Anomalous Behavior Of SSPC In Highly Crystallized Undoped Microcrystalline Si Films

Study Of Anomalous Behavior Of Study Of Anomalous Behavior Of Steady State Photoconductivity Steady State Photoconductivity

In In Highly Crystallized Undoped Highly Crystallized Undoped

Microcrystalline Si FilmsMicrocrystalline Si Films

Sanjay K. Ram Sanjay K. Ram Dept. of Physics, Dept. of Physics,

Indian Institute of Technology Kanpur, INDIAIndian Institute of Technology Kanpur, INDIA

OutlineOutline

MotivationMotivation

Sample preparation & structural characterizationSample preparation & structural characterization

Steady state photoconductivity (SSPC) Steady state photoconductivity (SSPC) measurementsmeasurements

Qualitative analysisQualitative analysis

Numerical simulation of SSPCNumerical simulation of SSPC

ConclusionConclusion

MOTIVATIONMOTIVATION

μμcc--Si:H thin filmsSi:H thin films

Promising material for large area electronicsPromising material for large area electronics

Good carrier mobilityGood carrier mobility

Greater stability under electric field and lightGreater stability under electric field and light--induced stressinduced stress

Good doping efficiencyGood doping efficiency

Possibility of low temperature depositionPossibility of low temperature deposition

Further development requires proper understanding of Further development requires proper understanding of carrier transport properties correlative with film carrier transport properties correlative with film microstructuremicrostructure

Complex microstructure & inhomogeneity Complex microstructure & inhomogeneity in the growth directionin the growth direction

ISSUESISSUES

Film growth

voids

substrate

grains grain boundaries

columnar boundariesconglomerate crystallites

surfaceroughness

Why is comprehensive description of its Why is comprehensive description of its optoopto--electronicelectronic

properties difficult ???properties difficult ???

Difference between Density of States (DOS) map of c-Si and amorphous Silicon (a-Si:H)

NonNon--availability of complete density of availability of complete density of state (DOS) map of state (DOS) map of µµcc--Si:H systemSi:H system

ISSUESISSUES

Electrical transport ???Electrical transport ???

Is it dominated by crystalline phase ???Is it dominated by crystalline phase ???oror

By interfacial regions between crystallites or grains???By interfacial regions between crystallites or grains???

A large number of studies claim that electronic A large number of studies claim that electronic transport in transport in μμcc--Si:H films is analogous to that Si:H films is analogous to that observed in aobserved in a--Si:H filmsSi:H films

GOAL GOAL

To study the To study the optoopto--electronic properties of well electronic properties of well

characterized characterized μμcc--Si:H films Si:H films

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

ISSUESISSUES

Sample preparationSample preparation

Parallel-plate glow discharge plasma deposition system

R=1/1R=1/1 R=1/5R=1/5 R=1/10R=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

Film characterization

Structural Properties Electrical Properties

Xray Diffraction

Raman Scattering

In-situ Spectroscopy Ellipsometry

Atomic Force Microscopy

σd(T) measurement15K≤T ≤ 450K

σPh(T,∅) measurement15K≤T ≤ 325K

CPM measurement

Hall effect

TRMC

Raman ScatteringRaman Scattering

400 450 500 550 600

μc-Si (X, SiF4) μc-Si Std.

c-Si

Inte

nsity

(a.u

.)

Raman shift (cm-1)

450 475 500 525 5500.40.81.21.62.02.42.83.2 Layer sideR (SiF4 / H2) = 1/10

t=950 nm

t=590 nm

t=422 nm

t=390 nm

t=170 nm FB22GF

FB11GF

F281GF

FB23GF

FB04GF

F152GBt=52 nm

Inte

nsity

(arb

. uni

t)

Raman Shift (cm-1)

Effect of thickness variation

Spectroscopy Spectroscopy ellipsometeryellipsometery studystudy

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)2.5 3.0 3.5 4.0 4.5 5.00

5

10

15

20

25 B23 (R=10, t=590 nm) B11 (R=10, t=390 nm) B22 (R=10, t=170 nm) F152(R=10, t=55 nm) F16 (R=20, t=35 nm)

Energy (eV)

< ε 2 >

Fig. Measured <ε2> spectrum for the µc-Si:H samples. The sample name, thickness and its 1/R value are shown in the graph.

