Residual photocurrent decay in amorphous chalcogenides

Journal of Non-CrystallineSolids77 & 78 (1985) 1253-1256 North-Holland,Amsterdam

1253

RESIDUAL PHOTOCURRENT DECAY IN AMORPHOUSCHALCOGENIDES K. SHIMAKAWA Department of E l e c t r o n i c s , Gifu U n i v e r s i t y , Gifu 501-11, Japan The long-term photocurrent decay f o l l o w i n g the steady state p h o t o e x c i t a t i o n was measured in amorphous As2Se3 f i l m as a function of temperature. The data are described e m p i r i c a l l y by the extended exponential law and are explained by d i s p e r s i v e d i f f u s i o n - c o n t r o l l e d recombination of excess Do ; 2D° + D+ + D- . I.

INTRODUCTION A nonexponential residual photocurrent decay (long-term decay) f o l l o w i n g the

steady p h o t o e x c i t a t i o n has been commonly observed in amorphous semiconductors! -3 In hydrogenated amorphous s i l i c o n , the data have been well explained by the Fermi l e v e l analysis by introducing a time-dependent recombination r a t e . 3 In amorphous chalcogenides, no u n i f i e d explanation f o r nonexponential long-term decay e x i s t s . The residual photocurrent decay was measured in amorphous As2Se3 f i l m as a function of temperature.

The decay f i t s

exp(-Ct m), where 0 < m < 1.0. fits

to the same law. 2

tion; dielectric

to the extended exponential law;

The decay observed in glassy Ge-Se system also

This functional form has r e c e n t l y found renewed a t t e n -

response, 4 photoluminescence, 5'6 photocurrent in c r y s t a l l i n e

GaAs, 7 and the dynamics of glassy r e l a x a t i o n . 8 A model is proposed to i n t e r p r e t the observed decay in amorphous chalcogenides.

The m a j o r i t y of excess electrons and holes w i l l

centers under non-equilibrium steady-state c o n d i t i o n .

be located on Do A f t e r stopping i l l u m i -

n a t i o n , Do centers decrease by recombination process; 2D° ÷ D+ + D- ( l o c a l i z e d l o c a l i z e d recombination).

I t is shown t h a t the data are well explained by

introducing d i s p e r s i v e d i f f u s i o n of Do p r i o r to recombination. 2. EXPERIMENTAL Thin f i l m s (~2.0 ~m) of As2Se3 were evaporated onto Corning 7059.

After

evaporation, samples were annealed at 200 °C f o r 30 min in an evacuated chamber.

Planar gap c e l l electrodes using Au contacts were f a b r i c a t e d (gap

spacing l = 50 ~m, gap width w= 6 mm).

Monochromatic l i g h t

mW/cm2) was used to e x c i t e photocurrent. sorption c o e f f i c i e n t is l x 103 cm- l , of c a r r i e r s through the f i l m .

(x=694 nm, 0.6

At t h i s wavelength, the o p t i c a l ab-

which could ensure the uniform c r e a t i o n

The e l e c t r i c f i e l d F is 600 V/cm.

0022-3093/85/$03.30 © Elsevier Science Pubfishe~ B.V. (No~h-Ho~and Physics Pubfishing Division)

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K. Shimakawa / Residual photoeurrent decay

Around this f i e l d , photocurrent Ip(t) is proportional to F. 3. RESULTAND DISCUSSION The decays of photocurrent Ip(t) after stopping steady state illumination at several temperatures are shown in Fig.l (solid circles).

Notethat Ip(t) is

net photocurrent, I p ( t ) = I ( t ) - I d, where I ( t ) is the total current and I d the d.c. current.

Ip(t) is described empirically by the function; A exp(-Ct ~).

This functional form is called the extended exponential.

The rate of change i~

instead i n i t i a l l y rapid, but becomescontinually slower as time progresses. This form has been widely used for describing various type of relaxations in condensed matters; 4-8 particularly for glassy materials. Phtoexcited electrons and holes are in quasi-equilibrium with D+ and Daccording to the reactions D+ + e~----D° and D- + h~---D°, respectively.

The

majority of excess electrons and holes can be located on DO centers under i l l u mination.

