Fabrication of WS₂/GaN p-n Junction by Wafer-Scale WS₂ Thin Film Transfer

High quality wafer-scale free-standing WS₂ grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS₂ was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS₂ onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS₂ and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS₂, which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS₂ onto an n-type GaN was demonstrated with a small leakage current density of 29.6 μA/cm² at -1 V which shows superior performances compared to the directly grown WS₂/GaN heterojunctions.