Showing papers by "Indian Association for the Cultivation of Science published in 1992"

VHF Plasma Deposition: A Comparative Overview

Abstract: The use of plasma excitation frequencies f in the VHF band (30-300 MHz), and particularly of f=70 MHz, for the high-rate deposition of amorphous hydrogenated silicon (a-Si:H) is described. Deposition rates, using monosilane (SiH4) as source gas, are thereby increased roughly five fold to over 10 A/s as compared with the conventional case of RF plasma enhanced chemical vapour deposition with f=13.56 MHz. This may possibly be attributed to an enhancement in the high-energy tail of the electron energy distribution function (EEDF) of the plasma. Thereby, no noticeable deterioration in film properties is found. Characteristics of VHF-deposited a-Si:H films are extensively reported, including properties like microstructure, hydrogen effusion behaviour and its low internal mechanical stress. High quality hydrogenated microcrystalline silicon (μc-Si:H) can be deposited at low substrate temperature and low plasma power densities thanks to VHF glow discharge. This can be linked to a reduction in sheath potential and to the energy of the ions arriving at the growing surface. Thereafter, use of VHF plasma in applications such as 100 μm thick a-Si:H layer for particle detectors and powder-free deposition of solar cells with efficiencies over 8% are reported.

Electrical conductivity of sol-gel derived metal nanoparticles

Abstract: Electrically conducting films of thickness ∼ 2 μm have been prepared on ordinary glass slides by growing ultra-fine particles of iron and copper, respectively, from a suitable precursor sol. The diameters of metal particles can be varied from 3–13 nm by controlling the heat-treatment schedule of the sol coating. Resistivity measurements (d.c.) have been carried out over the temperature range 80–300 K. The resistivity values in the range 0.0001–0.0039 Ω cm have been obtained depending on the particle diameter and the type of metal used. The effective Debye temperature θD for the different nanoparticle systems have been estimated by fitting the experimental data to the Ziman equation. θD is found to vary from 346–408 K for iron with the particle size in the range 3.4–9.5 nm. The values obtained for copper are 243–307 K with particle diameters covering a range of 5.9–12.6 nm.

The formation of intermetallics in Cu/In thin films

Abstract: The kinetics of the formation of intermetallics in the Cu–In bimetallic thin film couple have been studied from room temperature to 432 K by measuring the evolution of composite and contact electrical resistance with time and temperature. The resistivity measurements have been supplemented by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Copper reacts with indium even at room temperature to form CuIn intermetallic and assuming a model of defect assisted diffusion into the grains, the activation energy averaged over five different samples is found to be 0.40 eV. The grain boundary diffusion is found to occur with an average activation energy of 0.55 eV. XRD confirms the growth of CuIn intermetallic and on annealing at higher temperature, for copper-rich films copper further reacts with CuIn to form Cu9In4. Further evidences of solid state reactions and grain boundary diffusion through Cu grain boundaries have been obtained from SEM study. TEM indicates the growth of the grain size on annealing and confirms the presence of the CuIn phase.

Ultrasonic properties of V2O5P2O5 amorphous materials at different temperatures

Abstract: Investigations are reported of the acoustical properties of V2O5 P2O5 glass, on which other studies such as structural, spectroscopic, NMR and electrical conductivity have been reported in the literature. In the present investigation, the velocity and attenuation of longitudinal and transverse ultrasonic waves in four different compositions of (V2O5)x (P2O5)1−x glass system (x = 0.4769, 0.6020, 0.7487 and 0.8227) have been measured at temperatures ranging from 80 to 300 K with the help of an ultrasonic pulse echo apparatus operating at 8 MHz frequency. The velocity data have been used to find the elastic moduli and Debye temperature for each composition. The results indicate that for each composition the velocities decrease slowly and monotonically with increasing temperature and the attenuation as a function of temperature shows a broad peak which shifts to lower temperature as the V2O5 content increases. The temperature dependence of ultrasonic velocity and attenuation has been interpreted in terms of a thermally activated relaxation process which comes into play when the ultrasonic wave disturbs the equilibrium of an atom moving in a double-well potential in the glass network.

Correlated-barrier hopping in semiconducting tellurium molybdate glass.

Abstract: Measurements are reported for the real and imaginary parts of the ac conductivity of semiconducting tellurium molybdate glass in the frequency range ${10}^{2}$--${10}^{5}$ Hz and in the temperature range 80--350 K. The experimental results are interpreted in terms of the classical correlated-barrier hopping theory.

Microhardness studies in gel-grown ADP and KDP single crystals

Abstract: Microhardness studies were carried out on (100) faces of gel-grown ADP and KDP single crystals. Slip lines were observed on (100) face of ADP crystal at the corners of the impressions. Microcracks around the indentation were found on (100) face of KDP crystal from 10g load which spread out as the load increased. Vickers hardness numberH v decreased with increase in load. ΔH v at 50g load for solution-grown crystals and gel-grown crystals (present case) was determined. Work hardening indexn for both ADP and KDP crystals was less than 2 showing soft-material characteristics. Using Wooster’s empirical relation, values of C11 from hardness were calculated and found to be close to the reported ones.

Non-Markovian master equation for linear and nonlinear systems.

Abstract: Based on the standard system-heat-bath model, we have formulated a theory of the non-Markovian master equation for linear and nonlinear systems for a correlation time that is short but finite. We have applied the theory to linear systems, such as harmonic oscillators and two-level atoms and calculated the band-shape functions for transient optical absorption in the pump-probe experiments. The significant interplay of nonlinearity and non-Markovian decay has been illustrated by another application of the theory to a Morse oscillator. The theory is valid for arbitrary nonlinearity and finite temperatures but within the weak-coupling limit and has been derived without ad hoc implementation of a phenomenological memory function.

Electronegativity and Bader's bond critical point

Abstract: The electronegativities () of some 36 atoms/groups (including some 6 ionic ones) X are calculated from the atomic charges in the corresponding methyl species CH3X that were obtained by applying Bader's theory of atoms in molecules. The numerical values of the for the various groups studied are reasonable and correlate linearly with the two existing experimental scales for group electronegativity, Inamoto's i scale and the 1JCC (ortho-ipso) coupling constants in the monosubstituted benzenes, to satisfactory extents. The relations between the values and some “critical properties” of the various CH3X molecules considered are also studied. It is suggested that in a molecule PX, rP/R where rP is the distance of Bader's critical point on the bond PX of length R from the atom P or the binding atom of the group P can be a very good measure of the electronegativity of the atom/group X. © 1992 by John Wiley & Sons, Inc.

Molecular electrostatic potential: a tool for the prediction of the pharmacophoric pattern of drug molecules

Abstract: Molecular electrostatic potential (MEP) calculations were used to explain the diverse pharmacological properties of γ-aminobutyric acid (GABA), the celebrated inhibitory neurotransmitter, and several GABA mediators. MEP calculations were also performed on several bipyridine cardiotonics, to account for their observed cardiotonic properties.