Dispersive transport in amorphous arsenic telluride

Journal of Non-Crystalline Solids 59 & 60 (1983) 100%1010 North-Holland Publishing Company

1007

DISPERSIVE TRANSPORT IN AMORPHOUSARSENIC TELLURIDE Babar A. KHAN, Peiguong BAI, Ai-Lien JUNG, and David ADLER Center f o r M a t e r i a l s Science and Engineering, Massachusetts I n s t i t u t e of Technology, Cambridge, MA 02139, U.S.A. We have c a r r i e d out t i m e - o f - f l i g h t and t r a n s i e n t f i e l d - e f f e c t measurements on amorphous arsenic t e l l u r i d e (a-As2Tea). The results are i n t e r p r e t e d in terms of m u l t i p l e trapping by an.exponenti~l d i s t r i b u t i o n of l o c a l i z e d states. We show t h a t the long t r a n s i e n t s often observed in f i e l d - e f f e c t measurements can be understood by taking i n t o account the collapse of the applied f i e l d towards the gate oxide. I f t h i s e f f e c t is properly analyzed, t r a n s i e n t f i e l d - e f f e c t experiments can be used to study dispersive t r a n s p o r t in smallgap m a t e r i a l s , f o r which t i m e - o f - f l i g h t measurements are very d i f f i c u l t . 1. INTRODUCTION A great deal of work has been c a r r i e d out on dispersive t r a n s p o r t in a-As2Se 3 in which the model of m u l t i p l e trapping (MT) by an exponential d i s t r i b u t i o n of l o c a l i z e d states has been well established. 1

Less work has been done on

a-As2Te 3 and i t s r e l a t e d a l l o y s , m a t e r i a l s which are much more conductive under o r d i n a r y conditions. 2

However, these m a t e r i a l s are the only chalcogenide a l l o y s

in which the f i e l d e f f e c t has been measured, although such experiments are chara c t e r i z e d by very slow (~minutes) t r a n s i e n t s even under dc bias. 3 This is puzz l i n g , since a-As2Se 3 and a-As2Te 3 are both chalcogenides and thus would be expected to have very s i m i l a r e l e c t r o n i c s t r u c t u r e s . 4

We thus attempted to

e l u c i d a t e the e l e c t r o n i c properties of a-As2Te 3 by carrying out both t i m e - o f flight

and t r a n s i e n t f i e l d - e f f e c t

measurements on s i m i l a r l y prepared samples.

2. TIME-OF-FLIGHT EXPERIMENTS Ideally, time-of-flight

(TOF) experiments should be c a r r i e d out with blocking

contacts, so t h a t no charge i s i n j e c t e d during the d r i f t

process. 5

However,

this necessitates completion of the measurements in less than the t i ~ ,

z = RC, 6

required f o r the applied f i e l d to collapse to the regions near the contacts. a-As2Te 3, R is s u f f i c i e n t l y

In

low that TOF measurements are o r d i n a r i l y possible

only below 13OK. Since we wanted to study dispersive t r a n s p o r t over a broader temperature range, we could not use blocking contacts, but minimized i n j e c t i o n e f f e c t s by r e s t r i c t i n g experiments to the region in which the dark current is much less than the photocurrent. Amorphous As2Te3 f i l m s about lO~m t h i c k were r f - s p u t t e r e d onto polished aluminum substrates in an Ar atmosphere at 7mT and 50W using a I" diameter t a r g e t . 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan

B,A. Khan et al. / Dispersive transport in amorphous arsenic telluride

1008

The sputtering rate was approximately 4000 ~ 1 hr.

Semitransparent Au contacts

about 2 mm in diameter were evaporated onto the a-As 2 Te3.

TOF measurements

were c a r r i e d out as p r e v i o u s l y described. 5 The results over the temperature range I06-151K

are shown in Figure I. 1,5

I t is c l e a r t h a t the data are consis-

t e n t with the MT model.

tO3

,0

~

I

I

~

I

f

I

Time of Fhqh~ 10.5

I0 2 ~

294K

10 6 ~0-7 (z:

=o-s l o-g

I I8 K "•@,•"O

oc

,6,c

jO-3F 10~4

IO-dl

=0-s 0

10.8

%

.

t88 K

%1o6K

10_7

10.6

iO.S

10.4

TIME

10_3

i0_ 2

L 10- 4

i0. I

(SEC)

FIGURE l Time of f l i g h t transients at various temperatures. The current decreases as a power law and the slope decreases with increasing T.

J 10-3

I i0 "2

I i0-1

I I0 0

I

TIME

(SECS)

fOI

I I0 z

I 10 5

i0 4

FIGURE 2 Displacement current t r a n s i e n t s at various temperatures. The sharp f a l l o f f occurs when the f i e l d collapses across the oxide.

3. FIELD-EFFECT EXPERIMENTS To f u r t h e r t e s t the MT model over a broad time i n t e r v a l , we f a b r i c a t e d field-effect

t r a n s i s t o r s (FETs), using the inverted geometry described by Frye

and Adler. 3 A 5000 ~ oxide f i l m was t h e r m a l l y grown on h e a v i l y doped p-type Si (0.005-0.016 52-cm) substrates.

A1 was evaporated and a l l o y e d onto the

other side o f the substrate to provide a good gate contact.

