• Vikram Kumar

      Articles written in Bulletin of Materials Science

    • Vapour phase crystal growth under microgravity environment

      Vikram Kumar

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      Vapour phase crystal growth experiments performed in the Skylab and ASTP missions are reviewed. The basic vapour phase crystal growth technique is described and effect of gravity is discussed. The multipurpose furnace specially designed to carry out various experiments in flight conditions is described. Ge Se, Ge Te and GeS as well as ternary GeSe0·99 Te0·01 and GeS0·98 Se0·02 crystals have been grown in space showing improvement over similarly grown crystals on ground as determined by x-ray diffraction, chomical homogeneity and surface morphology studies. Mass flux rates under microgravity conditions have been found to be up to 10 times larger than expected indicating need for better theoretical and experimental understanding of the effect of gravity on crystal growth.

    • Characterization of deep levels in semi-insulating gallium arsenide

      Y N Mohapatra N Balasubramanyam Vikram Kumar

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      Two traps with activation energies ofEc – 0·47 eV andEv + 0·79 eV have been detected in semi-insulating GaAs:Cr through optical transient current spectroscopy (otcs) in the temperature range 300–450 K. The latter trap gives rise to rising current transients which result in a negative peak in theotcs spectrum. The theoretical expressions for current transients have been derived.

    • Characterization of defects in gallium arsenide

      Vikram Kumar Y N Mohapatra

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      It is well-known that the properties of semiconductor materials including gallium arsenide are controlled by defects and impurities. The characterization of these defects is important not only for better understanding of the solid state phenomena but also for improved reliability and performance of electronic devices. We have been investigating the defects in gallium arsenide for several years using deep level transient spectroscopy, photoconductivity, transient photoconductivity, photoluminescence etc. Results drawn from our recent studies are presented here to illustrate some of the problems concerning transition metal impurities, process-induced defects, occurrence of intracentre transitions and metastability of deep levels in gallium arsenide.

    • Bright red electroluminescence in diffused porous siliconp-n junction

      V K Jain Amita Gupta A Kumar G K Singhal O P Arora T Srinivasan D S Ahuja P P Puri Vikram Kumar

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      We report the first operation of light emittingp-n junction diode in porous silicon fabricated by diffusion.

    • Foreword

      Vikram Kumar

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    • Influence of deviation from stoichiometry on the photoluminescence in CdTe doped with indium

      Suma Gurumurthy K S R K Rao A K Sreedhar H L Bhat B Sundersheshu R K Bagai Vikram Kumar

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      Low temperature photoluminescence of vacuum and cadmium annealed CdTe:In is reported here. A new peak at ∼ 1·14 eV related to transitions from the conduction band to an acceptor involving a tellurium vacancy has been observed.

    • Some recent advances in bulk growth of mercury cadmium telluride crystals

      R K Sharma R K Bagai Vikram Kumar

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      The inherent metallurgical problems associated with the HgTe/CdTe pseudobinary alloy system render the standard crystal growth processes inapplicable to the preparation of mercury cadmium telluride crystals for infrared detector applications. A variety of rather nonconventional techniques have been developed to overcome these problems. Two such techniques, viz. asymmetrical Bridgman and horizontal casting for solid-state recrystallization, developed at Solid State Physics Laboratory for the bulk growth of mercury cadmium telluride crystals are reviewed in this communication.

      Due to the poor thermal conductivity of mercury cadmium telluride melts and solids, and the use of thick-walled quartz ampuoles, it is extremely difficult to obtain a flat solid-liquid interface during Bridgman growth of this material. The technique of asymmetrical Bridgman has been successful in overcoming this problem to a great extent. Solid-state recrystallization has been widely accepted as one of the most successful techniques for obtaining large quantities of acceptable-quality mercury cadmium telluride crystals for infrared detector applications. This is a two-step process—the melt is first quenched to obtain a good cast, which is then subjected to a grain-growth annealing. The horizontal casting procedure developed for solid state recrystallization growth has been successful in improving the overall quality and yield of bulk mercury cadmium telluride crystals.

    • Liquid phase epitaxial growth of pure and doped GaSb layers: morphological evolution and native defects

      P S Dutta H L Bhat Vikram Kumar

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      Undoped and Te-doped gallium antimonide (GaSb) layers have been grown on GaSb bulk substrates by the liquid phase epitaxial technique from Ga-rich and Sb-rich melts. The nucleation morphology of the grown layers has been studied as a function of growth temperature and substrate orientation. MOS structures have been fabricated on the epilayers to evaluate the native defect content in the grown layers from theC-V characteristics. Layers grown from antimony rich melts always exhibitp-type conductivity. In contrast, a type conversion fromp- ton- was observed in layers grown from gallium rich melts below 400 C. The electron mobility of undopedn-type layers grown from Ga-rich melts and tellurium doped layers grown from Sb- and Ga-rich solutions has been evaluated.

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