Semiconducting two dimensional (2D) transition metal dichalcogenides (TMDs) offer an alternative way to keeping up with the electronics industries demand, through smaller more powerful devices. Thin 2D materials possess sheet-like layered structures with the lateral dimension of a few micrometers, the thickness is only single- or few-atoms thick. 2D materials are absent of dangling bonds, have abrupt interfaces, and have potential application in electronics, optoelectronics and flexible electronics. Molecular beam epitaxy (MBE) is considered to be one of the best growth method to create high quality TMDs with control over the band gap size and whether it is a direct or indirect. We are working on the large-area uniform growth of WSe2 using MBE. The goal is to optimize the growth to achieve uniform WSe2 thin films on Al2O3(0001) substrates. The challenges to be overcome for MBE growth optimization: substrate choices, multitude of growth conditions, and post-growth treatment. The large vapor pressure difference between tungsten and selenium causes a narrower growth window, giving rise to defects in the material and smaller domain sizes. This was done through growing WSe2 through MBE then comparing with exfoliated WSe2 flakes. Using Raman spectroscopy, we compared the positions, intensities and the full-width-at-half-maxima of E12g and A1g peaks; the two characteristic peaks used to identify if the material is WSe2. Other spectroscopic methods were used to identify if WSe2 was successfully grown. X-ray photoelectron spectroscopy to identify the chemical composition of the grown WSe2 and photoluminescence (PL) spectroscopy was used to determine the band gap. The Raman spectra of monolayer to bulk exfoliated WSe2 shows a trend of the A1g peak decreasing, converging to the E12g peak, as layer thickness increases. The PL of the exfoliated WSe2 shows that as layer thickness increases the band gap energy decreases, and shifting from direct band gap to an indirect band gap. The MBE grown WSe2 did not show a uniform large area growth or match the purity of exfoliated WSe2. It was shown that at increased post growth annealing temperature, starting from 500°C and ending at 530°C, the E12g and A1g Raman peaks of MBE grown WSe2 annealed at 530°C matched the closest to the exfoliated WSe2 values. More work needs to be done on the optimization of WSe2, higher annealing temperatures, changing of other growth parameters to achieve uniform large area MBE growth of WSe2.

Molecular beam epitxay of transition metal dichalcogenides

2019 Spring National Meeting

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