Highly Responsive, Self-Powered <i>a</i>-GaN Based UV-A Photodetectors Driven by Unintentional Asymmetrical Electrodes

Abstract

Optoelectronic properties of nonpolar a-plane GaN are superior along the [0002] azimuth direction compared to other azimuth directions. We have grown a-GaN on r-sapphire, and interdigitated electrode patterns of Au were fabricated to restrict the carrier transport only along [0002] azimuth. Surprisingly, the Schottky barriers of Au/GaN were found to be asymmetric in nature as the current on the positive side was different than negative for the same bias. Polarization on the boundaries of the basal plane defects has been already investigated for the possible reason of the Schottky barrier inhomogeneity. Thus, it can be expected that the overall effect of these polarization centers would change the Schottky barrier height of one of the electrodes. Electronic band-alignment based on asymmetrical contacts reveals that the difference in the Schottky barrier height would create a net electrical field toward the higher Schottky barrier, which is exploited here for self-powered photodetection and also enhanced photodetection at higher applied bias. The spectral response of all the devices was studied within 300–700 nm, and it was found that spectral response enhances with the applied voltage. The maximum responsivity and detectivity for a 364 nm light source observed at 5 V was around 400 A W–1 and 6.6 × 1012 jones, while at 0 V, it was 4.67 mA W–1 and 3.0 × 1013 jones, respectively, which is the highest known responsivity for a-plane GaN to the best of our knowledge. The spectral response shows that the devices work for a very narrow band of radiation and hence can be used for selective UV-A photodetection. Overall, these results demonstrate much-improved UV photodetection properties compared to existing GaN-based photodetectors.