The Effect of a Vacuum Environment on the Electrical Properties of a MoS₂ Back-Gate Field Effect Transistor

Abstract

Adsorption of gas molecules on the surface of two-dimensional (2D) molybdenum disulfide (MoS₂) can significantly affect its carrier transport properties. In this letter, we investigated the effect of a vacuum environment on the electrical properties of a back-gate MoS₂ FET. Benefiting from the reduced scattering centers caused by the adsorbed oxygen and water molecules in a vacuum, the current I_on/I_off ratio of back-gate MoS₂ field effect transistor increased from 1.4 × 10⁶ to 1.8 × 10⁷. In addition, the values of field effect carrier mobility were increased by more than four times, from 1 cm²/Vs to 4.2 cm²/Vs. Furthermore, the values of subthreshold swing could be decreased by 30% compared with the sample in ambient air. We demonstrate that the vacuum process can effectively remove absorbates and improve device performances.

1. Introduction

Molybdenum disulfide (MoS₂), a layered semiconductor material, has attracted much attention due to its excellent electrical, mechanical and optical properties. The monolayer MoS₂ is found to have a direct bandgap of 1.8 eV. The field-effect transistor using the monolayer MoS₂ exhibits a high on/off ratio (~10⁸), low subthreshold swing (SS, ~70 mV per decade) and considerable field effect carrier mobility (~200 cm²/V·s) [1,2,3]. Additionally, the MoS₂ has an advantage over traditional silicon: the thickness of monolayer MoS₂ is thinner than the 2 nm of conventional silicon film and the dielectric constant of MoS₂ (ε = 7) is smaller than silicon (ε = 11.9). It is implied that using a monolayer of MoS₂ could reduce the “short channel effect” [4,5,6]. Moreover, considering its good compatibility with the silicon based complementary metal-oxide-semiconductor (CMOS) process, MoS₂ is recognized by researchers as a promising candidate for channel materials in the post-Moore era [7].

However, the electrical properties of MoS₂ are sensitive to adsorbents in the surrounding environment, which is attributed to its atomic thin layer. The monolayer MoS₂ is a strictly two-dimensional material, and as such, has its whole volume exposed to surface adsorbates, which maximizes their effect. Hence, the electrical properties of a MoS₂ transistor can be modulated by the charge transfer between the channel material and adsorbate molecules (oxygen/water) in ambient atmosphere [8,9,10,11]. In this letter, we investigate the effect of a vacuum environment on the electrical properties of a back-gate MoS₂ field effect transistor (FET). Benefiting from the reduced oxygen/water adsorbed molecules on the MoS₂ surface in the vacuum pumping process, the on/off ratio is increased about one order magnitude larger than the same MoS₂ transistor in an ambient atmosphere. The field effect carrier mobility (µ) of a MoS₂ transistor in a vacuum environment is three times as large as that in an ambient atmosphere. The corresponding subthreshold swing (SS) is reduced by about 30% compared with that in an ambient atmosphere. Our study provides a theoretical and experimental basis for reducing the interfacial adsorption effect of MoS₂ devices in the future.

2. Experimental Section

The monolayer MoS₂ triangular islands were grown inside a multitemperature-zone tubular furnace. Sulfur powder was placed outside the hot zone and was mildly sublimated with heating belts at 100 °C. MoO₃ powder and sapphire (0001) substrates with no pretreatments were placed in the hot center inside the tube furnace. Argon was used as the carrier gas to convey MoO_3-x vapor species to the downstream sapphire substrates. The tube was pumped down to a base pressure of 1 Pa and flushed with Ar carrier gas repeatedly to guarantee a favorable growth atmosphere. The as-grown MoS₂ film sample was transferred to a heavily P-doped silicon substrate with a 300 nm oxide layer by using a common wet-transfer process. Initially, the polymethylmethacrylate (PMMA) was spin coated on the as-grown MoS₂ film sample. The sample was then heated on a hot plate at 100 °C for 3 min to ensure complete curing of the PMMA. Subsequently, the sample was immersed in a potassium hydroxide (KOH) solution to detach the MoS₂ layer from the sapphire. The PMMA/MoS₂ sample was then cleaned in deionized water and transferred onto a Si/SiO₂ substrate. Following this, the sample was dried in an ambient atmosphere for several hours. To complete the wet-transfer process of MoS₂, the PMMA layer was removed by warm acetone, followed by rinsing with isopropyl alcohol (IPA) and drying with N₂ gas [12]. For the electrical measurement, the back-gate MoS₂ FET was placed in the Lakeshore probe station and analysis conducted by the Agilent semiconductor analyzer B5100. The source-drain contact electrode (20 nm Ti//80 nm Au) was fabricated by electron-beam lithography, electron-beam evaporation and a lift-off process. The back gate MoS₂ field-effect transistor (FET) with a channel length of 7.7 μm and a channel width of 30 μm was fabricated (seen in the Figure 1a). Figure 1b shows the schematic diagram of the back-gate MoS₂ FET.

