Epitaxial Growth and In Situ Studies of Ultra-Thin Chalcogenides Coupled to Graphene

Two-dimensional semiconductors can drive advances in fundamental science and advanced technologies. However, they should be free of any contamination; also, the crystallographic ordering and coupling of adjacent layers, and their electronic properties should be well-controlled, tuneable and scalable. Here, these challenges are addressed by a new approach, which combines molecular beam epitaxy and in-situ band engineering in ultra-high vacuum of semiconducting gallium selenide (GaSe) on graphene to form a heterostructure referred to as <i>2semgraphene. In-situ</i> studies by electron diffraction, scanning probe microscopy and angle-resolved photoelectron spectroscopy reveal that atomically-thin layers of GaSe align in the layer plane with the underlying lattice of graphene. The GaSe/graphene heterostructure features a centrosymmetric polymorph of GaSe, a band structure tuneable by the layer thickness, and a charge dipole at the GaSe/graphene interface. Both as-grown and defective GaSe layers are remarkably resilient to oxidation in a pure O₂ environment at room temperature, and chemisorption of O₂ molecules on the surface can effectively electronically neutralise the doping in the layer. Also, a high-temperature annealing of the grown layers in an O₂-rich environment can promote the chemical transformation and full conversion of GaSe into the crystalline oxide Ga₂O₃. These features are scalable, as demonstrated experimentally and modelled by density functional theory. The newly-developed <i>2semgraphene </i>is used to demonstrate ultrathin optical sensors that exploit the photoactive GaSe and the sensitivity of its interface with the graphene channel to photogenerated carriers. Versatile functionalities are demonstrated in GaSe- and Ga₂O₃-based photon sensors, ranging from electrical insulation to unfiltered deep ultraviolet optoelectronics, unlocking the technological potential of GaSe and its crystalline oxide.