a o . B e 3 of ese 1. Introduction H. Fritzsche said. The whole activity of many intelligent the first researches on the optical properties of vitreous selenium. Systematic research on materials based on chalc- ogens (sulphur, selenium and tellurium), called chalcogen- ide glasses, started at the middle of the twentieth century. In 1950 Frerichs [4] investigated the As2S3 glass and pub- by Professor R.L. Myuller from the University of Sankt – est number of citations, quoted by ISI, for many years [7]. 3. The past of chalcogenides Challenging problems faced scientists working on the glassy and amorphous chalcogenides in the 1970s and * Tel.: +40 214930047; fax: +40 214930267. E-mail address: mpopescu@infim.ro Journal of Non-Crystalline Solids men and women developed the field of glassy and amor- phous chalcogenides up to the bright level of today. 2. The dawn of the glassy chalcogenides The earliest experimental data on an oxygen-free glass was published by Schulz-Sellack in 1870 [1]. Later on, in 1902, Wood [2], as well as Meier [3] in 1910, carried out Petersburg. A little bit later, in 1968, S.R. Ovshinsky, who founded his own company, Energy Conversion Devices, in Troy, MI, USA, in 1960, made an outstanding discovery: the memory and switching effects exhibited by certain chal- cogenide films subjected to electrical fields. His paper, pub- lished in Physical Review Letters [21(20) (1968) 1450–1453] and entitled: Reversible electrical switching phenomena in disordered structures, soon became the paper with the high- International conferences play an important role in any field of research, especially when a rather young field is considered. New models and concepts are introduced, modified or discarded. A relationship must be created between the ideal systems studied by theorists and materi- als available for experimentation. Two-year interval meet- ings assess where we are and formulate new goals, as in the case of a long travel through the Universe, when a peri- odical evaluation of the coordinates is necessary. Science is not conducted by brains and hands but by individuals, as lished the paper: ‘New optical glasses transparent in infra- red up to 12 lm’. Several years later he started the study of the selenium glass and prepared several binary glasses with sulphur [5]. Winter-Klein [6] published papers on numer- ous chalcogenides prepared in the vitreous state. The first research program in the field of chalcogenide glasses was initiated in 1955 by two competing groups from Sankt – Petersburg: one group lead by Dr Phys. Boris T. Kolomiets and Dr Chem. N.A. Goryunova at the ‘A.F. Joffe’ Physico-Technical Institute and another group lead Chalcogenides – P Mihai P National Institute R&D of Materials Physics, P.O Available onlin Abstract A review of the evolution of the amorphous chalcogenide field sented. On the basis of recent achievements a bright future for th � 2006 Elsevier B.V. All rights reserved. PACS: 61.43.Bn; 61.46.+w Keyword: Chalcogenides 0022-3093/$ - see front matter � 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.11.126 st, present, future pescu * ox MG. 7, 77125 Bucharest-Magurele, Romania 0 March 2006 research from early times up to the present state of the art is pre- materials is predicted. www.elsevier.com/locate/jnoncrysol 352 (2006) 887–891 1980s. Why it is so difficult to remove the gap states from non-crystalline chalcogenides? Mott, Davis and Street [8] presented an ingenious new model in 1974. Their theory suggests that a lattice polarization around charged dan- gling bond sites D� and D+ gives rise to a negative corre- lation energy, which makes the reaction 2D! D+ + D� energetically favourable. It is not possible to remove the gap states by just making cleaner and more perfectly relaxed glasses, which the experimentalist can modify only slightly by quenching or annealing. Ovshinsky said: ‘the best amorphous silicon and germanium will be similar to crystalline silicon and germanium; however the best chalco- genide glasses will remain different’ To understand these