3. Results and Discussion

Firstly, the electrical properties of the back-gate MoS₂ FET were measured in an ambient atmosphere. Then, the chamber was evacuated to a vacuum level of 10⁻⁶ Torr and kept for eight hours before measuring the back-gate MoS₂ FET in a vacuum environment. This extended vacuum process ensures the removal of surface adsorbents induced by the ambient atmosphere. It is important to note that the vacuum pumping process in our study does not involve any heating. However, we acknowledge that heating can be an effective method for desorbing H₂O/OH molecules present in the ambient atmosphere. This method may be considered for future studies of the MoS₂ FET. Figure 2a shows the transfer characteristic curves of the back-gate MoS₂ FET for an ambient atmosphere, kept in a 10⁻⁶ Torr vacuum for 1 h, kept in a 10⁻⁶ Torr vacuum for 8 h, and 1 h after venting the chamber. The back gate voltage (V_GS) sweeps from −40 V to 100 V. The source to drain voltage (V_DS) is fixed at 0.5 V. The on current of the back-gate MoS₂ FET increases from 1.45 × 10⁻⁷ A to 1.27 × 10⁻⁶ A, after pumping the vacuum down to 10⁻⁶ Torr from the ambient atmosphere and maintaining this for 1 h. Furthermore, it increases to 1.36 × 10⁻⁶ A after maintaining 10⁻⁶ Torr for 8 h. When the chamber is vented and kept in an ambient atmosphere for 1 h, the on current of the back-gate MoS₂ FET decreases to 1.6 × 10⁻⁷ A, but it is still higher than that in an initial ambient atmosphere.

In an ambient atmosphere, for a back-gated molybdenum disulfide field-effect transistor, there exists an adsorbed water layer with dissolved oxygen on the SiO₂ surface, which leads to the suppression of electron conduction. This is caused by the electrochemical charge transfer reaction between the semiconductor channel and the dissolved oxygen in water. Charge transfer in the oxygen/water environment is achieved through electron transfer between the semiconductor channel and the oxygen electrochemical oxidation-reduction pair in water. This electron transfer process involves the transfer of electrons from the semiconductor channel to the oxygen in water, forming negatively charged chemical intermediates, subsequently leading to the generation and charging of acceptor states on the substrate surface. This charge transfer process may alter the electronic properties of many organic and inorganic semiconductors in humid air. Therefore, the charge transfer of the oxygen/water oxidation–reduction pair is an important phenomenon in charge transport in the oxygen/water environment.

The aforementioned process is a result of spontaneous electron transfer between a MoS₂ and oxygen solvated in water, mediated by the redox reaction [13]:

$$O_2+{4H}^{+}+4e^{-}\rightleftharpoons 2H_{2}O$$

(1)

Figure 2b illustrates the energy level diagram of the water/oxygen redox couple on the left, while on the right it presents the density of states of single-layer MoS₂. The chemical potential of this oxygen/water layer is located between −5.66 to −4.83 eV relative to the vacuum level, depending on the pH of the solution (ranging from 0 to 14). Charge transfer will continue to occur before reaching equilibrium, and the Fermi level of the semiconductor will align with the electrochemical potential of the oxygen/water layer. Carla M and others explicitly mention the possibility of electron transfer in two-dimensional materials exposed to moist air. Electrons located at the top of the valence band (−5.66 to −4.83 eV) will easily transfer from the semiconductor molybdenum disulfide to the oxygen/water layer due to their proximity to the adsorbed oxygen’s oxidation–reduction potential on the SiO₂ surface [14]. This electrochemical charge transfer process may generate negative chemical intermediates, such as peroxides, which, in turn, can induce charged acceptor states on the substrate surface. It is the electrochemical charge transfer reaction occurring between the dissolved oxygen in the adsorbed water layer on the SiO₂ surface and the semiconductor channel that suppresses electron conduction.

