Large-Area Ultraviolet Photodetectors Based on p-Type Multilayer MoS 2 Enabled by Plasma Doping

Two-dimensional (2D) MoS2 has recently become of interest for applications in broad range photodetection due to their tunable bandgap. In order to develop 2D MoS2 photodetectors with ultrafast response and high responsivity, up-scalable techniques for realizing controlled p-type doping in MoS2 is necessary. In this paper, we demonstrate a p-type multilayer MoS2 photodetector with selective-area doping using CHF3 plasma treatment. Microscopic and spectroscopic characterization techniques, including atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), are used to investigate the morphological and electrical modification of the p-type doped MoS2 surface after CHF3 plasma treatment. Back-gated p-type MoS2 field-effect transistors (FETs) are fabricated with an on/off current ratio in the order of 103 and a field-effect mobility of 65.2 cm2V−1s−1. They exhibit gate-modulated ultraviolet photodetection with a rapid response time of 37 ms. This study provides a promising approach for the development of mild plasma-doped MoS2 as a 2D material in post-silicon electronic and optoelectronic device applications.


Introduction
Two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as MoS 2 , have attracted considerable attention owing to the unique optical and electronic properties related to its 2D ultrathin atomic layer structure [1].MoS 2 is becoming prevalent in post-silicon digital electronics and in highly efficient optoelectronics due to its extremely low thickness and its tunable band gap (E g = 1-2 eV) [2,3].Sparsely-layered MoS 2 displays light absorbing and luminescence capabilities, enabling photodetector operation [1,3].Several efforts have been made to further develop 2D TMDC photodetectors with ultrafast response and high responsivity [4], owing to the longer lifetime of their photo-generated carriers and higher photosensitivity than traditional semiconductors [5][6][7].However, 2D TMDC photodetectors fabricated with transferred van der Waals heterostructures or chemical vapor deposition-grown hybrids are typically characterized with low responsivity.This is due to the lack of a photo-gain mechanism or by resilient photoconductivity in the heterojunction structure [8].In order to realize MoS 2 -based complementary electronic circuits and optoelectronic devices, it is necessary for there to be a development of up-scalable techniques to achieve controlled doping of MoS 2 .
Recently, field-effect transistors (FETs) fabricated with MoS 2 thin films have exhibited an excellent on/off current ratio (10 6 -10 8 ) with a high carrier mobility of around 200 cm 2 V −1 s −1 , making them suitable for next-generation transistors [9].For low-power, high-performance complementary logic applications, both pand n-type MoS 2 FETs (NFETs and PFETs) must be developed.NFETs with an electron accumulation channel can be obtained using unintentionally doped n-type MoS 2 .However, the fabrication of MoS 2 FETs with complementary p-type characteristics is challenging due to the significant difficulty of injecting holes into its inversion channel [10].One approach is to use unconventional contacts resulting in a low Schottky barrier height for hole injection in MoS 2 PFETs.For example, MoO x (2 < x < 3) contacts have been shown to be effective for hole injection into pristine MoS 2 [11].A proper interface between MoO x and MoS 2 layers is necessary for efficient hole injection.Alternatively, several p-type doping approaches for MoS 2 have been established.Examples initiated doping by incorporating substitutional niobium, Nb, atoms during chemical vapor deposition (CVD) growth and chemical doping with AuCl 3 [12][13][14].However, substitutional doping during CVD growth is lacking in area selectivity and the adoption of AuCl 3 would be hampered by the risk of Au contamination.Plasma treatments with different species (including CF 4 , SF 6 , O 2 , and CHF 3 ) have also been found to achieve the desired property modifications of MoS 2 [15][16][17].In the case of multilayer MoS 2 , which is exposed to energetic F-plasma treatment, p-type doping of the exposed area has been shown.However, plasma treatment may cause significant etching of the MoS 2 , which can directly affect the feasibility of the development of MoS 2 FETs with a thin channel region.Therefore, mild plasma treatments are essential to achieve the proper modification of MoS 2 .
In this work, we demonstrated a p-type multilayer MoS 2 enabled by selective-area doping using CHF 3 plasma treatment.Compared with single layer MoS 2 , multilayer MoS 2 can carry a higher drive current due to its lower bandgap and multiple conduction channels.Moreover, it has three times the density of states at its minimum conduction band [18].Back-gated MoS 2 PFETs were presented with an on/off current ratio in the order of 10 3 and a field-effect mobility of 65.2 cm 2 V −1 s −1 .The MoS 2 PFETs photodetector exhibited ultraviolet (UV) photodetection capability with a rapid response time of 37 ms and exhibited modulation of the generated photocurrent by back-gate voltage.Microscopic and spectroscopic characterization techniques, including atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), were used to investigate the morphological and electrical modification of the MoS 2 surface after CHF 3 plasma treatment.This work suggests the potential application of the mild plasma-doped p-type multilayer MoS 2 in UV photodetectors for environmental monitoring, human health monitoring, and biological analysis.

