Browsing by Author "Yildiz, A."

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    Contributions of impurity band and electron-electron interactions to magnetoconductance in AlGaN
    (Taylor & Francis, 2010-06-30) Tasli, P.; Yildiz, A.; Kasap, M.; Özbay, Ekmel; Lisesivdin, S. B.; Ozcelik, S.
    Low temperature electrical measurements of conductivity, the Hall effect and magnetoconductance were performed on a degenerate AlGaN sample. The sample exhibited negative magnetoconductance at low magnetic fields and low temperatures, with the magnitude being systematically dependent on temperature. The measured magnetoconductance was compared with models proposed previously by Sondheimer and Wilson [Proc. R. Soc. Lond. Ser. A 190 (1947) p. 435] and Lee and Ramakrishan [Rev. Mod. Phys. 57 (1985) p. 287]. Data were analyzed as the sum of the contribution of a two-band and electron-electron interactions to the magnetoconductance, applying these models to describe the observed behavior. Least-squares fits to the data are presented. In the sample, magnetoconductance can be explained reasonably well by assuming these contributions to the measured magnetoconductance. It was found that theoretical and experimental data were in excellent agreement.
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    Determination of the critical indium composition corresponding to the metal-insulator transition in InxGa1-xN (0.06 ≤ x ≤ 0.135) layers
    (Elsevier, 2009-10-13) Yildiz, A.; Lisesivdin, S. B.; Tasli, P.; Özbay, Ekmel; Kasap, M.
    The low-temperature conductivity of InxGa1-xN alloys (0.06 ≤ x ≤ 0.135) is analyzed as a function of indium composition (x). Although our InxGa1-xN alloys were on the metallic side of the metal-insulator transition, neither the Kubo-Greenwood nor Born approach were able to describe the transport properties of the InxGa1-xN alloys. In addition, all of the InxGa1-xN alloys took place below the Ioeffe-Regel regime with their low conductivities. The observed behavior is discussed in the framework of the scaling theory. With decreasing indium composition, a decrease in thermal activation energy is observed. For the metal-insulator transition, the critical indium composition is obtained as xc = 0.0543 for InxGa1-xN alloys.
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    DX-center energy calculation with quantitative mobility spectrum analysis in n-AlGaAs/GaAs structures with low Al content
    (ELSEVIER, 2009-03-17) Lisesivdin, S. B.; Altuntas, H.; Yildiz, A.; Kasap, M.; Özbay, Ekmel; Ozcelik, S.
    Experimental Hall data that were carried out as a function of temperature (60-350 K) and magnetic field (0-1.4 T) were presented for Si-doped low Al content (x=0.14) n-AlxGa1-xAs/GaAs heterostructures that were grown by molecular beam epitaxy (MBE). A 2-dimensional electron gas (2DEG) conduction channel and a bulk conduction channel were founded after implementing quantitative mobility spectrum analysis (QMSA) on the magnetic field dependent Hall data. An important decrease in 2DEG carrier density was observed with increasing temperature. The relationship between the bulk carriers and 2DEG carriers was investigated with 1D self consistent Schrödinger-Poisson simulations. The decrement in the 2DEG carrier density was related to the DX-center carrier trapping. With the simulation data that are not included in the effects of DX-centers, 17 meV of effective barrier height between AlGaAs/GaAs layers was found for high temperatures (T>300 K). With the QMSA extracted values that are influenced by DX-centers, 166 meV of the DX-center activation energy value were founded at the same temperatures.
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    Electrical conduction properties of Si δ-doped GaAs grown by MBE
    (2009) Yildiz, A.; Lisesivdin, S.B.; Altuntas H.; Kasap, M.; Ozcelik, S.
    The temperature dependent Hall effect and resistivity measurements of Si δ-doped GaAs are performed in a temperature range of 25-300 K. The temperature dependence of carrier concentration shows a characteristic minimum at about 200 K, which indicates a transition from the conduction band conduction to the impurity band conduction. The temperature dependence of the conductivity results are in agreement with terms due to conduction band conduction and localized state hopping conduction in the impurity band. It is found that the transport properties of Si δ-doped GaAs are mainly governed by the dislocation scattering mechanism at high temperatures. On the other hand, the conductivity follows the Mott variable range hopping conduction (VRH) at low temperatures in the studied structures. © 2009 Elsevier B.V. All rights reserved.