Figure 3a shows the Raman spectra of the monolayer MoS₂ film excited by a 473 nm laser line at room temperature. The E¹_2g model and A_1g was located at 386 and 405, respectively. The E¹_2g model arises from the vibration of an S–Mo–S layer against the adjacent layer. The A_1g model is associated with the out-of-plane vibration of only S atoms in opposite directions. The separation of E¹_2g and A_1g peaks was found to be 19 cm⁻¹, indicating the single layer nature of the MoS₂ film [15]. Figure 3b shows the transfer characteristic of back-gate MoS₂ FET in ambient atmosphere. The back-gate voltage (V_BG) scans from −40 V to 100 V in 1 V steps. The source-drain voltage (V_DS) is swept from 0.5 V to 2.5 V with a step of 0.5 V. The drain current (I_DS) increases with the increasing V_BG, indicating the n-type transistor behavior. The current on/off ratio is defined as the ratio of the current in the on-state to the current in the off-state for a field-effect transistor (FET). It is a crucial parameter for evaluating the performance of FETs. Enhancing the current on/off ratio has the potential to significantly improve the overall device performance and efficiency. A higher current on/off ratio can effectively reduce the leakage current in the off-state, thereby minimizing power consumption [16]. For the V_DS = 2.5 V, the on current (I_on) and off current (I_off) are observed at 7.4 × 10⁻⁷ A and 5.1 × 10⁻¹³ A, respectively. Additionally, the I_on/I_off ratio is determined as 1.4 × 10⁶. Then, the chamber vacuum was pumped down to 10⁻⁶ Torr and maintained for eight hours before the measurement of back-gate MoS₂ FET for the vacuum environment. As a result, the on current of the same back-gate MoS₂ FET can be increased to 6.3 × 10⁻⁶ A. Considering the I_off is 3.5 × 10⁻¹³ A, the I_on/I_off can be improved to 1.8 × 10⁷. The increasing drain current and on/off ratio can be attributed to the desorption of oxygen and water molecules benefiting from the vacuum environment. In addition, the threshold-voltage shows a positive shift under the vacuum environment, indicating the electronic depletion of n-type MoS₂ in an ambient atmosphere. This depletion can be owing to the electron transferred from the two-dimensional semiconductor to the aqueous oxygen redox couple originating from the oxygen/water layer on the MoS₂ surface. Figure 3c,d show the output characteristics of the back-gate MoS₂ FET in ambient air and vacuum, respectively. The near linear I_DS-V_DS curve at low bias and observed I_DS variation for different values of V_BG indicate that the field-effect behavior of our transistor is dominated by the MoS₂ channel and not the parasitic resistance [17].

The mobility of charge carriers is a crucial parameter in FETs as it determines their speed of movement in materials under the influence of an electric field. In the case of MoS₂ FETs, mobility refers to the rate at which electrons or holes move within the material. Higher mobility indicates that charge carriers can move faster, resulting in improved switching characteristics and increased current flow [18,19]. However, achieving high mobility is not a simple task as there are various factors that can limit the movement of charge carriers. One of the main reasons for low mobility in single-layer MoS₂ devices at room temperature is the presence of charge traps between the substrate and the MoS₂ layer. When carriers move through semiconductors, they can become trapped by these charge traps. The trapped carriers remain in the traps for a certain duration, leading to a decrease in effective carrier concentration and mobility [20]. Additionally, when the MoS₂ layer is exposed to ambient air, molecules of oxygen and water can adsorb onto its surface, resulting in an increase in the number of charge traps. In a vacuum environment, the surface of the MoS₂ layer is almost free from impurities, leading to a significant reduction in the density of charge traps. As a result, the mobility of carriers is higher in a vacuum since they are not limited or scattered by charge traps. Consequently, the reduction in charge traps improves the mobility of carriers, which is particularly advantageous for MoS₂ FETs as it enhances their performance and efficiency [21,22].

The field effect carrier mobility (µ_FE) of the back-gate MoS₂ FET can be extracted from the following expression:

$${\mu }_{FE}=\frac{L{g}_m}{W{C}_{ox}{V}_{DS}}$$

(2)

where g_m is the transconductance that can be determined by d(I_DS)/d(V_BG), C_ox is the back-gate capacitance per unit area, L is the gate length and W is gate width. The value of C_ox can be estimated to be 11.50 nF/cm⁻² through using the equation, C_ox = κε₀/t_ox, where κ is the SiO₂ relative dielectric constant with value of 3.9 and ε₀ is the permittivity of free space. The Keithley 4200 semiconductor parameter analyzer was used to measure the back-gate MoS₂ FET in a non-vacuum and vacuum environment.

