Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric & alpha; -In2Se3

Two-dimensional (2D) ferroelectric (FE) materials arepromisingcompounds for next-generation nonvolatile memories due to their lowenergy consumption and high endurance. Among them, & alpha;-In2Se3 has drawn particular attention due to its in-and out-of-plane ferroelectricity, whose robustness has been demonstrateddown to the monolayer limit. This is a relatively uncommon behaviorsince most bulk FE materials lose their ferroelectric character atthe 2D limit due to the depolarization field. Using angle resolvedphotoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenonappearing in 2H & alpha;-In2Se3 single crystals,the occurrence of a highly metallic two-dimensional electron gas (2DEG)at the surface of vacuum-cleaved crystals. This 2DEG exhibits twoconfined states, which correspond to an electron density of approximately10(13) electrons/cm(2), also confirmed by thermoelectricmeasurements. Combination of ARPES and density functional theory (DFT)calculations reveals a direct band gap of energy equal to 1.3 & PLUSMN;0.1 eV, with the bottom of the conduction band localized at the centerof the Brillouin zone, just below the Fermi level. Such strong n-typedoping further supports the quantum confinement of electrons and theformation of the 2DEG.