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Tuning the Threshold Voltage of MoS2 Field-Effect Transistors via Surface Treatment

Controlling the threshold voltage (Vth) of a field-effect transistor is important for realizing robust logic circuits. Here, we report a facile approach to achieve bidirectional Vth tuning of molybdenum disulfide (MoS2) field-effect transistors. By increasing and decreasing the amount of sulfur vacancies in the MoS2 surface, the Vth of MoS2 transistors can be left- and right-shifted, respectively. Transistors fabricated on perfect MoS2 flakes are found to exhibit two-fold enhancement in mobility and a very positive Vth. More importantly, our elegant hydrogen treatment is able to tune the large Vth to a small value without any performance degradation simply by reducing the atomic ratio of S:Mo slightly; in other words, creating a certain amount of sulfur vacancies in the MoS2 surface, which generate defect states in the band gap of MoS2 that mediate conduction of a MoS2 transistor in the subthreshold regime. First-principles calculations further indicate that the edge and width of defect band can be tuned according to the vacancy density. This work not only demonstrates for the first time the ease in tuning the Vth of MoS2 transistors, but also offers a process technology solution that is critical for further development of MoS2 as a mainstream electronic material.


Introduction
Among two-dimensional layered materials, transition metal dichalcogenides (TMDs) are being considered for their potential as active device materials due to their exceptional electronic and optical properties. Molybdenum disulfide (MoS2), in particular, has been extensively studied for a number of years. The use of MoS2 as the channel material in field-effect transistors (FETs) has been theoretically predicted to bestow several attractive traits that include reasonable room temperature mobility, almost ideal switching characteristics, and superior on/off current ratio (>10 10 ) due to the large direct bandgap (1.9 eV) for monolayer MoS2. [1][2][3] Nevertheless, despite all the hype about MoS2 being a promising candidate to replace silicon in such devices, performance inadequacies such as large variations in threshold voltage (Vth) for MoS2 transistors represent the present-day reality. As a reference, the Vth of silicon-based semiconductor technology at the 22 nm technology node is 0.289 V for high performance logic and 0.413 V for low operating power logic. 4 In contrast, values of Vth of back-gated MoS2 FETs being reported vary from -30 V to 40 V. [5][6][7][8][9][10][11][12][13][14][15][16] Considering the ultimate goal of implementing MoS2 in future electronic devices, the ability to adjust the Vth of MoS2 FETs to the desired level is of paramount importance.
Compared to conventional silicon-based transistors based on 3D bulk semiconductor channels, the physical origin of the Vth for 2D FETs is quite different. In the 3D case, an inversion layer forms under the gate to form a conducting channel as a result of long-range band bending within the bulk material from the substrate towards the gate. In the 2D case, the concept of band bending in the direction normal to the gate electrode does not apply; instead, a conducting channel is formed when the energies of states in MoS2 that contain majority carriers (i.e. conduction band) become aligned with the Fermi level of the source/drain electrodes. 17,18 This alignment is achieved by the gate voltage which induce shifts of the energy levels in the MoS2 channel relative to the Fermi level in the source/drain electrode, which is fixed by a constant drain-to-source voltage.
Here, we demonstrate that the Vth of MoS2 FETs can be easily tuned bi-directionally by controlling the amount of sulfur vacancies on the MoS2 surface. The sulfur vacancies (or defects) in the basal plane of MoS2 create states (i.e. defect bands) within the band gap that have been shown to mediate conduction in the subthreshold regime, 19,20 and these defect levels can be tuned according to the vacancy density. In this paper, we show that sulfur and hydrogen treatments are effective in right-and left-shifting of the Vth of MoS2 transistors, respectively. Specifically, the former treatment reduces sulfur vacancies that exist even in the intrinsic MoS2 flakes exfoliated from natural crystal and which are not uniformly distributed. 20,21 By annealing the freshly-exfoliated MoS2 flakes in a sulfur-rich environment, the dangling bonds of Mo are expected to bond readily with the sulfur atoms to reduce the density of sulfur vacancies in the MoS2 surface. On the other hand, the latter treatment creates sulfur vacancies in the basal plane of MoS2 through a gasification process: S (solid) + H2 (gas) → H2S (gas) 22 and this chemical reaction is unlikely to result in the formation of double vacancies in the MoS2 from both thermodynamic and kinetic points of view as predicted theoretically. 23 In other words, this chemical reaction does not require defects (i.e., sulfur vacancies) in the basal plane of MoS2 as initiation sites. Thus far, for most of the reported FETs fabricated on pristine MoS2 flakes, the Vth falls into the negative regime, while our FETs fabricated using sulfur-treated MoS2 flakes exhibit very positive Vth with respectable performance including high on-off current ratio (> 10 5 ) and 2-fold enhancement in electron mobility compared to that of intrinsic MoS2 flakes. More importantly, we also demonstrate that the large positive Vth of these MoS2 FETs can easily be tuned to a smaller value (in the opposite direction) without any degradation in terms of electron mobility and on/off current ratio. This work not only sheds light on the ease in tuning the Vth of MoS2 FETs, but also offers a promising approach that is compatible with silicon CMOS fabrication technology for threshold voltage tuning in the development of MoS2 devices.

