Mexican hat-like valence band dispersion and quantum confinement in rhombohedral ferroelectric alpha-In2Se3

Abstract: Two-dimensional (2D) ferroelectric (FE) 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 photo-response. 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 (ARPES) and density functional theory (DFT) calculations, we demonstrate that 3R alpha-In2Se3 phase exhibits a robust inversion of the valence band parabolicity at the Gamma point forming a bow-shaped dispersion with a depth of 140 +- 10 meV between the valence band maximum (VBM) along the GammaK direction of the Brillouin zone (BZ). Moreover, we unveil an indirect band gap of about 1.25 eV, as well as a highly electron doping of approximatively 5.1012 electrons per cmsquare 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.