Showing papers by "Seydi Doğan published in 2005"
TL;DR: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature.
Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...
10,260 citations
TL;DR: In this article, high quality n-ZnO films on commercial p-type 6H-SiC substrates have been grown by plasma-assisted molecular-beam epitaxy, and n- ZnO∕p-SiCs heterojunction mesa structures have been fabricated.
Abstract: High quality n-ZnO films on commercial p-type 6H–SiC substrates have been grown by plasma-assisted molecular-beam epitaxy, and n-ZnO∕p-SiC heterojunction mesa structures have been fabricated. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2×10−4A∕cm2 at −10V, a breakdown voltage greater than 20V, a forward turn on voltage of ∼5V, and a forward current of ∼2A∕cm2 at 8V. Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as 0.045A∕W at −7.5V reverse bias was observed for photon energies higher than 3.0eV.
140 citations
TL;DR: In this article, the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks was reported.
Abstract: We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situ annealing of thin Ti layers deposited in a metalization chamber, on the (0001) face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-ray diffraction shows that GaN grown on the TiN network has a smaller (102) plane peak width (4.6 arcmin) than the control GaN (7.8 arcmin). In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers.
54 citations
TL;DR: In this article, a highly n-doped GaN subcontact layer was inserted between the contact metal and the high resistivity SiC bulk to prevent current crowding, reduce the contact resistance, and avoid contact degradation.
Abstract: High-power photoconductive semiconductor switching devices were fabricated on 4H–SiC In order to prevent current crowding, reduce the contact resistance, and avoid contact degradation, a highly n-doped GaN subcontact layer was inserted between the contact metal and the high resistivity SiC bulk This method led to a two orders of magnitude reduction in the on-state resistance and, similarly, the photocurrent efficiency was increased by two orders of magnitude with the GaN subcontact layer following the initial high current operation Both dry etching and wet etching were used to remove the GaN subcontact layer in the channel area Wet etching was found to be more suitable than dry etching
27 citations
TL;DR: In this paper, the photoresponse properties of n-ZnO/p-SiC heterojunction mesa structures have been studied and their currentvoltage characteristics have been analyzed.
23 citations
TL;DR: In this article, the authors investigated the correlation between surface charging and current collapse in an AlGaN∕GaN heterostructure field effect transistor and found that surface charging due to applied biases was sensed by mapping the surface potential between the gate and drain using scanning Kelvin probe microscopy.
Abstract: This work investigates the correlation between surface charging and current collapse in an AlGaN∕GaN heterostructure field effect transistor. Surface charging due to applied biases was sensed by mapping the surface potential between the gate and drain using scanning Kelvin probe microscopy. Due to the bias, the surface band bending near the gate edge was observed to increase by as much as 1 eV. This increase of band bending is caused by an accumulation of excess charge near the surface during the applied bias. By varying the duration of the applied bias, we find that this accumulation of excess charge near the gate takes about 20 s to saturate. Continuous monitoring of the surface potential after switching off the bias shows that a complete relaxation of the excess band bending requires about 800 s. Drain current transient measurements show that the collapse and recovery of the drain current also occur on similar time scales. This correlation between time scales indicates that the accumulation of excess c...
21 citations
07 Mar 2005
TL;DR: In this article, the authors used deep level transient spectroscopy on Schottky diodes from 80 K to 700 K to characterize traps up to 1.2 eV.
Abstract: Deep levels in n-type GaN grown by molecular beam epitaxy, metalorganic chemical vapor deposition, and hydride vapor phase epitaxy were characterized for comparison between the different methods of growth. The deep level energies, capture cross sections, and concentrations were determined for each using deep level transient spectroscopy on Schottky diodes from 80 K to 700 K, to characterize traps up to ~1.2 eV. The capture kinetics and bias dependence were also measured for the main traps in each, in order to determine if they are related to threading dislocations, and if they are donor-type traps. Several traps were detected in samples from each growth method. The field dependence and the capture kinetics were not the same for peaks appearing in the same temperature in deep level spectra, associated with different growth method. Traps in HVPE GaN at 0.212 eV and 0.612 eV uniquely showed field dependence indicating singly charged donors. Overall, the thick hydride vapor phase epitaxy GaN samples showed the lowest concentration of traps.
18 citations
TL;DR: In this article, a study of GaN regrowth on micro-faceted GaN templates formed by in-situ thermal etching in a low-pressure metalorganic chemical vapor deposition system was performed.
Abstract: We report a study of GaN regrowth on micro-facetted GaN templates formed by in-situ thermal etching in a low-pressure metalorganic chemical vapor deposition system. First, the 1.5 µm-thick GaN epilayers were grown on c-plane sapphire substrates. This was followed by an in-situ thermal etching process under hydrogen and ammonia ambient with the etching time being a parameter. The thermally etched GaN templates showed hexagonal GaN pyramids, which were aligned along the growth direction on the c-plane substrate. The 3 µm-thick GaN regrowth was performed on these micro-facetted GaN templates. The surface of the overgrown GaN was atomically smooth. The full width at half maximum of (102) peak in the X-ray rocking curve profile decreased from 9.5 to 6.5 arcmin as the thermal etching time increased from 0 to 45 min. Etch pit density measurements revealed that the pit density of regrown GaN decreased by about one order of magnitude, compared to that of the control sample. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
14 citations
TL;DR: In this article, the authors have grown GaN layers by MOCVD on a set of nanoporous SiC templates with different porosity and morphology, produced by etching the anodized porous SiC starting material in a H2 environment at temperatures ∼1500 °C.
Abstract: We have grown GaN layers by MOCVD on a set of nanoporous SiC templates with different porosity and morphology, produced by etching the anodized porous SiC starting material in a H2 environment at temperatures ∼1500 °C, in an effort to attain improved films. The hydrogen etching serves to remove surface damage caused during mechanical polishing prior to anodization, remove the skin layer associated with anodization, tune the pore size, and consolidate pore geometry. Growth conditions favoring lateral overgrowth of GaN were employed on this set of samples to obtian GaN to a thickness of 2 µm. Atomically smooth surfaces were obtained for the epitaxial GaN layers. The GaN quality is highly dependent on the specifics of the porous templates used. An intensity increase of up to a factor of 30 was observed in the GaN excitonic peak compared to GaN grown on standard SiC substrate. The I-V data indicated significant reduction in the leakage current (in reverse bias) compared to GaN grown on standard SiC. The dependence of optical properties, crystalline quality, and surface morphology on the particulars of porous SiC templates is discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
6 citations
TL;DR: In this paper, TEM analyses performed for the GaN layer with 20 nm TiN porous netowrk indicate the effectiveness of TiN structure in blocking the threading dislocation from penetrating into the upper layer.
Abstract: nitridation of thin Ti layers (20 nm and 10nm) on a GaN template. TEM analyses performed for the GaN layer with 20 nm TiN porous netowrk indicate the effectiveness of TiN porous structure in blocking the threading dislocation from penetrating into the upper layer. Plane-view TEM indicated a reduction in the dislocation density by a factor of 10, compared to the GaN template without TiN network. Subsurface voids were formed during the TiN formation, which act as defect concentrators, and termination sites for dislocations. The reduction in defect density through the use of TiN porous networks is also confimed by X-ray diffraction data and time-resolved photoluminescence measurements at room temperature.
6 citations