The electrical transport and its correlation with the microstructural properties in single phase microcrystalline silicon may be very different from the transport in microcrystalline silicon with a mixed phase of amorphous silicon. We have shown that the transport in single phase microcrystalline silicon may be predicted by the large grain fraction.

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

2. 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

3. Optimization of μc-Si:H device applications:
Issues
columnar boundaries
grains grain boundaries
conglomerate crystallites
surface
roughness
voids
Film
growth
substrate
Complex microstructure of μc-Si:H
How is film microstructure related to transport
properties in µc-Si:H???

4. Objectives
• To study the optoelectronic properties of
well characterized μc-Si:H films
• Identify the role of microstructure in
determining the electrical transport behavior

5. 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
Flow ratio
High purity feed gases:
(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

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

7. Microstructural Properties

8. Spectroscopic Ellipsometry : measured imaginary part
of the pseudo-dielectric function <ε2> spectra
30 * Reference c-Si in BEMA model :
E2 (4.2 eV)
E1 (3.4 eV) LPCVD polysilicon with large
25 d=390 nm
(pc-Si-l) and fine (pc-Si-f) grains
d=170 nm
d=590 nm
20 d=950 nm
d=55 nm E2 (4.2 eV)
50
E1 (3.4 eV)
< ε2 >
15 40 c-Si
pc-Si-l
30
< ε2 >
pc-Si-f
10 a-Si
20
μ c-Si:H
5 10 (d = 950 nm)
0
0 (a)
-10
2 3 4 5
-5 Energy (eV)
2 3 4 5
Energy (eV) •Ram et al, Thin Solid Films 515 (2007) 7619.
thickness series of R=1/10 •Ram et al, Thin Solid Films (2008) in print.

9. Analyses of SE data: schematic view for two films
(initial and final growth stages)
Fcf = small grains
TSL (8.3 nm) Fcl = large grains
Fcf = 73.6 %, Fcl = 0 %, Fv = voids
Fv = 26.4 %, Fa =0 %
Fa = amorphous phase
TSL (7.9 nm)
MBL (918.9 nm)
d = 950 nm
Fcf = 32.3 %, Fcl = 0.6 %,
Fcf = 50.4 %, Fcl = 40.8 %,
Fv = 67.1%, Fa =0 %
Fv=8.8 %, Fa=0%
d = 55 nm
BL (48.2 nm)
BIL (27.7 nm)
Fcf = 88.4 %, Fcl = 0 %,
Fcf = 0 %, Fcl = 0 %,
Fv = 10.1 %, Fa = 1.5 %
Fv = 35.6 %, Fa =64.4 %

10. X-ray diffraction
Intensity (arb.unit)
(400)
(111)
(220)
(311)
20 30 40 50 60 70
Cu Kα 2θ (degrees)
thickness ~ 1 µm

11. X-ray diffraction analysis
Exp. XRD peak (400)
Exp. XRD peak (111)
Total Fit
Total Fit
Peak 1 (22.4 nm)
Intensity (arb. unit) Peak 1 (14.8 nm)
Intensity (arb. unit)
Peak 2 (9 nm)
Peak 2 (4.8 nm)
Exp. XRD peak (400) Exp. XRD peak(220)
Exp. XRD peak (111)
Total Fit
Total Fit (11.4 nm)
Total Fit
Exp. XRD peak (311)
Intensity (arb. unit)
Peak 1 (22.4 nm)
Intensity (arb. unit)
Peak 1 (14.8 nm)
Intensity (arb. unit)
Total Fit
Intensity (arb. unit)
Peak 2 (9 nm)
Peak 2 (4.8 nm) 69.5 70.0
(48 nm)
Peak 1 68.5 69.0
26 27 28 29 30 31 32 33 68.0
2θ (degree)
Peak 2 (11.4(degree)
2θ
nm)
Intensity (arb.unit)
(400)
(111)
thickness ~ 1 µm
(220)
(311)
20 30 40 50 60 70
Cu Kα 2θ (degrees) Exp. XRD peak (311)
Exp. XRD peak (220)
Total Fit
Total Fit (11.4 nm)
Peak 1 (48 nm)
Intensity (arb. unit)
Intensity (arb. unit)
Peak 2 (11.4 nm)
68.0 2768.5 4729 4869.5 49 70.0 50
46 28 69.0 30 5731 32 58
26 33
45
55 56
2θ (degree)
2θ (degree)
2θ (degree)
45 46 47 48 49 50 55 56 57 58
2θ (degree)
2θ (degree)

12. Surface morphology by AFM
σrms= 4 nm + 0.3 nm
d = 950 nm
σrms= 3.3 nm + 0.1 nm
d = 590 nm
Frequency (arb. unit)
σrms= 4.3 nm + 0.4 nm
d = 390 nm
σrms= 7 nm + 0.1 nm
d = 180 nm
σrms= 2.1 nm + 0.2 nm
d = 55 nm
0 100 200 300 400
Conglomerate surface grain size (nm)
•Ram et al, Thin Solid Films 515 (2007) 7619.
thickness series of R=1/10 •Ram et al, Thin Solid Films (2008) in print.

