Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Article Citation - WoS: 2Citation - Scopus: 2In-Situ Thin Film Copper-Copper Thermocompression Bonding for Quantum Cascade Lasers(Springer, 2021) Rouhi, Sina; Özdemir, Mehtap; Demirhan, Yasemin; Yiğen, Serap; Demirhan, Yasemin; Yiğen, Serap; Kosiel, Kamil; Kozubal, Maciej; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, Lütfi; Reno, John L.; Aygün, Gülnur; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyThe choice of metals, bonding conditions and interface purity are critical parameters for the performance of metal-metal bonding quality for quantum cascade lasers (QCLs). Here, we present a novel approach for the thermocompression bonding of Cu-Cu thin films on GaAs-based waveguides without having any oxide phase, contamination or impurities at the interface. We designed a hybrid system in which magnetron sputtering of Ta, thermal evaporation of Cu and Cu-Cu thermocompression bonding processes can be performed sequentially under high vacuum conditions. GaAs/Ta/Cu and Cu/Ta/GaAs structures were thermocompressionally bonded in our in-situ homebuilt bonding system by optimizing the deposition parameters and bonding conditions. The grown thin film and the obtained interfaces were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX) techniques. The optimum Ta and Cu films' thicknesses were found to be about 20 nm and 500 nm, respectively. EDX analysis showed that the Ta thin film interlayer diffused into the Cu structure, providing better adhesivity and rigidity for the bonding. Additionally, no oxidation phases were detected at the interface. The best bonding quality was obtained when heated up to 430 degrees C with an applied pressure of 40 MPa during bonding process.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, Mehtap; Özyüzer, Lütfi; Özdemir, Mehtap; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologySince 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; Özyüzer, Lütfi; Akça, Fatime Gülşah; Meriç, Ece; Tarhan, Enver; Tarhan, Enver; Özyüzer, Lütfi; Aygün, Gülnur; Özdemir, Mehtap; Tarhan, Enver; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn 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: 42Citation - Scopus: 46Importance of Cds Buffer Layer Thickness on Cu2znsns4-Based Solar Cell Efficiency(IOP Publishing Ltd., 2018) Cantaş, Ayten; Türkoğlu, Fulya; Aygün, Gülnur; Akça, Fatime Gülşah; Özdemir, Mehtap; Tarhan, Enver; Özyüzer, Lütfi; Özyüzer, Gülnur Aygün; Özdemir, Mehtap; Özyüzer, Lütfi; Tarhan, Enver; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyCu2ZnSnS4 (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, Gülnur; Özdemir, Mehtap; Aygün, Gülnur; Kurt, Metin; 04.05. Department of Pyhsics; 01. Izmir Institute of Technology; 04. Faculty of ScienceEfficiency 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: 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.; Aygün, Gülnur; Özyüzer, Lütfi; Aygün, Gülnur; Özyüzer, Lütfi; Bacaksız, Emin; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyIn 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.Article Citation - WoS: 60Citation - Scopus: 64Improvement of Optical and Electrical Properties of Ito Thin Films by Electro-Annealing(Elsevier Ltd., 2015) Köseoğlu, Hasan; Türkoğlu, Fulya; Aygün, Gülnur; Yaman, Mutlu Devran; Akça, Fatime Gülşah; Kurt, Metin; Özyüzer, Lütfi; Özyüzer, Lütfi; 01. Izmir Institute of Technology; 04.05. Department of Pyhsics; 04. Faculty of ScienceThe effect of electro-annealing in vacuum and air on the optical and electrical properties of ITO thin films grown by large area DC magnetron sputtering was investigated. Moreover, the performances of the electro-annealed ITO thin films in vacuum and air were compared. Electro-annealing was performed by applying 0.75, 1.00, 1.25 and 1.50 A constant ac current to the ITO thin films. It was observed that the crystallinity of the films was better for the ITO thin films electro-annealed in vacuum. The changes in sheet resistance of electro-annealed ITO thin films with applied currents were detailed. The transmittance of the films increased for both electro-annealing in vacuum and air. A correlation between band-gap and resistivity for all of the electro-annealed thin films was observed.Article Citation - WoS: 42Citation - Scopus: 44Growth of Cu2znsns4 Absorber Layer on Flexible Metallic Substrates for Thin Film Solar Cell Applications(Elsevier Ltd., 2015) Yazıcı, Şebnem; Aygün, Gülnur; Olgar, Mehmet Ali; Tarhan, Enver; Akça, Fatime Gülşah; Özyüzer, Lütfi; Cantaş, Ayten; Kurt, Metin; Kurt, Metin; Tarhan, Enver; Aygün, Gülnur; Tarhan, Enver; Yanmaz, Ekrem; Özyüzer, Lütfi; 01. Izmir Institute of Technology; 04.05. Department of Pyhsics; 04. Faculty of ScienceIn this work, Cu2ZnSnS4 (CZTS) absorber