Photo-induced absorption chance in a-AS2S3 films at 80K

Journal of Non.Cryskdlinc Solids 95 & 96 (1987) NortJl-Holland, Amsterdam

PHOTO-INDUCED

Hiroya

ABSORPTION

ECUCHl,Yoshiro

757 - 764

CHANGE

SUZUKI

I.

FILM

Masamitsu

HIRAI

and

Department of Applied Physi,cs, Tohoku University, Sendai, 980,

Absorption films with by illuminating -0.3eV. absorption the absorption independent shift reported quite similar sible reason

IN a-As2S3

Faculty Japan

of

AT 80K

Engineering,

change various

in the spectral range from -1.5eV to -6.OeV in a-As S3 thickness between 10.2nm and 797nm has been investiga t ed them with light peaking around 4eV with a band width of All experiments are carried out at 80K. After the illumination, around 5.5eV decreases, while that around 3eV increases. Namely, band becomes flat after the illumination. The flattening is of the film thickness, and seems to be the origin of the edge in previous papers. In addition to above, the flattening is to that resulting from temperature rise of the films. A posfor the flattening is discussed.

INTRODUCTION Amorphous

As2S3

phenomena,

and

The most

prominent

D.lleV

and

the

after

shift,

the

tigations, lfim

however, in

the

able

spectral

the

absorption

on

the

have

past.

been

was

at

and

out optical

and

photo-darkening,

be

on

a-As2S3

The

6.0eV

the

rather

density

obtain

should

measurement. 1.5eV

the

to a narrow to

change

2.5eV112. side

side of

by about

of

the

the

film

i.e.

the

films

such

thick

ab-

before

Most

amorphous. thicker

in

so-called

around

energy

volume

with

the

low energy

high

of

devices.

is edge

the

photo-induced

functional

material

the

structure

high

However, it

that

local

carried of

phenomena,

between

absorbed

restricted

edge.

absorption

light

of

photo

absorption toward

of

Because

range

photo-darkening spectra

change

the

this

way

suggest

the

in

of

diffraction

illumination from

shift

variety for

parallel of

The x-ray

arises

the

in almost

illumination

edge3.

presenting material

phenomena

from

occurs

under

chalcogenides

as a potential

arising shift

sorption

of

expected photo-induced

photo-darkening The edge

is one

is

than films,

edge Invesabout

observ-

range

such

as 2.0eV-2.7eV

around

a wide

and

exact

on

necessary

to

present

paper films

extend

knowledge the

reports

with

spectral the

thickness

the

range absorption

thinner

than

lfim.

2.

EXPERIMENTS Experimental

ments

and

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

conditions the

correction

Physics

0 Elsevier Publishing

on for

the

sample

the

interference

Science Publishers Division)

preparations, effects

B.V.

the due

absorption to

the

measurefilm

foru

are

stated

in

the

797nm.

The

preceding

film

on

a conventional light

the

position of

the

provided

of

4.0eV,

3.

RESULTS

in

sorption

the

the

spectra curves

3eV

O2

3 PHOTON

4 ENERG’

Absorption ;;~~~aio~:;r;OKand

As a typical

E as viously just

very show

after

in

of

a-As2S3

effects out

at

of

films

the

80K.

463K

illumination

at around

uith

for 80K

and

energy

thickness,

curves

with

the

t, All

2 hours.

5.5eV

solution

photon

corrected.

The broken

at

of a fixed

re-positioning

the

been

of

at

glass

films have

to

bottom

illumination

from

uith -2 .

10.2nm

the

sample

proper

6.3mU/cm

coefficient

example,

thick of

01

5 feV)

spectra after

narrow in

a part

of

the

the

arising

uith

from at

present

ab-

While,

the

I ight

decreased,

ab-

for

while

the that

increased.

