Photoinduced processes in chalcogenide glasses

567

Photoinduced processes in chalcogenide glasses Keiji Tanaka Studies of photoinduced

phenomena

in chalcogenide

glasses

appear to be entering a new phase, which can be specified by two recent advances. phenomenon Second,

First, the heat-mode

has been applied to erasable

a variety of photon-mode

to be manifestations disordered

phenomena

of electron-lattice

systems. However,

phase-change

optical memories. are understood

interaction

many fundamental

in problems

remain unresolved.

Addresses Hokkaiio University, Faculty of Engineering, Department of Applied Physics, Sapporo 060, Japan; e-mail: keijiQhikari4.huap.hokudai.ac.jp 0 Current

Science Ltd ISSN 1359-0266

Current Opinion in Solid State & 1:567-571

asymmetric motions of atoms, some structural changes may appear. Well known examples in crystals are the colourcenter formation in alkali-halides and the photographic reaction in silver-halides. Photoinduced phenomena are more commonly observed in amorphous semiconductors because the material is inherently quasi-stable and the disordered structure tends to localize electronic and atomic excitations [l]. Up to now, however, photoinduced effects in tetrahedral amorphous semiconductors have been treated as a nuisance because the materials are applied to photoelectric conductors which must be stable [3]. In contrast, in chalcogenide glasses, we have been positively approaching a variety of photoinduced phenomena from fundamental and technological viewpoint [4,5”].

Materials Science 1996,

Introduction A chalcogenide glass can be regarded as a kind of ‘soft semiconductor’: soft because its atomic structure is flexible and viscous (due to chalcogen atoms having two-fold coordination) and a semiconductor because it possesses a bandgap energy (- 2 eV) characteristic of semiconductor materials (l-3 eV). Accordingly, in atomic structure terms, a chalcogenide glass may be characterized as being in between an oxide glass composed of three-dimensional networks and an organic polymer possessing one-dimensional chain structures [ 11. Regarding semiconductor properties, such as the electronic mobility, a chalcogenide glass appears to possess intermediate properties between a crystalline material (e.g. Si) and a polymer (e.g. polyN-vinylcarbazole). These characteristics of chalcogenide glasses have been utilized in applications to infrared optical components and photoelectric materials [ 1,2]. In semiconductors and insulators, light exposure can excite electronic states and, if the excitation can be coupled to

The photoinduced phenomena observed in chalcogenide glasses can be classified into two groups, as illustrated in Figure 1 (4). The first group includes the so-called heat-mode phenomena ((1) in Fig. l), in which the heat generated through nonradiative recombination of photoexcited carriers triggers atomic structural changes. The most familiar may be the phase change between crystalline and amorphous phases, which is now applied to high-density (-1 GB) erasable optical memories. I review the update status of this research below. The second is the so-called photon-mode in which a variety of phenomena have been known, although underlying mechanisms are still largely speculative because the structural changes are vague (except those grouped under (3) in Fig. 1). Among the photon-mode phenomena, I will review the following: photodarkening and related changes, photoinduced anisotropy, and photoinduced photoconductive changes (grouped under (5) in Fig. l), photoinduced fluidity ((2) in Fig. l), and photoelectro-ionic phenomena ((3) in Fig. 1).

Figure 1

Photoinduced

phenomena

<

:,::,“~

~~tde:ri~ll~~in~:~~onn

F)Chemical Sulk

reactli;r;:i)sible change

(Reversible

(4) (5)

A classification of the photoinduced phenomena observed in chalcogenide glasses. Photoinduced phenomena can be classified into the heat mode (1: phase change, transmittance oscillation) and the photon mode, which can further be divided into the phenomena observed only after illumination (2: optical stopping, transitory absorption, photoinduced fluidity, stress relaxation) and after illumination. The latter memory phenomena are composed of photochemical reactions (3: photodoping, photosurface deposition, photochemical modification, photo-oxidation, light enhanced vaporization, photoinduced selective deposition) and bulk changes, which are irreversible (4: photopolymerixation, photoenhanced crystallization, giant photocontraction) or reversible (5: photodarkening, photoinduced bond-interchange, photoinduced ESR and mid-gap absorption, photoinduced photoconductive change, photoinduced anisotropy).

