Elsevier

Applied Surface Science

Volume 529, 1 November 2020, 147158
Applied Surface Science

Full Length Article
Envisaging radio frequency magnetron sputtering as an efficient method for large scale deposition of homogeneous two dimensional MoS2

https://doi.org/10.1016/j.apsusc.2020.147158Get rights and content

Highlights

  • Large scale deposition of 2D MoS2 using industry viable sputtering technique.

  • One step deposition by sputtering molybdenum in a sulphur sufficient environment.

  • MoS2 films from bulk to bilayer are formed on quartz substrates.

  • Optical characterisations ascertained the quality and purity of the samples.

Abstract

2D MoS2 has the potential to be used in demanding products such as transistors with capability of overcoming short-channel effects of silicon, ultra-thin solar cells because of huge optical absorption in 2D form, memristors, capacitors, sensors etc. However, the commercial use of 2D MoS2 is hindered by the lack of synthesis methods which provide with high quality large scale deposition. Here we demonstrate the deposition of large area 2D 2H-MoS2 from bulk to bilayers on quartz substrates by industry compatible RF sputtering technique in a sulphur sufficient environment provided by evaporation of sulphur flakes by an in-situ effusion cell. Absorbance spectra of sputtered samples showed peaks corresponding to A, B, convoluted C and D and E excitons. Raman signature peaks of MoS2, A1g near 410 cm−1 and E12g around 380 cm−1 was found in all samples. A1g-E12g value decreased with decreasing deposition time, indicating the formation of thin MoS2 layers up to bilayer which was further supported by the evolution of photoluminescence spectra. AFM measurements exhibited a uniform surface morphology. XPS ascertained the purity of the samples. Thus we could prove that radio frequency magnetron sputtering could be used as an efficient method for large area deposition of 2D MoS2.

Introduction

The exfoliation of graphene in 2004 led to a new era of two dimensional materials in the history of science [1]. The discovery of graphene not only resulted in the exploration and use of its intriguing properties, but also encouraged the scientific world to explore other two dimensional materials which have similar properties as that of graphene. In the family of two dimensional materials, transition metal dichalcogenides (TMDCs) have gained great interest due to excellent optical and electronic properties they possess [2]. They are metallic, semiconducting, insulating and even superconducting in nature. Semiconducting TMDCs have tunable bandgap, high charge carrier mobility, structural stability and flexibility making them a potential material for developing electronic, energy harvesting, energy storage, sensing and valleytronic devices [3]. One of the most promising TMDCs is two dimensional (2D) molybdenum disulphide (MoS2). Besides having remarkable optical and electronic properties, MoS2 also possesses a tunable band gap (1.2 eV in bulk to 1.9 eV in monolayer), accompanied by strong photoluminescence and large exciton binding energy, making it a potential candidate for post-silicon electronic applications including high mobility transistors, solar cells, photodetectors, photochemical devices etc. [3].

For exploiting the idiosyncratic properties of MoS2 in these applications, defect free atomic layers of MoS2 have to be developed in large scale on different substrates with precise control over the thickness. Although many synthesis methods like mechanical [4], [5] and chemical exfoliation [6], [7], [8], [9], chemical vapour deposition (CVD) [10], [11], [12], [13], atomic layer deposition (ALD) [14], [15], pulsed laser deposition (PLD) [16], [17] etc. have been tried, each method is having its own limitations. Mechanical exfoliation is the easiest way to produce few layer, high quality MoS2, but will provide only with small non-uniform flakes which cannot be used for industrial purposes [18], [19]. CVD methods including the sulfurization of molybdenum or molybdenum oxide [10], [11], [12], [13], [20], co-vaporisation of metal and chalcogen precursors [21], [22], [23], direct vaporisation and deposition of MoS2 powder [24], [25] etc. have been studied and is in use nowadays. But these processes also have the limitations of uncontrollable thickness, spatial non-uniformity, random growth and the use of toxic and expensive precursors. Also these methods produce only few micrometres of MoS2 layers consuming more time which makes this method undesirable for large scale manufacturing [10], [11], [21], [25]. ALD provides with scalable and uniform MoS2 films with controllable thickness, but the growth depends upon surface morphology of the substrate. The unavailability of suitable precursors is also a concern for ALD [14], [15].

