Improvements of transparent electrode materials for GaN metal–semiconductor–metal photodetectors

Abstract

Metal–semiconductor–metal (MSM) photodetectors based on GaN grown on (0 0 0 1) sapphire were fabricated and characterized. The responsivity of the Pt/GaN MSM device is low due to the blocking of incoming light by Pt electrodes. Although this problem can be partly solved by the transparent indium–tin oxide (ITO) contact, the range of operation voltage for ITO/GaN MSM devices is limited by the internal gain. Transparent multilayered electrode is proposed in this work by incorporating various intermediate layers (Ti, TiO₂, and Ti/TiO₂). The dark current of the ITO/TiO₂/GaN contact is two orders of magnitude lower than that of the ITO/Ti/GaN contact. The thin TiO₂ barrier also contributes the lower responsivity of the ITO/TiO₂/GaN structure. By introducing a thin Ti/TiO₂ interlayer at the ITO–GaN interface, a significant decrease in the dark current and an increase in responsivity can be achieved simultaneously. The photo-to-dark current contrast can reach 6×l0⁵, and the responsivity shows no discernible internal gain under a bias between 2.5 and 7.5 V.