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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.

ORDER-DISORDER transformations such as ordering of magnetic or ferroelectric domains are used in many information storage devices. The structural transformation of amorphous semiconductors from the amorphous to a more ordered state can be used for the same purpose if the structure transformation can be achieved fast and reversibly.1 Amorphous semiconductors have then the advantage over other information storage devices in that they can be cheaply produced and easily shaped in many different configurations. Furthermore, for semiconductors the difference between the physical properties of the amorphous and the crystalline state is particularly large. This enables one to retrieve and read the binary information, stored in the form of the structural state, with good signal to noise ratio despite extensive miniaturization.

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.

Ovonic threshold switches have been exposed to pulses of flash X-rays to levels of 1.8×1011 rads/sec and to fast neutron fluences of as high as nvt = 1.2×1017 n/cm2. The switching devices have continued to function during the transient of the X-ray flash and experienced no permanent damage or change of their electrical parameters in excess of the resolution of the experimental testing procedure which was about ±100% in the case of the neutron exposure.

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 have observed a high-speed crystallization of amorphous semiconductor films and the reversal of this crystallization back to the amorphous state using short pulses of laser light and evidenced by a sharp change in optical transmission and reflection. This optical switching behavior is analogous to the memory-type electrical switching effect in these materials which has received wide attention1 since the observation by S. R. Ovshinsky2 of both threshold and memory switching in amorphous semiconductors. In this letter, we propose a model which closely relates the optical and electrical switching behavior, and shows that the phase change from amorphous to crystalline state is not only a thermal phenomenon but is directly influenced by the creation of excess electron-hole carriers by either the light, or, for the electrical device, by the electric field. The reversibility of the phenomenon in this model is obtained through the large difference in crystallization rates with the light on or off.

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.

Performance and reliability of amorphous semiconductor devices that deal with the handling of information in the form of switching, modulation, storage, and display are discussed. Structural changes between a disordered and a more ordered state and the concomitant large change in many material properties offer the possibility of using amorphous semiconductors for high-density information storage and high-resolution display devices. The structural changes can be initiated by various forms of energy such as an electrical pulse, a short light pulse, or a brief light exposure. Many materials show good structural reversibility. The sensitivity of an amorphous photostructural film is amplified by several orders of magnitude by first forming a latent image by photonucleation and subsequent dry development by heat or radiation. Examples of optical contrast and resolution in image formation are given. The major differences between crystalline and amorphous semiconductors are briefly outlined.

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.

A tunnel triode was fabricated using A12O3 dielectric to form the hot-electron tunnel cathode and an amorphous film of As34Te28Ge16S21Se1 to separate the base and collector. The structure of the device is shown in Fig. 1. The electrodes were all aluminum. A 2000-A emitter electrode was evaporated onto a glass substrate and anodized to form an A12O3 layer 125 A thick as described by Onn and Silver.1 This was then coated with a 125-A Al layer to form the base electrode followed by a 1500-A sputtered layer of the amorphous material over which was subsequently deposited a 2000-A collector electrode.

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http://archive.org/details/aspeechnalysisnd1094514602 U.S. Navy (U.S.N.) author.