UV absorption shape between 3.5eV and 5.6eV in very thin a-As2S3 films at 80K

Journal oTNon-Crystalline Solids 95 & 96 (1987) North.HoUand. Amsterdam

UV ABSORPTION

Hideo

SHAPE

HOSHI,

Yoshiro

Department of Aoba Aramaki,

Optical

3.5eV

SUZUKI

Applied Sendai,

749

AND 5.6eV

and

spectra

for

of

a-As2S3

have

lowed seems served, tion.

the linear relation of (nE)1'2 linear above 3.5eV. Condisering a stepwise conduction state

a-As2S3

FILMS

AT BOK

HIRAI

Physics, Faculty 980, Japan

absorption

IN VERY THIN

Masamitsu

797nm sorption deduced

1.

Engineering,

films

vs. is

Tohoku

with

been measured at 80K after annealing coefficient (a) at the energies (E's) by correcting the interference effect.

University,

thickness

from

65nm

to

at 463K for 2 hours. Net abfrom 2.0eV to 5.6eV has been Although a below 3.5eV fol-

E, the the density suggested

relation of the to explain

of (uE)vs. E valence state obthe both rela-

INTRODUCTION The

photo-darkening

siderable devices.

the

The edge

optical

thin

because PO has

generally low

the

absorption

absorption

changes (-6.0eV)

seemed

useful

of to

distortion

of film

present

the

paper,

from

2.0eV

to

in

connection

state

and

induced

only

at

main

absorptin

the

the

the

for

a-As2S3 the

in

ragion

microscopic

at

the

absorption

In

and

the

463K,

with

also

and of

the

the

paper?

significant in

the

the In

energy

the

region

absorption of

the

we provide and

peak

difficult.

density

region,

the

the

measurement

PO, the

discuss the

very

that

reflection

in

abfrom

using

at

above

change

following energy

of

spectra

shape

above

but

multiple

absorption

of

showed

the

mechanism

the

region

(-2.5eV)

of

80K

shift

we recently

by

distribution

the

energy

caused

con-

However,

Although

band2.

corrected

state.

change

the

films,

analysis

with

conduction

of

extended

edge

received

photo-functional

photo-illumination'.

the

the

have to

as a parallel

the

(a-As2S3)

absorptionspectra

5.6eV

absorption

mechanism

not

glasses

application

by in

the

further

chalcogenide

characterized

side

trisulfide

we report

shape

in potential

been

energy

understand

the

made

the

measurement

arsenic

region

(PD)

of

to

amorphous

thin

effect

interest

sorption

2.

BETWEEN

749 - 756

valence the

discuss

the

photopossible

PD .

EXPERIMENTALS Thin

quartz deviation

films

of

from

0022-3093/87/$03.50 (North-Holland

a-As2S3

were

A slow

substrata. the

Physics

stoichiometry

0 Elsevier Publishing

prepared

deposition

by rate

in

vapor

deposition

(-O.lnm/s)

the

films.

Science Publishers Division)

B.V.

was Further

on used details

to synthesized to

prevent of

the

the prepara-

the

H. Hmhi

750

tion

procedure

have

tional

cryostat

higher

temperature

those

in

affected

by

been

and

the

Ref.

463K time

30min).

annealing

shape

in

at

longer

2 (443K,

absorption

with

described

annealed and

Ref.

Optical

E, al. / UV absorption

for

The films

used

the

obtain

well

The experimental

330

in

annealed were

be reported

is

about

the films

than

a little

elsewhere6.

spectrophotometer

accuracy

a conven-

work,

changes

will

Hitachi

kept

present

absorption

The detail with

were

In

to

Photo-induced

measured

a micro-computer.

2. 2 hours.

were

condition.

were

betweecn 3.5 c V owl 5.6 e V

controlled

0.003

in

optical

density. The the

following

observed

equation optical

was

density

(1 ODf

used

of

RS andTS

Rf and incident the

are

Tf are from

- RSRf)(l

a-As2S3

film

dispersion The

thickness

spectra, be

presented

3.

RESULTS

(a,),

refractive

those

the

the

with

Fig.1.

