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Although GeTe-${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ (GST) alloys are widely used in data storage, many fundamental issues are still under debate. Here, we demonstrate that the presence of vacancies in the crystalline phase has far-reaching consequences, namely, a triad of twofold coordinated Te atoms with lone-pair electrons generated around the vacancy enables the formation of soft three-center four-electron bonds, whose properties provide an explanation for the unusual characteristics of GST, in particular, the increase in local disorder upon crystallization, the co-existence of a very fast switching rate with a large property contrast, the possibility of a solid-solid amorphization process that excludes conventional melting, and the drastic difference in crystallization behavior between GST and the ideal binary GeTe. Anisotropy of the three-center bonds may serve as an additional degree of freedom for information recording and provide a unified explanation for a variety of unique effects observed in lone-pair semiconductors.

Neutral and ionic gas temperatures (Tn and Ti) are crucial parameters in the plasma processing of materials (etching and film deposition). Here we evaluate a method used for obtaining the temperatures, i.e., the Doppler broadening, of emission lines. We focus on Ar lines from microwave electron cyclotron resonance plasmas. We use the same technique for both ions and neutrals. Three factors are important in achieving accuracy: (1) knowing the correct instrument width at the central wavelength of each ion and neutral line, (2) understanding the noise‐dependent variation in the measured widths, and (3) knowing how the magnetic field affects the spectra. We find that T can be determined to within ∼100 °C. Ti and Tn are typically found to be ∼0.5 and

We report the effect that thermal annealing in inert and oxidizing atmospheres, and with and without encapsulating layers, has on the structure of tungsten/carbon [W/C] multilayer thin films. This study focuses on the tungsten component and deals mainly with multilayers where the ratio of thickness of tungsten layers is equal to or greater than for the carbon layers (that is, γ ≤ 0.5). This is in contrast to prior studies where the tungsten layer thickness was generally held constant and the carbon layer was varied. Thermal annealing in inert atmospheres produces reactions and other structural changes in the tungsten and carbide layers which depend on the as-deposited multilayer structure which depends, in turn, on the thickness of the tungsten layer. In samples where both the tungsten and carbide fractions of the multilayer are completely amorphous as deposited, which is the case for thin tungsten layers (thickness of tungsten (tw)4 nm/period), the reactions in the tungsten layer forming crystalline tungsten and tungsten carbide occur at annealing temperatures above 900°C. The layer pair spacing, or period, (d), in this group shows an expansion of up to 10-15% of the original value as has been reported in the past. Changes in both the tungsten and carbide layers, and their interfaces, contribute to changes in d spacing and relative thickness of the high and low Z components. When the tungsten layer thickness exceeds 4 nm per period the tungsten is partially crystallized in as-prepared samples. In such multilayers interfacial reactions, producing an oriented partially crystalline W2C/C superlattice, occur at temperatures of 600°C and below. The fact that W2C crystallites in one period can form a structure which is correlated to W2C crystallites in neighboring layers is somewhat surprising, since layers are presumably still separated by amorphous carbon which is still visible via Raman. The expansion of the layer pair spacing is relatively small (5%) in this group and, more importantly, mostly involves increases in the thickness of the high Z components. Samples annealed in air at temperatures below 300°C are progressively destroyed by the oxidation of both tungsten and carbide layers. Encapsulation of similar multilayers with a thin (30 nm) dielectric layer of any of several types can retard oxidation to 600°C. The silicon-containing encapsulants generally perform better. Failure at this temperature is seen to occur from pinhole formation.

We describe a pairing mechanism that is based on changes in the O-Cu-O bonds and orbital configurations that occur in the mixed valence planar superconducting ceramics. We discuss the mobility of these spin-zero bosons and their distinctive differences from other proposed real-space bosons.

High-quality epitaxial YBa 2 Cu 3 O 7−δ (F) films were grown directly on r-plane (1 1 02) sapphire by a pulsed laser deposition technique using a multiphase fluorinated target. X-ray diffraction data showed that the film is highly c -axis oriented and in-plane aligned. The c -axis and in-plane mosaic spread are 1.1° and 2.4°, respectively. The microwave surface resistance measured by a con-focal resonator configuration is 56 mΩ at 94.1 GHz at 77 K which is the best reported value for a YBCO film on sapphire. The improved film quality is attributed to the fluorine which is transported from the target to the growing surface of the film where it acts as an etching and/or catalytic agent to preferentially remove defects and second phase impurities and to promote better in-plane epitaxy.

