Browsing by Author "Kelekci, O."

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Open Access
Effect of growth pressure on coalescence thickness and crystal quality of GaN deposited on 4H-SiC
(Elsevier, 2010-09-25) Caban, P.; Strupinski, W.; Szmidt, J.; Wojcik, M.; Gaca, J.; Kelekci, O.; Caliskan, D.; Özbay, Ekmel
The influence of growth pressure on the coalescence thickness and the crystal quality of GaN deposited on 4HSiC 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.
Open Access
Electron transport properties in Al0.25Ga0.75N/AlN/GaN heterostructures with different InGaN back barrier layers and GaN channel thicknesses grown by MOCVD
(Wiley, 2012-01-24) Kelekci, O.; Tasli, P. T.; Yu, H.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
The 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.
Open Access
Improvement of breakdown characteristics in AlGaN/GaN/AlxGa 1-xN HEMT based on a grading Al xGa 1-xN buffer layer
(Wiley, 2010-08-03) Yu, H.; Lisesivdin, S. B.; Ozturk, M.; Bolukbas, B.; Kelekci, O.; Ozturk, M. K.; Ozcelik, S.; Caliskan, D.; Cakmak, H.; Demirel, P.; Özbay, Ekmel
To improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/Al xGa 1-xN double heterostructure (DH-HEMTs) were designed and fabricated by replacing the semi-insulating GaN buffer with content graded Al xGa 1-xN (x=x 1 → x 2, x 1 > x 2), in turn linearly lowering the Al content x from x 1=90% to x 2=5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive Al xGa 1-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.
Open Access
Investigation of AlInN HEMT structures with different AlGaN buffer layers grown on sapphire substrates by MOCVD
(ELSEVIER, 2012-06-01) Kelekci, O.; Tasli, P.; Cetin, S. S.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
We investigate the structural and electrical properties of Al xIn 1-xN/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 Al xIn 1 -xN/AlN/GaN/Al 0.04Ga 0.96N heterostructures when compared to the standard Al xIn 1 -xN/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 Al 0.04Ga 0.96N/GaN heterointerface and by the conduction band discontinuity formed at the same interface. If the growth conditions and design parameters of the Al xIn 1-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.
Open Access
Numerical optimization of In-mole fractions and layer thicknesses in AlxGa1-xN/AlN/GaN high electron mobility transistors with InGaN back barriers
(ELSEVIER, 2011-02-01) Kelekci, O.; Lisesivdin, S. B.; Ozcelik, S.; Özbay, Ekmel
The effects of the In-mole fraction (x) of an InxGa 1-xN back barrier layer and the thicknesses of different layers in pseudomorphic AlyGa1-yN/AlN/GaN/InxGa 1-xN/GaN heterostructures on band structures and carrier densities were investigated with the help of one-dimensional self-consistent solutions of non-linear SchrdingerPoisson 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.
Open Access
Temperature dependent negative capacitance behavior in (Ni/Au)/AlGaN/AIN/GaN heterostructures
(Elsevier, 2010-02-10) Arslan, E.; Safak, Y.; Altindal, S.; Kelekci, O.; Özbay, Ekmel
The temperature dependent capacitance voltage (C-V) and conductance voltage (G/omega-V) characteristics of (Ni/Au)/Al(0.22)Ga(0.78)N/AlN/GaN heterostructures were investigated by considering the series resistance (R(s)) effect in the temperature range of 80-390 K. The experimental results show that the values of C and G/omega are strongly functioning of temperature and bias voltage. The values of C cross at a certain forward bias voltage point (similar to 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/omega 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/omega values can be attributed to an increase in the polarization and the introduction of more carriers in the structure. R(s) 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 R(s) with temperature can be explained by the lack of free charge carriers, especially at low temperatures.