Back

ground

subtracted. region,

effect

in

at

are

the

can

be

represented

index

(n,),

substrata

(us, af(E),

(n,,

ns,

ts) the

quartz

substratum.

films

with

for

ts),

net

of

referring

the

4 and

interference

fringes of

the

absorption by

literatures

details

of

(t,)

deduced

the

light

functions

thickness

ns,

to

Further

the of

were

from

797nm.

in

This

interference

2,

seems coefficient

energy

region

and

the

to

assure is

the

various

reliability

considerably

(dashed

curve

the

5. in

analysis

in

IR will

Fig.2)

the

corrected

to

Net

an

reported also

close

ir. were

to

high

reflection

the

method

the

a-As2S3

absorption

correction.

that and

spectra

by

follow

presented windows

multiple

for

Fig.2.

heat-treated is

extended by

thickness our

for

cryostat

coefficients

of close

in

more

was

plotted

with

and

distorted

absorption

were

films

give

80K

thickness

substratum

the

effect

net

at

respective

obviously

the

thickness the

to films

were

respective

result

thinner

spectra

The

due

spectra

films.

from

absorption

thickness.

the

Section

obtained

optical

absorption

those

the

with

low

transmittance

energies,

to

of

and

determined

65nm

various

Although

energy

sorption

quartz

was

from

in

AND DISCUSSIONS Fig.1

This

effect

Ref.6.

films

films

transmittance

(1)

constants

films

ranged

a-As2S3

described

the photon

optical

in

cients

of

respective

in

Shown

and

reflectivity

coefficient

of and

reflectivity

eq.

of

reflectin

(1)

effective

and

multiple

- R:)TfTs

Since

at

the

- Rs12

substratum.

absorption

coefficient

the

the

correct (ODf).

= log (1

where

to

films

coeffi-

identical

curve.

The

present

by

Kosek

and

to

that

by

abTauc7

Drews

et

H. Hoshi

5

et al. /

a-As+, 797m

Optical at 80K.

at

high

5

ENERGY

3.5 e V ud

5.6 e V

751

energy

The

absorption

is

generally

constant

6

(eV)

FIGURE 1 spectra

absorption

8

the

between

at BOK 8OK arm 1al 463K ”

4 PHOTON

tems

shape

I

4 4-

al.

UV absorpfion

of

a-As2S3

region

(dotted

spectrum,

a(E),

represented transition

FIGURE 2 Corrected absorption spectra a-As2S3 at 80K. Dashed and curves are from Ref's.7 and respectively.

curve). for

by the

matrix

of dotted 8,

interband

transition

following

element

and

the

in

equation

from

relaxed

momentum

disordered

the

sys-

assumption

of

conservation

rule',

8m4e2a a(E)

=

~

I

ncm2E where

a is

states are of

the

average

(DOVS)

and

conventional energies

simple

lattice

the

spacing.

conduction

physical are

can

be

for deduced

a(E)

from

= A(E r = rv

where, and respect

E. rc

is

depend of

defined on a-As2S3,

as the

the

following

the

distribution

NC,

and

rc

power

following

(3)

gap,

relation

rv

the

valence notations

(2)".

+ 1,

and

of Other

with

respectively,

- Eojr, + rc

density

respectively.

the

NV and eq.

optical

constituents the

When the

NC are

(DOCS),

constants.

(2)

+ E)dE',

NV and

states

hypothesized

relation

Nv(E')Nc(E'

(4) and

A is

preparation has

a proportional of

been

amorphous generally

constant. systems. accepted

rv In according

H. Hoshi

152

to

the

assumption

et al. / UV abmrprimt

of

the

square

root

shape

berwem

function

3.5 eV md 5.6 rV

for

the

both

NV and

NC;

rv

= rc

l/Z. a(E).E Several edge

authors confirmed 1.7 . The present

region

energy

region

tion

around

coefficients

and

in

(EO)

of

2.47eV

after

linear

relation

absorption

a is

Kosek at shape

at

the

relation also

films

is

at

is

energies

low

curve

that If

3.5eV. the

Fig.2

were of

3.5eV.

coincident

curve

The

a function

below

a was

we pay

our

low

averaged the optical

that

of

from

the

attention

relation

the absorp-

The

with

a deviates

following

around at

Fig.3. in

as

shape relation

a in

energies

curve

ragion,

absorption above

(oE)"'

obvious

above energy

the the

thickness of

from It

high

with

different

linear

Tauk7.

the

5 from followed

shown

with

almost

high

(5)

a coordinate

determined and

.

result

with

2.51+0.02eV

- EO)'

This

the

Fig.3

Curve

(El.

gap

2.7eV. for

illustrated

eneyy

= A(E

to seems

the ap-

propriate. a(E)*E Curve

b in

Fig.3

versus

E.

The

cal

meanings

a-As&

describes linear

of

the

the

A'

and

spectrum from

E'O

in

with

3.5eV eq.

to (6)

a coordinate

5.5eV. are

of

Although

obscure,

a(E)*E

the

curve

physi-

b provides

al 80K

1

(oE)lj2vs CUE) a-As2S3

ranges

constant

(6)

- E'O). absorption

relation

/

2

= A'(E

3 PHOTON

VS:

a

I 4 5 ENERGY (eV)

F1~"~&e E (curve at 80K.

a) b)

and for

ENERGY

(eV)

6 FIGURE 4 OOVS (NV) observed by UPS12 (solid curve), and OOVS's (NC) assumed for a-As2S3. Dashed curve is a stepwise Nc. Chain curve is a NC symmetry to the NV .

