Phd Degree / Doktora

Permanent URI for this collectionhttps://hdl.handle.net/11147/2869

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Author: search.filters.author.Özyüzer, Lütfi

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Now showing 1 - 4 of 4
  • Doctoral Thesis
    Terahertz Imaging Applications and Characterization of Ito Thin Films Grown by Magnetron Sputtering
    (Izmir Institute of Technology, 2020) Köseoğlu, Hasan; Özyüzer, Lütfi
    THz radiation (0.1-10 THz) has unique properties not found in other parts of the electromagnetic spectrum. THz technological applications need high-performance THz imaging and spectroscopy system components such as THz filters, modulators, reflectors. Transparent conducting oxides (TCOs) meet these needs in the THz field. Indium tin oxide (ITO) thin film is one of the most used materials among the TCOs. However, the research on the THz properties of ITO film is currently at an early stage. In this thesis, ITO thin films were deposited on borosilicate (BS), fused silica (FS) and PET substrates using large area magnetron sputtering system. Optical, structural and electrical properties of the ITO films on different substrates were analyzed. Moreover, the effect of electro-annealing in vacuum and air on the properties of ITO films grown on BS and FS substrates were investigated. Furthermore, CW THz imaging system were designed to take THz images of some objects and to analyze THz properties of ITO films grown on FS and PET substrates. Our findings indicate that electrical, structural and optical properties of ITO films can be improved by electro-annealing in air and vacuum. We addressed that electro-annealing in vacuum is more appropriate technique for the applications in industry. We also addressed the importance of oxygen partial pressure and ITO film thickness to adopt them as a transparent electrode in flexible devices. Moreover, our analysis about the THz properties of ITO films on FS and PET substrates showed that fabricated ITO films have potential application in THz field such as THz filters, modulators and reflectors.
  • Doctoral Thesis
    X-Ray Photoelectron Spectroscopy Analysis of Magnetron Sputtered Cu2znsns4 Based Thin Film Solar Cells With Cds Buffer Layer
    (Izmir Institute of Technology, 2017) Cantaş Bağdaş, Ayten; Özyüzer, Lütfi
    Cu2ZnSnS4 (CZTS) is a novel quaternary compound which contains Cu, Zn, Sn and S elements. It is a p-type semiconductor which has been taken attention in the last years as an absorber layer. Since it consists of abundant, low cost and non-toxic elements, it is one of the most promising candidate as an absorber layer for thin film photovoltaic (PV) application. Having high absorption coefficient, low bandgap value which is theoretically in desired range make this material attractive for solar cell application. In this thesis, CZTS absorber layers were grown using two stages which are the magnetron sputtering of metallic precursors, followed by a heat treatment under sulfur vapor atmosphere. Two types of CZTS were grown such as SLG/Mo/Cu (55nm)/Sn/Zn/Cu (120nm) and SLG/Mo/Cu (120nm)/Sn/Zn/Cu (55nm). For the same stacking order, the effect of Cu thickness sequentially grown with Sn layer on the film quality were investigated. The optical properties, microstructure, surface and bulk composition of CZTS films were investigated in detail. This study revealed a correlation between the CZTS stacking order having different thickness of Cu layer and the improvement of film quality, which was also confirmed by the photo-conversion efficiency of the fabricated devices. In this work, the other investigated layer is CdS which is an n-type semiconductor with bandgap energy of 2.4 eV. Since CdS has well lattice match with the heterojunction between CdS and CZTS, it is one of the most preferred material as a buffer layer for solar cells. In this work CdS buffer layers were deposited by chemical bath deposition technique. The optimization of CdS layers were occurred and optical, structural, bulk and surface compositions were investigated in detail. Finally, SLG/Mo/CZTS/CdS/i-ZnO/AZO devices were fabricated. The effect of structure properties of CZTS films and the thickness of CdS buffer layer on efficiency of fabricated solar cells were investigated.
