Few-Layer MoS2 p-Type Devices Enabled

Jan 20, 2016 - ABSTRACT: P-type doping of MoS2 has proved to be a significant bottleneck in the realization of fundamental devices such as p-n junctio...
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Few-Layer MoS2 p‑Type Devices Enabled by Selective Doping Using Low Energy Phosphorus Implantation Ankur Nipane,† Debjani Karmakar,‡ Naveen Kaushik,† Shruti Karande,† and Saurabh Lodha*,† †

Department of Electrical Engineering, IIT Bombay, Mumbai 400076, India Technical Physics Division, BARC, Mumbai 400085, India

‡

S Supporting Information *

ABSTRACT: P-type doping of MoS2 has proved to be a significant bottleneck in the realization of fundamental devices such as p-n junction diodes and p-type transistors due to its intrinsic n-type behavior. We report a CMOS compatible, controllable and area selective phosphorus plasma immersion ion implantation (PIII) process for ptype doping of MoS2. Physical characterization using SIMS, AFM, XRD and Raman techniques was used to identify process conditions with reduced lattice defects as well as low surface damage and etching, 4X lower than previous plasma based doping reports for MoS2. A wide range of nondegenerate to degenerate p-type doping is demonstrated in MoS2 field effect transistors exhibiting dominant hole transport. Nearly ideal and air stable, lateral homogeneous p-n junction diodes with a gate-tunable rectification ratio as high as 2 × 104 are demonstrated using area selective doping. Comparison of XPS data from unimplanted and implanted MoS2 layers shows a shift of 0.67 eV toward lower binding energies for Mo and S peaks indicating p-type doping. First-principles calculations using density functional theory techniques confirm p-type doping due to charge transfer originating from substitutional as well as physisorbed phosphorus in top few layers of MoS2. Pre-existing sulfur vacancies are shown to enhance the doping level significantly. KEYWORDS: MoS2, CMOS, plasma doping, hole transport, p−n junction substitutional CVD doping.5−8 Among these, plasma treatment emerges as the most effective doping technique for few layer MoS2 due to a wide range of doping that can be achieved with good control and selectivity. However, reported plasma studies suffer from large surface damage, high rate of etching and lack of precise control over dopant concentration and location. This makes it challenging to dope monolayer or few layer (5−10) MoS2 flakes. In addition, the dopants are not compatible with complementary metal oxide semiconductor (CMOS) processes. For example, fluorine plasmas9 can easily remove (ash) resists used for selective doping in CMOS manufacturing. In this work, we demonstrate p-type doping of few layer MoS2 with phosphorus (P) using a CMOS compatible plasma immersion ion implantation (PIII) process. Raman spectroscopy, atomic force microscopy (AFM) and X-ray diffraction (XRD) characterization were first used to identify PIII process conditions with low surface damage and etching. Back-gated transistors fabricated using varying implant energy and dose

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ncreased short channel effects in conventional bulk semiconductors at scaled dimensions has led to extensive research on 2D semiconductors such as molybdenum disulfide (MoS2). Their atomistically thin dimensions enable excellent gate control.1 A large on-current/off-current ratio (ION/IOFF > 108) along with a low subthreshold slope have been demonstrated through experiments2 and simulations3 for MoS2 field effect transistors (FETs). Moreover, direct bandgap in monolayer form also enables high performance optoelectronics, flexible electronics and energy harvesting applications. However, these applications require complementary devices with p as well as n-type regions, which have been particularly challenging to realize using MoS2 because of its natural propensity for unipolar (n-type) transport. Strong Fermi level pinning near the conduction band, intrinsic defects and ambient interactions result in strong n-type conduction in MoS2.4 The use of traditional doping techniques for controlled chemical doping of MoS2, especially p-type, is challenging due to its atomistically thin dimensions. This has led to the exploration of different doping techniques. A few reports have demonstrated p-type doping of MoS2 using charge transfer, surface functionalization, plasma treatment and © 2016 American Chemical Society

Received: October 17, 2015 Accepted: January 20, 2016 Published: January 20, 2016 2128

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ACS Nano (plasma exposure time) show clear evidence of p-type conduction with transfer characteristics varying from ambipolar to complete p-type indicating nondegenerate to degenerate ptype doping. Gate tunable, air stable, lateral p-n junction diodes formed through area selective p-type doping of MoS2 with excellent rectifying characteristics have also been demonstrated. Finally, X-ray photoelectron spectroscopy (XPS) characterization of MoS2 flakes with and without the PIII process as well as ab initio simulations based on density functional theory (DFT) were used to validate and delineate the doping mechanism.

