Browsing by Subject "Temperature range"
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Item Open Access Current transport mechanisms and trap state investigations in (Ni/Au)-AlN/GaN Schottky barrier diodes(Elsevier, 2010-10-13) Arslan, E.; Bütün, S.; Şafak, Y.; Çakmak, H.; Yu, H.; Özbay, EkmelThe current transport mechanisms in (Ni/Au)-AlN/GaN Schottky barrier diodes (SBDs) were investigated by the use of current-voltage characteristics in the temperature range of 80-380 K. In order to determine the true current transport mechanisms for (Ni/Au)-AlN/GaN SBDs, by taking the Js(tunnel), E 0, and Rs as adjustable fit parameters, the experimental J-V data were fitted to the analytical expressions given for the current transport mechanisms in a wide range of applied biases and at different temperatures. Fitting results show the weak temperature dependent behavior in the saturation current and the temperature independent behavior of the tunneling parameters in this temperature range. Therefore, it has been concluded that the mechanism of charge transport in (Ni/Au)-AlN/GaN SBDs, along the dislocations intersecting the space charge region, is performed by tunneling. In addition, in order to analyze the trapping effects in (Ni/Au)-AlN/GaN SBDs, the capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics were measured in the frequency range 0.7-50 kHz. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming the models in which traps are located at the heterojunction interface. The density (Dt) and time constants (τt) of the trap states have been determined as a function of energy separation from the conduction-band edge (Ec - Et) as Dt≅ (5-8)×10 12eV-1 cm-2andτt≅(43-102) μs, respectively.Item Open Access Effective mass of electron in monolayer graphene: Electron-phonon interaction(AIP Publishing LLC, 2013-01-25) Tiras, E.; Ardali, S.; Tiras, T.; Arslan, E.; Cakmakyapan, S.; Kazar, O.; Hassan, J.; Janzén, E.; Özbay, EkmelShubnikov-de Haas (SdH) and Hall effect measurements performed in a temperature range between 1.8 and 275 K, at an electric field up to 35 kV m -1 and magnetic fields up to 11 T, have been used to investigate the electronic transport properties of monolayer graphene on SiC substrate. The number of layers was determined by the use of the Raman spectroscopy. The carrier density and in-plane effective mass of electrons have been obtained from the periods and temperature dependencies of the amplitude of the SdH oscillations, respectively. The effective mass is in good agreement with the current results in the literature. The two-dimensional (2D) electron energy relaxations in monolayer graphene were also investigated experimentally. The electron temperature (Te) of hot electrons was obtained from the lattice temperature (TL) and the applied electric field dependencies of the amplitude of SdH oscillations. The experimental results for the electron temperature dependence of power loss indicate that the energy relaxation of electrons is due to acoustic phonon emission via mixed unscreened piezoelectric interaction and deformation-potential scattering.Item Open Access 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.Item Open Access Energy relaxation probed by weak antilocalization measurements in GaN heterostructures(2009) Cheng H.; Bıyıklı, Necmi; Xie J.; Kurdak Ç.; Morko̧ H.Energy relaxation and electron-phonon (e-p) interaction are investigated in wurtzite Al0.15Ga0.85 N/AlN/GaN and Al0.83 In0.17 N/AlN/GaN heterostructures with polarization induced two-dimensional electron gases in the Bloch-Grüneisen regime. Weak antilocalization (WAL) and Shubnikov-de Haas measurements were performed on gated Hall bar structures at temperatures down to 0.3 K. We used WAL as a thermometer to measure the electron temperature Te as a function of the dc bias current. We found that the power dissipated per electron, P e, was proportional to Te4 due to piezoelectric acoustic phonon emission by hot electrons. We calculated Pe as a function of Te without any adjustable parameters for both the static and the dynamic screening cases of piezoelectric e-p coupling. In the temperature range of this experiment, the static screening case was expected to be applicable; however, our data was in better agreement with the dynamic screening case. © 2009 American Institute of Physics.Item Open Access Forward tunneling current in Pt/p-InGaN and Pt/n-InGaN Schottky barriers in a wide temperature range(Elsevier, 