2, U.S. Army Research Laboratory, Adelphi, Maryland, United States
Molybdenum disulfide (MoS2) is one of the most investigated 2D crystals. In order for MoS2 to be effectively used in electronic devices, such as top-gate field-effect transistors (FETs), a high-k dielectric layer must be deposited directly onto the 2D material – either as an integral part of the device or as a protective layer. The most common method to deposit a high-k dielectric is using atomic layer deposition (ALD). However, thin ALD films are not possible due to the inability of precursor molecules to uniformly react with the inert basal plane of MoS2. In order to achieve thin high-k dielectrics on MoS2 either the surface must be functionalized, a buffer layer must be first deposited, or a different method of film deposition needs to be employed. Recently, it has been shown that ultrathin high-k dielectrics can be realized on MoS2 using plasma-enhanced ALD (PEALD); however, the mechanism of the film growth and impact of the PEALD process, including its effects on the monolayer MoS2 crystal structure, have not been examined thoroughly. In this work, these effects are studied for the PEALD process on mono- and bi-layer MoS2 using Raman spectroscopy, photoluminescence (PL), and electrical characterizations.
In order to understand the impact of the PEALD process on MoS2 as well as gain insight on the PEALD growth mechanism, Raman and PL spectra were collected on chemical vapor deposition (CVD) grown monolayer and bi-layer MoS2. Raman and PL were carried out using a WiTec Raman setup at a wavelength of 532 nm. Spectra were collected at room temperature (under vacuum (in a N2 environment) and in ambient) and at liquid N2 temperatures. Characterization of the MoS2 was performed before and after the deposition of either ALD or PEALD HfO2 – study of film growth with and without the plasma enhancement provided further insight into the impacts of the plasma process. Raman spectra indicate that the PEALD process is damaging the top MoS2 layer, but is leaving the underlying layers intact. In addition to the spectroscopic studies, electrical characteristics of back-gated MoS2 FETs from CVD-grown mono- and bi-layer MoS2 were also tested before and after either the deposition of ALD or PEALD HfO2 to examine the impact of the process on the electrical properties of the 2D crystal. This work aids in the attainment of ultra-thin high-k dielectrics on mono- and bi- layer MoS2 and thoroughly examines the impact that the PEALD process has on their crystal structure and electrical properties.