We report on the fabrication and characterization of photoconductive ultraviolet detectors based on insulating single-crystal GaN. The active layer (GaN) was deposited over basal-plane sapphire substrates using a unique switched atomic-layer-epitaxy process. The sensors were measured to have a responsivity of 2000 A/W at a wavelength of 365 nm under a 5-V bias. The responsivity remained nearly constant for wavelengths from 200 to 365 nm and dropped by three orders of magnitude within 10 nm of the band edge (by 375 nm). We estimate our sensors to have a gain of 6×103 (for wavelength 365 nm) and a bandwidth in excess of 2 kHz. The photosignal exhibited a linear behavior over five orders of incident optical power, thereby implying a very large dynamic range for these GaN-based ultraviolet sensors.

The reverse breakdown voltage of p-GaN Schottky diodes was used to measure the electrical effects of high density Ar or H2 plasma exposure. The near surface of the p-GaN became more compensated through introduction of shallow donor states whose concentration depended on ion flux, ion energy, and ion mass. At high fluxes or energies, the donor concentration exceeded 1019 cm−3 and produced p-to-n surface conversion. The damage depth was established as ∼400 Å based on electrical and wet etch rate measurements. Rapid thermal annealing at 900 °C under a N2 ambient restored the initial electrical properties of the p-GaN.

We have confirmed the presence of a two-dimensional electron gas (2DEG) in a wide band-gap GaN-AlxGa1−xN heterojunction by observing steplike features in the quantum Hall effect. The 2DEG mobility for a GaN-Al0.13Ga0.87N heterojunction was measured to be 834 cm2/V s at room temperature. It monotonically increased and saturated at a value of 2626 cm2/V s at 77 K. The 2DEG mobility remained nearly constant for temperatures ranging from 77 to 4.2 K. Using Shubnikov–de Haas (SdH) measurements the two-dimensional carrier concentration was estimated to be 1×1011 cm−2. The peak mobility for the 2DEG was found to decrease with the heterojunction aluminum compositions in excess of 13%.

The effect of surface polarity on the growth of Mg-doped GaN thin films on c-plane sapphire substrates by molecular-beam epitaxy has been investigated. The doping behavior of Mg and resulting conductivity of the doped layers were found to strongly depend on the surface polarity of the growing GaN planes. The samples grown on the Ga-polar face (A face) exhibited a p-type conductivity with a free-hole concentration up to 5×1017 cm−3, while the samples grown on the N-polar face (B face) were highly resistive or semi-insulating. The incorporation of residual impurities (O, Si, and C) in the two different polar surfaces was studied by secondary ion mass spectrometry analysis and its effect on the Mg doping was discussed. Our results suggest that the A face (Ga face) is the favored surface polarity for achieving p-type conductivity during the growth of Mg-doped GaN.

In this letter we report the first observation of enhanced electron mobility in GaN/AlxGa1−xN heterojunctions. These structures were deposited on basal plane sapphire using low-pressure metalorganic chemical vapor deposition. The electron mobility of a single heterojunction composed of 500 Å of Al0.09Ga0.91N deposited onto 0.3 μm of GaN was around 620 cm2/V s at room temperature as compared to 56 cm2/V s for bulk GaN of the same thickness deposited under identical conditions. The mobility for the single heterojunction increased to a value of 1600 cm2/V s at 77 K whereas the mobility of the 0.3 μm GaN layer alone peaked at 62 cm2/V s at 180 K and decreased to 19 cm2/V s at 77 K. A 18-layer multiple heterojunction structure displayed a peak mobility of 1980 cm2/V s at 77 K.