Opticaily-induced phase change Although the principle of the optically-induced reversible phase change was demonstrated by Ovshinsky’s group [6] about a quarter of a century ago, optical memory systems utilizing the phenomenon were not commercialized until very recentty. The principle seems to be simple but the development of semiconductor lasers, which are compact and intense light sources, were necessary as a prerequisite for commercialization purposes. The essence of the phase-change optical memory depends upon the quasi-stability of amorphous states [7-IO,Il*,lZ-151. The material employed is Te-based thin film (-SO nm thick), such as GeSb-Te, which may be prepared through sputtering. The film is then crystallized photothermally. This is the initial state, which gives rise to OFF signals (i.e. high reflectivity) when monitored under reflection of a weak laser light. Incidentally, ‘ON state’ means a low reflectivity state caused by the amorphous state; ‘ON process’ means the transformation from the crystallized to the amorphous state and; OFF process means the transformation from the amorphous to the crystallized state. To write a bit signal, the film is exposed to a focused light pulse emitted from a semiconductor laser with an intensity of - 10 mW and a pulse width of - 50 ns. The irradiated point is heated above the melting temperature and then rapidly quenched into an amorphous state, which corresponds to an ON state. When erasing the signal, the spot is exposed to a light pulse with an intensity of -5 mW. The spot is then heated to a temperature between the crystallization and the melting point and cooled down so that the spot becomes crystallized again. Consequently, the system can operate as an erasable optical memory. The technique is being refined, even though the details remain to be studied. For the ON process, the material flow under viscous states is a problem as it limits the number of possible rewrites (i.e. the number of repeatable cycles between write [ON] and erase [OFF]) to be less than 104-106 times [8-lO,ll*]. In contrast, for the OFF process, the dynamic crystallization consisting of nucleation and crystal growth needs to be analyzed in more detail [ 13]. As described above, the phase change is believed to be thermally-induced, although it is plausible that there exists some electronic contribution. This is because the material is exposed to bandgap light (light with hw2 E,, where ho is the photon energy and E, is the optical bandgap energy of the material of interest), and subsequently electrons and holes are excited, which may cooperatively produce the structural transformations. In fact, it is known that chalcogenide glasses are liable to crystallize and amorphize under prolonged weak illumination [4,16*, 17,28’] and, accordingly, electronic contributions may also be envisaged in the phase-change disks. It will be interesting to investigate the phase changes in optical memory disks, with

time resolutions in the fs-ns range, which may manifest eEectronic contributions. It wouFd also be interesting if we could irradiate the film using pulsed infrared light which can excite only vibrational modes.

Photodarkening

and related phenomena

Chalcogenide glasses exhibit reversible structural changes in amorphous phases, when exposed to illumination and annealed at the glass-transition temperature [4,5”,19,20]. For this phenomenon, extensive studies have been done because the phenomenon is simple and inherent to bulk amorphous phases, and in addition, the reversibility is attractive with respect to applications such as erasable holographic memories. Photoinduced atomic changes are detected by structural measurements such as x-ray diffraction. Also, it is known that the structural changes cause some mod&cations in physical and chemical properties, such as the density and the optical bandgap energy the optical change being referred to as the photodarkening. The atomic structural change, however, cannot be identified without ambiguity, and the mechanism of the reversible change is still speculative [Zl-241. Recently, Nagels et a/. [ZS] and Shtutina et a/. [26] have examined the dependence of the photodarkening on preparation procedures of amorphous phases. They found that hydrogenation and spin-coating processes do not substantially alter the photodarkening characteristics. The photodarkening appears to be inherent to the chalcogen atoms. Another recent progress may be marked in the studies of photon-energy dependence. Effects of low- and high-energy photons have been investigated, as described below. Some researchers have emphasized that the reversible change can be induced with bandgap illumination. Subbandgap illumination, with the photon energy lying in the Urbach-tail region [1,2], however, can also exert substantial influence [4,27]. In fact, the quantum efficiency, normalized to an absorbed photon, seems to be comparable for bandgap and sub-bandgap illumination [27]. In addition, the penetration depth of sub-bandgap light is greater than that of bandgap light. This means that sub-bandgap light can provide more prominent optical and geometrical changes [28,29], which are promising for preparing micro-optic components [30,31]. It has also been demonstrated that so-called photoinduced scattering is not a scattering phenomenon, but is due to self-focusing structures which are formed through the photoinduced refractive-index change [32]. The fact that the glass is substantially influenced by the sub-bandgap illumination, or Urbach-tail light, Jeads us to believe that our understanding of the optical absorption edge in amorphous semiconductors -including the weak-absorption tail [ 1,2], the Urbach tail and the Taut optical gap [1,2] -is still incomplete.

