WoS İndeksli Yayınlar Koleksiyonu / WoS Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7150
Browse
11 results
Search Results
Now showing 1 - 10 of 11
Article Citation - WoS: 35Citation - Scopus: 38Influence of Sulfurization Temperature on Cu2znsns4 Absorber Layer on Flexible Titanium Substrates for Thin Film Solar Cells(IOP Publishing Ltd., 2018) Buldu, Dilara Gökçen; Cantaş, Ayten; Türkoğlu, Fulya; Akça, Fatime Gülşah; Meriç, Ece; Özdemir, Mehtap; Tarhan, Enver; Özyüzer, Lütfi; Aygün, GülnurIn this study, the effect of sulfurization temperature on the morphology, composition and structure of Cu2ZnSnS4 (CZTS) thin films grown on titanium (Ti) substrates has been investigated. Since Ti foils are flexible, they were preferred as a substrate. As a result of their flexibility, they allow large area manufacturing and roll-to-roll processes. To understand the effects of sulfurization temperature on the CZTS formation on Ti foils, CZTS films fabricated with various sulfurization temperatures were investigated with several analyses including x-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray photoelectron spectroscopy and Raman scattering. XRD measurements showed a sharp and intense peak coming from the (112) planes of the kesterite type lattice structure (KS), which is strong evidence for good crystallinity. The surface morphologies of our thin films were investigated using SEM. Electron dispersive spectroscopy was also used for the compositional analysis of the thin films. According to these analysis, it is observed that Ti foils were suitable as substrates for the growth of CZTS thin films with desired properties and the sulfurization temperature plays a crucial role for producing good quality CZTS thin films on Ti foil substrates.Article Citation - WoS: 2Citation - Scopus: 3Comparision of in Situ Spectroscopic Ellipsometer and Ex Situ X-Ray Photoelectron Spectroscopy Depth Profiling Analysis of Hfo2/Hf Multilayer Structure(IOP Publishing Ltd., 2018) Cantaş, Ayten; Özyüzer, Lütfi; Aygün, GülnurA HfO2 film was grown by RF magnetron sputtering technique on a Si substrate Using in situ Spectroscopic Ellipsometry (SE), the film thickness and refractive index were examined as a function of deposition time. Ex situ x-ray Photoelectron Spectroscopy (XPS) was used in depth profile mode to determine the phase evolution of HfO2/Hf/Si multilayer structure after the growth process. The chemical composition and the crystal structure of the film were investigated by Fourier Transform Infrared (FTIR) spectroscopic measurements and x-ray Diffraction in Grazing Incidence (GI-XRD) mode, respectively. The results showed that the film was grown in the form of HfO2 film. According to SE analysis, reactive deposition of HfO2 directly on Hf/Si results to SiO2 interface of about 2 nm. The final HfO2 films thickness is 5.4 nm. After a certain period of time, the XPS depth profile revealed that the film was in the form of Hf-rich Hf silicate with SiO2 interfacial layer. In reference to XPS quantification analysis from top to bottom of film, the atomic concentration of Hf element reduces from 19.35% to 7.13%, whereas Si concentration increases from 22.99% to 74.89%. The phase change of HfO2 film with time is discussed in details.Article Citation - WoS: 42Citation - Scopus: 46Importance of Cds Buffer Layer Thickness on Cu2znsns4-Based Solar Cell Efficiency(IOP Publishing Ltd., 2018) Cantaş, Ayten; Türkoğlu, Fulya; Meriç, Ece; Akça, Fatime Gülşah; Özdemir, Mehtap; Tarhan, Enver; Özyüzer, Lütfi; Özyüzer, Gülnur AygünCu2ZnSnS4 (CZTS) thin films were grown on Mo-coated soda lime glass (SLG) substrates by the sulfurization of DC magnetron-sputtered Zn, Sn and Cu metallic precursors under a sulfur atmosphere at 550 °C for 45 min. Understanding the composition and structure of the CZTS absorber layer is necessary to obtain efficient solar cells. With this aim, x-ray diffractometry, Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and x-ray photoelectron spectroscopy were used to investigate the CZTS absorber layers. CZTS absorber films were obtained and found to be Cu-poor and Zn-rich in composition, which are both qualities desired for efficient solar cells. CdS was used as a buffer layer and was grown by the chemical bath deposition technique. The optical properties of CdS films on SLG were searched for using a spectroscopic ellipsometer and the results revealed that the bandgap increases with film thickness increment. CZTS-based solar cells with different CdS buffer layer thicknesses were prepared using a SLG/Mo/CZTS/CdS/ZnO/AZO solar cell configuration. The influence of the CdS buffer layer thickness on the performance of the CZTS solar cells was investigated. Device analysis showed that electrical characteristics of solar cells strongly depend on the buffer layer's thickness. Highly pronounced changes in V OC, fill factor and J SC parameters, which are the main efficiency limiting factors, with changing buffer layer thicknesses were observed. Our experiments confirmed that decreasing the CdS thickness improved the efficiency of CZTS solar cells down to the lowest thickness limit.Article Citation - WoS: 21Citation - Scopus: 22Investigation of Electron Beam Lithography Effects on Metal-Insulator Transition Behavior of Vanadium Dioxide(IOP