Evolution of Direct and Indirect Band Gap by Inhomogeneous Strain in Monolayer MoS2-WSe2 and WSe2-MoSe2 Lateral Heterostructures
Wei-Ting Hsu1*, Li-Syuan Lu1, Dean Wang1, Ming-Yang Li2, Jing-Kai Huang3, Yi-Chia Chou1, Lain-Jong Li3, Wen-Hao Chang1
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
3Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
* presenting author:Wei-Ting Hsu, email:tc60169@yahoo.com.tw
Recently, semiconductor heterostructures formed by two-dimensional transition metal dichalcogenides (TMD) have attracted considerable interests. In particular, lateral heterostructures consisting of two laterally connected TMD monolayers have been demonstrated by chemical vapor deposition through edge epitaxial growth, paving the way for future planar device applications. In lateral heterostructrues, the outer material usually exhibits pronounced strain inhomogeneity caused by the large lattice mismatch, as has been observed in MoS2-WSe2 lateral heterostructures with strain variation up to ~1.59% in the outer MoS2. The strained lateral heterostructures thus provide a unique platform to study the impacts of strain on the electronic and optical properties of TMDs. In this work, the strain inhomogeneities in WSe2-MoS2 and MoSe2-WSe2 lateral heterostructures are investigated. Spatial-resolved photoluminescence (PL) shows a strong correlation between the PL intensity and energy caused by carrier occupation in valleys near the K valleys. This correlation can be modeled by considering the valley occupations according to the Boltzmann distribution, by which the energy difference and the ratio of deformation potential between K and vicinal valleys can be determined. Specifically, we found that the unstrained monolayer WSe2 is an indirect bandgap semiconductor, which can be tuned to direct bandgap under tensile strain. The local strain variation results in a spatial modulation of direct and indirect bandgap, with important implications for practical device application based on lateral TMD heterostructures.

Keywords: Transition metal dichalcogenide, Lateral heterostructure, MoS2, WSe2