MBE-Grown Two-Dimensional Transition Metal Dichalcogenides for Magnetic and Optical Applications

Two-dimensional transition metal dichalcogenides (2D TMDs) are a class of van der Waals (vdW) materials that have gained significant popularity in recent years due to their unique structural, optical, and electronic properties that are very relevant for applications like ultra-scaled transistors, magnetic memories, light absorbers, and monolithic 3D integrated devices. This research focuses on exploring the potential of 2D TMDs through molecular beam epitaxy (MBE) synthesis, targeting two specific applications: (1) magnetic impurity doped monolayers to create above-room-temperature ferromagnets, and (2) TMD based heterostructures for near-unity light absorbers. Guided by theoretical support, the first part of this research investigates substitutional doping of magnetic impurities in monolayer TMDs to create above room temperature 2D ferromagnets. Through careful selection of the substrates, optimization of the flux ratios of the source materials and using smart growth strategies, up to 35% doping of iron and vanadium in a monolayer of tungsten diselenide (WSe2) has been demonstrated and the resulting physical, chemical, and magnetic properties of the films have been studied. It has been found that these single-phase monolayers with minimal point defects show no ferromagnetism. The suppression of the theoretically predicted ferromagnetic order is due to clustering of the dopants which has been observed and quantified from transmission electron microscopy (TEM). Room-temperature ferromagnetism is observed when these doped monolayers contain a significant concentration of selenium vacancies (Sevac). These vacancies were intentionally created via a post-growth annealing process, and magnetism was seen to scale with heating time/vacancy concentration. Interestingly, even undoped WSe2 showed similar ferromagnetism for Sevac > 1E14 cm-2. These findings address the inconsistencies in the existing research on 2D ferromagnets and explain how ferromagnetism arises in TMD monolayers. The second part of this research is focused on MBE-grown TMD heterostructures to achieve near-perfect light absorption (NPLA) for optoelectronics and special-purpose defense applications. Different from previous approaches that rely on patterned meta-surfaces or plasmonic structures, our much simpler approach takes advantage of the strong band nesting in TMDs to harness NPLA without requiring any complex lithography or surface engineering. It has been shown that inserting a buffer layer in-between a bilayer TMD can be a feasible route to reduce the strong interlayer coupling that exists between the 2D layers and deteriorates its absorption response. By utilizing the digital thickness control capability of MBE, a variety of TMD/buffer layer/TMD tri-layer heterostructures were grown and later integrated with an optical cavity to demonstrate near the theoretically maximum light absorption in an active material just three atomic layers thin.