Paper SA+2D+AC+AS+TF-TuM5
Gas-source MBE Growth of 2D Materials Examined using X-ray Synchrotron Radiation

Tuesday, November 8, 2016, 9:20 am, Room 103C

Two-dimensional materials, in particular transition metal dichalcogenides, are attracting considerable interest from both fundamental and applied viewpoints. Here we report on the gas-source MBE growth of thin films of WSe₂ using W(CO)₆ and elemental Se, where we monitor the process in situ and in real time with X-ray synchrotron radiation. In this work, we have two independent means to introduce the thin film constituents: a (supersonic) gas-source of W(CO)₆ in a carrier gas of He, and an effusion cell containing elemental Se. We can collect both the scattered X-rays, and those emitted due to fluorescence. We have examined growth for a variety of conditions: growth with coincident fluxes of W(CO)₆ and Se, and growth in which one of the two reactant fluxes was gated. These experiments were conducted at several different substrate temperatures, on graphite (HOPG), and on so-called epitaxial graphene (graphene on SiC). First, for continuous exposure to W(CO)₆ and Se we observe, after an short incubation period, continuous growth of both the W and Se intensities. Analysis of this data indicates a stoichiometry consistent with WSe₂. We also find diffraction features consistent with the crystalline phase of WSe₂, where the basal plane is in the plane of the substrate. Particularly interesting is what we see when one of the reactants is gated: We have found that growth is halted by gating the incident flux of W(CO)₆, while the film is stable in the presence of a flux of Se_n. The physical properties of these thin films are essentially the same as those grown using continuous exposure to both reactants. We find a much different result when the flux of Se_n is gated: the absence of Se, but the presence of W(CO)₆, leads to loss of Se, and an increase in the amount of W. Since the termination of both fluxes does not lead to the loss of either element, it must be that W(CO)₆ is providing the species that leads to the loss of Se. The most likely scenario is that a ligand exchange reaction occurs producing the gas phase species, SeCO(g). Perhaps our most exciting result involves the growth of WSe₂ on epitaxial graphene. In these experiments, in addition to measuring the X-ray fluorescence, we also measured in situ and in real time the intensity at the anti-Bragg condition, which we have shown to be are very effective way to monitor thin film crystal growth. We observe strong and sustained oscillations, indicating layer-by-layer growth of WSe₂ of up to at least 5 monolayers. This is a very exciting result, and it indicates the effective use of synchrotron radiation to examine the growth of TMD thin films.