Simulation Study of Front-illuminated GaN Avalanche Photodiodes with Hole-initiated Multiplication

Abstract

III-N based Avalanche Photodiodes (APD) are of great interest for UV detection due to their low dark current density, high sensitivity, and high optical gain. In this paper, we propose and study via simulation a novel flip-chip (Al)GaN p-i-n-i-n APD which combines the features of the prevailing APDs-hole-initiated multiplication process and front-illumination. The proposed APDs could be obtained through a thin film transfer and substrate removal process, ending up with a cathode facing-up structure. Additionally, in order to reduce undesirable UV absorption ( $\lambda=340$ nm or shorter) in the optical path, AIGaN-APD with a UV-transparent n-Al 0.4 GaN layer was also designed for comparisons. With a 200-nm-thick multiplication layer, the calculated breakdown voltage was around 95 V for both GaN-APD and AlGaN APD, whose optical gain could reach 105 and 106, respectively, mainly contributed by hole-initiated multiplication process. Due to the absence of UV light absorption in the top layer, the AlGaN APD shows responsivity $(\mathrm{R}=25\mathrm{mA}/\mathrm{W}, \mathrm{EQE}=9.1 \%)$ twice as much as that of GaN APD with a reverse bias of 60 V. At the reverse bias of 94 V, the responsivity increases to 27.3 mA/W, corresponding to an EQE of 100% because of internal gain.