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.

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.

Shubnikov-de-Haas oscillation is observed in a polarization-doped three-dimensional electron slab in a graded AlxGa1-xN semiconductor layer. The electron slab is generated by the technique of grading the polar semiconductor alloy with spatially changing polarization. Temperature-dependent oscillations allow us to extract an effective mass of m*=0.21m(0). The quantum scattering time measured (tau(q)=0.3 ps) is close to the transport scattering time (tau(t)=0.34 ps), indicating the dominance of short-range scattering. Alloy scattering is determined to be the dominant mechanism-limiting mobility; this enables us to extract an alloy-scattering parameter of V-0=1.8 eV for the AlxGa1-xN material system. Polarization-doping presents an exciting technique for creating electron slabs with widely tunable density and confinement for the study of dimensionality effects on charge transport and collective phenomena.

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.

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 impact of threading dislocation density on Ni/n-GaN Schottky barrier diode characteristics is investigated using forward biased current-voltage-temperature (I-V-T) and internal photoemission (IPE) measurements. Nominally, identical metal-organic chemical vapor deposition grown GaN layers were grown on two types of GaN templates on sapphire substrates to controllably vary threading dislocation density (TDD) from 3x10(7) to 7x10(8) cm(-2). I-V-T measurements revealed thermionic emission to be the dominant transport mechanism with ideality factors near 1.01 at room temperature for both sample types. The Schottky barrier heights showed a similar invariance with TDD, with measured values of 1.12-1.13 eV obtained from fitting the I-V-T results to a thermionic emission-diffusion model. The I-V-T results were verified by IPE measurements made on the same diodes, confirming that the Ni/n-GaN barrier heights do not show a measurable TDD dependence for the TDD range measured here. In apparent contrast to this result is that the measured forward bias I-V characteristics indicate a shift in the observed forward bias turn-on voltage such that at the higher TDD value investigated here, a larger turn-on voltage (lower current) is observed. This difference is attributed to localized current blocking by high potential barrier regions surrounding threading dislocations that intersect the Ni/GaN interface. A simple model is presented that reconciles both the observed voltage shift and variations in the extracted Richardson constant as a function of threading dislocation density. With this model, an average local barrier surrounding dislocation of similar to 0.2 V is obtained, which diverts current flow across the forward biased Schottky interface to nondislocated regions. (c) 2006 American Institute of Physics.

N-p-n Al0.05GaN/GaN heterojunction bipolar transistors with a common emitter operation voltage higher than 330 V have been demonstrated using selectively regrown emitters. Devices were grown by metalorganic chemical vapor deposition on sapphire substrates. The n-type emitter was grown selectively on a 100-nm-thick p-base with an 8 mum n-collector structure using a dielectric mask. The shallow etch down to the collector mitigates damages induced in the dry etch, resulting a low leakage and a high breakdown. The graded AlGaN emitter results in a common emitter current gain of similar to18 at an average collector current density of up to 1 kA/cm(2) at room temperature.

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.