Alinhamento de bandas em heteroestruturas de TMDs / GaAs: um estudo ab initio

Abstract

In this work, we perform a theoretical study from first principles (ab initio) on the structural properties and electronic band alignments in structures formed by the stacking of transition metal dichalcogenides (TMDs) monolayers – MoS2, MoSe2, WS2, and WSe2 – on GaAs(110) surfaces. To do so, we initially present the methodology of density functional theory (DFT), from its mathematical foundations, initiated by Hohenberg and Kohn in the 1970s, to its implementation in the SIESTA computational code. We focus on the most relevant parameters for this implementation, as well as the approximations used: the linear combination of atomic orbitals (LCAO), pseudopotentials, and the generalized gradient approximation (GGA); and the plane-wave methodology – the linearized augmented plane wave (LAPW) method. Initially, we briefly discuss the scientific and industrial motivations for choosing these materials and how the employed models relate to possible and already completed works in van der Waals (vdW) heteroepitaxy. Subse- quently, we present relaxation calculations to determine the lattice parameters of MoS2, MoSe2, WS2, and WSe2, comparing them with experimental data. We also discuss the electronic structure of these materials, highlighting the transition phenomenon from an indirect gap (in bulk) to a direct gap (in monolayer), which makes them promising for optoelectronic applications. We also detail the geometry and electronic properties of the non-polar GaAs(110) surface via SIESTA, as well as the properties of the polar GaAs(001) and GaAs(111) surfaces using VASP. The text discusses the heteroepitaxy of thin films, explaining the concepts of lattice mismatch and strain, which are crucial for the formation of heterostructures. Finally, the vdW heterostructures (GaAs[110]/MoS2, GaAs[110]/MoSe2, GaAs[110]/WS2, and GaAs[110]/WSe2) are presented; we calculate the equilibrium distances between the layers, confirm the nature of the vdW interaction at the interface, and find the most stable structures. From these, we perform a band alignment analysis – based on the projected density of states onto each atomic orbital (fatbands) and the local density of states (LDOS) of the valence band maximum and conduction band minimum states to determine the type of alignment in each system. The results indicate a type II alignment (staggered gap) for the sulfur-containing heterostructures (MoS2 and WS2), with the valence band maximum (VBM) localized on the GaAs and the conduction band minimum (CBM) on the TMD, and a type I alignment (straddling gap) for those containing selenium (MoSe2 and WSe2), but with the formation of delocalized hybrid states.