Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Conference Object Terahertz Transmission Through Patterened Vanadium Oxide Thin Films on Dielectric Substrates(SCITEPRESS, 2017) Akkaya, M.; Demirhan, Yasemin; Yüce, Hürriyet; Aygün, Gülnur; Özyüzer, Lütfi; Sabah, Cumali; Altan, HakanPatterned and unpatterned films of vanadium oxide grown on dielectric substrates such as fused silica and sapphire were grown and analysed by varying the temperature using terahertz time domain spectroscopy. After investigating the critical transition temperature near 340K, a well-known cross-shaped pattern was studied to observe any resonances upon transmission. Due to the poor conductivity of the films the frequency selective nature of the structure was not observed, however an etalon effect could be seen in the sapphire substrate as opposed to the fused silica substrate above the critical temperature. Dependence of the refractive index difference between substrates upon transmission of the THz pulse is likely in explaining this observed difference.Article Citation - WoS: 21Citation - Scopus: 24Effect of Defects and Secondary Phases in Cu2znsns4 Absorber Material on the Performance of Zn(o,s) Buffered Devices(Elsevier Ltd., 2019) Türkoğlu, Fulya; Köseoğlu, Hasan; Cantaş, Ayten; Akça, Fatime Gülşah; Meriç, Ece; Buldu, Dilara Gökçen; Aygün, GülnurCopper zinc fin sulfide (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. In the present work, CZTS absorber layers were prepared following a two-stage process: firstly, a stack of metal precursors (Copper (Cu)/Tin (Sn)/Zinc (Zn)/Copper (Cu)) were deposited on molybdenum (Mo) substrate by magnetron sputtering, then this stack was annealed under S atmosphere inside a tubular furnace. CZTS thin films were investigated using energy dispersive X-ray spectroscopy, X-ray diffraction, scanning electron microscopy and Raman spectroscopy. 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. The importance of Cu thickness adjacent to Sn to avoid detrimental phases was addressed. The significance of sulfurization time to restrict the Sn and Zn losses, formation of oxides such as fin dioxide and zinc oxide, and formation of molybdenum disulfide and voids between Mo/CZTS interface was also addressed. Moreover, cadmium sulfide buffer layer, which is conventionally used in CZTS solar cells, is replaced by an environmentally friendly alternative zinc oxysulfide buffer layer.Article Citation - WoS: 33Citation - Scopus: 33Characterization of Thin Film Li0.5la0.5ti1-Xalxo3 Electrolyte for All-Solid Li-Ion Batteries(Elsevier, 2018) Ulusoy, Seda; Gülen, Sena; Aygün, Gülnur; Özyüzer, Lütfi; Özdemir, MehtapSince addition of Al in Li0.5La0.5TiO3 has enhanced ionic conductivity in bulk materials, it is important to apply this material on all solid state thin film batteries. Because some of the good ionic conductors such as Lithium Phosphorus Oxynitride (LiPON) are sensitive to oxygen and moisture and their application is limited, so amorphous Li0.5La0.5Ti1−xAlxO3 (LLTAlO) is a most promising candidate because of its stability. In this study, the crystalline LLTAlO targets were prepared changing the amount of x content by conventional solid state reactions. Using these targets, lithium lanthanum titanium oxide (LLTO) thin film electrolytes were deposited on ITO/SLG substrates by radio frequency magnetron sputtering system in Ar atmosphere. The structural and compositional properties of targets and thin films were characterized by SEM, XRD, Raman spectroscopy and XPS. It was found that all targets are crystalline while the thin films are amorphous. To understand the effect of Al doping on ionic conductivity, electrical measurements were done at room temperature by AC impedance spectroscopy forming ITO/LLTAlO/Al structure like capacitor. Highest ionic conductivity result, 0.96 × 10−6 S·cm−1, is obtained from the nominal thin film composition of Li0.5La0.5Ti1−xAlxO3 (x = 0.05) at room temperature measurements. Heat treatment is also conducted to investigate to understand its effect on ionic conductivity and the structure of the thin films. It is found that ionic conductivity enhances with annealing. Also, temperature dependent ionic conductivity measurements from 298 K to 385 K are taken in order to evaluate activation energy for Li-ion conduction.