XX--ray diffraction studyray diffraction study

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

.)

XRD study to see the effect of R (H2/SiF4)

Film deposited at SiF4/H2

flow ratio 1/1 shows a preferred orientation of (400).

film deposited at SiF4/H2

flow ratio of 1/5 shows a preferred orientation in (220)direction.

These results demonstrate the effectiveness of using fluorine based precursors in controlling the orientation of polycrystalline films on insulating glass substrates.

R

Structural FindingsStructural Findings

R =1/1

R =1/5

R =1/10

Random Orientation

(220) orientation

(400) orientation

Individual grains are bigger

Smooth top layerTightly packed

More Void fraction

Good crystallinity at bottom interface

Classification from coplanar electrical Classification from coplanar electrical transport point of viewtransport point of view

TYPE-A

TYPE-B

TYPE-C

More amorphous tissueSmall grains

Moderate amorphous tissueSmall grains

Tightly packed columnar crystalsLess amorphous tissueBig grains

Thickness (50-250 nm)



SSPC Process

absorption of photons and generation of free electron-hole pairs

recombination of excess free electrons and holes through recombination centers

transport of mobile carriers

We have measured temperature and light intensity We have measured temperature and light intensity dependent steady state photoconductivity (SSPC) dependent steady state photoconductivity (SSPC) for the samples of different microstructurefor the samples of different microstructure

γγ is a measure of characteristic width of tail states nearer to is a measure of characteristic width of tail states nearer to EEff

According to the Rose model: According to the Rose model: the exponentially distributed tail state shows: the exponentially distributed tail state shows: γγ = = kTkTCC/(kT+kT/(kT+kTCC))

In amorphous semiconductor 0.5<In amorphous semiconductor 0.5<γγ <1.0<1.0γγ=0.5 bimolecular recombination kinetics=0.5 bimolecular recombination kineticsγγ=1 monomolecular recombination=1 monomolecular recombination

Significance of Significance of γγ

In a disordered material: In a disordered material:

σσphph ((TT, , φφ)=)=ee[[μμnn((nn--nn00) ) + + μμpp((pp--pp00)])]

Light Intensity dependence:Light Intensity dependence:

where, Gwhere, GLL = = φφ (1(1--R)[1R)[1--exp(exp(--ααd)]/dd)]/d

γσ Lph G∝

Experimental ResultsExperimental Results

[[σσphph((φφ , T)] of sample #B22 of Type, T)] of sample #B22 of Type--AA

0 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)

1.2 x 1017 (100%) 8.4 x 1016 (75.4%) 7.6 x 1016 (65%) 5.5 x 1016 (49%) 2.0 x 1016 (15%) 1.6 x 1015 (1.25%)

1000 / T (K -1)

σ ph (Ω

−1cm

-1)

1014 1015 1016 1017

10-10

10-9

10-8

10-7

10-6

10-5

σ ph (Ω

−1cm

-1)

Intensity F (photons/cm2. sec)

310 K 275 K 250 K 225 K 175 K 125 K 80 K 50 K 30 K

Note: Note: σσPhPh (T) shows thermal quenching (T) shows thermal quenching (TQ) with an onset at ~ 225K(TQ) with an onset at ~ 225K

σph(T) vs φσph(φ ) vs T


[[σσphph((φφ , T)] of sample #B23 of Type, T)] of sample #B23 of Type--BB

Note: Note: σσPhPh (T) shows NO TQ(T) shows NO TQ

4 8 12 16 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

1012 1013 1014 1015 1016 1017

10-11

10-9

10-7

10-5

σ ph (Ω

−1cm

-1)

