Browsing by Subject "Crystal growth"

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    Anharmonicity of zone-center optical phonons: Raman scattering spectra of GaSe0.5S0.5 layered crystal
    (IOPscience, 2002) Gasanly, N. M.; Aydınlı, Atilla; Kocabas, C.; Özkan H.
    The temperature dependencies (10-300 K) of the eight Raman-active mode frequencies and linewidths in GaSe0.5S0.5 layered crystal have been measured in the frequency range from 10 to 320 cm-1. We observed softening and broadening of the optical phonon lines with increasing temperature. Comparison of the experimental data with the theories of the shift and broadening of the interlayer and intralayer phonon lines showed that the temperature dependencies can be explained by the contributions from thermal expansion, lattice anharmonicity and crystal disorder. The purely anharmonic contribution (phonon-phonon coupling) is found to be due to three-phonon processes. It was established that the effect of crystal disorder on the broadening of phonon lines is greater for GaSe0.5S0.5 than for binary compounds GaSe and GaS.
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    Crystal growth and investigations on the effects of hydrogen doping of VO2
    (2019-03) Yavuz, Koray
    Vanadium Dioxide(VO2) has been studied extensively for its interesting electronic structure that allows it to go through Metal-Insulator Transition(MIT) at 65 C. The nature of this phenomena is not entirely clear and more research is needed to firmly establish the science behind it and to realize possible applications; such as ultra-fast electrical and optical switching, sensor devices and Mott-Field Effect Transistors. One of the important experiments to understand the electronic structure of a material is Hall-effect measurements but due to acicular (needle like) nature of VO2 crystals, this subject is only studied either on millimeter sized samples which are not suitable for many device applications or on poly crystalline thin films that are under non-uniform stress due to the substrate effects which gives unsatisfactory results when performing experiments. This thesis suggest a new method of chemical vapour deposition(CVD) growth for low aspect ratio VO2 crystals that have lengths between 50-100 m and thicknesses between 40- 170 nm. These crystals can be mechanically removed from the substrate and transferred to use in different applications such as Hall-effect measurements or Transmission Electron Microscope(TEM) studies. Additionaly this work shows some aspects of the surface chemistry of the widely used Silica, Si, quartz and Sapphire substrates; relating with the control of oxygen saturation on the surface. Another VO2 growth method for c-plane sapphire that leads to considerably more crystal yield is shown. Hydrogenation of the VO2 crystals suppresses the MIT so understanding this phenomena might help us better understand the effects lying behind the transition. To study this phenomena a crystal is doped only from half by blocking the passage of hydrogen to other half so the interplay between the insulating phase and hydrogenated conductive phase can be observed. As the analysis tool, TEM is used on this sample. Using a two-terminal device of a VO2 crystal, the effects of hydrogenation on the electronic properties have also been studied. Overall this thesis introduces a new method for CVD growth of VO2 which is used in various applications such as Hall-effect experiments, two terminal devices and TEM studies. To control the growth process the interplay between oxygen and surface chemistry of sapphire, silica, Si and quartz substrates have been investigated. With these studies a better understanding of the mechanics of growth is intended.
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    Thermally stimulated current observation of trapping centers in undoped GaSe layered single crystals
    (Wiley, 2001) Gasanly, N. M.; Aydınlı, A.; Salihoglu, Ö.
    Undoped p-GaSe layered single crystals were grown using Bridgman technique. Thermally stimulated current measurements in the temperature range of 10-300 K were performed at a heating rate of 0.18 K/s. The analysis of the data revealed three trap levels at 0.02, 0.10 and 0.26 eV. The calculation for these traps yielded 8.8 × 10-27, 1.9 × 10-25, and 3.2 × 10-21 cm2 for capture cross sections and 3.2 × 1014, 1.1 × 1016, and 1.2 × 1016 cm-3 for the concentrations, respectively.
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    Trap levels in layered semiconductor Ga2SeS
    (Elsevier, 2004) Aydınlı, Atilla; Gasanly, N. M.; Aytekin, S.
    Trap levels in nominally undoped Ga2SeS layered crystals have been characterized by thermally stimulated current (TSC) measurements. During the measurements, current was allowed to flow along the c-axis of the crystals in the temperature range of 10-300 K. Two distinct TSC peaks were observed in the spectra, deconvolution of which yielded three peaks. The results are analyzed by curve fitting, peak shape and initial rise methods. They all seem to be in good agreement with each other. The activation energies of three trapping centers in Ga2SeS are found to be 72, 100 and 150 meV. The capture cross section of these traps are 6.7×10-23, 1.8×10-23 and 2.8×10-22cm2 with concentrations of 1.3×1012, 5.4×1012 and 4.2×1012cm-3, respectively.
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    Trapping centers in undoped GaS layered single crystals
    (Springer, 2003) Gasanly, N. M.; Aydınlı, Atilla; Yüksek, N. S.; Salihoglu, Ö.
    Nominally undoped p-GaS layered single crystals were grown using the Bridgman technique. Thermally stimulated current measurements in the temperature range 10-300 K were performed at a heating rate of 0.10 K/s. The analysis of the data revealed six trap levels at 0.05, 0.06, 0.12, 0.63, 0.71, and 0.75 eV. The calculations for these traps yielded 1.2 × 10-21, 2.9 × 10-23, 2.4 × 10-21, 8.0 × 10-9, 1.9 × 10-9 and 4.3 × 10-10 cm2 for the capture cross sections and 1.6 × 1013, 5.0 × 1012, 7.3 × 1012, 1.2 × 1014, 8.9 × 1013 and 2.6 × 1013 cm-3 for the concentrations, respectively.