Direct Integration and Interface Studies of 2D Materials and Dielectric Oxide Films

The mechanism of how any electronic device works relies on the intrinsic properties of the material layers that comprise the stack and the interfaces between them. In this work, we focus on studying the integration and properties of oxide/semiconductor interfaces for emerging field-effect transistor (FET) device architectures, specifically those used for memory and data storage. The materials that we focus our investigations on are HfO2 as the gate oxide, and 2D transition metal dichalcogenides (TMDCs) as the semiconducting channel.

Since the 2000’s, HfO2 has been established as an alternative linear dielectric material to SiO2, as it allows for dielectric layers that avoid leakage without sacrificing capacitance and device performance. Furthermore, in 2011, HfO2 was found to possess ferroelectricity in the thin film geometry, making it attractive for use in scaled memory devices, such as ferroelectric FETs (FeFETs). In this work, we interface HfO2 thin films with semiconducting TMDCs, which have amassed wide research interest due to their unique and versatile physical and electronic properties. TMDCs, with chemical formula MX2, are a class of 2D materials, in which X-M-X stacks form strongly bonded layers in-plane, which are held in the out-of-plane direction by weak van der Waals forces. Due to their van der Waals nature, TMDCs are stable at atomic-scale thicknesses, which allows device scalability, and are free of surface dangling bonds, which offers the promise of high-quality interfaces when applied to a device. However, as we demonstrate through in-depth spectroscopic studies of commercial TMDC single-crystals used by most of the 2D materials community, these single-crystal flakes suffer from high impurity concentrations which would impact their promising material and electronic properties. This necessitates methods for direct growth and integration of these TMDCs that allow for proper control of material properties.

Through direct integration studies, we aim to investigate and engineer the interface chemistry of semiconducting 2D TMDCs with dielectric and ferroelectric HfO2-based films, and to correlate this interface chemistry with the functional properties of FETs. Specifically, we have the following design schemes: 1) TMDC-on-Dielectric, where TMDCs are grown directly onto dielectrics by molecular beam epitaxy (MBE), and 2) Dielectric-on-TMDC, where we investigate interfaces formed by the deposition of dielectrics and ferroelectrics onto semiconducting TMDCs by atomic layer deposition (ALD). All of this work is carried out with the goal of providing fundamental insights into how device performance relates to processing and interface chemistry so that intelligent device optimization can be achieved.