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    Electronic transport characterization of AlGaN∕GaN heterostructures using quantitative mobility spectrum analysis
    (AIP Publishing LLC, 2007-09-06) Lisesivdin, S. B.; Yildiz, A.; Acar, S.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
    Resistivity and Hall effect measurements in nominally undoped Al0.25Ga0.75N/GaN heterostructures grown on sapphire substrate by metal-organic chemical vapor deposition are carried out as a function of temperature (20-350 K) and magnetic field (0-1.5 T). The measurement results are analyzed using the quantitative mobility spectrum analysis techniques. It is found that there is strong two-dimensional electron gas localization below 100 K, while the thermally activated minority carriers with the activation energies of similar to 58 and similar to 218 meV contribute to the electron transport at high temperatures.
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    Foraging swarms as Nash equilibria of dynamic games
    (IEEE, 2014) Özgüler, A. B.; Yildiz, A.
    The question of whether foraging swarms can form as a result of a noncooperative game played by individuals is shown here to have an affirmative answer. A dynamic game played by N agents in 1-D motion is introduced and models, for instance, a foraging ant colony. Each agent controls its velocity to minimize its total work done in a finite time interval. The game is shown to have a unique Nash equilibrium under two different foraging location specifications, and both equilibria display many features of a foraging swarm behavior observed in biological swarms. Explicit expressions are derived for pairwise distances between individuals of the swarm, swarm size, and swarm center location during foraging.
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    Grain boundary related electrical transport in Al-rich AlxGa1-xN layers grown by metal-organic chemical vapor deposition
    (M A I K Nauka - Interperiodica, 2011) Yildiz, A.; Tasli, P.; Sarikavak, B.; Lisesivdin, S. B.; Ozturk, M. K.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
    Electrical transport data for Al-rich AlGaN layers grown by metal-organic chemical vapor deposition (MOCVD) are presented and analyzed in the temperature range 135-300 K. The temperature dependence of electrical conductivity indicated that conductivity in the films was controlled by potential barriers caused by carrier depletion at grain boundaries in the material. The Seto's grain boundary model provided a complete framework for understanding of the conductivity behavior. Various electrical parameters of the present samples such as grain boundary potential, donor concentration, surface trap density, and Debye screening length were extracted.
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    Growth parameter investigation of Al0.25Ga0.75N/GaN/AlN heterostructures with Hall effect measurements
    (Institute of Physics Publishing Ltd., 2008-08-08) Lisesivdin, S. B.; Demirezen, S.; Caliskan, M. D.; Yildiz, A.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
    Hall effect measurements on unintentionally doped Al0.25Ga0.75N/GaN/AlN heterostructures grown by metal organic chemical vapor deposition (MOCVD) were carried out as a function of temperature (20–300 K) and magnetic field (0–1.4 T). Magnetic-field-dependent Hall data are analyzed using the quantitative mobility spectrum analysis (QMSA) technique. The QMSA technique successfully separated electrons in the 2D electron gas (2DEG) at the Al0.25Ga0.75N/GaN interface from other 2D and 3D conduction mechanisms of the samples. 2DEG mobilities, carrier densities and conductivities of the investigated samples are compared at room temperature and low temperature (20 K). For a detailed investigation of the 2DEG-related growth parameters, the scattering analyses of the extracted 2DEG were carried out for all of the samples. Using the results of the scattering analyses, the relation between the growth and scattering parameters was investigated. Increments in the interface roughness (IFR) are reported with the increased GaN buffer growth temperatures. In addition, a linear relation between the deformation potential and interface roughness (IFR) scattering is pointed out for the investigated samples, which may lead to a better understanding of the mechanism of IFR scattering.
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    Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method
    (Springer, 2009-12-03) Yildiz, A.; Lisesivdin, S. B.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel; Balkan, N.
    The electrical conduction mechanisms in various highly resistive GaN layers of Al x Ga1-x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal-insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln∈(ρ) is proportional to T -1/4 for the insulating sample and proportional to T -1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.