Φ (Photons/cm2-sec)

324 K 300 K 275 K 250 K 225 K 200 K 175 K 153 K 128 K 101 K 72 K 60 K 50 K 25 K

σph(φ ) vs T σph(T) vs φ


[[σσphph((φφ , T)] of sample #F06 of Type, T)] of sample #F06 of Type--CC

Note: Note: σσPhPh (T) shows TQ with an onset at 225 K(T) shows TQ with an onset at 225 K

0 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)

1014 1015 1016 1017

10-10

10-8

10-6

10-4

σ ph (Ω

−1cm

-1)

Φ (photons/cm2. sec)

15K 20K 30K 40K 50K 60K 70K 80K 90K 100K 150K 200K 250K 300K

σph(φ ) vs T σph(T) vs φ

0 10 20 30 40 50 60 70

0.2

0.4

0.6

0.8

1.0

γ

1000/T (K -1)

B22 F06 B23

temperature dependencies of light intensity exponent (γ)

Comparison of phototransport properties of all the three types of samples

TQ and 0.5<TQ and 0.5<γγ <1 : as <1 : as found in Typefound in Type--A: A:

NO TQ and 0.5<NO TQ and 0.5<γγ <1 : as <1 : as found in Typefound in Type--B: B:

TQ and TQ and γγ value value approaches to a lowest approaches to a lowest value of 0.14 at 225 K: as value of 0.14 at 225 K: as found in Typefound in Type--C: C:

Findings:Findings:


Causes of TQ :Causes of TQ :

The transformation of the recombination traffic The transformation of the recombination traffic from VBT states to DBfrom VBT states to DB

The asymmetry in band tails in the gap.The asymmetry in band tails in the gap.

Low value of defect densities or increasing nLow value of defect densities or increasing n--type type doping level may shift the onset of TQ to higher T. doping level may shift the onset of TQ to higher T.

Causes of sublinear behavior of Causes of sublinear behavior of γγ (<0.5) (<0.5)

The saturation of recombination centersThe saturation of recombination centers

The shift of The shift of EEFF towards band edges in doped towards band edges in doped material.material.

DISCUSSIONDISCUSSION

Phototransport properties of TypePhototransport properties of Type--A (TQ and 0.5< A (TQ and 0.5< γγ<1)<1)

This type of behavior is usually observed in typical This type of behavior is usually observed in typical aa--Si:HSi:H

Rose model works and width of CBT is deduced (Rose model works and width of CBT is deduced (kTckTc ~ 30 meV )~ 30 meV )

Possible explanation for Possible explanation for ““No TQ and 0.5< No TQ and 0.5< γγ<1 <1 ““ as found inas found in

TypeType--BB

Symmetric band tailsSymmetric band tails

Usually observed in typical Usually observed in typical µµcc--Si:HSi:H

Rose model works and width of CBT is deduced (Rose model works and width of CBT is deduced (kTkTCC ~ 25~ 25--28 28

meV )meV )

According to According to BalbergBalberg et. al (Phys. Rev. B 69, 2004, 035203): a et. al (Phys. Rev. B 69, 2004, 035203): a

Gaussian type VBT to be responsible for such behaviorGaussian type VBT to be responsible for such behavior


Phototransport properties of TypePhototransport properties of Type--C (TQ and C (TQ and γγ<0.5)<0.5)

Possible explanations for TQ behavior in TypePossible explanations for TQ behavior in Type--C materialC material

Rose model does not hold forRose model does not hold for TypeType--C materialC material

DBs unlikely to cause TQDBs unlikely to cause TQ

Possibilities of asymmetric band tail states in this type of Possibilities of asymmetric band tail states in this type of materialmaterial

lower DOS near the CB edge, i.e. a steeper CBT than VBT lower DOS near the CB edge, i.e. a steeper CBT than VBT (supported by defect pool model)(supported by defect pool model)

The CPM measurement supports the fact The CPM measurement supports the fact kTkTCC<<<<kTkTVV


Possible explanation for sublinear behavior of Possible explanation for sublinear behavior of γγ (<0.5) in Type(<0.5) in Type--C C

In TypeIn Type--C material, EC material, EFF is found to be very close to is found to be very close to EcEc (E(ECC--EEFF ~ ~ 0.34 eV)0.34 eV)

In doped aIn doped a--Si:H when Si:H when kTckTc << << kTvkTv δδn n ≈≈ nn00 then Rose model then Rose model doesndoesn’’t hold (by C. Main t hold (by C. Main …….).)

