Person: Kelekçi, Özgür
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Kelekçi
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Özgür
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Özgür KELEKÇİ
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Publication Electron transport properties in Al0.25Ga0.75N/AlN/GaN heterostructures with different InGaN back barrier layers and GaN channel thicknesses grown by MOCVD(Wiley, 2012-01-20) Özgür Kelekçi; Pınar T. Taşlı; HongBo Yu; Mehmet Kasap; Süleyman Özçelik; Ekmel Özbay; Kelekçi, ÖzgürAbstractThe electron transport properties in Al0.25Ga0.75N/AlN/GaN/InxGa1−xN/GaN double heterostructures with various indium compositions and GaN channel thicknesses were investigated. Samples were grown on c‐plane sapphire substrates by MOCVD and evaluated using variable temperature Hall effect measurements. In order to understand the observed transport properties, various scattering mechanisms, such as acoustic phonon, optical phonon, interface roughness, background impurity, and alloy disorder, were included in the theoretical model that was applied to the temperature‐dependent mobility data. It was found that low temperature (T < 160 K) mobility is limited only by the interface roughness scattering mechanism, while at high temperatures (T > 160 K), optical phonon scattering is the dominant scattering mechanism for AlGaN/AlN/GaN/InGaN/GaN heterostructures. The higher mobility of the structures with InGaN back barriers was attributed to the large conduction band discontinuity obtained at the channel/buffer interface, which leads to better electron confinement.Publication Classification of Killing magnetic curves in ℍ3(World Scientific Pub Co Pte Ltd, 2023-08-29) Özgür Kelekçi; Furkan Semih Dündar; Gülhan Ayar; Kelekçi, ÖzgürIn this paper, we study classification of magnetic curves corresponding to Killing vector fields of [Formula: see text]. First, we solve the geodesic equation analytically. Then we calculate the trajectories generated by all the six Killing vector fields, which are considered as magnetic field vectors, by using perturbation method up to first-order with respect to the strength of the magnetic field. We present a comparison of our solution with the numerical solution for one case. We also prove that 3-dimensional [Formula: see text]-Kenmotsu manifolds cannot have any magnetic vector field in the direction of their Reeb vector fields.Publication Locally conformally flat metrics on surfaces of general type(Duke University Press, 2020-04-01) Mustafa Kalafat; Özgür Kelekçi; Kelekçi, ÖzgürPublication Kähler Magnetic Curves in Conformally Euclidean Schwarzschild Space(Cumhuriyet University, 2024-03-28) Özgür Kelekçi; Kelekçi, ÖzgürIn this paper, we study the magnetic curves on a Kähler manifold which is conformally equivalent to Euclidean Schwarzschild space. We show that Euclidean Schwarzschild space is locally conformally Kähler and transform it into a Kähler space by applying a conformal factor coming from its Lee form. We solve Lorentz equation to find analytical expressions for magnetic curves which are compatible with the almost complex structure of the proposed Kähler manifold. We also calculate the energy of magnetic curves.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.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 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 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 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 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.