differences is a major task. What is the structure of simple, binary and more com- plex chalcogenide glasses? The structure of a glass is very important because many properties depend on it. In 1969 R. Grigorovici developed the first model (amorphon model) for amorphous germa- nium [9]. Nevertheless, the chalcogenides are very compli- cated systems and the amorphon model, which is specific to tetrahedral structures, fails. A specific feature in binary chalcogenides (As2S3, As2Se3) challenged the researchers: the first sharp diffraction peak (FSDP) observed in the X-ray diffraction pattern, and its anomalous dependence on temperature. In 1978 Popescu [10] and in 1979 Phillips [11] demonstrated that in many non-crystalline solids an ‘ordering at not too large scale but more extended than the short-range order’ must be admitted in order to explain the FSDP. Thus the concept of medium-range order, or intermediate-range order was born. All these challenging problems and others, including device fabrication, stimulated the exchange of ideas at inter- national conferences. The ICALS series of biennial confer- ences continued, especially for Western countries, while Eastern countries started a new series of conferences, orga- nized by the Academies of the former Socialist countries, entitled: amorphous semiconductors. The implications of the chalcogenide field in both series of conferences have been different. While at ICALS conferences the part dedi- cated to chalcogenides has diminished, especially after the discovery of the dopability properties of amorphous hydro- genated silicon in 1975 by Spear and LeComber [12], at the Amorphous Semiconductors conferences the part dedicated to chalcogenide papers remained still very large (81% in 1978) – see Table 1. Table 1 Number of papers at ICALS-ICAS-ICAMS-ICANS conferences Conference Year Place Total papers Chalcogenide papers Chalcogenide papers (%) ICALS-1 1965 Prague (Czech Republic) 17 7 41.1 ICALS-2 1967 Bucharest (Romania) 26 12 46.2 ICALS-3 1969 Cambridge (UK) 84 34 40.5 ICALS-4 1971 Ann Arbor (USA) 152 72 47.4 ICALS-5 1973 Garmisch-Part. (Germany) 180 ICALS-6 1975 Leningrad (Russia)a 175 97 55.4 50 06 45 00 60 38 65 27 44 83 77 22 70 87 44 888 M. Popescu / Journal of Non-Crystalline Solids 352 (2006) 887–891 ICALS-7 1977 Edinburgh (UK) 1 ICALS-8 1979 Cambridge (USA)a 2 ICALS-9 1981 Grenoble (France) 2 ICALS-10 1983 Tokyo (Japan) 3 ICALS-11 1985 Rome (Italy) 3 ICALS-12 1987 Prague (Czech Republic)a 3 ICALS/IAST-13 1989 Asheville (USA) 2 ICAS-14 1991 Garmisch-Part. (Germany)a 3 ICAS-15 1993 Cambridge (UK) 3 ICAS-16 1995 Kobe (Japan)a 2 ICAMS-17 1997 Budapest (Hungary)a 2 ICAMS-18 1999 Snowbird (USA) 3 ICAMS-19 2001 Nice (France) 2 ICAMS-20 2003 C. Jordaos (Brazil) 2 ICANS-21 2005 Lisbon (Portugal)b 4 Year Place Total papers Amorphous semiconductors 1972–1989 1972 Sofia (Bulgaria) 61 1974 Rheinhardsbrunn (Germany) 107 1976 Balatonfu¨red (Hungary) 88 1978 Pardubice (Czech Republic) 155 1980 Chisina˘u (Moldova Republic) 156 1982 Bucharest (Romania) 170 1984 Gabrovo (Bulgaria) 169 1986 Balatonsze´plak (Hungary) 139 1989 Uzhgorod (Ukraine) 232 a Published papers. b Number of papers as shown in Abstract book. 71 34.5 65 21.6 54 16.0 43 16.2 35 10.7 47 13.7 31 11.0 38 13.7 37 11.5 41 15.2 27 9.4 60 12.8 Chalcogenide papers Chalcogenide papers (%) 41 67.2 64 59.8 62 70.5 125 80.6 119 76.3 103 60.6 104 61.5 55 39.6 94 40.5 The systematical research of chalcogenide glasses has led to the discovery of numerous effects, of high fundamental and practical importance [13]. Table 2 lists some of these. The maturation of a field of research is reached when the synthesis of theories, experimental data, technologies and applications are presented as books or handbooks. For chalcogenides the master book was written by N.F. Mott and E.A. Davis (Electronic Processes in Non-Crystalline Materials, Clarendon Press, Oxford, UK) in 1971 with a second edition in 1979, which remained the basical book for scientists working in non-crystalline semiconductors, for more than 35-years. The collection of books on chalc- ogenides increased gradually up to day. The apogee of the field and the world-wide recognition of the importance of the research in amorphous semiconductors was repre- sented by the conferring of the nobel prize in physics in 1977 on Sir Nevill Mott, P.W. Anderson and John. H. Van Vleck for their fundamental theoretical investigations of the electronic structure of magnetic and disordered systems. 