layers. Secondary ion mass spectroscopy (SIMS) data from implanted bulk MoS2 samples in Figure 1c shows a sharp Gaussian profile for phosphorus compared to the background sulfur. This indicates shallow surface doping and reinforces the SRIM simulation results. The surface nonuniformity of the bulk crystal used for SIMS makes it difficult to measure the sputter depth accurately. However, a first level approximation of sputter depth can be estimated using SRIM simulations. An extracted implantation depth of 3.5 nm for a 2 keV implant was used to determine the sputter depth for the SIMS data. P-type doping of the top few MoS2 layers due to either substitutional and/or physisorbed species (P, PH3, PH+2 , PH2+, etc.) is likely to deplete/invert the underlying n-type MoS2 layers through a charge transfer mechanism as shown through cross-section schematics and energy band diagrams in Figure 1d. Implant process conditions resulting in minimal surface damage and etching were identified through AFM, Raman spectroscopy and XRD analysis of MoS2 implanted with different energies. Figures 2a and 2b show etching of 3−4 nm from the top of the MoS2 flake through comparative AFM line scans before and after implantation at 2 keV, the highest implant energy used in this work. This is nearly 4X lower when compared to previously employed plasma methods likely due to the pulsed DC bias (with variable on-time and frequency).13 Figures 2c and 2d show rms surface roughness values measured using AFM on the same flake before and after a 2 keV implant. A value of 1.14 nm for the unimplanted sample is consistent with reported numbers.14 2 keV implant decreases the roughness by 0.52 nm, again consistent with reports of decrease in Si surface roughness when etched in high density plasmas,15 similar to the one used in our PIII chamber. This can be attributed to a higher flux of etch radicals seen by the crests as opposed to the valleys on the MoS2 surface. Fluorine-based plasmas have been shown to increase the surface roughness of both Si16 and MoS2.17 In this context, the AFM results in this work are highly significant as the first report of reduced MoS2 surface roughness with plasma treatment. The reduced etching and improved surface roughness with PIII indicates that this method can also be extended to monolayer MoS2. The data for 2 keV implant suggests that 0 keV (no-bias) and 1 keV implants should result in lower MoS2 etch rates along with a smaller decrease in surface roughness. Nearly all the devices fabricated for electrical studies employed the 0 and 1 keV implant conditions for minimal surface etching and improved surface roughness. In addition, Raman spectra shown in Figure 3a indicate reduced implant-induced lattice distortions (defects) for lower energies.18 Significant broadening of the E12g peak indicates damage to the MoS2 flake for the 2 keV implant process. However, a 7X reduction in the full width half maxima (fwhm) of the E12g peak indicates lower damage for 0 and 1 keV implants as shown in Figure 3b. Further, thermal annealing at 300 °C after a 1 keV implant nearly recovers the fwhm for both E12g and A1g peaks to the values for a pristine, unimplanted flake as shown in Figure 3c and 3d. All p-type transistors and p-n junction diodes reported later in the manuscript were annealed at 300 °C after the implant and before contact formation. Raman fwhm and AFM surface roughness data summarized in Table 1 shows the impact of varying energy (0, 1, and 2 keV) and annealing at 300 °C on damage created in the MoS2 samples by the PIII process. Low implant energy and thermal annealing result in a low damage process. Moreover, presence of the {002} crystalline peak for all three (0, 1, and 2 keV)

RESULTS AND DISCUSSION This work employs an indigenously built PIII system designed for achieving shallow doping profiles.10 A schematic of the PIII system is shown in Figure 1a. PIII is a CMOS compatible

Figure 1. (a) Schematic of the plasma immersion ion implantation (PIII) system used in this work; (b) SRIM simulation data showing implantation depth of P in MoS2 for varying implant energies; (c) SIMS profiles obtained from P-implanted MoS2 showing shallow phosphorus doping with a sharp Gaussian nature compared to background sulfur; (d) cross-section schematics and energy band diagrams illustrating the impact of p-type doping in the top few layers of n-type MoS2.