Results and Discussion
In order to demonstrate that the Vth of MoS2 transistors can be tuned by modifying the amount of sulfur vacancies, we fabricated more than 50 back-gated FETs on 2 types of exfoliated MoS2 flakes: pristine (as-exfoliated) and surface-treated. For this study, all MoS2 flakes were first exfoliated from molybdenite crystal (SPI supplies ® ) on oxidized degenerately p-doped silicon substrates with 285 nm thick SiO2. It should be noted that all MoS2 flakes selected for the FETs fabrication are of similar thickness (5 nm) and uniform width (2-3 µm) as confirmed by atomic force microscopy ( Figure S1). After that, one-third of the selected MoS2 flakes were preserved as control samples, while the remaining parts were subjected to the surface treatment of using either sulfur ( Figure S2) or hydrogen (see Experimental for details).
Subsequently, transistors were fabricated on each MoS2 flake (Figure 1a) with the same device dimensions, where the channel length and contact length are both 1 μm, and the channel width and contact width are about 2-3 μm, being the natural width of the exfoliated MoS2 flakes. Electrical measurements for all MoS2 transistors were conducted in a high vacuum chamber at room temperature. Throughout this work, we have adopted the most common Vth extraction method for semiconductor device analysis, which is the extrapolation in the linear region (ELR) method. 24 In the ELR method, the Vth is defined by the gate voltage axis intercept of the linear extrapolation of the transistor's transfer characteristic (ID -VG) curve at its maximum first derivative point, which is also the point of maximum transconductance, gm = dID/dVG. We observe significant difference in the Vth of MoS2 transistors fabricated on both pristine and surfacetreated MoS2 flakes. Figure 1b shows the average Vth of transistors fabricated on different types of MoS2 flakes and their respective on/off current ratio. As can be seen in Figure 1b, the Vth of the transistors fabricated on pristine MoS2 flakes falls in the negative regime (-15 ± 5 V) with an average on/off current ratio of 10 5 . On the other hand, the Vth of transistors fabricated on hydrogen-treated MoS2 flakes, that are foreseen to contain more sulfur vacancies compared to that of pristine MoS2 flakes, is located in a more negative regime (-30.3 ± 5.7 V) but the average on/off current ratio of this group of transistors is about an order of magnitude smaller than that of pristine MoS2 flakes, which could be attributed to the chemical reaction between the electropositive Ti electrodes and the hydrogen-treated MoS2 surface that contains many sulfur vacancies prior to formation of Ti contacts. 25 In contrast, the Vth of transistors fabricated on sulfur-treated MoS2 flakes that are foreseen to have much fewer sulfur vacancies compared to that of pristine MoS2 flakes shows not only a right-shift, but a very positive value (18.5 ± 7.5 V). Furthermore, it is worth noting that the average on/off current ratio of transistors fabricated on sulfur-treated MoS2 flakes is comparable to that of transistors fabricated on pristine MoS2 flakes (Figure 1b).
where Cox is the gate capacitance (1.21 × 10 -8 F/cm 2 for 285 nm thick SiO2), 26 Lch and Wch are channel length and width, respectively, IDS is the drain current, VDS is the drain voltage and Vg is the gate voltage. In Figure 2, we compare the transfer characteristics of a MoS2 transistor that was first fabricated on a sulfur-treated MoS2 flake and electrically tested once followed by a hydrogen post-treatment and electrical testing (the output characteristics are shown in Figure S4a) In brief, the results signify that the Vth for a finished MoS2 transistor can be tuned to a smaller value by increasing the amount of sulfur vacancies in the uncovered MoS2 channel without any degradation in terms of electron mobility and on/off current ratio.
