Lateral heterostructures of two-dimensional semiconductors

Project description

Semiconductor heterostructures form the backbone of modern electronics: the combination of materials with different band structures enables transistors, solar cells or laser diodes. Two-dimensional materials can now be used to create ultra-thin lateral heterostructures in which monolayers are linked together in one plane - a step towards the ultimate miniaturization of components. In addition, effects occur in these structures that are not observed in the 3D world. The ideal interface in a heterostructure is atomically sharp, clean and coherent - meaning no broken bonds, no dislocations. While cleanliness and sharpness can be achieved through controlled preparation, coherence can in principle be limited by lattice mismatch of the materials involved. The electronic band structure of a 2D heterojunction is determined by the relative position of the Fermi level and band edges and is additionally influenced by charge traps at the interface. This band alignment controls the electronic properties and determines possible applications. The geometrical structure results from the lattice mismatch, which can cause epitaxial strain or dislocations.

The aim of this project is to experimentally determine the electronic and geometric structure of such heterojunctions on an atomic scale by applying surface research methods in order to understand their mutual interaction. Transition metal dichalcogenides (TMDCs) of the form MX2 (M: transition metal, here Mo, W; X: S, Se, Te) are a promising class of materials. The semiconducting representatives have a common crystal structure, but in some cases differ significantly in their lattice constants. The resulting mismatch is compensated for in narrow heterostructures by epitaxial strain; defects occur above a critical width: The elastic energy increases with width, but the energetic cost of dislocations is incurred only once at the interface. Dislocations can create charge traps, while strain changes the band structure both in terms of its size (e.g. band gap width) and its nature (e.g. direct vs. indirect transition). A description of the ligaments in transition must therefore go beyond the simple, tension-free and defect-free picture (e.g. Anderson's rule).

The project aims to develop such a description experimentally. To this end, molecular beam epitaxy (MBE) is used to produce heterostructures with varying composition and morphology. Atomic and electronic structures are resolved directly by varying the lattice mismatch and band alignment and analyzing them with scanning tunneling microscopy and spectroscopy (STM/STS). This provides a fundamental understanding of how atomic-scale geometry determines band structure in 2D heterostructures.

The project is funded by the German Research Foundation (DFG). The aim of the funding is to experimentally record the electronic and geometric structure of such heterojunctions on an atomic scale by applying surface research methods in order to understand their mutual interaction.