process which uses an inductively coupled plasma (ICP) source to generate low energy high density plasma. The amplitude (0− 2 keV), on-time and time period (frequency) of the pulsed DC bias applied to the sample holder can be varied to reduce etching and surface damage. Pulse biasing was preferred over continuous biasing as it creates less damage during the pulse on-time and neutralizes unwanted charges accumulated on the sample. CMOS compatible phosphorus was selected as the doping species due to its high affinity for molybdenum in sulfur deficient MoS2.11 We believe that a large number of naturally occurring sulfur vacancies in the exfoliated MoS2 used in this study, further enhanced by plasma exposure, facilitate phosphorus doping. Stopping range of ions in matter (SRIM) simulations were used to estimate phosphorus implant depths in MoS2 for varying implantation energies.12 A projected range of 0−5 nm obtained from SRIM simulations for an implant energy range of 0−3 keV as shown in Figure 1b indicates shallow phosphorus doping contained within the top few MoS2 2129

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Figure 2. AFM micrographs and line scans of the same MoS2−SiO2 edge (a) before and (b) after a 2 keV PIII phosphorus implant indicating etching of the top 3−4 nm of the flake. AFM surface scans of the same MoS2 flake (a) before and (b) after a 2 keV PIII phosphorus implant indicating a decrease of 0.52 nm in rms surface roughness.

Figure 3. (a) Raman spectra and (b) fwhm values of Raman peaks (E12g and A1g) for unimplanted, 0 keV (unannealed), 1 keV (unannealed), and 2 keV (unannealed) MoS2 samples. (c) Raman spectra and (d) fwhm values of Raman peaks (E12g and A1g) for unimplanted, 1 keV (unannealed), and 1 keV (annealed) MoS2 samples.

type) with a high ION/IOFF ratio (∼106) likely due to intrinsic sulfur vacancies, a low electron injection barrier due to Fermi level pinning near the conduction band and strong electrostatic coupling with gate voltage. However, we observe significant reduction in current along with a positive threshold voltage (VT) shift toward higher gate voltage after p-type doping of a few layers at the top as shown in Figure 4c. Figure 4d illustrates a cross-section schematic of the final device. A decrease in current with the P implant can be attributed to reduction in the number of n-type layers due to p-type counter doping as well as etching of the top few layers. Also, p-type layers at the top deplete the n-type MoS2 underneath of charge carriers and hence a larger gate voltage (positive VT shift) is now required to

implant conditions in the grazing and normal incidence angle XRD data of Figure S1 indicates that the plasma implant energy is too low to amorphize even the top few MoS2 layers. Impact of the phosphorus implant on transport properties was evaluated by fabricating transistors using two different process sequences, (i) implant (doping) after, and, (ii) implant before source-drain contact patterning and metallization. Figures 4a and 4b show the impact of implantation on transfer characteristics of transistors where the implantation was carried out after source-drain contact formation. Thermal annealing for dopant activation was intentionally avoided in these devices as this could also modify the source-drain contact resistance. Unimplanted MoS2 exhibits strong electron conduction (n2130

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ACS Nano Table 1. Comparison of Raman FWHM and AFM Surface Roughness Values Showing the Impact of Varying Energy (0, 1, and 2 keV) and Annealing at 300 °C on Damage Created in MoS2 Samples by the PIII Process fwhm (cm−1) process conditions unimplanted implanted

implanted and annealed

implant energy (keV)

E12g

A11g

0 1 2 1

5.6 6.5 11 71.7 6.5

6.9 7.5 10.7 12.2 7.3

RMS surface roughness (nm) 1.14

0.62

turn on (accumulate) the bottom n-MoS2 transistor channel. These effects are also depicted through energy band diagrams plotted in Figure 4e for A-A′ and B−B′ cuts of Figure 4d. Although this data suggests p-type doping of the top few MoS2 layers, significant p-type transport was not observed since the contacts are still on top of n-MoS2 and strongly pinned near the conduction band.19 Transistors where blanket implantation of the MoS2 flakes was done before source-drain contact formation were also fabricated and characterized. More efficient hole injection through tunneling from the contacts into p-MoS2 enables ptype transport in this case. The transistors were annealed at 300 °C after the implant and before contact formation. As shown in Figure 5a, high implant dose (time =35 s) at 1 keV results in weak gate control at negative VGS and high current values indicating degenerate p-type doping.20 Similar observation of weak gate modulation has been reported by Javey et al.21 for degenerate n-type doping of MoS2 and by Zhao et al.22 for degenerate p-type doping of WSe2. A quantitative estimate of the degenerate hole sheet doping concentration is given later in this manuscript. Nondegenerate doping obtained by lowering the dose (time =25 s) at 0 keV energy results in ambipolar characteristics (Figure 5a) with hole conduction dominating for negative gate voltages and electron conduction for positive gate voltages. The change from electron to hole transport at negative VGS confirms p-type doping. Figure 5b shows the p-