13. Bifacial Raman Spectroscopy
Expt. data (glass side)
collection
(a) excitation
Intensity (arb. unit)
glass
film
Expt. data (film side)
(b) collection
excitation
film
glass
400 425 450 475 500 525 550
-1
Raman Shift (cm )
R=1/1, thickness = 1200 nm

14. 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]

15. RS Data Deconvolution : Our Model
inclusion of crystallite size distribution (CSD)
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.

16. Bifacial Raman Study
1.2 1.2 glass side exp. data of F0E31
film side exp. data of F0E31
cd1
cd1
cd2
cd2
Intensity (arb. unit)
a
Intensity (arb. unit)
fit with - cd1cd2 fit with - cd1cd2a
0.9 0.9
0.6 0.6
0.3 0.3
0.0 0.0
450 475 500 525 550 400 425 450 475 500 525 550
-1
Raman Shift (cm ) -1
Raman Shift (cm )
RS(F) data bimodal CSD RS(G) data bimodal CSD +
amorphous phase
Small grain (cd1) Large grain (cd2) a-Si:H
Sample #E31
Fitting Model
(1200 nm, R=1/1) Size (nm) [σ (nm)] Size (nm) [σ (nm)]
XC1 (%) 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

17. RS analysis
fit model quot;cd1+cd2quot;
cd1
cd2
3 models:
d = 950 nm, RS(F)
cd1 + cd2
Intensity (arb. unit)
cd1
fit model quot;cd1+cd2+aquot;
cd1 +cd2 +a
d = 950 nm, RS(G) cd2
a
cd +a
fit model quot;cd+aquot; cd
d = 55 nm, RS(F) a
fit model quot;cd+aquot;
cd
a
d = 55 nm, RS(G)
400 450 500 550
-1
Raman shift (cm )

18. Fractional composition of films: Qualitative
agreement between RS and SE studies
100 Xc1 (%) 100
Fcf (b)
(a)
Fcl
Fcf , Fcl , Fv (%) by SE
Xc2 (%)
Xa, Xc1, Xc2 (%) by RS
80 Xa (%) 80
Fv
60 60
40 40
20 20
0 0
200 400 600 800 1000 1200 200 400 600 800 1000
Film Thickness (nm)
Film Thickness (nm)
Samples belong to thickness series of R=1/10

19. Summary of variation in fractional compositions and roughness with film growth
(a) (c)
(b)
Fc , Fcf and Fcl (%) by SE
Roughness by SE, σSE(nm)
100
Fc Fc Fc 8
Fcf
Fcf
80 Fcf
6
60
σSE
σSE
40 4
Fcl
σSE
20 Fcl 2
Fcl
R = 1/1 R = 1/10
R = 1/5
0
300 600 900 1200
300 600 900 1200 300 600 900 1200
d (nm) d (nm) d (nm)
σrms= 2.1 nm σrms= 7 nm σrms= 3.3 nm σrms= 4.3 nm σrms= 5 nm
d = 55 nm d = 390 nm
d = 170 nm d = 590 nm d = 950 nm
Surface morphologies of the samples belonging to thickness series of R=1/10

20. Dark Electrical Transport
Properties
– Above Room Temperature

21. Electrical transport in single phase µc-Si:H
• 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

22. Above room temperature dark electrical conductivity (σd) shows
Arrhenius type thermally activated behavior: σd(T)=σo e –Ea / kT
-2
10 R = 1/1 R = 1/10 (b)
(a) -3
10
-4
10 -4
10
-1
σd (Ω.cm)
-1
σd (Ω.cm)
-6 -5
10 10
d (nm); Ea (eV)
950; 0.33
d (nm); Ea (eV)
590; 0.44 -6
1200; 0.2
10
390; 0.44
-8 920; 0.15
10 170; 0.54
450; 0.55
150; 0.54 -7
180; 0.58
10
55; 0.54
62; 0.58
Fit
-10 Fit
10
2.1 2.4 2.7 3.0 3.3 2.1 2.4 2.7 3.0 3.3
-1 -1
1000/T (K ) 1000/T (K )
•Ram et al, Thin Solid Films 515 (2007) 7469.

