Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Characterization of Lattice Mismatch Induced Dislocations on Epitaxial Cdte Films(Izmir Institute of Technology, 2015) Bilgilisoy, Elif; Selamet, YusufMercury Cadmium Telluride (HgCdTe) is a widely used material for infrared focal plane array applications. In order to produce high quality infrared detecting material, HgCdTe needs to be grown on large area alternative substrates such as GaAs, GaSb, Si or Ge. GaAs is the best choice as an alternative substrate due to its surface polarity and commercially availability of high quality epi-ready wafers. However, there exists a lattice mismatch between HgCdTe and GaAs. To minimize the detrimental effect of the large lattice mismatch between the two materials, Cadmium Telluride (CdTe) is preferred as a buffer layer for HgCdTe IR material. The lattice mismatch between HgCdTe/CdTe and the GaAs substrate results in a large number of misfit dislocations. Dislocation density of the buffer layer limits and reduces the detector device performance. For this reason, the crystal quality and dislocation analysis of CdTe are examined in detail to produce large area and high performance HgCdTe IR devices. The aim of this thesis is the characterization of lattice mismatch induced dislocations on epitaxial CdTe buffer layers. CdTe epilayers which were grown on (211)B GaAs by molecular beam epitaxy (MBE) were subjected to two different etch treatments to quantify the crystal quality and dislocation density. The crystal quality was also obtained by using x-ray diffraction (XRD) measurements. The thicknesses of the samples were measured by ex-situ spectroscopic ellipsometry (SE). The surface morphologies of the CdTe buffer layers were analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM) and Nomarski microscopy before and after wet chemical etching. Vibrational phonon modes distributions of the as-grown and etched samples were examined by Raman spectroscopy mapping. The “triangle” and “trapezoid” shaped etch pits were compared due to the Everson and Nakagawa etching solutions, respectively. Measured etch pit density (EPD) values of “triangle” etch pits were found in 0.3x108 – 3.8x108 cm-2 range and “trapezoid” shaped etch pits were found in 0.03x108 – 0.6x108 cm-2 range for samples.Master Thesis Characterization of Molecular Beam Epitaxially Crown Cdte Layers Over Gaas by Spectroscopic Ellipsometry(Izmir Institute of Technology, 2014) Günnar, Merve; Selamet, YusufThe infrared detectors consist of two main parts that are optical elements and sensing elements. The sensor component is generally formed by semiconductor materials that can detect Infrared (IR) light which cannot be seen by human eye. Mercury Cadmium Telluride (MCT, HgCdTe) is widely used as a sensor material for this purpose. The adjustable bandgap (0-1.5 eV) which corresponds to energies of IR light can be obtained by changing the composition x of cadmium (Cd) in the ternary alloy Hg1-xCdxTe. HgCdTe has very high quantum efficiency for the detectible IR wavelengths in the atmospheric windows. HgCdTe which has a great importance in defense industry as an IR detecting material should be grown with high crystallinity in order to obtain high resolution images even under bad weather conditions. In addition, HgCdTe must be grown uniformly over a large area in order to have large format and high operability focal plane arrays. The defect density of HgCdTe strongly depends on the lattice mismatch between substrate and HgCdTe. In order to reduce the lattice mismatch which causes dislocations in HgCdTe the best suitable option is to grow Cadmium Telluride (CdTe) buffer layer on a substrate before growing HgCdTe. Studies have been focusing on semiconductors which are Gallium Arsenide (GaAs), Silicon (Si) and Germanium (Ge) as alternative substrates for CdTe growth. In this study, the CdTe films grown on (211) oriented GaAs wafers by molecular beam epitaxy (MBE) were characterized by ex-situ spectroscopic ellipsometry (SE). The properties of CdTe films such as thickness, surface roughness and optical constants were characterized by comparison with the growth conditions. It was also investigated that how these properties vary over the film surface. Characterization results were compared to those obtained by atomic force microscopy (AFM), Nomarski microscopy, Fourier transformation infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The temperature dependencies of the optical properties of the material obtained by SE were also investigated.