Figure 5. IDS vs VGS transfer characteristics at a fixed VDS of 3 V for (a) degenerately and nondegenerately p-doped MoS2 transistors with the implant and anneal carried out before source-drain metallization and (b) nondegenerately doped MoS2 transistor with varying VDS. (c) Cross-section schematic of the final device and (d) energy band diagrams along A−A′ and B−B′ cuts in the schematic of (c).

type transfer characteristics for the nondegenerately doped transistor with varying VDS and Figure 5c shows a cross-section schematic of the device with cuts along A-A′ and B−B′. The ambipolar transfer characteristics can be explained through energy band diagrams for A-A′ and B−B′ (Figure 5d). For negative VGS, free hole concentration in the top p-MoS2 layers (A-A′) exceeds free electron concentration in the bottom nMoS2 layers resulting in dominant p-type transport. Similar arguments explain n-type transport for positive VGS. The overlap of electron and hole conduction along with electrostatic screening due to the intermediate n-MoS2 layers leads to a relatively low ION/IOFF ratio (∼102) for the hole transport regime at negative VGS. Electrostatic screening can also be understood in terms of a difference in capacitive coupling between the channel and gate for unimplanted and implanted transistors as explained in Figure S2 (Supporting Information).

Figure 4. (a) Before and (b) after implant IDS vs VGS transfer characteristics of MoS2 transistors with the implant carried out after source-drain metalization. (c) Comparative analysis of transfer characteristics before and after the implant for a fixed VDS, (d) cross-section schematic of the final device, and (e) energy band diagrams along A−A′ and B−B′ cuts in the schematic of (d). 2131

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Figure 6. (a) 3D schematic of a lateral p−n junction diode at the time of the phosphorus implant and (b) SEM image of a fully processed p−n junction diode sample.

Figure 7. (a) I−V characteristics of lateral p−n junction diode for varying bottom gate bias exhibit high rectification ratio (up to 2 × 104) and good ideality (∼1.2), (b) diode rectification ratio as a function of applied gate bias, (c) cross-section schematic of the final p−n junction device, and (d) energy band diagrams along A−A′ and B−B′ cuts in the schematic of (c).

Electron transport through the bottom n-MoS2 layers for positive VGS is similar to the case of implanting after contact formation where reduction in number of n-channel layers, counter doping and charge depletion due to p-type MoS2 layers on top leads to a reduction in current level with the implant. Quantitative analysis of transfer characteristics for the degenerately and nondegenerately doped p-type transistors in Figure 5a yields source-drain resistance (RSD) corrected23 peak field effect hole mobilities of 8.4 cm2/V-s and 137.7 cm2/V-s, respectively. These correspond to sheet hole concentrations of 2.4 × 1012 cm−2 and 1.3 × 1010 cm−2 respectively at VGS = 0 V. Similar combinations of mobility and sheet concentrations have been reported previously for nondegenerate24 as well as degenerate25 doping. The decrease in mobility with increasing doping is consistent with previous reports on Nb-doped pMoS2.26 Assuming a range of 7 (entire flake)- 0.7 (monolayer) nm for thickness of the p-type doped MoS2 layers, we get 1.9 × 1016 - 1.9 × 1017 cm−3 and 3.4 × 1019 - 3.4 × 1020 cm−3 as the ranges for hole doping concentrations in the nondegenerately and degenerately doped transistors, respectively. The classification of these sheet concentration values as nondegenerate and degenerate is consistent with the theoretical threshold of 7.7 × 1017 cm−3 for degenerate hole doping in MoS2.27 Calculations for arriving at this theoretical value along with the

