Monolayers by Molecular-Beam Epitaxy – Phase Tuning by Changing the Growth Conditions*

Monolayer transition-metal dichalocogenides (TMDs) have been under extensive research attention in recent years. Many of the studied TMDs are semiconductors or semimetals for their attractive physical properties and application promises. MoTe₂ is an interesting material that can exist in both the hexagonal (2H) and monoclinic (1T’) phases at experimentally accessible conditions, which show respectively semiconducting and metallic properties. Controlled growth and tuning of its structural phases can thus lead to new applications such as phase-change electronics. In this work, we epitaxially grow monolayer MoTe₂ on highly oriented pyrolytic graphite by molecular-beam epitaxy and obtain both 2H and 1T’ phases. The ratio between the two domain areas is derived by reflection high-energy electron diffraction measurements and is found to vary with the MBE conditions. Low-temperature and high Te flux both lead to increasing 1T’ domain size, which is explained by an effect of Te adsorption on surface. The 1T’ and 2H domain interface forms a metal-semiconductor junction and the property is probed by scanning tunneling spectroscopy.