As shown in Fig.2, the quasi-Fermi levels Efn for electrons and Efp

for holes are defined, where Wl is the energy needed to take an electron from the valence band (V.B.) to turn DO into D-, and W2 is the energy needed to take an electron from Do to the conduction band (C.B.). 9

The density of Do, n~ ,

is given by

n~ = Ant + nt = Nt exp(-E/kT) ,

(I)

where Ant is the excess densilo_l d

ty of Do, nt the equilibrium density of Do, and ~=Efp-Wl ,

i

and Nt the total density of charged defects (=[D+]+[D-]). Holes could dominate photocurrent since the mobility of

10-12"~

|

323 K 313 303

holes is larger than that of electrons-9

293 K

lO-131

283K 273 K

I

I

I0 I00 Time (sec) FIGURE I Decays of photocurrent Ip(t) after stopping the steady illumination as a function of temperature

K. Shimakawa / Residual photocurrent decay

1255

The density of free holes, p' , is given by p' = Ap + Po = Nv exp(-Efp/kT) ,

(2)

where Ap is the excess density of free holes, Po the e q u i l i b r i u m density of free holes, and Nv the e f f e c t i v e d e n s i t y - o f - s t a t e s of V.B.

Ap is then obtained

from eqs. ( I ) and (2): Ap = Ant Nv exp(-Wl/kT)/N t .

(3)

Equation (3) predicts that photocurrent (= Ap) is proportional to the excess density of Do (Ant). After stopping steady i l l u m i n a t i o n , Ant decreases by recombination process; 2D° ÷ D+ + D-.

I f the i n t e r - p a i r separation of trapped electron-hole is larger

than the i n t r a - p a i r separation, the monomolecular recombination (geminate-like p a i r ) would predominate; pair of Do centers. hole could hop (tunneling) from Do to D-. as

Before recombination, a trapped A trapped electron can be regarded

immobile; coexistence of mobile D°(h) and immobile D°(e) centers.

The Bohr radius for trapped electrons may be smaller than that for holes because the energy depth W2 has been predicted to be larger than WI.9

Dispersive

d i f f u s i o n of D°(h) by tunneling could control the recombination rate b as I0 b : B(T) t - ( l - m ) ,

(4)

where B(T) is the temperature-dependent parameter and m the dispersion parameter.

l.OI

C.B. m

I

w2

0+

I

~

~

Efn

6=0.04

eV

O.l

Efp ,

D

W1 I

V.B. FIGURE 2 Thermal energy levels associated with D+ and D-

I 3.2

I 3.4 IO00/T (K - l )

I 3.6

FIGURE 3 Temperature depen~nc~ of B appeared in the rate b=B(T)t-U -~J

1256

K. Shimakawa/Residualphotocurrent decay

Then the rate equation for excess Ant is given by dAnt/dt = -B(T) t - ( I - ~ ) ~nt .

(5)

Hence the excess density of free holes, Ap , is given as ~p : Ant(O ) exp(-Bt ~/~) Nv exp(-Wl/kT)/N t .

(6)

Note that eq.(6) is just the extended exponential. Solid lines in Fig.l show the calculated results by taking m=O.16, WI=0.52 eV, and some choice of B(T), which replicate well experimental data. plotted in Fig.3. with ~=0.04 eV.

B(T) is

As shown in the figure, B(T) is proportional to exp(-6/kT) This may be the potential barrier for l o c a l i z e d - l o c a l i z e d

tunneling recombination (2D° ÷ D+ + D-). tent with those from other studies. 9

WI=0.52 eV obtained here is consis-

4. CONCLUSION The long-term photocurrent decay in amorphous As2Se3 was well described by the extended exponential; exp(-Ct ~). The dispersive d i f f u s i o n of excess Do centers dominates the recombination process of 2D° ÷ D+ + D-.

This process

could dominate the recombination in other amorphous chalcogenides. ACKNOWLEDGEMENT The author wishes to thank M. Miwa and Y. Yano for t h e i r help in experiment. REFERENCES I) W. Fuhs and D. Meyer, phys. stat. sol. (a)24 (1974) 275. 2) J.M. Chamberlain and A.J. Moseley, Jpn. J. appl. Phys. 21 (1982) 13. 3) K. Shimakawa and Y. Yano, Appl. Phys, Lett. 45 (1984) 862. 4) K.L. Ngai, Comments Solid State Phys. 9 (1979) 127. 5) K. Murayama and T. Ninomiya, Jpn. J. appl. Phys. 21 (1982) L512. 6) K. Shimakawa, Phys. Rev. B3I (1985) 4012. 7) H.J. Queisser, Phys. Rev. Lett. 54 (1985) 234. 8) R.G. Palmar, D.L. Stein, E. Abraham, and P.W. Anderson, Phys. Rev. Lett. 53 (1984) 958. 9) N.F. Mott and E.A. Davis, Electronic Processes in Non-crystalline Materials, 2nd ed. (Oxford University Press, 1979). lO) Z. Vardeny, P. O'Connor, S. Ray, and J. Tauc, Phys. Rev. Lett. 44 (1980) 1267.