detailed methodology used to extract mobilities and sheet concentrations are shown in the Supporting Information. The demonstrated doping concentration range of 1017−1020 cm−3 (considering monolayer MoS2) corresponding to hole sheet densities of 1010−1012 cm−2 is the widest reported until date using plasma treatment. Lateral p-n junction diodes were also fabricated through area selective PIII doping of EBL patterned high quality multilayer flakes (∼5−10 nm) exfoliated on Si-SiO2 substrates as shown in the 3D schematic (Figure 6a), followed by resist stripping and annealing in nitrogen (N2) ambient at 300 °C for 15 min. Finally, contacts were formed using EBL patterning, metallization and liftoff. Figure 6b shows a scanning electron microscopy (SEM) image of a sample diode. Lateral homojunction p-n diodes can support higher levels of current density due to the lack of band-offsets and higher in-plane mobilities when compared to vertical heterojunction diodes.28 I-V characteristics in Figure 7a show a rectification ratio (RR) as high as 2 × 104 (Vbias = ± 3 V) and an ideality factor of 1.2 at VBG = −80 V. The diode rectification ratio can be modulated using VBG as shown in Figure 7b. Figure 7c shows a crosssection schematic of the final device while Figure 7d shows the energy band diagrams along the A-A′ and B−B′ cuts for the top p-n junction diode and the bottom n-MoS2 transistor, 2132

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Figure 8. (a) IDS−VGS data of transistors implanted with varying energies but fixed phosphorus dose, (b) normalized change in transistor ION and shift in VT versus implant energy, and (c) I−V characteristics of p−n diodes fabricated using different implant doses at fixed energy of 0 keV. Longer implant time (40 s) corresponds to higher dose.

Figure 9. (a) XPS spectrum for MoS2 implanted with phosphorus showing presence of the P 2p3/2 peak. XPS spectra for (b) molybdenum (Mo 3d5/2 and Mo 3d3/2) and (c) sulfur (S 2p3/2 and S 2p1/2) peaks before and after implantation showing a downward shift of 0.67 eV in binding energy for all four peaks.

formation of a stable p-n junction. Impact of thermal annealing on diode and transistor characteristics can be seen in Figure S4 (a, b). A 9X increase in diode rectification ratio and decrease in ideality factor indicates significant repair of implant-induced defects. For back-gated transistors, 13% increase in on-current and 3.5X increase in hole sheet concentration along with reduced gate control indicates increase in dopant activation and defect reduction due to thermal annealing. However, a 3X reduction in peak field effect mobility was also observed which can be attributed to increased scattering due to higher sheet concentration. The implanted diodes show good air stability with minimal change in I−V characteristics over 7 days of ambient exposure as shown in Figure S4c. PIII process offers the ability to control dopant profile and concentration by varying energy and dose conditions. Transistors that received a fixed implantation dose at different energies after source-drain contact formation were fabricated to demonstrate this capability of PIII. Figure 8a shows the IDS-VGS data indicating a monotonic decrease in transistor ION with increasing implantation energy and a significant increase in VT for the 1 and 2 keV higher energy implants. Figure 8b shows the average normalized change in ION and VT obtained from measurements on multiple devices for each energy. Larger drop in current level and increase in VT with increasing implant energy can be explained by the reduction in number of n-type layers and their electron charge density due to the deeper ptype counter doping (see Figure 4 and accompanying text). Figure 8c shows the impact of varying implant dose (time) on electrical performance of p-n diode samples. Increasing the dose at constant implant energy of 0 keV increases (4X) the rectification ratio due to the higher doping concentration resulting in higher diffusion on-current and reduction in series resistance. The high degree of controllability offered by the PIII process makes it useful for a large variety of device applications.

respectively. Variable diode rectification can be explained by comparing the current components of the top p-n junction and the bottom n-type transistor at different gate and diode voltages. With increasing positive VBG the transistor channel turns on with high electron density and adds to the reverse bias leakage current of the diode. At the same time the high electron density reduces series resistance of the quasi-neutral n-MoS2 region resulting in higher forward bias on-current. As VBG is made zero and then negative, the bottom transistor turns off exponentially below VT resulting in an exponential decrease in diode reverse leakage that leads to an exponential increase in rectification ratio as seen in Figures 7a and 7b. This is also accompanied by a smaller decrease in on-current of the diode due to an increase in series resistance because of lower electron density. This suggests that true electrical performance of the p-n junction is observed for VBG = −80 V when the bottom transistor is completely switched off. Hence 2 × 104 refers to the absolute rectification ratio of the diode, which is also the highest achieved with plasma based doping of MoS2 until date. To rule out the possibility of rectification due to different Schottky barrier heights at the contacts, output characteristics of unimplanted (n-type) and doped (p-type, implantation before source-drain contact formation) transistors were measured as shown in Figure S3. Ti/Au contacts in both cases are ohmic in nature with symmetric I−V characteristics over ±1 V of applied VDS. They are either p-type (hole transport modulated with negative VBG) or n-type (electron transport modulated with positive VBG) depending on whether the channel is doped p-type or unimplanted n-type, respectively. The I−V characteristics indicate efficient electron (hole) transport/injection across the source-drain contact barriers for the unimplanted (implanted) sample for positive as well as negative VDS. Hence, rectification observed in the diode sample (Figure 7a) can only be attributed to the 2133