Photoinduced

In add&n to the low-energy photon effect, high-energy photons can also provide srsbstantial influence upon photodarkening. Soft x-my irradiation whieli can, induce resonaraee core-eleetmrexcitation seems to puovide highen quantum &ciencies, which will be beneficial in qtickly inducing photodarkening [33]. Gamma-ray irradiation can also induce photodarkening [34]. In zhe course of scudlying sub-bandgap illumination effects, HisaknuGi and Tanaka [35”] tpanrCenoticed that chalcogenidk films and fibers attain appreciable fluidity when exposed to illuminat& (a phenomenon belonging under (2) in Fig, 1:): ?%e mechanism of the fIuidity is speculative au m although it may be similar to that deceeted under bandgap illumination ]36]. Cornpa&of bandgap and sub-bandgap effcu~, however, reveals that the sub+bandgap effects may be more pnnmising for micm-fabrication and sx11forth, because they can induce more pronounccdl volumeric effecssdue to longer peneanaian deptha ofsub-bandgap l&e. &c&es of chalcogenide glaaaes under illumination appean ao be very interest&g f&r understanding the prow of excited &etmnic systems having disordered8 lastice sss11p~tnm [33]. The results will be understood in a unified way with relaxation dynamics detected i,n the fs-ms interval afees pulsed ilhminaticm [38,3,9].

Photoinduced im4setmpy k seems to be rarfier universal that isotropic

disordered sOrids e&G&it some aniaocropy when lrominated wirh linearly-polarized tiglhr In fact, for chalcogenide glasses such a phenomenon has been discovered by Zhdamov ct aL [Ml, and the mechanism has. been studied extensively. We v speculate smrdly that same structural elements are aligned by linearly-polarized light. Accords ir& a central problem may be the entity of the strucamral cl’ements. Ps&mrsly, the element had been assumed to be the .same as that respernsible for photodarkening. Ki.mura et al.[41], however, demonstrating some different features between she two phenomena, suggested different structural osigina. The aligned structure seems to be subtle and w accordingly, the structural element has not yet .been identified [42*,43,44]. With regards to the anisotropic phenomenon, two recent advances must be pointed out. The first is that the anisosropy can be induced with unpolarized (or circularlypolarized) light, if k ..is -incident upon a sample from directions normal so the light probing the anisotropy. The ~pbenomemm was predicted by Fritz&e [42*], and recently demonstrated [45,46]. This fact clearly indicates that the ekctric field of mducing light, not the propagation direction, governs the stsuetural change. The second is thaF, similar to the rcvecsible change, sub-bandgap illumination can fn&ree the anisotropic structure [32,46,47]. Studies of the anisotropy in glasses will lead to. preparation of anisotropic amorphous semiconductors, which may be

processes in chelcagenide

interesting in the application liquid crystals.

