The Effect of a Vacuum Environment on the Electrical Properties of a MoS 2 Back-Gate Field Effect Transistor

: Adsorption of gas molecules on the surface of two-dimensional (2D) molybdenum disulﬁde (MoS 2 ) 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 MoS 2 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 MoS 2 ﬁeld effect transistor increased from 1.4 × 10 6 to 1.8 × 10 7 . In addition, the values of ﬁeld effect carrier mobility were increased by more than four times, from 1 cm 2 /Vs to 4.2 cm 2 /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.


Introduction
Molybdenum disulfide (MoS 2 ), a layered semiconductor material, has attracted much attention due to its excellent electrical, mechanical and optical properties.The monolayer MoS 2 is found to have a direct bandgap of 1.8 eV.The field-effect transistor using the monolayer MoS 2 exhibits a high on/off ratio (~10 8 ), low subthreshold swing (SS, ~70 mV per decade) and considerable field effect carrier mobility (~200 cm 2 /V•s) [1][2][3].Additionally, the MoS 2 has an advantage over traditional silicon: the thickness of monolayer MoS 2 is thinner than the 2 nm of conventional silicon film and the dielectric constant of MoS 2 (ε = 7) is smaller than silicon (ε = 11.9).It is implied that using a monolayer of MoS 2 could reduce the "short channel effect" [4][5][6].Moreover, considering its good compatibility with the silicon based complementary metal-oxide-semiconductor (CMOS) process, MoS 2 is recognized by researchers as a promising candidate for channel materials in the post-Moore era [7].
However, the electrical properties of MoS 2 are sensitive to adsorbents in the surrounding environment, which is attributed to its atomic thin layer.The monolayer MoS 2 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 MoS 2 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 MoS 2 field effect transistor (FET).Benefiting from the reduced oxygen/water adsorbed molecules on the MoS 2 surface in the vacuum pumping process, the on/off ratio is increased about one order magnitude larger than the same MoS 2 transistor in an ambient atmosphere.The field effect carrier mobility (µ) of a MoS 2 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 MoS 2 devices in the future.

Experimental Section
The monolayer MoS 2 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. MoO 3 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 MoS 2 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 MoS 2 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 MoS 2 layer from the sapphire.The PMMA/MoS 2 sample was then cleaned in deionized water and transferred onto a Si/SiO 2 substrate.Following this, the sample was dried in an ambient atmosphere for several hours.To complete the wet-transfer process of MoS 2 , the PMMA layer was removed by warm acetone, followed by rinsing with isopropyl alcohol (IPA) and drying with N 2 gas [12].For the electrical measurement, the back-gate MoS 2 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 MoS 2 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 MoS 2 FET.
Crystals 2023, 13, x FOR PEER REVIEW 2 of 10 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.

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 MoO3-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.

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.

Results and Discussion
Firstly, the electrical properties of the back-gate MoS 2 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 MoS 2 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 H 2 O/OH molecules present in the ambient atmosphere.This method may be considered for future studies of the MoS 2 FET. Figure 2a shows the transfer characteristic curves of the back-gate MoS 2 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 Crystals 2023, 13, 1501 3 of 10 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 MoS 2 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 MoS 2 FET decreases to 1.6 × 10 −7 A, but it is still higher than that in an initial 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 (VGS) sweeps from −40 V to 100 V.The source to drain voltage (VDS) 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 oxidationreduction 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]: 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 In an ambient atmosphere, for a back-gated molybdenum disulfide field-effect transistor, there exists an adsorbed water layer with dissolved oxygen on the SiO 2 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 oxidationreduction 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 MoS 2 and oxygen solvated in water, mediated by the redox reaction [13]: 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 MoS 2 .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 SiO 2 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 SiO 2 surface and the semiconductor channel that suppresses electron conduction.
Figure 3a shows the Raman spectra of the monolayer MoS 2 film excited by a 473 nm laser line at room temperature.The E 1 2g model and A 1g was located at 386 and 405, respectively.The E 1 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 E 1 2g and A 1g peaks was found to be 19 cm −1 , indicating the single layer nature of the MoS 2 film [15].Figure 3b shows the transfer characteristic of back-gate MoS 2 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 × 10 6 .Then, the chamber vacuum was pumped down to 10 −6 Torr and maintained for eight hours before the measurement of back-gate MoS 2 FET for the vacuum environment.As a result, the on current of the same back-gate MoS 2 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 × 10 7 .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 MoS 2 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 MoS 2 surface.Figure 3c,d show the output characteristics of the back-gate MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 devices at room temperature is the presence of charge traps between the substrate and the MoS 2 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 MoS 2 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 MoS 2 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 MoS 2 FETs as it enhances their performance and efficiency [21,22].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 (µFE) of the back-gate MoS2 FET can be extracted from the following expression: where gm is the transconductance that can be determined by d(IDS)/d(VBG), Cox is the backgate capacitance per unit area, L is the gate length and W is gate width.The value of Cox can be estimated to be 11.50 nF/cm −2 through using the equation, Cox = κε0/tox, 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 field effect carrier mobility (µ FE ) of the back-gate MoS 2 FET can be extracted from the following expression: 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 = κε 0 /t ox , where κ is the SiO 2 relative dielectric constant with value of 3.9 and ε 0 is the permittivity of free space.The Keithley 4200 semiconductor parameter analyzer was used to measure the back-gate MoS 2 FET in a non-vacuum and vacuum environment.The µ FE of the back-gate MoS 2 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 cm 2 /Vs to about 4.2 cm 2 /Vs.This obviously enhanced µ FE is explained below.When the MoS 2 layer is exposed to the ambient air, the oxygen and water molecules will be absorbed on the MoS 2 surface, especially on defects and edges.The vacancies in the plane and edges of MoS 2 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 MoS 2 FET in a large area of integrated circuits holds significant importance.MoS 2 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 MoS 2 FET also presents promising prospects in the field of biosensors and flexible electronic devices.In the realm of biosensors, MoS 2 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 MoS 2 FET with high mobility can find applications in high-speed electronic devices.For instance, in high-frequency amplifiers and communication systems, MoS 2 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.MoS 2 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 MoS 2 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 MoS 2 FET.By focusing on enhancing the mobility of MoS 2 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.
Crystals 2023, 13, x FOR PEER REVIEW 6 of 10 The µFE of the back-gate MoS2 FET derived from gm in amb ient air and vacuum are shown in Figure 4a,b, respectively.After the vacuum pumping process, the values of µFE can be increased by more than four times, from about 1 cm 2 /Vs to about 4.2 cm 2 /Vs.This obviously enhanced µFE 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 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 MoS 2 -SiO 2 interface and other material interfaces, the presence of interface states significantly impacts the performance and stability of MoS 2 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 MoS 2 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 MoS 2 FETs.The utilization of a vacuum environment effectively mitigates the adverse effects of interface states on the performance of MoS 2 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 MoS 2 FETs [26,27].
The subthreshold swing (SS) of a MoS 2 -SiO 2 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: 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/cm 2 (Cox = ε 0 ε r /d; ε r = 3.9; d = 300 nm) for the MoS 2 FET.
In the vacuum state, the presence of interface states on the surface of a MoS 2 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 MoS 2 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: Figure 5 shows the values of SS as a function of I DS for the back-gate MoS 2 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 MoS 2 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 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 subthreshold 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, MoS 2 FETs can potentially achieve higher operating frequencies and shorter switch delays.This makes them suitable for highfrequency 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, MoS 2 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 MoS 2 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].