=

H. Hoshi

cl cd. / UV absorption

shape

berwem

3.5 e V and 5.6 eV

-al

80K

2 PHOTON

ENERGY

(1 . 9050 . 02)x106cm-'

and In

respectively. the

actual UPS for

a-As2S3

at

373K

The

Fig.4. the

linear

Namely,

this been

has

been

confirmed

and

r = P,

the

suggested

from fact,

most

absorption

result

is

cient,

while shown

in

circles the

with the

above

top

of

The are

valence

region

the

leading

with rv

of

solid the

line

on on

The state,

the

DOCS,

the

distribution with

the

side

right

leading

side edge

is

defined

of

of

In

this Fig.4.

of

Fig.4

as

step

fol-

latter r = 2

Since

rv

rc

= 0 is

the

DOCS,

eq.(2).

case,

to

Fig.3,

absorption

of

the

square

Fig.4.

the

is,

anof

seems in

a in

using

measured

ones. left

the

line

that

were

side

edge

energies.

reproduced

the

which

low

the

with

curve

be

is

on

films

left

although

with

eq.(4).

calculated

shown

= l/2,

of

curve

the

a dotted

at

for

E'O,

a-As2S3

of

the

absorption

could

curve

as shown

function

and

consideration

(-O.leV) for

find

shown

distribution

77K

turn,

of

A'

the

resolution at

a solid

hand,

optical

UPS as a dashed

6

necessary.

result

= 1 instead

a step

calculation. the

as

of

that

spectral

In

consideration

Fig.5.

open

for

hardly

observed

from

DOCS (NC) the

brief

we assume

shown

we can

oter

energy

spectra

obtained

function

the if

rv

the

stepwise

with

energy

On the

from

DOCS is

shown

value

suggest

a excellent 11 . Their

the

implies

assumed.

empirical

DOVS observed. to

result

long

the results

OOVS and

that

the

relation

has

In

is in

the

Harada 12 is

2 hours

remarkable

above

with

and

for

distribution

low

was

films

4 5 ENERGY (eV)

FIGURE 6 spectra of a-As2S3 at 463K (dotted curve) (solid curve).

Absorption measured and 8OK

as

the

of

by Takahashi

nealed

3.10'0.03eV case,

distribution

reported

root

any

3 PHOTON

(e'f)

FIGURE 5 absorption spectra for with stepwise NC (open and symmetric NC (closed Solid curve is the measured.

Calculated a-As2S3 circles) circles).

753

= 1

The coeffi-.

the

DOVS

(Nvl

The step is

employed

function

an extrapolated

as is

the

2.55eV energy

H. Hoshi

754

from

the

straight

spectrum with

dotted

calculated

a chain

curve

absorption

is

between

state

was

at

low

edge

of

is

also

different

DOCS's

This

seems calculated

edge

shapes

high

energies,

contribution leading

becomes does

prominent. not

following

edges,

those

relation

6.

DOCS rises decreases

becomes

as r=l,

of

present

a-As2S3

films

and

has

reported

for

clarify

a-Ge

asymmetric the

the

is

relation

sharper the

and

a-Si

reason.

by

the

probable the

leading

Spicer13 of

systems.

the

of

is and

DOVS and Further

DOVS.

than

for

DOVS and in

the for the

the to

the

is

3.5eV.

the

actual

the

both

that

the

DOVS,

and

that

the

similar

shape

the has

Jackson common

given

eq.(4)

above

from

is

DOVS

the

situation Thus,

region

by

-l.OeV

since

determining

DGCS seems

the

with

Although

to

a-Si:H

investigation

the

DOVS below

region.

conclusion

notable

regions

that

the

UPS by Takahashi edge

It

however,

the

3.5eV,

energy

from

explanation

similar

the

the

region

contributes

energy

and

edge.

the

eq.(2)

result

DOCS to

hardly

incapable

of

is

at

energies,

would

above

of

almost

6 in is

absorption

a much

top

to hand,

edge

in

edges

from

decrease

interesting

in

a possible

shape

eq.(Z) Accordingly,

other

high

symmetric give

the

in

sensitive

DOCS

the

contribution

situation

Nevertheless,

amorphous physical

such

the

distribution

various

above

analysis

following

the

the

the

This

absorption

-l.OeV,

in

gives

from

might

2.55eV

This

optical

region

Fig.4,

optical

at

rv=rc=D

DOCS.

constant

for

up

primitive

the

the

This

in

the

at

DOVS and

obscure. with

energies.

below

of

The present

This

calcurated

integral.

by a condition

tic

regions

for

stepwise

shape

contribution

above

As seen

gradually

of

the

the

high

important

overlapping

since

the

the region.