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Figure 10. OPDOS and APDOS plots for the three cases: (a) P-substituted, (b) P-adatom, and (c) PH3-adsorbed. For each part, the top panel represents the unimplanted 1H-MoS2 DOS for comparison. The middle and bottom panels depict the DOS without and with SV, respectively. For all three cases, P partial DOS (green) has been multiplied 10× to improve its visibility over the same scale. States close to the Fermi level have been highlighted to indicate possible changes resulting from P incorporation in the system.

the P-substituted, P-adatom and PH3-adsorbed cases result in a lowering of the total energy compared to 1H-MoS2, adsorption of PH2+ and PH2+ ions results in an unstable system with much higher total energy. In addition, the relaxed system with these ions, without any selective dynamics, results in only the Padatom adsorbed on MoS2 surface with the H-ions getting stripped off from P and placed closer to S. Therefore, for the third case, we have only dealt with PH3-adsorption. A comparison of the total energy for all these cases indicates that P-substitution and P-adatom are energetically more favorable than PH3-adsorption. Presence of a nearest neighbor (NN) SV favors the P-substituted, P-adatom and PH3-adsorbed cases by 981, 937, and 35 meV respectively than the corresponding without-vacancy systems. Presence of MoV favors formation of only the first two cases by 655 and 562 meV respectively. Comparison of ionic binding energies (IBE) for these three cases with pristine 1H-MoS2, as presented in Table S1, indicates a considerable reduction in the IBE of Mo and S for all three cases, which gets enhanced further in the presence of vacancies. This reduction in IBE is consistent with the reduction in BEs observed in XPS results. A detailed discussion on IBE is presented in the Supporting Information. In addition to this, impact of interlayer separation on p-type doping of MoS2 using phosphorus is also discussed in Supporting Information. Here, P atom and PH3 molecule placed in the interlayer space of bilayer MoS2 show significant doping in the presence of SV as shown in Figure S5. We have investigated the orbital (OPDOS) and atomprojected (APDOS) density of states for all three cases mentioned above. Figure 10a depicts OP and APDOS plots for the P-substituted case. We have substituted one single S in 1HMoS2 by a P atom corresponding to a doping percentage of 3.125%. In Figure 10a-(i), total DOS and OPDOS for Mo-4d and S-3p are plotted for 1H-MoS2, showing a bandgap of ∼1.8 eV with strongly hybridized Mo-4d and S-3p orbital levels constituting the top of the valence band (VBT) and bottom of the conduction band (CBB). For P-substituted case, Figure