γγ=T/=T/TvTv for low excitationfor low excitation

γγ= = TTc/c/TTvv at high excitationat high excitation

According to Polycrystalline Si model two different VBT is According to Polycrystalline Si model two different VBT is also possible; also possible;

A sharper shallow tail near the edgeA sharper shallow tail near the edge--> originating from grain boundary > originating from grain boundary defectsdefects

A less steeper deeper tail associated with the defects in columnA less steeper deeper tail associated with the defects in columnar ar boundary regions. boundary regions.

Capture cross section for the deeper tail is smaller than the shCapture cross section for the deeper tail is smaller than the shallower allower one. one.


Numerical SimulationNumerical SimulationMotivationMotivation

Experimental results cannot discern the states where the recombiExperimental results cannot discern the states where the recombination nation actually occursactually occurs

SS--RR--H mechanism and SimmonsH mechanism and Simmons--TylorTylor Statistics are extensively used to Statistics are extensively used to understand recombination mechanism in steady state processunderstand recombination mechanism in steady state process

RR11RR22

RR33

RR44

RR55

RR66

RR77 RR88

RR1212

RR99

RR1111

RR1616

RR1515

RR1313

RR1010

RR1414

GGLL

EEVV

EECC

UU

VBTVBT CBTCBTDB DB ++ DB DB 00 DB DB --

VBTVBT

CBTCBT

Schematic illustration of DOS in amorphous semiconductor and Schematic illustration of DOS in amorphous semiconductor and representation of electron (solid lines) and hole transitions (drepresentation of electron (solid lines) and hole transitions (dotted lines)otted lines)

[ ] [ ] ( ) ( )[ ] ( ) ( )[ ] ( ) 022,,,, 000000000 =−−++−−−+−−− −−

DBDBDBDBDBVTVTCTCT FFFFNpnQpnQpnQpnQppnn

DBVTCTL UUUG ++=

Charge neutrality equationCharge neutrality equation

Recombination equationRecombination equation

( ) ( ) ( )pnQdEEFENQ CTCT

E

ECTCT

C

V

,=∫=

( ) ( )[ ] ( )pnQdEEFENQ VTVT

E

EVTVT

C

V

,1 =−∫=

( ) ( ) ( )−− −−+=− DBDBDBDBDBDBDB FFFFNpnQpnQ 22,, 00000

( ) ( ) ( )[ ]dEFFSFSFnENU nDBnDBnDBnDB

E

EDBDB

C

V

−−++ +−+∫= εε 0000

( ) ( ) ( )''

'

ppSnnSpSnS

EF CTp

CTn

CTp

CTn

CT +++

+=

( ) ( ) ( )''

'

ppSnnSpSnS

EF VTp

VTn

VTp

VTn

VT +++

+=

CBT

VBT

DB

( )( )( )

( )( )⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎥⎦

⎤⎢⎣

⎡ −−−+

⎥⎦

⎤⎢⎣

⎡ −−−

×⎥⎦

⎤⎢⎣

⎡ −−×=

2

1

2

1

1

011

exp1

expexp

c

tcc

c

tcc

c

cctct

kTEEE

kTEEE

kTEENN

( )( )( )

( )( )⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎥⎦

⎤⎢⎣

⎡ +−−+

⎥⎦

⎤⎢⎣

⎡ +−−

×⎥⎦

⎤⎢⎣

⎡ −−×=

2

1

2

1

1

011

exp1

expexp

v

tvv

v

tvv

v

vvtvt

kTEEE

kTEEE

kTEE

NN

( )( )

