Defect-induced doping and chemisorption of O₂ in Se deficient GaSe monolayers

Owing to their atomically thin nature, structural defects in two dimensional materials often play a dominating role in their electronic and optical properties. Here, we grow epitaxial GaSe monolayers on graphene/SiC by molecular beam epitaxy and characterise the layers by in situ scanning tunnelling microscopy and angle-resolved photoemission spectroscopy extracted from k-resolved photoemission electron microscopy mapping. We identify an electric dipole at the GaSe/graphene interface, with electrons accumulating on the GaSe, that cannot be compensated by p-type doping through the creation of defects formed by annealing in ultrahigh vacuum. Additionally, we demonstrate that both as-grown and defective GaSe layers are remarkably resilient to oxidation in a pure O₂ environment, and chemisorption of O₂ molecules on the surface can effectively electronically neutralise the doping in the layer. This work demonstrates the robust interlayer interaction in the GaSe/graphene van der Waals heterostructure and the role of defects on the doping for nanoelectronics.