Wafer-Scale Substitutional Doping of Monolayer MoS2 Films for High

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Functional Inorganic Materials and Devices

Wafer-Scale Substitutional Doping Of Monolayer MoS2 Films For High-Performance Optoelectronic Devices Youngchan Kim, Hunyoung Bark, Byunggil Kang, and Changgu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20714 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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ACS Applied Materials & Interfaces

Wafer-scale Substitutional Doping of Monolayer MoS2 Films for High-performance Optoelectronic Devices Youngchan Kim1,†, Hunyoung Bark2,†, Byunggil Kang2, and Changgu Lee1,2,*

1Department

of

Mechanical

Engineering,

2SKKU

Advanced

Institute

of

Nanotechnology (SAINT), Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Korea

†These

authors contributed equally to this work.

*Corresponding author, [email protected]

KEYWORDS: doping, substitutional, wafer scale, MoS2, electronics, optoelectronics, monolayer

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ABSTRACT

The substitutional doping method is ideally suited to generating doped 2D materials for practical device applications as it does not damage or destabilize such materials. However, recently reported substitutional doping techniques for 2D materials have given rise to discontinuities and low uniformity which hamper the extension of such techniques to large-scale production. In the current work, we demonstrated uniform substitutional doping of monolayer MoS2 in the area of a two-inch wafer (>13 cm2). The devices based on doped MoS2 showed extremely high uniformity and stability in electrical properties in ambient condition for 30 days. The photodetectors based on the doped MoS2 samples showed ultrahigh photoresponsivity of 5 x 105 A/W, detectivity of 5 x 1012 Jones and fast response rate of 5 ms than did that based on undoped MoS2. This work showed the feasibility of real-life applications based on functionalized 2D semiconductors for next-generation electronics and optoelectronics.

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INTRODUCTION A high performance and low power consumption of electronic systems are required for their integration into advanced modern solid-state electronic and optoelectronic devices. The dim ensions of electronic devices have been shrunk dramatically to meet these requirements, but silicon technology has reached its limit in this regard. New materials and device architectures have been developed to carry on Moore’s law. Two-dimensional (2D) semiconducting materials, for example, provide high potential for scalability down to the nanoscale and superior performance of electronic devices compared with three-dimensional semiconductors including silicon due to the atomic-scale thickness, surfaces free of dangling bonds, and controllable band gap.1-9 MoS2, the most widely studied transition metal dichalcogenide (TMDC) 2D semiconductor, represents one of the alternatives for future electronics owing to its high flexibility, indirect to direct band gap transition, and optically controllable valley polarization.6-7, 10-12 MoS2 has been applied to versatile electronic applications such as field effect transistors (FETs),3, 8, 13 photodetectors,14 gas sensors,15 integrated circuits,15 and energyharvesting devices.16 Doping of 2D materials including MoS2 is one of the main available ways to control the functionality of semiconductors and enhance the electrical performances of their devices. However, in contrast to the case of semiconductors such as silicon, strategies for the facile and effective doping of MoS2 and other 2D semiconducting materials have not yet been developed. Ion implantation is commonly used for the doping of silicon in conventional semiconductor processes,

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and allows for the precise control of the carrier concentration and type from

nondegenerate p- and n-types to degenerate p- and n-types However, this conventional doping technique can hardly be extended to 2D materials because it introduces unintentional layer thinning and severe defects damaging to the crystal structure of the 2D material.20-22 While 5



multilayer MoS2 has been effectively doped using ion implantation, doping of monolayer MoS2 is still challenging.22 Instead of ion implantation, various other doping techniques have been developed for the modification of the electrical properties of MoS2. One of the representative techniques is surface charge transfer doping using wet chemicals such as AuCl3 (p-type) and benzene viologen (BV) (n-type). 23-24 Also, the control of doping using gas molecules such as NH3 (n-type)25 and NO2 (p-type)26 has also been demonstrated. These methods are simple and effective at adjusting the carrier type of MoS2, and have been applied to the functionalization of electronic devices based on MoS2. However, these methods result in products with very low stability in air because doping is made by a weak reversible physisorption of gas or liquid ionic molecules instead of strong chemical bonding. In contrast to surface charge doping, substitutional doping is likely to be an ideal technique for practical applications and industrial mass production due the covalent bonding of this technique endowing its products with high stability. Previously, Nb was doped substitutionally in bulk MoS2 grown using chemical vapor transport (CVT), and resulted in a transition of MoS2 from an n-type to p-type semiconductor, and Mn was doped substitutionally in flake-type monolayer MoS2 through chemical vapor deposition using powders as the precursors27-28 However, these methods have been shown to result in the degradation of the optical properties and have been limited by strong substrate effects. To improve the optical and electrical properties of the doped material, the in situ synthesis for Nb doping (p-type) of monolayer WS2 and Re doping (n-type) of monolayer MoS2 have been reported.29 Also, the doping of a high concentration (1 at%) of Re into

