Si-doped Al_xGa_1-xN photoconductive detectors

We report on the fabrication and characterisation of ultraviolet photoconductive detectors based on Si-doped AlxGa1-xN (0 x0.35) epitaxial layers grown on sapphire. The peak responsivity shifts from 365 nm to 300 nm for increasing Al contents. The devices present high responsivities (~100 A W-1 for Popt = 1 W m-2), which strongly depend on the incident optical power. This behaviour is explained by a light-induced modulation of the conductive area. In addition, the responsivity increases with carrier concentration and mobility, as also predicted by this model. Persistent photoconductivity decreases with doping, whatever the Al content, because of the enhancement of tunnel recombination across the potential barriers generated by surface and defects. Photocurrent decays have been successfully modelled by taking into account both the thermoionic and tunnel relaxation mechanisms, the latter being dominant even for undoped GaN.