Electronically Decoupled Films of InSe Prepared by van der Waals Epitaxy: Localized and Delocalized Valence States

Abstract

Submonolayer to several monolayer thick films of the layered semiconductor InSe were deposited on highly oriented pyrolytic graphite by van der Waals epitaxy and probed by energy dependent angle resolved photoelectron spectroscopy. The layers show a transition from two-dimensional bands with atomiclike states to molecularlike states localized along the c direction normal to the surface. The extended band structure showing band dispersion was observed for thicker films.[ S0031-9007(97) In contrast to the common elemental (Si, Ge) or compound semiconductors (e.g., GaAs) with diamond, zinc blende, or related structures the layered metal chalcogenides like MoS 2 or InSe exhibit strong anisotropic chemical bonding and can be considered as quasi-twodimensional materials. They are characterized by covalently bound X-M-X or X-M-M-X (M metal; X chalcogen) sandwich layers with only weak van der Waals interactions along the crystal c axis, perpendicular to the layers [1]. The absence of directional chemical bonds at interfaces formed with the van der Waals planes allows heteroepitaxial growth despite lattice mismatches as high as 20% [2]. In addition, the absence of dangling bonds at the van der Waals planes results in a rather strong anisotropy of the surface tension of these materials which favors growth of two-dimensional layers rather than of three-dimensional islands.We have investigated the thickness dependence of the electronic structure of thin films of the semiconductor InSe grown on semimetallic highly oriented pyrolytic graphite (HOPG). Both materials belong to the nonsymmorphic space group D 4 6h [3,4]. The crystallographic structure of InSe is shown in Fig. 1(a). Film growth proceeds along the c direction which corresponds to the D symmetry line of the hexagonal Brillouin zone [ Fig. 1(b)]. The electronic band structure along this direction as adopted from band structure calculations of Doni et al. [5] for InSe and of Tatar and Rabii [3] for graphite are shown in Fig. 1(d) together with their symmetry notations. Four well separated groups of valence bands [labeled A D in Fig. 1(d)] are present for InSe. They are derived from Se 4p z ͑A͒, Se 4p xy ͑B͒, antibonding ͑C͒, and bonding ͑D͒ In 5s orbitals, respectively [4,5]. For graphite the bands A and B belong to s 1,2 and to p states, respectively [3,6]. The uppermost valence bands of InSe (group A) are formed by D 1 and D 2 bands which have no correspondence in the graphite band structure at comparable binding energies. This implies that these states can form no strong chemical bonds with the substrates or, in other words, are electronically decoupled. Considering the films as a quantum well structure embedded between graphite and vacuum we are dealing with large confinement potentials on either side in contrast to common semiconductor quantum well structures with confinement potentials of about 1 eV or less [7]. It should therefore, in principle, be possible to study the electronic properties of semiconductor quantum films and to follow the evolution of band st...

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