The thermal activation energy, AE, of the conductivity in amorphous chalcogenide semiconductors is usually about half of the optical gap energy E. This is commonly interpreted as (intrinsic) conduction at the mobility edge by carriers whose concentration is governed by the Fermi energy EF which is pinned near the gap center. Small changes in glass composition and the presence of impurities usually have little effect on the properties except for small changes in EO and AE with composition. The insensitivity to impurities is ascribed to the fact that in a material lacking long-range order each foreign atom can satisfy its valency requirement,(1) and, thus, will not act as an electrically active center. The origin of the various states in the gap (DECs) (2,3) and the pinning of EF is attributed to those DECs which are inherent valence alternation defect centers.

The relationship between electronic conduction and stereochemical local bonding arrangements is illustrated by four examples: the unusual high conductance ON-state of the Ovonic Threshold Switch resulting from nonequilibrium excitation processes the high conductivity connected with the oxidation states of the Ovitron, the chemical modification of electronic transport in amorphous semiconductors and insulators, and the high temperature copper oxide ceramic superconductors. We discuss the effect of fluorination on raising the superconducting transition temperature to Tc=155K or higher, on eliminating oxygen diffusion and on orienting the crystallites in the YBaCuO superconductors.

During the past year a large number of models have been proposed to explain high Tc superconductivity in Cu-0 ceramic compounds. A nearly equal number of novel charge carrier pairing mechanisms have been invoked in these models as alternatives to the conventional phonon mediated process described in the BCS theory. Some of these models have recently been reviewed by Rice.(1)

It is fitting to discuss in this festschrift for Hellmut Fritzsche the phenomena which first interested him in the amorphous field.

An important difference between crystalline and amorphous structures is that while in crystals the local environment is the same everywhere, in amorphous materials one finds a large spectrum of 3-dimensionally varying spatial and bonding relationships. This gives rise to unique energy interactions, orbital overlaps and bonding configurations of the lone pair electrons associated with the group VI chalcogen atoms which do not occur in crystals. We present the ‘ball and spoke’ models we have built to test out the concepts of lone pair relationships discussed earlier (Ovshinsky 1972, Ovshinsky 1973, Ovshinsky and Fritzsche 1973).

Imbedded in the structural chemistry of the high-temperature cupric superconductors lies the new physics which can answer the unsolved fundamental problem of pairing. A mechanism is offered which uniquely shows how pairing can occur by virtue of intimate Cu III valence relationships that are generated in the mixed valence copper-oxygen planes. This mechanism leads to condensation to the lowest free energy superconducting state.

Amorphous Si:F:H with desirable properties for photovoltaic applications can be fabricated by the glow discharge of SiF4 and H2. The preparation conditions influence the properties of the resultant alloy. For instance, altering the ratio of SiF4 to H2 from 80 to 5 can alter the localized state density from 1019cm−3eV−1 to ≃ 1016cm−3eV-1, respectively. The conduction mechanisms are altered and there are vast changes in the photoconductivity as the density of recombination centers is decreased. The lower density of states achieved in the a-Si:F:H alloy reflects in the ease of doping. In addition, the lower density of states in a-Si:F:H alloy should result in a wider depletion region than reported for the a-Si:H alloy when fabricated within the device configuration. Results of C-V measurements using Au Schottky barrier devices confirm this.

We have grown amorphous silicon-germanium alloys by rf glow discharge decomposition of disilane, germane, silicon tetrafluoride and hydrogen. Films with an optical band gap in the range of 1.72 to 1.2e V have been made. The optimized films show the following properties: 1) incorporation of germanium is uniform across the length of the sample; 2) hydrogen is bonded to silicon and germanium uniformly without any preference; 3) the slope of the valence band tail remains constant as the optical gap is lowered down to 1.25eV and 4) use of silicon tetrafluoride in the gas mixture improves the quality of the material. Photothermal deflection spectroscopy and space charge limited conduction were used to obtain information about the deep states. Even for the lowest band gap material, the density of states at the Fermi level is about 10 17 cm −3 eV −1 which demonstrates the high quality of the material.

