Browsing by Author "Teke, A."

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Open Access
Buffer optimization for crack-free GaN epitaxial layers grown on Si(111) substrate by MOCVD
(Institute of Physics Publishing Ltd., 2008) Arslan, E.; Ozturk, M. K.; Teke, A.; Ozcelik, S.; Özbay, Ekmel
We report the growth of GaN films on the Si(1 1 1) substrate by metalorganic chemical vapour phase deposition (MOCVD). Different buffer layers were used to investigate their effects on the structural and optical properties of GaN layers. A series of GaN layers were grown on Si(1 1 1) with different buffer layers and buffer thicknesses and were characterized by Nomarski microscopy, atomic force microscopy, high-resolution x-ray diffraction (XRD) and photoluminescence (PL) measurements. We first discuss the optimization of the LT-AlN/HT-AlN/Si(1 1 1) templates and then the optimization of the graded AlGaN intermediate layers. In order to prevent stress relaxation, step-graded AlGaN layers were introduced along with a crack-free GaN layer of thickness exceeding 2.6 νm. The XRD and PL measurements results confirmed that a wurtzite GaN was successfully grown. The resulting GaN film surfaces were flat, mirror-like and crack-free. The mosaic structure in the GaN layers was investigated. With a combination of Williamson-Hall measurements and the fitting of twist angles, it was found that the buffer thickness determines the lateral coherence length, vertical coherence length, as well as the tilt and twist of the mosaic blocks in GaN films. The PL spectra at 8 K show that a strong band edge photoluminescence of GaN on Si (1 1 1) emits light at an energy of 3.449 eV with a full width at half maximum (FWHM) of approximately 16 meV. At room temperature, the peak position and FWHM of this emission become 3.390 eV and 58 meV, respectively. The origin of this peak was attributed to the neutral donor bound exciton. It was found that the optimized total thickness of the AlN and graded AlGaN layers played a very important role in the improvement of quality and in turn reduced the cracks during the growth of GaN/Si(1 1 1) epitaxial layers.
Open Access
Comparison of the transport properties of high quality AlGaN/AlN/GaN and AllnN/AlN/GaN two-dimensional electron gas heterostructures
(AIP Publishing LLC, 2009-01-07) Tülek, R.; Ilgaz, A.; Gökden, S.; Teke, A.; Öztürk, M. K.; Kasap, M.; Özçelik, S.; Arslan, E.; Özbay, Ekmel
The transport properties of high mobility AlGaN/AlN/GaN and high sheet electron density AlInN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures were studied. The samples were grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The room temperature electron mobility was measured as 1700 cm(2)/V s along with 8.44 X 10(12) cm(-2) electron density, which resulted in a two-dimensional sheet resistance of 435 Omega/square for the Al(0.2)Ga(0.8)N/AlN/GaN heterostructure. The sample designed with an Al(0.88)In(0.12)N barrier exhibited very high sheet electron density of 4.23 X 10(13) cm(-2) with a corresponding electron mobility of 812 cm(2)/V s at room temperature. A record two-dimensional sheet resistance of 182 Omega/square was obtained in the respective sample. In order to understand the observed transport properties, various scattering mechanisms such as acoustic and optical phonons, interface roughness, and alloy disordering were included in the theoretical model that was applied to the temperature dependent mobility data. It was found that the interface roughness scattering in turn reduces the room temperature mobility of the Al(0.88)In(0.12)N/AlN/GaN heterostructure. The observed high 2DEG density was attributed to the larger polarization fields that exist in the sample with an Al0.88In0.12N barrier layer. From these analyses, it can be argued that the AlInN/AlN/GaN high electron mobility transistors (HEMTs), after further optimization of the growth and design parameters, could show better transistor performance compared to AlGaN/AlN/GaN based HEMTs. c 2009 American Institute of Physics.
Open Access
Current-Transport mechanisms in the AlInN/AlN/GaN single-channel and AlInN/AlN/GaN/AlN/GaN double-channel heterostructures
(Elsevier, 2013) Arslan, E.; Turan, S.; Gökden, S.; Teke, A.; Özbay, Ekmel
Current-transport mechanisms were investigated in Schottky contacts on AlInN/AlN/GaN single channel (SC) and AlInN/AlN/GaN/AlN/GaN double channel (DC) heterostructures. A simple model was adapted to the current-transport mechanisms in DC heterostructure. In this model, two Schottky diodes are in series: one is a metal-semiconductor barrier layer (AIInN) Schottky diode and the other is an equivalent Schottky diode, which is due to the heterojunction between the AlN and GaN layer. Capacitance-voltage studies show the formation of a two-dimensional electron gas at the AlN/GaN interface in the SC and the first AlN/GaN interface from the substrate direction in the DC. In order to determine the current mechanisms for SC and DC heterostructures, we fit the analytical expressions given for the tunneling current to the experimental current-voltage data over a wide range of applied biases as well as at different temperatures. 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. At both a low and medium forward-bias voltage values for Schottky contacts on AlInN/AlN/GaN/AlN/GaN DC and AlInN/AlN/GaN SC heterostructures, the data are consistent with electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80-420 K.