monolayer MoS2 has been achieved.30

However, these syntheses and doping methods using only powders as the precursor are not appropriate for practical electronic and optoelectronic device applications due to resulting discontinuities, low uniformity, and critical size limitations on the roughly micrometer scale of 6


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the products. Moreover, these products have not been shown to be useful for practical applications such as photodetectors, gas sensors, and photocatalysts of the hydrogen evolution reaction. In the current work, we synthesized substitutionally doped continuous monolayer MoS2 films using a gas-phase precursor and metal deposition in an area of more than 13 cm2 for both p-type (Nb) and n-type (Mn) charge doping. An apparently perfect transition of n-type MoS2 into the p-type semiconductor resulting from doping it with Nb, and the production of more ntype carriers in MoS2 resulting from substituting in Mn, were both confirmed using Kelvin probe force microscopy (KPFM). Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray photoemission spectroscopy (XPS) were performed to characterize the physical properties of the doped MoS2 samples. The substitutional doping of Mn in MoS2 was clearly confirmed using transmission electron microscopy (TEM), since atoms with such different atomic number (Z) values as Mo and Mn was distinguished using this techniqueTo analyze the uniformity, we fabricated several hundred electronic device arrays in a two-inch wafer. These devices presented highly uniform electrical properties and high stability even after 30 days in air. In addition, photodetectors based on the doped MoS2 were fabricated to show the feasibility of practical applications of the doped product. The photodetectors of the doped MoS2 samples showed significantly better photoresponse, detectivity and response rate levels than did those of undoped MoS2. Most notably, the average responsivity level of the photodetectors made using Nb-doped MoS2 was 104 times better. Also, their average response rate was improved to 4.9 ms, 103 times higher than that of the detector based on undoped MoS2. These results showed the potential of the present doping technique to be used for controlling functionalization in versatile opto-electronic devices applications.

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RESULTS AND DISCUSSION Figure 1a illustrates the overall process we used to synthesize doped MoS2. The transition metals niobium (Nb) and manganese (Mn), which were used as the dopants, were each first deposited on a separate SiO2/Si substrate sample using an e-beam evaporator. MoS2 was synthesized from MoO3 and H2S gas on each of the two dopant-deposited substrates using the CVD method.31 MoS2 was substitutionally doped with the deposited metal during the synthesis process. The synthesis conditions are shown in figure S1a. The furnace was heated to 600 oC at a rate of 20 oC min-1 under a flow of Ar gas. The evaporated MoO3 powder was sulfurized with injected H2S gas for over 30 minutes, as illustrated in figure S1b. After the synthesis process, the temperature of the furnace was increased to 850 oC and maintained at this temperature for 60 minutes to improve the substitutional doping and crystallinity of the doped MoS2. The sample was then rapidly cooled to room temperature. Optical microscope (OM) images of the synthesized Nb-doped and Mn-doped monolayer MoS2 films are shown in figure 1b. Inspection of these images revealed the synthesized doped MoS2 to contrast sharply with the bare SiO2 substrate, and showed continuous and uniformly doped MoS2 films. Our samples were also characterized by carrying out Raman and photoluminescence (PL) analyses using an excitation laser wavelength of 532 nm. The laser intensity was fixed at less than 1 mW to avoid thermal damage during the experiments and Raman spectra were calibrated by the Si peak at 520 cm -1. For comparison, MoS2 without dopant was also synthesized under the same conditions and characterized at the same time in all of the experiments. The Raman spectrum of undoped MoS2 exhibited typical E2g and A1g vibration modes at 385.02 cm-1 and 404.25 cm-1 (figure 1c), with 19.23 cm-1 difference between the two modes, indicating the presence of monolayer MoS2. Also, the thickness of MoS2 was confirmed by a cross sectional image of TEM (figure S2). Doping MoS2 yielded no significant 8