We have observed that an rf current of frequency 1–5 MHz, passing through a sputtered binary superconducting film, can induce a dc voltage as large as 1 mV across the film for temperatures near the critical temperature T C . The largest induced voltage has been observed in films which are microscopically inhomogeneous, such as Mo 63 C 37 films which exhibit columnar microstructure. We suggest that the effect may be due to an inverse ac Josephson effect in a randomly connected series array of many microscopic superconducting weak links that are expected to exist close to T C .

Previously we have shown that amorphous silicon-based alloys containing F and H (1–7) possess desirable properties for photovoltaic application. Amorphous tetrahedral semiconductors ordinarily possess a very large density of states which act as traps leading to low values for drift mobility and low recombination lifetimes of free carriers. However, Spear and his group (8–10) reported that a-Si decomposed from SiH4 gas by r.f. glow discharge and deposited on a heated substrate produces a film which has a comparatively low density of states. However, materials prepared in this way possess a large concentration of H.(11) Because of the relatively low density of states, these types of films can be doped n type, albeit using relatively large concentration of the dopant. However, p-type doping using B2H6 is generally accompanied with the decrease in the band gap, indicating alloying action rather than conventional doping.(9) Furthermore, the material has apparently a sufficiently large carrier lifetime that efficiencies of photovoltaic devices exceeding 5% have been reported.(12)

We have fabricated Mo x C 1-x thin film samples by sputtering, and have studied the dependence of superconducting critical temperatures, fields, and currents on composition and microstructure. For x in the range from 40-60%, the preferred nonequilibrium Mo-C Bl phase (with T c up to 13K) grows in an oriented columnar structure, with column diameters of 100-1000A. This is correlated with sharply enhanced perpendicular critical fields (with extrapolated values of H c2 (O) that can exceed 300kOe) together with a critical current density that may be as large as 106A/cm2. We also present data on artificial layered structures based on Mo-C, fabricated either by alternately sputtering Si layers or by pulsed gas injection of N 2 .

It is fitting that we honor Norbert Kreidl by considering the future of glass for there is a dramatic “phase” transformation presently taking place in the area of glass science and technology. The glass meetings of old were centered on the much debated subject, “What is glass?” In a sense, this reflected not only scientific uneasiness but a search for identity, for the very basis of glassy materials, their inherent disorder, was the cause of much insecurity. The crystalline field had as its bedrock the crystal lattice, the order, if not the boredom, of repetitive atoms which looking in all directions saw sharply definable and identifiable neighbors over relatively long distances. It is no wonder that glassy materials from time immemorial through the 1950’s appeared to be associated only with wide band gap materials whose optical and mechanical properties were of paramount importance.

The way we work, indeed, our entire industrial civilization, depends upon materials. We have long used noncrystalline solids in the form of glasses for their passive properties, such as their inertness, transparency, and in everyday use as containers. They are artifacts of an ancient time, not only on earth but in the heavens—on the moon glassy materials are widespread.

We have made a study of the resistivity, ρ(T), and the structural and superconducting properties of amorphous Mol-xSix Besides the usual interest in studying a metal to nonmetal transition, this system has the additional feature that Si changes from acting metallic for x << 1 to acting nonmetallic for x ~ 1. Murarka et al. (1) measured the resistivity of both amorphous and crystallized cosputtered films of MoSi and a comparison of their data with ours will be made below. We have observed a large increase in dρ(297)/dx with increasing x at x = x0 ≡ 0.63 ± 0.05 which is most likely associated with an electronic transition involving the Si valence electrons. A tentative model for this transition is presented below. Besides observing this increase in d ρ(297)/dx, the specific results pertinent to superconductivity are: 1. The initial linear decrease of the superconducting transition temperature T­c is 0.075 K/at.% Si. This is consistent with the e/a dependence found by Collver and Hammond (2) for near neighbor amorphous 4d metals if one assumes that each Si atom contributes 4 electrons to the conduction band. 2. Supperconductivity persists up to at least 64 at.% i.e., slightly into the concentration region where the resistivity begins to increase. As x→xo ­from below, the superconducting transition width ΔT­c increases rapidly until at x ≃ xo the width ΔTc becomes comparable to Rc. 3. For x > xo, Tc, if it exists, is less than 1.5K.

