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    AlGaN/AlN MOVPE heteroepitaxy: pulsed co-doping SiH4 and TMIn
    (Iop Publishing Ltd, 2019) Demir, Ilkay; Kocak, Yusuf; Kasapoglu, A. Emre; Razeghi, Manijeh; Gur, Emre; Elagoz, Sezai
    We report a new growth approach pulsed co-doping growth of AlxGa1-xN (x > 0.5) epilayers on AlN/Al2O3 templates by metal organic vapor phase epitaxy (MOVPE). Using this approach SiH4 (silane) and TMIn (trimethylindium) supplied to the growth chamber alternately and pulsed during the growth of AlGaN epilayers. Structural and morphological quality of AlGaN epilayers were investigated by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM), Raman spectroscopy, and scanning electron microscopy (SEM) techniques. It has shown that higher crystalline quality with low full width at half maximum (FWHM) and smoother surface morphology with reduced hexagonal hillock density has been obtained by the pulsed codoping growth approach. Volcano like hillock structures has been confirmed by Raman mapping.
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    Distributed contact flip chip InGaN/GaN blue LED; comparison with conventional LEDs
    (ACADEMIC PRESS LTD- ELSEVIER SCIENCE LTD, 2019) Genc, M.; Sheremet, V.; Elci, M.; Kasapoglu, A. E.; Altuntas, I.; Demir, I.; Egin, G.; Islamoglu, S.; Gur, Emre; Muzafferoglu, N.; Elagoz, S.; Gulseren, O.; Aydinli, A.
    This paper presents high performance, GaN/InGaN-based light emitting diodes (LEDs) in three different device configurations, namely Top Emitting (TE) LED, conventional Flip Chip (FC) and Distributed Contact (DC) FC. Series resistances as low as 1.1 Omega have been obtained from FC device configurations with a back reflecting ohmic contact of Ni/Au/RTA/Ni/Ag metal stack. A small shift has been observed between electroluminescence (EL) emissions of TE LED and the FC LEDs. In addition, FWHM value of the EL emission of DCFC LED has shown the minimum value of 160 meV (26.9 nm). Furthermore, DCFC LED configuration has shown the highest quantum efficiency and power output, with 330 mW at 500 mA current injection, compared to that of traditional wire bonded TE LEDs and the conventional FC LEDs.
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    The effects of two-stage HT-GaN growth with different V/III ratios during 3D-2D transition
    (IOP PUBLISHING LTD, 2018) Altuntas, Ismail; Demir, Ilkay; Kasapoglu, Ahmet Emre; Mobtakeri, Soheil; Gur, Emre; Elagoz, Sezai
    The aim of the study is to understand the effects of two stages of HT-GaN growth with different V/III ratios on optical, chemical and structural characteristics of the HT-GaN layer. In addition, the effects of two-stage growth on the transport properties has been presented through fabricated Schottky diodes. As the V/III ratio in the 1st stage growth of HT-GaN layer is varied, it has been observed that recovery of reflectance is faster with higher V/III ratios, while no significant effect has been observed by changing the V/III ratios in the 2nd stage of HT-GaN growth. The 2nd stage growth is caused to decrease in screw dislocation densities obtained from the high-resolution x-ray diffraction measurements. The lowest edge type dislocation densities have been obtained for samples with lower V/III ratios in both the 1st and 2nd stages of HT-GaN growth. Well-defined terraces and a few GaN atomic layer surface roughness have been realized through the atomic force microscopy measurements on all the samples. Ga atom bound oxygen states has been investigated through the x-ray photoelectron spectroscopy to find out the V/III ratio effect on the impurity incorporation. An increase in the V/III ratio has given rise to a higher percentage of oxygen incorporation during the 2nd stage of growth. The lowest internal quantum efficiency has been obtained for the samples grown at the highest V/III ratio for both the 1st and 2nd growth stages. Fine excitonic transitions have been indicated by the low temperature high-resolution photoluminescence measurements. Current-voltage measurements performed on the Schottky diodes have shown the effects on the diode parameters.