Open Access
The effect of AlN interlayer thicknesses on scattering processes in lattice-matched AlInN/GaN two-dimensional electron gas heterostructures
(Institute of Physics Publishing Ltd., 2009) Teke, A.; Gökden, S.; Tülek, R.; Leach, J.H.; Fan, Q.; Xie, J.; Özgür, Ü.; Morkoç, H.; Lisesivdin, S. B.; Özbay, Ekmel
The scattering mechanisms governing the transport properties of high mobility AllnN/AlN/GaN two-dimensional electron gas (2DEG) heterostructures with various AIN spacer layer thicknesses from zero to 2 nm were presented. 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 alloy disorder limits the electron mobility for samples having spacer layer thicknesses up to 0.3 nm. On the other hand, alloy scattering is greatly reduced as the AlN spacer layer thickness increases further, and hence the combination of acoustic, optical and interface roughness become operative with different degrees of effectiveness over different temperature ranges. The room-temperature electron mobility was observed to increase gradually as the AlN spacer layer increases. A peak electron mobility of 1630 cm2 V-1s -1 was realized for the sample consisting of a 1 nm AlN spacer layer. Then, the electron mobility decreased for the sample with 2 nm AlN. Moreover, the measured 2DEG densities were also compared with the theoretical predictions, which include both piezoelectric and spontaneous polarization components existing at AlN/GaN interfaces. The experimental sheet carrier densities for all AllnN/AlN/GaN HEMT structures were found to be in excellent agreement with the theoretical predictions when the parasitic (unintentional) GaN layer deposited between AlN and AllnN was taken into account. From these analyses, 1 nm AlN spacer layer thickness is found to be the optimum thickness required for high electron mobility and hence low sheet resistance once the sheet carrier density is increased to the theoretically expected value for the sample without unintentional GaN layer.
Open Access
The effect of GaN thickness inserted between two AlN layers on the transport properties of a lattice matched AlInN/AlN/GaN/AlN/GaN double channel heterostructure
(Elsevier BV, 2014-01-31) Tülek, R.; Arslan, E.; Bayraklı, A.; Turhan, S.; Gökden, S.; Duygulu, Ö.; Kaya, A.; Fırat, T.; Teke, A.; Özbay, Ekmel
One AlInN/AlN/GaN single channel heterostructure sample and four AlInN/AlN/GaN/AlN/GaN double channel heterostructure samples with different values of the second GaN layer were studied. The interface profiles, crystalline qualities, surface morphologies, and dislocation densities of the samples were investigated using high resolution transmission electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. Some of the data provided by these measurements were used as input parameters in the calculation of the scattering mechanisms that govern the transport properties of the studied samples. Experimental transport data were obtained using temperature dependent Hall effect measurements (10-300 K) at low (0.5 T) and high (8 T) magnetic fields to exclude the bulk transport from the two-dimensional one. The effect of the thickness of the second GaN layer inserted between two AlN barrier layers on mobility and carrier concentrations was analyzed and the dominant scattering mechanisms in the low and high temperature regimes were determined. It was found that Hall mobility increases as the thickness of GaN increases until 5 nm at a low temperature where interface roughness scattering is observed as one of the dominant scattering mechanisms. When GaN thicknesses exceed 5 nm, Hall mobility tends to decrease again due to the population of the second channel in which the interface becomes worse compared to the other one. From these analyses, 5 nm GaN layer thicknesses were found to be the optimum thicknesses required for high electron mobility. (C) 2013 Published by Elsevier B.V.
Open Access
The electrical, optical, and structural properties of GaN epitaxial layers grown on Si(111) substrate with SiNx interlayers
(ELSEVIER, 2009-10-12) Arslan, E.; Duygulu, Ö.; Kaya, A. A.; Teke, A.; Özçelik, S.; Özbay, Ekmel
The effect of the in situ substrate nitridation time on the electrical, structural and optical properties of GaN films grown on Si(111) substrates by metal organic chemical vapor deposition (MOCVD) was investigated. A thin buffer layer of silicon nitride (SiNx) with various thicknesses was achieved through the nitridation of the substrate at different nitridation times ranging from 0 to 660 s. The surface roughness of the GaN film, which was grown on the Si substrate 10 s, exhibited a root mean square (RMS) value of 1.12 nm for the surface roughness. However, further increments in the nitridation times in turn cause increments in the surface roughness in the GaN layers. The number of threading dislocation (TD) was counted from plan-view TEM (Transmission Electron Microscopy) images. The determined density of these threading dislocations was of the order of 9×109 cm-2. The sheet resistances of the GaN layers were measured. The average sheet resistance significantly increases from 2867 Ω sq-1 for sample A (without nitridation) to 8124 Ω sq-1 for sample F (with 660 s nitridation). The photoluminescence (PL) measurements of the samples nitridated at various nitridation times were done at a temperature range of 10-300 K. A strong band edge PL emission line, which was centered at approx. 3.453 eV along with its phonon replicas which was separated by approx. 92 meV in successive orders, was observed at 10 K. The full width at half maximum (FWHM) of this peak is approx. 14 meV, which indicates the reasonable optical quality of the GaN epilayers grown on Si substrate. At room temperature, the peak position and FWHM of this emission became 3.396 eV and 58 meV, respectively.
