| GaN-AlN alloys are an important class of materials for ultraviolet photodetectors. The focus of this work was to develop photoconducting and photovoltaic detectors based on AlxGa1-xN films, grown by molecular beam epitaxy.; GaN photoconducting detectors were fabricated and characterized. Mobility-lifetime products of the films were determined from measurement of photoconductive gain. They varied from 10-2cm2/V to 10 -7cm2/V as resistivity changed from 10 2W-cm to 107W-cm. Spectral response showed a sharp transition at 365nm and three orders of magnitude visible light rejection. Semi-insulating films exhibited fast response time of 20ns. Dynamic range was linear over five orders of magnitude. Low frequency noise spectra were dominated by l/f noise in the 0--100Hz range and by generation-recombination noise in the 100Hz--10KHz range. A trap at DE = 0.32eV was identified from the temperature dependence of the noise spectrum. The noise equivalent power was determined to be 3 x 10-14W/Hz 1/2 at 10KHz for the semi-insulating detectors.; Solar-blind UV photoconductive detectors were fabricated using Al xGa1-xN films with Al mole fraction x = 0--0.45. Detectors fabricated on films with Al mole fraction x = 0.45, corresponding to true solar-blind detection, exhibited mobility-lifetime products of 10 -5cm2/V, which is two orders of magnitude higher than that of GaN films of comparable resistivity. These films also exhibited the highest degree of long range atomic ordering. The enhanced photoconductive gain is accounted for by a model, which considers band offsets between the ordered and random domains in the partially ordered alloys, causing spatial separation of photo-carriers and consequently, long recombination lifetimes.; Schottky barrier photovoltaic detectors (n-GaN/Ni-Pt), with a mesa-etched vertical geometry, were fabricated. These photodiodes exhibit a responsivity of 0.18A/W at 325nm and NEP of 1.3 x 10-9W/Hz 1/2 at 1KHz. Doping concentrations and barrier heights were determined to be 2--9 x 1016 cm-3 and 0.95--1.0eV respectively from C-V measurements. Reverse saturation current densities were measured to be in the 1--10 x 10-9 A/cm2 range. Using the diffusion theory of Schottky barriers, vertical mobility values were determined to be 950cm2/V-s. Lateral mobility in films grown under similar conditions was determined to be in the 150--200cm2/V-s range, by Hall effect measurements. The significant increase in vertical mobility is attributed to reduction in electron scattering by charged dislocations. |
