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.

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.

High-voltage Al0.22Ga0.78N-GaN high-electron mobility transistors have been fabricated using multiple field plates over dielectric passivation layers. The device breakdown voltage was found to increase with the addition of the field plates. with two field plates, the device showed, a breakdown voltage as high as 900 V. This technique is easy to apply, based on the standard planar transistor fabrication, and especially attractive for the power switching applications.

We report AlGaN-GaN high electron mobility transistors (HEMTs) grown by molecular beam epitaxy (MBE) on SiC substrates with excellent microwave power and efficiency performance. The GaN buffers in these samples were doped with carbon to make them insulating. To reduce gate leakage, a thin silicon nitride film was deposited on the AlGaN surface by chemical vapor deposition. At 4 GHz, an output power density of 6.6 W/mm was obtained with 57% power-added efficiency (PAE) and a gain of 10, dB at a drain bias of 35 V. This is the highest PAE reported until now at 4 GHz in AlGaN-GaN HEMTs grown by MBE. At 10 GHz, we measured an output power density of 7.3 W/mm with a PAE of 36% and gain of 7.6 dB at 40-V drain bias.

Titanium dioxide (TiO2, with the rutile structure) was grown on (0001) oriented GaN and (0001) Al0.33Ga0.67N/GaN heterostructure field effect transistor (HFET) structures by molecular beam epitaxy. X-ray diffraction showed (100)TiO2 parallel to (0001)(GaN(AlGaN)) and [001](TiO2) parallel to (GaN(AlGaN)) with three rotational variants of the TiO2. Transmission electron microscopy of 50 nm thick TiO2 films on GaN and AlGaN/GaN showed sharp interfaces with no intermixing or reaction between the oxide and semiconductor. The TiO2 exhibited a columnar film microstructure with a lateral domain size of a few nanometers parallel to (10 (1) over bar)(TiO2) and a few tens of nanometers parallel to 00160 2 2 Metal-oxide HFIAs with 50 nm thick TiO2 dielectric layers under the gate were processed and compared to HFETs without the TiO2 dielectric layer. The transconductance of the HFETs with TiO2 was 140 MS/mm, approximately 20% less than HFETs with no dielecric, and the pinchoff voltages of the two stuctures were comparable. The dielectric constant of the TiO2 was similar to 70. The gate leakage current of the HFETs with ,TiO2, similar to 4 X 10(-6) mA/mm at. 50 V, was approximately 4 orders of magnitude lower than that of the. HFETs with no dielectric. Band offset measurements were performed using x-ray photoelectron spectroscopy, and the valence band of the rutile TiO2 and the GaN nearly line up. (c) 2005 American Vacuum Society.