean) sizes of crystallites

• Conventional deconvolution:– Single mean crystallite size – A peak assigned to grain boundary material– An amorphous phase : asymmetric tail

• Previous efforts – Deconvolution based on two asymmetric Lorentzian peaks

[Touir et al, J. Non-Cryst. Solids 227-230 (1998) 906]

– Method of subtracting the amorphous contribution and fitting the resulting crystalline part of spectrum with three or five Gaussian peaks [Smit et al, J. Appl. Phys. 94 (2003) 3582]

Page 15: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

In absence of amorphous phase

• Asymmetry in the Raman lineshape of RS profiles (low energy tail) distribution of smaller sized crystallites

• Incorporation of a bimodal CSD in the deconvolution of RS profiles avoids:– Overestimation of amorphous content

– Inaccuracy in the estimation of the total crystalline volume fraction •Islam & Kumar, Appl. Phys. Lett. 78 (2001) 715.

•Islam et al, J. Appl. Phys. 98 (2005) 024309.

•Ram et al, Thin Solid Films 515 (2007) 7619.

RS Data Deconvolution : Our Model

inclusion of crystallite size distribution (CSD)

Page 16: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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)

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

RS(F) data bimodal CSD RS(G) data bimodal CSD +amorphous phase

Page 17: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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

3 models:

cd1 + cd2

cd1 +cd2 +a

cd +a

Page 18: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

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

Page 19: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Summary of variation in fractional compositions and roughness with film growth

300 600 900 12000

20

40

60

80

100(a)

R = 1/1

FcFcf

Fcl

F c , F cf

and

Fcl (%

) by

SE

σSE

d (nm)300 600 900 1200

(b)

R = 1/5

FcFcf

Fcl

σSE

d (nm)300 600 900 1200

2

4

6

8

(c)

Fcl

FcfFc

R = 1/10

d (nm)

σSE

Rou

ghne

ss b

y SE

, σSE

(nm

)

d = 390 nm

σrms= 3.3 nmσrms= 7 nm

d = 170 nm

σrms= 2.1 nm

d = 55 nm d = 590 nm

σrms= 4.3 nm

d = 950 nm

σrms= 5 nm

Surface morphologies of the samples belonging to thickness series of R=1/10

Page 20: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Dark Electrical Transport Properties

–Above Room Temperature

Page 21: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

• Microstructure:

– >90% crystallinity, no amorphous phase• Electrical transport:

– Crystallinity ?– Crystallite size ? – Interfacial regions between crystallites or columns ?

• Carrier transport is influenced by:– Film morphology – Compositional variation in constituent crystallites

• large grain fraction?• Microstructure ↔ Electrical transport:

– Need for investigation of correlation with large grain fraction

Electrical transport in single phase µc-Si:H

Page 22: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

2.1 2.4 2.7 3.0 3.310-10

10-8

10-6

10-4

10-2

(a)

1200; 0.2 920; 0.15 450; 0.55 180; 0.58 62; 0.58 Fit

d (nm); Ea (eV)

R = 1/1

σ d (Ω

.cm

)-1

1000/T (K -1)2.1 2.4 2.7 3.0 3.3

10-7

10-6

10-5

10-4

10-3

σ d (Ω

.cm

)-1

(b)

d (nm); Ea (eV)

R = 1/10

1000/T (K -1)

950; 0.33 590; 0.44 390; 0.44 170; 0.54 150; 0.54 55; 0.54 Fit

Above room temperature dark electrical conductivity (σd) shows

Arrhenius type thermally activated behavior: σd(T)=σo e –Ea / kT

•Ram et al, Thin Solid Films 515 (2007) 7469.

