Advances in Condensed Matter Physics
Volume 2018 (2018), Article ID 3485380, 5 pages
https://doi.org/10.1155/2018/3485380

Research Article

Fabrication and Characterization of Two-Dimensional Layered MoS₂ Thin Films by Pulsed Laser Deposition

Lei Jiao,¹ Yuehui Wang,¹ Yusong Zhi,¹ Wei Cui,¹ Zhengwei Chen,¹ Xiao Zhang,¹ Wenjing Jie,² and Zhenping Wu¹

¹State Key Laboratory of Information Photonics and Optical Communications and School of Science, Beijing University of Posts and Telecommunications, Beijing 100876, China
²College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China

Correspondence should be addressed to Zhenping Wu; zhenpingwu@bupt.edu.cn

Received 25 December 2017; Revised 25 January 2018; Accepted 7 February 2018; Published 15 April 2018

Academic Editor: Zhixin Hu

Copyright © 2018 Lei Jiao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Direct growth of uniform wafer-scale two-dimensional (2D) layered materials using a universal method is of vital importance for utilizing 2D layers into practical applications. Here, we report on the structural and transport properties of large-scale few-layer MoS₂ back-gated field effect transistors (FETs), fabricated using conventional pulsed laser deposition (PLD) technique. Raman spectroscopy and transmission electron microscopy results confirmed that the obtained MoS₂ layers on SiO₂/Si substrate are multilayers. The FETs devices exhibit a relative high on/off ratio of 5 × 10² and mobility of 0.124 cm²V⁻¹S⁻¹. Our results suggest that the PLD would be a suitable pathway to grow 2D layers for future industrial device applications.

1. Introduction

Atomically layered two-dimensional (2D) materials are intriguing for both electronic and optoelectronic applications, because of their unique electrical, optical, and mechanical properties [1–4]. Among these 2D layered materials, molybdenum dichalcogenides (MoS₂), where the layered S–Mo–S are bonded by van der Waals interactions with Mo and S atoms which are bonded by strong covalent interactions, have attracted extensive interests due to their considerable band gap and high carrier mobility. Both theoretical estimations and experimental observations indicate an indirect to direct energy bandgap transition when MoS₂ is thinned from bulk to monolayer, while its bandgap increases from ~1.2 eV to ~1.8 eV [5–8]. Owing to the extraordinary layer-dependent bandgap behavior, MoS₂ is considered a promising candidate to overcome the shortages belonging to zero-bandgap graphene, providing a possible solution for next-generation electronic applications [9]. For instance, monolayer and few-layer MoS₂ based field effect transistors (FETs) have been reported, possessing high on/off ratios exceeding 10³ [10–12]. The mobility in monolayer MoS₂ FETs in vacuum reported so far is 8 cm²v⁻¹s⁻¹ [13–15], which is lower than the theoretical value (410 cm²v⁻¹s⁻¹) [15]. The mobility in few-layer FETs at room temperature could reach 470 cm²v⁻¹s⁻¹[16]; the value is close to the theoretical value (200~500 cm²v⁻¹s⁻¹) estimated through phonon scattering limit [17]. Even with the desirable requirement for industrial applications, to date, most of works have focused on exploring the methods of large scale, high quality MoS₂ thin films. Conventional methods to obtain MoS₂ layers include mechanical exfoliation, chemical vapor deposition (CVD), and PLD [18–20]. However, it is inconvenient to control the scale size and thickness of samples using mechanical exfoliation method. The layer nucleation in CVD methods is difficult with a rigor experimental condition, resulting nonuniform MoS₂ layers. Up to now, PLD method has successfully been employed for the deposition of large-scale layered materials, including graphene, black phosphorous (BP), MoS₂, and InSe [21–23]. One of the main advantages of using PLD is the convenience to control the layer numbers of the large-scale 2D materials by solely varying the pulses numbers. In this work, we report the structural and transport characterization of few-layer MoS₂ back-gated FETs fabricated via PLD technique.

2. Materials and Methods

Few-layer MoS₂ films with the dimension of 10 mm × 10 mm were fabricated on the surface-cleaned SiO₂ (300 nm)/p⁺-Si wafer by PLD (KrF 248 nm) with a laser energy of 200 mJ/pulses and a frequency of 5 Hz. Figure 1(a) illustrates the experimental setup of the PLD system. A commercial MoS₂ pellet (HeFei Crystal Technical Material Co., Ltd.) was used as the target. The target and substrate distance was set at 5 cm. Before deposition, the chamber was evacuated to a base pressure of ~1 × 10⁻⁵ Pa to avoid the oxidation of the MoS₂ films. The SiO₂/Si substrates temperature was maintained at 700°C during the growth. Both target and substrate were rotated during the deposition to obtain a uniform film. When the laser pulses strike the MoS₂ target, the formed plasma including atoms and ions will reach substrates. The thickness of MoS₂ films can be controlled by the number of laser pulses. Here the pulse numbers for growth MoS₂ films are set to be 1200 pulses. The atoms valence in MoS₂ was measured by X-ray photoelectron spectroscopy (XPS). Raman spectroscopy and transmission electron microscopy (TEM) were employed to confirm the thickness of MoS₂ films. To construct the FETs structure, radio frequency magnetron sputtering technique was used to deposit Au/Ti electrodes on surface of MoS₂ layers and back of SiO₂/Si substrates. The deposition time of Au/Ti electrodes is 2 min and 20 s, respectively. The schematic diagram of the fabricated FETs device is shown in Figure 1(b). The transport and transfer measurements were conducted in a Keithley 4200 semiconductor characterization system. All the tests were executed at 300 K in atmosphere.

