Mexican hat–like valence band dispersion and quantum confinement in rhombohedral ferroelectric α−In2Se3

Two-dimensional ferroelectric materials offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, alpha-In2Se3 has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, in-and out-of-plane ferroelectricity, and high photoresponse. Precise experimental determination of the electronic structure of rhombohedral (3R) alpha-In2Se3 is needed for a better understanding of potential properties and device applications. Here, combining angle-resolved photoemission spectroscopy and density-functional theory calculations, we demonstrate that 3R alpha-In2Se3 phase exhibits a robust inversion of the valence band parabolicity at the P point forming a bow-shaped dispersion with a depth of 140 +/- 10 meV between the valence band maximum along the PK direction of the Brillouin zone. Moreover, we find an indirect band gap of about 1.25 eV, as well as a highly electron doping of approximately 5x1012 electrons per cm2 at the surface. This leads to surface band bending and the formation of a prominent electron accumulation layer. These findings allow a deeper understanding of the rhombohedral alpha-In2Se3 electronic properties underlying the potential of III-VI semiconductors for electronic and photonic technologies.