Fabrication of MoS2 Thin Film at Low Temperature By Atmospheric-Pressure Solution Based Mist CVD
Authors: Sato Shota , Kawaharamura Toshiyuki 
Published: February 2020

Recently, layered transitional metal dichalcogenide (TMD) materials which are bound by vdW forces between the layers have received much attention owing to their excellent properties. In this study, we focused on layered molybdenum disulfide (MoS2) thin film fabrication by mist CVD. MoS2 is a semiconductor with a non-zero energy band gap of 1.8 eV which is close to the typical band gap energy of a-Si:H (1.7 eV), contrary to graphene with a zero energy band gap [1]. The fabrication of MoS2 layered thin film by mechanical exfoliation (ME), chemical vapor deposition (CVD), and molecular beam epitaxy (MBE) have been previously studied and reported in literatures. However, there are some problems to be solved for scaling up those processes to industrial level. In ME, size of layered MoS2 thin films are smaller than 1×1 mm2. In CVD and MBE, large amount of energy is consumed by the heater set at 700-1000°C and the vacuum pump keeping the low pressure of 10-5 Pa. [2-5]. Therefore, the development of a new method which is suitable for industrial expansion is very important. Thus, in this study, we demonstrate the fabrication of MoS2 by mist CVD. mist CVD is one of functional thin-film fabrication methods carried out under open-air atmospheric-pressure, which is developed for advanced control of precursor streams in order to achieve the fabrication of uniform thin films.

In the experiment, MoS2 was fabricated by a homemade fine-channel type mist CVD system, which was designed to achieve the uniform flow and homogeneous temperature in the reactor, details of which were reported elsewhere [6]. The quartz substrates were used. Prior to the thin-film growth, substrates were cleaned by acetone, isopropanol, and deionized water each for 2 min. Thin films were fabricated by using a precursor solution containing a mixture of (NH4)6Mo7O24・4H2O and thiourea dissolved in methanol. The growth temperature was varied from 200 to 450°C at 25°C intervals. Raman spectra of samples were evaluated using a micro-Raman system with incident laser wavelengths of 532 nm (2.33 eV). The binding states of the samples were investigated by X-ray Photoelectron spectroscopy (XPS) operated at 15 kV and 26.7 mA with the unmonochromated Al Kα source. The crystal structure of the samples was identified by grazing incidence X-ray diffraction (GIXD). In this paper, results of Raman spectroscopy and XPS are shown and discussed.

In the results, the slight change of the color on the substrate surfaces of some samples was seen after fabrication of the MoS2 thin film and was seemed that uniform thin films were obtained on 30×30 mm2 square substrate. Raman peaks around 380 cm-1 (E1
2g) and 410 cm-1 (A1g) were observed from the samples prepared at 300 - 450°C, except the samples prepared at temperature lower than 300°C. Raman peaks around 383.0 cm-1 (E1
2g) and 407.8 cm-1 (A1g) in bulk MoS2 are reported in the references [7].

XPS analysis of the samples prepared at 350°C shows binding energies corresponding to Mo3d (229.25, 232.58, and 235.48 eV) and S2p (162.22 and 162.96 eV) levels in Fig.2. According to the literature, binding energy peaks corresponding to Mo-S bonds; Mo3d5/2 (229.25 eV), Mo3d3/2 (232.2 eV), S2p3/2 (162.1 eV), and S2p1/2 (163.1 eV) as well as corresponding to Mo-O bonds; Mo3d5/2 (232.8 eV) and Mo3d3/2 (235.5 eV) has been observed in the case of XPS analysis of bulk MoS2 powders [8]. A small amount of MoO3 is included in MoS2 powder, as evident from low intensity of those binding energy peaks corresponding Mo-O bonds compared with that of Mo-S bonds in this literature. Similar results were obtained in XPS analysis of MoS2 thin film prepared by mist CVD at 350°C. From these results, it is suggested that layered MoS2 thin films with high quality were successfully fabricated by mist CVD. Remarkably, the growth temperature of MoS2thin-films by mist CVD is comparatively lower temperature than that of the traditional CVD.

In summary, we have demonstrated the successful fabrication of MoS2 thin-film at atmospheric pressure and relatively low growth temperature by mist CVD. 

Reference

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[8] M.A. Baker, et. al., Appl. Surface Sci., 150 (1999) 255