Experimental Section
The large-area growth of MoS 2 films on 300 nm thick SiO 2 /Si substrate was carried out by thermal decomposition of ammonium tetrathiomolybdate, (NH 4 ) 2 MoS 4 (Sigma-Aldrich, St. Louis, MO, USA), in a tube furnace (Home-built).A precursor solution in 3.0 wt% was prepared by dissolving 0.292 g (NH 4 ) 2 MoS 4 in 10 mL dimethylformamide (DMF) solvent (Sigma-Aldrich, St. Louis, MO, USA).The SiO 2 /Si substrates were cleaned using a standard cleanroom protocol and subsequently treated with 100 W O 2 plasma for 5 min to improve the adhesion of the precursor solution to the substrates.After the plasma treatment, the precursor solution was promptly spin-coated onto the substrates at 1000 rpm for a period of time to form the dried (NH 4 ) 2 MoS 4 films.Afterwards, a two-step annealing process was conducted to synthesize MoS 2 films.For the first step, the temperature was set to 280 • C for 30 min in a N 2 rich environment at 1.8 Torr.This was done to transform (NH 4 ) 2 MoS 4 into MoS 3, as shown in previous literature [19].To further reduce MoS 3 into MoS 2 , the second step of annealing was performed.For the second step, the temperature was set to 750 • C for 30 min in a reducing atmosphere consisting of 90% Ar and 10% H 2 at 1.8 Torr.Once the heating process was completed, the furnace was allowed to cool down naturally, and a large-area sample of MoS 2 films was obtained.

Results and Discussion
The MoS 2 films fabricated from the thermal decomposition process were uniform and continuous based on optical microscopy (3D laser microscope, VK-X250, Keyence, Osaka, Japan), as shown in Figure 1a.This demonstrates the feasibility of the large-scale growth of homogeneous MoS 2 films by thermal decomposition for practical electronic applications.The grown MoS 2 films were subjected to out-of-plane doping by CHF 3 plasma treatment using a dry-etching system (ULVAC original NLD-570).The radiofrequency power of this dry-etching system was set to 100 W and the pressure was set to 7.5 mTorr.The final thickness of the treated samples was obtained by etching for 30 s. Figure 1b shows the optical micrograph (OM) image of the selective-area MoS 2 films with and without CHF 3 plasma treatment.On the right, the untreated region exhibits a light blue color, while on the left, the plasma-treated region displays a dark blue color.Raman spectroscopy is being used widely to study 2D materials and to identify their thicknesses.Figure 1c shows the Raman spectra of the MoS 2 films in the untreated region and in the CHF 3 plasma-treated region, respectively.Figure 1c clearly shows two main Raman features which correspond to E 1 2g (approximately 381 cm −1 ) and A 1g (approximately 406 cm −1 ) modes [20].The intensity of the two peaks increases with an increase in MoS 2 film thickness.The strong, sharp peaks displayed in the Raman spectrograph of the untreated region indicate a thick layer of high crystalline MoS 2 film.On the other hand, the E 1 2g and A 1g peaks of the CHF 3 -treated region are weaker due to a reduction in thickness during the plasma treatment.The energy difference between the two Raman peaks can be used to identify the number of MoS 2 layers.This energy difference value was obtained in the untreated region and equated to about 25.9 cm −1 , indicating a bulk-like multilayer MoS 2 .However, in the CHF 3 -treated region, the energy difference equated to a smaller value of 23.2 cm −1 , indicating a lower thickness of MoS 2 film than in the untreated region.The difference in thickness between the untreated and the CHF 3 treated region was further indicated by the difference in the E 1 2g and A 1g peak intensity between these two regions.