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changes to the E2g and A1g peaks, except for a slight broadening of the A1g peak, which has been shown to be affected by a change in carrier concentration.32 The full width at half maximum (FWHM) values of the A1g peaks of undoped MoS2, Mn-doped MoS2, and Nb-doped MoS2 were measured to be 5.68, 6.424, and 6.811 cm-1 respectively. The A1g peak broadening resulted from doping-induced change in carrier concentration. In contrast to the very minor effect of doping MoS2 on the Raman peaks, the doping resulted in a considerable effect on the photoluminescence (PL) spectrum, as shown in figure 1d. Here, the A-exciton PL peak, which arises from direct-gap recombination, exhibited a 40 meV red-shift for Nb-doped MoS2 and 15 meV red-shift for Mn-doped MoS2, as shown in figure 1d. This trend of PL shifts corresponded to the results described in previous reports of CVD-synthesized Nb and Mn-doped MoS2 samples using powders .28, 33 Our transition metal-doped MoS2 films were further analyzed using X-ray photoelectron spectroscopy (XPS). All XPS spectra were calibrated using the C 1s peak at 284.3 eV as a reference. The Nb-doped MoS2 binding energy peaks were observed at lower values than those of the undoped MoS2. The amount of the downshift was 0.35 eV for the Mo 3d peak and 0.55 eV for the S 2p peak, as shown in figures 2a and 2 b. This shift was attributed to the lowering of the Fermi level (EF) upon the p-type doping of MoS2 with Nb, due to Nb having one less electron than molybdenum (Mo). 27, 29 Distinct binding energy peaks associated with the Nb 3d core levels were detected only from the Nb-doped MoS2 samples (figure 2c). The Nb 3d5/2 and Nb 3d3/2 signature peaks were observed at 203.2 eV and 205.9 eV, respectively, consistent with the previous report about the Nb 3d binding energies in NbS2.34 This pair of Nb 3d peaks could be attributed to substitutional doping of Nb as previously reported.29 The calculated atomic concentration of Nb in the doped MoS2 samples was 1.7%. The binding energy peaks of Mndoped MoS2 showed no shift compared to those of undoped MoS2, as shown in figures 2d and 9



2e. The static binding energies of Mn-doped MoS2 could be attributed to a small amount of EF shifting and this result was also consistent with the literature results for Mn-doped MoS2.33 As shown in figure 2f, Mn-doped MoS2 showed distinct Mn 2p binding energy peaks not observed in the spectrum of the undoped MoS2, and the calculated atomic concentration of Mn was 1.4%. In contrast to Nb, Mn can bond with sulfur (S) in the rock-salt (-MnS), zinc-blende (-MnS), and wurtzite (-MnS) structures, which are not 2D hexagonal layered structures.35-37 Hence, it is difficult to distinguish the Mn bonding state using XPS alone. To show that metal substitution doping occurred during the synthesis process and to identify the crystal structure, scanning transmission electron microscopy (STEM) was performed. The Mn-doped MoS2 exhibited a hexagonal lattice as seen from the fast-Fourier transform (FFT) pattern in the inset of figure 2g, and the atomic structure of MoS2 well matched the STEM image. This result indicated that the crystal structure was not significantly damaged by the Mn substitution and still retained its pristine 2H-type MoS2 hexagonal structure after the synthesis and doping process. The contrast scales were roughly proportional to the square of the atomic number (Z). Compared to Mo (Z = 42), the Mn (Z = 25) atoms were confirmed to have ~77% of the intensity in HAADF as shown in the intensity spectrum (figure 2h) of the dotted rectangle in figure 2g. Due to the considerable difference between of atomic number (Z) of Mn and that of Mo, their atoms were distinguished in the STEM image, as shown by comparing the contents of the dotted rectangle and dashed circle of figure 2g. The atomic structure of Nb-doped MoS2 was also evaluated using STEM. The FFT pattern in the inset of figure 2i also indicated a hexagonal lattice structure and the STEM image (figure 2j) matched the atomic structure of MoS2. In contrast to Mn-doped MoS2, Nb-doped MoS2 showed a uniform atomic intensity distribution (figure 2j), attributed to the very similar atomic numbers of Nb (Z = 41) and Mo (Z = 42). Atoms differing by an atomic number of only one have not yet been distinguished 10


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using STEM.27-29 In addition, Nb and Mn dopants were detected using energy dispersive spectroscopy (EDS), as shown in figures S3 and S4. The results of these chemical and structural analyses taken together indicated the replacement of Mn and Nb atoms with Mo atoms. To establish that Nb and Mn acted as p-type and n-type dopants, respectively, work functions, which indicate the position of EF, were measured by carrying out Kelvin probe force microscopy (KPFM). The work function can be easily calculated from the contact potential difference (CPD) between the KPFM tip and sample by using Equation (1).

