Coherently strained in-plane atomic layer heterojunctions

Semiconductor heterojunctions (HJs) have played a critical role in many modern electronic and photonic devices. The emergence of transition metal dichalcogenides (TMDs) as a new class of two-dimensional semiconducting materials creates exciting new opportunities to push semiconductor heterostructures toward a new frontier. Vertically stacked van der Waals heterostructures have been quickly recognized as a powerful platform to create atomically thin heterostructures.1 Recently, attention has been directed to the creation of monolayer ‘lateral heterojunctions’ with a line interface between two different TMDs, pushing semiconductor HJs to a new dimension. In late 2014, three papers published simultaneously, demonstrating direct growth of lateral heterojunctions using chemical vapor deposition.2, 3, 4 The papers demonstrated growth including both lattice matched and mismatched HJs; however, the interfaces were not chemically abrupt (that is, diffuse interface). In particular, the lattice mismatched system (for example, WS2-WSe2) displayed a compositional transition width of 40 nm. Presumably, the gradual composition alleviates the abrupt change of the lattice constant, similar to the epitaxial growth of conventional heterostructures with a large lattice mismatch. Conventional wisdom suggests a chemically abrupt lateral interface in a lattice mismatched system would be difficult to achieve. In contrast, in 2015 by using a two-step process, Li et al.5 reported the successful growth of lattice-mismatched lateral HJs of WSe2–MoS2 with an atomically abrupt compositional interface as shown schematically in Figure 1a. This system is coherently strained judging by the high Photoluminescence efficiency of individual TMDs. This scenario is similar to the growth of coherently strained InAs quantum dots in GaAs, although in this case, compositional mixing at the interface has been difficult to eliminate. Thus, an atomically abrupt compositional interface in a system with such a large lattice mismatch comes as a pleasant surprise.