Importance of the substrate's surface evolution during the MOVPE growth of 2D-transition metal dichalcogenides.

In this paper, we explore the impact of changing the growth conditions on the substrate surface during the metal-organic vapor phase epitaxy of 2D-transition metal dichalcogenides. We particularly study the growth of molybdenum disulfide (MoS2) on sapphire substrates at different temperatures. We show that a high temperature leads to a perfect epitaxial alignment of the MoS2 layer with respect to the sapphire substrate underneath, whereas a low temperature growth induces a 30° epitaxial alignment. This behavior is found to be related to the different sapphire top surface re-arrangement under H2S environment at different growth temperatures. Structural analyses conducted on the different samples confirm an improved layer quality at high temperatures. MoS2 channel-based metal-oxide-semiconductor field-effect transistors are fabricated showing improved device performance with channel layers grown at high temperature.

Effects of buried grain boundaries in multilayer MoS2.

Two-dimensional transition metal dichalcogenides have been the focus of intense research for their potential application in novel electronic and optoelectronic devices. However, growth of large area two-dimensional transition metal dichalcogenides invariably leads to the formation of grain boundaries that can significantly degrade electrical transport by forming large electrostatic barriers. It is therefore critical to understand their effect on the electronic properties of two-dimensional semiconductors. Using MoS2 as an example material, we are able to probe grain boundaries in top and buried layers using conductive atomic force microscopy. We find that the electrical radius of the grain boundary extends approximately 2 nm from the core into the pristine material. The presence of grain boundaries affects electrical conductivity not just within its own layer, but also in the surrounding layers. Therefore, electrical grain size is always smaller than the physical size, and decreases with increasing thickness of the MoS2. These results signify that the number of layers in synthetically grown 2D materials must ideally be limited for device applications.