Sputtering, on the other hand, is a widely used industrial standard fabrication method which provides with repeatable, large scale deposition of high quality, crystalline films with fair uniformity and thickness controllability. This method is also desirable since all kinds of substrates whether it is insulating, semiconducting or metallic could be used for the deposition and is less costly and less time consuming compared to the alternatives. Since the sputtered atoms or molecules have high kinetic energy which helps them in adhering and rearranging on the substrate, this method doesn’t need growth promoters as in the case of CVD [26]. Even though sputtering is an industry viable growth method for thin films it has not been exploited well for the production of TMDCs. Researchers adopted two different methods to sputter MoS2. First is a one step process in which either molybdenum metal target is sputtered in sulphur ambience [27] or MoS2 target is sputtered directly [28], [29], [30]. In the direct sputtering of MoS2 sputtered films were sub-stoichiometric and of poor quality. Tao et al. provided sulphur ambience by leaking vaporized sulphur from an evaporator into the sputtering chamber. But since the sputtering power was as low as 6 W, it resulted in molybdenum deficient films [31]. Second method is a two-step process in which the sputtered films were sulfurized or annealed in another chamber by a CVD process [32], [33], [34], [35], [36]. This combined sputtering and CVD process generally results in impurities and defects in the films. By sputtering in a sulphur rich environment and optimising the sputtering parameters which influence the deposition, it is expected to form 2D MoS2 without the defects and impurities.

In this work, we used radio frequency (RF) magnetron sputtering method to produce highly uniform MoS2 layers ranging from bulk to bilayer. Molybdenum atoms were sputtered from a pure molybdenum metal target in sulphur-sufficient environment to produce stoichiometric MoS2 films. The sulphur sufficient environment was provided by simultaneous evaporation of sulphur flakes using an in-situ effusion cell. The deposition parameters like working pressure, RF power, substrate temperature, temperature of effusion cell, distance between the target and substrate etc. were optimised. Keeping all other sputtering parameters constant, we were able to control the number of layers of MoS2 by only varying the duration of the sputtering process. From UV–Vis spectroscopy and Raman spectroscopy we were able to identify that the sputtered films were of pure 2H-MoS2. The bandgaps of these films, calculated from Tauc plots were found to be near 1.75 eV depending on the number of layers of the films. The difference between the Raman shift of the A1g and E12g peaks of each film, helped us to identify the layer number of sputtered MoS2. Thickness measured by atomic force microscopy matched with the layer numbers in each film. Photoluminescence (PL) spectra showed the enhancement of PL emission corresponding to the decrease in the thickness of the film. XPS studies revealed the composition and surface electronic states of the films. The MoS2 films thus produced were highly homogeneous and their size was limited only by the size of our substrate (1 cm × 1 cm). In order to further confirm the large scale deposition, we sputtered MoS2 in larger microscopic glass substrates of 1 in. × 2 in. We got a uniform deposition all over these substrates and were confirmed by the characterisations as given in Fig. S1 of the supplementary information. These observations demonstrate that radio frequency magnetron sputtering could be used as an efficient and reliable method for large scale synthesis of TMDCs and in particular MoS2.

Section snippets

Methods

Quartz substrates of 1 cm × 1 cm size were used as substrates for depositing 2D MoS2 films. The substrates were cleaned by soap solution and water followed by ultra-sonication in acetone, deionised water, isopropyl alcohol and then again in deionised water each for 20 min at a temperature of 50° C. The cleaned substrates were dried by purging high purity nitrogen and were loaded immediately into the vacuum chamber.

We used a custom made radio frequency magnetron co-sputtering system equipped

Results and discussion

Fig. 1(a) shows the absorbance spectra of the deposited films in the range 250 to 800 nm. Four major peaks around 280 nm, 430 nm, 605 nm and 650 nm were observed in each sample. The decrease in intensity of these peaks with decreasing number of layers in MoS2 films is attributed to the decrease in amount of the absorbing material. The peaks observed around 650 nm and 605 nm are identified as the A and B excitonic peaks respectively [39]. Due to the combined effect of spin–orbit coupling (in

Conclusion

In summary, we demonstrated a scalable production method for two dimensional semiconducting form of MoS2, using radio frequency magnetron sputtering technique. The sputtering system is incorporated with an in-situ effusion cell, which helps in the evaporation of sulphur flakes to provide a sulphur rich environment while sputtering molybdenum to produce stoichiometric MoS2 films. This one step approach also helped in reducing impurities and defects in the samples. Optical studies like UV-Vis-NIR

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Authors would like to acknowledge SERB, Govt. of India for financial support (EMR/2016/007140) to carry out this research and DST-PURSE, Govt. of India for the financial support for the purchase of sputtering equipment.

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