Publishing Ltd., 2017) Yüce, Hürriyet; Alaboz, Hakan; Demirhan, Yasemin; Özdemir, M.; Özyüzer, Lütfi; Aygün, GülnurVanadium dioxide (VO2) shows metal-insulator phase transition at nearly 68 °C. This metal-insulator transition (MIT) in VO2 leads to a significant change in near-infrared transmittance and an abrupt change in the resistivity of VO2. Due to these characteristics, VO2 plays an important role on optic and electronic devices, such as thermochromic windows, meta-materials with tunable frequency, uncooled bolometers and switching devices. In this work, VO2 thin films were fabricated by reactive direct current magnetron sputtering in O2/Ar atmosphere on sapphire substrates without any further post annealing processes. The effect of sputtering parameters on optical characteristics and structural properties of grown thin films was investigated by SEM, XRD, Raman and UV/VIS spectrophotometer measurements. Patterning process of VO2 thin films was realized by e-beam lithography technique to monitor the temperature dependent electrical characterization. Electrical properties of VO2 samples were characterized using microprobe station in a vacuum system. MIT with hysteresis behavior was observed for the unpatterned square samples at around 68 °C. By four orders of magnitude of resistivity change was measured for the deposited VO2 thin films at transition temperature. After e-beam lithography process, substantial results in patterned VO2 thin films were observed. In this stage, for patterned VO2 thin films as stripes, the change in resistivity of VO2 was reduced by a factor of 10. As a consequence of electrical resistivity measurements, MIT temperature was shifted from 68 °C to 50 °C. The influence of e-beam process on the properties of VO2 thin films and the mechanism of the effects are discussed. The presented results contribute to the achievement of VO2 based thermochromic windows and bolometer applications.Article Citation - WoS: 13Citation - Scopus: 13Fourcross Shaped Metamaterial Filters Fabricated From High Temperature Superconducting Ybco and Au Thin Films for Terahertz Waves(IOP Publishing Ltd., 2017) Demirhan, Yasemin; Alaboz, Hakan; Nebioğlu, Mehmet Ali; Mulla, B.; Akkaya, M.; Altan, Hakan; Sabah, Cumali; Özyüzer, LütfiIn this study, we present a new, unique fourcross shaped metamaterial terahertz (THz) filter fabricated from both gold thin films and YBa2Cu3O7-δ high Tc superconducting thin films. A commercial electromagnetic simulation software, CST Microwave Studio, is used to design and optimize the metamaterial filter structures. The proposed fourcross shaped rectangular filter structure consists of periodic metallic rings where strip lines are located at the sides of the ring. Fourcross metamaterial filters are fabricated by using e-beam lithography and ion beam exhing techniques. Terahertz time-domain spectroscopy measurements validated the design predictions for both the center frequencies and bandwidths of the resonances due to the fourcross structures. The resonance switching of the transmission spectra was investigated by lowering the temperature below the critical transition temperature. This resonance switching effect is not observed in filters made up of metals. This novel fourcross rectangular resonator with a temperature-dependent resonance behavior holds great potential for active, tunable and low loss THz devices for imaging, sensing, and detection applications.Article Citation - WoS: 39Citation - Scopus: 40Interferometer Measurements of Terahertz Waves From Bi 2sr 2cacu 2o 8+d Mesas(IOP Publishing Ltd., 2012) Türkoğlu, Fulya; Köseoğlu, Hasan; Demirhan, Yasemin; Özyüzer, Lütfi; Preu, Sascha; Malzer, Stefan; Şimşek, Yusuf; Müller, P.; Yamamoto, T.; Kadowaki, K.We fabricated rectangular mesa structures of superconducting Bi 2Sr 2CaCu 2O 8+d (Bi2212) using e-beam lithography and Ar ion beam etching techniques for terahertz (THz) emission. c-axis resistance versus temperature (R-T), current-voltage (I-V) characteristics and bolometric THz power measurements were performed to characterize Bi2212 mesas. The emission frequency of mesas was determined using a Michelson interferometer setup which also demonstrates polarized emission. Interference patterns of THz radiation from Bi2212 mesas were detected by various detectors such as a liquid helium cooled silicon composite bolometer, a Golay cell and a pyroelectric detector. An emitted power as high as 0.06mW was detected from Bi2212 mesas. For the first time, most of the pumped power was extracted as THz emission from a Bi2212 mesa. The radiation at 0.54THz was detected using the Michelson interferometric setup.Article Citation - WoS: 266Citation - Scopus: 292High Quality Ito Thin Films Grown by Dc and Rf Sputtering Without Oxygen(IOP Publishing Ltd., 2010) Tuna, Öcal; Selamet, Yusuf; Aygün, Gülnur; Özyüzer, LütfiHigh quality indium tin oxide (ITO) thin films were grown without oxygen by both dc and RF magnetron sputtering techniques on glass substrates. The effects of substrate temperature, film thickness and sputtering method on the structural, electrical and optical properties of the as-grown films were investigated. The results showed that the substrate temperature had substantial effects on the film properties, in particular on the crystallization