CPD

= (Φtip ― Φsample)/𝑒

(1)

In Equation (1), e, tip, and sample are the electron charge, work function of the KPFM tip, and work function of the sample, respectively. In accordance with previous reports, the work function of our KPFM tip was measured in advance by scanning graphite ( ~4.6eV) as a reference (figure S5).38 Figure 3a shows the 4 m x 4 m KPFM images, which are expressed in terms of work function values. The work function of Nb-doped MoS2 was determined to be 6.19 eV, i.e., 1.27 eV higher than that of undoped MoS2, and Mn-doped MoS2 was determined to have a work function of 4.40 eV, i.e., 0.52 eV lower than that of undoped MoS2. To determine the position of EF, the band diagram was depicted based on the KPFM results (figure 3b). The electron affinity and the band gap of monolayer MoS2 are 4.25 eV and 1.84 eV respectively.39-40 MoS2 usually exhibits n-type semiconductor behavior and the EF of undoped MoS2 has also been shown to be close to the conduction band. The EF of Nb-doped MoS2 was determined to be located inside the valence band, indicating degenerate doping because Nb changed the electronic properties of MoS2 by donating holes. This result demonstrated the transition of MoS2 from an n-type to p-type semiconductor as a result of the substitution of 11



some of the Mo atoms with Nb atoms during the synthesis. For Mn-doped MoS2, EF was slightly shifted to the conduction band by the donated electrons from Mn and the small amount of EF shifting was also confirmed in the static binding energies according to the XPS results described above and shown in figure 2d. To characterize the electrical properties of the doped MoS2 samples, bottom-gated field effect transistors (FETs) using Nb-doped MoS2, Mn-doped MoS2 and undoped MoS2 were separately fabricated, with one such FET shown in the inset of figure 4a. The typical transfer characteristics (drain current versus gate voltage plot) are presented in figure 4a and transfer curves in the form of logarithmic coordinates are shown in figure S6. The undoped MoS2 FET showed a strong, n-type gate voltage dependence of the drain current. In contrast, the Nb-doped MoS2 FETs exhibited a drastically different behavior, specifically a p-type conduction, a much weaker gate voltage dependence, and much higher on- and off-currents. As expected from the KPFM results, the Nb-doped MoS2 FETs showed degenerately p-type doped MoS2 behavior. On the other hand, the Mn-doped MoS2 FETs, which was expected to be an n-type doped MoS2 according to the KPFM results, exhibited an n-type conduction, weaker gate voltage dependence, and higher on current. Figure 4b shows the output characteristics (drain current versus drain voltage) of the Mn-doped MoS2 FET. The resistivity values for Nb-doped MoS2, Mn-doped MoS2, and undoped MoS2 were determined to be 18.3 k, 382.6 k, and 1.46 M, respectively. Notably, the fabricated FET devices were stable in ambient air even without any additional treatment like a capping layer deposition, i.e., minimal degradation in the current was observed over time, as shown in figure 4c. This result contrasted with those of otherwise currently available approaches using chemical treatment doping, which usually suffers from volatility and reactivity with the surrounding environment. The superior stability arose from the 12


