Suppression of Sulfur Desorption of High-Temperature Sputtered MoS2 Film By Applying DC Bias
Authors: Hibino Yusuke , Ishihara Seiya , Sawamoto Naomi , Ohashi Takumi , Matsuura Kentarou , Wakabayashi Hitoshi , Ogura Atsushi 
Published: February 2020

As the research field of 2D layered materials expands and progresses, especially for the transition metal dichalcogenaides (TMD), the demand for high quality thin films is also rising. There have been many techniques proposed to fabricate TMD films [1-2]. Among the fabrication techniques, RF magnetron sputtering is expected to realize MoS2 device operating in normally-off mode due to large-area uniformity and suppression of impurity [3]. On the other hand, during sputtering deposition using compound MoS2 target, it is known that the film suffers from sulfur desorption especially when the substrate is heated to obtain sufficient crystallization of the film. We have reported the 2-step technique with the post-deposition sulfurization (PDS) to achieve the S implementation [4-5], however the crystallization is still insufficient due to the lack of migration of Mo atoms. Therefore prevention of sulfur desorption with 1-step method is desired in order to fabricate high quality MoS2 thin film. In this study, we proposed applying positive DC bias to the substrate during high-temperature sputter deposition in order to prevent sulfur desorption and obtain superior crystallinity by the enhanced Mo migration.
MoS2 was deposited on SiO2 substrate by RF sputtering. The substrate temperature was varied from 250°C to 600°C. Ar partial pressure was also varied from 0.09 to 1.0 Pa with RF power of 50 to 225 W. The sputtering duration was fixed to 600 s. In order to investigate the effect of the sputtering parameters, the sputter-deposited MoS2 flms were evaluated with X-ray electron spectroscopy (XPS) to confirm Mo-S bond formation. The S/Mo ratio of the film was calculated from the ratio of Mo 3d and S 2p spectra. The samples with elevated deposition temperature were evaluated with Raman spectroscopy and compared to the as-depo. sample and the PDS sample [4].
As a result of applying bias voltage, higher positive bias voltage yielded films with higher S/Mo. Stoichiometric S/Mo ratio of 2.0 was achieved by applying positive bias voltage even with high-temperature sputtering. This is considered to be the attracting effect of S2- to the substrate due to the positive bias. When the Ar partial pressure was increased, the S/Mo ratio decreased. This result shows that the mean free path of the sputtered particles have large effect on the amount of S2- reaching the substrate determining S/Mo ratio of the film. As the sputter RF power was increased S/Mo also increased, which may also provide larger Mo migration energy, therefore result in the better crystallinity. By optimizing each sputtering parameters, S/Mo of 2.0 was achieved at 400-600°C.
This work was partly supported by JST CREST Number JPMJCR16F4, Japan. This work was also partly supported by JSPS KAKENHI Grant Number 16J11377.

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Figure 1