23. 0
TS =250 C
-2
10 P =1.5 Torr
-4
10
-1
σd (Ω cm)
0
TS =150 C (a)
-6
10 R, TS
0
1/10, 200 C
0
1/5, 200 C
-8
10 1/5, variable TS
0
TS =100 C 0
1/1, 200 C
0
1/20, 200 C
-10
10
200 400 600 800 1000 1200
Film thickness (nm)
5
10
0.7 (c)
(b)
0
0.6 TS =100 C 3
0
TS =100 C 10
-1
0.5
σ0 (Ω cm)
Ea (eV)
0
TS =150 C
0
TS =150 C 1
0.4 10
0
TS =250 C
0.3
-1
10
0.2
0
TS =250 C P=1.5 Torr
0.1 P=1.5 Torr -3
10
200 400 600 800 1000 1200
200 400 600 800 1000 1200
Film thickness (nm)
Film thickness (nm)

24. Ea (eV)
0.1 0.2 0.3 0.4 0.5 0.6
1200 1200
d = 950 nm
1000 1000
Film Thickness (nm)
Film Thickness (nm)
d = 590 nm
Frequency (arb. unit)
800 800
600 600
d = 390 nm
400 400
d = 180 nm
200 200
d = 55 nm
0 0
0 100 200 300 400 0
-7 -6 -5 -4 -3 -2 20 40 60 80
10 10 10 10 10 10 Fcl (%)
Conglomerate surface grain size (nm)
σd (Ω cm)
-1
Samples belong to thickness series of R=1/10

25. Classification of films: electrical transport behavior and Fcl
σ0
4
10
Ea 0.5
3
10
2
10 0.4
-1
σ0 (Ω cm)
Ea (eV)
1
10
0.3
1
0.2
-1
10
-2
10 0.1
type-B
type-A type-C
0 20 40 60 80 100
Fcl (%) •Ram et al, J. Non-Cryst. Solids 354
(2008) 2263.

26. σ0
4
10
Ea 0.5
3
10
2
10 0.4
-1
σ0 (Ω cm)
Classification
Ea (eV)
1
10
0.3
1
of films 0.2
-1
10
-2
10 0.1
Type-A material Type-B material type-B
type-A type-C
0 20 40 60 80 100
• Small grains (SG) • Rising fraction of LG. Fcl (%)
• Low amount of Type-C material
• Marked
conglomeration morphological • Highest fraction of LG.
(without column variation: column
• Well formed large
formation) formation
columns
• High density of • Moderate amount of
• Least amount of
intergrain boundary disordered phase in
disordered phase in the
regions containing the columnar
columnar boundaries.
disordered phase. boundaries.

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

28. Meyer Neldel Rule (MNR)
Observed in:
Materials: Processes:
Activated process:
Annealing Phenomena
Ionic Materials Y=A.exp (-B/X)
Trapping in crystalline
Chalcogenide glasses MNR A=A’.exp(GB)
Semiconductors
Organic thin films where G and A’ are
Aging of insulating polymers
Amorphous Silicon MNR parameters
Biological death rates
doped μc-Si:H
Chemical reactions
Electrical conduction
microscopic origin of MNR
& physical meaning of G ??
Statistical shift of Fermi level
electrical transport in a-Si:H/
σ0=σ00 eGEa ,
σd=σ0.exp(-Ea/kT) MNR
disordered semiconductor:
where G or EMN (=1/G)
and σ00 are MNR parameters

29. Statistical Shift Model
According to Mott: σd(T) =σM exp{-(EC - EF)/kT}
EC(T ) = EC0 - γCT ; EF(T ) = EF0 - γFT
Ea= EC0 - EF0, at T=0 K
σd=σo exp (–Ea / kT )
σo=σM exp {(γC - γF) / k}
σ0=σ00 exp (GEa) --- MNR

30. 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

31. The reason for observed anti MNR
in doped µc-Si:H
Lucovsky & Overhof (LO) model
in a degenerate case Ef
moves 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)

32. σ0 vs. Ea Findings
σo and Ea follow linear
relationship Type-A and
MNR parameters
type-A
4 Type-B samples.
type-B -1
G=25.3 eV (EMN=39.5 meV)
10 type-C
σ00=7.2x10 (Ωcm)
-4 -1
γf ~ 0 Type-A samples are
anti MNR parameters
having high values of Ea
-1
2 -1
10 G = -44.6 eV
γf ~ γc
σ0 (Ω cm)
and σ0
or EMN=-22.5 meV
σ00= 87 (Ωcm)
-1
γF
This shows is
0
10 extremely small in Type-A
samples due to its pinning
-2
10 The values of MNR
parameters nearly the same
0.8 as found in a-Si:H.
0.0 0.2 0.4 0.6
Ea (eV)
Correlation between σo
and Ea appears to change
MNR & anti MNR in single phase μc-Si:H sign for type-C samples:
anti-MNR
•Ram et al, Phys. Rev. B 77 (2008) 045212.
•Ram et al, J. Non-Cryst. Solids 354 (2008) 2263.

