Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
Browse
3 results
Search Results
Master Thesis Defect Reduction Study of Molecular Beam Epitaxially Grown Cdte Thin Flims by Ex-Situ Annealing(Izmir Institute of Technology, 2015) Bakali, Emine; Selamet, YusufMolecular Beam Epitaxy (MBE) grown CdTe thin films were annealed in this study to decrease the number density of defects. For annealing, a system was designed and constructed. During anneals; anneal temperature, anneal time, anneal cycle and hydrogen gas effects were analyzed. The effects of annealing parameters were analyzed by Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), Everson Etch method, resonance Raman spectroscopy and Photoluminescence measurement. In our studies, dislocation density decreased for 5 min. annealing when annealing temperature increased. Dislocation density decreased with increasing annealing time. Besides, dislocation density decreased when cycle number increased. Te precipitation decreased with annealing. Raman mode at 144 cm-1 was investigated and that mode was decided as Te E mode. Also I2LO/ILO ratio decreased with increasing annealing temperature and annealing time. I2LO/ILO ratio were approached to 1 at 80oK due to so called ‘resonance Raman scattering’. Extra peaks were also observed by Raman scattering. On the surface, small pits occurred when annealing temperature increased. Surface roughness decreased with increasing cycle number.Master Thesis Characterization of Defect Structure of Epitaxial Cdte Films(Izmir Institute of Technology, 2014) Özden, Selin; Selamet, YusufMercury Cadmium Telluride (HgCdTe) is widely used material for infrared detection. Epitaxial growths carried on Gallium arsenide (GaAs) substrates gained more attention in recent years due to commercially availability of epi-ready wafers. However, large lattice mismatch between the HgCdTe epilayer and GaAs substrates, and Gallium (Ga) diffusion into HgCdTe layers during growth limit the device performance. In order to decrease large lattice mismatch and hereby dislocations formed at HgCdTe epilayer, a closely lattice matched Cadmium Telluride (CdTe) is preffered buffer layer for Molecular Beam Epitaxial (MBE) growth of HgCdTe. This thesis focuses on a study of defects on (211)B CdTe buffer layers grown on (211)B oriented GaAs substrates by MBE. Prior to epitaxial growth of CdTe layers, to understand the effect of wet cleaning procedure on chemical composition of epi-ready GaAs wafers, piranha solution-based wet chemical etching and oxide removal processes using diluted hydrofluoric acid (HF) were performed on undoped 625≤25 μm thick GaAs(211)B wafers. The surfaces of GaAs wafers were investigated by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The variation of As2O3 and Ga2O3 contents on GaAs (211)B wafers studied by Raman spectroscopy. Following the growth of CdTe (211)B epitaxial films, the quality of CdTe layers were investigated in detail by various characterization techniques such as AFM, SEM, Nomarski Microscopy, X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy. Thicknesses of CdTe layers were calculated via intensity oscillations in the transmittance spectrum of the films.Master Thesis Influence of Ni Thin Flim Structural Properties Over Graphene Growth by Cvd(Izmir Institute of Technology, 2013) Özçeri, Elif; Selamet, YusufThis thesis work focused on the effect of polycrystalline Nickel (Ni) TM thin film structure on the growth graphene by chemical vapor deposition (CVD). TM films were deposited by magnetron sputtering technique on Si/SiO2 substrates. To grow 1-2 layer graphene on Ni thin film catalyst by methane decomposition thermal CVD method was carried out using various growth parameters. To reduce the TM film surface roughness and grow larger size graphene layers on Ni film, Si/SiO2 substrates were coated by a thin Al2O3 buffer layers and Cr adhesive layers by magnetron sputtering. Ni film crystal structure and surface roughness, which affected the number of graphene layers, were examined by X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM) techniques, respectively. The thickness and columnar structure of the films were measured from Surface Profiler and Scanning Electron Microscopy (SEM) images. Ni films were annealed at 800 oC, 900 oC and 950 oC in order to improve their crystal quality and to evaluate the effect of the crystallinity on graphene growth at atmospheric pressure. Samples were studied using XRD and AFM also to assess their crystal quality after the annealing process. It was observed that the calculated grain sizes depended on the film thickness and the annealing temperature. Surface roughness of the films was increased by increasing film thickness. A sole thin Al2O3 buffer layer reduced the surface roughness significantly. However, sole Cr adhesive layer or Cr/Al2O3 buffer layers did not reduce the surface roughness, but increased the crystallinity of Ni films in (111) direction. Argon, Hydrogen or a mixture of these two gases was added to methane during graphene growth at ambient pressure by CVD. The Raman spectroscopy was utilized in order to determine the number of the layers and quality of graphene growth over the Ni catalyst film.