To understand the mechanism of how the Vth of a finished MoS2 transistor can be tuned to a smaller value by increasing the amount of sulfur vacancies in the MoS2 channel, we perform density functional theory (DFT) calculations to model the essential physics in these transistors. In these calculations, we consider only bilayer MoS2 films as the band gap and qualitative band structure features do not change significantly when the number of MoS2 layers increases from two layers 28 and most of the current flows through the top few layers in a MoS2 transistor. 29 As discussed earlier, an applied gate voltage will shift the energy levels in the MoS2 channel regions, while the voltages of the source/drain electrodes are fixed according to the experiment. Here, we consider the Ti-covered-MoS2 system as the source/drain electrode in a transistor configuration, because Ti binds strongly to MoS2, with metal-induced gap states in the MoS2 band gap ( Figure S5a); the metallic screening in the Ti-covered-MoS2 system implies that a gate voltage will not affect the energy levels of this system. Due to charge redistribution, 16 The energy band diagrams in Figure 3 are obtained by extracting the DFT calculated energy levels of both Ti-covered-MoS2 electrode and MoS2 channel regions ( Figure S5). For the vacancy density considered in Figures 3a and 3b (NVs = 4.7 x 10 13 cm -2 ), the transistor is in the off-state at Vg = 0, and the defect levels in MoS2 channel must be shifted down to align with the Fermi level in the source/drain electrodes, i.e. Vth > 0. Comparing cases 1 and 2 in Figure 3c, we see that the defect band edge shifts down with increasing density of sulfur vacancies NVs, implying that as long as the transistor is still in the offstate at Vg = 0 (i.e. the Fermi level in the source/drain electrodes is below the defect level), ΔE is smaller for larger vacancy densities, which means smaller positive Vth is required, in good agreement with experiment ( Figure 1b).
Although ΔE is predicted to be only 0.1 and 0.2 eV for cases 1 and 2 respectively, the change in ΔE with vacancy density is robust. This trend is consistent across different vacancy densities as summarized in Table S1. Furthermore, the values of ΔE as well as the corresponding differences are 10 times larger than the precision in our calculations; in particular, we have also verified that ΔE and other relevant energy levels do not change with higher kinetic energy cutoffs and denser Brillouin Zone samplings, indicating that these values are converged in our calculations (Table S2 and Figure S6). In addition, the Our measurements on the group of S+H transistors give a Vth of -2 ± 5 V (as indicated in Figure 1b), meaning that the Vth is very small, and can sometimes be positive or negative depending on which transistor is being measured. The small positive Vth corresponds to a case where ΔE as discussed above is extremely small, consistent with a vacancy density NVs similar to or slightly larger than 13 x 10 13 cm -2 (case 1). When Vth is small and negative, the transistor is already in the on-state when Vg = 0 (e.g. see the red curve in Figure 2a for S+H), indicating that the Fermi level in the source/drain electrodes is already in the defect band, allowing conduction in this subthreshold regime. A positive gate voltage further increases the current in these n-type transistors, while the small magnitude of Vth indicates that this Fermi level is close to the defect band edge. We note that for all the different vacancy densities considered in our calculations, the defect band is centered closer to the conduction band (Table S1) The mechanism that we discuss here on how the Vth of a MoS2 transistor changes with vacancy density is consistent with a recent study that varies the vacancy density in MoS2 monolayer films by controlling the chemical vapour deposition (CVD) growth process. 35 Here, we show that the vacancy density in mechanically exfoliated MoS2 can also be easily controlled to tune the Vth using CMOS-compatible processes. Importantly, the room temperature hydrogen treatment proposed here can be used to fine-tune the Vth of any finished MoS2 transistor (including those made with CVD-grown MoS2 films), without degradation in performance.
The photoluminescence (PL) intensity of a MoS2 flake with uniform thickness ought to be significantly higher wherever there are sulfur vacancies in the basal plane of MoS2 due to the suppression of nonradiative recombination of excitons at the sulfur vacancies. 36 Through a series of PL analyses, we have verified that the sulfur and hydrogen treatments reduce and increase the amount of sulfur vacancies in the basal plane of MoS2, respectively. Figure 4a shows the optical image of a piece of freshly-exfoliated (pristine) MoS2 flake while Figure 4b shows the respective PL intensity map for the region marked in Figure 4a. Subsequently, the sulfur treatment was performed on the sample and Figure 4c shows the corresponding PL intensity map. After that, the sample was subjected to hydrogen treatment and its corresponding PL intensity map is shown in Figure 4d. It should be noted that all the PL intensity maps in Figure 4 share the same color intensity bar and as such, the intensity level measured is a representative indicator of the defect density throughout the MoS2 surface after undergoing different processes. As expected, the PL intensity of the MoS2 flake is more uniform after the sulfur treatment compared to when it was as-exfoliated. As shown in Figure 4c, the PL intensity is almost uniform throughout the MoS2 flake, As mentioned earlier, our sulfur treatment involves annealing the MoS2 sample at ~435 °C for 2 h which is expected to repair the sulfur vacancies in the MoS2 through bonding of sulfur atoms to the dangling Mo bonds. It is important to note that the annealing process must be conducted in a sulfur-rich environment, and failure to do so would result in the introduction of more rather than fewer sulfur vacancies. In Figure   5, we demonstrate the impact of annealing the MoS2 sample without sulfur vapor. A sample (exfoliated MoS2 flakes on a p + Si / SiO2 substrate) was first sulfur-treated. Its optical image and PL intensity map over the energy range of 1.7 to 2.0 eV are shown in Figure 5a. Subsequently, the sample was annealed using the same experimental setup and conditions as the sulfur treatment except that no sulfur powder was loaded and no Ar gas was introduced. In other words, the sample in this case was simply annealed at ~435 °C for 2 h in vacuum (2 x 10 -4 mbar). The few-layer MoS2 flake shows uniform PL intensity ( Figure   5a) and weak PL signal (Position 1 in Figure 5c) after being sulfur-treated similar to the case shown in acting on each ion is less than 0.01 eV/Å. For the Ti-covered-MoS2 system, 4 layers of Ti atoms are used and the lattice constants are set to that of the optimized bilayer MoS2 system. In the self-consistent calculation, the convergence threshold for energy is set to 10 −5 eV.
