Introduction

Two-dimensional (2D) materials with their layered structures have attracted much attention as next generation device materials due to their extraordinary properties such as mechanical flexibility, large surface to volume ratio, and their easy integration in heterostructure junction devices1,2,3. Graphene is well known example among these materials, which shows very rich physics resulting from its linear dispersion relation and massless Dirac Fermion4. Owing to remarkable properties such as very high mobility, large electrical and thermal conductivity, high Young’s modulus and small spin-orbit coupling (SOC), graphene became a promising candidate for wide range of applications, including high speed electronics, sensors, energy generation and storage devices as well as spintronics5,6,7. However, semi-metallic nature (gapless band structure) of pristine graphene limits its application in semiconductor electronics as zero band-gap leads a low on/off ratio in graphene-based field effect transistors (FETs)4,8. In addition to this, small SOC in graphene does not allow this material to have better control on generation and electrical manipulation of spins in spintronic devices.

Unlike graphene, molybdenum disulfide (MoS2), which belongs to the family of transition metal dichalcogenides (TMDs) shows semiconducting nature with a sizable band-gap3. The type and value of band-gap in MoS2 can be changed by varying the number of layers—MoS2 shows an indirect bandgap (~1.2 eV) in the bulk form (multilayers) and a direct band-gap (~1.8 eV) when reduced to monolayer3. In addition to non-zero band-gap, MoS2 also possesses considerable SOC along with unique spin-valley coupling to manipulate the spins, which makes the material very attractive for the next generation spintronic and other technological applications. The interest in mono-layer MoS2 has further increased after the demonstration of a high on/off ratio (~108) and high carrier mobility at room temperature FETs9,10. However, the large electrical potential drop due to high contact resistance between MoS2 and metal contacts may strongly limit the performance of MoS2-based devices. Previous reports show large Schottky barrier heights (SBHs) for various metal/MoS2 contacts11, which can be reduced by different approaches such as insertion of insulating layers (h-BN12, MgO13, TiO214,15, Al2O316) between metal and MoS2, chemical doping17,18 of MoS2 and electrical gating13,16. To study spin injection from ferromagnetic (FM) metal and spin transport in MoS2, it is very important to investigate the contact behavior between FM metal and MoS2 and to suppress the SBH that hinders efficient spin injection/detection. There are only few reports in the literature, which discussed behavior of FM/monolayer MoS2 contacts and estimated SBH12,13,16. From our knowledge, the MoS2 in previous reports is either exfoliated from the bulk single crystal of MoS2 or transferred from MoS2 sample grown via chemical vapor deposition (CVD) technique. However, it is notable that these methods are not suitable for mass production of electronic devices and exfoliation and/or transfer methods can induce unwanted changes in physical and electronic properties of MoS2. In addition, the FET device characteristics strongly depend on the growth methods of MoS2 and FM electrodes. Hence, it will be interesting to study the behavior of FM/MoS2 contacts, where MoS2 is directly grown on substrate by CVD technique.

In this paper, we study the device characteristics of FETs fabricated using monolayer MoS2 channels, directly grown on SiO2/Si substrate using salt-assisted CVD technique. 20 nm thick Ni80Fe20 (Py) electrodes were used as ferromagnetic contacts. The work function of Py is 4.83 eV19. To understand contact behavior of Py/MoS2 contacts, I-V characteristics were studied with temperature and back gate voltages as controlling parameters. The SBH of Py/MoS2 contacts was determined to be +28.8 meV at the zero-gate voltage. Such contacts with low SBH and ohmic nature can play a key role in future spin-based devices because the tuning of the SBH allows circumventing the conductance mismatch problem for injecting spins in semiconductors20.