DOCS.

the

the

probable.

energy

On the

the

a sharper

more with

high

DOCS above

obtained

of

is

integral

the

to follow

that

Fig.4

rather

Fig.5.

conduction

becomes

integral

overlapping

shape

deviation

DOVS and

are

in

and

shapes

shape

DOCS at

becomes

the

the

at

the

energies

region

the

from

absorption

stepwise the

reflects

the

similar

to

of

shown

almost

overlapping

edges

low as

The absorption

directly

DOCS makes

at

OOCS,

edge

the

the

energy

the

calculated

other

shown

the

of

implies in

shapes

as

manifestation

also curve

each

energies,

shapes

the

result

absorption

case,

Fig.5

region,

chain

with

this bottom

in

energy

DOVS, optical

similar

circles

This

the

low

OOVS and

the

the

between

absorption

DOVS below

of

identical

At

the

high

In the

the

The absorption

the

calculated

and

with

at

DOVS. to

Fig.5.

closed

3.5eV.

almost

by those

of

defined

that that

are

only

the

in

the

the

The

examined.

state

below

of

symmetry

circles

DOCS than

3.5 CV and 5.6 eV

edge

valence

the

curve)

reasonable.

predominated

also

was

interesting

leading

closed

(solid

the

befawn

OOCS being

Although

energies

shape

of

The latter

absorption

It

top

above.

the the

was

with the

2.20eV.

rising

at

using

Fig.4,

plotted

evident

the

in

mentioned

observed

UV obsorprimr

line

with

separation

former

e, al. /

DOCS for an almost

results et

and

expected

al.14. characteristo

are

Absorption with almost the in

spectra

a dotted parallel low the

of details

reported

in

to

flatten

edge

the

shape temperature

following

paper,

absorption

at from

with at

high

temperatures

Fig.6

absorption

the

DOVS by

might

be

8OK

that

of

Since

discussed

by

in

is

reported

that

connection

in

changes

Takahashi 15 , the of

absorption

shown

Although

absorption

UPS measurement

the

65nm

curve). was

the

band. the

of

(solid

predominated

dependence and

thickness

that

main

in

the

film

with

IS apparent

absorption of

a-As2S3

absorption

flattening

the

the

for together

it

similar

Further

induced

of

region

the

463K

Fig.6,

region7,

whole

ma1 change

in

shift

energy

observed

at

curve

the

et

DOVS.

spectra

will

the

photo-

with

al.

ther-

be

change.

ACKNOWLEDGMENTS Authors

greatly

UPS data

and

appreciate

helpful

Dr.

T.

Takahashi

for

providing

the

unpublished

discussions.

REFERENCES 1)

K. Tanaka, in

Fundamental

Solid-State

Physics

of

Vol.

25,

Sciences,

Amorphous ed.

Semiconductors,

F. Yonezawa

Springer

Series

(Springer-Verlag,

1981)

pp.104-118. 2)

14. Hirai,

3)

H. films

4)

at

D.J.

H.R.

6)

H.

and

S.

Suzuki

and

M. Hirai,

this

volume.

8OK,

Treaty,

(academic 5)

Y. Suzuki

Eguchi,Y.

Handbook Press,

Philipp, Eguchi,

of

1985)

H.

Takeuchi,

J.

Phys.

Sot.

Photo-induced

Optical

Constants

Jpn.

53

absorption

of

Solids,

ed.

(1984)

4009.

change

in

E.D.

a-As2S3

Palik

pp.641-664.

ibid.,

pp.749-764.

Hoshi,

Y. Suzuki

and

M. Hirai,

to

be

published

in

J.

Phys.

Sot.

Jpn. 7)

F.

8)

R.E.

Kosek

(1972) 9)

10)

J.

J.

Taut,

R.L.

Czech.

Emerald,

J. M.L.

Phys.

820

Slade

(1970)

and

94.

R. Zallen,

Solid

State

Commun.

10

293.

N.F. 2nd

and

Drews,

Mott ed.

and

E.A.

(Clarendon

Taut,

Davis,

Electronic

Press,

Oxford

Amorphous

and

Liquid

Process 1979)

in

Non-crystalline

Materials,

pp.272-306.

Semiconductors

(Plenum

Press,

1974)

pp.171-178. 11)

T.

Takahashi

12)

T.

Takahashi,

13)

W.E.

Spicer,

Garmisch,

14)

W.8.

Jackson,

S.M.

(1985) 15)

T.

and

Y.

Harada,

Solid

(1974) Kelso,

Cotnnun.

35

(1980)

Allen

and

S.J.

191.

p.499.

C.C.

5187.

Takahashi,

State

unpublished.

private

communication.

Tsai,

J.W.

Oh,

Phys.

Rev.

831