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Graduate / 0548

Single nickel crystals were grown from the melt by the Czochralski technique. Domain wall movements under applied magnetic fields were observed by using the Lang technique for X-ray topography. Magnetization measurements at room temperature were recorded and the variation of coercivity as a function of temperature was examined. Although low dislocation-density crystals were obtained, dislocation free growth was not achieved, copper single crystals were grown to compare the perfection with that of nickel. The effect of geometrical dimensions of crystal, neck, seed and neck angle on the temperature gradient at the interface of crystal and melt was examined theoretically using the model developed by Buckley-Golder and Humphreys. The model was extended to consider conical crystal and seed. It was found that for nickel the diameter had little effect on the thermal gradient. Increase of crystal cone angle led to a significant decrease in the thermal gradient at the interface. Real time movement of haematite domains under applied magnetic fields has been studied by synchrotron X-ray radiation topography.

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In the present thesis, optical and electrical properties of Ga0.75In0.25Se layered single crystals have been studied. The optical properties of the crystals have been investigated by means of visible and infrared reflectivity and transmittance, ellipsometry, Raman spectroscopy, photoluminescence (PL) and thermoluminescence (TL) measurements. The analysis of the absorption data at room temperature revealed the existence of indirect transitions in the crystal. Moreover, the rate of change of the band gap energy with temperature was calculated from the analysis of the temperature dependence of transmission measurements. The spectroscopic ellipsometry measurements on Ga0.75In0.25Se crystals were also performed to get detailed information about the real and imaginary parts of the pseudodielectric function, pseudorefractive index and pseudoextinction coefficient. The critical point analysis of the second derivative spectra of the dielectric function was done to reveal the interband transition energies. The vibrational spectra of Ga0.75In0.25Se crystals were studied by means of infrared reflectivity and transmittance and Raman scattering. The refractive and absorption indices, the frequencies of transverse and longitudinal optical modes, high- and low-frequency dielectric constants were obtained from the analysis of the IR reflectivity spectra. PL experiments were carried out as a function of temperature and excitation laser intensity to get detailed knowledge about the recombination levels in Ga0.75In0.25Se crystals. The observed emission bands in PL spectra were interpreted as the transitions from donor levels to an acceptor level. Electrical characterization of the crystal have been performed using dark electrical conductivity, space charge limited current, photoconductivity and thermally stimulated current (TSC) measurements. The detailed information about the localized levels in the band gap has been obtained from the analysis. The photoconductivity measurements were performed to determine the dominant recombination mechanism in the crystal. Defect centers in the crystal were characterized from TSC and TL measurements accomplished in the low temperature range. The activation energies, attempt-to-escape frequencies, concentrations and capture cross sections of the traps were calculated from the analysis of the experimental data.

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