(a)

(b)

Figure 1: Schematic diagram of (a) PLD equipment; (b) MoS₂ back-gated FETs structure.

3. Results and Discussion

Raman spectroscopy was a commonly used technique to estimate the thickness of the 2D materials [24]. As shown in Figure 2(a), two prominent characteristic peaks are attributed to the mode of MoS₂ atomic layers vibration (in-plane mode: ~382 cm⁻¹ and out-of-plane mode: ~407 cm⁻¹). The distance between and    is proportional to thickness of MoS₂ films, the peak exhibits a red shift with a blue shift of the peak when the MoS₂ thickness decreased, and follows an empirical formula:  cm⁻¹ [25]. According to the formula, the thickness of MoS₂ layers can be deduced: monolayer ~18 cm⁻¹, bilayer ~22.4 cm⁻¹, trilayer ~23 cm⁻¹, and few-layer ~25 cm⁻¹. The cross-sectional TEM image (shown in Figure 2(b)) exhibits a sharp and well-defined MoS₂ layers. A 7 nm thick MoS₂ layer was grown on SiO₂/Si wafer with an interlayer spacing ~0.68 nm, which is close to the theoretical value of the MoS₂ monolayer thickness (~0.65 nm) [26]. These results are consistent with the Raman spectral results, implying that the obtained MoS₂ films ( = 25 cm⁻¹) in this work are few-layer sample.

(a)

(b)

Figure 2: (a) Raman image of MoS₂ films. The distance between and is 25 cm⁻¹. (b) Cross-sectional TEM image of few-layer MoS₂ film.

In order to examine the sulfur deficiency in the MoS₂ films, XPS analysis was carried out. As shown in Figure 3, there are three peaks of Mo 3d, located at 229.18 eV, 232.67 eV, and 235.97 eV. The first two peaks could be assigned to Mo 3d5/2 and Mo 3d3/2, respectively. They are correlating to Mo⁴⁺ state in MoS₂. The third core level peak is correlating to Mo⁶⁺ 3d3/2 state, and the fitted curves are shown in blue in the image. Due to the existence of Mo⁶⁺, there should be S vacancies in our samples. One possibility is the S missing during the process of the growth of the films. Other possibility is the reaction with oxygen to form MoO₃ after the sample taking out of the chamber. The existence of MoO₃ will exert influence on the optical and electrical characteristics of MoS₂ thin films.

Figure 3: The XPS spectra of Mo 3d, S 2s core levels in MoS₂ films.

To investigate the electric properties of the MoS₂ film, back-gated FETs were fabricated based on few-layer MoS₂ films grown on SiO₂/Si substrates. As seen from Figure 4(a), the relationship between drain current () and drain voltage () at different back-gated voltage () was investigated, and the step of back-gated voltage is 5 V. A clear -type semiconducting property was depicted which is consistent with previous reports [27]. The -type behavior of MoS₂ sample might be due to the impurities or existence of interstitial atoms in the interlayer gap. As shown in Figure 4(b), the off-state leakage current () is ~10 nA, while the on-state current () is ~5 μA; the on/off ratio is about 5 × 10², and the ratio is compared to the ratio (~10³) of monolayer FETs [10]. The mobility of multilayer MoS₂ FETs can be estimated based on the formula:

and the channel length is 0.1 mm, the channel width is 1.5 mm, is 8.854 × 10–12 F/m, is 3.9 [17],   is the thickness of SiO₂ (300 nm), and in our experiment is 5 V. The value of can be obtained from the linear fit in Figure 4(b). The mobility of the FETs calculated is 0.124 cm²v⁻¹s⁻¹, and the value is similar to other multilayer back-gated MoS₂ FETs [28]. A bit low mobility may be due to the impurity phase in the film or the impurities and traps on the surface of SiO₂ films [17]. Moreover, the nonlinear rectifying behavior in - curves and the saturation of current in the high positive gate voltage imply a contact resistance between the electrodes and the film, which could also result in an underestimated mobility.

(a)

(b)

Figure 4: FETs characterization of few-layer MoS₂ film. (a) Transport characteristics at different . (b) Transfer curves of MoS₂ thin films in logarithmic and linear scales.

4. Conclusions

In summary, we have reported the PLD grown few-layer MoS₂ based FETs showing good transport characteristics with relatively high ON/OFF ratio ~500, which is comparable to monolayer and few-layer MoS₂ FETs fabricated using CVD-grown samples. Our results suggest that the PLD grown MoS₂ films not only achieve large-scale size, but also present moderate electric properties. The developed PLD method for growth of wafer-scale 2D layered materials may provide a new insight for further electronic applications.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (nos. 61604100, 11404029, 51572033, and 51172208) and the Fund of State Key Laboratory of Information Photonics and Optical Communications (BUPT).

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