Carbon tetrabromide (CBr4) was studied as an intentional dopant during rf plasma molecular beam epitaxy of GaN. Secondary ion mass spectroscopy was used to quantify incorporation behavior. Carbon was found to readily incorporate under Ga-rich and N-rich growth conditions with no detectable bromine incorporation. The carbon incorporation [C] was found to be linearly related to the incident CBr4 flux. Reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction were used to characterize the structural quality of the film's postgrowth. No deterioration of structural quality was observed for [C] from mid 10(17) to similar to10(19) cm(-3). The growth rate was also unaffected by carbon doping with CBr4. The electrical and optical behavior of carbon doping was studied by co-doping carbon with silicon. Carbon was found to compensate the silicon although an exact compensation factor was difficult to extract from the data. Photoluminescence was performed to examine the optical performance of the films. Carbon doping was seen to monotonically decrease the band edge emission. Properties of carbon-doped GaN are interpreted to be consistent with recent theoretical work describing incorporation of carbon as function of Fermi level conditions during growth. (C) 2004 American Institute of Physics.

We report on the use of in-situ SiNx nanomask for defect reduction in nonpolar a-plane GaN films, grown by metal-organic chemical vapor deposition. High-resolution x-ray diffraction analysis revealed that there was a monotonic reduction in the full width at half maximum, both on-axis and off-axis, with the increase in the SiNx thickness. Atomic force microscopy images revealed a significant decrease in the root-mean-square roughness and the density of submicron pits. Cross-section and plan-view transmission electron microscopy on the samples showed that the stacking fault density decreased from 8x10(5) to 3x10(5) cm(-1) and threading dislocation density decreased from 8x10(10) to 9x10(9) cm(-2). Room temperature photoluminescence measurement revealed that the band-edge emission intensity increased with the insertion of the SiNx layer, which suggests reduction in the nonradiative recombination centers. (c) 2006 American Institute of Physics.

Polarization gradients in graded AlGaN alloys induce bulk electron distributions without the use of impurity doping. Since the alloy composition is not constant in these structures, the electron scattering rates vary across the structure. Capacitance and conductivity measurements on field effect transistors were used to find mobility as a function of depth. The effective electron mobility at different depths calculated from theory closely matched the measured mobility. Local bulk mobility values for different AlGaN compositions were found, and the electron mobility in AlGaN as a function of alloy composition was deduced. These were found to match with theoretical calculations. (c) 2006 American Institute of Physics.

The electrical conductivity and deep level spectrum of GaN grown by molecular beam epitaxy and codoped with carbon and silicon were investigated for substrate temperatures T-s of 650 and 720 degrees C as a function relative carbon and silicon doping levels. With sufficiently high carbon doping, semi-insulating behavior was observed for films grown at both temperatures, and growth at T-s=720 degrees C enhanced the carbon compensation ratio. Similar carbon-related band gap states were observed via deep level optical spectroscopy for films grown at both substrate temperatures. Due to the semi-insulating nature of the films, a lighted capacitance-voltage technique was required to determine individual deep level concentrations. Carbon-related band gap states underwent substantial redistribution between deep level and shallow acceptor configurations with change in T-s. In light of a T-s dependence for the preferential site of carbon incorporation, a model of semi-insulating behavior in terms of carbon impurity state incorporation mediated by substrate temperature is proposed. (c) 2006 American Institute of Physics.

We discuss an AlGaN/GaN metal-semiconductor field-effect transistor (MESFET) structure grown without any impurity doping in the channel. A high-mobility polarization-induced bulk channel charge was created by grading the channel region linearly from GaN to Al0.3Ga0.7N over 1000 Angstrom. This polarization-doped FET (PolFET) was fabricated and tested under dc and rf conditions. Current density of 850 mA/mm and transconductance of 93 mS/mm was observed under dc conditions. The 0.7 mum gate length devices had a cutoff frequency, f(tau)=19 GHz, and maximum oscillation frequency, f(max)=46 GHz. We demonstrate that the PolFETs perform better than comparable MESFETs with impurity-doped channels, and are suitable for high power microwave applications. An important advantage of these devices over AlGaN/GaN high electron mobility transistors is that the transconductance versus gate voltage profile can be tailored by compositional grading for better large-signal linearity. (C) 2004 American Institute of Physics.

This letter reports AlGaN/GaN high-electron mobility transistors with capless activation annealing of implanted Si for nonalloyed ohmic contacts. Source and drain areas were implanted with an Si dose of I x 10(16) cm(-2) and were activated at similar to 1260 degrees C in a metal-organic chemical vapor deposition system in ammonia and nitrogen at atmospheric pressure. Nonalloyed ohmic contacts to ion-implanted devices showed a contact resistance of 0.96 Omega (.) mm to the channel. An output power density of 5 W/mm was measured at 4 GHz, with 58% power-added efficiency and a gain of 11.7 dB at a drain bias of 30 V.

AlGaN/GaN current aperture vertical electron transistor (CAVET) was fabricated and optimized for band gap selective photoelectrochemical wet etching. The large polarization induced voltage offset (around 2.5-4 eV) observed in the first generation CAVET was reduced to 0.7 V in this structure by employing a delta Si doping layer buried 60 Angstrom below the In0.03Ga0.97N (60 nm thick) and bottom GaN interface to screen the polarization fields. Other sample structures were studied to achieve an aperture with both good undercut etching and a small voltage offset. It was clearly demonstrated that etch selectivity in the GaN/InGaN/GaN undercut structures was influenced by hole confinement and the chemical activity of the N-face GaN. (C) 2004 American Institute of Physics.

Mg doping has been found in some situations to invert growth on Ga-face GaN to N-face. In this study, we clarified the role the Ga wetting layer plays in rf plasma molecular beam epitaxy of GaN when Mg doping, for [Mg] from similar to2 X 10(19) to similar to1 X 10(20) cm(-3) corresponding to the useful, accessible range of hole concentrations of p similar to 10(17)-10(18) cm(-3) Structures were grown in the N-rich and Ga-rich growth regime for single Mg doping layers and for multilayer structures with a range of Mg concentrations. Samples were characterized in situ by reflection high-energy electron diffraction and ex situ by atomic force microscopy, transmission electron microscopy, convergent beam electron diffraction, and secondary ion mass spectroscopy. Growth on "dry" surfaces (without a Ga wetting layer) in the N-rich regime completely inverted to N-face upon exposure to Mg. No reinversion to Ga-face was detected for subsequent layers. Additionally, Mg was seen to serve as a surfactant during this N-rich growth, as has been reported by others. Growth initiated in the Ga-rich regime contained inversion domains that nucleated with the initiation of Mg doping. No new inversion domains were found as the Mg concentration was increased through the useful doping levels. Thus the Ga wetting layer was found to inhibit nucleation of N-face GaN, though a complete wetting layer took time to develop. Finally, by establishing a complete Ga wetting layer on the surface prior to growth, we confirmed this finding and demonstrated Mg-doped GaN completely free from inversion domains to a doping level of [Mg] similar to2 X 10(20). (C) 2003 American Vacuum Society.

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9-1.0 eV for GaN, and an interface polarization charge density of 1.6x10(13)-1.7x10(13) cm(-2), were extracted from the simulations. (C) 2003 American Institute of Physics.