271 OPTICAL PROPERTIES AND PHOTOINDUCED ... - Science Direct

Thin Solid Films, 66 (1980) 271-279 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

271

OPTICAL PROPERTIES AND PHOTOINDUCED CHANGES IN AMORPHOUS As-S FILMS KEIJI TANAKA Department of Engineering Science, Faculty of Engineering, Hokkaido University, Sapporo 060 (Japan) (Received April 20, 1979; accepted July 23, 1979)

The dependence on composition of optical properties and photoinduced changes in thin films of As~S~o0_~ with x between 15 and 45 were investigated. The optical band gap and the single oscillator fitting constants show extrema at the stoichiometric composition x = 40 and they vary almost linearly with sulphur content in specimens containing an excess of chalcogen. In contrast, irreversible and reversible photoinduced changes of the optical properties accompany a characteristic transition at x ~ 30. This suggests that the origin of these effects is attributable to transformations of As--S and S--S bonds in films containing an excess of chalcogen. This speculation is supported by thermal analyses.

1. INTRODUCTION There has been considerable interest in the development of amorphous chalcogenide semiconductors for optical applications, such as lenses mad windows in infrared optics, waveguiding materials, photoacoustic devices and image recording media. This interest is augmented by the fact that the materials are physically robust and relatively cheap in comparison with single-crystal specimens. Moreover, the relevant properties can be altered continuously by compositional changes, since there are no severe stoichiometric restrictions. Among the many binary alloys, the As-S system is one of the most interestin~ partly because stable bulk specimens ofAsxS 1o0 -x can be formed over a fairly wide range of composition 1, namely 5 ~ x ~ 43. In addition, the thin films of this material exhibit remarkable photoinduced effects 2. Although many reports have been published on the optical properties and photoinduced changes in AsxSlo0-x, most of them have dealt with the stoichiometric composition As4oS6o. A few papers 3-7 are available on the influence of composition on the photoinduced changes but many unresolved problems remain. In a previous paper 7, the present author investigated the compositional variation of the refractive index and its photoinduced changes in ASxSloo_ ~ films. There seemed to be a characteristic boundary at x ~ 30. The purpose of the present work is therefore to extend this investigation and to discuss the photoinduced changes in connection with optical properties in AsxS10o_ ~ films. The results are considered in terms of the underlying microscopic structural configurations of these solids. Another purpose is to report the optical properties as a function of x.

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It may at this point be worth summarizing some physical properties of these compounds in relation to their compositional variation. For amorphous specimens, standard procedures for determining the probable atomic configuration, which are based on chemical and thermal analyses, have been applied to As-S glasses by several authors 1'8'9. Myers and Felty 1 have proposed that for glassy As-S specimens single sulphur rings exist in compositions containing excess chalcogen, whereas in chalcogen-deficient compositions As4S4 molecules occur. Tsuchihashi and Kawamoto 8 and Maruno and Noda 9 have confirmed this speculation. Raman and IR studies 1°-13 have also partially reconfirmed this proposal. Furthermore, it has been established that most physical properties exhibit extrema at the stoichiometric composition As40S60, e.g. a minimum of the electrical conductivity14 and a maximum of dielectric constant 15. These observations indicate that, in terms of the distribution of bonds, heteropolar As--S bonds are in principle favoured at all compositions. Additionally, S--S bonds exist in specimens with an excess of chalcogen and As--As bonds in specimens deficient in chalcogen. As regards the optical properties, measurements of the refractive index 7' 8 and optical absorption16 have been reported. From the theoretical standpoint, the electrical conductivity17 and electronic structure18 have been investigated. It should be noted that the structure and properties of the materials are sensitive to the method of preparation. For instance, it is well known that evaporated films have properties which differ markedly from those of the corresponding bulk glasses 2' 19.20. The existence of structural differences due to sample preparation methods or to treatment of the sample after preparation is essentially the result of the inherent nature of amorphous materials, namely quasiequilibrium or metastability of the state. One of the most dramatic manifestations of this nature is that of photoinduced structural changes 2 which unavoidably accompany certain modifications of the physical properties such as the optical constants.

randomness

Fig. 1. Schematic diagram of photoinduced transformations.

