Electric field dependent photodecomposition of a-As2Se3

Solid State Communications, Vol. 13, Pp. 1721—1724, 1973.

Pergamon Press.

Printed in Great Britain

ELECTRIC FIELD DEPENDENT PHOTODECOMPOSITION OF a-As2Se3 W.R. Salaneck and J.S. Berkes Xerox Corporation, Xerox Square

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Wi 14, Rochester, New York 14644, U.S.A.

(Received 20 August 1973 by R.H. Silsbee)

Electric field dependent photodensification of electroded, thin a-As2Se3 films is observed in the field range 0 ~ E ~ 1.4 X iO~V/cm. The applied field decreases the amount of ultimate photodensification, and, a nontransient increase in conductivity of the film occurs in the illuminated area. The field dependent optical absorption coefficient, a(E), cannot account for the decrease in photodensification. An extension to the concept of bond breaking in molecular solids is proposed to account for the observed phenomenon.

INTRODUCTION

compared to those employed by Thornberg, such that

THE PHOTODECOMPOSITION of amorphous (a-) and crystalline As2Se3 and hasstudies, bene studied 1’2 As2S3 In these regionsby Berkes and co-workers. of the materials illuminated with hv ~ band gap light exhibited an increase in optical density. The optical

electric field switching could not occur. EXPERIMENTAL PROCEDURES a-As 2Se3 films about i~in thickness were prepared on NESA glass (chemically coated with tin oxide), Corning Pyrex brand 7059 EC coated glass, common microscopic slides, and KC1 single crystal substrates. Flash and open boat evaporation were employed, while holding the substrates at either 25°Cor 180°C (T Tg). Semitransparent counter electrodes of vapor deposited gold, on the order of several millimeters square were employed on conducting substrate samples. Contact to the Au electrode was accomplished via pressure contact with a fine gold wire. A total of six electroded pin hole free samples and twenty one nonelectroded samples were investigated. A focused HeNe laser was used as the light source. Since at 6328A, a ~ 2 X 104cm’ in a-As 2Se3, the HeNe light is essentially bulk absorbed within the lM film, and the optical transmission can be used as a probe of the optical densification due to exposure. The focused beam measured less than 0.32.mm TheinAu diameter and corresponded to 1 W/cm electrodes, however, were only about250in per cent electroded transmitting, resulting in 1/2 W/cm samples. Since the Au contact area, A 0, was large

darkening in a-As2Se3 was attributed to the reaction As2Se3 x As + As2_~Se3,and then verified by identifying the products of subsequent secondary oxidation reactions. From light intensity and thermal cycling studies, the phenomena was characterized by photodecomposition followed by nucleation and growth of arsenic clusters. The reaction was, under certain conditions, thermally reversible; and increase in temperature resulting in a decrease in effective optical absorption coefficient a. —~—-~‘

Phase separation in a-As2Se3 in the presence of an 3’4 He electric field has beendecomposition studied by Thornberg. proposed a spinodal mechanism which was enhanced by the application of high electric fields. Precipitation of free arsenic was evoked to explain increases in conductivity, leading to a memory switching phenomena. In theupon present investigation, the affect of an electric field the photodecomposition in a-As 2Se3 was observed. Applied voltages were kept small 1721

ELECTRIC FIELD DEPENDENT PHOTODECOMPOSITION OFa-As2Se3

1722

compared with the illumination area,AL, i.e.,AL/AO ~ 2.5 X l0_2, many densification curves were run on the sample. All data was independent of location within any given sample. By using very low level, defocused illumination and/or pulsed illumination, the temperaturethe and electric fieldbe dependence of the optical density of samples could observed without significant photodecomposition.

RESULTS AND ANALYSIS

in the figure, two parameters can be used to characterize the densification. The dgreee of densification [flO) T(°o)]/7’0= AT/T0 = T_, is taken as a measure of the ultimate photodecomposition of the sample. The shape of the 1densiflcation curve has pre. and compared with thebeen silver halide viously discussed process. When a d.c. voltage is applied across the sample, the ultimate amount of optical densification is decreased. The field dependence of ~.. is illustrated for a 0.7p sample in Fig. 2. Additionally, the initial transmission (at t = 0) is electric field dependent. This effect at t = 0 corresponds to a Franz—Keldysh type electric field shift of the optical absorption edge. Data at 6328A, for which a(X) a~,~ 2 X l0~cm~, —

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Dark currents at low voltages (E i0~V/cm) were only slightly larger than those reported on thick a-As 5 on aluminium substrates and with Au 2Se3 films electrodes. At a given d.c. bias, ±V 0,the current is constant with time in the dark, but increases due to optical distributed densification. In the the entire dark, the is urnformly over area,current A 0, of the electrode. The increase in current due to decompo sition is attributed only to the exposed area,AL. The ‘~

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reduced to the optical absorption co..efficient, a(E), is shown in Fig. 3. The effect is reversible and5%) goesin 312 (to within better than approximately at E -_________________________________

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optical densification, is shown Theduring exposure relative increase in current overinanFig. area1.AL, was interrupted several times to illustrate the nonsaturates as the optical densification saturates. Even though the currents are emission limited, to within I per cent of i0, noof photoconductive current transient nature the current increase. Theincrease increase appears at this wavelength in our samples at E 10 V/cm since the light is essentially bulk absorbed.

