Electronic structure and interfaces of ultrathin [beta]-tellurium film and WSe₂-MoSe₂ heterojunction grown by molecular beam epitaxy

Abstract

Two-dimensional (2D) materials have been the subject of intensive research in recent years. New 2D materials are continuously put forward by theory and realized by experiments. In this family, elemental thin layer such as graphene, silicene, phosphorene, stanene, and borophene, and typical compound film exampled by transition-metal dichalcogenides (TMDs) all exhibit attractive physical characteristics covering from metals, semiconductors to insulators, which own huge potentials and invite further investigations. In particular, single-layer (SL) heterojunctions, termed as the interface between dissimilar materials, realize one-dimensional (1D) electronic systems at their hetero-interfaces, which expectedly give rise to new and interesting properties and promise new applications. Molecular-beam epitaxy (MBE), which is known for its precise control in deposition coverage (thickness), can be a great technique in pursuit of new material and heterostructures.

A theoretical study has predicted some layered structures of tellurium (Te), a group VI element that has not been studied before, to exist under certain conditions. In the first part of this thesis, we employ MBE to fabricate ultrathin Te films on highly oriented pyrolytic graphite (HOPG). By utilizing scanning tunneling microscopy (STM), the as-grown Te ultrathin layers reveal rectangular surface cells with the cell size being consistent with the predicted β-tellurene. While for thicker films, the cell size is found more consistent with that of the [1010] surface of bulk Te crystal. Scanning tunneling spectroscopy (STS) measurements suggest the β-tellurium films are semiconductors with energy bandgaps narrowing with increasing film thickness and predominantly occurring at the valence-band maximum (VBM). The latter can be explained by the strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory (DFT) calculations.

Another work of this thesis is to achieve a diverse interface structure by depositing WSe2 and MoSe2 sequentially by MBE. It also reports a strong anisotropic behavior of the hetero-interface formation process. Specifically, a sharp interface is obtained only when WSe2 deposition precedes MoSe2 (denoted as WSe2-MoSe2), whereas an alloy (Mo1-xWxSe2) without a noticeable boundary between the two materials is found when MoSe2 is grown first. The process is not very temperature sensitive and can be attributed to an 'edge segregation' effect and supported by the first-principles total energy calculations.

Besides, the electronic bands and their alignment at the hetero-interfaces are examined by STS, revealing the type-II alignment for both monolayer (ML−ML) and ML−bilayer (ML-BL) lateral junctions irrespective of the presence or not of step states.