Araştırma Çıktıları / Research Outcomes
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Publication Improvement of breakdown characteristics in AlGaN/GaN/AlxGa1 xN HEMT based on a grading AlxGa1 xN buffer layer(2010-01-01) Kelekçi, ÖzgürTo improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/AlxGa1 xN double hetero structure (DH-HEMTs) were designed and fabricated by replacing the semi-insulating GaN buffer with content graded AlxGa1 xN(x¼x1!x2, x1>x2), in turn linearly lowering the Al content x from x1¼90% to x2¼5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive AlxGa1 xN epilayer suppresses the parasitic conduction in the GaN buffer, and the band edge discontinuity limits the channel electrons spillover, thereby reducing leakage current and drain current collapse. In comparison with the conventional HEMT that use a semi-insulating GaN buffer, the fabricated DH-HEMT device with the same size presents a remarkable enhancement of the breakdown voltage.Publication Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1 xN/AlN/GaN high electron mobility transistors with InGaN back barriers(2011-01-01) Lisesivdin, S.B.; Kelekçi, Özgür; Ozcelik, S.; Ozbay, E.The effects of the In-mole fraction (x) of an InxGa1 xN back barrier layer and the thicknesses of different layers in pseudomorphic AlyGa1 yN/AlN/GaN/InxGa1 xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non linear Schrodinger–Poisson equations. Strain relaxation limits were also calculated for the investigated ¨ AlyGa1 yN barrier layer and InxGa1 xN back barriers. From an experimental point of view, two different optimized structures are suggested, and the possible effects on carrier density and mobility are discussed.Publication Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AlN/GaN heterostructures(2010-01-01) Arslan, Engin; Kelekçi, Özgür; Şafak, Yasemin; Altındal, Şemsettin; Özbay, EkmelThe temperature dependent capacitance–voltage (C–V) and conductance–voltage (G/x–V) characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures were investigated by considering the series resistance (Rs) effect in the temperature range of 80–390 K. The experimental results show that the values of C and G/x are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point ( 2.8 V) and then change to negative values for each temperature, which is known as negative capacitance (NC) behavior. In order to explain the NC behavior, we drawn the C vs I and G/x vs I plots for various temperatures at the same bias voltage. The negativity of the C decreases with increasing temperature at the forward bias voltage, and this decrement in the NC corresponds to the increment of the conductance. When the temperature was increased, the value of C decreased and the intersection point shifted towards the zero bias direction. This behavior of the C and G/x values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. Rs values increase with increasing temperature. Such temperature dependence is in obvious disagreement with the negative temperature coefficient of R or G reported in the literature. The intersection behavior of C–V curves and the increase in Rs with temperature can be explained by the lack of free charge carriers, especially at low temperatures.Publication Anomalous Nernst Effects of [CoSiB/Pt] Multilayer Films(2013-01-01) Lee, H. N.; Kelekçi, Özgür; Kim, T. W.; Noh, H.We report a measurement for the anomalous Nernst effects induced by a temperature gradient in [CoSiB/Pt] multilayer films with perpendicular magnetic anisotropy. The Nernst voltage shows a characteristic hysteresis which reflects the magnetization of the film as in the case of the anomalous Hall effects. With a local heating geometry, we also measure the dependence of the anomalous Nernst voltage on the distance d from the heating element. It is roughly proportional to 1/d1.3, which can be conjectured from the expected temperature gradient along the sample from the heat equation.Publication An alternative IIB embedding of F(4) gauged supergravity(2013-01-01) Jeong, Jaehoon; Kelekçi, Özgür; Colgâin, Eoin ÔThrough the construction of a complete non-linear Kaluza-Klein reduction ansatz from type IIB supergravity to Romans’ F(4) gauged supergravity, we identify a recently discovered supersymmetric AdS6 solution as the IIB uplift of the supersymmetric vacuum of Romans’ theory. We present new IIB uplifts of a number of known solutions of Romans’ theory and comment on supersymmetry in higher-dimensions where it is expected.Publication Effect of growth pressure on coalescence thickness and crystal quality of GaN deposited on 4H–SiC(2011-01-01) Caban, Piotr; Kelekçi, Özgür; Strupinski, Wlodek; Szmidt, Jan; Wojcik, Marek; Gaca, Jaroslaw; Caliskan, Deniz; Ozbay, EkmelThe