and resistivity. When the substrate temperature was increased to 150 °C, crystallization in the (2 2 2) plane started appearing for both dc and RF sputtered films. We additionally found that with further increments of substrate temperature, the preferred crystallization orientation changed differently for dc and RF sputtered films. Optical transmission in the visible region for a film thickness of 70 nm was found to be above 85%. The bandgap was calculated to be about 3.64 eV for the substrate temperature of 150 °C for a 70 nm thick film. The value of the bandgap increased with respect to the increment in film thickness as well as substrate temperature. We also measured the temperature dependence of the resistivity and Hall coefficient of the films, and calculated the carrier concentration and Hall mobility. Very low room temperature resistivities for dc and RF magnetron sputtered grown films of about 1.28 × 10-4 Ω cm and 1.29 × 10-4 Ω cm, respectively, were obtained. © 2010 IOP Publishing Ltd.Article Citation - WoS: 40Citation - Scopus: 45Terahertz Wave Emission From Intrinsic Josephson Junctions in High- Tc Superconductors(IOP Publishing Ltd., 2009) Özyüzer, Lütfi; Şimşek, Yılmaz; Köseoğlu, Hasan; Türkoğlu, Fulya; Kurter, Cihan; Welp, U.; Koshelev, A. E.; Gray, Kenneth E.; Kwok, W. K.; Yamamoto, T.; Kadowaki, K.; Koval, Yu I.; Wang, Huabing; Müller, Paul H.Recently, we experimentally demonstrated that rectangular mesa structures of intrinsic Josephson junctions (IJJ) in Bi2Sr2CaCu 2O8+d (Bi2212) can be used as a compact solid-state generator of continuous, coherent and polarized terahertz (THz) radiation. In the present work, we will exhibit tall mesas (over 600 junctions) which were fabricated using UV lithography, e-beam lithography with photoresist and e-beam lithography with a Ti selective etching technique. We will present measurements of the c-axis resistance as a function of temperature and of current-voltage characteristics of THz emitting mesas with lateral sizes ranging from 30 × 300 to 100 × 300νm2. Furthermore, we will discuss the dependence of the characteristics of the mesa structures on the oxygen doping level of the Bi2212 crystals. We will also experimentally show that the voltage-frequency relation of the ac Josephson effect has to match the cavity resonance for successful emission.Article Citation - WoS: 6Citation - Scopus: 11Electrical and Mechanical Properties of Superconducting Mgb2/Mg Metal Matrix Composites(IOP Publishing Ltd., 2006) Eğilmez, Mehmet; Özyüzer, Lütfi; Tanoğlu, Metin; Okur, Salih; Kamer, Orhan; Öner, YıldırhanMgB2/Mg composites were prepared using a metal matrix composite fabrication method that offers the potential to produce superconducting wires as an alternative approach to the powder in tube process. To obtain composites, MgB2 and Mg powders were mixed at different weight fractions and uniaxially pressed in a cylindrical die under the pressure of 0.5 and 1.0GPa for two hours at various temperatures. The x-ray diffraction technique was used for phase identification. Temperature dependence of resistivity and magnetization measurements were carried out to determine superconducting properties. The effects of composite fabrication temperature and the addition of the Mg on the mechanical properties of MgB2/Mg composites were investigated. For this purpose, the compressive mechanical testing was performed to measure the elastic modulus and fracture strength values of the composites. It was found that the relative weight fraction of the Mg and the fabrication conditions of the composites have considerable effect on the superconducting and mechanical properties of the composites.Article Citation - WoS: 22Citation - Scopus: 22Oxidation of Si Surface by a Pulsed Nd: Yag Laser(IOP Publishing Ltd., 2004) Özyüzer, Gülnur Aygün; Atanassova, Elenada A.; Alaçakır, Ali; Özyüzer, Lütfi; Turan, RaşitSiO2 thin films have been obtained by 1064 nm Nd: YAG laser oxidation of p-Si in the presence of O2. The thickness uniformity, dielectric and electrical properties of the layers have been studied. The effect of both the laser beam energy density and the substrate temperature on the oxide growth is also discussed. It was established that there exists an interval of laser beam energy density in which the oxidation occurs without surface melting. The oxidation process is controlled by the laser beam energy density rather than by the substrate temperature (673-748 K) and the higher laser power results in a thicker oxide. X-ray photoelectron spectroscopy (XPS) was used to provide information on the oxide composition. XPS results revealed that the as-grown oxide is a mixed layer of SiO2 and Si2O, which are distributed nonuniformly through the depth. MOS capacitors fabricated on the grown oxide exhibited typical capacitance-voltage, conductance-voltage characteristics. However, the density of interface states and oxide charge density were found to be higher than the typical values of thermally grown oxides. The quality of the oxide layers can be further improved by optimization of the process parameters and/or by post-processing of the grown films. It is concluded that the SiO2 films formed by the technique of Nd: YAG laser-enhanced oxidation at low temperature are potentially useful for device applications.