It has been shown that the changes can be classified into three processes 6' 21. The features can be illustrated schematically, as shown in Fig. 1. In this figure four quasi-equilibrium states I, X, Y and Z are plotted according to their internal energy and structural randomness. I and Y denote the states of as-deposited and annealed films. X refers to a film which was exposed to band gap illumination and then stored

OPTICAL PROPERTIES OF AMORPHOUS

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in the dark for sufficient time. Films illuminated with light of energy above the band gap are denoted by Z. Solid lines represent physical changes induced by exposure to radiation with energy above the band gap and changes induced by heat treatment are represented by broken lines. The transition from I to X is an irreversible change, whereas the transition between X and Y is reversible since these states can be interconverted by annealing and exposure to radiation. It is noted 2~ that definite differences exist, as indicated by X and Z, between a film under illumination and a film which was illuminated in advance and stored in the dark. Furthermore, the characteristics of a film which was irradiated by band gap illumination can be modified by subsequent illumination at an energy lower than that of the band gap of the material under study6. These changes are termed dynamic changes and the relation between these and the reversible changes will be discussed elsewhere 22. 2. EXPERIMENTAL The method of preparation is as follows: bulk glasses of AsxStoo_ x were synthesized in vacuum-sealed ampoules by heating the elements at 500-700 °C in a rocking furnace with subsequent quenching. Thin film specimens were prepared from the crushed ingot by evaporation in a vacuum of 1 x 10- 3 Pa onto microscope slides. The boat temperature was controlled so that the deposition rate was about 5 A s- t, irrespective of the composition. The arsenic:sulphur ratios of the evaporated films were investigated by electron microprobe X-ray analyses with an accuracy of + 2 at.%. Except for compositions with x > 40, the films become richer in arsenic than the parent bulk material~ 9. For measurements of the refractive index dispersion and thickness of the films, an optical instrument based on the prism coupling method z3 was constructed. Low intensitylight from a xenon lamp or a He-Ne laser is employed in this investigation. The accuracies are 0.001 and 0.001 I~m for the refractive index and film thickness 24. Optical absorption coefficients were determined from the transmitted light intensity for several films of different film thickness. Subsidiary thermal measurements using differential scanning calorimetry (DSC) were also performed. Band gap irradiation at a photon energy of 2.81 eV was obtained from a He--Cd laser. Heat treatments were performed in a flowing argon ambient for 1 h at the reversible temperature, which had been determined in previous work 7. 3.

RESULTS AND DISCUSSION

3.1. Optical absorption and refractive index dispersion In this section, optical properties of annealed AsxSloo_x films are described. It is to be noted that the structure of the annealed specimens has been assumed to be similar to that of the corresponding bulk glasses 2. Figure 2 shows the optical absorption ct in the region of band edges as a function of the photon energy hog. It is well established 25 that the relation (cthog)1/2 oz h t o - C holds for A%oS6o films, where C is a constant. This relation is confirmed in Fig. 2. However, in cases of sulphur-rich specimens, e.g. AstsSas, the experimental results are fitted better by the relation 0thtooc hog-C. Such a functional dependence has been observed in amorphous selenium and it has been

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suggested that its origin is connected with the one-dimensional nature of the atomic configuration 26. This explanation can be applied to the present results for sulphurrich specimens, sincq, sulphur exists in twofold coordination similarly to selenium. The transition from the square root to the linear dependence corresponds to the compositional variation in the excess chalcogenides. Thus, it might be reasonable to define the optical band gap energy E~o by means of (CthtD) 1/20E h(.0- E~o for AS4oS6o and otha) oc h c o - E s o for sulphur (see Fig. 4). It is interesting that for As4aSs7 the linear relation holds better. The present results are in agreement with the data of Kosek-and Tauc 16 on the Urbach region.

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Fig. 2. Spectral dependence of the absorption coefficient ~t for annealed AsxSloo_x films plotted as (ethos)1/2 vs. hoJ (solidlines)and as otho~vs. h(o (brokenlines).

Fig. 3. Refractiveindexdispersionin annealedAS~Sloo_x films. The refractive index dispersion n(og)of amorphous materials can be fitted by the Wemple-DiDomenico relation 27 /12(O)) = 1 + EdEo/{Eo 2

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where E o and E d are single oscillator fitting constants which measure the oscillator energy and strength, respectively. By plotting (n 2 -- 1)- 1 against (h~o)2 and fitting a straight line as shown in Fig. 3, E d and E o can be determined directly from the slope (EdEo)-1 and the intercept E o / E d on the vertical axis. A negative curvature deviation 27 which is observed in the high energy portion in Fig. 3 is due to interband absorption (see Fig. 2). The dependences of the optical band gap Ego, the single oscillator energy E o and its dispersion energy E d on composition are plotted in Fig. 4 using several sets of published data 16' 28-34. The salient features of these results are the maximum in E~ and the minima in E o and EgO at the stoichiometric composition. The correspondence between E o and Ego over the compositional range is apparent and is expressed functionally as E o ~ 1.9 Ego. It has been assumed that E o for these chalcogenides approximates closely to the peak in e2(o~) and can be identified with

