Anisotropic Responsivity of AlGaN Metal–Semiconductor–Metal Photodetectors on Epitaxial Laterally Overgrown AlN/Sapphire Templates

Abstract

Al_0.4Ga_0.6N metal–semiconductor–metal photodetectors on epitaxial laterally overgrown (ELO) AlN/sapphire templates show anisotropic device characteristics depending on the orientation of the electrode stripes with respect to the stripe pattern onto which the underlying ELO AlN buffer layers have been grown. With electrodes perpendicular to the stripes, a quantum efficiency (QE) of ∼140 was found for 20-V bias at room-temperature. This gain is explained by carrier transport along channels with increased Ga content resulting from faceted growth at the steps of the ELO template. The resulting potential barrier is confirmed by the activation energy found for the temperature dependence of the QE. In contrast, photodetectors with electrodes running parallel to these channels do not show gain but have an enhanced QE at elevated bias voltage compared to devices on planar AlN buffer layers. This effect is attributed to different densities of threading dislocations in the absorber layer.