Ekl

In the

carried annealing

those

error

a pouer

curves

The absorption

and

keeping

photo-reaction

interference

were the

present

photo-reaction.

and

The

after

prevent

the

0.3eV

sample

by

lamp

absorption

figure.

measurements

sorption

around

of

ranged holder

out

to

for

thickness the

measurement

carried

mercury-xenon

light

on

absorption

were

uidth

illustrates

listed

solid

the

a band

Fig.1 as

The

A 5OOU

film

was set

spectrophotometer

sample.

filters

The

substratum

photo-reaction

in

the

the

cryostat.

for

4 .

paper

films Fig.3

m a-As2S3

results

range

Fig.2.

of i I-

from This as

discussed

in

films

-2.7eV result

shown

* 2.0

to is

by open next.

with 3.0eV,

almost and

’ ’ ’ 2.2 2.4 26 PHOTON ENERGY

at

GCZGvs.EIn EOK.

-6Onm JIG@j identical

closed

’ ’ 26 3.0 f eV 1

circles5.

thickness is to

are

a linear those Figure

discussed. function

reported

of pre-

2 presents

H. Eguchi

Now,

we turn

curves

in

the

our

Fig.3

attention

quite

wide

in

and

cussed

range

the

connection

the

preceding

spectral

range.

can

ot 80 K

The

of

-3.5eV

curve,

is

not

EK

E*a(E)

In

Fig.4,

are to

this

vs.

and

5.5eV. for

A and

the

broken

the

density

respectively. eq.Cl>

clear

EO are curve,

depend

on

Is not

true

that

the

smed changed

fro1

the

films

aaCE>

transmitted

light

where, as the

t,

large

is as

penetration

sample

a’

as shown

the

fiI1

-105ca“, depth,

I’

meaning,

of

states

5 (eV)

EiERGY

curves

appeared if

we plot

On correcting

illuminated

10*expC-

absorption films

thickness

were to

=

the

below.

intensity I’

between for

as shown

es1.80f

dis-

6

&qJ Difference spectra between absorption spectra before and after illumination on a-As2S3 at 80K.

spectra

illustrated

the

yet.

PHO:ON

a-As2S3

after

and

The physical

and

(eV)

E in

difference

illumination seem

ENERGY

solid

and

by an equation,

2 PHOTON

and

before

(1)

between ’

759

broken

E*a(E)

be expressed

3.10f0.03eV

solid

paper

/i/m

- EO),

and the

in a-AsJ,

distribution

between

1.90f0.02 for

words, in

a wide

spectral

= A*CE

of

3.00f0.03eV

other

change

E*aCE>

a(E)

spectral

constants

0.02

the

respectively. E*

in

to

present

illumination,

pirlcal

et 01. / Absorption

is

wholly

the in

by Figs.SCA) given

before

in

Fig.1.

them

as

and

and

that

CB).

Then,

illumination

t,,

as

shown

we asabsorption

the

apparent

(2) since light

in

the

by an equation,

However,

the

the

However,

effects,

a ’ t,)

thickness.

after difference

observed.

interference

depth,

and The

Fig.5CC).

can

the

absorption

penetrate Then,

I’

coefficient the

in

this

fila case

is only is

given

by

H. Eguchi

760

e, 01. / Absorprimr

change

in a-As,S,

/i/m

01 80 K

by an equation,

I’ a d is

where,

= I *exp 0

the

From-eqs.(Z)

real

and

The apparent da d isa -a. a

The real

from ad

In other ratio

- ad

t,1

after

,

(3)

the

illumination

)tp/ts.

given

due

by eq.(4)

to

the

interference

- cta,

of

effects

as

- aa)tp/ts.

- aa

words,

= caa

we have the

sample

-

(5)

the

absorption

a

‘)ts/tp

q

to correct

coefficient

Au

in

to

*ts/tp.

the’apparent

thickness

(C)

is

given

as

t,

and

(6)

difference the

(aa

penetration

- a’>

depth

by

the

tp.

llluminallon

2

3

4

PHOTON

Illustration the absorption to the penetration

Fig.6

presents

tp=108nI. pendent

tp.

eq.(S),

between

u ing due

in

(4)

by correction is

= (ad

ad

- aa

obtained difference

change,

follows

- tp)

s

coefficient

a - (ad

difference

a - a’

(t

we obtain

=a

This

ua

absorption

(31,

a’

{-

It of

the

for correctcoefficient depth.

Aa*ts/tp is film

evident thickness.