OPTICAL PROPERTIES OF AMORPHOUS

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the mean energy of transitions from the valence band of the lone pair state to the conduction band state 27. This is also confirmed in the present results, i.e. E o for As4oS6oand an extrapolated E0 for sulphur are in good agreement with e2(co) data in ref. 35 and in refs. 30 and 36, respectively. Wemple 27 has also pointed out that E d obeys the simple empirical relation E a = flNcZaN e eV (2) where/~ is a constant, No the number of the nearest neighbour cations to the anion and Ne the total number of valence electrons per anion. In the amorphous As-S system, it is reasonable to assume the proportionality E a oc Nc, in which N~ ~ 3 for As4oS6o and 2 for sulphur. The present results agree closely with this quantitative prediction and this therefore confirms the validity of eqn. (2) in the ehalcogenide system. It is in turn concluded that the variation of E a with composition in the As-S system is due to the nearest neighbour atomic configuration. The observed prominent features in Ego, E o and Ea, i.e. that the quantities have extrema at the stoichiometric composition and depend linearly on x for specimens with an excess of chalcogen accord well with compositional trends of other physical properties such as the glass transition temperature1' s, the dielectric constant 15 and the electrical conductivity 14. This reconfirms that strong As--S bonds are favoured in these specimens.

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Fig. 4. Compositional trends of E~., E 0 and E a. The present results are indicated by open circles. Also plotted are data obtained by various authors: a, Glaze et al. 2s; b, Rodney et al. 29 ; c, Cook and Spear 3°; d, Kosek and Cermak31; e, Kosek and TauC6; f, Street et al?2; g, DeNeufville et al.33; h, Gottlieb and I s a a c s 3'*.

Fig. 5. Compositional variation of irreversible (O) and reversible (O) changes AEso, AEo and AEd in the optical energy gap Ego, the single oscillator energy E o and its dispersion energy E a.

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3.2. Irreversible and reversible photoinduced changes

Results on irreversible and reversible changes in the refractive index measured with 1.96 eV light have been reported in the previous publication 7. We shall discuss here their dispersion characteristics. With the aid of the Wemple-DiDomenico relation of eqn. (1), the irreversible and reversible changes in the refractive index dispersion can be analysed into changes in E o and E d. The results are depicted in Fig. 5. It should be noted that although the absolute magnitudes of E o and E d vary from sample to sample, as shown in Fig. 4, the photoinduced changes AE o and AE~ in a specimen are well defined. Photoinduced changes AEgo in the band gap energy are also shown in Fig. 5. These can be determined by the method described in Section 3.1, since the shape of the absorption curve, such as that shown in Fig. 2, does not alter with illumination or annealing. It is evident in Figs. 4 and 5 that the compositional variations of the changes AEo, AEgo and A E a are unrelated to those of E0, Ego and E a. Furthermore, it is clear that AEgo and AE o are not always correlated with each other, in contrast to the close connection between EgOand E o. Rather, it seems that AEgo is influenced by both AE o and A E a. There are regions, however, in which AEd ~ 0, and in these compositional ranges the approximate proportionality AEgo oc AE o holds. Comparing the present results with those of ref. 7 we also confirm another proportionality for change An in the refractive index, namely An ~: - A E o, ~¢hich can be derived analytically from eqn. (1) with the assumption AE d = 0. The detailed shapes of the curves in Fig. 5 are of some interest. It is noted that the reversible - A E o curve has a minimum at x ~ 35 and that the reversible AE d curve increases as the concentration of sulphur increases above that composition. The results for the irreversible changes are not very clear, although a minimum in A E a is observed at x ~ 30 and at this composition an inflection point exists in the characteristics of AE o. For the irreversible process, the phenomenon of so-called photodepression is observed 5' 21, 3 7. Its dependence on composition is shown in Fig. 6. A result obtained by Shimizu and Fritzsche 37 is also indicated in this figure. The scatter from sample to sample is fairly large but an extremum at x ~ 30 is beyond doubt. These observations suggest that photoinduced changes in As-S films cannot be ascribed only to As--S bonds. If they were, the changes should have a linear, or at least monotonic, dependence on the arsenic content. Therefore, the present results imply that the changes originate from transformations of both As--S and S--S bonds. To investigate this hypothesis further, thermal analyses by DSC were performed with films evaporated onto aluminium foils. Typical results for As 21$79 films are shown in Fig. 7. In this figure, small endothermic peaks around 110 °C are assigned to the melting of sulphur microcrystals 1' 38. However, the existence of this peak was not reproducible from sample to sample, in contrast with the observations of Peason and Bagley 3a on hot-pressed As7593 specimens. More prominent features in Fig. 7 are that the as-deposited film shows two broad endothermic peaks, while the illuminated and annealed films reveal a single peak. It seems that the width of the peak in the illuminated film is broader than that for the annealed film. It is tempting to speculate that these broad peaks are caused by sulphur polymerization, the temperature of which decreases abruptly from 160 to 120 °C at around As2S98with increasing arsenic content in glassy specimens 1' lo.