glasses Tanaka

569

of these semiconductors

as

Photoconductive chm The photoconductive degradation induced with light illumination in a&H, that is, the so-called Staebler-Wronki e&ct, may be one of rhe most serious problems for device. applications of amorphous semiconductors, and hence a number of stuckas have been carried out [3]. It may be caused by some property inherent to amorphous semieonduewrPs~]3,5’,48]~ and in this context, illumination ef&crs. upon chalcogenide glasses have also attracted consitierable interest [.24’,49*,50,51.]. Actually, Shimakawa d a/.[49*], have demonstrated that the characteristics of the photoinduced degradasimn in a-Si:H and A@3 are almosn the same. The photoinduced degradation can be regarded as a# kind of radiation damage. Quantitatively, however, the degradation in chalcogenide glasses occurs much fgswr, probably because of the flexible atomic stmsesrsms. In short, illumination (electronic excitation) has been believed: to degrade amorphous semiconductors. In conmast, “&yosawa and: Tanaka [52’] have demonstrated very recently that illumination upon AszS3, which is held at temperatures below 200K, can increase the photocurrent. The dark current remains below a detection limit, and accordingly the photoconductive characteristic seems to be improved with light illumination. The mechanism is speculative at present, although it may be assumed that illumination is effective in reducing the density of trapping and/or recombination centers. If we may follow the charged-defect concept [2], the trarnaformation from D- to Do ma,y be envisaged (where D-, is a negatively charged dangling bond and Do is a neutral dangling bond); which is consistent with the appearance of the m&-gap absorption [4,5**,39]. Should a similar plnenomenon exist in a-Si:H, the degradation problem might be overcome. The charged-defect concept is frequently employed in order to understand these .photoinduced electronic changes. It shouM be remembered, however, that the crrmeept has trura been confirmed experimentally - the concept is. a good working hypothesis. It seems difficult to identify local defective structures in amorphous materials, which may resemble point defects in crystals, if the density is less than -1%. Specifically, if the defect is ESR-inaesi;ae, we, at present, have no techniques feasible for revealing the~structure.

Phetoelectro-lank

processes

Chalcogcnide g&asses can be optically alloyed with metals such as Ag, and in addition, Ag-alloyed chalcogenide glasses exhibit interesting photoinduced phenomena [4,5”,53,54]. Among these, the most famous may be she so-called photodoping phenomenon, discovered by Kostyshin et al. [55],in which an alloying process of a metal, such as Ag, and a chalcogenide glass is dramatically

570

Amorphous

materials

enhanced with illumination. The phenomenon may be promising for ultrahigh-resolution photolithographic processes and for fabricating optical components [26,56,57].

References

. ..

The mechanism of photodoping, however, has been controversial for more than a quarter of a century (B-611. A process similar to the photographic reaction occurring in Ag-halide crystals has been proposed [53], although detailed studies upon electronic properties of Ag-As(Ge)-S(Se) glasses suggest that diffusion and drift of photoexcited holes induce migration of Ag+ ions [57]. It should be noted that in the chalcogenide glasses of interest, holes are more mobile than electrons in marked contrast to the electron-dominated characteristic of Ag-halide crystals [54].

Ag-As(Ge)-S(Se) glasses also exhibit photosurface deposition [62-65] and photochemical modification [66]. The mechanisms of these phenomena can also be understood in a similar way to that of the photodoping phenomenon (54,65,66].

Having understood the motive force behind Ag+-ion migration, scientists in this field would like now to see single-ion motion so that atomic manipulation will be possible.

Conclusions Studies of photoinduced phenomena in chalcogenide glasses continue to be carried out with the aim of completely understanding the electron-atom(ion) interaction in amorphous semiconductors and of developing novel applications. The present status for respective phenomena can be summarized as follows. For the phase-change phenomenon, the initial and the fina! structural state are known, and the main problem is shifting towards understanding transient characteristics appearing in the ps-ns regions. For the bulk reversible changes, it has been demonstrated that the configuration-coordinate models provide useful insights, although real structural entities are difficult to identify. Sub-bandgap illumination effects open up the possibilities of novel applications, although these effects cast doubts upon the nature of the bandgap in amorphous semiconductors. For the photoelectro-ionic phenomena, Lhe mechanisms seem to be understood at a phenomenological level, and, in the future, these studies will focus upon microscopic features.

Acknowledgements This work is partially supported by grants from the Suhara Memorial Foundation and the Casio Science Foundation.

and recommended

reading

Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest

1.

Zallen R: The physics & Sons; 1963.

of amorphous

solids. New York: John Wiley

2.

Elliott SR: Chalcogenide glasses. In Materials science and technology, vol. 9. Edited by Zarzycki J. Weinheim: VCH; 1991.

3.

Tanaka Kazunobu: Photo-induced metestability in amorphous semiconductors. J Non-Cryst So/ids 1991, 137-I 36:1-l 0.

4.

Tanaka K: Photoinduced structural changes in chalcogenide glasses. Rev Solid State Sci 1990, 4:51 I-529.

Shimakawa K, Kolobov A, Elliott SR: Photo-induced effects and metestebility in amorphous semiconductors and insulators. Adv Phys 1995,44:475-566. The most recent review about photon-mode phenomena in amorphous semiconductors and oxide glasses. 5. ..

