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 (A_1g) (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).