Electronic Band Structure of In-Plane Ferroelectric van der Waals beta '-In2Se3

Layered indium selenides (In2Se3) have recently been discovered to host robust out-of-plane and in-plane ferroelectricity in the alpha- and beta'-phases, respectively. In this work, we utilize angle-resolved photoelectron spectroscopy to directly measure the electronic band structure of beta'-In2Se3 and compare to hybrid density functional theory (DFT) calculations. In agreement with DFT, we find the band structure is highly two-dimensional, with negligible dispersion along the c-axis. Because of n-type doping we can observe the conduction band minima and directly measure the minimum indirect (0.97 eV) and direct (1.46 eV) bandgaps. We find the Fermi surface in the conduction band is characterized by anisotropic electron pockets with sharp in-plane dispersion about the (M) over bar points, yielding effective masses of 0.21 m(0) along (KM) over bar and 0.33 m(0) along (Gamma M) over bar. The measured band structure is well supported by hybrid density functional theory calculations. The highly twodimensional (2D) band structure with moderate bandgap and small effective mass suggests that beta'-In2Se3 is a potentially useful van der Waals semiconductor. This, together with its ferroelectricity makes it a viable material for high-mobility ferroelectric-photovoltaic devices, with applications in nonvolatile memory switching and renewable energy technologies.