Experimental Details

The MoS2 is grown on the thermally oxidized SiO2/n+-Si substrate with SiO2 thickness of ~285 nm via salt-assisted CVD technique (please see ref.21,22 for the growth procedure). The monolayer crystal grown by this method were found to be almost free from defects/vacancies and any unintentional doping with alkali and/or halogen atoms22. Moreover, the sharp photoluminescence spectrum and the largest mobilities observed for these samples confirm its excellent crystal quality over exfoliated and CVD grown samples22,23. Figure 1(a) shows an optical microscope image of as-grown monolayer MoS2, which was processed in different channels shown in Fig. 1(b). Afterwards, Py electrodes of thickness 20 nm were deposited on the top of MoS2 channels, capped by Ti(3 nm)/Au(50 nm) as shown in Fig. 1(c). The monolayer MoS2 was confirmed by the Raman spectroscopy with a laser light of wavelength of 488 nm. Only monolayer MoS2 was processed for the fabrication of FET devices. Figure 1(d) shows Raman spectra for MoS2 samples. The two prominent peaks in the Raman spectra appear due to an in-plane (E2g) mode located around 382.2 cm−1 and an out-of-plane (A1g) mode located around 398.2 cm−1. The difference between these two modes is found to be ~18 cm−1, which confirms the monolayer MoS224. The MoS2 channels [Fig. 1(b)] of length (l) 6–8 µm and width (w) 2–4 µm have been prepared by electron-beam (EB) lithography using TGMR resist (negative tone). In the first step, the MoS2 channels were covered by TGMR resist and unwanted MoS2 is etched by O2 plasma etching for 20 seconds. In the next step, TGMR resist was lifted by wet etching using N-Methyl-2-pyrrolidone and then cleaned by acetone and isopropyl alcohol (IPA). Py(20 nm)/Ti(3 nm)/Au(50 nm) electrodes were deposited by EB deposition technique after patterning them by EB lithography using Poly(methyl methacrylate) (PMMA) resist. The center-to-center distance between two electrodes was 0.65 µm. The electrical measurements have been performed using helium-free cryostat in the temperature range 10–300 K. The gate voltage was applied from the backside of the Si substrate.

Figure 1
figure 1

Optical microscope images of monolayer MoS2—(a) As grown MoS2 (triangular shaped), (b) MoS2 channels of different dimensions after processing (rectangular shaped) and (c) MoS2 device with permalloy (Py) electrodes. (d) Raman spectra of monolayer MoS2 performed with laser light of wavelength of 488 nm. (e) The schematic of a FET device and measurement configuration for two-probe source-drain current-voltage (IDS-VDS) curves.

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Results and Discussion

Figure 1(e) shows the schematic for FET device and measurement configuration. Source-drain current-voltage (IDS-VDS) characteristics were performed by applying DC voltage and recording the current between two probes. We measured five devices (device #A1, #A2, #B1, #B2 and #C1) from three different samples (A, B and C) to check the consistency of our experimental results. In the main text, we mainly discuss FET characteristics of device #A1 while results of other devices are used as supporting information and are shown in Supplementary Material.

To understand the electrical behavior of the Py/MoS2 contacts, we carried out systematic two-probe IDS-VDS curves measurements as a function of Vg and temperature for device #A1, and the results are shown in Fig. 2. Figure 2(a) shows IDS-VDS characteristics at 300 K as a function Vg. The IDS-VDS curves are linear and symmetric under a small range (±0.1V) of VDS (inset figure), however show small deviation from linearity and asymmetric behavior under high range (±1V) of VDS (main figure). IDS-VDS curves measured for device #A2 are almost linear and symmetric even under high range (±1V) of VDS (see Supplementary Fig. S1), which reflects better device quality (we note that this device was unfortunately broken during the temperature dependent measurements). It can also be noted from Fig. 2(a) that IDS increases with increasing Vg—for a fixed applied VDS, at Vg = 20 V, obtained IDS is at least 15 times higher than that of Vg = 0 V. It shows the tunability of FET characteristics with the application of Vg and suggests that the Schottky barrier is modified at the Py/MoS2 interface, which can result in the reduction of the SBH. As temperature is lowered, IDS-VDS characteristics show strong deviation from linearity and significant reduction in IDS, as can be seen in Fig. 2(b). This suggests that the device goes in the off state at low temperatures.

Figure 2
figure 2

Source-drain current-voltage (IDS-VDS) characteristics of a MoS2-based FET (a) at T = 300 K as a function of back gate voltage (Vg), (b) at T = 10 K as a function of Vg, (c) at Vg = 0 V as a function of temperature, and (d) at Vg = 20 V as a function of temperature. The inset in (a) shows IDS-VDS characteristics under VDS of range ±0.1V. The arrow in (c,d) shows the direction of increase of IDS with increasing temperature.

