Growth of chalcogenides on non van der Waals substrates by molecular beam epitaxy

Abstract

Since the success of graphene isolation, two-dimensional (2D) semiconductor materials have attracted keen research interest in materials science and physics because of the superior electronic and optoelectronic properties. For example, graphene shows high thermal conductivity and carrier mobility. Molybdenum disulfide (MoS2) monolayer material is predicted as a low-consumption transistor. Phosphorene is a semiconductor material which is suitable for electronic and optical devices. Though huge progress in 2D materials has been reported, achieving high-quality epitaxial transition-metal dichalcogenides (TMDCs) remains an important but challenging problem. While progress has been made in growth by molecular-beam epitaxy (MBE), films grown by MBE often contain rotation and/or mirror twin domains, point, and line defects, etc., which need to be addressed before they can meet the application demands.

Till now, the majority of the MBE growth experiments are done on substrates of van der Waals (vdW) interaction, e.g., HOPG and graphene. These vdW substrates have the advantage of being versatile and strain-accommodating, but they suffer from the problem of high propensity of rotation and/or twin domain formation and thus the presence of domain boundaries. In this work, we experiment TMDCs growth on GaAs and InP, non-vdW substrates and achieve crystallographically aligned MoSe2 and MoTe2 monolayers with suppressed rotation domains in the epifilms. Evidence provided by low-energy electron diffraction (LEED) will be given. Unlike that on the vdW substrates, the interface interactions between substrates and epitaxial TMDCs are mainly covalent interactions and partly ionic interactions, which is proved by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) measurements show that the binding energy shift comparing with MoSe2 grown on graphene, indicating a suppressed electron transfer from epitaxial TMDCs layer to the substrates.

Except for some traditional TMDCs, other layered materials also arise interest in the physical area. Among them, gallium selenide (GaSe) is a promising semiconductor material with a band gap of 2.11eV. It can be applied in nonlinear optics and optoelectronics area. In this thesis, two-dimensional GaSe has been grown successfully by MBE. Reflection High Energy Electron Diffraction (RHEED) and LEED pattern provide symmetry and structure information for the evidence of successful growth. XPS tests prove that the Ga 3d core level is corresponding to GaSe rather than Ga2Se3. Based on the UPS spectra, the work function of GaSe is calculated for further evidence.