Insight into the electronic structure of semiconducting ε− GaSe and ε− InSe

Metal monochalcogenides (MX) have recently been rediscovered as two-dimensional materials with electronic properties highly dependent on the number of layers. Although some intriguing properties appear in the few-layer regime, the carrier mobility of MX compounds increases with the number of layers, motivating the interest in multilayered heterostructures or bulk materials. By means of angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory calculations, we compare the electronic band structure of bulk ε−GaSe and ε-InSe semiconductors. We focus our attention on the top valence band of the two compounds along main symmetry directions, discussing the effect of spin-orbit coupling and contributions from post-transition-metal (Ga or In) and Se atoms. Our results show that the top valence band at Γ point is dominated by Se pz states, while the main effect of Ga or In appears more deeply in binding energy, at the Brillouin zone corners, and in the conduction band. These findings explain also the experimental observation of a hole effective mass rather insensitive to the post-transition metal. Finally, by means of spin-resolved ARPES and surface band structure calculations we describe Rashba-Bychkov spin splitting of surface states in ε−InSe.
The support by the Fundamental Research Program of the State Academies of Sciences (line of research III.23.2.9), and the project EUROFEL-ROADMAP ESFRI, are gratefully acknowledged. This work has been partly performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA—MIUR Italy Progetti Internazionali) facility. M. Castriota and F. Ciuchi are acknowledged for having performed, respectively, Raman and XRD characterization of GaSe and InSe samples. This work was supported by Saint Petersburg State University (Grant No. ID 51126254).