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OPTICAL PROPERTIES OF AMORPHOUS A s - S FILMS

Thus, the present results can be understood as follows. In the as-deposited films sulphur molecules are inhomogeneously mixed up with arsenic atoms, i.e. sulphur atoms are likely to be clustered. Hence, the higher peak in Fig. 7(a) is due to the clustered sulphur, and the lower to the As--S network. Heat treatment or band gap illumination makes the structure more homogeneous. Thus, the single peak due to sulphur polymerization of the As--S network appears. Differences in the shapes for the annealed and the illuminated films indicate that certain structural changes of sulphur bonds are also included in the reversible transformations. It follows that configurational change of sulphur bonds is one of the mechanisms of the photostructural transformations in sulphur-rich As-S films. This conclusion is consistent with the results of the optical measurements.

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Fig. 6. Compositional variation of irreversible photodepression, i.e. the irreversible thickness change divided by thickness. A result by Shimizu and Fritzsche 37 is denoted by a solid circle. Fig. 7. DSC thermograms for As21$79 films: (a) as-deposited; (b) illuminated; (c) annealed. The data were obtained with a heating rate of 10 °C min- 1.

The real vapour-deposited films are almost certainly composed of a mixture of more or less cross-linked regions, together with individual molecular species. Furthermore, the molecular species of sulphur are numerous 39. Therefore, it seems that there is no simple method of characterizing the photoinduced configurational changes of sulphur bonds. It should be mentioned, however, that traces of sulphur molecules are detected when x ~ 30 by thermal 1 and chemicals analyses as well as Raman scattering experiments t°' 12. Hence transformations of sulphur bonds, e.g. between ring and chain configurations, are envisaged. It is plausible for the photoinduced changes that when x ~ 30 transformations of S--S bonds are dominant, while As--S bonds play an essential role in specimens with x > 30. It is to be noted that in the specimens with x ~ 30 an annealing at slightly below the glass transition temperature did not produce any appreciable change in the DSC thermograms. This fact agrees with the anomalous behaviour ~ of the reversible temperature, i.e. the temperature is nearly constant at around 110 °C when x ~ 30. In consequence, it is reasonable to understand that the reversible temperature at this composition corresponds to the melting of sulphur crystallites1' 3s.

