Tailoring Graphene/GaN Photodetector Performance by GaN Crystal Orientation and h-BN Interfacial Engineering

Graphene/GaN heterojunctions on conventional Ga-plane GaN exhibit low responsivity and photoelectric conversion efficiency (PCE). While employing hexagonal boron nitride (h-BN) interlayers and alternative GaN orientations are promising, their regulation effects lack systematic comparison, hindering device optimization. Here, we investigated the effects of GaN crystallographic orientation (polar Ga- and N-planes, semipolar r-plane, nonpolar a-plane) and h-BN interlayer thickness on the photoelectric performance of graphene/GaN heterojunction devices across the visible-to-UV spectrum (325-525 nm). The heterojunction devices were fabricated via wet transfer, and their morphological and electrical properties were characterized using scanning electron microscopy and a probe station. Heterojunctions on N- and r-planes GaN, possessing higher neutral levels and lower interface trap densities, exhibited enhanced reverse current under shorter wavelengths and higher bias, along with superior responsivity (717.3 mA/W and 619.3 mA/W) and PCE (11.97% and 11.79%), especially in the UV regime. In contrast, devices on Ga- and a-planes GaN showed saturated reverse current and inferior performance. Insertion of monolayer h-BN reduced both operating and dark currents for graphene/GaN devices and induced a current convergence effect. However, only Ga-plane GaN devices benefited from a monolayer h-BN with improved output power and PCE; further increasing h-BN thickness degraded photoelectric performance. For N-, r-, and a-planes GaN devices, thin h-BN intercalation (1-5 layers) generally reduced optoelectronic metrics, e.g., causing a 5~7-fold decrease in responsivity and PCE for N-plane GaN devices with 5-layer h-BN. This study provides critical guidance into interface engineering for high-performance graphene/GaN optoelectronics.