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A typical optical densification curve is shown for the zero field case in the inset of Fig. 2. As illustrated 0.810

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ELECTRIC FIELD, IO~ V/cm 8______

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FIG. 2. A typical optical densification curve is shown in the inset. The2ultimate densification for a 0.77p ifim is given by T.. and is plotted versus at I ~ 1/2 applied field. W/cm agreement with theoretical predictions.6 The maxi-

° u~ 1

TIME 20 seC/div

FIG. 1. The non-transient nature of the current inci~~ase during optical densification 2. The is shown increaseforinEconductivit’ ~ 1.2 X 1 o~ V/cm of the and filmIin~ the 1/2illuminated W/cm regions is consistent with the precipitation of free arsenic.

mum change in a(E) at £ l0~V/cm is only I per cent of a 0, while total densification leads to an effective change in a0by 2X. —.

indicate Densifi a drastic cationreduction measurements in apparent with near optical u.v. densifilight cation. This occurs because of the decrease in penetration

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ELECTRIC FIELD DEPENDENT PHOTODECOMPOSITION OF a-As2Se3

depth in the u~v.and illucidates the bulk nature of the effect.

1723 I

DISCUSSION The photodensification curves for electroded and non-electroded thin films of a-As2Se3 are essentially similar, (with V0 =0) with the exception that the decrease in transmission is greater for non-electroded samples. Without electrodes, floo)/flO) 0.2, while for electroded samples and zero applied field floe)/flO) 0.5. Thus, the bulk effect is surface influenced. Since the 100A gold electrodes had the same conductivity as those on glass, it is not expected that any chemical or electrochemical reactions occur at the contacts. The ‘encapsulation’ of the sample by dec. troding, has an inhibiting effect on the ultimate photodensification. The temperature at which the substrate is held during vapor deposition of the films, however, does not have any first order affect on the photodensification characteristics, The application of the electric field leads to a reversible shift in optical absorption coefficient at 6328A. Since this effect increases the apparent optical density of the sample while the application of the field decreases the ultimate apparent photodensification, the field dependence of the photodensification is not a direct consequence of increased optical absorption with applied electric field. ‘-‘

Dexter has considered photodecomposition mechanisms in solids, and in particular, bond breaking in molecular crystals.7 During illumination, electrons make optical transitions from a (bonding) or lone pair (nonbonding) states to a~(antibonding) states comprisinc the ‘conduction band’ in As 8 In the presence of light the system is, however,2Se3. no longer in thermodynamic equilibrium. Additionally, the a” states in a-As 9 Therefore bonds remain 2Se3 are highly localized.

102

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APPLIED FIELD V/cm

FIG 3. The reversible electric field dependence of the effective optical absorption coefficient in a-As2Se3 is312. shown. Franz—Keldysh expected to as The A slope in the above shift data is corresponds togo1.43. E ‘broken’ long enough on a time scale consistent with the dynamics of phase separation such that arsenic diffusion and clustering10 and the corresponding optical densification can occur. We propose that an applied electric field then leads to increased transfer probability among a~’states, tending to remove electrons from their initial excitation sites, thus leading to a decrease in bond breaking probability in the molecular solid. Optical densification rates, which are a function of the rate of bond breaking are thus inhibited. -

Thornberg’s mechanism3 is considered inappropriats since applied fields in this case are small compared to fields applied in his case, and such an effect is not observable in the photodensified areas.

REFERENCES I.

BERKES J.S., ING Jr. S. and HILLEGAS W~J.,J.App!. Phys. 42,4908 (1971).

2.

BERKES J.S. and SHORT J.M.,Bull. Am. Ceram. Soc. 51, 338 (1972).

3.

THORNBERG D.D. and WHITE R.M.,J. App!. Phys. 43,4609(1972).

4.

THORNBERG D.D., J. Non-Crystalline Solids to be published.

5.

PM D.M. and SCHARFE M.E.. .1. Non-Crystalline Solids 8—10, 752 (1972).

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ELECTRIC FIELD DEPENDENT PHOTODECOMPOSITIONOF a-As~Se3

6.

FRANZ W.,Z. Naturf. 13,484(1958); KELDYSH L.V.,Zh. Eksperim. Teor. Fiz. 34, 1138 (1958) [Soviet Phys. JETP7, 788 (1958).

7.

DEXTER D.L.,Nuovo am. 32,90 (1964).

8.

DREWS R.E., EMERALD R.L., SLADE M.L. and ZALLEN R., Solid State Commun. 10,293 (1972) and KASTNER M.,Phys. Rev. Lett. 28, 355 (1972). SCHARFE M.E.,Phys. Rev. B12, 5025 (1970). BERKES J.S., SHORT J. and JOHNSON K.,Proc. 5th mt. Conf on Amorphous and Liquid Semiconductors,

9. 10.

(to be published).

Es wurde beobachtet, dass die lichtinduzierte Erhohung der Dichte von dUnnen, mit Elektroden versehenen amorphen As2Se3 Filmen durch angelegte elektrische Felder im Bereich von 0 ~E ~ 1.4 X i0~V/cm vermindert wird. Die Dunkelleitfähigkeit in bestrahlten Proben wird irreversibel erhöht. Die feldabhangige Absorptionskonstante kann diese Resultatenicht erklären. Die Dissoziation chemischer Bindungen in molekularen FestkOrpern wird zur Rationalisierung der beobachteten Ergebnisse vorgeschlagen.

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