Effect of topological thresholds on thermal behaviour of germanium telluride glasses containing metallic additive

Differential Scanning Calorimetric (DSC) studies on Ag_xGe₁₅Te_85-x glasses have been undertaken over a wide range of compositions, to understand the effect of topological thresholds on thermal properties. It is found that the compositional dependence of glass transition temperature (T _g ), crystallization temperature (T _c ), activation energy for crystallization and thermal stability show anomalies at the rigidity percolation threshold. Unusual variations also observed in different thermal properties at the composition x = 20, clearly establishes the occurrence of chemical threshold in these glasses. Received: 27 January 1998 / Revised: 12 June 1998 / Accepted: 3 July 1998     

Photoconductivity studies on bulk As-Te-In glasses

Photocurrent measurements have been undertaken on bulk As₄₀Te_60-xIn_x glasses of different compositions. It has been found that the photocurrent increases with illumination, which can probably be understood on the basis of the large dielectric constant of these materials and also due to the presence of a larger number of positive defect states. Further, the composition dependence of the conductivity activation energy and the photosensitivity exhibit a maximum at x=12.5(〈r〉=2.65) and a minimum at x=15(〈r〉=2.70). Earlier investigations on As-Te-In glasses, such as high-pressure resistivity behavior, electrical switching, etc., have identified the compositions x=12.5 and x=15 as the rigidity percolation threshold (RPT) and the chemical threshold (CT) respectively. Based on this, the maximum and the minimum seen in activation energy and photosensitivity of As₄₀Te_60-xIn_x glasses at x=12.5 and x=15 can be attributed to RPT and CT respectively. PACS 72.40.w; 71.55.Jv; 72.80.Ng     

High pressure studies on AgI-Ag2O-MoO3 glasses

The effect of pressure on the conductivity of AgI-Ag₂O-MoO₃ glasses has been reexamined. A conductivity maximum is observed around 0·7 GPa. No variation of the sample temperature is noted under pressure. The results are found to agree well with the cluster-tissue model.     

Optical,photo-acoustic and electrical switching studies of amorphous GeS2 thin films

This paper reports optical, photo-acoustic and electrical switching investigations of GeS₂ amorphous thin films of different thicknesses, deposited on glass substrates in vacuum. The Tauc parameter (B ^1/2) and Urbach energy (E _U) have been determined from the transmittance spectra, to understand the changes in structural disorder; it is found that B ^1/2 increases whereas E _U decreases as the thickness of the films increases. Based on the results, it is suggested that bond re-arrangement, i.e. transformation from homopolar bonds to heteropolar bonds, takes place with increase in thickness. The thermal diffusivity values of GeS₂ thin films also show the presence of a chemically ordered network in the GeS₂ thin films. Further, it is found that these films exhibit memory-type electrical switching. The observed variation in the switching voltages has been understood on the basis of increase in chemical order.     

Structural,optical and transport properties of 4-hydroxy coumarin: an organic Schottky diode

In the present paper, 4-hydroxy coumarin compound was synthesized. The crystallographic phase of this molecule was carried out by X-ray diffraction and confirms its monoclinic structure. Associated functional groups were predicted by Fourier transform infrared spectroscopy. To see its potential utility, a Schottky diode of this material was fabricated. The current–voltage (I–V) and optical characteristics of this Schottky diode were also investigated. Various well-established methods related with the device were used to get information about various diode parameters. From its optical measurement, this compound shows an indirect allowed transition. From optical data analysis, a band gap (E _g) of around 3.78 eV was shown by this compound. The observed features exhibited by this molecule give a vital chance to explore its application for various optoelectronic devices.     

Study on the ferromagnetism in Co and N doped ZnO thin films

Present investigation reports the structural, optical and magnetic properties of co-doping of Co and N ions in ZnO samples, prepared by two distinct methods. In the first method, samples are synthesized by Sol–gel technique in which the Co and N are co-doped simultaneously during the growth process itself. In the second case, N ions are implanted in the Co doped ZnO thin films grown by Pulsed Laser Deposition (PLD). Structural studies showed that the nitrogen implantation on Co doped ZnO samples developed compressive stress in the films. X-ray photoelectron spectroscopy confirmed the doping of Co and N in ZnO matrix. In the Resonant Raman scattering multiple LO phonons up to fifth order are observed in the (Co, N) co-doped ZnO. Photoluminescence spectra showed that there is reduction in the bandgap due to the presence of Co in the lattice and also the presence of Zn vacancies in the films. All samples showed ferromagnetic behavior at room temperature. The magnetic moment observed in the implanted films is found to be varied with the different dosages of the implanted N ions. First principle calculations have been carried out to study the possible magnetic interaction in the co-doped system. Present study shows that the ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states in the (Co, N) co-doped ZnO and is very sensitive to the geometrical configurations of dopants and the vacancy in the ZnO host lattice.     

Electrical and structural properties of the swift heavy ion irradiated Au/n-GaAs schottky barrier diodes

Abstract

Au/n-GaAs Schottky Barrier Diodes (SBDs) have been fabricated on LEC grown silicon doped (100) GaAs single crystals. The SBDs were irradiated using high energy (120 MeV) silicon ion with fluences of 1 × 10 ¹¹ and 1 × 10¹² ions/cm². Current-Voltage (I-V) characteristics of unirradiated and irradiated diodes were analyzed. The change in the reverse leakage current increases with increasing ion fluence. This is due to the irradiation induced defects at the interface and its increase with the fluence. The diodes were annealed at 573 and 673 K. to study the effect of annealing. The rectifying behavior of the irradiated (fluence of 1 × 10¹² ions/cm¹²) SBDs improves upon as the annealing temperature increases and is attributed to the in situ self-annealing during irradiation. Scanning Electron Microscopic analysis was carried out on the irradiated samples to delineate the projected range and to observe defects.     

Effect of pressure on the electrical resistivity of bulk Ge20Te80 glass

An irreversible pressure induced semiconductor-to-metal transition in bulk Ge₂₀Te₈₀ glass is observed at about 5 GPa pressure. The high pressure phase has a face centered cubic structure with a lattice constant 6.42 A° as deduced by X-ray diffraction studies on the pressure quenched samples. The temperature and pressure dependence of the electrical resistivity confirms the observed transition to be a semiconductor-to-metal transition. The temperature dependence of thermo electric power is also reported.     

Composition dependence of photoconductivity of Al20AsxTe80−x glasses

The time dependent photocurrent of Al₂₀As_xTe_80−x glasses has been studied at low temperatures. It is found that the photocurrent of all the Al₂₀As_xTe_80−x samples studied does not decrease appreciably during illumination, which is consistent with the behavior of other narrow band gap amorphous chalcogenides. Further, the photosensitivity is found to be maximized for the composition x=25, which can be associated with rigidity percolation.     

A facile method for the synthesis of nicotinonitriles from ketones via a one-pot chloromethyleneiminium salt mediated three-component reaction

Simple enolizable ketones such as acetophenones and benzalacetones were treated with malononitrile under Vilsmeier-Haack reaction conditions to afford 2-chloronicotinonitriles. The reaction proceeds via a one-pot chloromethyleneiminium salt mediated three-component reaction followed by sequential cyclization and aromatization under Vilsmeier-Haack reaction conditions.