High-speed visible-blind GaN-based ITO-Schottky photodiodes

In this paper we present our efforts on the design, fabrication and characterization of high-speed, visible-blind, GaN-based ultra-violet (UV) photodiodes using indium-tin-oxide (ITO) Schottky contacts. ITO is known as a transparent conducting material for the visible and near infrared part of the electromagnetic spectrum. We have investigated the optical properties of thin ITO films in the ultraviolet spectrum The transmission and reflection measurements showed that thin ITO films had better transparencies than thin Au films for wavelengths greater than 280 mn. Using a microwave compatible fabrication process, we have fabricated Au and ITO based Schottky photediodes on n-/n+ GaN epitaxial layers. We have made current-voltage (I-V), spectral quantum efficiency, and high-speed characterization of the fabricated devices. I-V characterization showed us that the Au-Schottky samples had better electrical characteristics mainly due to the larger Schottky barrier. However, due to the better optical transparency, ITO-Schottky devices exhibited higher quantum efficiencies compared to Au-Schottky devices. ITO-Schottky photodiodes with ∼80 nm thick ITO films resulted in a maximum quantum efficiency of 47%, whereas Au-Schottky photodiode samples with ∼10 nm thick Au films displayed a maximum efficiency of 27% in the visible-blind spectrum. UV/visible rejection ratios over three orders of magnitude were obtained for both samples. High-frequency characterization of the devices was performed via pulse-response measurements at 360 nm. ITO-Schottky photodiodes showed excellent high-speed characteristics with rise times as small as 12 psec and RC-time constant limited pulse-widths of 60 psec.