Selective Area Epitaxy Growth of 2D Materials by Molecular Beam Epitaxy

Two-dimensional (2D) materials have exploded in popularity due to their wide range of properties and the fact that they can be stacked into devices. The vast majority of devices made from 2D materials use exfoliated flakes, which works well for creating prototypes and to understand basic physics, but exfoliation is difficult to scale up. For traditional materials, devices are fabricated using wafer-scale films and subtractive manufacturing processes. Unfortunately, wafer-scale films of 2D materials grown by physical vapor deposition or chemical vapor deposition techniques often have a high density of grain boundaries and twin defects, which lead to unwanted conducting channels in electronic devices, non-radiative recombination pathways in optical devices, and decoherence in quantum devices. In addition, etching often damages the edges of the 2D material, further degrading device performance. One way to solve this problem is to take a new approach to the wafer-scale manufacturing of 2D material devices. Instead of growing a wafer-scale film and using subtractive techniques to fabricate devices, we use selective area epitaxy (SAE), an additive technique, to build devices from the bottom up.<br/> In SAE, a mask is patterned on the substrate before growth with precisely placed holes through to the substrate. The growth conditions are selected such that the film only nucleates and grows within the openings in the mask, thus defining the location, size, and shape of the film. By choosing the growth conditions correctly, we can ensure that only a single grain of 2D material grows in each opening, eliminating issues with grain boundaries and film coalescence. Multiple materials can be stacked together using SAE to create functional device stacks. If contacts to the stack are needed, they can be made as edge contacts before epitaxy and/or as top contacts post-growth. Using SAE, we can synthesize quantum-confined 2D materials without the need for etching, thus preserving the intrinsic properties of the material. Overall, SAE has the potential to transform growth of 2D materials for devices by enabling the synthesis of 2D materials free of grain boundaries with precise locations and sizes.<br/> In this talk, I will discuss our recent results on SAE of 2D materials using molecular beam epitaxy (MBE). MBE is an ultra-high vacuum physical vapor deposition technique, in which high-purity elemental source materials are thermally evaporated. The atoms impinge upon the substrate and react to form the film. In the past decade, MBE has been used to grow a wide range of 2D materials and heterostructures and due to its high purity, is a good choice for SAE. To begin, we explored SAE of the topological insulator Bi2Se3 on GaAs substrates using Al2O3 masks deposited by electron beam evaporation and atomic layer deposition. We find that we can obtain SAE in a narrow but reproducible growth window. We also find differences in SAE growth windows for electron beam deposited Al2O3 compared to atomic layer deposited Al2O3, likely due to the difference in smoothness and the number and density of dangling bonds. I will further show results for SAE of Ga2Se2 on GaAs and for Bi2Se3 on Si. Overall, SAE is a promising technique for the growth of high-quality 2D material devices.