Page 23: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

200 400 600 800 1000 120010-10

10-8

10-6

10-4

10-2

(a)

P =1.5 TorrTS =250 0C

TS =150 0C

TS =100 0C

σ d (Ω c

m)-1

Film thickness (nm)

R, TS

1/10, 200 0C 1/5, 200 0C 1/5, variable T

S

1/1, 200 0C 1/20, 200 0C

200 400 600 800 1000 12000.1

0.2

0.3

0.4

0.5

0.6

0.7 (b)

TS =250 0C

P=1.5 Torr

TS =150 0C

TS =100 0C

Film thickness (nm)

E a (eV

)

200 400 600 800 1000 1200 10-3

10-1

101

103

105

(c)

P=1.5 Torr

TS =250 0C

TS =150 0C

TS =100 0C

Film thickness (nm)

σ 0 (Ω

cm

)-1

Page 24: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

0 20 40 60 800

200

400

600

800

1000

1200

Fi

lm T

hick

ness

(nm

)

Fcl (%)0

200

400

600

800

1000

1200

10-7 10-6 10-5 10-4 10-3 10-2

Film

Thi

ckne

ss (n

m)

σd (Ω cm)-1

0.1 0.2 0.3 0.4 0.5 0.6Ea (eV)

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

Samples belong to thickness series of R=1/10

Page 25: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

0 20 40 60 80 100

10-2

10-1

1

101

102

103

104

type-Ctype-Btype-A

σ0Ea

0.1

0.2

0.3

0.4

0.5

Fcl (%)

E a (eV

)

σ 0 (Ω c

m)-1

Classification of films: electrical transport behavior and Fcl

•Ram et al, J. Non-Cryst. Solids 354 (2008) 2263.

Page 26: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Classification of films

Type-A material• Small grains (SG)

• Low amount of conglomeration (without column formation)

• High density of intergrain boundary regions containing disordered phase.

Type-C material• Highest fraction of LG.

• Well formed large columns

• Least amount of disordered phase in the columnar boundaries.

Type-B material• Rising fraction of LG.

• Marked morphological variation: column formation

• Moderate amount of disordered phase in the columnar boundaries.

0 20 40 60 80 100

10-2

10-1

1

101

102

103

104

type-Ctype-Btype-A

σ0Ea

0.1

0.2

0.3

0.4

0.5

Fcl (%)

Ea (e

V)

σ 0 (Ω c

m)-1

Page 27: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Meyer Neldel Rule (MNR) & anti-MNR behaviors in dark electrical transport

Page 28: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Meyer Neldel Rule (MNR)Observed in:

Materials:

Ionic Materials Chalcogenide glassesOrganic thin filmsAmorphous Silicondoped μc-Si:H

Processes:

Annealing Phenomena Trapping in crystalline SemiconductorsAging of insulating polymersBiological death ratesChemical reactionsElectrical conductionmicroscopic origin of MNR

& physical meaning of G ??

electrical transport in a-Si:H/disordered semiconductor: MNR σ0=σ00 eGEa ,

where G or EMN (=1/G)

and σ00 are MNR parameters

σd=σ0.exp(-Ea/kT)

Statistical shift of Fermi level

Activated process: Y=A.exp (-B/X)

MNR A=A’.exp(GB)where G and A’ are MNR parameters

Page 29: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Statistical Shift Model

According to Mott: σd(T) =σM exp{-(EC - EF)/kT}

EC(T ) = EC0 - γCT ; EF(T ) = EF

0 - γFT

Ea= EC0 - EF

0, at T=0 K

σd=σo exp (–Ea / kT )

σo=σM exp {(γC - γF) / k}

σ0=σ00 exp (GEa) --- MNR

Page 30: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Anti Meyer Neldel Rule

Correlation between σ0 and Ea appears to change sign– a negative value of MN energy (EMN) is seen

Experimentally observed in:– Heavily doped μc-Si:H

– Heterogeneous Si (het-Si) thin film transistor

– Organic semiconductors

Page 31: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Lucovsky & Overhof (LO) model

in a degenerate case Efmoves above Ec in the crystalline phase

consequently Ef can move deeply into the tail states in the disordered region, giving rise to anti MNR behavior.

Energy band diagram as proposed by Lucovsky et al, J.N.C.S. 164-166, 973 (1993)

The reason for observed anti MNR in doped µc-Si:H

Page 32: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

0.0 0.2 0.4 0.6 0.8

10-2

100

102

104

anti MNR parametersG = -44.6 eV-1

or EMN=-22.5 meVσ00= 87 (Ωcm)-1

MNR parametersG=25.3 eV-1 (EMN=39.5 meV)σ00=7.2x10-4 (Ωcm)-1

γf ~ 0

γf ~ γc

σ 0 (Ω c

m)-1

Ea (eV)

type-A type-B type-C

σ0 vs. Eaσo and Ea follow linear

relationship Type-A and Type-B samples.

