Investigation of photodoping kinetics in amorphous Ge-chalcogenides

Journal of Non-CrystallineSolids 97&98 (1987) 1247-1250 North-Holland, Amsterdam

INVESTIGATION OF PHOTODOPING Ge-CHALCOGENIDES R~diger

RESSEL,

GHnther

Dresden University Dresden, GDR*

1247

KINETICS

KLUGE

IN AMORPHOUS

and Peter

of Technology,

SUPTITZ

Department

of Physics,

Photodoping is c o n s i d e r e d as an i n t e r c a l a t i o n reaction. Light absorption stimulates both the generation and the diffus i o n a l t r a n s p o r t of e l e c t r o n / m e t a l - i o n pairs. A p e r c o l a t i o n m e c h a n i s m causes the concentration-dependence of the diffusion coefficients and the appearance of step-like doping profiles. Generation and diffusion are quantitatively characterized. I. INTRODUCTION To the p r o c e s s hides

of p h o t o d o p i n g

contribute

induced

two

generation

metal/doped

main

chalcogenide

the

amorphous

network.

as a p h o t o i n d u c e d

kinetics

its

three

period

period

tion),

the

Here

acceleratory diffusion

of

reaction

the

chalcogenide insertion

is to

into

be identified

of an a m o r p h o u s

sub-

by (I)

an

limited period

solid state

induction

exhib-

a so-called

While

interface

is either

reaction

period,

period.

by the

of the

the

reaction

accele(genera-

limited

by diffusion

of

source

or by ceasing

of

metal

on a q u a n t i t a t i v e silver

characterization

in amorphous

investigations

of

the

germanium doping

stage and of the d i f f u s i o n a l

limited

*8027 Dresden,

second,

of the elec-

doped

or temporary

and a deceleratory

by exhaustion

we r e p o r t on

at

absorption.

kinetics

especially

and,

photodoping

chalcogeradiation-

at the interface

migration

already

type of

stages:

deceleratory

light

a

CTAE +.

is rate

the reactants,

doping

--~

of this special

ratory

actinic

Therfore,

intercalation

distinct

acceleratory

the

permanent

can be described

C~ + e- + Ag + The

through

first

pairs

(photodissolution)

and their

stance I ,2 which

At

quasi-particle-like

pairs

(photodiffusion)

and A s - b a s e d

of electron/silver-ion

radiation-stimulated tron/silver-ion

in Ge-

mechanisms:

stage.

:,:ommsenstr.

13, GDR

0022-3093/87/$03.50 @Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

of p h o t o -

chalcogenides,

rates

during

the

transport

during

the

R. Ressel et al. / lnuestigation of photodoping kinetics

1248

2. EXPERI I,IENTAL For

the

determination

resistance

deposition

layer onto

glass

silver

The

~ to the

silver

thickness

by

characteristics

rates

samples

by means

respectively.

were

thickness

using

of

obtained

the

d

the

were

limited

were (the

3. RESULTS

function

calculated doped

stage

of the

generated

to the m e t a l

j is the

current

stant. The d e p e n d e n c e T

resulting

is

known

with

E B is the b a r r i e r investigating

from

silver-ion

the

concen-

of the i n t e r c a l a t i o n

height

represents

a metal/semiconductor

the electron/silver-ion

= lJAg+[ =

pairs are

density,

C-rAg+

from

(2)

r the d o p i n g

of the e l e c t r o n i c thermionic

rate

current

and C a con-

density

on t e m -

emission-diffusion

theory 6

= C*T2exp(-EB/kT).

height

(3)

and k B o l t z m a n n ' s

the temperature

ing the generation-limited

dependence

constant.

Hence,

of the doping

stage of doping

the

by

rate dur-

effective

barrier

can be determined.

Corresponding

experimental

In(r/T 2) vs.

determined FIGURE

sequence.

(2) into

rAg+(T)

form

layer

transparencies

relative

be

with lJelectronicl

height

could

(micropho-

as a solid solution 5.

barrier

at which in photodoping

perature

into

AND DISCUSSION

The i n t e r f a c e

where

the

diffusion

by c o n s i d e r a t i o n

chalcogenide)

3.1. Determination contact

of the transverse

of the m e a s u r e d

spread

and

of the

E = -0.3(~7.

diffusion

edge tracing technique 4) with a reversed

a deconvolution

product

by a

thermal

transfered

tometric

trations

prepared

calibration-relationship

by investigation

microphotometer

electrical

ratios

obtained After

the

of conventional

The

d = A(8/d-~Z)~' w i t h A = 29.6 nm and The

doping

of a silver- and a thin amorphous GexSel_ x-

substrates

evaporation,

resistivity a

the

m e t h o d 3 w a s applied.

successive flash

of

and

straight

were

lines

barrier heights and pre-factors

1. T h e y

electron

1/T

results

reflect

affinity

the

variation

of G e x S e l _ x

energy

of light e x c e e d s

region

and is sufficient

with

were

in the

obtained.