Results and Discussion
The MoS2 films fabricated from the thermal decomposition process were uniform and continuous based on optical microscopy (3D laser microscope, VK-X250, Keyence, Osaka, Japan), as shown in Figure 1a.This demonstrates the feasibility of the large-scale growth of homogeneous MoS2 films by thermal decomposition for practical electronic applications.The grown MoS2 films were subjected to out-of-plane doping by CHF3 plasma treatment using a dry-etching system (ULVAC original NLD-570).The radiofrequency power of this dry-etching system was set to 100 W and the pressure was set to 7.5 mTorr.The final thickness of the treated samples was obtained by etching for 30 s. Figure 1b shows the optical micrograph (OM) image of the selective-area MoS2 films with and without CHF3 plasma treatment.On the right, the untreated region exhibits a light blue color, while on the left, the plasma-treated region displays a dark blue color.Raman spectroscopy is being used widely to study 2D materials and to identify their thicknesses.Figure 1c shows the Raman spectra of the MoS2 films in the untreated region and in the CHF3 plasma-treated region, respectively.Figure 1c clearly shows two main Raman features which correspond to E 1 2g (approximately 381 cm −1 ) and A1g (approximately 406 cm −1 ) modes [20].The intensity of the two peaks increases with an increase in MoS2 film thickness.The strong, sharp peaks displayed in the Raman spectrograph of the untreated region indicate a thick layer of high crystalline MoS2 film.On the other hand, the E 1 2g and A1g peaks of the CHF3-treated region are weaker due to a reduction in thickness during the plasma treatment.The energy difference between the two Raman peaks can be used to identify the number of MoS2 layers.This energy difference value was obtained in the untreated region and equated to about 25.9 cm −1 , indicating a bulk-like multilayer MoS2.However, in the CHF3-treated region, the energy difference equated to a smaller value of 23.2 cm −1 , indicating a lower thickness of MoS2 film than in the untreated region.The difference in thickness between the untreated and the CHF3 treated region was further indicated by the difference in the E 1 2g and A1g peak intensity between these two regions.AFM (Dimension Icon, Bruker, Billerica, MA, USA) was used to examine the surface morphology and the thickness of the MoS 2 films.Figure 2a,b show the AFM image and the height profile of the as-grown MoS 2 films on the substrate, respectively.The difference in contrasts in the AFM image indicates the different heights in the image.From the height profile, the thickness of the as-grown MoS 2 was found to be about 10 nm, suggesting that there are 15 layers of MoS 2 .The different contrasts in the AFM image shown in Figure 2c further indicated the thinning effect after plasma treatment.From the height profile in Figure 2d, the difference in height between the untreated and CHF 3 -treated MoS 2 film was found to be approximately 4 nm.Further doping could be applied by increasing the plasma etching time; however, that would result in poor and uncontained MoS 2 films due to MoS 2 chemical modification and the formation of defects introduced by energetic ions.
AFM (Dimension Icon, Bruker, Billerica, MA, USA) was used to examine the surface morphology and the thickness of the MoS2 films.Figures 2a,b show the AFM image and the height profile of the as-grown MoS2 films on the substrate, respectively.The difference in contrasts in the AFM image indicates the different heights in the image.From the height profile, the thickness of the as-grown MoS2 was found to be about 10 nm, suggesting that there are 15 layers of MoS2.The different contrasts in the AFM image shown in Figure 2c further indicated the thinning effect after plasma treatment.From the height profile in Figure 2d, the difference in height between the untreated and CHF3-treated MoS2 film was found to be approximately 4 nm.Further doping could be applied by increasing the plasma etching time; however, that would result in poor and uncontained MoS2 films due to MoS2 chemical modification and the formation of defects introduced by energetic ions.XPS analysis was carried out to investigate the binding energies of Mo, S, and F in the CHF3treated and untreated MoS2 samples.Figures 3a,b show the detailed binding energy profiles of Mo and S for the CHF3-treated and untreated MoS2 samples, respectively.Two peaks at 