Migration-Enhanced MOCVD of Fully-Coalesced WS₂ Monolayers

The well-controlled synthesis of monolayer (ML) TMDC targeting large-scale fabrication is still technically challenging. Although metal-organic chemical vapor deposition (MOCVD) has stood out as a scalable and reproducible technique, details of growth are not completely understood, and some issues need to be addressed before realizing the “Lab-to-Fab” transition. One concern is C incorporation, which not only hinders lateral growth but also affects film quality. Furthermore, it is challenging to close a ML without premature bilayer (BL) formation. Coherent ML films with sparse BL nucleation are a prerequisite for growing planar 2D-2D heterostructures. In this study, we focus on WS₂ as a 2D TMDC well-suited e.g. for red light emission. In S-rich conditions (S/W molar ratio &gt;4,000), W adatoms are considered to be the growth-limiting species. W accumulation and thus BL nucleation will inevitably start when the size of ML nuclei exceeds the on-WS₂ migration length. Analyzing the morphology after the onset of BL nucleation suggests this length on a scale &lt;100 nm. Therefore, a two-stage process is proposed aiming at: (i) achieving a sufficient nucleation density in the nucleation stage and (ii) enhancing the migration of W adatoms (especially close to ML coalescence) in the lateral-growth stage.<br/>All processes are performed on (0001)-oriented sapphire in AIXTRON CCS reactors in 7×2" and 1×4" configuration, equipped with LayTec in-situ temperature measurement and spectroscopic reflectance monitoring. Tungsten hexacarbonyl (WCO) and di-tert-butyl sulfide (DTBS) were chosen as precursors with H₂ as carrier gas. Processes were started with a 15 min H₂sapphire desorption (150 hPa, 1050 °C on wafer surface). Scanning electron microscopy (SEM), Raman and photoluminescence (PL) spectroscopy were used for characterization.<br/>To reduce C incorporation, different surface temperatures (from 700 to 840 °C) and chamber pressures (50 and 20 hPa) were investigated, revealing that both parameters affect C incorporation. This can be explained by complex multi-step precursor pyrolysis driven by collisions between precursor and H₂ molecules at high temperature. A pressure of 20 hPa leads to vanishing C-related Raman modes up to 840 °C and was fixed for further experiments.<br/>Temperatures of 700 and 750 °C were investigated to optimize the initial nucleation density for later closing a ML. The temperature-dependent migration length of W adatoms on sapphire defines the maximum distance between neighboring nuclei. Nucleation at 700 °C for 15 min yields a high nucleation density of ~210 µm^-2 (50% higher than at 750 °C), with typical size of ML triangles ≤50 nm and a total ML coverage ~24%.<br/>To suppress further ML nucleation, the lateral-growth stage is initiated by a temperature ramp of +10 K/min. By comparing the morphology and optical properties of the WS₂ films, the optimal temperature is determined to be 820 °C. In parallel, the WCO flux is ramped down (-75% over 144 min), aiming at reducing the W arrival rate and thus extending migration length. The impact of this ramp is clearly visible in the in-situ transient data, indicating a decelerated growth. This approach leads to a fully-coalesced (&gt;99%) ML with very small BL coverage (~20%). The predominant ML nature of WS₂ was confirmed by Raman and PL studies. This MOCVD process can be used in both 7×2" and 1×4" configuration, showing excellent homogeneity and only small deviations in surface coverage for 2" and 4" substrates. Nevertheless, more advanced characterization methods like X-ray photoelectron spectroscopy (XPS) are necessary to study the stoichiometry, e.g. revealing the dependence of sulfur vacancy density on growth parameters.<br/>In conclusion, a novel two-stage migration-enhanced MOCVD process is proposed to prepare fully-coalesced WS₂ ML films with suppressed BL formation. These results pave the way for the direct growth of 2D-2D heterostructures in a single process.