Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Research Project Üstüniletken magnezyum borür (MgB2) tellerinin yüksek akım ve yüksek manyetik alan uygulamaları için geliştirilmesi(2008) Okur, Salih; Tarhan, Enver; Büyükköse, Serkan; Özyüzer, Lütfi; Tanoğlu, Metin; Emirdağ, Mehtap[No Abstract Available]Research Project Yüksek Kalite İnce Kataliz Filmler Üzerine Kimyasal Buhar Biriktirme Yöntemleri ile Kontrollü Grafen Büyütülmesi, Karakterizasyonu ve Uygulamaları(2016) Selamet, Yusuf; Öztürk, Orhan; Tarhan, EnverGrafen hem çok hafif hem de çelikten kat kat sağlam bir malzemedir. Aynı zamanda görünür bölgede geçirgendir ve metalik özellik gösterir. Grafen değişik şekillerde kesildiğinde bir bant aralığı oluşur ve bunlardan üstün özellikli devre elemanları üretilebilir. Grafen’de elektronik alanında Si’un yerini alacak potansiyel görülmektedir. Bunlarla tek atom kalınlığında elektronik, nano-boyutlu aygıtlar yapılabilir ve normal yarıiletkenlerin boyutlarının daha da küçülmesini sınırlayan kuantum tünelleme olayından etkilenmezler. Grafen kuantum ve rölativistik fizik ile ilgili birçok deneyleri, milyarlarca dolar harcanarak oluşturulan hızlandırıcılara gerek kalmadan basit laboratuvar ortamında yapma olanağı sunar. Grafen’in içerdiği benzersiz fizik tamamen yeni cihazların tasarımlanmasına yol açabilecek yeniliktedir.Research Project Metal-yalıtkan Geçiş Özellikli Vo2 ile Gate Oksitli ve Gate Oksitsiz Alan Etkili Aygıt(2017) Aygün, Gülnur; Tarhan, EnverVanadyum dioksit (VO2) yaklaşık olarak 68 °C'de metal-yalıtkan geçiş özelliği göstermektedir. VO2 düşük sıcaklıklarda yalıtkan fazda bulunurken, geçiş sıcaklığına göre yüksek sıcaklıklarda metalik fazda bulunur. Geçiş sıcaklığında, VO2'nin özdirenci ani bir şekilde 10^4 oranında değişim gösterir ve bu özelliğinden dolayı hızlı elektronik cihazlarda kullanılma potansiyeli oldukça yüksektir. VO2 alan etkili transistör uygulamalarında önemli bir rol oynamaktadır. VO2 gösterdiği elektriksel özellikler sebebiyle alan etkili transistörlerde kanal görevi görmektedir. VO2 malzemesinin elektrik alan altında değişen elektriksel özellikleri, VO2'yi alan etkili transistörlerde kanal malzemesi olarak kullanılmasını mümkün kılmaktadır. Bu özellikler göz önünde bulundurulduğunda VO2 kanal görevi görecek şekilde iki farklı tipte FET üretilmiştir. Birinci tip FET üretiminde, VO2'ye temas etmeksizin, arada hava bırakılarak VO2'nin altına ve üstüne, iki adet altın gate tabakası buharlaştırılmıştır. Altın gate tabakalarının VO2'ye hiçbir şekilde fiziksel teması olmadığından, VO2 içerisine malzeme difüz etmesi de söz konusu değildir. Bu ise VO2'nin MIT özelliğini etkilemeden, e-demeti litografi tekniği sayesinde çok detaylı hatlara sahip olan FET üretebilmemizi sağlamıştır. İkinci tip FET üretiminde ise VO2'nin üretilip e-demeti ile şekillendirilmesi aşamasına kadarki yapılacak işlemler birinci tip FET üretimi ile aynıdır. HfO2 ince filmi gate dielektrik olarak kullanılmıştır. VO2 kanalları üzerine, DC saçtırma yöntemi ile HfO2 gate dielektrik büyütülmüştür. HfO2 gate dielektrik tabakası üzerine Au/Al gate tabakası buharlaştırılmıştır. Devamında ise, iki farklı tipte üretilen FET yapılarındaki (tip 1 ve tip 2) VO2 kanal tabakasının, prob istasyonu kullanılarak vakum ortamında sıcaklığa bağlı olarak, voltaj altında MIT karakterizasyonları yapılmıştır.Article Citation - WoS: 3Citation - Scopus: 3Local Vibrational Modes of Natural Isotopes of Substitutional Oxygen in Cdte(TÜBİTAK - Türkiye Bilimsel ve Teknolojik Araştırma Kurumu, 2020) Tarhan, Enver; Ramdas, Anant K.We investigated the localized vibrational modes (LVM) of natural oxygen containing ${}^{16}O,\;{}^{17}O$ and ${}^{18}O$ isotopes at a substitutional tellurium site in cadmium telluride using infrared absorption spectroscopy at cryogenic temperatures. The main absorption peak observed at 350 cm ?1 was formerly attributed to a fundamental LVM mode (?0) of oxygen at a tellurium site. The relatively weak absorption peaks observed at 331 $cm^{-1}$ and 340 $cm^{-1}$ are assigned as the same $\nu_0$ mode of the ${}^{17}O$ and ${}^{18}O$ isotopes, respectively, based on their relative intensities and spectral positions. The spectral positions were confirmed with theoretical calculations using a linear chain model where the peak position at 350$cm^{-1}$ was taken as the reference for the ${}^{16}O$ isotope. From a least square analysis of the observed peak positions we were able to calculate the force constants from perturbation theory. A Lorentzian line shape analysis of each $\nu_0$ absorption