Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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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; Ekmekçioğlu, Merve; Yiğen, Serap; Demirhan, Yasemin; Szerling, Anna; Kosiel, Kamil; Kozubal, Maciej; Kruszka, Renata; Prokaryn, Piotr; Ertuğrul, Mehmet; Reno, John L.; Aygün, Gülnur; Özyüzer, LütfiThe 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: 5Citation - Scopus: 5Comparison of Characteristic Properties of Al, Ga, and In-Doped Zno Thin Films Formed by Sol-Gel Method(Academic Press, 2021) Horzum, Şeyda; Bulduk, Emel; Şener, Deniz; Serin, TülayHerein, we examine the effect of doping with Indium (In), Gallium (Ga), and Aluminum (Al) (group III elements) on the structural, optical, and vibrational properties of ZnO thin films. The characteristic properties of the ZnO films prepared by the sol-gel dip-coating method are explored by utilizing X-ray diffraction, optical spectroscopy, and Raman scattering measurements. XRD analyzes exhibit that the crystallite size reduces upon doping by Ga and Al, while it increases with In, and all films have hexagonal wurtzite structure. Additionally, Raman measurements indicate that the dominant two peaks at around 104 and 445 cm(-1) are related to E(2)(low )and E-2(high) phonon modes of ZnO, respectively. The low-frequency mode (E-2(low)) is affected by dopant atoms, whereas the high-frequency mode (E-2(high)) of the wurtzite phase is not influenced by the dopant. Moreover, E-dop.atom phonon mode appears at low frequencies and the intensity ratio, I(E-dop.atom)/I(E(2)low), decreases as the ionic radius of dopant atoms increases. UV-Vis spectra reveal that the film transparency, optical band gap, Urbach energy, and refraction index can be effectively tuned by dopant atoms.Article Citation - WoS: 3Citation - Scopus: 3Compact Multilayer Thin-Film Color Filters and Direct Integration on White-Light Diodes for Color Conversion(Optical Society of America, 2021) Yiğen, Serap; Ekmekçioğlu, Merve; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiWe present highly efficient green, yellow, and red filters based on a metal-dielectric structure. The filters encompass only five layers of alternating zinc tin oxide and silver thin films that are grown on soda lime glass and white light-emitting diodes (LEDs) using direct current magnetron sputtering at room temperature. The designed filters provide efficient color filtering in the visible spectrum. High purity colored light is obtained by direct application of filters on LEDs as color converters. The presented method offers an easy way for realizing different colors by tuning the thicknesses of layers in the structure. (C) 2021 Optical Society of AmericaArticle Citation - WoS: 7Citation - Scopus: 8Investigation of the Structural and Optical Properties of Copper-Titanium Oxide Thin Films Produced by Changing the Amount of Copper(Elsevier Ltd., 2019) Horzum, Şeyda; Gürakar, Sibel; Serin, TülayWe examine how the structural, morphological and optical properties of TiO2 thin films are changed with heavily copper (Cu) content. Variations in characteristic properties of the films with 0, 12.5, 25 and 50 wt% Cu contents, grown by sol-gel dip coating method, are observed by using X-ray diffraction (XRD), Raman scattering, atomic force microscopy, energy dispersive X-ray analysis and optical spectroscopy measurements. The XRD and Raman spectra indicate that pure TiO2 film forms in the anatase structure. At high Cu concentrations, XRD results also reveal the substitution of Ti with Cu and formation of extra compound Copper-Titanium oxide. Raman measurements also show that Cu is incorporated homogeneously into TiO2 matrix up to 12.5 wt% concentration and this uniformity is distorted at higher Cu contents. In addition, optical spectroscopy measurements show that the optical band gap energy decreases from 3.26 eV to 2.05 eV with increasing Cu concentration. Furthermore, it is observed that the refractive index values obtained by means of transmittance spectra at 550 nm wavelength; increases from 2.47 to 3.39 when the Cu concentration increases from 0 to 50 wt %.Article Citation - WoS: 6Citation - Scopus: 6Experimental and Density Functional Theory Study on Humidity Sensing Properties of Copper Phthalocyanine (cupc)(IOP Publishing, 2019) Farzaneh, Amir; Esrafili, Mehdi D.; Okur, SalihThe quartz crystal microbalance (QCM) technique was applied to investigate humidity sensing properties of a copper phthalocyanine (CuPc) thin film prepared by drop cast method. The humidity adsorption and desorption kinetics of (CuPc) thin film was evaluated. The QCM and electrical measurements results showed that humidity sensing properties of CuPc is very sensitive to humidity changes and reversible adsorption/desorption performance which is an indicative of a good humidity sensor even at room temperature. Reproducible experimental results indicated that CuPc thin films have an abundant potential for humidity sensing applications at ambient temperature. According to the first-principle density functional theory calculations, the promising humidity sensing properties of CuPc can be attributed to the considerable charge transfer from the water molecule into Cu atom.Article Citation - WoS: 50Citation - Scopus: 52Monitoring the Characteristic Properties of Ga-Doped Zno by Raman Spectroscopy and Atomic Scale Calculations(Elsevier, 2019) Horzum, Şeyda; İyikanat, Fadıl; Senger, Ramazan Tuğrul; Çelebi, Cem; Sbeta, Mohamed; Yıldız, Abdullah; Serin, TülayWe experimentally and theoretically study how the structural and vibrational properties of zinc oxide (ZnO) are modified upon Gallium (Ga) doping. The characteristics of Ga-doped ZnO thin films which are synthesized by sol-gel spin coating method on glass substrates are monitored by using X-ray diffraction (XRD) and Raman scattering measurements. For atomic-level understanding of the experimental findings state-of-the-art density functional