New approach for the molecular beam epitaxy growth of scalable single-crystalline WSe$_2$ monolayers

TL;DR

This paper presents a novel molecular beam epitaxy method to grow large-area, high-quality monolayer WSe$_2$ on mica, achieving controlled domain size, orientation, and transferability, with insights into grain formation.

Contribution

It introduces a new MBE growth process for scalable WSe$_2$ monolayers with detailed analysis of grain orientation and methods to improve crystalline quality.

Findings

High-temperature, low-deposition rate growth yields large, high-quality WSe$_2$ domains.

WSe$_2$ monolayers are air-stable, transferable, and exhibit strong photoluminescence.

Presence of misoriented grains explained by atomic coincidence model.

Abstract

The search for high-quality transition metal dichalcogenides mono- and multi-layers grown on large areas is still a very active field of investigation nowadays. Here, we use molecular beam epitaxy to grow 15$\times$15 mm large WSe$_2$ on mica in the van der Waals regime. By screening one-step growth conditions, we find that very high temperature ($>$900$^{\circ}$C) and very low deposition rate ($<$0.015 nm/min) are necessary to obtain high quality WSe$_2$ films. The domain size can be larger than 1 $\mu$m and the in-plane rotational misorientation less than $\pm$0.5$^{\circ}$. The WSe$_2$ monolayer is also robust against air exposure, can be easily transferred over 1 cm$^2$ on SiN/SiO$_2$ and exhibits strong photoluminescence signal. Moreover, by combining grazing incidence x-ray diffraction and transmission electron microscopy, we could detect the presence of few misoriented grains. A two-dimensional model based on atomic coincidences between the WSe$_2$ and mica crystals allows us to explain the formation of these misoriented grains and gives suggestion to remove them and further improve the crystalline quality of WSe$_2$.