Closing address

Journal of Non-CrystallineSolids77 & 78 (1985) 1493-1496 North-Holland,Amsterdam

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CLOSING ADDRESS

J. TAUC

Division of Engineering and Department of Physics, Brown University, Providence, Rhode Island 02912, USA

Mr. Chairman, dear Colleagues: In the short lime allocated for this talk I cannot properly summarize the many contributions of this rich and very successful conference. Instead I will share with you a few thoughts about the conference and the field. tIellmut Fritzsche gave au overview of the history of the field at the Tokyo conference and I will only supplement his story with a few reminiscences associated with the first Conference on Amorphous and Liquid Semiconductors which took place twenty years ago in Prague. This was a small informal meeting which we organized under the auspices of the Czechoslovak Academy of Sciences. It was more like a Gordon Conference with emphasis on discussions and no published proceedings. There were about twenty five participants. The purpose of the meeting was to provide an opportunity for the physicists and chemists interested in this emerging field from the East and West to meet. At that time, in Leningrad there was on-going research on chalcogenide glasses originated by Ioffe who was probably the first physicist who thought about what would happen to a semiconductor when the long range order is absent. From his school, Regel, Goriunova, Kolomiets and others attended the meeting. In Germany, Stuke was a leading expert on selenium and there were a few groups ill England. Owen and Edmond worked on chalcogenide glasses and Enderby on liquid semiconductors. Grigorovici in Romania and myself in Czechoslovakia had been studying for a couple of years various properties of amorphous Ge films in close collaboration. Ziman, who was primarily interested ill liquid metals attended the Prague meeting and gave a closing speech in which he predicted that a new series of conferences will follow our meeting, and that in ten years there will be three hundred people discussing this field. It was considered as a joke; however, the actual growth of the field excc(~dcd the most optimistic expectations. I invited Nevill Mott to attend the meeting but at that time he was not specifically interested in amorphous semiconductors; his interest started shortly after the meeting. Mott's participation significantly increased the respectability of the field and, more importantly, provided a true leadership. An event that happened simultaneously and stirred a strong interest was Stan Ovshinsky's switching and memory devices and his enthusiastic predictions about the bright future of a technology based on amorphous semiconductors. After the second meeting in Bucurest Mott invited the third conference to Cambridge where it took the current form. The most important event at that time was the discovery by Chittick and his collaborators of amorphous hydrogenated Si, the material that now dominates our field. uu22-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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I would like to stress that the series of our conferences has helped to establish amorphous semiconductors as a new field which shares some characteristics with gla~ses and some with crystalline semiconductors. When the field started it did not have a conceptual and mathematical background that crystalline semiconductors had at their start and the glass science did not provide much help with the key problems - the structure, nature of electron states and electron transport. Even today, after so much progress in an understanding of various effects in amorphous semiconductors there are still many fundamental issues that have not been resolved. It is not clear whether a detailed description as we have of crystalline semiconductors will ever by possible, or even useful. Perhaps a general framework such as for example the one we have today (Anderson transition, mobility edge, etc.) may be all one can hope for. However, it may be possible to describe some kinds of disorder in more detail. In recent years a remarkable progress has been made in understanding chaos which in fact is the result of a deterministic motion. A conceptually similar approach to spacial disorder has been developed by Sadoc, Mosseri and their collaborators. For a local configuration that cannot perfectly tile 3D space an ideal structure (a polytope) is defined on a curved surface embedded in 4D space. The actual structure is obtained by a projection (mapping) into 3D space. This mapping produces some ordered regions separated by regions of defects. It is not clear whether this procedure is able to give a picture of the structure of a real amorphous solid; however, in any case, it is an intellectually very fascinating concept with exciting possibilities. For example, there have been attempts to calculate the electronic properties in the ideal structure and "project" the results into real space (Brodsky and others). Recently, significant progress has been made in understanding the shape of the absorption edge of amorphous semiconductors, both in the Urbach part and the higher energy region. I will comment here ~)nly on the latter for which I proposed a formula w a ~ i w - Eg)2 that was discussed at the Prague. Conference and then extensively used as an empirical procedure for determining the optical edge Eg in various amorphous semiconductors. The formula was based on experimental data obtained on amorphous Ge films using two assumptions (i) the density of states close to the band edges is a square root function of energy (ii) the linear momentum matrix element is constant. These assumptions were made by analogy with crystals but there was no justification for them in amorphous solids. W. Paul expressed doubts about assumption (ii) and tentatively suggested that the linear matrix element should be proportional to ca, in other words that the dipole matrix element should be constant. Cody and collaborators did detailed experimental studies of the absorption edge of a-Si:H and among other results showed that for energies above the Urbach edge the edges are best described by a/cv--~0iw - Eg) 2 which implies that assumption (i) holds and the dipole matrix element is constant. Morrel Cohen and collaborators presc~lted at this Conference some results of their theory aimed at obtaining the universal features of electronic properties of amorphous semiconductors, independent of structure, composition and details of disorder. They found that the density of states in the region considered does have the square root dependence and the average dipole matrix element is constant, in agreement with Cody's experimental data. Jackson and his collaborators reported on their X-ray studies (photoemission and inversed photoemission) from which they determined the densities of states in the valence and conduction bands of a-Si:H. They can be piecewise approximated by linear or square root functions depending on the frequency range. By combining their data with the optical data they concluded that in the range from 0.6 to 3.5 eV the average