The µ_FE of the back-gate MoS₂ FET derived from g_m in ambient air and vacuum are shown in Figure 4a,b, respectively. After the vacuum pumping process, the values of µ_FE can be increased by more than four times, from about 1 cm²/Vs to about 4.2 cm²/Vs. This obviously enhanced µ_FE is explained below. When the MoS₂ layer is exposed to the ambient air, the oxygen and water molecules will be absorbed on the MoS₂ surface, especially on defects and edges. The vacancies in the plane and edges of MoS₂ have high catalytic activity for oxygen and water chemisorption at room temperature, which could act as extra Coulomb scatters for electrons. After the vacuum process, the weakened scattering induced by partial oxygen and water molecules’ desorption will lead to the promotion of the carrier mobility [23].

The application of high mobility MoS₂ FET in a large area of integrated circuits holds significant importance. MoS₂ FET exhibit excellent electron transport performance, enabling high-speed and low-power operation, thereby enhancing the efficiency and performance of integrated circuits. The high mobility of MoS₂ FET also presents promising prospects in the field of biosensors and flexible electronic devices. In the realm of biosensors, MoS₂ FET with high mobility can serve as sensitive detectors for biochemical substances. These transistors can convert chemical signals into electrical signals, facilitating the sensitive detection of biomolecules. Such biosensors hold great potential for applications in medical diagnosis, environmental monitoring, and food safety, playing a crucial role in these areas [24]. Furthermore, a MoS₂ FET with high mobility can find applications in high-speed electronic devices. For instance, in high-frequency amplifiers and communication systems, MoS₂ FETs can exhibit fast switching characteristics and response speeds, enabling high-frequency operation and reduced switching delays. This makes them highly suitable for such applications. MoS₂ FET with high mobility also offer potential applications in flexible electronic devices. Their thin film structure and high mobility allow them to maintain their performance even under bending and stretching conditions, making them suitable for wearable devices, flexible display screens, and other related fields. These flexible electronic devices provide a more comfortable and portable user experience and have broad application prospects. High mobility plays a critical role in MoS₂ FETs. By increasing the mobility, the speed of electron movement within the transistor can be enhanced, leading to improved response speed and working frequency [25]. Additionally, high mobility reduces scattering losses, resulting in reduced energy consumption and heat generation, thus enabling low-power operation. Consequently, increasing mobility enhances the performance and efficiency of MoS₂ FET. By focusing on enhancing the mobility of MoS₂ FET, researchers and engineers can unlock their full potential in various applications, ranging from integrated circuits to biosensors and flexible electronic devices. This will contribute to advancements in technology and pave the way for future innovation in these fields.

SS refers to the relationship between the input voltage and the output current when a field effect transistor (FET) operates in the sub-threshold region, which is below the threshold voltage. It is a crucial parameter for assessing the performance of low-power electronic devices and integrated circuits. In the sub-threshold region, the switching characteristics and energy consumption of the FET are influenced by the sub-threshold swing. A smaller SS indicates that the FET can effectively control the output current with minimal input voltage variation, thereby reducing power consumption. However, when considering the MoS₂–SiO₂ interface and other material interfaces, the presence of interface states significantly impacts the performance and stability of MoS₂ FETs, particularly the stability of the SS. Interface states refer to defects or impurities formed at the material interface. These interface states increase the SS, leading to slower switching speeds and higher static power consumption. Thus, investigating the formation and control of interface states is crucial for optimizing the performance of MoS₂ FETs. In a vacuum environment, the formation of interface states is reduced due to the absence of gas molecule adsorption. This reduction in interface states leads to a decrease in the SS of MoS₂ FETs. The utilization of a vacuum environment effectively mitigates the adverse effects of interface states on the performance of MoS₂ FETs. Furthermore, the vacuum environment also reduces impurity diffusion and surface oxidation, further minimizing the number of interface states and subsequently reducing the SS. Therefore, a vacuum environment can be considered as a favorable condition for enhancing the performance of MoS₂ FETs [26,27].

