Solar-blind AlGaN photodiodes with very low cutoff wavelength

We report the fabrication and characterization of AlxGa1-xN photodiodes (x similar to 0.70) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The peak responsivity for -5 V bias is 0.11 A/W at 232 nm, corresponding to an internal quantum efficiency greater than 90%. The device response drops four orders of magnitude by 275 nm and remains at low response for the entire near-ultraviolet and visible spectrum. Improvements were made to the device design including a semitransparent Ni/Au contact layer and a GaN:Mg cap layer, which dramatically increased device response by enhancing the carrier collection efficiency. (C) 2000 American Institute of Physics. [S0003-6951(00)01204-3].