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Figure 2(c,d) show IDS-VDS characteristics as a function of temperature at Vg of 0 V and 20 V, respectively. The arrows in these figures show the direction of increase of IDS with increasing temperature. At fixed temperature and VDS, the IDS is higher at Vg = 20 V than Vg = 0 V. Indeed, the Schottky barrier is lowered at 20 V, therefore more carriers can be thermally activated and overcome the barrier.

SBH can be extracted using an activation energy method, the commonly used one. The advantage of the activation energy method is that we do not need information of electrically active area under the contacts to extract the SBH25. The SBH for 2D materials can be extracted by employing the 2D thermionic emission equation26,27,

$${I}_{DS}=A{A}^{\ast }{T}^{\frac{3}{2}}\exp [-\frac{q}{{k}_{B}T}({\varphi }_{B}-\frac{{V}_{DS}}{n})],$$
(1)

where A is the contact surface area, A* is the effective Richardson constant, q is the electronic charge, kB is the Boltzmann constant, ϕB is the Schottky barrier height and VDS is source-drain voltage, and n is the ideality factor. From above equation, the activation energy is given by \({E}_{A}=q({\varphi }_{B}-\frac{{V}_{DS}}{n})\). After rearranging few terms, the equation can be written as

$$ln({I}_{DS}/{T}^{\frac{3}{2}})=\,\mathrm{ln}\,A+ln{A}^{\ast }-\frac{{E}_{A}}{{k}_{B}}(\frac{1}{T})$$
(2)

From Eq. (2), it is clear that EA can be estimated from the slope of \(ln({I}_{DS}/{T}^{\frac{3}{2}})\) vs. 1/T, called the Arrhenius plot. Once EA is estimated, the SBH (ϕB) can be extracted by simply taking the intercept of EA vs. VDS plot, which takes into account an effect of band bending of MoS2 by an application of the source-drain voltage.

IDS-VDS curves recorded in high temperature range (290 −190 K) were employed to calculate the SBH. At low temperatures, the device remains in off state because the thermal energy supplied to carriers is not enough to overcome the barrier and therefore the thermionic emission theory cannot be applied successfully. Figure 3(a–c) show Arrhenius plots for various VDS at different Vg. To determine the slope of the plot, the experimental data were fitted by Eq. (2) as shown in Fig. 3 as solid black lines. It is worth to note that the slope of the Arrhenius plot for Vg = 0 V is negative [Fig. 3(a)] and changes its sign from negative to positive with the application of Vg [Fig. 3(b,c)]. This indicates that the SBH decreases from a positive value (at Vg = 0 V) to negative values for Vg = 10 V and 20 V.

Figure 3
figure 3

Arrhenius plots, \(ln({I}_{DS}/{T}^{\frac{3}{2}})\) vs. 1000/T as a function of source-drain voltage (VDS) for different back gate voltages, (a) Vg = 0 V, (b) Vg = 10 V, and (c) Vg = 20 V. The solid black line is a linear fit to Arrhenius plot to extract the slope.

