K. S. Sangunni

Chemical ordering in Ge20Se80−χInχ glasses

Bulk glasses of GeSeIn have been prepared by quenching the melt and have been subjected to thermal crystallization and X-ray structure analysis of devitrified materials. These glasses show a transformation from a single-stage to a double-stage crystallization with respect to composition. The present investigations indicate that these materials are structurally inhomogeneous at a molecular level, in which In2Se3 microclusters are dispersed in the GeSe matrix. These results are discussed using the chemical threshold in glasses.

Structural correlations in GexSe1−x glasses – a neutron diffraction study

Neutron diffraction measurement is carried out on GexSe1-x glasses, where 0.1 less than or equal to x less than or equal to 0.4, in a Q interval of 0.55-13.8 Angstrom(-1). The first sharp diffraction peak (FSDP) in the structure factor, S(Q), shows a systematic increase in the intensity and shifts to a lower Q with increasing Ge concentration. The coherence length of FSDP increases with x and becomes maximum for 0.33 less than or equal to x less than or equal to 0.4. The Monte-Carlo method, due to Soper, is used to generate S(Q) and also the pair correlation function, g(r). The generated S(Q) is in agreement with the experimental data for all x. Analysis of the first four peaks in the total correlation function, T(r), shows that the short range order in GeSe2 glass is due to Ge(Se-1/2)(4) tetrahedra, in agreement with earlier reports. Se-rich glasses contain Se-chains which are cross-linked with Ge(Se-1/2)(4) tetrahedra. Ge-2(Se-1/2)(6) molecular units are the basic structural units in Ge-rich, x = 0.4, glass. For x = 0.2, 0.33 and 0.4 there is evidence for some of the tetrahedra being in an edge-shared configuration. The number of edge-shared tetrahedra in these glasses increase with increasing Ge content.

Sulphur passivation of gallium antimonide surfaces

Improvement in optical and electrical properties were observed after sulphur passivation of gallium antimonide surface. Enhancement of photoluminescence intensity up to 60 times, reduction in surface state density by two orders of magnitude, and reverse leakage currents by a factor of 20-30 were obtained as a result of surface passivation. While the reduction of surface recombination is attained, the surface is not unpinned.

Glass formation in germanium telluride glasses containing metallic additives

Differential scanning calorimetric (DSC) studies have been carried out on germanium telluride glasses containing Cu and Ag. Both CuxGe15Te852xO2 # x # 10U and AgxGe15Te852xO2:5 # x # 21:5U glasses are found to exhibit single glass transition and single crystallization. On the basis of the devitrification behaviour of these glasses one can conclude that the network connectivity of the parent Ge‐Te matrix is not improved by the addition of Cu whereas Ag improves the connectivity. Overconstraining of the structural network for x . 5 is rapid in Cu added glasses and more gradual in Ag added glasses. The difference in the glass formation in the Cu‐Ge‐Te and Ag‐Ge‐Te systems is understood in light of the above differences in the structural network. q 1999 Elsevier Science Ltd. All rights reserved.

Growth of gallium antimonide by vertical Bridgman technique with planar crystal-melt interface

High quality single crystals of GaSb were grown using vertical Bridgman technique with a planar melt-solid interface. Various factors affecting the interface shape during growth were investigated. In general, the shape of the freezing isotherm was found to depend on the furnace temperature profile near the melt-solid interface, the ampoule lowering rate, the ampoule geometry, the mode of heat extraction from the tip of the ampoule and the extent of lateral heat loss from the side walls of the ampoule. A critical ratio of temperature gradient of the furnace at the melting point to ampoule lowering rate was found to be necessary for planar interface shape during the growth. The sensitivity of the interface shape was found to decrease with increasing temperature gradient of the furnace and ampoule diameter. Crystals grown by employing the flat melt-solid interface exhibited superior quality than those with non-planar interfaces

Coexistence of fast photodarkening and slow photobleaching in Ge19As21Se60 thin films

We experimentally demonstrate the coexistence of two opposite photo-effects, viz. fast photodarkening (PD) and slow photobleaching (PB) in Ge(19)As(21)Se(60) thin films, when illuminated with a laser of wavelength 671 nm. PD appears to begin instantaneously upon light illumination and saturates in tens of seconds. By comparison, PB is a slower process that starts only after PD has saturated. Both PD and PB follow stretched exponential dependence on time. Modeling of overall change as a linear sum of two contributions suggests that the changes in As and Ge parts of glass network respond to light effectively independent of each other.

