Large Scale Growth of MoS2 Monolayers by Low Pressure Chemical Vapor Deposition

Monolayers of molybdenum disulphide MoS2, a two dimensional (2D) semiconductor with a direct band gap of 1.9 eV, have been proposed as a candidate for next generation nanoscale electronic and opto-electronic devices. Controlled synthesis of MoS2 monolayers is critically important since the thickness uniformity and grain size are major concerns for the fabrication of opto-electronic devices.
In this study, we demonstrated the growth of wafer scale uniform MoS2 monolayers on SiO2 covered silicon wafers, at a range of growth temperatures (650 oC-850 oC) with optimum grain sizes as large as 400 μm, using low pressure chemical vapor deposition (LPCVD). By controlling the partial pressure of the reactant species at the growth surface and the limiting time, we can achieve prefered monolayer growth over multilayer growth.
The MoS2 monolayer crystals follow a lognormal size distribution, consistent with random crystal nucleation, with single crystal domains as large as 400 μm. We estimated the thermal expansion coefficient to be (2.5±1.2) ×10-6 /oC, which is at least double that of the bulk.
We have found film growth can be clearly classified into the reaction limited, feed limited and desorption limited regimes. With the help of COMSOL simulations, we have related the local growth environment such as growth temperature, MoO2 concentration, sulphur chemical potential and growth time with the macroscopic growth parameters such as Ar flux. In the feed limited regions, it is the supply of Mo that is the rate limiting factor. In the desorption regions, the growth is controlled by thermal stability of MoS2 monolayers. The growth modes also can be used to tune the grain morphology from perfect triangles to hexagons.
Finally, we have also compared our approach with an LPCVD approach based on MoO3 as the Mo source. MoO3 has a higher vapor pressure than MoO2 which was used in the previous approach. By tuning the the S:MoO3 ratio, we could grow controllably planar MoS2 monolayers, vertically aligned MoS2/MoO2 and planar MoO2 crystals.