4. The state of art in chalcogenides The amorphous chalcogenide field continued to develop both from fundamental and applications points of view. New phenomena were discovered, new applications emerged. An international race occurred for denser memo- ries, including the phase-change ones, developed firstly at ECD, under the leadership of Stanford and Iris Ovshinsky. After 1989, the series of Conferences ‘Amorphous Semi- conductors’ was stopped due to the revolutions in the Socialist countries and Soviet Union. In the first year of Table 2 Effects discovered in chalcogenide glasses [13] 1 Elongation effect (Vonwiller, 1919) 2 Switching (Ovshinsky) effect (Ovshinsky, 1959) 3 Polarization effect (electret effect) (Kolomiets and Lyubin, 1962) 4 Photo-dissolution and photo-doping (Kostishin et al., 1966) 5 Photo-crystallization (Dresner and Stringfellow, 1968) 6 Photo-polarization (Andreichin, 1970) 7 Photo-darkening (Berkes et al., 1971, Keneman, 1971; de Neufville, Moss, Ovshinsky, 1973) 8 Photo-bleaching (Berkes et al., 1971; de Neufville, Moss and Ovshinsky, 1973; Averyanov et al., 1980) 9 Photo-vaporisation (Janai and Rudman, 1974) 10 Photo-polymerization (de Neufville, 1975) 11 Photo-expansion (Hamanaka et al., 1976) 12 Oscillatory transmission (Hajto et al., 1977) 13 Ultrasonic induced modifications of the optical properties (Okano, 1978) 14 Anisotropy of the transmittance (Hajto and Ewen, 1979) 15 Photo-induced softening and hardening (Tanaka, 1980) 16 Photo-contraction (Bhanwar Singh et al., 1982) 17 Thermo-dissolution (Kolobov et al., 1985) 18 Gamma-radiation induced effect (Shpotyuk et al., 1987) 19 Photo-amplified oxidation (Tichy et al., 1987) 20 Photo-induced change of ac transport (Shimakawa et al., 1987) 21 Photo-anisotropy (Weigert, 1920; Lee, 1990) 22 Photo-elastic birefringence (Ke. Tanaka, 1990) (Tan iott and Lyu hug ati et and nd nd E t (K eter tocr ana ikin d, 2 shi, s e (Ma (Ko M. Popescu / Journal of Non-Crystalline Solids 352 (2006) 887–891 889 23 Mechanically induced anisotropy 24 Photo-induced amorphisation (Ell 25 Photo-induced girotropy (Lyubin 26 Photo-induced scattering of light ( 27 Photo-plasticity (Trunov and Anc 28 Athermal photo-induced transform 29 Acoustic-optical effect (Abdulhalim 30 Giant photo-expansion (Hisakuni 31 Photo-induced fluidity (Hisakuni a 32 Optomechanical effect (Krecmer a 33 Anisotropic opto-mechanical effec 34 Photo-induced anisotropic nanom 35 Laser-induced suppression of pho 36 Anisotropic surface corrugation (T 37 Light stimulated interdiffusion (K 38 Self-organization effect (Boolchan 39 Ion induced interdiffusion (Kikine 40 Photo-induced ductility (Kastrissio 41 Thermo-stimulated inter-diffusion 42 Deuteron-radiation induced effect 43 Polarization dependent photo-plasticit 44 Multistate switching effect (Ovshinsky aka, 1990) and Kolobov, 1991) Tikhomirov, 1991) bin and Tikhomirov, 1991) in, 1992) on (Kolobov et al., 1992) al., 1993; Laine and Seddon, 1995) Tanaka, 1994) Tanaka, 1995) lliott, 1997) recmer et al., 1997) scale expansion (Krecmer et al., 1997) ystallization (Roy et al., 1998) ka et al., 1999) eshi, 2001) 001) 2001) t al., 2002) lyovanik, Kikineshi et al., 2003) kenyesi, 2003) y (Trunov and Bilanich, 2004) , 2004) orida). IS anized in ep ). ce ill p (In a optical enabled h nf tic ge o ve vic di s o m switching effect) that transcends the possibility of inscrib- ing information as binary states 0 and 1. The possibility of controlling the electrical resistance of a thin chalcogen- ide film subjected to electrical pulses opened the way to trigger various steps of resistance. The multistate storage capability of the OUM