Direct hexagonal transition of amorphous (Ge2Sb2Te5)0.9Se0.1 thin films

Ge2Sb2Te5 (GST) is well known for its phase change properties and applications in memory and data storage. Efforts are being made to improve its thermal stability and transition between amorphous and crystalline phases. Various elements are doped to GST to improve these properties. In this work, Se has been doped to GST to study its effect on phase change properties. Amorphous GST film crystallized in to rock salt (NaCl) type structure at 150 degrees C and then transformed to hexagonal structure at 250 degrees C. Interestingly, Se doped GST ((GST)(0.9)Se-0.1) film crystallized directly into hexagonal phase and the intermediate phase of NaCl is not observed. The crystallization temperature (T-c) of (GST)(0.9)Se-0.1 is around 200 degrees C, which is 50 degrees C higher than the T-c of GST. For (GST)(0.9)Se-0.1, the threshold switching occurs at about 4.5V which is higher than GST (3 V). Band gap (E-opt) values of as deposited films are calculated from Tauc plot which are 0.63 eV for GST and 0.66 eV for (GST)(0.9)Se-0.1. The E-opt decreases for the films annealed at higher temperatures. The increased T-c, E-opt, the contrast in resistance and the direct transition to hexagonal phase may improve the data readability and thermal stability in the Se doped GST film

Non-isothermal transformation of Pb-modified Ge-Se glasses

Abstract Differential scanning calorimetric studies have been carried out on GeXSe80-x Pb20(17≤×≤24) bulk glasses, which exhibit a change from p- to n-type conduction at x=21at.%. All the samples show a single-stage crystallization followed by a single-stage glass transition. The dependence of glass transition temperature (T g) on composition shows a maximum at x=21at.%. Structural relaxation around the glass transition has been studied by measuring Tg as a function of heating rate (β). Values of activation enthalpies (E g) of the glass transition for different samples reveal that chemical bond breaking in the process of structural relaxation at T g cannot be ignored. Crystallization kinetics have been analysed using the Kissinger method. The crystallization activation energy (E c) and the frequency factor (C o), calculated using the above method for different compositions, show anomalies at x=21 at.%. All the samples show the existence of two distinct melting points. The observed compositional dependence...

Structural transition and enhanced phase transition properties of Se doped Ge2Sb2Te5 alloys

Amorphous Ge2Sb2Te5 (GST) alloy, upon heating crystallize to a metastable NaCl structure around 150°C and then to a stable hexagonal structure at high temperatures (≥250°C). It has been generally understood that the phase change takes place between amorphous and the metastable NaCl structure and not between the amorphous and the stable hexagonal phase. In the present work, it is observed that the thermally evaporated (GST)1-xSex thin films (0 ≤ x ≤ 0.50) crystallize directly to the stable hexagonal structure for x ≥ 0.10, when annealed at temperatures ≥ 150°C. The intermediate NaCl structure has been observed only for x < 0.10. Chemically ordered network of GST is largely modified for x ≥ 0.10. Resistance, thermal stability and threshold voltage of the films are found to increase with the increase of Se. The contrast in electrical resistivity between the amorphous and crystalline phases is about 6 orders of magnitude. The increase in Se shifts the absorption edge to lower wavelength and the band gap widens from 0.63 to 1.05 eV. Higher resistance ratio, higher crystallization temperature, direct transition to the stable phase indicate that (GST)1-xSex films are better candidates for phase change memory applications.

A consistent approach towards Bi doping mechanism in chalcogenide glasses from Cp measurement in Ge-Se-Te-Bi system

In this paper, we are proposing a consistent approach to understand the mechanism of Bi doping in Ge‐Se glasses by carrying out specific heat (Cp) measurement in Ge20Se702xTe10Bix system for 0 # x # 11: The two softening temperatures exhibited by heattreated samples are explained on the basis of ‘microscopic phase separation’. The minimum in ‘configurational heat capacity’ (DCp )a txa 7:5 for as-prepared glasses is related to be a feature of chemical threshold. A supporting evidence for three-fold coordination for Bi in line with the earlier structural studies is deduced. q 2000 Elsevier Science Ltd. All rights reserved.