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covalently bonded metal dopants in the MoS2 atomic structure. To confirm the stability more precisely, the work function of the Nb-doped MoS2 was measured after 30 days and compared with that 30 days before (figure S7). This result shows high stability with almost negligible variation of work function for 30 days in the air. Furthermore, the synthesized doped MoS2 was uniform over a large area with dimensions of up to two inches as shown in figures 4d-f. Figure 4d shows a photographic image of a transferred uniform Nb-doped MoS2 film over an area of 13 cm2 on a 2-inch SiO2/Si wafer. To confirm the uniformity of the doping in the Nbdoped MoS2 film over a large area, hundreds of FET device arrays were fabricated. Resistivity and work function values of 100 regions of a 4.0 mm x 5.3 mm area of an Nb-doped MoS2 film were measured, with histograms of the resistivity and work function mappings shown in figures 4e and 4f, respectively. A large-area FET device array was fabricated on each region as shown in figure S8. The average resistivity and work function values were 32.6 k and 6.19 eV, respectively, with standard deviations of only 1.2 k and 0.029 eV, which indicated high to extremely high levels of uniformity of doping in the MoS2 film. Doping in TMDCs is required to reduce contact resistance, which should lead to enhanced performance of electronic and optoelectronic devices based on TMDCs. Previous doping studies reported enhanced electronic device performances such as lower contact resistance and higher carrier mobility.41-43 Notably, doping has been applied to fabricate next-generation photodetectors displaying high photoresponsivity and response rate levels originating from their higher carrier concentration and lower contact resistance.44-45 However, optoelectronic devices based on substitutionally doped MoS2 have not been explored as much so far. Figure 5 shows a schematic diagram of the doped MoS2 device under illumination of a focused laser beam (figure 5a) and photoresponse properties of doped MoS2 (figures 5b-5f). The transfer characteristics of Nb-doped MoS2 photodetectors at a constant drain voltage (3 V) are shown 13



in figure 5b. The photodevices based on Nb-doped MoS2 exhibited hole-dominated photocurrents with higher photocurrent generation under negative gate voltage (figure 5b). This result indicated an essentially perfect transition of MoS2 to a p-type semiconductor. Also, the photocurrent of the Nb-doped MoS2 device exhibited a strong dependence on gate voltage, as shown in figure S9. As shown in figure S10, detectable photocurrents (Iillumination-Idark) were also measured from the Nb-doped MoS2 photodetectors exposed to various laser powers at a constant gate voltage and 550-nm-wavelength laser. As the optical power of the laser illuminated onto each device was increased, a higher photocurrent was generated. For the Nbdoped MoS2 device, a sublinear relationship between photoresponsivity and optical power was observed, and can be described as R ∝ 𝑃𝛼 , where R is the photoresponsivity, P is the optical power and  represents a constant. An  value of -0.9 was calculated for Nb-doped MoS2 by applying the power function fitting method as shown in figure 5c. The photoresponsivity, which is a critical parameter for characterizing the performance of a photodetector, was estimated by using Equation (2).

Photoresponsivity (R) = Iph/Pin (2)

In Equation (2), Iph, and Pin are the photocurrent and input optical power, respectively. The photoresponsivity of the Nb-doped MoS2 device was 2 x 105 A/W, i.e., 106 times higher than the 0.52 A/W value for undoped MoS2 at a 550nm excitation wavelength. To check the repeatability of photoresponsivity, 10 photodetectors were evaluated as shown in figure S10. The average photoresponsivity was 6.84 x 104 A/W with 4.37 x 104 standard deviation under the illumination of 550nm wavelength laser. Conventional MoS2-based photodetectors 14


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exhibited photoresponsivity ranging from 0.1 to 103 A/W. The photoresponsivity value of our photodetector was 100 times higher than the maximum photoresponsivity value of the conventional MoS2 photodetector. Even though the photoresponsivity value from the current work is not the maximum among the all 2D material based photodetectors, still our photodetector performance is competitive with other 2D material based photodetectors.46-48 Especially, this value is even higher than the photodetectors based on doped MoS2.23, 49 The photoresponsivity and external quantum efficiency (EQE) were calculated over the visible spectrum range as shown in figure 5d and figure 5e, respectively. The maximum photoresponsivity of the Nb-doped MoS2 device was 4.83 x 105 A/W at a 750nm excitation wavelength. The EQE, which is the ratio between the number of electron-hole pairs contributing to the photocurrent and the number of incident photons, was calculated using Equation (3).

EQE

=

Iph/𝑒 P𝑖𝑛/ℎ𝑣

× 100(%)

(3)

In Equation (3), Iph, e, Pin, h, and  are the photocurrent, the elementary charge, Pl anck’s constant, and the frequency of the illuminated laser. The maximum EQE for N b-doped MoS2 was 9.31 x 107 %, which was 106 times higher than the 107% value r esulting from undoped MoS2 at a 450nm excitation wavelength. These values of responsivity and EQE were comparable to the values obtained from photodevices based on MoS2 doped by other methods.23, 50-51 As shown in figures 5f, we also estimated the detectivity values of the two kinds of MoS2 devices over the visible-light range. The detectivity, which is a useful parameter for comparing the detection performances of photodetectors, was 15



also calculated using Equation (4).

