Evidence of direct electronic band gap in two-dimensional van der Waals indium selenide crystals

Metal monochalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photoresponse. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy, we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Gamma point of the Brillouin zone. STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction-band minimum. This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle-resolved photoemission spectroscopy (ARPES) investigation. In fact, a hole effective mass of about m* / m(0) = -0.95 ((Gamma K) over bar direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic technologies.