dc.description.abstract | Since the discovery of graphene, various two-dimensional (2D) materials have been explored. Graphene has a two-dimensional structure and unique physical properties, like high carrier mobility (≥200000 cm2V-1s-1). However, graphene is a gapless material, which limits its applications in electronic devices. Other 2D materials have been explored. Two-dimensional transition metal dichalcogenides (2D TMDs) is one of 2D materials. The absence of dangling bonds, and finite and distinctive band gaps, making it a good candidate for electronics and optoelectronics.
In experiment, we focused on the synthesis of tungsten ditelluride (WTe2) and molybdenum ditelluride (MoTe2). There are two common methods to obtain TMDs thin films. First is mechanical exfoliation, which is hard to create a large and uniform thin film.
This method is most used in research. Second is chemical vapor deposition (CVD), which needs tellurization to achieve high quality thin film. However, tellurization process is hard to control, and tellurium is harmful for health. We develop a three-step process. Comprising of physical vapor deposition, capping layer deposition and annealing process. First, we used ultra-high vacuum sputter system (UHV-sputter system) to deposit WTe2 or MoTe2 thin film. Second, we deposited a silicon oxide capping layer, to prevent oxidation of thin film oxidize or sublimate during the following annealing. Finally, the annealing process will improve thin film quality and crystallinity.
Various analysis methods, including Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy were used to characterize the thin film quality. The different phases of thin films like 1T’-WTe2, 1T’-MoTe2 and 2H-MoTe2 were confirmed by Raman analysis. Our thin films show similar properties as compared with CVD and exfoliation methods. Moreover, The thin film quality is very sensitive to sputtering temperature and annealing temperature. Without proper time and temperature, it’s hard to synthesize thin film successfully.
We developed a new process to synthesize WTe2 and MoTe2 thin films. The PVD method without tellurization process is safe and easily controlled. This process can also expand to other TMDs materials. Our research shows the potential of PVD process for TMDs thin film in the future. | en_US |