Open Access
Energy relaxations of hot electrons in AlGaN/AlN/GaN heterostructures grown by MOCVD on sapphire and 6H-SiC substrates
(E D P Sciences, 2011-08-18) Ilgaz, A.; Gökden, S.; Tülek, R.; Teke, A.; Özçelik, S.; Özbay, Ekmel
In this work, we investigated the hot-electron dynamics of AlGaN/GaN HEMT structures grown by MOCVD on sapphire and SiC substrates at 80 K. High-speed current-voltage measurements and Hall measurements over the temperature range 27-300 K were used to study hot-electron dynamics. At low fields, drift velocity increases linearly, but deviates from the linearity toward high electric fields. Drift velocities are deduced as approximately 6.55 × 10 6 and 6.60 × 106 cm/s at an electric field of around E ∼ 25 kV/cm for samples grown on sapphire and SiC, respectively. To obtain the electron temperature as a function of the applied electric field and power loss as a function of the electron temperature, we used the so-called mobility comparison method with power balance equations. Although their low field carrier transport properties are similar as observed from Hall measurements, hot carrier energy dissipation differs for samples grown on sapphire and SiC substrates. We found that LO-phonon lifetimes are 0.50 ps and 0.32 ps for sapphire and SiC substrates, respectively. A long hot-phonon lifetime results in large non- equilibrium hot phonons. Non-equilibrium hot phonons slow energy relaxation and increase the momentum relaxation. The effective energy relaxation times at high fields are 24 and 65 ps for samples grown on sapphire and SiC substrates, respectively. They increase as the electron temperature decreases.
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, Ekmel
The 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.
Open Access
Temperature dependent hot electron transport in slightly lattice mismatched AlInN/AlN/GaN heterostructures
(National Institute of Research and Development for Optoelectronics, 2014-10) Ilgaz, A.; Gökden, S.; Tülek, R.; Teke, A.; Özçelik, S.; Özbay, Ekmel
In this work, the hot-electron transport properties of AlInN/AlN/GaN HEMT structures with a high sheet electron density of 4.84x10(13) cm(-2) grown by MOCVD (Metal Organic Chemical Vapor Deposition) on sapphire substrate were investigated at lattice temperature ranging from 10 K to 300 K. High speed current voltage measurements and Hall measurements were used to study hot-electron transport. Current-voltage characteristics show that current and drift velocity increase linearly but deviate from the linearity towards high voltages, as would be expected from the increased scattering of hot electrons with LO phonons. However, no saturation of current and drift velocity were observed at the highest voltage reached. Drift velocities were deduced as approximately 6.7x10(6) and 6.1x10(6) cm/s at an electric field of around E similar to 23 kV/cm at lattice temperatures T-L = 10 K and 300 K, respectively. To obtain the electron temperature as a function of the applied electric field and power loss as a function of the electron temperature, the so-called mobility comparison method with power balance equations were used. The effect of hot-phonon production on the phonon lifetime and effective energy relaxation of hot electrons was investigated as a function of lattice temperature.
Open Access
Transport properties of epitaxial graphene grown on SiC substrate
(National Institute of Optoelectronics, 2017) Ağızaçmak, S.; Tülek, R.; Gökden, S.; Teke, A.; Arslan, E.; Aygar, A. M.; Özbay, Ekmel
In this study, the Hall effect measurement of graphene on SiC substrate was carried out as a function of temperature (12-300 K). Hall data were first analyzed to extract the temperature dependent mobilities and carrier densities of the bulk (3D) and two dimensional (2D) channels using a Simple Parallel Conduction Extraction Method (SPCEM) successfully. High carrier mobility 2.296 cm2/V.s from the graphene layer and low carrier mobility 813 cm2/V.s from the SiC were obtained at room temperature. By using SPCEM extracted data, 3D and 2D scattering mechanisms were analyzed and the dominant scattering mechanisms in low and high temperature regimes were determined. It was found that the transport was mainly determined by scattering processes in 2D graphene.