Photolithographic

techniques were used to form an i n t e r d i g a t a t e d pattern on a Mo f i l m sputtered on the oxide; t h i s formed the source and drain of the FET. The a-As 2 Te 3 f i l m was then r f - s p u t t e r e d on top of the Mo contacts. Transient FET experiments were c a r r i e d out by applying a constant voltage between the source and drain and step voltage to the gate.

The displacement

current component was measured independently by applying a negative voltage step between the gate and drain and observing the response across a series resistance (see Fig. 2).

The actual t r a n s i e n t FET response is shown in Fig. 3.

B,A. Khan et al. / Dispersive transport in amorphous arsenic telluride

1009

1.0 0.9 lo 3[ I

!

• T 7 ~ , ~ Monomoleculor

i

X

I

q----

0.8

.................

o Transient Field Effect z~ Time of Flight

0.7 Q

0.5

297K ~w I 0 -

0.6

O4

232K

0.3 ~

J

"~'~=d

3 ~

9

'2~

K

0.2 O.

I0 4~_ I

~0 51 iO-4

L 10-3

i I0-2

I 10+I

I I0 0

I I01

I 10 2

i I0 3

104

I

I

I00

200

300

T(K)

TIME (SECS)

FIGURE 3 F i e l d e f f e c t t r a n s i e n t s at various temperatures. The f i r s t kink is the monomolecular recombination time. The second kink is the time at which the f i e l d collapses to the oxide.

FIGURE 4 Dispersion parameter m vs T obtained from the data of Figure 1 ( t r i a n g l e s ) and Figure 3 ( c i r c l e s )

4. DISCUSSION I t is c l e a r from Fig. 3 t h a t two kinks e x i s t in the t r a n s i e n t FET response. The two regions before and a f t e r the f i r s t

kink e x h i b i t t - ( I - ~ )

decays, c o n s i s t e n t with MT by an exponential d i s t r i b u t i o n FET geometry, the f i r s t

kink could be e i t h e r the t r a n s i t

and t -(l+m)

of t r a p s . 1

In the

time or the monomole-

c u l a r recombination time, 1 but since i t does not vary with s o u r c e - t o - d r a i n v o l tage, we can i d e n t i f y

i t as the l a t t e r .

This kink is not e v i d e n t in Fig. 2

because the MOS geometry tends to enhance the g a t e - t o - d r a i n c u r r e n t over t h i s region of time.

The d i s p e r s i v e t r a n s p o r t probably r e s u l t s from the k i n e t i c s of

the excess free holes induced near the oxfde by the negative gate voltage step. However, another p o s s i b i l i t y

is t h a t hole accumulation regions near the source

and drain contacts e x i s t and the holes d r i f t of the gate v o l t a g e . possibilities.

towards the oxide a f t e r a p p l i c a t i o n

Experiments are under way to d i s t i n g u i s h between the two

In e i t h e r event, the values o i

~ = T/To obtained from the FET

measurements agree with those found from TOF experiments (see Fig. 4). A f t e r a time,

T = RC, where R is the s o u r c e - t o - d r a i n r e s i s t a n c e and C is

the MOS capacitance, the a p p l i e d gate f i e l d the oxide.

has collapsed to the region near

Since the second kink in Fig. 3 v a r i e s with temperature i n v e r s e l y

1010

B.A. Khan et al. /Dispersive transport in amorphous arsenic telluride

to the dark c o n d u c t i v i t y of a-As 2 Te 3, we can i d e n t i f y i t with ~ . longer times, the slow decay p r e v i o u s l y discussed is evident. 3 t h i s regime does not e x h i b i t any simple power-law behavior.

At s t i l l

The current in

This behavior can-

not be explained in terms of any continued exponential density of states since i t is s t i l l equal to

evident at times corresponding to energies of about 0.9 eV, almost the o p t i c a l gap of a-As 2 Te 3.

One possible explanation is the e x i s -

tence of a p o t e n t i a l b a r r i e r to n e u t r a l - d o n o r i n t e r c o n v e r s i o n , as p r e v i o u s l y proposed by Frye and Adler. 3 Another p o s s i b i l i t y is an increase in the t r a p release energy r e s u l t i n g from a local r e l a x a t i o n around trapped c a r r i e r s . 7 any event, the f u l l

In

three-dimensional problem in the presence of a c o l l a p s i n g

f i e l d must by solved before a complete q u a n t i t a t i v e understanding of these effects can be a t t a i n e d . 5. CONCLUSIONS We have shown t h a t t r a n s i e n t FET experiments provide a s t r a i g h t f o r w a r d means f o r observation of sidpersive t r a n s p o r t in a-As 2 Te 3.

The values obtained f o r

are the same as in TOF measurements and the monomolecular r e l a x a t i o n time was e a s i l y i d e n t i f i e d . ACKNOWLEDGEMENT This research was sponsored by the United States National Science Foundation under Grant No. DMR 81-19295. REFERENCES I) J. Orenstein and M. Kastner, Phys. Rev. L e t t . 46 (1981) 1421. 2) D. Adler, J. Phys. (Paris) 42, (1981) C4-3. 3) R.C.Frye and D. Adler, Phys. Rev. B24 (1981) 5812. 4) M. Kastner, D. Adler, and H.H.Fritzsche, Phys. L e t t , 37 (1976) 1504. 5) B.A. Khan, M. Kastner, and D. Adler, Solid State Commun, 45 (1983) 187. 6) Under some c o n d i t i o n s , the time f o r the f i e l d to collapse is longer than RC. See M. Kastner, Solid State Commun. 45 (1983) 191. 7) O. Adler, Solar Cells 9 (1983) 133.