Abstract
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 $\ensuremath{\epsilon}\text{\ensuremath{-}}\mathrm{GaSe}$ and $\ensuremath{\epsilon}$-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 $\mathrm{\ensuremath{\Gamma}}$ point is dominated by Se ${p}_{z}$ 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 $\ensuremath{\epsilon}\text{\ensuremath{-}}\mathrm{InSe}$.
Highlights
The electronic properties of layered monochalcogenides (MX, where M stands for the post-transition metal, and X represents the chalcogen atom), among them GaSe and InSe, have recently been the focus of extensive research, mainly devoted to the discovery of unique properties in the few-layers regime [1,2,3,4,5,6]
Through angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations, the present study aims at clarifying the role of the metal and Se chalcogenide atoms in the bulk electronic structure of two related MX compounds, -GaSe and -InSe
We show that the post-transition metal has a key role in tuning the energy position of the conduction band minimum (CBM) and the energy width of the band gap, leaving unaffected the shape of the top valence band
Summary
The electronic properties of layered monochalcogenides (MX , where M stands for the post-transition metal, and X represents the chalcogen atom), among them GaSe and InSe, have recently been the focus of extensive research, mainly devoted to the discovery of unique properties in the few-layers regime [1,2,3,4,5,6]. Through angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations, the present study aims at clarifying the role of the metal and Se chalcogenide atoms in the bulk electronic structure of two related MX compounds, -GaSe and -InSe. The single tetralayer of each compound consists of four covalently bonded Se-M-M-Se atoms (M stands for Ga or In), while different tetralayers are held together by vdW forces, making the compounds exfoliable. The bulk polytype [Fig. 1(a)] is made up by superposition of a pair of tetralayers with AB stacking. It belongs to the P6 ̄/m2 noncentrosymmetric space group (D13h), regardless of the number of layers
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