Type-A samples are having high values of Eaand σ0

This shows γF is extremely small in Type-A samples due to its pinning

The values of MNR parameters nearly the same as found in a-Si:H.

Correlation between σoand Ea appears to change sign for type-C samples:anti-MNR

Findings

MNR & anti MNR in single phase μc-Si:H•Ram et al, Phys. Rev. B 77 (2008) 045212.

•Ram et al, J. Non-Cryst. Solids 354 (2008) 2263.

Page 33: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

MNR: type-A μc-Si:H

• Consists mainly of SG with an increased number of SG boundaries. – No question of formation of

potential barrier (i.e., transport through crystallites)

– transport will be governed by the band tail transport.

0.0 0.2 0.4 0.6 0.8

10-2

100

102

104

anti MNR parametersG = -44.6 eV-1

or EMN=-22.5 meVσ

00= 87 (Ωcm)-1

MNR parametersG=25.3 eV-1 (EMN=39.5 meV)σ00=7.2x10-4 (Ωcm)-1

γf ~ 0

γf ~ γc

σ 0 (Ω c

m)-1

Ea (eV)

type-A type-B type-C

• Ea saturates (≈ 0.55 eV) and σo ≈ 103 (Ωcm)-1.– EF is lying in the gap where the DOS does not vary much and there is a

minimal movement of EF, or γF ≈ 0 • The initial data points for type-A have higher σo [≈ 104 (Ωcm)-1] and Ea (≈

0.66 eV)– because of a shift in EC and/or a negative value of γF, as happens in

a-Si:H for Ea towards the higher side.

Page 34: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

MNR: type-B μc-Si:H

Improvement in film microstructuredelocalization of the tail states– EF moves towards the band edges,

closer to the current path at EC. – γF depends on T and initial position

of EF, and when EF is closer to any of the tail states and the tail states are steep, γF is rapid and marked.

Transition between Type-A and Type-B materials– Nearly constant σo [70-90 (Ωcm)-1] with the fall in Ea (0.54-0.40 eV),– γF ≈ γC, canceling each other out in σo=σM exp [(γC - γF) / k]– EF pinned near the minimum of the DOS between the exponential CBT

and the tail of the defect states (DB–)– With increasing crystallinity and/or improvement in microstructure,

minimum shifts towards EC leading to a decrease of Ea.

0.0 0.2 0.4 0.6 0.8

10-2

100

102

104

anti MNR parametersG = -44.6 eV-1

or EMN=-22.5 meVσ

00= 87 (Ωcm)-1

MNR parametersG=25.3 eV-1 (EMN=39.5 meV)σ

00=7.2x10-4 (Ωcm)-1

γf ~ 0

γf ~ γc

σ 0 (Ω c

m)-1

Ea (eV)

type-A type-B type-C

Page 35: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Is the model by Lucovsky et al applicable for

explaining Anti MNR in type-C μc-Si:H ?

• The value of EMN = -22.5 meV is close to the value reported in heavily doped µc-Si:H (-20meV)

• EB diagram as suggested by LO model seems inapplicable to

our undoped µc-Si:H case

– Calculated free electron concentrations do not suggest

degenerate condition.

– Consideration of equal band edge discontinuities at both ends of

c-Si and a-Si:H interface Doubtful

– Also, in a degenerate case, the conductivity behavior of

polycrystalline material is found to exhibit a T 2 dependence of σd

Page 36: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

• Considering transport through the encapsulating disordered tissue, a band tail transport is mandatory.

• The large columnar microstructure in a long range ordering delocalizes an appreciable range of states in the tail state distribution.

• In addition, higher density of available free carriers and low value of defect density can cause a large increase in DB–

density together with a decrease in DB+ states in the gap a lower DOS near the CB edge possibility of a steeper CB tail.

• In this situation, if Ef is lying in the plateau region of the DOS, it may create an anti MNR situation.