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4. SUMMARIZING REMARKS

The compositional trends of the optical properties and the p h o t o i n d u c e d changes in a m o r p h o u s A s - S films have been investigated. The results have been discussed in terms of the underlying microscopic structural configuration. The dependences of optical properties, i.e. the optical b a n d gap energy and the single oscillator fitting constants, on composition show extrema at the stoichiometric composition As40S60. In specimens containing an excess of chalcogen the optical properties change continuously and depend linearly on the arsenic content. This feature is consistent with the dependence of other physical properties on composition and reconfirms the chemical preference for A s - - S bonds. The irreversible and reversible p h o t o i n d u c e d optical changes have been discussed in connection with the dependence on composition. Unlike the optical properties, the p h o t o i n d u c e d changes are n o t linearly dependent on the composition. Characteristic transitions exist at a r o u n d ms30STo. A contribution of S - - S bonds to the p h o t o i n d u c e d effects in sulphur-rich specimens is p r o p o s e d to explain this feature. This speculation is consistent with the results of thermal analyses. ACKNOWLEDGMENTS The a u t h o r would like to thank Professors A. O d a j i m a and K. M u r a t a for their continuous support and Professor Y. O h t s u k a for helpful comments. This work is partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education. REFERENCES 1 M.B. Myers and E. J. Felty, Mater. Res. Bull., 2 (1967) 535. 2 J.P. DeNeufville, in E. O. Seraphin (ed.), Optical Properties of Solids--New Developments, NorthHolland, Amsterdam, 1976, p. 437. 3 R.G. Brandes, F. P. Laming and A. D. Peason, Appl. Opt., 9 (1970) 1712. 4 Y. Ohmachi and T. Igo, Appl. Phys. Lett., 20 (1972) 506. 5 M. Kasai, H. Nakatsui and Y. Hajimoto, J. Appl. Phys., 45 (1974) 3209. 6 K. Tanaka, Solid State Commun., 28 (1978) 541. 7 K. Tanaka and Y. Ohtsuka, Thin Solid Films, 57 (1979) 59. 8 S. Tsuchihashi and Y. Kawamoto, .L Non-Cryst. Solids, 5 (1971) 286. 9 S. Maruno and M. Noda, J. Non-Cryst. Solids, 7 (1972) 1. 10 A.T. Ward and M. B. Myers, .L Phys. Chem., 73 (1969) 1374. 11 G. Lucovsky, F. L. Galeener, R. H. Geils and R. C. Keezer, in P. H. Gaskell (ed.), The Structure of Non-Crystalline Materials, Taylor and Francis, London, 1977, p. 127. 12 P.J.S. Ewen, M. J. Sik and A. E. Owen, in P. H. Gaskell (ed.), The Structure of Non-Crystalline Materials, Taylor and Francis, London, 1977, p. 231. 13 A. Bertiluzza, C. Fagnano, P. Monti and G. Semerano, J. Non-Cryst. Solids, 29 (1978) 49. 14 R. B. South and A. E. Owen, in J. Stuke and W. Brenig (eds.), Amorphous and Liquid Semiconductors, Taylor and Francis, London, 1974, p. 305. 15 G. Lucovsky, R. H. Geils and R. C. Keezer, in G. H. Frishat (ed.), The Physics of Non-Crystalline Solids, Trems-Tech., Zellerfeld, 1977, p. 299. 16 F. Kosek and J. Tauc, Czech. J. Phys. B, 20 (1970) 94. 17 K. Shirakawa and S. Nitta, Phys. Rev. B, 17 (1978) 3950.

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R.M. White, J. Non-Cryst. Solids, 16 (1974) 387. A.J. Apling, A. J. Leadbetter and A. C. Wright, J. Non-Cryst. Solids, 23 (1977) 369.

R. A. Street and A. D. Yoffe, J. Non-Cryst. Solids, 8-10 (1972) 745. K. Tanaka and Y. Ohtsuka, J. Appl. Phys., 49 (1978) 6132. K. Tanaka, J. Non-Cryst. Solids, to be published. F. Zernike, in T. Tamir (ed.), Integrated Optics, Springer, Berlin, 1975, p. 202. K. Tanaka, Appl. Phys. Lett., 34 (1979) 674. J. Tauc, in J. Tauc (ed.), Amorphous and Liquid Semiconductors, Plenum, London, 1974, p. 159. E.A. Davis and N. F. Mott, Philos. Mag., 22 (1970) 903. S.H. Wemple, Phys. Rev. B, 7 (1973) 3767. F.W. Glaze, D. H. Blackburn, J. S. Osmalov, D. Hubbard and M. H. Black, J. Res. Natl. Bur. Stand., 59 (1957) 2774. W, S. Rodney, I. J. Malitson and T. A. King, J. Opt. Soc. Am., 48 (1958) 633. B.E. Cook and W. E. Spear, J. Phys. Chem. Solids, 30 (1969) 1125. F. Kosek and J. Cermak, Cesk. Cas. Fyz., ,4 19 (1969) 271. R.A. Street, T. M. Seade, I. G. Austin and R. S. Sussmann, J. Phys. C, 7 (1974) 1582. J.P. DeNeufville, R. Seguin, S. C. Moss and S. R. Ovshinsky, in J. Stuke and W. Brenig (eds.), Amorphous and Liquid Semiconductors, Taylor and Francis, London, 1974, p. 305. M. Gottlieb and T. J. Isaacs, Appl. Opt., 17 (1978) 2482. R.E. Drews, R. L. Emerald, M. L. Slada and R. Zallen, SolidState Commun., 10 (1972) 293. W.R. Salaneck, N. O. Lipari, A. Paton, R. Zallen and K. S. Liang, Phys. Rev. B, 12 (1975) 1493. I. Shimizu and H. Fritzsche, J. Appl. Phys., 47 (1976) 2969. A.D. Peason and B. G. Bagley, Mater. Res. Bull., 6 (1971) 1041. B. Myers, M. Gouterman, D. Jensen, T. V. Oommen, K. Spitzer and T. Stroyer-Hansen, Sulfur Research Trends, American Chemical Society, Washington D.C., 1972, p. 53.