Z Tauc / Closing address

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dipole matrix element is constant. They also noted that although the different power laws may often be better descriptions of the absorption edge, the original formula czo~('~w- Eg)2 apparently gives the best estimate of E~ (defined as the difference between energies at which the exponential tails start) because the error in the matrix element roughly compensates the error in the density of states extrapolation. A substantial progress has been made in understanding the mechanism of doping of a-Si:H and the metastable defects produced by prolonged illumination (Staebler-Wronski effect). In the Mott lecture, Street presented a self-consistent picture of the processes that accompany doping and compensation, based on data obtained from a number of t.xperimental techniques. A remarkable feature of this picture is a relatively small number of tile basic defect states: band-tail states, dangling bonds, and impurity states. For example, Street and collaborators can explain why phosphorus and arsenic doping introduces dangling bonds while in a compensated sample the density of dangling bonds is small but the tail states are enhanced, and they can calculate the doping efficiency. A simple assumption that only dangling bonds are created during illumination enabled Stutzman and collaborators to explain the pertinent features of the Staebler-Wronski effect, including its saturation. Chenevas-Paul and collaborators observed lattice dynamics effects during the illumination of a-St:H, using small angle neutron scattering. The relatively simple picture of states in the gap as worked out by the Palo Alto group is very useful, although, as pointed out by Fritzsehe, it is unlikely that it will emerge as the complete picture of defects in a complex material such as a-Si:H. In particular, he stressed that our current knowledge of states in the lower part of the gap is very limited. Experimental techniques that have made possible the recent progress in our understanding of the states in the gap and associated trapping and recombination processes are based oil photoexeited effects in the steady or transient modes (FL, PC, PA, LESR, photocapacitance, photo DLTS and others). Of particular interest are the methods based on a combination of a simple photoexcitod effect with microwave resonance in a magnetic field (ODMR, PCDESR, PADESR and others), because they are a tool for identifying the studied state. Morigaki reviewed the use of some of these methods for studying recombination in a-St:H; his results cannot be completely explained by the Palo Alto model and may point out the need for a larger variety of defect centers. From the numerous contributions on electron transport I will only mention Kastner's and Thomas' thoughts concerning the mobility edge. Kastner interprets his transport data on chalcogenide glasses as evidence for conductivity below the mobility edge at non-zero temperature due to phonon assisted tunneling. Thomas discussed the shift of the mobility edge caused by electron-phonon coupling. The answer to the question "what is completely new at this Conference" is amorphous multilayer structures, commonly referred to as superlattices. Perhaps the most exciting feature about them is that they are so surprisingly good, the layers being well defined and sharply separated. Four groups currently working on this subject were represented at this Conference (Exxon, University of Chicago, Max Planck Institute in Stuttgart and Hiroshima University}. Tiedje reviewed the work on compositional superlattices, Dohler on doping superlattices. These remarkable structures promise to provide a new handle on amorphous materials. Perhaps the most exciting prospect is that they will enable us to actually measure the extent of the wavefunction. There are many other very interesting effects associated with electron-hole confinement in some structures and electron-h(.l~, separation in others; the former effect leads to recombination enhancement (most clearly set,n in strongly enhanced photoluminescence), the latter to

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very long recombination times. Hundhausen and Ley discussed an intriguing effect of "persistent photoconductivity" in doping superlattiees. Another application of superlattices of fundamental interest is their use for studying interfaces; the increased sensitivity will hopefully enable this important area to be developed beyond the current status reviewed by Evangelisti. In the last days of the Conference we have learned about an impressive progress in industrial applications of a-Si:H. We have seen a real full color liquid crystal TV set addressed by amorphous silicon thin film transistors (Yamano and Takesada). Shimizu discussed the current status of photoelectric application of a-Si:H, in particular electrophotography (a promising system with an amorphous silicon drum has been recently marketed) and image pick-ups devices which are approaching the marketable stage. The steady process of improving solar cells is continuing in several laboratories. As an example of recent achievements, let us quote clever designs in which the degradation process is minimized and ~ tandem configuration increases the efficiency (Guha; Nakamura et al.) It may well be that the most important development for industrial applications (and perhaps science as well) is a surprisingly large number of contributions on novel deposition techniques. An immediate impact should have the highly increased deposition rate produced by microwave glow discharge process (Hudgens et al.; Kato and Aoki; Mejia et al.) and other methods (Anderson and Biswas). Weiser is developing a method for producing a-Si:H using a direct reaction of silicon with atomic hydrogen. Lucovsky and his collaborators are working on a novel deposition method for a-SiO 2 and silicon nitride in which the plasma production and chemical reactions are separated; this method is of much interest for devices because the deposition occurs at low temperatures (200 to 300°C). On behalf of us all, I would like to congratulate the organizers of this Conference and thank them for the excellent job. It was a most stimulating and most enjoyable conference. I would like to wish our colleagues in Prague much success with the next Conference and conclude with: Arrivederci in Prague!