Conclusions
In this study, we demonstrated the efficacy of vacuum treatment in preserving the electrical characteristics of back-gate MoS 2 FETs.Through the meticulous removal of oxygen and water molecules from the surface of MoS 2 , we have achieved a substantial improvement in device performance [33].Specifically, we report a remarkable increase in the switching ratio of back-gate MoS 2 devices, elevating it from 1.4 × 10 6 to an impressive 1.8 × 10 7 .Moreover, we have observed a significant enhancement in the field effect carrier mobility, soaring from approximately 1 cm 2 /Vs to an impressive 4.2 cm 2 /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 MoS 2 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].

Figure 1 .
Figure 1.(a) Optical image of the back-gate MoS2 field effect transistor; (b) Schematic diagram of the back-gate MoS2 field effect transistor.

Figure 1 .
Figure 1.(a) Optical image of the back-gate MoS 2 field effect transistor; (b) Schematic diagram of the back-gate MoS 2 field effect transistor.

Figure 2 .
Figure 2. (a) Transfer characteristic curves of MoS2 field effect transistor in an ambient atmosphere, kept at a 10 −6 Torr vacuum for 1 h, kept at a 10 −6 Torr vacuum for 8 h, and 1 h after venting the chamber.(b) Energy level diagram of the water/oxygen redox couple (left) compared to the density of states of single-layer MoS2 (right).The arrow indicates the direction of the charge transfer reaction.

Figure 2 .
Figure 2. (a) Transfer characteristic curves of MoS 2 field effect transistor in an ambient atmosphere, kept at a 10 −6 Torr vacuum for 1 h, kept at a 10 −6 Torr vacuum for 8 h, and 1 h after venting the chamber.(b) Energy level diagram of the water/oxygen redox couple (left) compared to the density of states of single-layer MoS 2 (right).The arrow indicates the direction of the charge transfer reaction.

Figure 3 .
Figure 3. (a) Raman spectra of the monolayer of MoS2.(b) Transfer characteristic curve of MoS2 field effect transistor in an ambient atmosphere and under vacuum.(c) Output characteristic curve of the MoS2 field effect transistor in an ambient atmosphere.(d) Output characteristic curve of the MoS2 field effect transistor in a vacuum.

Figure 3 .
Figure 3. (a) Raman spectra of the monolayer of MoS 2 .(b) Transfer characteristic curve of MoS 2 field effect transistor in an ambient atmosphere and under vacuum.(c) Output characteristic curve of the MoS 2 field effect transistor in an atmosphere.(d) Output characteristic curve of the MoS 2 field effect transistor in a vacuum.

Figure 4 .
Figure 4. (a) The field-effect carrier mobility of the MoS2 field effect transistor as a function of backgate voltage in an ambient atmosphere.(b) The field-effect carrier mobility of the MoS2 field effect transistor as a function of back-gate voltage in a vacuum.

Figure 4 .
Figure 4. (a) The field-effect carrier mobility of the MoS 2 field effect transistor as a function of back-gate voltage in an ambient atmosphere.(b) The field-effect carrier mobility of the MoS 2 field effect transistor as a function of back-gate voltage in a vacuum.

Figure 5 .
Figure 5.Comparison of the sub-threshold swing as a function of source-drain current for the backgate MoS2 FET in ambient air and a vacuum.

Figure 5 .
Figure 5.Comparison of the sub-threshold swing as a function of source-drain current for the back-gate MoS 2 FET in ambient air and a vacuum.