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.
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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.
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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.
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.
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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).