Low Temperature WS2 Metal-Organic Chemical Vapor Deposition Using N-Bunc-w(co)5 for W Precursor

Tungsten disulfide (WS2) is one of the representative two-dimensional layered materials, and its applications in various fields have been actively studied due to its preferable properties such as reasonably wide band gap (monolayer: 2.03 eV, bulk: 1.32 eV) [1] and excellent stability. In particular, large area, a few layer WS2 has a high potential as channel material for MOSFET in the next generation LSI. In most of the WS2 CVD reported so far, WF6 or W(CO)6 has been widely used for tungsten precursor as well as H2S for S precursor. However, there are critical problems for the precursors: WF6 is toxic, and W(CO)6 is solid and hard to supply to the reaction chamber. Sublimation supply of solids is not stable in terms of transport volume, and sometime clogging of pipes is also a concern. H2S is also highly toxic. In this study, a tungsten precursor of n-BuNC-W(CO)5 was investigated for WS2 deposition. One of its favorable characteristics is that it is liquid above 35 °C with stable vapor supply. It is also expected to interact with the substrate efficiently due to its polarity in the molecule. The vapor pressure is 1.2 torr at 115 °C, which is an appropriate for a CVD precursor. As a sulfur precursor, (t-C4H9)2S2 [2], which is non-toxic in contrast to H2S, was employed. WS2 thin films were grown by MOCVD in a cold-wall reactor, using the organic compounds introduced above as precursors for W and S, respectively, on silicon (001) or sapphire (0001) substrates. The deposited films were evaluated by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (wavelength: 355 nm), and transmission electron microscopy (TEM). As a result, the continuous films were successfully obtained at the deposition temperature over 250 °C. XPS measurements showed the S/W ratio were slightly larger than the stoichiometric value of two (Fig. 1(a) and (b)). Residual C contamination was significantly reduced by elevating the deposition temperature over 300 °C. The Raman spectra confirmed the film was multi-layer WS2 with peak at 356 cm-1 (E1 2g) and 419 cm-1 (A1g) (Fig. 1(c)). TEM observation also revealed the film had a layered structures as is shown in the typical 2D materials (Fig. 1(d)). This study was partly supported by JST CREST Number JPMJCR16F4, Japan. Reference [1] J. Gusakova, X. Wang, L. L. Shiau, A. Krivosheeva, V. Shaposhnikov, V. Borisenko, V. Gusakov, and B. K. Tay, Phys Status Solidi. 214, 12 (2017). [2] C. Kirito, K. Yamazaki, Y. Hibino, Y. Hashimoto, H. Machida, M. Ishikawa, H. Sudoh, H. Wakabayashi, R. Yokogawa and A. Ogura, ECS Trans. 104, 3 (2021). Figure 1