increases its memory storage den- sity manyfold. Fig. 1 shows how is possible to store memory state, in this case, sixteen. The resistance values are obtained with the programming current pulse indicated before the steps. Each state is non-volatile until it is over-written by a differ- ent current pulse. In Fig. 2 the first four electrical pulses do not change the resistance, but the energy is accumulated such that the fifth pulse achieves the change of state. Each of the sub-pulses produces some crystallization and each sub-pulse increases the crystalline volume fraction until the percolation thresh- old is reached at which the resistance drops by one or two orders of magnitudes. The behaviour of the device reminds us of a biological neuron that receives energy inputs at its dendritic synaptic terminals and accumulates them until a threshold is reached and it fires [15]. stalline Solids 352 (2006) 887–891 5. Future of amorphous chalcogenides A quantitative increase in the knowledge in the field of chalcogenides is obvious. We are waiting now for a great discovery in this field, similarly to the case of amorphous silicon when the discovery of the effect of hydrogenation in 1975 triggered the way to extended studies and mass scale applications. In the last two years Stanford Ovshinsky has moved the research on chalcogenide switching much further. In the cogenide glasses are far fro newly created Ovonic Unive new effect observed in cha exhausted. ory and cognitive de es in cate that application rsal Memory [14], he lcogenide glasses (mu f chal- electrical phase-chan mem ries. His new multile l mem- states in response to the large industry of igh i and electrical pulses ormation-density op al and thorough study of the unique chemistry and the interesting electro-optical properties of chalcogenide glasses. His dis- covery of certain chalcogenide compositions that transform easily and reversibly between amorphous and crystalline April 2006. Ovshinsky’s invention of variety of devices initiated a series, ISNOG-15, w take lace in Bangalore dia) in Cape Canaveral, Fl Pardubice (Czech R ublic NOG-13 was org The next conferen in this ANC-1 (2001) ANC-2 (2005) No. of participants 110 No. of participants 80 No. of abstracts 125 No. of abstracts 100 No. of published papers 84 No. of published papers 80 The last two Symposiums ISNOG-13 and ISNOG-14 are characterized by: ISNOG-13 ISNOG-14 No. of papers 181 No. of papers 262 No. of papers on chalcogenides 125 No. of papers on chalcogenides 75 We must note that ISNOG-14 was included in the Fall 2004 Meeting of the Glass and Optical Materials of the American Ceramic Society (Radisson Resort at the Port the 21st century, a new series of conferences appeared in Eastern Europe entitled: ‘amorphous and nanostructured chalcogenides’. In parallel the ICAS conferences contin- ued. ICAS became ICAMS and, thereafter, ICANS (Amorphous and nanocrystalline semiconductors). An other biennial series of Symposia, ISNOG (Internatiomal symposium on non-oxide glasses) reached in 2004 its 14th edition. The first two workshops on amorphous and nano- structured chalcogenides ANC-1 and ANC-2, organized in Romania, are characterized by: 890 M. Popescu / Journal of Non-Cry used a ltistate Fig. 1. Multi-state programming of OUM device. Each resistance state can be reached from any other state with the programming pulse current shown before each step. Fig. 2. Pulse accumulation mode of the OUM device. The device has been cycled and programmed 1000 times. The minimum and maximum resistance value of these 1000 cycles is shown. The cognitive device is on the way! Future computers will be smart and will allow easy manipulation due to small size associated with the three-dimensional integration of the memory core. The recent discovery of intermediate, self-organized phases in chalcogenides and other glasses [16–18] will have probably as J.C. Phillips believes, high impact not only in glass science but also in many other fields of research as, for example, protein folding, thin-film gate dielectrics and