influence of growth pressure on the coalescence thickness and the crystal quality of GaN deposited on 4H–SiC by low pressure metalorganic vapor phase epitaxy was studied. It was shown that growth pressure has an impact on the surface roughness of epilayers and their crystal quality. GaN coalescence thicknesses were determined for the investigated growth pressures. The GaN layers were characterized by AFM and HRXRD measurements. HEMT structures were also fabricated and characterized. Among the growth pressures studied, 50, 125 and 200 mbar, 200 mbar was found to be most suitable for GaN/SiC epitaxy.Publication Supersymmetry and non-Abelian T-duality in type II supergravity(2015-01-01) Lozano, Yolanda; Kelekçi, Özgür; Macpherson, Niall T; Colgáin, Eoin ÓWe study the effect of T-duality on supersymmetry in the context of type II supergravity. For both U(1) Abelian and SU(2) non-Abelian T-duality, we demonstrate that the supersymmetry variations after T-duality are related to the variations before T-duality through the Kosmann spinorial Lie derivative, which vanishes when the Killing spinors are independent of the T-duality directions. As a byproduct of our analysis, we present closed expressions for SU(2) T-duality in a class of spacetimes with diagonal Bianchi IX symmetry and comment on specific examples of T-dual geometries, including a novel AdS3 geometry with large = (4, 4) superconformal symmetry.Publication Carrier Density and Electric Field Dependent Nonlinear Transport of Chemical Vapor Deposition Graphene(2013-01-01) KHALIL, Hafiz M. W.; Kelekçi, Özgür; NOH, HwayongWe report on the measurements of nonlinear current-voltage characteristics of graphene fabricated by chemical vapor deposition. The current-voltage characteristic is described by a power law with a superlinear dependence of the current on the voltage, and the nonlinearity depends on the carrier density and the excitation level. The nonlinearity is strongest at the Dirac point and becomes weaker as the carrier density increases. At the Dirac point, we also observe a crossover to a much stronger nonlinear transport when the electric field increases above 104 V/m.Publication The structural and electrical evolution of chemical vapor deposition grown graphene by electron beam irradiation induced disorder(2013-01-01) Iqbal, M.Z.; Kelekçi, Özgür; Iqbal, M.W.; Eom, JonghwaThe defect formation mechanism in chemical vapor deposition grown single layer graph ene devices has been investigated by increasing electron beam (e-beam) irradiation doses gradually up to 750 e /nm2 . The evolution of D peaks in Raman spectra provides an evi dence of strong lattice disorder due to e-beam irradiation. Particularly, the trajectory of D and G peak intensities ratio (ID/IG) suggests that the transformation of graphene from crys talline to the nanocrystalline and then towards amorphous form with increasing irradia tion dose. The defect parameters were calculated by phenomenological model of amorphization trajectory for graphitic materials. The mobility decreasing gradually from 1200 to 80 cm2 /V s with gradual increase of irradiation dose, which implies the forma tion of localized states in e-beam irradiated graphene. The Dirac point is shifted towards negative gate voltage which indicates the n-doping in graphene with increasing e-beam irradiation dose.Publication Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD(2012-01-01) Kelekçi, Özgür; Tasli, P.; Cetin, S.S.; Kasap, M.; Ozcelik, S.; Ozbay, E.We investigate the structural and electrical properties of AlxIn1exN/AlN/GaN heterostructures with AlGaN buffers grown by MOCVD, which can be used as an alternative to AlInN HEMT structures with GaN buffer. The effects of the GaN channel thickness and the addition of a content graded AlGaN layer to the structural and electrical characteristics were studied through variable temperature Hall effect measurements, high resolution XRD, and AFM measurements. Enhancement in electron mobility was observed in two of the suggested AlxIn1 xN/AlN/GaN/Al0.04Ga0.96N heterostructures when compared to the standard AlxIn1exN/AlN/GaN heterostructure. This improvement was attributed to better electron confinement in the channel due to electric field arising from piezoelectric polarization charge at the Al0.04Ga0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same inter face. If the growth conditions and design parameters of the AlxIn1 xN HEMT structures with AlGaN buffers can be modified further, the electron spillover from the GaN channel can be significantly limited and even higher electron mobilities, which result in lower two-dimensional sheet resistances, would be possible.