X-ray photoelectron spectroscopy (XPS) characterization was done on MoS2 samples before and after implantation to confirm and understand the doping mechanism. Figure 9a confirms the presence of phosphorus after implantation. Additionally, the P 2p3/2 peak occurs at a binding energy (BE) of 133.4 eV which matches the peak position for P 2p3/2 in molybdenum phosphide (MoP).29 This indicates that the implanted phosphorus interacts chemically with MoS2 and is present at a substitutional/interstitial site. BE shifts in XPS spectra for core level electrons have been used previously to identify the doping type in MoS2.30,31 Figures 9b and 9c compare the BEs for Mo 3d5/2, Mo 3d3/2, S 2p3/2 and S 2p1/2 peaks before and after implantation on the same flake to avoid ambiguities arising from flake-to-flake variations. All four peaks shift by ∼0.67 eV to lower BEs with the implant indicating a shift in the Fermi level toward the valence band. This is equivalent to doping the MoS2 layers p-type. Hence, the XPS data confirms that phosphorus acts as a p-type dopant in MoS2 primarily occupying substitutional/interstitial sites. First principle calculations using DFT were also used to gain additional insights into the doping mechanism. Phosphorus has been theoretically predicted to result in p-type doping of MoS2 as a substitutional dopant.32 However, the use of a PH3 plasma in our experiments to dope MoS2 p-type suggests three possibilities, viz. (1) P substitutional doping either by replacing S or occupying a S-vacancy (SV), (2) P-adatom adsorbed on the MoS2 surface, and/or, (3) PH3, PH2+ and PH2+ ions adsorbed on the MoS2 surface. Presence of S or Mo-vacancies (MoV) will also influence the doping nature of all these three cases. Since MoS2 has a higher propensity for SV, the simulation study focuses more on analyzing these doping cases in the presence of SVs. All these three cases have been constructed for a free-standing MoS2 monolayer (1H-MoS2) and the ionic positions and lattice parameters of the full assembly have been geometrically optimized following the conjugate gradient algorithm until the Hellmann−Feynman forces acting on each atom are less than 0.01 eV/Å.33 Although 2134

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Figure 11. Charge-density plots for (a) P-substituted without SV, (b) P-adatom without SV, and (3) PH3-adsorbed without SV.

resulting from Mo-4d and P-3p states. In the presence of SV, similar to the P-adatom case, the PH3 molecule also tries to occupy the SV, albeit remaining at a slightly higher level than the S-plane, leading to little charge transfer from the nearest neighbor Mo resulting in only a small shift of EF toward the valence band, as seen in Figure 10c (bottom panel). Comparison of parts b and c of Figure 11 shows that the amount of charge transfer to P-adatom is far larger than for the PH3-adsorbed case. This is consistent with the lesser IBE of P for case (3) (Table S1). Thus, phosphorus implantation using a low energy PIII process leads to a mixed state of all the above three possible configurations with more favorable chances for P-substitution, resulting in p-type doping of 1H-MoS2.

10a-(ii) in the same panel depicts a shift of the Fermi level (EF) toward the valence band indicating p-type doping. This system shows a difference in the Mo−S (2.42 Å) and Mo−P (2.39 Å) bond-lengths after geometrical optimization. The impact of shorter bond-length can be clearly observed from the overlapping charge spheres around Mo and P in the chargedensity plot of Figure 11a. There is no such overlap for Mo−S bonds in the same isosurface scale. This difference in bondlength increases in the presence of a NN SV. The Mo−P bond length reduces to 2.37 Å which leads to more charge sphere overlap. Thus, although the presence of SV initially leads to ntype doping within the system, vacancy-induced structural distortion leads to significant charge transfer from Mo to P. To obtain a visible effect on OPDOS of Figure 10a-(iii), we have kept the concentration of P-dopant (6.25%) as double the concentration of SV (3.125%). Due to charge transfer from Mo-4d to P-3p, Mo-4d levels (marked red in figure) become partially unoccupied compared to Figure 10a-(i). These unoccupied levels are highly hybridized with P-3p levels (green) indicating the charge transfer direction. Presence of SV results in an additional shift in EF toward the valence band indicating increased p-type doping, as can be seen from Figure 10a-(iii). The levels are highlighted near EF in all the figures to indicate the extent of p-type doping. DOS plots for adsorption cases (2) and (3) are also presented in Figures 10b and 10c, respectively. For P-adatom, the charge-density plot (Figure 11b) implies that a significant amount of the charge transfer happens from Mo to P via NN S atoms due to van der Waalstype bonding of P over S-layer of MoS2. Figure 10b-(ii) indicates that Mo and S occupied levels shift to a more bound state because of this charge transfer. This particular case shows no p-type doping without the presence of a vacancy. In presence of SV, after relaxation, the P-adatom is seen to settle at the SV position but with a shorter bond-length (∼2.37 Å) with the Mo-ion and thereby charge transfer from Mo to P leads to partially unoccupied Mo-4d levels hybridized with P-3p near EF. Qualitatively, this case becomes exactly similar to a Psubstituted system, as can be inferred from the close resemblance between the DOS figures presented in Figures 10a-(ii) and 10b-(iii). The extent of p-type doping appears to be slightly larger for the latter case because of shorter Mo−P bond length compared to the P-substituted case. From an experimental perspective this also implies that presence of SV promotes conversion of any adsorbed P into a substituted one. Adsorption of PH3, however, is not energetically favorable for p-type doping. PH3 does not lead to significant charge transfer from 1H-MoS2 because of its flattened nature of self-charge distribution. As can be seen from Figure 10c (middle panel), adsorbed PH3 only leads to some additional gap states mostly