layers were fabricated using a two-stage process. Sequentially deposited Cu-Zn-Sn thin film layers on metallic foils were annealed in an Ar + S2(g) atmosphere. We aimed to investigate the role of flexible titanium and molybdenum foil substrates in the growth mechanism of CZTS thin films. The Raman spectra and X-ray photoelectron spectroscopy analyses of the sulfurized thin films revealed that, except for the presence of Sn-based secondary phases, nearly pure CZTS thin films were obtained. Additionally, the intense and sharp X-ray diffraction peak from the (112) plane provided evidence of good crystallinity. Electron dispersive spectroscopy analysis indicated sufficient sulfur content but poor Zn atomic weight percentage in the films. Absorption and band-gap energy analyses were carried out to confirm the suitability of CZTS thin films as the absorber layer in solar cell applications. Hall effect measurements showed the p-type semiconductor behavior of the CZTS samples. Moreover, the back contact behavior of these metallic flexible substrates was investigated and compared. We detected formation of cracks in the CZTS layer on the molybdenum foils, which indicates the incompatibility of molybdenum's thermal expansion coefficient with the CZTS structure. We demonstrated the application of the magnetron sputtering technique for the fabrication of CZTS thin films on titanium foils having lightweight, flexible properties and suitable for roll-to-roll manufacturing for high throughput fabrication. Titanium foils are also cost competitive compared to molybdenum foils. © 2015 Elsevier B.V.Article Citation - WoS: 18Citation - Scopus: 20Impact of Incorporated Oxygen Quantity on Optical, Structural and Dielectric Properties of Reactive Magnetron Sputter Grown High-? Hfo2/Hf Thin Film(Elsevier Ltd., 2014) Cantaş, Ayten; Aygün, Gülnur; Aygün, Gülnur; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyHigh-κ hafnium-oxide thin films have been fabricated by radio frequency (rf) reactive magnetron sputtering technique. To avoid formation of an undesired interfacial suboxide layer between Si and high-κ film, prior to HfO2 deposition, a thin Hf buffer layer was deposited on p-type (1 0 0) Si substrate at room temperature. Effect of oxygen gas quantity in the O2/Ar gas mixture was studied for the optical and structural properties of grown HfO2 high-κ thin films. The grown thin oxide films were characterized optically using spectroscopic ellipsometer (SE) in detail. Crystal structure was studied by grazing incidence X-ray diffractometer (GIXRD) technique, while bonding structure was obtained by Fourier transform infrared spectroscopy (FTIR) analyses. In agreement with GIXRD and FTIR analyses, SE results show that any increment above ideal quantity of oxygen content in the gas mixture resulted in decrements in the refractive index and thickness of HfO2 dielectric film, while increments in SiO2 thickness. It is apparent from experimental results that oxygen to argon gas ratio needs to be smaller than 0.2 for a good film quality. The superior structural and optical properties for grown oxide film were obtained for O2/Ar gas ratio of about 0.05-0.1 combined with ∼30 W constant rf sputtering power. © 2014 Elsevier B.V. All rights reserved.Article Citation - WoS: 25Citation - Scopus: 26Structural and Optical Characteristics of Tantalum Oxide Grown by Pulsed Nd:yag Laser Oxidation(AVS Science and Technology Society, 2006) Atanassova, Elenada A.; Aygün, Gülnur; Turan, Raşit; Babeva, T.; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyTantalum pentoxide (Ta2 O5) thin films (20-50 nm) have been grown by 1064 nm Nd:YAG laser oxidation of Ta film deposited on Si. The chemical bonding, structure, and optical properties of the films have been studied by Fourier transform infrared spectroscopy, x-ray diffraction, and reflectance measurements at normal light incidence in the spectral range of 350-800 nm. The effect of the substrate temperature (250-400 °C) during oxidation and its optimization with respect to the used laser beam energy density (3.2-3.4 J cm2 per pulse) is discussed. It is established that the substrate temperature is a critical factor for the effectiveness of the oxidation process and can be used to control the composition and amorphous status of the films. The film density explored by refractive index is improved with increasing film thickness. The refractive index of the layers grown under the higher laser beam energy density and at substrate temperature of 350-400 °C was found to be close to the value of bulk Ta2 O5. The films are amorphous at substrate temperature below 350 °C and possessed an orthorhombic (Β- Ta2 O5) crystal structure at higher temperatures. The thinner layers crystallize at a little higher temperature.