2D metal chalcogenide semiconductors have received considerable attention due to their compelling properties and layered crystal structure. Much of this work has focused on transition metal dichalcogenides (TMDs, MX2 where M=Mo, W and X=S, Se, Te), but there is growing interest in group III (Ga, In) and group IV (Sn, Ge) chalcogenides to expand the suite of layered materials. Much of the research to date has been carried out using flakes exfoliated from bulk crystals but techniques for wafer-scale epitaxial growth of single crystal monolayer and few layers films are rapidly developing.
Our work has focused on the epitaxial growth of 2D layered chalcogenides using gas source chemical vapor deposition (CVD) also referred to as metalorganic CVD (MOCVD). The process is carried out in cold wall reactor geometries at moderate pressures (100-700 Torr) using hydrides (H2Se, H2S) as the chalcogen source in a H2 carrier gas. In the case of TMDs, a multi-step method involving modulation of the metal hexacarbonyl precursor source flow rate was developed to independently control nucleation and lateral growth of domains at elevated temperature (>700oC) which is beneficial to enhance the surface diffusivity of transition metal adatoms. Using this approach, uniform, coalesced monolayer and few-layer TMD films (MoS2, WS2, MoSe2 and WSe2) were obtained on 2” sapphire substrates at growth rates on the order of ~1 monolayer in 10-60 minutes. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire. Post-growth dark-field transmission electron microscopy carried out on monolayers transferred from the sapphire reveals that the films consist of highly ordered micron-sized single crystal regions bounded by low angle grain boundaries. Epitaxial growth of indium selenide was also investigated using trimethyl indium (TMIn) and H2Se in H2. In this case, lower growth temperatures (<500oC) and lower reactor pressures (100 Torr) were necessary to reduce the extent of gas phase pre-reaction of precursors and obtain a stoichiometric film. Under these conditions, epitaxial growth of beta-In2Se3 was demonstrated on c-plane sapphire and (111) Si substrates with both island growth and step-flow growth modes observed. Prospects for utilizing MOCVD for the growth of epitaxial 2D heterostructures will also be discussed.