Detectivity = R 𝐴/ 2𝑒𝐼𝑑𝑎𝑟𝑘 (4)

In Equation (4), R, A, e, and Idark are the photoresponsivity, area of the photoactive channel, elementary charge, and dark current, respectively. Equation (4) is only valid in the event that the shot noise from the dark current dominated the total noise of the photodetector. The devices based on undoped MoS2 exhibited detectivity values of 3.93 x 108 Jones, at a 750 nm excitation wavelength. At this wavelength, the device using Nb-doped MoS2 showed the detectivity of 5.0 x 1012 Jones, 104 times higher than that of undoped MoS2. This relative improvement in detectivity resulting from doping MoS2 with Nb was less than the corresponding relative improvement in the photoresponsivity, a result attributed to the dark current level having been increased significantly as a result of the high doping concentration. To compare the performances of the doped and undoped MoS2 photodetectors more clearly, their output characteristics were plotted as shown in figure 6a. This figure specifically shows, for each of the devices, the dependence of the induced photocurrent on drain voltage at an illuminating optical power of 80 nW and excitation wavelength of 550 nm. The device based on Nb-doped MoS2 exhibited the highest photocurrent, with a value of 0.74 A when the drain voltage was 1 V. The undoped MoS2 devices exhibited photocurrent values of 1.14 x 10-4 A, at this drain voltage (1V). This trend, i.e., the approximately 104 greater photoresponses of the Nb-doped MoS2 device than of the undoped MoS2 device, was similar to the trend of the transfer characteristics.

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Subsequently, the photoswitching behaviors of the Nb-doped MoS2 and undoped MoS2 devices were tested, as shown in figure 6b. Photocurrents generated and recombined in accordance with the illuminated light, turning on and off at a period of 10 s. The reversible photocurrent response confirmed the excellent reproducibility of the photodetector under continuous cycling. The amount of time it took for the photocurrent for each device to rise to near its maximum level and to decay back to near zero is shown in figure S12. Specifically, the rise and decay times were calculated from 10% to 90%, and 90% to 10% of the maximum photocurrent after the light was turned on and off, respectively. These rise and decay times for undoped MoS2 were found to be 0.63 s and 1.63 s, respectively. In contrast, these photoresponse times for the rise and decay of the device based on Nb-doped MoS2 were dramatically lower, with values of 4.9 ms and 5.7 ms, respectively. In general, undoped-MoS2 shows clear shift of threshold voltage by photogating effect and high gate dependence of photocurrent under illumination.(figure S13) However, superior photoresponsivity of Nb-doped MoS2 resulted from high current level and low contact resistance by degenerate doping rather than photogating effect. Photocurrent mechanism of Nb-doped MoS2 is more dependent on photoconductive effect compared to undoped MoS2. To analyze photo mechanism systematically, we derived a model to explain the enhancement of the photoresponse characteristics resulting from doping MoS2, an enhancement that was especially large with Nb. figure 6c shows proposed energy band diagrams of MoS2 upon the illumination of the two MoS2 photodetectors with a positive drain-source bias, and shows photoexcited electrons and holes generated in the MoS2 channel. For undoped MoS2, according to our model, a large Schottky barrier impeded the injection of photoexcited electrons into the metal contacts, hence preventing an efficient collection of these electrons and diminishing the performance of the photodetector. Doping MoS2 with Nb, however, was modeled (figure 6c) 17



by degenerate doping to shift the Fermi level of MoS2 toward the valence band, and hence significantly reduce the thickness of the Schottky barrier to the transfer of holes via thermally assisted tunneling. Therefore, the photoexcited holes could pass through the Schottky barrier directly by the tunneling effect, resulting in the great observed enhancement of photoresponse properties.

CONCLUSION We have demonstrated a strategy for carrying out wafer-scale substitutional doping of monolayer MoS2. The doping method was found to be superior in terms of uniformity, stability, and device performance over other substitutional doping methods involving monolayer TMDCs. Raman, XPS, STEM, KPFM, and electronic transport characterizations all confirmed that the n-type MoS2 was successfully converted into a p-type semiconductor by being doped with Nb and to a semiconductor with more n-type carriers by being substitutionally doped with Mn. The doped MoS2 was uniform over a region with dimensions of two inches and yielded stable device performance for 30 days. Furthermore, the photodetectors based on Nb-doped MoS2 exhibited dramatically enhanced photoresponse, detectivity, and response time values than did the other tested photodetectors. Future work should focus on tuning the doping levels by controlling the deposition thickness of dopant in addition to detailed electrical characterizations. The method described here and used to effect ultra-uniform substitutional doping can help expand the possible applications of 2D TMDCs in practical electronic and optoelectronic devices.

