AlGaN-based photodetectors for solar UV applications

In the recent years, the depletion of the stratospheric ozone layer has alerted the scientific community about the risks of a solar ultraviolet (UV) radiation overexposure. Biological research has confirmed the very important role of the W-B (320-280 nm) and UV-A (400-320 nm) bands on the Earth biosystem. AlxGa1-xN semiconductor alloys, with a bandgap tunable between 3.4eV and 6.2eV, are the most suitable materials for the fabrication of solar UV detectors. In this paper we describe the fabrication and characteristics of AlGaN photoconductive and Schottky barrier photodetectors, with AI mole fractions up to 35%. Photoconductive detectors show very high gains, that decrease with increasing incident optical power. They present persistent photoconductivity effects, and a significant below-the-gap response. The physics of this behavior is discussed. On the other hand, AlGaN Schottky barrier photodetectors show a very fast response that is independent of the optical power, and their UV/visible rejection ratio is rather high. As the Al content increases, the evolution of the responsivity and cut-off wavelength is presented. Al0.22Ga0.78N Schottky barriers are very good candidates to monitor the W-B band. The prospective applications of AlGaN photodiodes to determine the biological action of the solar UV radiation are also discussed.