high-temperature superconductivity. The problem of defects in chalcogenides is still challeng- ing. The recent discovery of a new type of charged defect in As2S3 by Simdyankin et al. [19] open the way to other dis- coveries in this field, proving that the potentiality of the ate the evolution of the field, which is still rich in surprising discoveries. References [1] C. Schulz-Selack, Ann. Phys. 139 (1870) 182. [2] R.W. Wood, Philos. Mag. 3 (1902) 607. [3] W. Meier, Ann. Phys. 31 (1910) 1017. [4] R. Frerichs, Phys. Rev. 78 (1950) 643. [5] R. Frerichs, J. Opt. Soc. Am. 43 (1955) 1153. [6] A. Winter-Klein, Verres et Refractaires 9 (1955) 147. [7] S.R. Ovshinsky, Phys. Rev. Lett. 21 (20) (1968) 1450. [8] N.F. Mott, E.A. Davis, R.A. Street, Philos. Mag. 32 (1975) 961. [9] R. Grigorovici, J. Non-Cryst. Solids 1 (1969) 303. M. Popescu / Journal of Non-Crystalline Solids 352 (2006) 887–891 891 appearance of internet sites dedicated to chalcogenides (e.g., Forum of Chalcogeniders), and the appearance of specialized electronic Journals (e.g., Chalcogenide Letters, Journal of Ovonic Research). Important papers in the field of chalcogenides have been published in a rather new and specialized journal: Journal of Optoelec- tronics and Advanced Materials [20–28]. 6. Conclusions There are strong arguments for a bright future for amor- phous chalcogenides. Research studies are more and more directed to rapid applications of fundamental findings. Meetings related to chalcogenides are more frequent and the appearance of organizations with worldwide participa- tion speaks in favour of a new trend in the chalcogenide field: self-control of the research. These trends will acceler- chalcogenides is not exhausted. Finally, we point out four trends: 1. the development of the conferences dedicated mainly to chalcogenides; 2. the development of transnational teams of research; 3. the organization of the research at the European and international scale (e.g., North-Atlantic Consortium on Non-Oxide Glasses NACNOG) and International Materials Institute: Functionality in Glass (IMI)); 4. the use of world-wide communications by e-mail, the [10] M. Popescu, in: Proc. Intern. Conf. ‘Amorphous Semiconductors ‘78’, Pardubice, Czechoslovakia, vol. 1, 1978, p. 185. [11] J.C. Phillips, J. Non-Cryst. Solids 34 (1979) 153. [12] W.E. Spear, P.G. LeComber, Solid State Commun. 17 (1975) 1193. [13] M. Popescu, J. Optoelectron. Adv. Mat. 7 (4) (2005) 2211. [14] S.R. Ovshinsky, Non-crystalline materials for optoelectronics, in: Optoelectronic Materials and Devices, vol. 2, INOE Publishing House, Bucharest, Romania, p. 1. [15] H. Fritzsche, in: Comm. To ISNOG-14 Meeting, Cocoa Beach, Florida, November 2004, Phys. Chem. Glasses, in press. [16] P. Boolchand, D.G. Georgiev, B. Goodman, J. Optoelectron. Adv. Mat. 3 (3) (2001) 703. [17] F. Wang, S. Mamedov, P. Boolchand, M. Chandrashekar, Phys. Rev. B 71 (2005) 174201. [18] S. Chakravarty, D.G. Georgiev, P. Boolchand, M. Micoulaut, J. Phys. Condens. Mat. 17 (2005) L1. [19] S.I. Simdyankin, T.A. Nieuhaus, G. Natarajan, Th. Frauenheim, S.R. Elliott, J. Phys. Condens. Mat. 1 (2004) 0409441. [20] E.V. Emelianova, G. Adriaenssens, J. Optoelectron. Adv. Mat. 6 (4) (2004) 1105. [21] Ke. Tanaka, T. Gotoh, K. Sugawara, J. Optoelectron. Adv. Mat. 6 (4) (2005) 1133. [22] A. Zakery, J. Optoelectron. Adv. Mat. 7 (3) (2005) 1143. [23] D. Strand, J. Optoelectron. Adv. Mat. 7 (4) (2005) 1679. [24] G. Lucovsky, J. Optoelectron. Adv. Mat. 7 (4) (2005) 1691. [25] F. Yonezawa, H. Ohtani, T. Yamaguchi, J. Optoelectron. Adv. Mat. 7 (4) (2005) 1707. [26] V.S. Minaev, S.P. Timoshenkov, V.V. Kalugin, J. Optoelectron. Adv. Mat. 7 (4) (2005) 1717. [27] T. Aoki, D. Saito, K. Ikeda, S. Kobayasshi, K. Shimakawa, J. Optoelectron. Adv. Mat. 7 (4) (2005) 1749. [28] N. Qamhieh, G. Adriaenssens, J. Optoelectron. Adv. Mat. 7 (4) (2005) 1785. Chalcogenides - Past, present, future Introduction The dawn of the glassy chalcogenides The past of chalcogenides The state of art in chalcogenides Future of amorphous chalcogenides Conclusions References