METHODS

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Synthesis of the doped MoS2 film on a SiO2/Si substrate by using the LPCVD system: Prior to dopant deposition, the 300-nm-thick SiO2/Si substrate was cleaned using sonication for ten minutes each in acetone and isopropyl alcohol (IPA), and then rinsed in deionized water (DIW) several times. Nb (p-type) or Mn (n-type) metal was deposited on the SiO2/Si substrate. For both of the metals, we deposited about 0.1nm, which corresponds roughly to one atomic layers for each metal, using e-beam evaporator. The resulting synthesized film is the single layer doped MoS2. If all the deposited atoms are consumed for doping, either the thickness of the film should be thicker or the doping level should be 100%. However, it seems that not all the deposited metal atoms were incorporated into the film as the dopant. We speculate that some portion of the deposited metals evaporated during the synthesis due to the high temperature and low pressure. Metals are oxidized easily once they are exposed to air, and we can reasonably expect that the oxidized metal atoms evaporated and only the remaining atoms were used for doping. The wafer-scale monolayer MoS2 was synthesized in a custom-made CVD chamber in a low-pressure state (~10-3 Torr). To synthesize the MoS2, a mass of 5 mg of molybdenum trioxide (MoO3) (≥99.999% , Sigma-Aldrich) in a quartz boat was placed at the center of a furnace, and the SiO2/Si substrate was placed downstream from the MoO3 powder. During the synthesis process, the flow rate of Ar gas was set at 200 standard cubic centimeters per minute (sccm) and that of H2S gas was set at 1 sccm. Characterizations of MoS2: All of the kinds of MoS2 that we made were analyzed using Raman spectroscopy, PL, XPS, TEM, and KPFM. For the Raman spectroscopy and PL, an NTMDT Ntegra atomic force microscope (AFM)-Raman system was used with an excitation wavelength of 532 nm. XPS data were measured with a monochromated Al Kα X-ray source using a K-alpha (Thermo U.K.) system. The TEM images were taken using a Cs-corrected STEM (JEOL, JEM-ARM 200F) with an acceleration voltage of 200 kV. In the KPFM analysis, 19



the KPFM tip, an SI-DF3-A microcantilever type with a resonance frequency of 29 KHz and a spring constant of 2.0 N/m, was used to measure the contact potential difference between the tip and MoS2 surfaces. The KPFM analysis was performed by using a Seiko Instrument AFM. Device fabrication and characterizations: To fabricate each FET, photolithography was used to define the source/drain patterns, and a gold film (50 nm) was deposited using an e-beam evaporator followed by lift-off process to form the source-drain electrode. Subsequently, the channel area was defined by carrying out additional photolithography followed by plasma etching to remove unnecessary MoS2 area.

All devices were back-gated FETs and were

evaluated using a semiconductor parameter analyzer (Agilent, 4155C) in a vacuum environment (2 mTorr). The photoelectrical behaviors of the MoS2 devices were investigated under illumination of a solid-state laser with a wide wavelength range, from 700 nm to 450 nm.

ACKNOWLEDGEMENTS This research was supported by the Basic Science Research Program (2016K1A1A2912707, 2016R1A2B4012931, 2018R1A6A3A11047867) and the Global Frontier Research Center for Advanced Soft Electronics (2011-0031630) through the National Research Foundation of Korea(NRF) and by an Institute for Information & Communications Technology Promotion (IITP) grant (B0117-16-1003) funded by the Ministry of Science, ICT & Future Planning of Korea.

Supporting Information Available: Schematic illustration of synthesis condition and setup (Figure S1), a cross sectional image of TEM (Figure S2), EDS results of Nb doped (Figure S3) and Mn doped (Figure S4) MoS2, KPFM image of graphite (Figure S5), Transfer curves of 20


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doped and undoped MoS2 (Figure S6), Comparison of work functions of Nb doped MoS2 using the KPFM before and after 30 days in ambient air (Figure S7), Optical image of Nb doped MoS2 device array (Figure S8), Photocurrents depending on gate voltage (Figure S9) and drain voltage (Figure S10), Photo-responsivities of 10 photodetectors based on Nb-doped MoS2(Figure S11), Photoresponse time (Figure S12) and Photo-response properties of undoped MoS2(Figure S13). This material is available free of charge via internet at http://pubs.acs.org.