229.8 and 232.9 eV are shown in Figure 3a.They were attributed to the doublet Mo 3d5/2 and Mo 3d3/2 of the untreated MoS2, respectively, while the peaks of the S 2p3/2 and S 2p1/2 orbitals of the divalent sulfide ions were observed at 162.7 and 163.8 eV, respectively.These results are consistent with the reported values for untreated MoS2 crystals [21,22].In comparison, all relevant peaks of the CHF3-treated sample were broader, and red-shifted by 0.57 eV.This was because of the shift in Fermi levels of the CHF3-treated samples towards the valence band edge [23].This red-shift of peaks indicates the proper p-type doping of MoS2 films.The specific types of dopants introduced by the plasma processes were confirmed by the XPS spectra in Figure 3c.A prominent binding energy peak associated with F was XPS analysis was carried out to investigate the binding energies of Mo, S, and F in the CHF 3 -treated and untreated MoS 2 samples.Figure 3a,b show the detailed binding energy profiles of Mo and S for the CHF 3 -treated and untreated MoS 2 samples, respectively.Two peaks at 229.8 and 232.9 eV are shown in Figure 3a.They were attributed to the doublet Mo 3d 5/2 and Mo 3d 3/2 of the untreated MoS 2 , respectively, while the peaks of the S 2p 3/2 and S 2p 1/2 orbitals of the divalent sulfide ions were observed at 162.7 and 163.8 eV, respectively.These results are consistent with the reported values for untreated MoS 2 crystals [21,22].In comparison, all relevant peaks of the CHF 3 -treated sample were broader, and red-shifted by 0.57 eV.This was because of the shift in Fermi levels of the CHF 3 -treated samples towards the valence band edge [23].This red-shift of peaks indicates the proper p-type doping of MoS 2 films.The specific types of dopants introduced by the plasma processes were confirmed by the XPS spectra in Figure 3c.A prominent binding energy peak associated with F was observed in CHF 3 -treated samples, while the F peak was absent for untreated samples.The excess electrons were preferentially transferred from the MoS 2 layers onto F atoms with strong electronegativity when incorporating F dopants into MoS 2 layers.This surface charge transfer process suggests that F atoms are the critical dopants responsible for the p-type doping in MoS 2 film [24].
observed in CHF3-treated samples, while the F peak was absent for untreated samples.The excess electrons were preferentially transferred from the MoS2 layers onto F atoms with strong electronegativity when incorporating F dopants into MoS2 layers.This surface charge transfer process suggests that F atoms are the critical dopants responsible for the p-type doping in MoS2 film [24].In order to measure the electrical characteristics of the CHF3-treated multilayer MoS2, a backgated FET device was fabricated as shown in the schematic illustrated in Figure 4a.Photolithography was carried out to define the exposed area of MoS2 films for plasma treatment.After plasma treatment, 80 nm of Au was deposited as the source and as drain contacts for the FET by sputtering and was followed by lift-off.Due to the screening of the electric field in MoS2, only the top few layers of the MoS2 encountered a surface charge transfer process.Therefore, a thinner n-type MoS2 channel at the untreated bottom layers could result in effective gate modulation and higher on/off ratios for FET applications.Figure 4b shows the output characteristics, drain current (Ids)-drain voltage (Vds), of the MoS2 FET under varying gate voltages (Vg, from 0 to −10 V).Ids decreased with the increasing Vg values, indicating typical p-type behavior.The Ids-Vds characteristic of the device exhibited a low onset voltage and a linear increase of Ids versus Vds up to a value of Vds approximately equal to 0.6 V, where a kink in the Ids-Vds curves was observed.This was followed by saturation of the Ids-Vds curve for Vds greater than 0.6 V. Figure 4c  In order to measure the electrical characteristics of the CHF 3 -treated multilayer MoS 2 , a back-gated FET device was fabricated as shown in the schematic illustrated in Figure 4a.Photolithography was carried out to define the exposed area of MoS 2 films for plasma treatment.After plasma treatment, 80 nm of Au was deposited as the source and as drain contacts for the FET by sputtering and was followed by lift-off.Due to the screening of the electric field in MoS 2 , only the top few layers of the MoS 2 encountered a surface charge transfer