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: 2Citation - Scopus: 2Comparison of Photocatalytic Properties of Tio2 Thin Films and Fibers(EDP Sciences, 2016) Özdemir, Mehtap; Kurt, Metin; Özyüzer, Lütfi; Aygün, GülnurEfficiency of solar panels degrades as a result of organic contamination such as airborne particles, bird droppings and leaves. Any foreign object on photovoltaic panels reduces the sunlight entering the absorbing surface of the solar panels. Since this leads to a major problem decreasing in energy production, solar panels should be cleaned. The self-cleaning method can be preferred. There are some methods to clean the surface of solar panels. Among the self-cleaning materials, TiO2 is the most preferable ones because of its powerful photocatalytic properties. In this study, photocatalytic TiO2 were produced in two different nanostructures: nanofibers and thin films. TiO2 nanofibers were successfully produced by electrospinning. TiO2 thin films were fabricated by reactive magnetron sputtering technique. Both TiO2 nanofiber and thin film structures were heat-treated to form TiO2 in anatase phase at 600 °C for 2 h in air. Then, they were evaluated by SEM analyses for morphology, X-ray diffraction (XRD) analyses for phase structures, X-ray photoelectron spectroscopy (XPS) for the chemical state and atomic concentration, and UV-spectrometer for photocatalytic performance. The results indicate that photocatalytic and transmittance properties of TiO2 thin films are better than those of nanofibers. Consequently, TiO2 based thin films exhibit better performance for solar cell applications due to the surface cleanliness.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: 7Citation - Scopus: 7Comparative Study of Annealing and Gold Dopant Effect on Dc Sputtered Vanadium Oxide Films for Bolometer Applications(Springer Verlag, 2017) Alaboz, Hakan; Demirhan, Yasemin; Yüce, Hürriyet; Aygün, Gülnur; Özyüzer, LütfiVanadium oxide (VOx) thin film has been widely used for IR detectors and it is one of the promising materials for THz detectors due to its high temperature coefficient of resistance (TCR) values. VOx films with proper TCR values have also high resistance and it restricts bolometer performance especially for uncooled bolometers. To overcome this problem, deposition at elevated temperatures or annealing approach has been accepted and used but gold co-deposition approach has been proposed recently. In this study, vanadium oxide films were fabricated on high resistivity silicon substrates by reactive direct current magnetron sputtering in different O2/Ar atmosphere at room temperature. We investigated influence of oxygen partial pressure during deposition process and fabricated VOx thin films with sufficient TCR values for bolometer applications. In order to decrease resistivity of the deposited films, post annealing and gold doping approaches were performed separately. Effect of both post annealing process and gold doping process on structural and electrical properties of VOx thin films deposited at room temperature were investigated and detailed comparison between these methods were presented. We obtained the best possible approach to obtain optimum conditions for the highly reproducible VOx thin films which have the best resistivity and suitable TCR value for bolometer applications.Article Citation - WoS: 22Citation - Scopus: 23Effect of Heat Treating Metallic Constituents on the Properties of Cu2znsnse4 Thin Films Formed by a Two-Stage Process(Elsevier Ltd., 2017) Olgar, Mehmet Ali; Başol, B. M.; Atasoy, Y.; Tomakin, Murat; Aygün, Gülnur; Özyüzer, Lütfi; Bacaksız, EminIn this study Cu2ZnSnSe4 (CZTSe) thin films were grown by a two-stage process that involved sputter deposition of a Cu/Sn/Zn/Cu metallic stack, annealing the stack at various temperatures for 30 min, evaporation of a Se cap over the metallic stack thus forming a precursor layer, and subjecting the precursor layer to a final high temperature reaction step at 550 °C. Different samples were prepared with annealing temperatures of the metallic stacks ranging from 200 °C to 350 °C. The results showed that heat treatment of the metallic stacks did not cause much change in their morphology and elemental composition, however their phase content changed noticeably when the anneal temperature was raised to 250 °C. Specifically, while the metallic films were dominated by CuSn and Cu5Zn8 phases at low temperatures, the dominant phase shifted to Cu6Sn5 at the annealing temperature of 250 °C and higher. Also formation of a distinct Cu3Zn2 phase was observed upon annealing at temperatures at or above 250 °C. After reaction with Se, the CZTSe layer obtained from the metallic film, which was annealed at 250 °C was found to be the best n terms of its composition, crystalline quality and purity, although it contained a small amount of CuSe. The other layers were found to contain small amounts of other secondary phases such as SnSe, CuSe2, ZnSe and Cu2SnSe3. SEM micrographs showed denser structure for CZTSe layers grown from metallic films annealed at or above 250 °C. Optical band gap, resistivity and carrier concentration of the best quality CZTSe film were found to be about 0.87 eV, 2 Ω-cm and 4 × 1017 cm− 3, respectively.