p-i-p-i-n Separate Absorption and Multiplication Ultraviolet Avalanche Photodiodes

Front-illuminated GaN-based separate absorption and multiplication (SAM) ultraviolet (UV) avalanche photodiodes (APDs) with various photon detection areas are demonstrated grown by metalorganic chemical vapor deposition on bulk GaN native substrates with low dislocation density. By adopting a front-illuminated UV-APD structure with a thin AlGaN window layer, no additional etching of the substrate for the reduction of strong UV absorption is required. The epitaxial layer structure of the p-i-p-i-n SAM UV-APDs consists of a Mg-doped ${p}$ -Al 0.05 Ga 0.95 N window layer to minimize UV absorption at the top surface region and a Mg-graded ${p}$ -GaN charge layer to serve as a field-termination layer. The onset point of the breakdown voltage ( $V_{\text {BR}}$ ) is around 73 V for all SAM-APDs with different mesa areas ranging from 1963 to 10 $000~\mu \text{m}^{2}$ , which is a lower $V_{\text {BR}}$ than the typical p-i-n UV-APDs with the similar thickness of undoped layer, where the photon absorption and multiplication processes take place simultaneously. Under UV-light illumination at $\lambda =340$ nm, the SAM-APDs exhibit high avalanche gains greater than $8.0\times 10^{5}$ at a reverse bias of $V_{R}> 72.5$ V