Diameter-Dependent Photoresponse and Surface Effects in Top-Down Fabricated GaN p−n Nanowire Photodetectors

Abstract Top-down fabrication of GaN-based nanowires offers a promising alternative to bottom-up approaches, providing enhanced control over geometry and doping profiles. In this work, single nanowire photodetectors were realized from planar GaN p−n junction structures, including a single p−n junction (SPN) and a structure incorporating additionally a tunnel junction (SPNTJ). Nanowires with diameters in the 150−300 nm range were fabricated using nanosphere lithography followed by dry and crystallographic wet etching. Electrical and optoelectronic characterization of individual nanowires contacted on Si3N4 membranes reveal rectifying behavior and ultraviolet photoresponse with a sharp cut-off at ∼364 nm. The devices exhibit responsivities in the range of 8−20 mA/W under reverse bias, increasing by 1−3 orders of magnitude under forward bias. The photocurrent follows a power-law dependence on optical irradiance, Iph∝Poptβ, with the exponent β strongly influenced by nanowire diameter and bias conditions. Linear response is achieved under reverse bias for thinner nanowires (150 nm), while larger diameter nanowires (300 nm) display pronounced sublinear behavior. The device operation is interpreted using a model combining junction-driven photocurrent generation and surface-state-controlled series resistance, accounting for the role of nanowire depletion and surface effects in determining linearity and dynamics. Results demonstrate that the performance of top-down fabricated nanowire photodetectors is comparable to that of their bottom-up counterparts in terms of responsivity, linearity, and temporal response. These results demonstrate the viability of top-down approaches for the fabrication of high-performance nanowire-based optoelectronic devices with improved design flexibility.