Ej.& Corrected between absorption and after illunination 80K.

for that

all the

films

photo-induced

appeared change

ENERGY

5

6 (eV)

difference spectra spectra before on a-AsZS3 at

in

Fig.1, of

absorption

by

assusing is

inde-

H. Eguchi

Results

in

thinner

films

According

films. 10.2na

and

coverage

30.3nm

of

thickness

of

Fig.? density

the

are

in

the

-90%

on

and

the

10.2na

presents

the

(OD) observed

films

could

of

the by

not

form

differed

the

of substrata,

of

for

the

curves)

was

them.

be obtained

slightly

from

islands

respectively.

curve

film

(II 80 K

observation,

aggregation

aftercsolid

10.2nm

films

761

those

in

the

films

and

stripes

thick to

Therefore,

be with

the

real

be -2Onm.

absorption

piling

in a-As,S,

aicroscope

must

and

change

-30nm

electron

fila

curves>

were

than

to

-50%

beforecbroken

et al. / Absorption

so

The

because

small

that

real of

10.2nm

the

OD of

absorption

and

30.3nm

illumination. three

identical

coefficient

transparent

space

films

The optical samples

of

between

these

thin

islands

and

stripes.

2

3 PHOTON

4 5 ENERGY (eV)

m Absorption spectra and after illumination a-As2S3 films at 80K.

The tween films, 5.5eV tively tion

broken

and

absorption

chain curves

quantitatively

similar. change

thin

Ej& Difference sorption spectra illumination on at 80K.

curves

in and

films

Fig.8

present

after

increase

different Although

in

before thin

before The

respectively. are

on

PHOTON

6

and from

the

previous

with

about

the

the

paper’

difference

of

in

the reported

1Onm thickness,

(eV 1

spectra between before and after thin a-As2S3 films

illumination

decrease that

ENERGV

curve on

film,

and

around but

no photo-induced the

(AOD>

10.2nm

absorption 65nm

photo-darkening

ab.

are

be-

30.3nm 3eV

and

qualitaabsorpseems

762

to be

real

even

I

0 2

in such

thin

films,

too.

I 3 PHOTON

2

4 5 6 ENERGY (eV 1

An

lnterestlng

ference

spectra

several

between

the

shown

in

Figs.!3 other,

curves

are

the

between

illumination

each

the

similarity

due

to

and

in

the at

flattened

by

figure.

at

curve

the

the

present

the

way

65nm

other

by

both

the dif-

measured

at

the

illumination

are

In

same

those

to the

presents

of

distributions

identical.

almost

and

was

due

9 presents

Fig.10

before

thickness

6

between and after film at

changes

80K

While,

spectral

almost

absorption Figure

curve

the

Sample

The

are

the

illumination.

absorption 92K.

10.

but

of

the

absorption

indicated

spectra

after

is and

temperatures

ference

from

result difference

4 5 ENERGY ( eV)

W Difference spectra absorption curves before illumination on a-As2S3 92K.

Ej,& Difference spectra between absorption curves of aAs2S3 film at 80K and several temperatures indicated.

temperature

3 PHOTON

dif-

and for

those

both

different

cases slightly

words,

the

absorption

thermal

and

optical

ef-

fects.

4.

DISCUSSIONS

AND SUHHARV

Followings (1)

resulted

Under

the

sorption

(2)

(3)

(4)

band

and

the

The

increase

the

absorption

The

photo-induced

from

from

illumination

with

of

a-As2S3

increase

-1Onm

The flattening

around of

the edge

to

works.

light films

3.0eV

absorption

of

absorbed

in

are

flattened

the

absorption

around

reported

by other

absorption

change

the

absorption

by

the

edge,

decrease

the

around

ab5.5eV

band.

3.0eV

is

the

origins

of

the

shift

of

papers. is

independent

of

the

fila

thickness

-8OOnm. of

the

absorption

band

occurs

almost

in

the

same

way by

H. Eguchi

the thermal

et al. / Absorptim

and optical

The final result have been discussed

change

in a-As,&

Jilnrr

ot 80 K

763

effects.

