A PHOTO-INDUCED MEMORY EFFECT OBSERVED ON In-Si-Se SYSTEM

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A PHOTO-INDUCED MEMORY EFFECT OBSERVED ON In-Si-Se SYSTEM

M. Okuda, T. Matsushita, A. Suzuki, H. Naito, T. Nakau

To cite this version:

M. Okuda, T. Matsushita, A. Suzuki, H. Naito, T. Nakau. A PHOTO-INDUCED MEMORY EFFECT OBSERVED ON In-Si-Se SYSTEM. Journal de Physique Colloques, 1982, 43 (C1), pp.C1-241-C1-246.

�10.1051/jphyscol:1982133�. �jpa-00221790�

JOURNAL DE P H Y S I Q U E

Colloque C I , supple'ment au nO1O, Tome 43, octobre 1982 page C1-241

A PHOTO-INDUCED MEMORY EFFECT OBSERVED ON In-Si-Se SYSTEM M. Okuda, T. Matsushita*, A. ~uzuki*., H. Naito and T. Nakau

College of Engineering, University of Osaka Prefecture, Mozu, Sakai, Osaka, Japan 591

*College ofEngineering,Osaka I n d u s t r i a l University, Nakagaito, Daito, Osaka, Japan 574

Rgsum6

.-

Un effet mgmoire photo-induit (ph6nomSne similaire S la photoconductivit& persistante) a 6t6 trouvs dans des couches polycristallines photo-induitesdu syst5me In-Si-Se. Le processus polycristallin photo-induita St& obtenu en irradiant lf&chantil- lon (intensit6 lumineuse de 6 mw/cm2) 2 70°C. La conductivitg augmente fortement aprgs irradiation par lumisre pulsse (intensit6 de 20 p~/cm2) 2 basse tempgrature (153 K ) et un effet mGmoire apparaft. En augmentant la tempgrature au-dessus d'un point cri- tique (T=263 K), la conductivit6 revient S sa valeur premigre.

Comrne l'effet m6moire photo-induit est reproductible, il peut Stre considgr6 comme un phgnomgne Glectrique et para?t s'expliquer par un modGle de barrigre tenant compte des inhomog6n6it6s diver- ses des couches polycristallines.

Abstract

.-

A photo-induced memory effect (a phenomenon similar to the persistent photoconductivity) was found in the photo- induced polycrystalline films of the In-Si-Se system. The photo-induced polycrystalline process was performed by irra- diating the specimen ( the light intensity is 6 m ~ / c m 2 ) at tem- perature 70°C. The conductivity drastically increased by ir- radiation of the light pulse (its intensity is 20yw/cm2) at low temperature (153 K) and entered a memory state. With increasing the temperature above a critical point (T=263 K), the conductiv- ity returned to the original value. As the photo-induced memory effect is a reproducible phenomenon, it may be considered as an electronic effect and seems to be explained by the barrier model with various inhomogeneities in the polycrystalline films.

1. Introduction.- A photo-induced memory effect of annealed films under illumination is described for the In-Si-Se systemtll.

The Arrhenius' plot of conductivity of the film annealed under

illumination results in a nearly straight line at the cooling process and its activation energy is 0.4 eV. The moment the specimen was irradiated by a light pulse (its intensity is 20 pW/cm2), the conduc- tivity changed drastically from 3xl0-'~(0hm-cm)-' to 6.3x10-~ (ohm-cm)-'.

This low resistivity is a stationary state, that is, a memory state because the value of the conductivity does not change although the temperature increases from T=153 K to T=263 K. As the photo-induced memory effect is a reproducible phenomenon, it may be taken as an electronic effect.

Several physical models for the photo-induced memory effect have been proposed recentlyC2J. First, the persistent photoconductivity is observed in Te-doped AlxGa, -xAs crystal C31. The dominant features

.. - .-

of this type are; (a) apparently enormous Stokes shift, and (b) a very small thermally activated electron-capture cross section for tempera- tures below about 77 K. These features could be semi-quantitatively explained by a somewhat unorthodox configuration coordinate model.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982133

C1-242 JOURNAL DE PHYSIQUE

Second, the barrier model which is induced by the inhomogeneities of the microcrystalline semiconductors is reportedC43.

For the photo-induced memory effect observed in the In-Si-Se system, the barrier model associa.ted with various inhomogeneities will be ac- cepted and we will further examine it.

2. Experimental.- In, Si and Se elements of different atomic percents were first melted in evacuated quartz ampoules at llOO°C for 10 hours.

