GaN p-i-n photodiodes with high visible-to-ultraviolet rejection ratio

Ultraviolet photodetectors are critical components in many applications, including ultraviolet astronomy, flame sensors, early missile threat warning and space-to-space communications. Because of the presence of strong infrared radiation in these situations, the photodetectors have to be solar blind, i.e. able to detect ultraviolet radiation while not being sensitive to infrared. Al_xGa_1-xN is a promising material system for such devices. Al_xGa_1-xN materials are wide bandgap semiconductors, with a direct bandgap whose corresponding wavelength can be continuously tuned from 200 to 365 nm. Al_xGa_1-xN materials are thus insensitive to visible and infrared radiation whose wavelengths are higher than 365 nm. We have already reported the fabrication and characterization of Al_xGa_1-xN-based photoconductors with a cut-off wavelength tunable from 200 to 365 nm by adjusting the ternary alloy composition. Here, we present the growth and characterization of GaN p-i-n photodiodes which exhibit a visible-to-ultraviolet rejection ratio of 6 orders of magnitude. The thin films were grown by low pressure metalorganic chemical vapor deposition. Square mesa structures were fabricated using dry etching, followed by contact metallization. The spectral response, rejection ratio and transient response of these photodiodes is reported.