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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.

In this letter we report the fabrication and dc characterization of a high electron mobility transistor (HEMT) based on a n-GaN-Al0.14Ga0.86N heterojunction. The conduction in our low pressure metalorganic chemical vapor deposited heterostructure is dominated by two-dimensional electron gas at the heterostructure interface. HEMT devices were fabricated on ion-implant isolated mesas using Ti/Au for the source drain ohmic and TiW for the gate Schottky. For a device with a 4 μm gate length (10 μm channel opening, i.e., source-drain separation), a transconductance of 28 mS/mm at 300 K and 46 mS/mm at 77 K was obtained at +0.5 V gate bias. Complete pinchoff was observed for a −6 V gate bias.

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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.

Recently several research groups, including ours, have reported on the deposition of extremely high quality single crystal GaN layers over sapphire substrates. One of the keys to obtaining the high quality was the use of a thin AlN or GaN buffer layer between the sapphire substrate and the grown film. In this communication, we discuss the crystallinity and the influence of the buffer layer in controlling the crystalline, optical, and the electrical properties of the GaN depositions. We also compare the use of GaN and AlN as the buffer layer material. Our results indicate that the buffer layer thickness and the total film thickness are the key factors controlling the electrical, optical, and crystalline properties of the GaN depositions over sapphire substrates.

We fabricated a 0.25 μm gate length AlGaN/GaN heterostructure field effect transistor (HFET) with a maximum extrinsic transconductance of 27 mS/mm (at room temperature) limited by the source series resistance. The device exhibited an excellent pinch-off and a low parasitic output conductance in the saturation regime. We measured the cutoff frequency fT and the maximum oscillation frequency fmax as 11 and 35 GHz, respectively. These values are superior to the highest reported values for field effect transistors based on other wide band-gap semiconductors such as SiC. These results demonstrate an excellent potential of AlGaN/GaN HFETs for microwave and millimeter wave applications.

Chemically assisted ion beam etching (CAIBE) characteristics of gallium nitride (GaN) have been investigated using a 500-eV Ar ion beam directed onto a sample in a Cl2 ambient. Enhanced etch rates were obtained for samples etched in the presence of Cl2 over those etched only by Ar ion milling at a substrate temperature of 20 °C. The CAIBE etch rates were further enhanced at higher substrate temperatures whereas etch rates for Ar ion milling were not influenced by substrate temperature. Etch rates as high as 210 nm/min are reported. The etch rates reported here are the highest so far reported for GaN. Anisotropic etch profiles and smooth etched surfaces in GaN have been achieved with CAIBE.