C * in (3) are given

of the

band

composition

the b a r r i e r h e i g h t to stimulate

represented

gap and

x. The

in the w h o l e

the generation.

The in the

photon visible

R. Ressel et al. / Investigation of photodoping kinetics

10~

~

600

0.4 o ss-evoporetec system

1 b: after exposure

*°t

&'0

x

50

400

0.4[ DI=07 101°~-sm2]

\ t

DU 17 101°Q--~2 !

3.2. Spectral At

the

dependence

interface

ties of gencration Jgeneration diffusion

smaller

k,nm

o!

600

FIGURE 2 Relative spectral sensitivity of photodoping in Oe~r, S e ~ normalized to the light intensity absorbed at t h e interface within the metal

FIGD!IE I Dependence of the barrier height E L and of t~e prefactor C" in (3) on composition

(D:

~

, 30

(x=O)

of the

doping

continuity

and diffusion

rate

coefficient,

silver-ion

limits

concentration).

volume

course

initial

contact

chalcogenide

the a b s o r p t i o n

to the

With increasing

the a c c e l e r a t o r y

the efficiency

met al/doped

The

absorpboth the

of D). sensitivity

2). This

above

extension

transformation

of

of the

contract into a metal/doped increase

at the interface chalcogenide rate

absorption

of

at w a v e l e n g t h s be d i f f u -

also D increases

p e r i o d the s p e c t r a l generation

light

to the metal.

will initially

also in this

of the spectral

chaleogenlde.

- enhances

corresponding

in u n d o p e d the d o p i n g

becomes generation-limited

During flects

(FIGURE

to a p r o g r e s s i v e

and

580 nm is s m a l l

elements

an increasing

within the ehalcogenide

sion-limited. system

relate

metal/undoped

absorption Lecause

of photodoping

of 580 nm is observed we

chalcogenide

above

diffusion

densi-

(zl)

the rate (4). Light

(photoactivation

and the

sensitivity

current

is to be demanded:

tion - within the appropriate the

the

= - Dx=+O(dc/dx)x=+O

c:

~6

X,prn

AT-ion concentrat i o n vs. position in transverse diffusion (de25Se75' exposure: ~76 run; 20.7 kJ/cm ~)

between

generation In

8

= Jdiffusion,x=+O

one of b o t h c u r r e n t s

a wavelength

1249

spectral

and the region.

sensitivity

re-

at the interface

R. Ressel et al. / Investigation of photodoping kinetics

1250

3.3. Investigation of the diffusion-limited In the course

of p h o t o d o p i n g

dc/dx

system becomes diffusion-limited.

In transverse

ments 4 this stage is ensured by the applied the

increase

of

and diffusional

the ratio

between

Although a non-conducting edge

chalcogenide

substrate

by a

solution

of the

switches

between

absorption

a treshold

to t h e r m a l

consists

electron/silver-ion

by

reduced,

generation.

pairs.

The

the

pairs

(light

period,

absorption

by

at the

elec-

interface

within

chalcogenide

the diffuwhich in-

of light a b s o r p -

with transition

to the diffu-

stage and thermal emission may become

sufficient to

photodoping process.

REFERENCES

I)R.

light of the

generating

by a c t i v a t i n g

the undoped

the first three steps. The i m p o r t a n c e

sion-limited

of

transport

influences the photodoping kinet-

induction

for the generation diminishes

carry the

the d i f f u s i o n

importance

hence Cp is a percolation treshold.

sion and by p h o t o - a n n e a l i n g tion

by

which

Cp. In the dark

however

The

the hopping

the metal and the doped chalcogenide), fluences

equation

coefficient

switching of D at Cp we relate to

light absorption

overcoming

tron/silver-ion

diffusion

diffusion

concentration

in a c t i v a t i n g

a percolation mechanism; In summary,

on the pre-history of

a low value D I and a high value DII , when the

reaches

both D I and DII are d i s t i n c t l y

ics

was used typical step-like

copper diffusion 5 the step-profiles

a concentration-dependent

due

area

and to be proportional to t 1/2.

of thermal

be d e s c r i b e d

proceeds

time and

interface

(FIGUI~E 3). The time variation of the

assuming

concentration

diffusion experi-

long doping

the generating

position Xp was found to be depending

As in the case can

and the

cross-section to a value larger than one.

profiles could be confirmed the amorphous

stage

in (z~) d e c r e a s e s

SchOllhorn,

Angew. Chemie 92(1980)1015

2) G. Kluge, phys. star. sol. (a) 101(1987)105

3) D. Goldschmidt J. Non-Cryst.

and

P. S. Rudman,

Solids

22(1976)229

4) M. Yamaguchi, I. Shimizu and E. Inoue, J. Non-Cryst.

Solids 47(1982)341

5) ~. Sebastian, G. Kluge and P. Sflptitz, to be published 6) L. J. Brillson,

Surf. Sci. Reports 2(1982)123