The subthreshold swing (SS) of a MoS₂–SiO₂ interface is affected by the density of interface states (Dit). The presence of interface states causes an elevation in the subthreshold swing, resulting in a decrease in switching speed and an increase in static power consumption. The relationship between SS and density can be described by the following equations:

$$SS=\mathrm{l}\mathrm{n}10\times \frac{kT}{q}\times \left(1+\frac{C_{it}}{C_{OX}}\right)$$

(3)

$$D_{it}=C_{it}/q^2$$

(4)

where k is the Boltzmann’s constant, T is the measurement temperature, q is the electron charge, and Cox is the gate dielectric capacitance density which is 11.5 nF/cm² (Cox = ε₀ε_r/d; ε_r = 3.9; d = 300 nm) for the MoS₂ FET.

In the vacuum state, the presence of interface states on the surface of a MoS₂ FET is diminished, resulting in a decrease in interface state density. This phenomenon can be attributed to the interaction between interface states and the vacuum environment. In a vacuum, the formation of interface states is less influenced by the surrounding environment, leading to a reduction in their density. The decrease in interface state density has a positive impact on the performance of the MoS₂ FET. It mitigates the influence of interface traps on electron transport, thereby reducing the subthreshold swing. Consequently, the transistor exhibits enhanced sensitivity and linearity in current variation at low voltages, thereby improving its switching characteristics and current amplification ability [28]. Hence, the values of SS can be obtained from the transfer characteristics in Figure 3, according to the equation:

$$SS=\frac{\partial V_{BG}}{\partial lg{I}_{DS}}$$

(5)

Figure 5 shows the values of SS as a function of I_DS for the back-gate MoS₂ FET in ambient air and a vacuum, respectively. In the I_DS range of 10⁻¹³ A to 10⁻⁶ A, the value of SS increases with the increasing I_DS and decreases with the increasing V_DS. The minimum value of SS for the back-gate MoS₂ FET in ambient air is 400 mV/dec. However, it decreases by 30%, down to 280 mV/dec after the vacuum process. The decrease in SS can be attributed to the less interfacial states beneficiating from the partial desorption of oxygen and water molecules.

In the field of low-power electronic devices, as electronic devices become smaller and the demand for power consumption decreases, reducing the sub-threshold swing has become an important technical method. The reduction in sub-threshold swing can be achieved by reducing the threshold voltage of the transistor. When the threshold voltage of the transistor is reduced to the sub-threshold level, the current change in the transistor will be more sensitive and linear, thereby enabling the transistor to exhibit stable operating characteristics even at low voltage. This characteristic is very advantageous for the design of low-power electronic devices because it can achieve higher switching efficiency and lower power consumption at lower voltage. In addition, reducing sub-threshold swing can also effectively reduce the leakage current of the transistor, thereby improving the reliability and long-term stability of the device. Therefore, the technique of reducing sub-threshold swing has become an indispensable part of the design of low-power electronic devices, with broad application prospects and research value [29]. Similarly, in high-frequency electronic devices, fast switching characteristics and response speed are crucial. By reducing the subthreshold swing, MoS₂ FETs can potentially achieve higher operating frequencies and shorter switch delays. This makes them suitable for high-frequency electronic devices such as RF amplifiers and communication systems. Moreover, the reduction in subthreshold swing can enhance the sensitivity and accuracy of sensors [30]. By decreasing the interface state density in a vacuum environment, MoS₂ FETs may possess higher signal detection capabilities. This opens up possibilities for their application in various sensors, including optical sensors, chemical sensors, and biological sensors. In conclusion, the study and application of MoS₂ FETs in a vacuum environment hold promise for improved performance and applications in fields such as low-power electronic devices, high-frequency electronic devices, and sensor technology [31,32].

4. Conclusions

In this study, we demonstrated the efficacy of vacuum treatment in preserving the electrical characteristics of back-gate MoS₂ FETs. Through the meticulous removal of oxygen and water molecules from the surface of MoS₂, we have achieved a substantial improvement in device performance [33]. Specifically, we report a remarkable increase in the switching ratio of back-gate MoS₂ devices, elevating it from 1.4 × 10⁶ to an impressive 1.8 × 10⁷. Moreover, we have observed a significant enhancement in the field effect carrier mobility, soaring from approximately 1 cm²/Vs to an impressive 4.2 cm²/Vs. Meanwhile, we identified a 30% decrease in the sub-threshold swing value when compared to measurements made in ambient air. Overall, this study provides a theoretical and experimental basis for reducing the interfacial adsorption effect of MoS₂ devices in the future. It was demonstrated that by optimizing the performance of MoS2 field effect transistors in vacuum environments, device degradation and instability are addressed and device performance and efficiency are improved [34].