PL analysis. For this study, all PL measurements were conducted using a WITec alpha 300R confocal Raman system with a 532 nm laser excitation source and a laser spot size of ~320 nm was used. The laser power at the sample was kept below 0.1 mW to avoid laser-induced damage at the sample. 39 For PL mappings, the sample was placed on a piezostage and scanned with a step size of 100 nm, unless otherwise specified.

Conclusion
We have introduced two facile approaches, namely sulfur treatment and hydrogen treatment, which allow one to fine-tune the threshold voltage of MoS2 FETs while being Si-CMOS process compatible at the same time. Using a sulfur treatment process to reduce the number of sulfur vacancies on as-exfoliated MoS2, we demonstrate that the Vth of fabricated FETs can be shifted into the positive regime. On the other hand, left shifting of the Vth by populating the exposed MoS2 channel with sulfur vacancies can be achieved by a hydrogen treatment. In addition, we show that not only could Vth be tuned in both directions, the treatment is also able to marginally improve the intrinsic electrical properties of the MoS2, with the treated FET exhibiting respectable electrical performance -high on-off current ratio (> 10 5 ) and 2-fold enhancement in electron mobility; the performance does not degrade even after multiple treatments.
Hence as such, our work provides a means to adjust the critical threshold voltage parameter needed in the development of MoS2 as a channel material for next generation transistors.    Table S1. We note that for negative Vth, the device is already in the on-state at Vg = 0 (see red curve in Figure 2a).         As mentioned in the main text, the proposed sulfur and hydrogen treatments are expected to repair and create sulfur vacancies in the basal plane of MoS2, respectively. Here, we attest the hypothesis by investigating stoichiometry changes of the MoS2 prior to and following the proposed treatments through an X-ray photoelectron spectroscopy (XPS) study. A piece of bulk MoS2 flake was first cleaved from molybdenite crystal (SPI supplies ® ) and its XPS spectrum is shown in Figure S9 (labelled as P). It should be noted that all XPS spectra in this study were obtained by exciting the MoS2 sample with an Mg Kα source (XR 50, SPECS GmbH), and detected with a PHOIBOS 150 Hemispherical Energy Analyzer equipped with a Delay Line Detector (SPECS GmbH) at a pass energy of 30 eV. Subsequently, the MoS2 sample was subjected to the sulfur treatment and its XPS spectrum is plotted in Figure S9 too (labelled as S). The same MoS2 sample was then exposed to the hydrogen treatment and its XPS spectrum is also included in Figure S9 (labelled as S+H) for comparison purposes. As can be seen in Figure S9, the Mo 3d and S 2p XPS spectra of the MoS2 sample show slight changes after the sulfur treatment followed by the hydrogen treatment. Table S3 summarizes the stoichiometry (atomic ratio of Mo:S) of the same MoS2 sample when it was pristine (P), sulfur-treated (S), and following the hydrogen treatment (S + H), which were identified through components'peak fitting (an example is shown in Figure S10). As expected, for the same MoS2 sample, the stoichiometry changes from 1.89 to 1.96 after the sulfur treatment, which indicates a reduction of sulfur vacancies in the MoS2 surface, and more importantly, stoichiometry of the same MoS2 sample changes to 1.90 after the hydrogen treatment, which signifies an increase in sulfur vacancies in the MoS2 surface. Overall, the results agree well with our hypothesis that the proposed sulfur / hydrogen treatment reduces / increases the amount of sulfur vacancies in the basal plane of MoS2.