Paper 2D-TuA3
Direct, Real-Time Observation of Layer-by-Layer Growth of a 2D Semiconductor using In Situ X-ray Synchrotron Radiation

Tuesday, October 31, 2017, 3:00 pm, Room 15

Two-dimensional materials, in particular transition metal dichalcogenides, are attracting considerable interest from both fundamental and applied viewpoints. In a number of studies, thin films of these materials are produced by techniques such as exfoliation. While these techniques have been invaluable concerning the measurement of electronic and other important physical properties, growth of thin films of these materials is essential to fully exploit their promise in a variety of devices. In particular, there is a great need to develop an understanding of the growth process such that one can at will deposit an arbitrary number of layers (e.g., 1, 2, 3 etc.) on a variety of possible substrates. Here we report on the metal-organic MBE growth of thin films of WSe₂ on sapphire substrates 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, both in situ and in real time. We have examined growth for a variety of conditions, including the flux and incident kinetic energy of the W(CO)₆, and the substrate temperature. After a short incubation time, we observe steady-state epitaxial growth of crystalline WSe₂ thin films. X-ray fluorescence confirms formation of a stoichiometric thin film of WSe₂. Perhaps most interestingly, during epitaxial growth we observe strong and sustained oscillations at the “anti-Bragg” condition for X-ray scattering, consistent with layer-by-layer growth. These results enable a direct determination of how each successive layer forms^[1], and how these depend on process conditions. Concerning process conditions, we observe a spectacular change in the growth as the incident kinetic energy of the W(CO)₆ is varied. At sufficiently low values of the incident kinetic energy we observe no growth, while at higher values we observed sustained 2D LbL growth. These results point to the importance of incident kinetic energy in driving the dissociative chemisorption of the W(CO)₆ precursor.

[1] See, e.g., A. R. Woll, T. V. Desai and J. R. Engstrom, Quantitative modeling of in situ x-ray reflectivity during organic molecule thin film growth, Phys. Rev. B 84, 075479/1-14 (2011).