Characterization and Growth Mechanism of APCVD Grown 2D monolayer WS2

M.B. Azim, M. Adachi
Simon Fraser University,
Canada

Keywords: TMDs, XPS, Mechanism, CVD, WS2

Summary:

Abstract: Two-dimensional Transitional Metal Dichalcogenides (TMDs) such as MX2 (M= Mo, W; X= S, Se) have gained tremendous attention for use in a variety of electronic and optoelectronic applications because of their indirect-direct band gap transition for thin layers, and strong light emission. Moreover, monolayer TMDs have exceptional other properties such as piezoelectricity, superconductivity, high carrier mobility, tunable bandgap etc. Like 2H-MoS2, it is possible to construct monolayer 2H-WS2 by sandwiching two atomic layers of S and one atomic layer of W through covalent W-S bonds. A cost effective and reliable means is necessary to achieve large-area, high quality single crystalline materials for the growth of monolayer TMDs for future technologies. Mechanical exfoliation, hydrothermal method, electrochemical exfoliation, chemical vapor deposition (CVD) etc. are the most widely used methods for preparing monolayer TMDs. Among these methods, CVD is regarded as the most promising approach because of its numerous advantages including large area, high crystallinity and uniformity. The problems associated with other methods are either small flake size or low quality with lower carrier mobility restricting its incorporation in electronic devices. In this study, we demonstrate growth of monolayer triangular WS2 crystals using a 3-heating zone furnace using a bottom-up CVD process. The average lateral crystal size is ~10-25 µm and the largest crystal size is ~75 µm. Although, several research groups have reported WS2 growth using WO3 and S precursors, specific parameters such as precursor amount, growth substrate, growth pressure and flow rate, temperature, use of gases (e.g. N2, Ar, Ar+H2), growth time, use of promoter (e.g. PTAS, NaCl, KBr), pre-surface treatment of substrate etc. can vary widely from lab to lab effecting the growth morphology, mechanism, luminescence yield, Raman spectra and light absorption/transmission. Atomic Force Microscopy (AFM) measurements indicate that the thickness of the monolayer WS2 is ~1 nm. We also performed SEM to investigate surface morphology of monolayer WS2 and EDX to perform elemental analysis of monolayer WS2.X-ray Photoelectron Spectroscopy (XPS) has been performed to investigate the chemical composition in terms of binding energy of pristine monolayer WS2. Photoluminescence spectroscopy revealed a sharp emission peak at ~626 nm confirming indirect (bulk) to direct band-gap (monolayer) transition in the monolayer. Moreover, the PL intensity for bi/tri-layer is relatively weak compared to monolayer. Here, we are also focusing to study the effect of different parameters such as temperature, precursor’s ratio, growth time etc. on the different growth features on the basis of different mechanism.