33. MNR: type-A μc-Si:H
• Consists mainly of SG with an MNR parameters
type-A
4 type-B -1
G=25.3 eV (EMN=39.5 meV)
10
increased number of SG type-C
σ00=7.2x10 (Ωcm)
-4 -1
γf ~ 0
boundaries. anti MNR parameters
-1
2 -1
10 G = -44.6 eV
γf ~ γc
σ0 (Ω cm)
– No question of formation of or EMN=-22.5 meV
σ00= 87 (Ωcm)
-1
potential barrier (i.e., transport 0
10
through crystallites)
– transport will be governed by -2
10
the band tail transport.
0.0 0.2 0.4 0.6 0.8
Ea (eV)
• 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.

34. MNR: type-B μc-Si:H
Improvement in film microstructure MNR parameters
type-A
4 type-B -1
G=25.3 eV (EMN=39.5 meV)
10
delocalization of the tail states
type-C
σ00=7.2x10 (Ωcm)
-4 -1
γf ~ 0
– EF moves towards the band edges, anti MNR parameters
-1
2 -1
G = -44.6 eV
10 γf ~ γc
σ0 (Ω cm)
or EMN=-22.5 meV
closer to the current path at EC. σ00= 87 (Ωcm)
-1
– γF depends on T and initial position 0
10
of EF, and when EF is closer to any of
-2
10
the tail states and the tail states are
steep, γF is rapid and marked. 0.0 0.2 0.4 0.6 0.8
Ea (eV)
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.

35. 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

36. Applying the statistical shift model
in explaining Anti MNR in type-C μc-Si:H
• 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.

37. Evidence of Anti MNR in μc-Si:H
in
Literature

38. Undoped µc-Si:H
5
10
#1 (rH=21) MNR line of types: A & B μc-Si:H
#1 (rH=32) MNR line of a-Si:H
#2
3
10 #3 (a-Si:H)
-1
this work
σ0 (Ω.cm)
1
10
# 1 undoped µc-Si:H
-1
10 # 2 lightly p-doped µc-Si:H
anti-MNR line of type-C μc-Si:H
-3
10
0.0 0.2 0.4 0.6 0.8
Ea(eV) •Ram et al, Phys. Rev. B 77 (2008) 045212.

39. Doped µc-Si:H
anti MNR line (#7)
[heavily doped μc-Si:H]
3
10
-1
σ0 (Ω.cm)
MNR line (#7)
1
10 [a-Si,C:H+μc-Si,C alloy]
-1
10 #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)
-3 #7
10
0.0 0.2 0.4 0.6 0.8
Ea (eV) •Ram et al, Phys. Rev. B 77 (2008) 045212.

40. MNR parameters Anti MNR parameters
σ00 σ00
EMN EMN
G G
-1 -1
-1
(eV-1)
Samples (Ω.cm) (Ω.cm)
(eV ) (meV) (meV)
This work
7.2×10-4
Type-A&B 25.3 --
39.5 --
--
Type-C -- -- -44.6
-- -22.5
87
Published
Data
4×10-3 1.26×1010
Case#1 20.7 -97.7
48.4 -10.2
(rH=21)
3.2×10-6
Case#1 --
36.6 --
27.3 --
(rH=32)
1.7×10-4
Case#2 6
23.4 -32.5
42.7 -30.8
7.7×10-3
Case#3 --
24 --
41.6 --
Case#4 0.32 59
15.4 -66.1
65.1 -15.1
4.2×10-3
Case#5 21
15.3 -64.9
65.4 -15.4
3.2×10-6
Case#6 2.4
31.3 -39.9
31.9 -25.1
2.3
Case#7 309
8.5 -49.5
118.3 -20.2
0.5
Case#8 --
11.8 --
84.5 --
7.2×10-3
Case#9 --
20 --
50 --

41. If one has a collection of G and σ00 then:
a-Si,C:H alloy (#7) #1 (rH=21)
σ00=σM exp [(γC- γF)/k –GEa]
0
10 #1 (rH=32)
Porous Si (#9)
#2
#3
σ00=σM exp [(γC- γF)/k –G(EC0 –EF0)]
a-Si:H (#3) #4
-1
#5
σ00 (Ω.cm)
-2
10 #6
At a position of EF in DOS where
#7
#8
p-nc-Si-SiC:H alloy (#5)
γF(EC0-Emin)=0
#9
-4
10 this work
-1
σM=100 (Ωcm) (at γf=γc) Fit
σ00=σM exp [(γC/k) –GEmin]
Emin=0.61 eV
3 -1
σ0=1.2x10 (Ωcm) (at γf=0)
-6
The quantity Emin is a measure for the
10
5 10 15 20 25 30 35 40
-1
G (eV ) 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.

42. 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

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

44. Thank you