  • Doctoral Thesis
    Magnetron Sputtering Growth of Azo/Zno Multilayers for Cu2znsns4 Thin Film Solar Cells: Material and Device Characterization
    (Izmir Institute of Technology, 2017) Köseoğlu, Fulya; Özyüzer, Lütfi; Aygün Özyüzer, Gülnur
    Cu2ZnSnS4 (CZTS) absorber layer attracts so much attention in photovoltaic industry since it contains earth abundant, low cost and non-toxic elements contrary to other chalcogenide based solar cells such as CuInGa(S,Se)2 (CIGS) and CdTe. Although, CZTS studies have been newly started, recently 9.4 % efficiency has been achieved. In the present thesis, all layers used in the CZTS device structure were investigated using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrophotometry and Raman spectroscopy. For CZTS absorber layer, CZTS films based on a stacked precursor (Cu/Sn/Zn/Cu) were prepared. The effect of sulfurization time and the thickness of top and bottom Cu layer in precursors on the properties of CZTS thin films were investigated. We addressed the importance of Cu layer thickness adjacent to Sn to avoid developing detrimental phases and to get complete formation of kesterite CZTS absorber layer. We also addressed the importance of sulfurization time to restrict the Sn and Zn losses, formation of oxides such as SnO2 and ZnO, formation of MoS2 and voids between Mo/CZTS interface. Effect of the sulfur concentration on the properties of Zn(O,S) thin films were investigated. We showed that key parameters such as energy gap and crystal structure of the Zn(O,S) thin films can be tuned by changing the sulfur concentrations of the films. We succeed substitute conventionally used CdS buffer layer with environmentally friendly alternative Zn(O,S) buffer layer in CZTS solar cells. Effect of substrate position and rotation speed during the deposition of AZO thin films were investigated. We addressed that stress on the films can be significantly reduced by off-center deposition and rotating the sample holder during the deposition. In this way, high transmission in the visible range and metal like resistivity were achieved simultaneously at room temperature. We observed strong dependence of device performances on both sulfurization time and the thickness of Cu layer adjacent to Sn in CZTS absorber. The best device was based on CZTS films sulfurized for 30 minutes and having thicker Cu layer adjacent to Sn layer in precursors.
  • Doctoral Thesis
    Fabrication of Metamaterial Filters for Terahertz Wave Applications by E-Beam Patterning
    (Izmir Institute of Technology, 2017) Demirhan, Yasemin; Özyüzer, Lütfi; Aral, Gürcan
    In the electromagnetic spectrum, terahertz (THz) radiation falls into the region among microwaves and the far-IR. The unique properties of superconducting materials allow them to be utilized in a number of ways for THz device applications. In the first part of this thesis, the spectral performance of THz bandpass filters were produced from titanium, copper and indium tin oxide thin films with a metal-mesh shape on fused silica substrates by uv lithography were investigated. For metamaterial filter fabrication, Bi2Sr2CaCu2O8+x thin films were deposited by DC and RF magnetron sputtering system on saphirre and MgO substrates. After thin film characterizations, it is obtained that the films were not in single phase and could not be used for filter fabrication. A new, unique fourcross shaped metamaterial THz filter was designed and fabricated from both gold thin films and YBa2Cu3O7-d high Tc superconducting thin films. The designed fourcross shaped rectangular filter structure consists of periodic metallic rings where strip lines are located at the sides of the ring. CST Microwave Studio, is used to design and optimize the metamaterial filter structures. To investigate the temperaturedependent resonance behavior and confirm the measurements, simulations are carried out at temperatures above and well below Tc. Fourcross metamaterial filters are fabricated by using e-beam lithography and ion beam etching techniques. The resonance switching of the transmission spectra was investigated by lowering the temperature below the critical transition temperature. This novel fourcross filter holds great potential for active, tunable and lowloss THz devices for imaging, sensing, and detection applications.