The ampoules were then quickly quenched to the room temperature. The film specimens were prepared by evaporating, for example, the melt- quenched Inz(Si0~1Se0~9)1-z onto the glass substrate. Its temperature was about 30°C, followed by another deposition of an upper metallic comb-type Au electrode. The deposition rate was 50 x/sec and the pressure was 2x10-STorr. The thickness of the film was about 0.5 to 1.0 pm. The annealing process was performed by irradiating the spec- imen on a heated plate with an incandescent lamp through an IR absorp-

tion filter (its intensity is 6 mW/cmz). The annealing tempera- ture for the Si rich samples was 70°C and that for the In rich ones was 1 0 0 ~ ~ . When the "as deposited" film was heated in darkness, the nucleation for crystallization was formed at the interface between the substrate and the film. The speed of crystallization was very slow. But, under illumination, the nucleation generated at the inter- face rapidly reached the free surface side as a result of fast growth.

Following this, the spectral response of the photocurrent in the In0~1(Si0~1Se0~9)0~9 film before and after the annealing process (for 15 minutes) was measured. The shift of the peak value from 2.68 eV to 2.03 eV was clearly observedl51.

3. Results.- It is shown in Fig.1 that the value of the optical gap E~~~ taken from the intercepts of the (dhY)ll2 vs hY curves extra-

g

polated to d = O , decreases towards lower photon energy with increas- ing illumination time; where d is the absorption coefficient

( E E ~ ~

value of 1.92 eV, 1.80 eV and 1.65 eV were obtained by illumi-

Fig.1. Curves of (I$~v)~/~ ,S Fig.2. Observation of the annealing Wbefore and after annealing effect using X-ray diffraction

for the In0~1(SiO~1Se0~9)0~9 film.

film.

onto the glass substrate varied with the annealing process. This variation was examined using X-ray diffraction. From Fig.2(a) and (b), the occurence of crystallized peaks was rec- ognized in the InO.l(SiOal Se0.9)0.9 film annealed for

15 minutes. No identifi- cation of the crystalline Se and the crystalline Si was obtained. However, it i considered from a comparison of Fig.2(b) with (c) that some of the crystallization effect related to the Si-Se system is certainly induced by the annealing process.

It is shown in Fia.3 that

nation for 0 min., 10 min., Temperature ('C)

and 15 min. duration, re- 0 -60 -80 -100 -120 spectively). The proper-

Arrhenius ' plot of conduc tiv- itY of the film annealed for

15 minutes results in a nearly Fig.3. Photo-induced memory straight line in the cooling'pro- effect. The memory state cess from point A to point B is kept in the region be- (the activation energy is 0.4 eV). tween D and E points.

The moment the specimen was irra- diated by a light pulse (its inten- sity is 20 pw/cm2) at point B in

Fig.3, the conductivity changed

-

drastically from point R to point C

+

after annealing) ties of the InO.l(SiO.l

10-4- Se0.9)0.9 film evaporated

and then gradually settled down to point D with a decay time of few minutes. Point D is a stationary state, that is, a memory state be- cause the value of the conductivity does not change although the tem- perature increases from point D to point E. With increasing temper- ature above point E , the conducti- vity returned to the original value.

Figure 4 shows the transient re- sponse of the photo-induced memory effect at T=153 K. It is clear that the transient response is af- fected remarkably by the wavelength of illumination, and for the short wavelength of light ( h=6000

A;

hV

=2.07 eV), the transient response is rapidly changed. The transit time from low conductivity to the high one is about 1 msec. Also, it is interesting that the conductivity of photo-induced state illuminated by A=6000

A

increases as compared with that illuminated by )\=8000 %1

I I I I I I I 1 .. I

1n0.1(Si0.1Se0.9)0.9 C ( after annealing)

?

E

-

o 6000 light off

x

E

I

I

o

1 2 3 4 5 6 time t (sec) Fig. 4. The transient

response of the photo- induced memory effect at T=153 K.

C1-244 JOURNAL DE PHYSIQUE

( h V =1.55 eV)

.

The conductivity rati.0 before and after illumi- nation by a light pulse

(at T=183 K) for the samples with different components are shown in Fig.5. It is interest- ing that the samples of 1n0.1(Si0.1Se0.9)0.9 and I n o m 2 (Si0.1Se0.9)0.8 show the largest value for the conductivity ra- tio in the In-Si-Se system.

4. Discussion.-

As mentioned above, the photo-induced crystal- lization (the annealing process with light illu- mination) is necessary for the memory effect, because the inhomoge- neities due to photo- crystallization play an important role.

