(Invited) In-Vacuo Studies of Transition Metal Dichalcogenide Synthesis and Layered Material Integration
Authors: Freedy Keren M , Litwin Peter M , McDonnell Stephen John 
Published: February 2020

The last decade has seen fevered interest in 2D materials for nanoelectronics. Previous studies focused primarily on tribological and energy storage applications due to the weak van der Waals bonding that exists between the atomic layers. These materials, which include graphene, hexagonal boron nitride, and the plethora of transition metal dichalcogenide (TMD) combinations, have electronic structures exhibiting metallic, semiconducting, and insulating properties. This promises devices with scalability to the atomic limit combined with defect free interfaces. Realizing this promise has not proved trivial. Defects in the naturally occurring material can dominate their properties, and even synthesized materials can suffer from high impurity concentration. Process residues such as photo-resists can impact device performance. Metal depositions can result in the formation of unexpected interface compounds that can dominate the contact behavior.
We study both the synthesis and integration of 2D materials for nano- and optoelectronic applications. Using an in-vacuo MBE-ARPES cluster tool, we grow TMDs from elemental sources using van der Waals epitaxy. The growth can be carried out on a range of substrates include insulating, semiconducting, and metallic materials. The layer-by-layer nature of these growths can be verified in-situ using reflection, high-energy electron diffraction. We use in-vacuo angle resolved photoemission spectroscopy to study the electronic structure of these materials as a function of composition, substrate, and processing conditions. The in-vacuo set-up is vital to determining the intrinsic properties of these materials, many of which readily oxidize in air. Detailed oxidation studies allow us to correlate our in-vacuo determined properties with the properties determined using ex-situ methods. Complementary ex-situ characterization, including photoluminescence, is used to determine the optical properties of novel ternary TMD alloys, and TMD/TMD heterostructures.
In addition to studying the synthesis of these materials, we also investigate how these materials interface with metals that are commonly used as contacts in electronic devices. It is well known that low work-function metals can react with TMDs to form metal-chalcogen compounds at the metal-semiconductor interface. Recently it has been shown that the process conditions during the electron beam deposition of metals can control the interface reaction. In particular, the reactor base pressure and deposition rate can be used to tune the interface chemistry. For example, the partial pressure of oxidizing species, that are typically present in a high-vacuum reactor, is sufficiently high to completely oxidize titanium as it is deposited by e-beam. This results in a sharp interface between TiO2 and the semiconductor interfaces; however, it is the properties of TiO2 rather than Ti that are dominating electrical and thermal conduction across the interface.
We will present a summary of our work on the synthesis of novel TMD alloys, the determination of their electronic and optical properties as a function of composition and process condition, and the interfaces formed between these materials and typical device contact metals.