6.

Feinleib J, deNeufville J, Moss SC, Ovshinsky SR: Rapid reversible light-induced crystellization of amorphous semiconductors. Appl Phys Lett 1971, 16:254-257.

7.

Jiang F, Xu Y, Jiang M, Gan F: Film preparation and structure analysis of optical recording domains of amorphous Ge-Sb-Te. J Non-Cryst Solids 1995, 164:51-56.

6.

Hirotsune A, Miyauchi Y, Terao M: New phase-change rewriteble opticel recording film having well suppressed material flow for repeated rewriting. Appl Phys Lett 1995, 66:2312-2314.

9.

Matsushita T, Suzuki A, Nishiguchi T, Shibata K. Okuda M: Phasechange optical recording films with AQlnTel-Sb-Te system. Jpn J Appl Phys 1995, 34:51 Q-520.

10.

Nobukuni N, Takashima M, Ohno T. Horie M: Microstructural changes in GeSbTe film during repetitious overwriting in phase-change optical recording. J Appl Phys 1995. 76:6960-6966.

Ohta T, Akahira N, Ohara S, Satoh I: High-density phese-chanQe optical recording. Optoelectronics 1995, IO:361 -360. in update technological review of phase-change rewritable optical memo11.

12.

Coombs JH, Jongenelis APJM, Van Es-Spiekman W, Jacobs BA: Laser-induced crystallization phenomena ‘in GeTe-based alloys. J Appl Phys 1995, 76:4906-491 Z

13.

Naruse A, lkuta I, Andoh H, Sato Y, Minemura H: Crystallization mechanism and recording properties of.lnsSbTeP phasechange optical disks. Jpn J Appl Phys 1995, 34:156-l 60.

14.

Sripathi Y, Malhotra LK, Reddy GB: GaGeTe films as phasechanQe optical recording media. Thin So/id Films 1995, 270160-64.

15.

Ide T, Suzuki M, Okada M: Spectroscopic ellipsometry studies on optical constants of GelSblTes used for phase change optical disks. Jpn J Appl Phys 1995, 34:L529-L532.

Kolobov AV, Elliott SR: Reversible photo-amorphization of a 16. . crystallized As&es,, alloy. Phi/ Mag 8 1995, 71 :1-l 0. The first demonstration of athermal photo-amorphlzation in a chalcogenlde glass. 17

Frumar M, Firth AP, Owen AE: Optically induced crystal-toamorphous state transition in As& J Non-Cryst Solids 1995, 7 92-l 93:447-450.

Matsuda 0, Oe H, lnoue K, Muraae K: Electronic and thermal processes during the photo-induced crystellization of amorphous GeSep J Non-Cryst Solids 1995, 192-l 93:524-526. Matsuda et a/. demonstrate, using Raman-scattering spectroscopy, that the crystalline species produced through electronic and thermal excitation in amorphous GeSe2 are different. 16. .

19.

Pfeiffer G, Paesler MA, Agawal SC: Reversible photodarkening of amorphous arsenic chalcogens. Phys Rev B 1991, 130:111-143.

Photoinduced

processes in chalcogenide

glasses Tanaka

571

20.

Utsugi Y, MizumimaY: Lattice-dynamical aspects in photoexcited chakogenide glasses. Jpn J Appl Phys 1992, 31:3922-3933.

43.

TikhomirovVK, Elliott SR: Models for photoinduced anisotropy and its dark relaxation in chakogenide glasses. Phys Rev B 1995,51:5538-5541.

21.

Shpotyuk01: Photostructural transformation in amorphous chalcogenide semiconductors. Phys Status Solidi B - Basic Re 1994, 103~365-374.

44.

Baidakova MV, Faleev NN, Mazets TF, Smorgonskaya EA: Nano-scale medium-range order in semiconducting glassy chakogenides. J Non-Cryst Soilds 1995, 192-l 93:149-l 52.

22.

Vlcek M, Raptis C, Wagner T, Vidourek A, Frurnsr M, Kotsskss IP, PapadimitriouD: Photo- and thermally induced phenomena in amorphous Ge&b,& films. J Non-Cryst Solids 1995, 192-l 93:669-673.

