WoS İndeksli Yayınlar Koleksiyonu / WoS Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7150
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Article Citation - WoS: 3Citation - Scopus: 4Terahertz Wavefront Engineering Using a Hard-Coded Metasurface(Springer, 2023) Noori, Aileen; Akyürek, Bora; Demirhan, Yasemin; Özyüzer, Lütfi; Güven, Kaan; Altan, Hakan; Aygün, GülnurDuring the past few years, coding metamaterials (MM) drew significant attention, where the far-field scattering/transmission pattern of the electromagnetic wave (particularly in the THz regime) can be encoded into a single or few-bit digitized phase-response of the metasurface, thereby enabling a full digital control. Single-bit MMs contain two types of unit cells where the phase becomes 0 and 1 (in units of ?), respectively. By arranging these unit cells into a 2D surface pattern, the THz wavefront can be shaped. In this work, a novel hard-coded metasurface was designed, fabricated, and experimentally investigated for multi-beam reflection of incident THz beam. The design employs stripe and checkerboard patterns of bilayer MM unit cells consisting of square gold patches with a polymer spacing layer from a gold backplane. Experimental and simulation results show that the incident wave in the 0.500–0.750 THz range can be reflected with > 95% efficiency in uniform amplitude and 1-bit coded phase. For the checkerboard metasurface pattern, the measured and analytically calculated reflection angle shows good agreement. The metasurface design is suitable for large-scale fabrication and can potentially be used as a template in the development of actively coded metasurfaces. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Article Citation - WoS: 6Citation - Scopus: 5Structural and Optical Characteristics of Antimony Selenosulfide Thin Films Prepared by Two-Step Method(Springer, 2022) Türkoğlu, Fulya; Ekren, Memduh Emirhan; Cantaş, Ayten; Yakıncı, Kübra; Gündoğan, Hazal; Aygün, Gülnur; Özyüzer, LütfiAntimony triselenide (Sb2Se3) is one of the most promising absorber material choices among the inorganic semiconductors that has attracted much attention today. However, highest recorded efficiencies for Sb2Se3 solar cells are still lower than ideal. Exploring antimony selenosulfide (Sb-2(SxSe1-x)(3)) to increase device performance is one option because some features of alloyed Sb-2(SxSe1-x)(3) depend on composition such as bandgap and band position. In this study, two-step process was used to grow Sb-2(SxSe1-x)(3) thin films. In the first stage, Sb2Se3 thin films were deposited on soda lime glass substrates using direct current magnetron sputtering technique. In the second stage, Sb2Se3 thin films were exposed to sulfurization process in a quartz ampoule to obtain Sb-2(SxSe1-x)(3) thin films. Characterization results showed that morphological, optical, and structural properties of Sb-2(SxSe1-x)(3) thin films grown by presented method were highly dependent on amount of sulfur in the films. By the adjustment of the S/S + Se atomic ratio, Sb-2(SxSe1-x)(3) absorber materials with suitable bandgap, favorable orientation and compact morphology can be obtained for photovoltaic applications.Article Citation - WoS: 4Citation - Scopus: 5Enhanced Optoelectronic Properties of Magnetron Sputtered Ito/Ag Multilayers by Electro-Annealing(AVS, 2022) Uyanık, Zemzem; Türkoğlu, Fulya; Köseoğlu, Hasan; Ekmekçioğlu, Merve; Ata, Bengü; Demirhan, Yasemin; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiIndium tin oxide/silver/indium tin oxide (ITO/Ag/ITO) multilayers have attracted much attention to fulfill the growing need for high-performance transparent conducting oxide electrodes. To make these transparent multilayers work better, electro-annealing, which is a method of self-heating by electric current, can be effective. Moreover, the effect of current on ITO/Ag/ITO multilayers should be investigated to make sure that electronic devices will be reliable over their lifetime. In this study, ITO/Ag/ITO multilayer electrodes with varying Ag thicknesses were grown by DC magnetron sputtering at room temperature. Structural, optical, and electrical properties of these multilayers were investigated before and after electro-annealing. Measurement results revealed that improved optical transmittance and sheet resistance can be obtained by the optimization of Ag thickness for the as-grown ITO/Ag/ITO layers. The highest figure of merit (FoM) value of 17.37 × 10−3 Ω−1 with optical transmittance of 85.15% in the visible region and sheet resistance of 11.54 Ω/□ was obtained for the Ag thickness of 16.5 nm for as-grown samples. The electro-annealing of as-grown ITO/Ag/ITO multilayers led to improved optical behavior of the multilayer structure over a wide spectral range, especially in the near-infrared range. Electro-annealing also provided an improvement in the crystallinity and sheet resistance of the electrodes. The improvement of the electrical and optical properties of the structure enabled a FoM of 23.07 × 10−3 Ω−1 with the optical transmittance of 86.80% in the visible region and sheet resistance of 10.52 Ω/□. The findings of this work provide proper knowledge of the properties of ITO/Ag/ITO multilayers under electrical current and suggest that the overall performance of the multilayers can be improved by the electro-annealing process.Article Citation - WoS: 8Citation - Scopus: 8Development of Zto/Ag Transparent Electrodes for Thin Film Solar Cells(Springer, 2022) Türkoğlu, Fulya; Köseoğlu, Hasan; Ekmekçioğlu, Merve; Cantaş, Ayten; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiThis article presents the optimization of Zinc Tin Oxide/Silver/Zinc Tin Oxide (ZTO/Ag/ZTO) multilayers to implement them in thin film solar cells as transparent electrodes. To achieve