( )⎥⎦

⎤⎢⎣

⎡ −= 2

2

2/1 2exp

2 WEE

WNEN DBD

DB π

Steps in Numerical SimulationSteps in Numerical Simulation

DOS distribution is first assumedDOS distribution is first assumed

Guess values of n and p are givenGuess values of n and p are given

Charge neutrality equation & recombination rates equation Charge neutrality equation & recombination rates equation

are simultaneously solved for a fixed value of T and Gare simultaneously solved for a fixed value of T and GLL

SS--RR--H mechanism and SimmonsH mechanism and Simmons--TylorTylor Statistics are appliedStatistics are applied

NewtonNewton--RaphsonRaphson method for finding roots of n and pmethod for finding roots of n and p

SimpsonSimpson’’s method for numerical integrations method for numerical integration

n and p are obtainedn and p are obtained

We calculatedWe calculated σσphph ((TT, , φφ)=)=ee[[μμnn((nn--nn0) 0) + + μμpp((pp--pp0)]0)]

The corresponding The corresponding γγ values are obtained as in experimental values are obtained as in experimental

casecase

4 6 8 10

10-6

10-5

10-4

1000/T (K -1)

σ ph (Ω

-1cm

-1)

G=1019 cm-3sec-1

G=1020 cm-3sec-1

G=1021 cm-3sec-1

4 6 8 100.3

0.4

0.5

0.6

1000/T (K -1)

γ

γ

100 150 200 250 3001011

1013

1015

1017

1019R

ecom

bina

tion

rate

s (c

m-3se

c-1)

T (K)

Uct1 Uvt1 Uvt2 UDB

0.0 0.3 0.6 0.9 1.2 1.5 1.81013

1015

1017

1019

1021

DO

S (c

m-3eV

-1 )

EC- EF=0.34 eV

DB

CBT

VBT2

VBT1

ECEV

(E-EV) eV

Simulation results for Type-C material (ex. #F06)

μμnn = 10 = 10 cmcm22VV--11ss--11

μμpp == 0.5 0.5 cmcm22VV--11ss--11

Simulation results for Type-B material (#B23)

0.0 0.3 0.6 0.9 1.2 1.5 1.81013

1015

1017

1019

1021

DB

EC- EF=0.42 eV

CBT2

CBT1

VBT2

VBT1

DO

S (c

m-3eV

-1 )

EV EC(E-EV) eV4 6 8 1010-8

10-7

10-6

10-5

1000/T (K -1)

σ ph (Ω

-1cm

-1)

G=1018 cm-3sec-1

G=1019 cm-3sec-1

G=1020 cm-3sec-1

G=1021 cm-3sec-1

4 8 12 16 200.5

0.6

0.7

0.8

0.9

γ

γ

1000/T (K -1)100 150 200 250 3001011

1013

1015

1017

1019

T (K)

Rec

ombi

natio

n ra

tes

(cm

-3se

c-1)

Uct1 U

ct2 Uvt1 Uvt2 UDB

μμnn = 10 = 10 cmcm22VV--11ss--11

μμpp == 0.5 0.5 cmcm22VV--11ss--11

SummarySummaryThe qualitative as well as quantitative analysis of the The qualitative as well as quantitative analysis of the

study of our phototransport properties of undoped study of our phototransport properties of undoped µµcc--

Si:H thin films are in good agreement Si:H thin films are in good agreement

MicroMicro--structural differences leads to totally different structural differences leads to totally different

phototransport behavior. phototransport behavior.

The recombination rate of deeper valence band tail is The recombination rate of deeper valence band tail is

higher in percolated grains than in unpercolated grainshigher in percolated grains than in unpercolated grains

We propose different effective DOS distribution forWe propose different effective DOS distribution for

micromicro--structurally different structurally different μμcc--Si:H thin filmsSi:H thin films