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    Formation of carbon nanowalls by pulsed filtered cathodic vacuum arc deposition
    (ELSEVIER SCIENCE SA, 2019) Tuzemen, Ebru Senadim; Kilic, Merhan; Zeyrek, Birsen Kesik; Kasapoglu, Ahmet Emre; Gur, Emre; Alaydin, Behcet Ozgur; Esen, Mehmet; Esen, Ramazan
    In the present study, Carbon Nanowalls (CNWs) were produced by Pulsed Filtered Cathodic Vacuum Arc Deposition (PFCVAD) on glass substrates. A high purity graphite rod was used as a cathode target. Growth was performed by varying the distance between the target and substrate, namely 1 cm (1A250), 2 cm (2A250) and 3 cm (3A250). No CNW formation has been observed on the substrate, when the target and substrate distance is 3 cm. Amorphous nature of carbon structures has been shown with the Raman measurements. Raman mapping has been also performed to show the vertical wall structures of the CNWs. Surface morphology of the CNWs was investigated by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) measurements. It has been found that the wall widths are roughly 1.8 mu m and 1.0 mu m for the samples 1A250 and 2A250, respectively. It has also shown that curvier vertical wall structures have formed for samples 2A250 compared to that of 1A250. X-ray Photoelectron Spectroscopy measurements have shown that 40% sp3 C-C bonding and C-O bonding which indicates the amorphous nature of the structure. Optical studies have shown that CNWs have low reflectance in visible region when the substrate to target distance is 1 cm which makes this thin coating very suitable for dark coating applications.
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    Influence of Highly Efficient Carbon Doping on AlxGa1-xAs Layers with Different Al Compositions (x) Grown by MOVPE
    (Springer, 2023) Perkitel, Izel; Kekul, Reyhan; Altuntas, Ismail; Gur, Emre; Demir, Ilkay
    Carbon (C)-doped aluminum gallium arsenide (AlxGa1-xAs) epitaxial layers with different aluminum (Al) concentrations have been grown on gallium arsenide (GaAs) substrates by metalorganic vapor phase epitaxy (MOVPE) technique. The impact of varying carbon tetrabromide (CBr4) flow rates on the electrical properties of AlxGa1-xAs materials with different Al compositions has been investigated. High-resolution x-ray diffraction (HRXRD) measurement and a Hall effect measurement system have been used to determine the Al compositions and to evaluate the electrical properties. It has been found that the carrier density increases and the mobility decreases by increasing the flow rate of CBr4 and changing Al compositions up to a certain point. In contrast, at higher Al compositions, a decrease in carrier density and an increase in mobility have been observed with increasing CBr4 flow rate. Since these observed trends require to be analyzed in more detail, x-ray photoelectron spectroscopy (XPS) has been used to analyze the elements in the structure. From the XPS results, it has been shown that the atomic concentration of the arsenic in the structure decreased with the increase in CBr4 flow rates. In addition, it has been shown that the Al composition in the AlxGa1-xAs material obtained from the XRD results increases with the increase in the atomic concentration of the arsenic. Accordingly, a linear increase in carrier concentration is shown with increasing Al composition. This increase is explained by the effect of the Al-C bond content on the electrical properties of Al(x)Ga(1-)xAs.
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    Influences of thickness and temperature of low temperature GaAs buffer layer on two-step MOVPE grown GaAs/Ge heterostructures
    (Edp Sciences S A, 2020) Demir, Ilkay; Kasapoglu, Ahmet Emre; Budak, Hasan Feyzi; Gur, Emre; Elagoz, Sezai
    We investigate influence of GaAs buffer layer (BL) growth parameters such as temperature and thickness on the structural, morphological, crystalline and optical quality of metal organic vapor phase epitaxy (MOVPE) grown heterostructures of GaAs on Ge. It was found that the optimal BL conditions significantly decrease the effects of anti-phase boundaries (APBs) even when grown on offcut Ge substrate by two-step growth technique with AsH3 pre-flow to promote double atomic step formation. It is observed that as the growth temperature increases, the growth rate of the GaAs BL increases, too. Improvement on the structural quality is observed up to BLs temperature of 535 degrees C, then it decreases. On the other hand, as the different thick BLs, 12, 25, 75nm are considered, the epilayer grown on the 25nm thick BL has shown the lowest full width at half maximum (FWHM) value, large photoluminescence peak intensity and internal quantum efficiency (IQE).
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    Layered Transition Metal Sulfides for Supercapacitor Applications
    (Wiley-V C H Verlag Gmbh, 2024) Ozturk, Ozan; Gur, Emre
    Supercapacitor (SC) devices holds an important position between traditional capacitors and ion batteries in terms of energy density and power density values. In particular, SC's greater power density values than Li-ion batteries make them useful for some specific applications, such as storing energy in hybrid cars while the car slows down. Increasing energy density values is one of the key challenges for the SC community. Transition metal dichalcogenides (TMDCs), are one of the new developing important material systems to have this potential, compared to their counterparts' transition metal oxides and conductive polymers. This review is about giving insight into the electrochemical performances of two-dimensional (2D) layered transition metal sulfides (TMS) such as MoS2, WS2, TaS2, NbS2, VS2, TiS2 and ZrS2 materials. On the other hand, the methods mostly used in synthesizing these materials are presented.