John P. Deneufville

Chemical aspects of glass formation in telluride systems

Abstract The compositional dependence of the electrical activation energy, the optical absorption and the glass transition temperature in typical binary, pseudo-binary and ternary glass forming telluride systems has been measured in an effort to correlate chemical binding with physical properties. A modification of the valency satisfaction model to include chemical ordering effects is required to account for the property singularities observed in the GeTe system at the composition GeTe2. The ordering effects in Te-based alloys containing Ge are further verified by an endothermic disordering reaction in these liquids peaking between 400–475°C depending on composition.

Neutron and X-ray diffraction radial distribution studies of amorphous Ge0.17Te0.83

Abstract Bulk amorphous samples of amorphous Ge 0.17 Te 0.83 have been prepared by quenching of small moltens droplets. Radial distribution functions have been constructed from both X-ray and neutron diffraction intensity data. These indicate first neighbor peaks at 2.75 A and 2.65 A, respectively, indicating the greater sensitivity of the neutron RDF to atoms pairs containing the smaller Ge atom. The first peak area in both RDFs is not consistent with twofold coordination of the Ge. It can be fit by two types of models. In the first, the Ge is fourfold and the Te twofold coordinated. In the second, the Ge is threefold coordinated, while some Te are threefold and others twofold coordinated. It is shown that both types of models are consistent with RDFs performed on other compositions.

Laser-induced fluorescence of the HSiF radical

Abstract HSiF has been detected by laser-induced fluorescence, following the formation of the ground-state radical in the flow reaction of SiH4 with F2 at a total pressure of 0.05-0.15 Torr. We report here the radiative lifetime of the first vibronic state of HSiF ( A 1A″) as well as a tentative assignment of the prominent bands of the A 1A″- X 1A

Thermal crystallization of selected Te-based sputtered thin films

transition.

LIF and OES detection of radical species in SiF4 + H2 plasmas

Abstract A series of X-ray diffraction analyses were performed of the crystallization behavior of thin ∼ 2 μm) sputtered films of a typical memory glass based on the TeGe binary eutectic composition. The predominant crystallization products are related to pure Te and GeTe. The Te phase is much the more easily crystallized and appears as the sole product after anneals of as long as one hour at 200°C. The remaining phase(s) appear at higher temperatures. The details of the crystallization rates, the morphologies of the various phase(s) and the thermodynamic considerations that govern their formation will be discussed. The influence of minor constituents will also be treated.

Studies of amorphous GeSeTe alloys (II): Thermal crystallization of sputtered Ge20Se40Te40

Abstract We have used laser-induced fluorescence (LIF) and optical emission spectroscopy (OES) to measure the relative concentrations of various radical species (in particular, SiF and HSiF) in glow-discharge plasmas of SiF 4 + H 2 used to deposit photovoltaic quality a-Si:H:F thin films. By varying the partial pressure ratio of SiF 4 versus H 2 , we can qualitatively relate the concentrations of certain ground-state radical species (detected by LIF) and excited-state species (detected by OES) with the electronic properties of the deposited films.

Reversible Optical Effects in Amorphous Semiconductors

Abstract RF sputtered films of the amorphous alloy Ge20Se40Te40 were subjected to a variety of thermal annealing treatments and examined by X-ray diffraction and transmission electron microscopy. The electrical conductivity changes which accompany the initial stages of Te crystallization were observed dynamically. At the first crystallization transition, which occurs at about 130°C, the number of Te crystals per unit volume grows rapidly from zero to about 3 × 1017 cm−3. Annealing the amorphous films at successively higher temperatures produced comparable volume fractions of Te crystals, but a progressive increase in crystallite size and a concomitant reduction in the number of crystals per unit volume was observed. Up to 340°C the structure consisted of Te crystals dispersed on a very fine scale in a glassy matrix with the approximate composition Ge29Se57Te14. The glass transition temperature of this phase is 340°C and the crystallization temperature is 350°C. The amorphous film crystallizes completely to Te and GeSe2 after annealing for 30 min at 375°C. We believe that the high density of Te crystals arises through two stages of homogeneous nucleation: the sputtered film first segregates into Te-Rich and GeSe2-rich amorphous phases, and, subsequently, Te crystals nucleate within the Te-rich amorphous phase or at the interface of the two amorphous phases. However, these nucleation events are essentially superimposed on each other, and were not temporally resolved in this study.

Superconductivity and structural features of the TeGeAs system

Abstract 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.

Progress in Large Area Photovoltaic Devices Based on Amorphous Silicon Alloys

Abstract Alloys in the TeGeAs system are superconducting below 1 K. X-ray studies show a correlation between a distorted GeTe lattice and elevated transition temperature. Scanning electron micrographs show an inverse trend between typical microstructure dimensions and the slope of the critical field curve. These observations suggest a model of interconnected superconducting GeTe regions in a background of normal Te.

Mechanism of Reversible Optical Storage in Evaporated Amorphous AsSe and Ge 10 As 40 Se 50

Photovoltaic devices based on amorphous alloys of silicon with H and/or F have recently generated substantial interest in the photovoltaic community and the larger solid state physics and electronics communities as well. These developments have been reviewed recently by D. Carlson of RCA Labs(1). It was Carlson’s early collaboration with C. Wronski(2) which served to rekindle scientific and technological interest in a-Si:H produced by the glow discharge induced decomposition of SiH4 gas and the deposition of Si-rich fragments of this gas onto substrates held at 200–300°C(3–5).