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

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 lowerthan 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-clark current contrast can reach 6 × 10⁵, and the responsivity shows no discernible internal gain under a bias between 2.5 and 7.5V.