45.

TikhomirovVK, Elliott SR: Metastable optical anisotropy in chalcogenide glasses induced by unpolarized light Phys Rev B 1994,49:17476-l 7479.

46.

23.

KolobovAV, AdriaenssensGJ: On the mechanism of photostructural changes in As-based vitreous chalcogenides: Microscopic, dynamic and electronic aspects. Phil Akg B 1994, 69:21-30.

Tanaka K, Notsni M, Hisakuni H: Optical anisotropy in As& glass induced with sub-bandgap illumination. Solid State Commun 1995, 95:461-464.

47.

Mikla VI: Photoinduced structural changes and related phenomena in amorphous arsenic chakogenides. J Phys Condens Matier 1996,0:429-448.

TikhomirovVK, Elliott SR: The anisotropic photorefractive effect in bulk As& glass induced by polarized subgap laser light J Phys-Condens Matter 1995, 7:1737-l 747.

48.

Nrqels P, Sleeckx E, Callsetis R, Tichy L: Structural and opticcll properties of amorphous selenium prepared by plasmaenhanced CVD. Solid State Commun 1995,94:49-52.

Fritzsche H: Photo-induced structural changes associated with the Staebler-Wronski effect in hydrogenated amorphous Si. Solid State Commun 1995, 94:953-955.

49. .

24.

25.

26.

Shtutina S, KlebanovM, LyubinV, Rosenwsks S, Volterra V: Photoinduced phenomena in spin-coated vitreous As& and AsSe films. Thin Solid Films 1995, 261:263-265.

27.

Tanaka K, Hisakuni H: Photolnduced phenomena in As& glass under sub-bandgap excltatlon. J Non-Ctyst Solids 1996, in press. Shirsmine K, Hisskuni H, Tanaka K: Photoinduced Bmgg mftector in As& glass. Appl Phys Let! 1994,6&l 771-l 773.

28. 29.

Hisakuni H, Tanaka K: Giantphotoexpansion in As&

Shirnakawa K, Elliott SR: Photoinduced effects in amorphous chalcogenldes and a-SI:H. In Electronic, Optoelectronic and Magnetic Thin Films. Edited by Marshall JM, Kirov N, Vavrek A. New York:John Wiley 6 Sons; 1995:95-l 04. A unified interpretationof the photoinduced photoconductingdegradationin amorphous semiconductors. 50.

KounavisP, Mytilineou E: The kinetics of the retvenibla lightInduced phenomena of sputtered amorphous As+,. Phil hfag Lett 1995,72:117-l 22.

51.

ltoh M, Tanaka K: Time-of-flight photocurrents In amorphous semiconductors under nonequilibrium trap states. Jpn J Appl Phys 1995,34:2216-2222.

glass.

Appl Phys Letf 1994,65:2925-2927.

Toyosawa N, Tanaka K: Photocurrent enhancement in lightsoaked AssSs glass. Solid State Commun 1996,97:623-626. ;)ernonstrationof photocurrent enhancement (induced by light illumination), which may be caused by defect transformation. 52.

30.

Hisakuni H, Tanaka K: Optical fabrication of microlenses in chakogenlde glasses. Optics Letters 1995, 20:958-960.

31.

Tanaka K, Toyosswa N, Hisakuni H: Photoinduced Bragg grating in As& optical fibers. Optics Letters 1 Q95,20:1976-1978.

53.

Kolobov AV, Elliott SR: Photodoplng of amorphous chalcogenides by metals. Adv phvs 1991,40:625-684

32.

Hisakuni H, Tanaka K: Laser-induced persistent self-focusing in As& glass. Solid State Commun 1994, 90:463-486.

54.

33.

Hayashi K, Ksto D, Shimakswa K: Photoinduced effects In amorphous chalcogenide films by vacuum ultra-violet light. J Non-Crysr Solids 1996, in press.

Tanaka K, ltoh M: Photo-electro Ionic phenomena in ionconducting amorphous semiconductors. Optoelectronics 1994, 9:299-304.

55.

KostyshinMT, MikhailovskayaEV. RornanankoPF: Photographic sensitivity in thin semlconductlng films on metal substrates. Sov Phys - Solid State 1966,8:451-452.