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The slope of the Arrhenius plot as a function of VDS is depicted in Fig. 4 for different Vg and fitted by linear function. The intercept from the linear fit gives the value of SBH, which is found to be +34.5 meV for Vg = 0 V. The SBH estimated for Vg = 10 and 20 V from the intercept in Fig. 4(b,c) are found to be −6.8 and −3.2 meV, respectively. To confirm the reproducibility of our results, we also estimated SBH for other FET devices fabricated on different samples. The SBHs estimated for devices #B1, #B2 and #C1 were found to be +26.0, +24.6 and 30.1 meV, respectively (see Supplementary Material), which strengthens the central claim of this study. It is quite important to estimate the SBH in TMD-based devices for future spintronic applications, because formation of the Schottky barrier strongly hinders efficient spin injection and spin detection. This is the reason we have been focusing on the SBH of the Py/MoS2 by modulating the source-drain and gate voltages, and indeed, it is significant that we have clarified appearance of the approximately zero barrier height. Meanwhile, in a fundamental point of view, it is also important to estimate SBH at flat band gate voltage. Hence, we fabricated device #C1 and measured both the zero- gate voltage and the flat-band SBH in the same device. As can be noted from Fig. S4 in the supplementary Material that the zero- gate voltage and flat band SBHs are 30.1 and 21.8 meV, respectively. The observed SBH value of +28.8 meV (average value) at Vg = 0 V is 64% lower than the previously reported SBH in Py/MoS2 contacts16. The small zero gate bias SBH observed in our devices indicates the ohmic nature of Py/MoS2 contacts. In the previous reports the SBH for ferromagnetic contacts such as Py/MoS2 and Co/MoS2 was reported to be 80.2 and 60.6 meV, where in the first case, the authors fabricated FET devices after transferring CVD grown MoS2 to SiO2/Si substrate via PMMA stamping method16 and in the latter case, the FET devices were fabricated using the MoS2 channels exfoliated from bulk MoS2 crystals13. The values of SBH estimated in previous reports for various FM and non-magnetic (NM) metal contacts on monolayer MoS2 channel are compared in Table 1. From the Table 1, it is clear that the observed SBH in our study is the smallest value reported so-far in any direct FM (non-magnetic)/monolayer MoS2 contact. The best value reported previously for direct metal/monolayer MoS2 contact, patterned by EB lithography was ~38 meV12,28. In fact, the SBH becomes small for transferred Ag electrodes with multilayer MoS2 channels29. However, multilayer MoS2 is not a direct band-gap semiconductor and spin-valley locking effect is somewhat suppressed30. Since one of the purposes of our study is exploring a potential of combination of ferromagnet and monolayer TMDs in spintronics viewpoints, realization of low SBH in Py/monolayer MoS2 is crucial, although the low SBH formation to a multilayer TMD is also notable. It can be noted that MoS2 channel used in previous reports were either exfoliated and/or transferred from CVD grown MoS2. This implies that FET devices fabricated using MoS2 directly grown on substrate via CVD technique decreases the chance of introducing distortion-induced defects during the exfoliation or the transferring methods and/or that of surface contamination, unlike exfoliated and transferred MoS2 techniques. It is worth to recall that in our case MoS2 is grown by salt-assisted CVD technique, which shows excellent crystal quality over exfoliated and CVD MoS2 as demonstrated by the smallest SBH reported so-far in exfoliated and/or CVD MoS2. High performance devices fabricated on directly grown CVD MoS2 confirms the possibility of mass production of MoS2-based devices. As aforementioned, circumventing the formation of the Schottky barrier is quite significant to realize efficient spin injection and detection in FM/semiconductor heterostructure, our results demonstrate the importance of an integration of directly grown MoS2 channels.

Figure 4
figure 4

Source-drain voltage (VDS) dependence of slopes (−EA/1000kB) extracted from Fig. 3 for (a) back gate voltages, Vg = 0 V, (b) Vg = 10 V and (c) Vg = 20 V. The solid red line is a linear fit to the experimental data to extract the intercept and therefore Schottky barrier height (ϕB). (d) Vg dependence of Schottky barrier height, showing positive value for Vg = 0 V and negative values for Vg = 10 and 20 V.

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Table 1 Comparison of Schottky barrier heights (SBHs) for direct metal (magnetic and non-magnetic) contact on monolayer MoS2 channel.
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Conclusions

In conclusion, we fabricated FET devices using directly grown monolayer MoS2 channels on SiO2/Si substrate via salt-assisted CVD technique. The electrical properties of FET devices were studied by measuring two-probe I-V characteristic as a function of temperature and back gate voltage. The SBH estimated at Vg = 0 is found to be +28.8 meV, which is the smallest SBH reported so-far for any direct ferromagnetic as well as non-magnetic metal contact on monolayer MoS2. The small zero- gate voltage SBH indicates ohmic Py/MoS2 contacts. Ferromagnetic contacts with the smallest SBH and controllable ohmic contacts studied in the present study can open a route for practical realization of high performance MoS2 based spintronic devices.