Lateral heterostructures of semiconducting single-layer transition metal dichalcogenides

• Applicant: Professor Dr. Carsten Busse
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 569845622

Modern electronics makes extensive use of semiconductor heterostructures, where the functionality of devices (transistors, solar cells, laser diodes) arises from connecting different materials. With 2D materials it is now possible to create ultrathin lateral heterostructures from monolayers stitched together in-plane, forming the building blocks for monolayer electronics. This enables us to reach the ultimate limit of device miniaturization. More fundamentally, the structures show effects not present in their 3D counterparts. The ideal interface is atomically sharp, clean, and coherent (no dangling bonds, no dislocations). While the first two can be controlled through preparation, coherency may be limited by the inherent lattice mismatch of the constituents. The band structure at a 2D semiconductor heterojunction is determined by Fermi level and band edge alignment and can be further influenced by interface traps. This alignment governs the electronic properties and thus potential applications. The geometry is set by lattice mismatch, leading to strain and/or dislocations. The main objective of this proposal is to use surface science techniques to experimentally determine the electronic and geometric structure of 2D heterojunctions at the atomic scale, as a prerequisite for revealing their interdependence. A promising class of 2D materials are the transition metal dichalcogenides (TMDCs) of the form MX2 (M: transition metal, here Mo or W; X: S, Se, Te). The semiconducting members of this family share a crystal structure but generally differ in lattice constant. The resulting, often significantly large misfit in heterostructures must be compensated, either by epitaxial strain, ripple formation, or the formation of dislocations. In narrow heterostructures, epitaxial strain minimizes energy, but beyond a critical width, defects emerge as the strain-related elastic energy increases with width, while the energetic cost of forming defects is incurred only once at the interface. The geometric structure shapes the electronic properties of the material: dislocations can introduce interface charge traps, and strain alters the band structure both quantitatively (e.g., bandgap size) and qualitatively (e.g., direct vs. indirect gap). This calls for a description of band structure transformations across the interface that goes well beyond the idealized, strain-free, and defect-free case typically treated by Anderson’s Rule. The central objective of this proposal is to experimentally establish this description by preparing heterostructures via molecular beam epitaxy (MBE) with varying compositions and morphologies, systematically tuning lattice mismatch and band alignment, and using scanning tunneling microscopy and spectroscopy (STM/STS) to map both geometric and electronic structure at the atomic scale. This will provide fundamental understanding of how atomic-scale geometry dictates band formation in 2D systems.

• DFG Programme: Research Grants