(Invited) Metal-Organic Chemical Vapor Deposition of WS2 on Patterned Substrate
Authors: Ogura Atsushi , Cho Kirito , Yokogawa Ryo , Sawamoto Naomi , Machida Hideaki , Ishikawa Masato , Sudoh Hiroshi , Wakabayashi Hitoshi 
Published: February 2024

Transition metal dichalcogenides (TMDs) have attracted much attention in recent years due to their excellent electrical properties and unique nature. Tungsten disulfide (WS2) , as a representative TMD, is a typical two-dimensional stacking material with moderately wide band gap (single layer: 2.03 eV, bulk: 1.32 eV) and, therefore, has led to active application research in various fields. In particular, large-area, few-layer WS2 is expected to have high potential as a channel material for MOSFETs in the next-generation LSIs because of relatively high carrier mobility even in the extremely thin film. Recently, we have succeeded in fabricating continuous WS2 thin films on Si substrates at low temperatures using newly developed organometallic precursor [1]. In this study, we further report the film characteristics and our attempts to deposit films on Fin structures in order to evaluate their applicability to the next-generation channel materials, which should have complicated 3-dimesional structure.

In the experiment, n-BuNC-W(CO)5 was synthesized for WS2 deposition. This novel precursor is a 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, which is non-toxic in contrast to H2S, was adopted. 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 flat and patterned silicon (001) substrates at 300 to 350 °C for 5 to 15 minutes. The deposited films were evaluated by X-ray photoelectron spectroscopy (XPS), UV-Raman spectroscopy (excitation wavelength: 355 nm), and transmission electron microscopy (TEM).

Figure 1 summarizes the evaluation results of the WS2 film deposited on the flat substrate. W 4f XPS spectrum (Fig. 1 (a)) show that the film is mostly composed of 2H with a possible slight amount of 1T/1T' structure. In conjunction with S 2p spectrum (Fig. 1 (b)), it can be calculated using the relative sensitivity coefficient that the composition ratio of W to S is approximately 2.1, which is close to the stoichiometric ratio. Moreover, it was confirmed the film was stable more than 6 month in the room atmosphere. In the Raman spectrum shown in Fig. 1(c), two peaks appeared at approximately 350 cm-1 and 420 cm-1 corresponding to in-plane and out-of-plane vibrations, respectively, which are consisted of 2H-WS2 specific peaks of A1g (418 cm-1) and E1
2g (351 cm-1). Here, E1
2g might be broadened by the presence of another smaller mode called 2LA(M) (second-order longitudinal elastic mode), which may be attributed to the various distortions in the grown WS2 thin film. These results are also consistent with previous reports. This Raman results indicating the 2H structure of the film is also consistent with the XPS results as well as the previously reported literatures. Fig. 1(d) shows bright-field TEM images. It can be observed that the film continuously covers the substrate, with no voids larger than micrometer scale. The higher magnification image also shows approximately 10 layers of WS2 film grown with a layered structure parallel to the surface. The WS2 layers are stacked with a face spacing of approximately 0.7 nm, whose distance correlates well with the theoretical tungsten-tungsten interlayer distance of 0.62 nm for the bulk material. The domain size can be estimated to be roughly 50-100 nm.

Figure 2 shows cross-sectional TEM images of WS2 thin films deposited on Si Fin structures. It can be observed that a continuous layered film is formed along the 3D Fin structure, and the layered structure is parallel to the surface all over the entire Fin structure. However, the number of layers differs depending on the position, with fewer layers on the sidewalls than those on the top surface, and even fewer layers toward the bottom. This may be due to the insufficient supply of precursor deep in the Fin structure. To improve the uniformity and controllability of layer number, further optimization of the deposition conditions such as precursor supply and deposition temperature, including atomic layer deposition technique, is desired.

This research was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) under the project JPJ011438 for the creation of a center for the creation of next-generation X-nics semiconductors.

[1] 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