process.Therefore, a thinner n-type MoS 2 channel at the untreated bottom layers could result in effective gate modulation and higher on/off ratios for FET applications.Figure 4b shows the output characteristics, drain current (I ds )-drain voltage (V ds ), of the MoS 2 FET under varying gate voltages (V g , from 0 to −10 V).I ds decreased with the increasing V g values, indicating typical p-type behavior.The I ds -V ds characteristic of the device exhibited a low onset voltage and a linear increase of I ds versus V ds up to a value of V ds approximately equal to 0.6 V, where a kink in the I ds -V ds curves was observed.This was followed by saturation of the I ds -V ds curve for V ds greater than 0.6 V. Figure 4c represents the transfer characteristic of the back-gated MoS 2 FET at V ds of 4 V.The back-gated MoS 2 FET showed excellent gating control capability.The field-effect mobility, µ h , of this MoS 2 device can be estimated based on the following equation: where the channel length, L, is 1 mm, the channel width, W, is 1.2 mm, and the gate capacitance, C g , is 115 aF/µm 2 for a 300 nm thick SiO 2 layer [9].The mobility of the device was calculated to be 65.2 cm 2 V −1 s −1 , which is comparable with previous results of similar back-gated FET devices [18].However, the mobility was lower than that obtained from the top-gated FET.This difference in the mobility was due to the existence of trap states within the SiO 2 dielectric layer at the bottom gate.Moreover, the device exhibited an on/off current ratio in the order of 10 3 , and a threshold gate bias of about −5 V, whereas the MoS 2 PFET was positively biased under 4 V.This indicates that a large negative V g (less than or equal to −5 V) is needed to tune the electron-rich layers of the bottom untreated MoS 2 layers into hole-rich layers.The energy band structures of the device under a negative gate with applied bias are shown in Figure 4d-f, where the conduction band (CB) and Fermi level (E F ) of multilayer MoS 2 were assumed to be 4.2 eV and 4.7 eV, respectively.From Figure 4d, the work function of Au was found to be about 5.1 eV.Although the E F of the MoS 2 was shifted toward the valence band (VB) due to p-type doping, the E F of Au still lay on the top half of the MoS 2 band.This led to a large Schottky barrier for holes along the Au/MoS 2 interface.As shown in Figure 4e, for V g(th) < V g < 0, holes could only pass through the barrier at a high V ds due to the bending of the MoS 2 channel.As shown in Figure 4f, as the negative V g increased, for V g < V g(th) < 0, the hole barrier became thinner and allowed holes to penetrate through.This was consistent with the threshold gate bias (−5 V) obtained in Figure 4c.The drain current was greatly enhanced under a negative gate bias less than −5 V.
Appl.Sci.2019, 9, 1110 6 of 9 where the channel length, L, is 1 mm, the channel width, W, is 1.2 mm, and the gate capacitance, Cg, is 115 aF/μm 2 for a 300 nm thick SiO2 layer [9].The mobility of the device was calculated to be 65.2 cm 2 V −1 s −1 , which is comparable with previous results of similar back-gated FET devices [18].However, the mobility was lower than that obtained from the top-gated FET.This difference in the mobility was due to the existence of trap states within the SiO2 dielectric layer at the bottom gate.Moreover, the device exhibited an on/off current ratio in the order of 10 3 , and a threshold gate bias of about −5 V, whereas the MoS2 PFET was positively biased under 4 V.This indicates that a large negative Vg (less than or equal to −5 V) is needed to tune the electron-rich layers of the bottom untreated MoS2 layers into hole-rich layers.The energy band structures of the device under a negative gate with applied bias are shown in Figures 4d-f, where the conduction band (CB) and Fermi level (EF) of multilayer MoS2 were assumed to be 4.2 eV and 4.7 eV, respectively.From Figure 4d, the work function of Au was found to be about 5.1 eV.Although the EF of the MoS2 was shifted toward the valence band (VB) due to p-type doping, the EF of Au still lay on the top half of the MoS2 band.This led to a large Schottky barrier for holes along the Au/MoS2 interface.As shown in Figure 4e, for Vg(th) < Vg < 0, holes could only pass through the barrier at a high Vds due to the bending of the MoS2 channel.As shown in Figure 4f, as the negative Vg increased, for Vg < Vg(th) < 0, the hole barrier became thinner and allowed holes to penetrate