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Figure 1. Schematic of the synthesis, and fundamental characterizations, of the doped MoS2. (a) Schematic illustration of the procedure used to synthesize either Nb- or Mn-doped MoS2. (b) Optical microscopy images of Nb-doped (top) and Mn-doped (bottom) MoS2. The scale bar indicates 20 m. (c) Raman spectra of Nb-doped (red curve), Mn-doped (blue curve), and undoped (black curve) MoS2. (d) Photoluminescence spectra of Nb-doped (red curve), Mndoped (blue curve), and undoped (black curve) MoS2.

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Figure 2. Chemical and structural analyses of the doped MoS2 samples. (a-f) XPS scans of Mo 3d (a,d), S 2p (b,e), Nb 3d (c), and Mn 2p (f) core-level binding energies of Nb-doped MoS2 (red curves), Mn-doped MoS2 (blue curves) and undoped MoS2 (black curves). (g) Aberrationcorrected STEM image from Mn-doped MoS2 with the corresponding atomic structure overlaid on the image and FFT in the inset. The white dashed circle represents an Mn atom. The scale bar indicates 1 nm. (h) Experimental intensity line profile inside the red rectangle marked in panel (g). (i) Aberration-corrected STEM image of Nb-doped MoS2 with the corresponding 26


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atomic structure overlaid on the image and FFT in the inset. The scale bar indicates 0.5 nm. (j) Experimental intensity line profile inside the yellow rectangle marked in panel (i).

Figure 3. KPFM work function measurements and corresponding energy band diagrams. (a) KPFM images of Nb-doped MoS2 (left), undoped MoS2 (middle), and Mn-doped MoS2 (right). The scale bars each indicate 1 m. (b) Band diagrams of Nb-doped MoS2 (left), undoped MoS2 (middle), and Mn-doped MoS2 (right).

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Figure 4. Electrical properties of doped MoS2. (a) Transfer curves of Nb-doped MoS2 (red curve), Mn-doped MoS2 (blue curve), and undoped MoS2 (black curve). The inset shows an optical microscopy image of one of the fabricated FET devices from which the electrical properties were determined. The scale bar indicates 50 m. (b) Output curves of Nb-doped MoS2 (red curve), Mn-doped MoS2 (blue curve), and undoped MoS2 (black curve). (c) Comparison of transfer curves of Nb-doped MoS2 before and after 30 days in ambient air. (d) Photographic image of a large-area film of Nb-doped MoS2 transferred onto a two-inch SiO2/Si wafer. (e) Resistivity distribution across a 4.0 mm x 5.3 mm area divided into 100 regions. (f) Work function distribution across a 4.0 mm x 5.3 mm area divided into 100 regions

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Figure 5. Characterization of doped MoS2 photodetectors. a) Schematic diagram of the photodetector based on doped MoS2. b) Transfer curves of photodetectors based on Nb-doped MoS2 resulting from exposure of the photodetectors to 282nW illumination powers at 550 nm wavelength laser c) Responsivity of Nb-doped MoS2 at VD = 3 V and VG = -100 V as a function of 550-nm-wavelength laser excitation power. d-f) Comparison of photoresponse characteristics of the device based on Nb-doped MoS2 and that based on undoped MoS2, each subjected to an illumination power of 0.22 nW. Photoresponsivity (d), EQE (e), and detectivity (f) values of the devices, each as a function of wavelength of the illumination.

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Figure 6. Comparison of photoresponse characteristics of the detectors made using Nb-doped and undoped MoS2 with corresponding energy band diagrams. a) Generated photocurrent as a function of drain voltage under an illumination power of 80 nW and wavelength of 550 nm. b) Temporal photoresponse behaviors at VD = 1 V with a pulsed 550-nm-wavelength 1.5-nW laser. c) Energy-level alignment for undoped MoS2 (left) and Nb-doped MoS2 (right) with a positive drain-source bias and exposed to the laser illumination.

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Table of Contents Graphic

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Wafer-Scale Substitutional Doping of Monolayer MoS2 Films for High

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