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    Large zero-field spin splitting in AlGaN/AlN/GaN/AlN heterostructures
    (AIP Publishing LLC, 2009) Lisesivdin, S. B.; Balkan, N.; Makarovsky, O.; Patanè, A.; Yildiz, A.; Caliskan, M. D.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
    This work describes Shubnikov-de Haas (SdH) measurements in Al0.22 Ga0.78 N/AlN/GaN/AlN heterostructures. Our experiments coupled with the analysis of the Hall data at various temperatures confirm the formation of a two-dimensional electron gas (2DEG) at the AlN/GaN interface. A beating pattern in the SdH oscillations is also observed and attributed to a zero-field spin splitting of the 2DEG first energy subband. The values of the effective spin-orbit coupling parameter and zero-field spin-split energy are estimated and compared with those reported in the literature. We show that zero-field spin-split energy tends to increase with increasing sheet electron density and that our value (12.75 meV) is the largest one reported in the literature for GaN-based heterostructures.
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    Partially informed agents can form a swarm in a nash equilibrium
    (Institute of Electrical and Electronics Engineers, 2015) Yildiz, A.; Ozguler, A. B.
    Foraging swarms in one-dimensional motion with incomplete position information are studied in the context of a noncooperative differential game. In this game, the swarming individuals act with partial information as it is assumed that each agent knows the positions of only the adjacent ones. It is shown that a Nash equilibrium solution that exhibits many features of a foraging swarm such as movement coordination, self-organization, stability, and formation control exists. © 1963-2012 IEEE.
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    Scattering analysis of two-dimensional electrons in AlGaN/GaN with bulk related parameters extracted by simple parallel conduction extraction method
    (American Institute of Physics, 2010-07-15) Lisesivdin, S. B.; Yildiz, A.; Balkan, N.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
    We carried out the temperature (22-350 K) and magnetic field (0.05 and 1.4 T) dependent Hall mobility and carrier density measurements on Al 0.22Ga0.78N/GaN heterostructures with AlN interlayer grown by metal-organic chemical-vapor deposition. Hall data is analyzed with a simple parallel conduction extraction method and temperature dependent mobility and carrier densities of the bulk and two-dimensional (2D) electrons are extracted successfully. The results for the bulk carriers are discussed using a theoretical model that includes the most important scattering mechanisms that contribute to the mobility. In order to investigate the mobility of two-dimensional electron gas, we used a theoretical model that takes into account the polar optical phonon scattering, acoustic phonon scattering, background impurity scattering, and interface roughness scattering in 2D. In these calculations, the values are used for the deformation potential and ionized impurity density values were obtained from the bulk scattering analysis. Therefore, the number of fitting parameters was reduced from four to two. © 2010 American Institute of Physics.
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    The substrate temperature dependent electrical properties of titanium dioxide thin films
    (2010) Yildiz, A.; Lisesivdin, S.B.; Kasap, M.; Mardare, D.
    Titanium dioxide thin films were obtained by a dc sputtering technique onto heated glass substrates. The relationship between the substrate temperature and the electrical properties of the films was investigated. Electrical resistivity measurements showed that three types of conduction channels contribute to conduction mechanism in the temperature range of 13-320 K. The temperature dependence of electrical resistivity between 150 and 320 K indicated that electrical conductioninthe films was controlled by potential barriers caused by depletion of carriers at grain boundaries. The conduction mechanism of the films was shifted from grain boundary scattering dominated band conduction to the nearest neighbor hopping conduction at temperatures between 55 and 150 K. Below 55 K, the temperature dependence of electrical resistivity shows variable range hopping conduction. The correlation between the substrate temperature and resistivity behaviorisdiscussed by analyzing the physical plausibility of the hopping parameters and material properties derived by applying different conduction models. With these analyses, various electrical parameters of the present samples such as barrier height, donor concentration, density of states at the Fermi level, acceptor concentration and compensation ratio were determined. Their values as a function of substrate temperature were compared. © Springer Science+Business Media, LLC 2009.
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    Well parameters of two-dimensional electron gas in Al0.88In 0.12N/AlN/GaN/AlN heterostructures grown by MOCVD
    (Wiley, 2009-12-01) Tasli, P.; Lisesivdin, S. B.; Yildiz, A.; Kasap, M.; Arslan, E.; Özcelik, S.; Özbay, Ekmel
    Resistivity and Hall effect measurements were carried out as a function of magnetic field (0-1.5 T) and temperature (30-300 K) for Al0.88In 0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two-dimensional electron gas (2DEG) channel located at the Al0.88In 0.12N/GaN interface with an AlN interlayer and a two-dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al 0.88In0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al 0.88In0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA