Thin Ga₂O₃ Layers by Thermal Oxidation of van der Waals GaSe Nanostructures for Ultraviolet Photon Sensing

Two-dimensional semiconductors (2DSEM) based on van der Waals crystals offer important avenues for nanotechnologies beyond the constraints of Moore's law and traditional semiconductors, such as silicon (Si). However, their application necessitates precise engineering of material properties and scalable manufacturing processes. The ability to oxidize Si to form silicon dioxide (SiO₂) was crucial for the adoption of Si in modern technologies. Here, we report on the thermal oxidation of the 2DSEM gallium selenide (GaSe). The nanometer-thick layers are grown by molecular beam epitaxy on transparent sapphire (Al₂O₃) and feature a centro-symmetric polymorph of GaSe. Thermal annealing of the layers in an oxygen-rich environment promotes the chemical transformation and full conversion of GaSe into a thin layer of crystalline Ga₂O₃, paralleled by the formation of coherent Ga₂O₃/Al₂O₃ interfaces. Versatile functionalities are demonstrated in photon sensors based on GaSe and Ga₂O₃, ranging from electrical insulation to unfiltered deep ultraviolet optoelectronics, unlocking the technological potential of GaSe nanostructures and their amorphous and crystalline oxides.