56.

Dumford SA, Latine JM: Ag2Te/As2Ss a top-surface, highcontrast negative-tone resist for deep ultraviolet submicron lithography. J Vat Sci Technol B 1994,12:44-47.

57.

TrunovML: Structural relaxation and the photoplastic effect in amorphous semkonducton. J Non-Cryst Solids 1995, 192-l 93:431-434.

Kosa Tl, Wagner T, Ewen PJS. Owen AE: Index of refraction of Ag-doped Ass&, films: measurements and analysis of dispersion. Phil Mag B 1995, 71:31 l-31 8.

58.

KolobovAv, Oysnagi H, Tanaka Ka, Tanaka Ke: Photo&u&ml changes In amorphous selenium: An in-situ EXAFS study at low temperature. J Nor&r@ Solids 1996, in press.

Lee J, Ogawa T, Wakaki M, Uehara F, Arsi T: In-situ observation of photodoplng process by infrared attenuated total mflection method. Jpn J Appl l%ys 1994,33:4773-4770.

59.

Lee J, Ogawa T, Hudo H, Arai T: Volume expansion and Ag doping amounts in the photodoping process in amorphous As+% Jpn J Appl Phys 1994,33:5865-5869.

60.

Dale G, Ewen PJS: Kinetics of Ag dlssolution into evaporated As&s, glass films. Solid Stata Commun 1995, 93:685-689.

61.

lndutnyi l2, Dan’ko VA, KudryavtsevAA, MichailovskayaEV, Min’ko VI: Photodoping in As2Ss-A9 structures. J Non-Cryst Solids 1995,185:176-l 82.

62.

Kswaguchi T, Msruno S: Revenible photowriting and thermal erasing of A9 patterns on Ag-rich Ag-Ge-S film% Jpn J Appl Phys 1994, 33~6470-6474.

63.

Kawaguchi T, Msruno S: Photoinduced surface deposition of metallic Ag: Bask research for its applkation to optical recording devices. Jpn J Appl Phys 1994, 33:4521-4525.

64.

Kawaguchi T, Matuno S: Kinetic study of metallic Ag photoinduced surface deposition phenomenon Jpn J Appl Phys 1994, 33:3417-3418.

65.

Kawaguchi T, Maruno S, Elliott SR: Photoelectro-ionic processes in photoinduced surface deposition of metallic silver on a chakogenide glass. J Non-Cryst Solids 1996, in press.

66.

Yoshida N, Tanaka K: Photoinduced Ag migration in Ag-As-S glasses, J Appl Phys 1995, 78:1745-l 750.

34.

Shpotyuk 01, MatkovskiiAO: Radiation-stimulated processes in vitreous As&. J Non-Cryst Solids 1994,176:45-50.

35. Hisakuni H, Tanaka K: Optical microfabrication of chakogenide .. glasses. Science 1995,270:974-975. Demonstrationof photoinduced(athermal)fluidityin glasses and its application to microfabrications.The glass transition may be electronically-induced. 36.

37.

38.

F&o E, Hulin D, ChurnsshV, Michelotti F, Andriesh AM, Bertolotti M: On-off resonance femtosecond non-linear absorption of chalcogenide glassy films. J Non-Cryst Solids 1994, 168:213-222.

39.

Andriash AM, Enachi NA, Culeac IP, Copaci TN, Binchevici VA: The mechanism of enhancement of photoinduced absorption in As& glass at sub-bandgap Illumination. J Non-Cryst Solids 1995,189:147-153.

40.

41.

Zhdanov VG, KolomietsBT, LyubinVM, M&no&ii VK: Photoinduced optical anisotropy in chakogenide vitreous semiconducting films. Phys Status Solidi B-Basic Re 1979, 52:621-626. Kimura K, Murayatna K, NinomiyaT: Change in optical anisotropy of localized states associated with reversible photostructural change In a-As+3 films. J Non-Crysr Solids 1985,77-7&l 203-l 206.

42. Fritzsche H: Optical anisotropies in chelcogenide glasses . induced by bandgap light Phys Rev B 1995,52:15854-l 5861. An interpretationof negative photoinduced anisotropyand a predictionthat the anisotropycan be induced by illuminationof unpolarizedlight.