In order to clarify the mechanism of the anneal- ling process, the photo- induced crystallization was observed at the var- ious stage of the anneal- ing process under the illumination or in darkness by the optical microscope. It was rec- ognized from these re- sults that the crystal- lization rate induced by illumination was greater than that for heatinn in

Fig.5. Component dependence of the conductivity ratio before and after illumination by a light pulse (T=183 K).

In d a r k n e s s

Ir!.,,fSb.se,,),

Illuminated

darkness.

-

(3) ~ m i n

{ J) l3"lttl

Figure 6 shows the crys-

tallization process under Fig.6. The crystallization process under the illumination for the the illumination and in darkness

amorphous In 0~1(Si0~1Se0~9)0.9 for the amorphous I n 0 ~ 1 ( S ~ 0 ~ 1 S e 0 ~ 9 ) 0 ~ 9 f ilms.

film. It has been clear that

the nucleation for photo-induced crystallization is smaller and the number of the nucleation is much larger than that in darkness.

A qualitative explanation for these results would appear to derive from the special characteristics of the defects, which is described in terms of the valence alternation pairs by KastnerL63. The positive- ly charged threefold-coordinated C: and the negatively charged, singly- coordinated C; a.toms occur in pairs in the chalcogenide semiconductor.

We shall first consider the case of the amorphous Se to understand the process of the photo-crystallization. Under the illumination, the reaction

may be considered. In due course, the squilibrium between the direct and the reverse processes defining the new structural state of the material is established. We may suggest that the photo-crystalliza- tion proceeds when the direct process is dominant in the reaction (1).

An admixture of the fourfold Si and the threehold In atom to the two- hold Se atom leads to the more complex structure. Thus, it must be considered that these atoms are involved in the process described by the reaction (I), because the Si-Se bonds play a dominant role in the photo-crystallization process in the In-Si-Se system.

There have been many papers of the photo-induced memory effect in the various semiconductor mateials. However, it is accepted in the In-Si-Se system that the observed memory effect is due to the presence of macroscopic potential barriers associated with various polycrystal- line inhomogeneities.

Figure 7 shows the schematic representation of the macroscopic potential barrierC43. In this model, we shall assume that the conduc- tivity is governed by the current flowing across the effective barrier of some typical interface, located at z=0. Here,

qsd

and

ys

are the heights of the equilibrium and nonequilibrium drift barriers, respec- tively. E c and Ev are the conduction and valence band edge.

According to the barrier model, the ratio of the current across the drift

barriers in the residual conductivity state (I,)

to the current in dark-

9 s /-.--

ness (Id) is Ir/Id N

exp (

9

^{sd/ kT)}

.

^When

- - _- - -

ⁱ _-+

^- - ^-

y s d > > k ~ = 0.013 eV

-

_T

- -

(T=153 K), the current ratio 1.-/I, is very

L' U

large and the conduc- tivity change observed in Fig.3 seems to be

well ;nderstood. t

For the photo-induced ^I

-

memory effect in the ^I

In-Si-Se system, it is

z = O

clear that the enhance-

ment of the inhomoge- Fig.7. The schematic representation of the neities due to the photo- m a c r o s c o p i c potential barrier. Here, crystallization and the and are the heights of the equilibrium

(?

^sd

increase of the barrier

height in darkness are and nonequilibrium drift barrier.

necessary for the photo- induced memory effect.

In summary, a photo-induced memory effect is found in the micro- crystalline films of the In-Si-Se system. The conductivity ratio of

a,/

ad

⁼ 2.lXl08 is obtained at T=153 K. As this memory effect is a reproducible phenomenon, it may be considered as an electronic effect.

The observed memory effect will be explained by the barrier model associated with various polycrystalline inhomogeneities.

References

113 A. Suzuki, T.Matsushita, M.Okuda, H.Naito and T.Nakau: Jpn. J.

C1-246 JOURNAL DE PHYSIQUE

Appl. Phys. 2 (1982) 407.

C21 M.K.Sheinkman and A.Ya.Shik: Sov. Phys. Sernicond. 1 0 (1976) 128.

131 D.V.Lang and R.A.Logan: Phys. Rev. B-19 (1979) 101c

1 4 3 V.B.Sandornirski, A.G.Zhdan, M.A.Messerer and 1.G.Gulyaev: Sov.

Phys. Semicond.

2

(1974) 881.

I53 T.Matsushita, M.Okuda, H.Naito, A.Suzuki and T.Nakau: this conf.

Group 8.

263 M.Kastner and H-Fritzsche: Phil. Mag.

3

^{(1978) 199.}