improvements on the performance of these transparent multilayers, effect of Ag and ZTO thicknesses, and position of Ag layer within the multilayer were investigated. Electrical and optical characterization of these multilayers revealed that reduced sheet resistance and improved optical transmittance can be acquired for solar cells by the optimization of thin film thicknesses and position of the Ag within the multilayer. The improvement of the electrical and optical behavior of the ZTO/Ag/ZTO structures enabled figure of merit (FoM) values up to 69.69 × 10–3 Ω−1. The performance of our multilayer electrodes was also compared with ITO and AZO electrodes. The obtained results suggest that fabricated multilayer electrodes can be a good choice for thin film solar cells.Article Citation - WoS: 33Citation - Scopus: 39High Transparent, Low Surface Resistance Zto/Ag Multilayer Thin Film Electrodes on Glass and Polymer Substrates(Pergamon-Elsevier Science Ltd, 2021) Ekmekçioğlu, Merve; Erdoğan, Nursev; Astarlıoğlu, Aziz Taner; Yiğen, Serap; Aygün, Gülnur; Özyüzer, Lütfi; Özdemir, MehtapZinc tin oxide (ZTO)/Ag/ZTO multilayer thin films were grown by direct current (DC) magnetron sputtering technique at room temperature on soda lime glass (SLG) and different polymer substrates such as polycarbonate (PC) and polyethylene terephthalate (PET) for transparent conductive electrode (TCE) applications. The effect of substrate on the structural, optical and electrical characteristics of ZTO/Ag/ZTO multilayers was investigated. All prepared ZTO/Ag/ZTO films presented amorphous structure as expected from room temperature deposition process and smooth surface quality with very low surface roughness. We found that ZTO/Ag/ZTO multilayer films grown on SLG, PET and PC substrates have very high optical transmission and low surface resistance. Moreover, after ZTO/Ag/ZTO multilayer thin film deposition on polymer substrates, the optical transmission was found to be enhanced because the higher absorption due to Ag layer is compensated by lower reflectance. Our results suggest that ZTO/Ag/ZTO multilayer thin films on any substrate can be a promising alternative to indium tin oxide (ITO) films as a cost-effective, indium-free, flexible and transparent electrode for various applications.Article Citation - WoS: 6Citation - Scopus: 6Nanolitography Based on Electrospun and Etched Nanofibers(Elsevier, 2021) Noori, Aileen; Döğer, Hilal; Demirhan, Yasemin; Özdemir, Mehtap; Özyüzer, Lütfi; Aygün, Gülnur; Sağlam, ÖzgeIn this study, we propose a new type of nanolithography procedure to fabricate orderly patterned metallic nanostructures using the electrohydrodynamic method and the reactive ion etching process. The electrohydrodynamic process parameters were tuned so as to create patterning with precision, and fibers in nanoscale on silver-coated substrates. We also studied reactive ion etching with different durations on the well-patterned samples. The experiments show that applying a voltage of 400 V resulted in straight patterned fibers with a diameter of 208.7 ? 30.3 nm. The statistical analysis on scanning electron microscope (SEM) images showed a significant difference in the diameter of the fibers fabricated at 400 V compared to those at 500 V and 600 V. We also confirm that the etching process has no affect on the fiber diameter. Moreover, electron dispersive X-Ray spectrometer (EDX) results suggest that an etching duration of 7 min is sufficient to remove the silver coating that is not covered with the fibers, and protect the silver nanostructures underneath the fibers. Utilizing a lowcost nanolithography procedure, we obtain the orderly patterned silver nanostructures for possible integration into miniaturized devices.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: 39Citation - Scopus: 46Surface Free Energy Analysis of Ito/Au Multilayer Thin Films on Polycarbonate Substrate by Apparent Contact Angle Measurements(Elsevier, 2020) Özbay, Salih; Erdoğan, Nursev; Erden, Fuat; Ekmekçioğlu, Merve; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiA detailed surface free energy (SFE) knowledge of transparent conducing oxide (TCO)/metal/TCO electrodes is necessary for their applications related to surface wettability. However, SFE analysis of these surfaces has not been performed systematically previously. In this study, ITO and ITO/Au/ITO multilayer thin films were coated onto O-2 plasma treated polycarbonate (PC) substrates by magnetron sputtering. The wettability characteristics of untreated PC, O-2 plasma treated PC, ITO, Au interlayer, and ITO/Au/ITO multilayer thin films were evaluated by apparent contact angle measurements of nine different test liquids having various surface tensions. Following this, Lifshitz-van der Waals, acidic, basic, dispersive, and polar components of SFE were calculated using acidbase, geometric and harmonic mean approaches. In the present study, in which the significance of calculation methods and selected liquid pairs on SFE parameters were investigated, the effect of Au interlayer presence on SFE parameters were also evaluated simultaneously. The results showed that the total SFE values of ITO/Au/ITO multilayer thin films were found to be higher than that of ITO surface. The reasons behind this difference were discussed in terms of SFE components obtained using various liquid pairs by different methods. The results were also supported with XRD, XPS, AFM, and TEM analysis.Conference Object Characterization of Vo2 Films Grown by Magnetron Sputtering(Institute of Electrical and Electronics Engineers Inc., 2015) Yüce, Hürriyet; Aygün, Gülnur; Özyüzer, Lütfi; Köklü, Mehtap[No abstract available]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.
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