Applying the statistical shift modelin explaining Anti MNR in type-C μc-Si:H

Page 37: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Evidence of Anti MNR in μc-Si:H in

Literature

Page 38: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

# 1 undoped µc-Si:H# 2 lightly p-doped µc-Si:H

0.0 0.2 0.4 0.6 0.810-3

10-1

101

103

105

Ea(eV)

anti-MNR line of type-C μc-Si:H

MNR line of types: A & B μc-Si:HMNR line of a-Si:H

#1 (rH=21) #1 (rH=32) #2 #3 (a-Si:H) this work

σ 0 (Ω

.cm

)-1

•Ram et al, Phys. Rev. B 77 (2008) 045212.

Undoped µc-Si:H

Page 39: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

0.0 0.2 0.4 0.6 0.810-3

10-1

101

103

MNR line (#7) [a-Si,C:H+μc-Si,C alloy]

anti MNR line (#7) [heavily doped μc-Si:H]

#4 (thickness series) #4 (doped series) #5 dope series, p-nc-Si-SiC:H alloy#5 dilution series, p-nc-Si-SiC:H alloy #6 (Boron doped μc-Si:H) #7

σ 0 (Ω.c

m)-1

Ea (eV) •Ram et al, Phys. Rev. B 77 (2008) 045212.

Doped µc-Si:H

Page 40: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

MNR parameters Anti MNR parameters

Samples σ00

(Ω.cm)-1 G

(eV-1) EMN

(meV)σ00

(Ω.cm)-1 G

(eV-1) EMN

(meV) This workType-A&B

Type-C Published

Data Case#1 (rH=21) Case#1 (rH=32) Case#2 Case#3 Case#4 Case#5 Case#6 Case#7 Case#8 Case#9

7.2×10-4

--

4×10-3

3.2×10-6

1.7×10-4 7.7×10-3

0.32 4.2×10-3 3.2×10-6

2.3 0.5

7.2×10-3

25.3

--

20.7

36.6

23.4 24

15.4 15.3 31.3 8.5

11.8 20

39.5

--

48.4

27.3

42.7 41.6 65.1 65.4 31.9

118.384.5 50

-- 87

1.26×1010

--

6 -- 59 21 2.4 309 -- --

--

-44.6

-97.7

--

-32.5 --

-66.1 -64.9 -39.9 -49.5

-- --

--

-22.5

-10.2

--

-30.8 --

-15.1 -15.4 -25.1 -20.2

-- --

Page 41: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

5 10 15 20 25 30 35 4010-6

10-4

10-2

100a-Si,C:H alloy (#7)

a-Si:H (#3)

p-nc-Si-SiC:H alloy (#5)

Porous Si (#9)

#1 (rH=21) #1 (rH=32) #2 #3 #4 #5 #6 #7 #8 #9 this work FitσM=100 (Ωcm)-1 (at γf=γc)

Emin=0.61 eVσ0=1.2x103 (Ωcm)-1 (at γf=0)

σ 00 (Ω

.cm

)-1

G (eV-1)

If one has a collection of G and σ00 then:

σ00=σM exp [(γC- γF)/k –GEa]

σ00=σM exp [(γC- γF)/k –G(EC0 –EF

0)]

At a position of EF in DOS where

γF(EC0-Emin)=0

σ00=σM exp [(γC/k) –GEmin]

The quantity Emin is a measure for the

position of the DOS minimum within

the mobility gap.

If γC is known then for such a value of

σ00 where G=0, one can obtain σM•Ram et al, Phys. Rev. B 77 (2008) 045212.

Page 42: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Summary• In single phase µc-Si:H films, film morphology shows

correspondence with large grain fraction independent of film thickness and deposition conditions

• Percentage fraction of constituent large crystallite grains can be used as an empirical parameter to correlate a wide range of microstructures to the electrical transport properties

• Both MNR and anti MNR can be seen in the dark conductivity behavior of this material, depending on the microstructure and the correlative DOS features.

• The statistical shift model can successfully explain both the MNR and anti MNR behavior in our material.

• Corroborative evidence of similar electrical transport behavior of µc-Si:H in literature is present

Page 43: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Acknowledgements

• Dr. Satyendra Kumar (I.I.T. Kanpur, India)• Dr. Pere Roca i Cabarrocas (LPICM, France)

Page 44: Variation of Electrical Transport Parameters with Large Grain Fraction in Highly Crystalline Undoped Microcrystalline Silicon

Thank you