Mexican hat–like valence band dispersion and quantum confinement in rhombohedral ferroelectric α − In 2 Se 3

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Two-dimensional ferroelectric materials offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ 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) $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ 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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ phase exhibits a robust inversion of the valence band parabolicity at the $\mathrm{\ensuremath{\Gamma}}$ point forming a bow-shaped dispersion with a depth of $140\ifmmode\pm\else\textpm\fi{}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ between the valence band maximum along the $\mathrm{\ensuremath{\Gamma}}K$ 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 $5\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}\mathrm{electrons}\phantom{\rule{0.16em}{0ex}}\mathrm{per}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}$ 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 $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ electronic properties underlying the potential of III--VI semiconductors for electronic and photonic technologies.

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