theory (DFT) based calculations are also performed. DFT calculations reveal that both the substitution and adsorption of Ga atoms in ZnO are energetically possible and substitutional doping in ZnO is the most favourable scenario. XRD measurements show that all the films are in wurtzite structure and the crystallite size of the films decreases with increasing Ga doping. In addition, Raman analysis show that strong vibrational modes at about 100 and 441 cm(-1) are associated with E-2(low) and E-2(high) phonon branches of ZnO, respectively. While the frequency of the E-2(low) mode downshifts with increasing Ga concentration, the E-2(high) phonon mode is not affected by the Ga doping. Furthermore, E-Ga phonon branch, stemming from the substituted Ga atoms, emerges at low frequencies. It is also seen that the Raman intensity of the E-G(a) peak linearly increases with increasing Ga concentration. Experimental results on the vibrational properties are in good agreement with the ab initio phonon calculations. (C) 2018 Elsevier B.V. All rights reserved.Article Citation - WoS: 22Citation - Scopus: 27Mechanisms Behind Slow Photoresponse Character of Pulsed Electron Deposited Zno Thin Films(Elsevier, 2020) Özdoğan, Mehmet; Çelebi, Cem; Utlu, GökhanSemiconducting Zinc Oxide (ZnO) is ideal candidate for ultraviolet (UV) photodetector due to its promising optoelectronic properties. Photoconductive type ZnO photodetectors, which is fabricated in metal-semiconductor-metal configuration, show mostly very high photoconductivity under UV light, but they are plagued by slow photoresponse time as slow as several tens of hours, even more. Most of the studies claimed that atmospheric adsorbates such as water and oxygen create charge traps states on the surface and remarkably increase both the photoconductivity and response time. There are also limited studies, which claim that the defect states acting as hole trap centers prolong response time significantly. However, the underlying physical mechanism is still unclear. Here we study the effects of both adsorbates and defect-related states on the photoresponse character of Pulsed Electron Deposited ZnO thin films. In order to distinguish between these two mechanisms, we have compared the time-dependent photoresponse measurements of bare-ZnO and SiO2 encapsulated-ZnO thin film samples taken under UV light and high vacuum. We show that the dominant mechanism of photoresponse in ZnO is the adsorption/desorption of oxygen and water molecules even when the measurement is performed in high vacuum. After the encapsulation of sample surface by a thin SiO2 layer, the adsorption/desorption rates can significantly improve, and the effects of these molecules partially removed.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: 4Citation - Scopus: 5Identifying Threading Dislocations in Cdte Films by Reciprocal Space Mapping and Defect Decoration Etching(American Institute of Physics, 2018) Polat, Mustafa; Bilgilisoy, Elif; Arı, Ozan; Öztürk, Orhan; Selamet, YusufWe study threading dislocation (TD) density of high-quality cadmium telluride (CdTe) layers grown on a (211) oriented GaAs substrate by molecular beam epitaxy. High-resolution X-ray diffraction was performed to calculate the density of screw-type TDs by measuring the broadening of the asymmetrical (511) Bragg reflections of CdTe epilayers. In addition, total TD densities were determined by the Everson-etching method and were compared with screw TDs. Our results show that the total TD densities in CdTe films were dominated by those with screw character. The screw component TDs are estimated to account for more than 90% of the total TD density. CdTe layers grown at a thickness of less than 3.0 μm typically exhibit the screw TD densities in the 106 cm-2 and 107 cm-2 range. It can be noted that as the nucleation temperature increases, i.e., ≥222 °C, both the area density of TDs with the screw component of the CdTe films and the total TD density are roughly four times larger than those of the epilayer grown at the nucleation temperature of 215 °C. Furthermore, we discuss the influence of the II/VI flux ratio on the density of threading dislocations. The contribution of screw TDs to the total TD density showed a significant decrease in roughly 30% in the case of a high II/VI flux ratio. We further examine the reciprocal space maps in the vicinity of the (422) reflections.Article Citation - WoS: 6Citation - Scopus: 10Effect of Annealing on the Density of Defects in Epitaxial Cdte (211)/Gaas(Springer, 2018) Bakali, Emine; Selamet, Yusuf; Tarhan, EnverCdTe thin films were grown on GaAs (211) wafers by molecular beam epitaxy as the buffer layer for HgCdTe infrared detector applications. We studied the effect of annealing on the density of dislocation of these CdTe thin films under varying annealing parameters such as annealing temperature, annealing duration, and number of cycles. Annealings were carried out using a homemade annealing reactor possessing a special heater element made of a Si wafer for rapid heating. The density of dislocations, which were made observable with a scanning electron microscope after etching with an Everson solution, were calculated by counting the number of dislocations per unit surface area, hence the term etch pit density (EPD). We were able to decrease EPD values by one order of magnitude after annealing. For example, the best EPD value after a 20-min annealing at 400°C was ∼ 2 × 107 cm−2 for a 1.63-μm CdTe thin film which was about 9.5 × 107 cm−2 before annealing. We also employed Raman scattering measurements to see the changes in the structural quality of the samples. From the Raman measurements, we were able to see improvements in the quality of our samples from the annealing by studying the ratio of 2LO/LO phonon mode Raman intensities. We also observed a clear decrease in the intensity of Te precipitations-related modes, indicating a decrease in the size and number of these precipitations.