2012-07-27) Arslan, E.; Çakmak, H.; Özbay, EkmelThe current-transport mechanisms of the Pt contacts on p-InGaN and n-InGaN were investigated in a wide temperature range (80-360 K) and in the forward bias regime. It was found that the ideality factor (n) values and Schottky barrier heights (SBHs), as determined by thermionic emission (TE), were a strong function of temperature and Φb0 show the unusual behavior of increasing linearly with an increase in temperature from 80 to 360 K for both Schottky contacts. The tunneling saturation ( JTU(0)) and tunneling parameters (E 0) were calculated for both Schottky contacts. We observed a weak temperature dependence of the saturation current and a fairly small dependence on the temperature of the tunneling parameters in this temperature range. The results indicate that the dominant mechanism of the charge transport across the Pt/p-InGaN and Pt/n-InGaN Schottky contacts are electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80-360 K.Item Open Access 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.Item Open Access Investigation on braze joint strength and microstructure of Ti-CP with Ag and Ti base filler alloys(American Welding Society, 2012) Ganjeh, E.; Khorsand H.; Sarkhosh H.; Ghaffari, M.; Sabet H.; Dehkordi, E.H.This research investigates influences of brazing parameters (brazing alloy, temperature and time) on microstructures and mechanical properties of a commercially pure (CP) titanium sheet which is brazed with CBS 34 (Ag-based) and STEMET 1228 (Ti-based) braze-filler foils. Brazing was performed in a conventional inert furnace at temperature ranges of 800-870°C and 10-30 minutes for holding times. Qualities of the brazed joints were evaluated by ultrasonic testing, and then, microstructure and phase constitution of the bonded joints were analyzed by means of metallography, scanning electron microscope (SEM), and X-ray diffraction (XRD). Mechanical properties of brazed joints were evaluated by shear testing. Diffusion of titanium from substrate to filler alloy developed a strong reaction between them. A number of phases such as TiCu, Ti 2Cu, TiAg, Ag-Zn solid solution matrix (for Ag-based brazed samples) and Ti 2Cu, (Ti,Zr) 2Ni, Zr 2Cu (for Ti-based brazed samples) have been identified. The optimum brazing parameters were achieved at a temperature of 870 °C-20 min for CBS 34 and 870 °C-30 min for STEMET 1228. The specimen using STEMET 1228 braze alloy demonstrates best bonding strength (equal to Ti-CP tensile strength). Copyright 2012 ASM International® All rights reserved.Item Open Access Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel(IOP Publishing, 2010-03-16) Gökden, S.; Tülek, R.; Teke, A.; Leach, J. H.; Fan, Q.; Xie, J.; Özgür, Ü.; Morkoç, H.; Lisesivdin, S. B.; Özbay, EkmelThe scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3-1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In 0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.Item Open Access Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition(2012) Ozgit, C.; Donmez I.; Alevli, M.; Bıyıklı, NecmiWe report on the self-limiting growth and characterization of aluminum nitride (AlN) thin films. AlN films were deposited by plasma-enhanced atomic layer deposition on various substrates using trimethylaluminum (TMA) and ammonia (NH 3). At 185 °C, deposition rate saturated for TMA and NH 3 doses starting from 0.05 and 40 s, respectively. Saturative surface reactions between TMA and NH 3 resulted in a constant growth rate of ∼ 0.86 Å/cycle from 100 to 200 °C. Within this temperature range, film thickness increased linearly with the number of deposition cycles. At higher temperatures (≤ 225 °C) deposition rate increased with temperature. Chemical composition and bonding states of the films deposited at 185 °C were investigated by X-ray photoelectron spectroscopy. High resolution Al 2p and N 1s spectra confirmed the presence of AlN with peaks located at 73.02 and 396.07 eV, respectively. Films deposited at 185 °C were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction. High-resolution transmission electron microscopy images of the AlN thin films deposited on Si (100) and glass substrates revealed a microstructure consisting of nanometer sized crystallites. Films exhibited an optical band edge at ∼ 5.8 eV and an optical transmittance of > 95% in the visible region of the spectrum. © 2011 Elsevier B.V. All rights reserved.Item Open Access 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.