A NEW TYPE of amorphous semiconductor alloy has recently been reported [1]. This new material has silicon and fluorine as its main structural components. It is multi-elemental and includes hydrogen and can also include other elements such as oxygen without deleterious effects. This Si: F: H alloy, prepared by the glow-discharge decomposition of SiF4 mixed with hydrogen, has been reported to overcome a number of problems of a-Si and a-Si: H. The a-Si: F: H alloy is highly photoconductive, possesses a lower density of states in the upper half of the band gap than the a-Si: H alloy and is devoid of any photostructural changes. In order to gain further information about the electronic and vibrational states of this amorphous semiconductor, we have investigated the electrolyte electroreflectance (EER) and Raman spectra of several samples of this material.

THE AMORPHOUS-CRYSTALLINE transition in a-Si and a-Ge has been studied for many years [1,2]. However, unlike electrical and optical properties, no substantial modifications on the RDF (radial distribution function) for the amorphous phases have been reported for different methods of preparation or annealing conditions short of crystallization. More recently, Barna et al. [3] have found a small shift of the second peak in the RDF towards smaller r, and a more pronounced peak at 5 A for the a-Si: H prepared by glow discharge technique. These authors concluded that a higher degree of local order is present in the glow discharge a-Si: H although they stated that there exist only minor differences in amorphous silicon samples with different preparations. Earlier Raman measurements [4] have not been shown to be sensitive to preparation conditions, however, recent results indicated that Raman measurements can indeed differentiate various types of amorphous silicon samples. Tsu et al.

Since the publication of the paper announcing the discovery of reversible switching phenomena in certain types of chalcogenide glasses [1], much effort has been devoted towards the characterization, improvement, and understanding of the properties of these materials [2]. This interest has been stimulated both by the basic scientific questions that have to be answered in order to understand the structures and properties of these alloys, and by their obvious potential as reversible threshold and memory switching elements in a wide variety of electronic devices [2,3].

There are many dogmas which have been uncritically accepted in the amorphous field that are basically unsound, the chief one being that the short-range order surrounding each atom is the same as in the “corresponding” crystal and that all that is different is some fluctuations in bond lengths and angles. We propose to show that short-range order is not conserved in amorphous materials, since space is related to energy in a basically different manner. The consequences of this lead to entirely new local structures than are found in crystalline materials and to the fact that many of these structures have no analogs in crystals and are inherently anticrystalline in nature.

The importance of fundamental chemical bonding considerations, based on an examination of the chemical bonds expected to be present, in the determination of the structures and properties of a wide variety of glasses, is discussed. The average number of near neighbors of each type expected to surround a given central atom in an alloy glass, can often be estimated by using simple considerations based on chemical ordering, coordination numbers and bond energies. The bonding patterns thus obtained provide insight into the average local order present in these glasses, as well as enabling the explanation and prediction of various physical, chemical and electrical properties. Several examples are given to illustrate these ideas.

Photovoltaic quality films of a-Si:F:H prepared by the plasma-induced reaction of, for example, SiF4 and H2 appear to contain unusual bonding configurations involving the intimate association of silicon with both fluorine and hydrogen atoms.(1) The creation of radical sub-fluorides of silicon has been suggested as a mechanism for the deposition of such films.(2) Indeed, the presence of various excited-state neutral and ionic silicon radical species has been demonstrated in the optical emission of plasmas containing SiH4, SiF4 and H2,(3,4) and the silylene radical, SiH2, has been proposed to play a dominant role in the deposition of a-Si:H from the H0M0CVD of SiH4.(5)

I feel that the best way for me to write a paper that honors Sir Nevill Mott is to continue a discussion on points of our mutual concern for Mott is not one who rests on his laurels but is in ceaseless struggle with nature, using the power of his mind as a searchlight into the unknown; in the case of amorphous materials, into the world of disorder. It is unfortunate that this negative emotive word, disorder, is used to describe amorphous materials since I believe that the operative description of the uniqueness of amorphous materials is really “freedom”-freedom from restrictions of crystalline symmetry, freedom from stoichiometry, and especially freedom of choices of bonding and nearest neighbor relationships. The multiple bonding and orbital relationships permitted by the removal of crystalline restrictions, while numerous, are specific and, in the manner and number in which they co-exist in an amorphous material, are definable as well as being unique.

The application of sufficiently high electric fields to any material eventually results in deviations from linearity in the observed current-voltage I(V) characteristic. There are two general classes of explanations for such non-Ohmic effects— thermal and electronic. Thermal effects arise because the electrons accelerated by the field always emit phonons in an attempt to return to equilibrium. Electronic effects are due to changes in the response of the charged carriers to high applied fields. In general, both effects must be considered in any quantitative analysis, and the two can produce a coupled response ofter called “electrothermal.” The use of the terminology electrothermal encompasses predominantly thermal and predominantly electronic processes as well as all intermediate cases, and therefore should not prejudice the casual observer into concluding that both effects are necessarily important. In a discussion of the physical mechanism in a particular sample, the major parameters controlling its operation must be identified and separated out from the less significant features.

As glow discharge amorphous silicon alloys become more important for commercial production of photovoltaic, xerographic, optical sensing, and microelectronic devices, economic considerations have motivated considerable research aimed at increasing deposition rate and gas utilization efficiency. Recently, a low pressure microwave glow discharge process was reported (1) which produces good quality a-Si:F:H films, with essentially 100% gas utilization at deposition rates > 100A/sec.

The structural, optical, and electrical properties of amorphous multilayer films containing up to 180 double layers of a-Si:H/a-SiNx and a-Si:H/a-SiOx were studied. For a-Si:H layer thickness d >100 A the transport properties are dominated by space-charge doping with a-SiNx positive and a-SiOx negatively charged. For d A quantum-well effects increase the optical and electrical gaps. A d =12 A multilayer film shows no evidence for the predicted loss of extended states in two-dimensional disordered systems.

In the first paper of this conference, Prof. Stuke presented a detailed review of the optical and electrical properties of the basic materials, Se, Ge and Te, which can be formed in both the amorphous and crystalline form1). In the final day of this meeting we would like to tie the basic physics learned from these discussions to more material ends — that is to the design of materials which can be utilized for the basic differences between crystal and amorphous forms and, in particular, the differences in optical properties.

Description of a double-injection space-charge process which can account for the principal features of amorphous threshold switching, and discussion of its applicability to chalcogenide glasses under various operating conditions.

This paper describes structural studies of the reversible memory type electrical transition1) in amorphous films of Ge-Te2) based alloys. The films, as evaporated, are high resistance intrinsic-like semiconductors. After subjection to a sufficiently high electrical field to bring about a transition to a low resistance state, and a further delay during which there is current flow, a portion of the material is transformed to a semipermanent low resistance state. That is, the material remains in a low resistance state without further application of an electric field. The material may be switched back to a high resistance state through application of a current pulse with a rapid turn off. This paper reports initial results of studies designed to elucidate structural aspects of the phase transition in these materials.

Radial distribution studies of amorphous Ge χ Te 1−χ alloys have been calculated from X-ray diffraction data for x = 0.11 and 0.54. These show peaks at approximately 2.7 A and 4.2 A. The absence of a peak at the crystalline GeTe first neighbor separation of 3 A is shown to imply that the local coordinations in the amorphous materials are different from those in crystalline GeTe. The areas under the first neighbor peaks indicated that it is very unlikely that the coordination numbers of the Ge and Te atoms are the same. These areas are consistent with models in which the average Ge and Te coordinations are four and two, respectively. This implies that the similarity in the optical absorption edges of crystalline and amorphous GeTe cannot be explained by similarities in short range structures. The similarities in bonding over a wide range of compositions in these amorphous alloys appear to lend support to Mott's picture for the failure of many impurities in many amorphous semiconductors to add significantly to the conductivity. The form of bonding postulated appears to be inconsistent with a microcrystalline picture for these alloys.

High field breakdown often leads (i) to a regenerative structural change in the material, or (ii) without any material change, to a conducting state which is maintained only above a certain holding current value. It is suggested that breakdown can be triggered by different processes depending on the temperature and the electrode configuration. The observed high field and non-equilibrium phenomena will be discussed in relation with the several proposed breakdown mechanisms. The material characteristics yielding the different switching effects will be discussed.

A three-fold classification of amorphous semiconductors into (i) elemental, (ii) covalent alloys, and (iii) ionic and tightly bound amorphous materials is proposed. The experimental evidence supporting a simple band model for the amorphous covalent alloys is presented. The present understanding of the reversible switching effects and of the switching with memory is discussed.

This paper will briefly review our past work in amorphous switching and discuss some of the operational concepts underlying our continued effort. In the past, periodicity of atomic structure has been the basis of the understanding of solid state physics. Part of the problem in understanding the amorphous state is the terms themselves, for the words “disordered” and “amorphous” for non-crystalline materials have misleading connotations as they project an image of a material with lack of specific structure. In this paper we will describe some of our working hypotheses which pay particular attention to spatial relationships of the localized structural units as a means of developing unusual electronic and electrical changes in amorphous materials.

We describe here a rapid and reversible transition between a highly resistive and a conductive state effected by an electric field which we have observed in various types of disordered materials, particularly amorphous semiconductors1,2 covering a wide range of compositions. These include oxide- and boron-based glasses and materials which contain the elements tellurium and/or arsenic combined with other elements such as those of groups III, IV, and VI.

Differential thermal analysis (DTA) and differential calorimeter (DCA) measurements were performed between 300 and 800°K on chalcogenide alloy glasses of various compositions. Fast and slow cooling cycles were used to determine the rates of structure formation. After each thermal cycle the electrical conductivity was determined. These studies were supplemented by thermal expansion measurements. The calorimetric studies distinguish between materials (i) which show no memory effect but only ovonic switching action and (ii) those which show switching effect with memory action. The memory materials (ii) show an exothermic reaction at a temperature T1 between 200–300°C and an endothermic reaction at a higher temperature. The bistable resistance state of these materials is associated with the formation near T1. The other materials (i) do not show any reaction and remain highly resistive even after slow cooling from the melt.

The influence of illumination and annealing on thin films of amorphous As2Se3 and As2S3 has been studied through their effects on the structure and optical properties. It is shown that the dominant photostructural change in evaporated films is the polymerization of the as-deposited molecular (As4S6 or As4Se6) glass through either illumination or heating. This irreversible polymerization gives rise to a large absorption edge shift to smaller energy and to an appreciable index of refraction change. In addition there is a small reversible absorption edge shift, due presumably to trapping of photo-generated electrons and holes which is larger in As2S3 than in As2Se3. This reversibility persists in both well-annealed evaporated films and in sputtered films which appear to be structurally indistinguishable both from each other and, aside from defects or voids, from the bulk glass. The reversible effect, called photo-darkening, is accompanied by a negligible refractive index change when compared with the irreversible polymerization change. The application of the present results to photochemical effects is discussed.

Assuming that a critical temperature must be reached for initiating of the electronic switching process, the time necessary to reach this temperature by Joule's heating after switching-on a certain voltage (delay time) is calculated, using the thermal balance equation in an essentially one-dimensional sandwich of a semiconducting glass, which is imbedded in a spherical material of high heat conductivity (electrodes). It is shown that the delay time for high overvoltages decreases exponentially with the same slope as the conductivity increases with voltage. Near the critical voltage a steep increase of the delay time results. The thermal conductivity of the glass can be obtained by adjusting the theoretical curve to the experiments. Excellent agreement between theory and experiment can be achieved.

Amorphous covalent alloys particularly of group-IV, -V, and -VI elements are readily formed over broad ranges of composition.1–6 They have been described as low-mobility electronic intrinsic semiconductors with a temperature-activated electrical conductivity σ = σ 0×exp(-ΔE/kT) which sometimes extends well into the molten state.2,3,7 They remain intrinsic with changed ΔE when their composition is changed.1,5,7 These alloys transmit infrared light up to an exponential absorption edge from which an energy gap E g is estimated.1,2 The value of E g usually is smaller than 2ΔE, often by as much as 10–20%.7,8 Photoconductivity9 and recombination-radiation10 measurements have been interpreted as giving evidence for the presence of localized states in the gap.

An analysis of the switching mechanism in semiconducting glasses is given which indicates that the actual switching involves electronic processes which prevent a thermal runaway. The rapid and reversible transition between the highly resistive and conductive states can have a thermal origin from Joule's heating of a current channel. Double tunneling, forced by the characteristic temperature profile, and current‐density saturation are suggested as an explanation of the characteristics in the conductive state.

Several long chained based alloy amorphous systems show a reversible optical effect with exposure to a sharply focused laser pulse. SEM and optical microscopic examination, along with chemical analysis, show that the basic optical effect is caused by scattering centers induced by flow in the amorphous phase. Reversibility can be achieved under the influence of less intense laser radiation. The investigations also show that as one of the effects, the ring/chain polymer ratio is altered by the light cycling and that, in the microseconds available during laser pulsing, photopolymerization must play an important role.

One of our major approaches to information storage and control through the years (1–5) has been to have external information alter the physical structure of amorphous material and have the change of structure detected unambiguously by the concomitant electrical, chemical, optical and other changes that accompany the structural transformation. In this manner, external information is encoded into an internal structural change.

Amorphous Zr-Rh alloys produced by melt spinning have a higher superconducting transition temperature Tc than materials generated by r.f. cosputtering techniques. However, the temperature dependence of the upper critical field Hc2(T is consistent with the GLAG theory for both materials. The Tc differences, as well as variations in the electronic density of states, can be partially understood in terms of slight differences in the average density, perhaps due to the inclusion of Ar gas during sputtering.

The accomplishment of high-temperature superconductivity is of immense scientific and technological importance. Several critical transition-temperature barriers have recently been breached since the long-standing record temperature of 23.2 K for Nb3Ge was exceeded. The most important milestones were the announcement of T c ≈ 30 K in lanthanum barium copper oxide by Bed-norz and Muller,1 whose work was based upon materials developed by Michel and Raveau,2 and the work of Chu, Wu, and others,3 based upon the replacement of lanthanum by yttrium, which resulted in superconductivity at temperatures of approximately 95 K.

What are the chemical and structural bases of localized states in amorphous materials? In the case of tetrahedral amorphous materials, chemistry and structure can be closely related and the localized states can be clearly distinguished, for example, materials which have not reached their lowest free energy can have dangling bonds, structurally dependent voids and other kinds of vacancy defects. We classify these as extrinsic localized states which can be annealed out. When the materials are at their lowest free energy, as represented by the ideal model of Polk,(1) the localized states which remain and are the result of chemical bonding and positional disorder we consider intrinsic. In such materials, substitutional impurities can insert new extrinsic localized states and the possibility of ordinary doping exists, as shown by the work of Spear and LeComber.(2) While this is an important subject, we believe that it will follow the concepts of classical crystalline tetrahedral materials.

Fluorine substitution for oxygen in the high Tc superconducting ceramic oxides is not only fundamentally but also technologically interesting. We have previously reported the observation of superconducting zero resistance transition at 155–168K, subsequently confirmed by several other groups. We have also observed diamagnetic signals and flux trapping at temperatures as high as 305K. The samples were made from solid-state reaction in a pressed pellet form. Because of the BaF2 formation and the small volume fraction, we have not yet been able to identify the specific high Tc phases.In the course of elucidating and optimizing the fluorination effect, we have carried out other means of low-temperature fluorination experiments. The zero resistance at 154K was repeated.In addition to the observation of these high 1c results, we found that a dopant amount of fluorine promotes the oriented crystal growth. We also found that the replacement of weakly bonded oxygen by fluorine can greatly enhance the material's stability by eliminating the serious problem of oxygen diffusion.