through.This was consistent with the threshold gate bias (−5 V) obtained in Figure 4c.The drain current was greatly enhanced under a negative gate bias less than −5 V. MoS2 is sensitive to light illumination and is able to generate photoexcited electron-hole pairs from incident light.Figure 5a   MoS 2 is sensitive to light illumination and is able to generate photoexcited electron-hole pairs from incident light.Figure 5a displays the photoinduced I ds -V ds output curves of the MoS 2 PFET excited by ultraviolet (λ = 365 nm) light (UV lamp SLUV-8, intensity 1407 µW/cm 2 ).The photocurrent generated at V g from −10 to 0 V suggests that the MoS 2 PFET can be used as a phototransistor for UV light detection.Based on the photocurrent generated under different values of V g , the photoresponsivity as a function of V g is plotted in Figure 5b.The photoresponsivity increased from 0.45 A/W under zero gate voltage to 9.3 A/W under the gate voltage of 10 V, with an incident light power of 16 µW and V ds set at 4 V.The gate voltage-dependent photoresponsivity in the MoS 2 FET was due to the p-type doping of MoS 2 .Under the negative V g , E F moved from the CB to the VB of the p-MoS 2 , forming a smaller Schottky barrier between the CB of MoS 2 and the E F of the Au electrode.This led to photogenerated charges which efficiently transferred to the external circuit and produced a large photocurrent.This photocurrent was enhanced by further increasing the negative V g due to a thin hole Schottky barrier at the Au/MoS 2 interface.The photocurrent was also enhanced by the application of a higher positive bias V ds due to the bending of the MoS 2 channel.Figure 5c shows the transfer curves of the MoS 2 device with V ds equal to 4 V under UV illumination and dark conditions, respectively.Compared with dark conditions, the V g(th) obtained from the UV illumination had a greater absolute value.This indicates that the photoexcited holes are transferred to the electrodes due to the p-type behavior of the MoS 2 , while the photoexcited electrons are trapped within the MoS 2 .This is consistent with the results shown in previous literature, indicating that MoS 2 is an effective charge trapping layer [25].The time-resolved characteristics revealed a reliable photoresponse with a stabilized photocurrent, as shown in Figure 5d.Under 365 nm illumination, the photocurrent of the MoS 2 PFET increased rapidly after exposure to UV radiation, with a rise time of 37 ms, significantly faster than those of other reported MoS 2 photodetectors [3,4,15,26], as shown in Figure 5e.This photocurrent remained nearly constant during the UV exposure (10 s) and decayed quickly during dark conditions with a decay time of approximately 39 ms.The current fully recovered after repeated cycles, which shows excellent repeatability and optical response for this MoS 2 PFET photodetector.This led to photogenerated charges which efficiently transferred to the external circuit and produced a large photocurrent.This photocurrent was enhanced by further increasing the negative Vg due to a thin hole Schottky barrier at the Au/MoS2 interface.The photocurrent was also enhanced by the application of a higher positive bias Vds due to the bending of the MoS2 channel.Figure 5c shows the transfer curves of the MoS2 device with Vds equal to 4 V under UV illumination and dark conditions, respectively.Compared with dark conditions, the Vg(th) obtained from the UV illumination had a greater absolute value.This indicates that the photoexcited holes are transferred to the electrodes due to the p-type behavior of the MoS2, while the photoexcited electrons are trapped within the MoS2.This is consistent with the results shown in previous literature, indicating that MoS2 is an effective charge trapping layer [25].The time-resolved characteristics revealed a reliable photoresponse with a stabilized photocurrent, as shown in Figure 5d.Under 365 nm illumination, the photocurrent of the MoS2 PFET increased rapidly after exposure to UV radiation, with a rise time of 37 ms, significantly faster than those of other reported MoS2 photodetectors [3,4,15,26], as shown in Figure 5e.This photocurrent remained nearly constant during the UV exposure (10 s) and decayed quickly during dark conditions with a decay time of approximately 39 ms.The current fully recovered after repeated cycles, which shows excellent repeatability and optical response for this MoS2 PFET photodetector.

