Advanced Semiconductor Materials and Devices Group

2D Quantum materials form a category of ultrathin (atomic scale) materials that exhibit many exotic fundamental properties due to the strict confinement of carriers in one dimension, such as high carrier mobility, tunable bandgap, strong spin-orbit coupling, large magnetoresistance and high excitonic binding energy. Graphene, being the first and most intensively researched material of this category, has paved the way for various other quantum materials such as hBN, black phosphorus, and different types of metal chalcogenides. Transition metal dichalcogenides (TMDCs) are this category's second most probed materials after graphene owing to their diverse optical, electronic, superconducting, and topological properties. In addition to these fundamental properties, stacking the same or different 2D materials provides another degree of freedom for tuning their properties, thus making it one of the potential categories of materials for future digital and quantum device applications.

Molecular Beam Epitaxy (MBE) is one of the sophisticated techniques for the growth of high-quality 2D materials. It employs molecular beams of source material to deposit on the heated substrate under an ultra-high vacuum (UHV) environment with base pressure ~10^-10–10^-11 mbar to produce high-quality epitaxial 2D films. Unlike some other techniques, the use of UHV and high-purity elemental material sources leads to extreme cleanliness of the grown 2D material. The use of individual shutters for each material cell and small precursor fluxes lead to precise control over the amount of depositing material and, thus, the film thickness (down to a single monolayer), making MBE one of the important techniques for realizing vertical heterostructures.

One of the most potent aspects of MBE is its ability to monitor the details of the growth precisely and simultaneously, with the help of different in-situ facilities and characterization techniques such as quartz crystal microbalance (QCM) for measuring real-time thickness and growth rate, reflection high-energy electron diffraction (RHEED) for observing the film quality, growth orientation as well as the growth rate, spectroscopic ellipsometer for the optical properties and layer thickness; most of which necessarily require a high level of vacuum for their smooth operation, hence benefit from the UHV environment inside an MBE system.

Our group focuses on the growth of the following large area and high-quality, few-layer thick 2D TMDC materials using molecular beam epitaxy. The growth is optimized for different materials to achieve a pure phase of a particular material. Various characterization techniques, as shown in the figure below, have been utilized for investigating different properties of the grown ultrathin films.

Materials grown using the 2D MBE:

·       MoTe₂ on silicon and sapphire

·       MoSe₂ on silicon and sapphire

·       WSe₂ on silicon and sapphire

·       GaSe on silicon and sapphire

·       WTe₂ on Sapphire and MoTe₂/sapphire

·       GaTe on sapphire