peak, considering the effects of isotopic mass and natural abundance variations of the host Cd atoms, was also carried out to further confirm their assignments. Reasonably good line shape fittings were obtained for $\nu_0$ modes of all isotopes of oxygen.Article Citation - WoS: 9Citation - Scopus: 7Epitaxial Characteristics of Mbe-Grown Znte Thin Films on Gaas (211)b Substrates(Springer, 2019) Özçeri, Elif; Tarhan, EnverHighly crystalline ZnTe thin films were grown on GaAs (211)B substrates by molecular beam epitaxy (MBE) for potential applications such as MCT detectors and optoelectronic devices. We investigated the effects of Te to Zn (VI/II) flux ratio on the quality of ZnTe films in terms of crystal orientation, elemental composition, surface roughness, and dislocation density. Atomic concentrations of Zn, Te, and oxygen complexes due to oxygen contamination on the film surfaces were analyzed by X-ray photoelectron spectroscopy. X-ray double crystal rocking curve full width half maximum (FWHM) of ZnTe (422) peak was observed as 233 arcseconds for a 1.66 mu m thick film, which indicates high crystallinity. Wet chemical etching was applied to the films to quantify the crystal quality by calculating etch pit densities (EPD) from scanning electron microscope images. A very low EPD value of 1.7 x 10(7) cm(-2) was measured. Additionally, the root mean square roughness values, obtained from atomic force microscopy topography images were in the range of 10-25 nm. These values were supported by FWHM values of red green blue color intensity histograms obtained from Nomarski Microscope images. The results of our analyses indicate that the VI/II flux ratios of 4 and 4.5 produce the best quality ZnTe films on GaAs (211)B substrates.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.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: 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: 2The Effect of Thickness of Silver Thin Film on Structural and Optical Properties of Porous Silicon(World Scientific Publishing Co. Pte Ltd, 2017) Çetinel, A.; Özdoğan, M.; Utlu, G.; Artunç, N.; Şahin, G.; Tarhan, EnverIn this study, porous silicon (PS) samples were prepared on n-type silicon (100) wafers by electrochemical etching method, varying the current density from 20 to 100mA/cm2 and keeping constant HF concentration (10%) and etching time of 15min. Then, Ag thin films, which have 10, 50 and 100nm film thicknesses, were deposited on PS layers by using thermal evaporation to investigate the influence of Ag film thickness on structural and optical properties of PS. The structural and optical properties of PS and Ag deposited PS layers have been investigated by XRD, FE-SEM, Raman and photoluminescence (PL) spectroscopy. FE-SEM XRD and Raman analyzes indicate that average pore size and porosity of PS layers increase with the increasing current density. Further, Ag nanoparticles have embedded in pore channel. PL measurement reveals that higher porosity of PS would be better to form the Ag-PS nano-composite material leading to stronger PL band. The PL spectra of PS and Ag-PS samples indicate that PL bands show blue shift with increasing current density and film thickness. Consequently, it has been found that the structural and optical properties of PS depend on current density and Ag film thickness individually.Article Citation - WoS: 24Citation - Scopus: 25Investigation of the Structure of Alpha-Lactalbumin Protein Nanotubes Using Optical Spectroscopy(Cambridge University Press, 2014) Tarhan, Özgür; Tarhan, Enver; Harsa, ŞebnemAlpha-lactalbumin (α-la) is one of the major proteins in whey. When partially hydrolysed with Bacillus licheniformis protease, it produces nanotubular structures in the presence of calcium ions by a self-assembly process. This study presents investigation of α-la protein structure during hydrolysis and nanotube formation using optical spectroscopy. Before spectroscopic measurements, nanotubes were examined with microscopy. The observed α-la nanotubes (α-LaNTs) were in the form of regular hollo strands with a diameter of about 20 nm and the average length of 1 μm. Amide and backbone vibration bands of the Raman spectra displayed remarkable conformational changes in α and β domains in the protein structure during nanotube growth. This was confirmed by the Fourier-transform infrared (FTIR) spectroscopy data. Also, FTIR analysis revealed certain bands at calcium (Ca++) binding sites of COO- groups in hydrolysed protein. These sites might be critical in nanotube elongation.