The photo-darkening effects such as D’D- creation’l, the bond SWi tch i ng* and the creation of As-As clusters in As& filmso. We also interpreted the photo-induced absorption change from a view point of the creation of localized centers in the previous paper ‘? However, we might have to

dismiss believed

(4) is discussed in detail. by the local structure change

such bond breaking that the heating

Fig.3, sorption

the

results hanced

suggest randou of

uodels if we remark causes such drastic

the result effects.

(4). It can be hardly Furthermore, as seen in

photo-darkening seems to be caused not by the shift but by the flattening of the main absorption band n ore

local

“temperate” structure’1*‘2

At first, let us consider raising the film temperature, caused by thermal vibration.

models for rather

the photo-darkening than bond breaking

of the tail abitself. These such as the enmodels.

the thermal effect on the absorption band. By a number of microscopic structural changes are (i)The distance between the layers in a-As2S3 films

having the layer-like structure becomes large when we see the distance by the time average. Simultaneously, the. layers wave at random by making themselves rugged. As a result, (iijthe distance between layers becomes short locally at random In some time, while becomes distant in other time. (iii)The bond length of As-S bonds and (iv)the bond angle between As-S bonds also temporally fluctuate in the layer. Such change and temporal fluctuation of the microscopic structure sulfurs)

make varied,

the potential and result

for the valence in the broadening

electron (lone pair electrons and tailing of the valence

to flatten the absorption band, as suggested in the preceding paper. to the calculation with LCAO method13, (i) and (ii) are not effective ing the character of the valence band. Consequently, the intralayer

in band

According for changfluctuation

of bond lengthsciii) and bond anglesciv) might predominate the thermal effect on the absorption shape. On the other hand, In the case of the photo-induced effects at low temperature such tween the explalned between intralayer

as 80K, if we assume layers are teared off If such as follows. layers, local

the average distortion

Al though the hanced. it does not reaction, photo-induced absorptlon tion of the bond length a layer,

but

uould

former affect

that the bonds due to van der Uaals’ force belocally by the photo-reaction, the result(4) is small tear-off occurs at random at many places

distance (change

between layers would be expanded, and the of the bond length and angles) would be en-

causes the the optical

volume expansion after absorption as mentioned

the photoabove. The

change would be attributed to the latter. The distorand angle would be fixed locally at various locations in Such distortion distributed in not fluctuate temporally.

H. Eguchi

164 layers

uould

bring

nearly

when we see the distortion

er al. / Absorption

the same effect by the spatial

tening of the absorption band, The recovery of the absorption can be explained

by the

restoration

change

in a-As,S,

as the thermal average,

too. band by heating

/ibns

af 80 K

effect

and would films

of the van der Uaal’s

on the potential result

after

the

in the flatillumination

bonds.

REFERENCES 1) S.A.Keneran, Awl. Phys. Lett. 19 (1971) 205. 2) K.Tanaka, J.Non-Crystalline Solids 35 8 36 (1980) 201. 3) K.Tanaka, J.Non-Crystalline Solids 46 (1981) 259. 4) ~H.Hoshi,V.Suzuki and M.Hirai, UV absorption shape between 3.5eV and 5.6eV in very thin a-As2S3 films at 80K, this volume. 5) K.Tanaka, Awl. Phys. Lett. 26 (1975) 243. 6) K.Tanaka, Thin Sollds Films 111 (1984) 195. 7) R.A.Street, Proc. 7th International Conference on Amorphous and Liquid Semiconductors (Edinburgh, 1977). 8) A.V.Kolobov, B.T.Kolomiets, O.V.Konstantinov and V.M.Lyvbin, J.Non-Cryst. Solids 45 (1981) 335, 343. 9) N.Frumer, A.P.Flrth and A.E.Ouen, J.Non-Cryst. Solids 59 8 60 (1983) 921. 10) M.Hirai. V.Suzuki and S.Takeuchi, J. Phys. Sot. Jpn. 53 (1984) 4009. 11) V.Utsugi and V.Mlzushlma, J. Appl. Phys. 49 (1978) 3470. 12) K.Tanaka, Fundamental Physics of Amorphous Semiconductors, Springer Series In Solid-State Sciences, Vol.25, ed. F.Vonezaua (Springer-Verlag, 1981) pp.104-118. 13) T.Shimizu and N.lshi I, J.Non-Cryst. Solids 27 (1978) 109.