Conclusions
In summary, multilayer MoS 2 photodetectors, enabled by CHF 3 plasma treatment, have been fabricated.These out-of-plane doped MoS 2 PFETs displayed an on/off current ratio in the order of 10 3 and a field-effect mobility of 65.2 cm 2 V −1 s −1 .The MoS 2 PFETs exhibited improved UV light photodetection capability with a fast photoresponse time of 37 ms.This indicates that a vertical design could pave the way for faster MoS 2 -based photodetectors.The photocurrent generation greatly depends on the back-gate voltage.This study provides a promising approach to the development of mild plasma-doped MoS 2 as a 2D material in post-silicon electronic and optoelectronic device applications.

Figure 1 .
Figure 1.(a) Optical micrograph (OM) image of the as-grown MoS2 films on the SiO2/Si substrate.(b) OM image of the selective-area MoS2 films with and without CHF3 plasma treatment.(c) Raman spectra taken of the untreated and CHF3-treated regions.

Figure 1 .
Figure 1.(a) Optical micrograph (OM) image of the as-grown MoS 2 films on the SiO 2 /Si substrate.(b) OM image of the selective-area MoS 2 films with and without CHF 3 plasma treatment.(c) Raman spectra taken of the untreated and CHF 3 -treated regions.

Figure 2 .
Figure 2. (a) Atomic force microscopy (AFM) height image of the as-grown MoS2 films.(b) Line scan of the as-grown MoS2-substrate interface.(c) AFM height image and (d) line scan across the CHF3treated and untreated interface of the MoS2 film.

Figure 2 .
Figure 2. (a) Atomic force microscopy (AFM) height image of the as-grown MoS 2 films.(b) Line scan of the as-grown MoS 2 -substrate interface.(c) AFM height image and (d) line scan across the CHF 3 -treated and untreated interface of the MoS 2 film.

Figure 3 .
Figure 3. X-ray photoelectron spectroscopy (XPS) spectra for (a) Mo and (b) S signals before and after plasma treatment, showing a downshift of 0.57 eV in binding energy for all peaks.(c) XPS spectra for MoS2 exhibit the presence of the F 1s peak in CHF3 plasma-treated MoS2.

Figure 3 .
Figure 3. X-ray photoelectron spectroscopy (XPS) spectra for (a) Mo and (b) S signals before and after plasma treatment, showing a downshift of 0.57 eV in binding energy for all peaks.(c) XPS spectra for MoS 2 exhibit the presence of the F 1s peak in CHF 3 plasma-treated MoS 2 .

Figure 4 .
Figure 4. (a) Schematic diagram of the back-gated field-effect transistor (FET).Source and drain metal contacts are deposited on the untreated MoS2 region.F atoms are incorporated into the upper regions of the multilayer MoS2.(b) Ids-Vds graph of the FET device at different gate bias values ranging from 0 to −10 V. (c) Ids-Vg graph displaying transfer characteristic of the MoS2 p-type FET for Vds = 4 V. (df) Band diagram of Au/MoS2 (d) under equilibrium condition, (e) with Vg(th) < Vg < 0, and (f) with Vg < Vg(th) < 0.

Figure 4 .
Figure 4. (a) Schematic diagram of the back-gated field-effect transistor (FET).Source and drain metal contacts are deposited on the untreated MoS 2 region.F atoms are incorporated into the upper regions of the multilayer MoS 2 .(b) I ds -V ds graph of the FET device at different gate bias values ranging from 0 to −10 V. (c) I ds -V g graph displaying transfer characteristic of the MoS 2 p-type FET for V ds = 4 V. (d-f) Band diagram of Au/MoS 2 (d) under equilibrium condition, (e) with V g(th) < V g < 0, and (f) with V g < V g(th) < 0.
from −10 to 0 V suggests that the MoS2 PFET can be used as a phototransistor for UV light detection.Based on the photocurrent generated under different values of Vg, the photoresponsivity as a function of Vg is plotted in Figure 5b.The photoresponsivity increased from 0.45 A/W under zero gate voltage to 9.3 A/W under the gate voltage of 10 V, with an incident light power of 16 μW and Vds set at 4 V.The gate voltage-dependent photoresponsivity in the MoS2 FET was due to the p-type doping of MoS2.Under the negative Vg, EF moved from the CB to the VB of the p-MoS2, forming a smaller Schottky barrier between the CB of MoS2 and the EF of the Au electrode.

Figure 5 .
Figure 5. (a) Ids-Vds output curves of the MoS2 PFET under ultraviolet (λ = 365 nm) illumination (symbols lines) and in dark conditions (solid lines) with varying gate voltages.(b) Photoresponsivity as a function of Vg at Vds value of 4 V. (c) Ids-Vg graph displaying transfer behavior of the MoS2 PFET for Vds = 4 V under dark and UV illumination.(d) Photoresponse of the p-type MoS2 device revealing

Figure 5 .
Figure 5. (a) I ds -V ds output curves of the MoS 2 PFET under ultraviolet (λ = 365 nm) illumination (symbols lines) and in dark conditions (solid lines) with varying gate voltages.(b) Photoresponsivity as a function of V g at V ds value of 4 V. (c) I ds -V g graph displaying transfer behavior of the MoS 2 PFET for V ds = 4 V under dark and UV illumination.(d) Photoresponse of the p-type MoS 2 device revealing the stable and repeated changes in response to the light at on and off conditions at V ds = 6 V. (e) Photoresponse rate of the p-type MoS 2 device.