Dept. of Materials Science and Engineering
Pennsylvania State University
Epitaxy of 2D Materials: Challenges and New Approaches
Location: EB1 Room 1011
Friday, November 4th 2016 - 11:00 am
The spectrum of two-dimensional (2D) materials "beyond graphene" has been continually expanding driven by the compelling properties of monolayer films compared to their bulk counterparts. Device applications, however, require the ability to deposit single crystal 2D films over large areas necessitating the use of epitaxy or templated growth for film synthesis. Our studies have focused on the epitaxial growth of layered chalcogenide films, including WSe2 and WS2, by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The choice of precursor has a significant impact on the nucleation of 2D domains on sapphire. When organic chalcogen sources are used such as dimethylselenium or diethylsulfur, a thin defective graphene layer forms on the sapphire surface during growth at elevated temperature (>700oC) which impedes W(Se,S)2 nucleation and lateral growth. This problem was eliminated through the use of hydride sources such as H2Se and H2S thereby enabling epitaxial growth of single crystal monolayer and few layer films on sapphire.
We also developed a process to synthesize ultra-thin layers via intercalation and confined reaction in epitaxial graphene templates and demonstrated this process for the synthesis of 2D gallium nitride. MOCVD was used to deposit Ga atoms on the surface of epitaxial graphene formed on SiC. The Ga atoms intercalate beneath the graphene via defects and wrinkles and diffuse along the graphene/SiC interface forming a stable bilayer of Ga atoms. The Ga atoms are converted to gallium nitride via exposure to ammonia (NH3). 2D gallium nitride formed by this process exhibits a rhombohedral structure (R3m space group) with stoichiometry Ga2N3 in contrast to the wurtzite structure of bulk GaN. First principles calculations supported by optical spectroscopy measurements indicate that 2D Ga2N3 is a direct gap semiconductor with a wide bandgap energy of ~4.8-4.9 eV making it of interest for heterostructure devices and UV emitters. Since many other elements can intercalate into graphene, it is possible that other 2D semiconductors that don't naturally exist as layered materials or are unstable and air sensitive can be synthesized using graphene encapsulated growth.