Nucleation, growth, and stability of WSe2 thin films deposited on HOPG examined using in situ, real-time synchrotron x-ray radiation

Abstract

We have examined the nucleation and growth of WSe2 thin films in ultrahigh vacuum on highly oriented pyrolytic graphite (HOPG) using in situ real-time x-ray fluorescence (XRF), and ex situ x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. We employed W(CO)6 as the W source delivered via a supersonic molecular beam, Sen delivered via an effusion cell, and we examined substrate temperatures from 400 to 540 °C. Crystalline, near stoichiometric thin films were formed at temperatures Ts ≤ 470 °C, whereas those formed at 540 °C were very W-rich. The thin films were not continuous but consisted of filamentlike features with spikelike edges. A focus of our work was to examine the initial stages of growth and the effects of extinguishing one of the species (W or Se) both before and during growth. First, in all cases examined, there was a delay in the onset of a measurable rate of growth on the clean HOPG surface following the introduction of both species, W(CO)6 and Sen. In cases where the incident flux of W(CO)6 was gated, once WSe2 growth had commenced, extinguishing the flux of W(CO)6 quenched growth immediately and did not result in the deposition of additional Se. Once the incident flux of W(CO)6 was re-started, growth began essentially immediately. The pattern with Sen gating was strikingly different. In this case, once WSe2 growth had commenced, extinguishing the flux of Sen resulted in a continuing uptake of W essentially unabated, while the amount of Se in the thin film decreased, which resulted in an oscillation in the Se-to-W content in the thin film. As the thin films were stable in UHV in the absence of both species, the incident W(CO)6 is responsible for the etching of Se, which we postulate is due to a ligand exchange reaction forming volatile SeCO.