CONCLUSION In conclusion, we have demonstrated p-type doping of MoS2 using a low damage phosphorus PIII process. High level of control and area selectivity enabled by PIII is shown to result in a wide range of doping concentration and potential use in various device applications. Dominant p-type transport in field effect transistors as well as air-stable, lateral p−n junction diodes with a high rectification ratio (2 × 104) demonstrate electrical proof of selective p-type doping of MoS2. XPS analysis confirms the p-nature of phosphorus doping which is further reinforced by DFT simulation data for substitutional and adsorbed P in MoS2 layers, especially in the case of pre-existing S vacancies. Although data for monolayer MoS2 has not been shown, the low-damage PIII process can be easily adapted to fabricate monolayer devices. These findings open up the roadmap for development of complementary (p and n) and p− n junction-based MoS2 devices as well as the use of PIII doping technique for other emerging 2D materials. METHODS Transistor fabrication started with reactive ion etching of thermally grown SiO2 on a degenerately doped Si substrate. MoS2 was mechanically exfoliated from molybdenite crystals (SPI supplies) using the scotch tape method. Large area, multilayer flakes were identified using optical, Raman and atomic force microscopy. Transistors were fabricated on the selected flakes using bilayer resist (PMMA and EL-9) with electron beam lithography (EBL, Raith 150Two) for source-drain contact patterning. In this work, gold (Au) has been used as the primary contact metal with a thin Ti adhesion layer, since it has been shown to form ohmic contacts on MoS2. Samples were then transferred to the PIII chamber for the phosphorus implant through PH3/He plasma exposure either before or after source-drain contact patterning and metallization according to the sample type. The samples were kept at an appropriate distance from the plasma source to minimize surface damage. The indigenously built PIII system uses a pulsed DC bias source with a bias range of 0 to 5 kV. Chamber 2135

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ACS Nano pressure was kept constant at 1.1 × 10−1 mbar with RF power set at 1000 W for all the implant runs. A process flow similar to the one used for transistor fabrication was followed for the diode samples. Multilayer flakes exfoliated on SiO2/Si substrates were first patterned for area selective plasma exposure using EBL for forming the p−n junction. PIII chamber was used to dope MoS2 p-type using phosphorus through a PH3/He plasma implant. The resist was then stripped by soaking the samples in acetone for 2 h. After resist stripping, the samples were annealed in N2 ambient at 300 °C for 15 min using a 4 in. quartz furnace. Finally, contacts to the p and n regions of the MoS2 flake were fabricated using EBL, metallization, and liftoff. Room-temperature electrical transport studies were conducted using a probe station connected to an Agilent BX-51 parameter analyzer. Photoemission XPS studies were done using a PHI Versaprobe system equipped with an Al kα source. SIMS doping profiles were acquired using TOF-SIMS (PHI Trift V Nano) equipped with LMIG and Cesium ion guns. XRD analysis was performed using RIGAKU Smartlab where both normal and GIA modes were used. Experimental observation of p-type doping of MoS2 through a phosphorus implant was verified and analyzed with the help of spinpolarized density functional calculations. We have used projectoraugmented wave (PAW) potentials with Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional within generalized gradient approximation (GGA), as implemented in the Vienna Ab-initio Simulation Package (VASP).34 The plane wave cutoff energy was 500 eV, and Monkhorst−Pack (MP) meshes of 17 × 17 × 1 and 5 × 5 × 3 were used for complete geometry optimization and electronic structure calculations of substitutional and adsorbed cases, respectively. van der Waals interactions were incorporated using DFT-D2 method of Grimme as in VASP. We have used a 4 × 4 × 1 MoS2 monolayer to simulate different cases resulting from the various possible experimental scenarios.

Government of India, for funding this work. D.K. acknowledges the support of the BARC-ANUPAM supercomputing facility.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b06529. Impact of phosphorus implant on the crystallinity of MoS2 using XRD characterization, capacitor models to explain gate voltage dependence of the transfer characteristics of unimplanted (n-type) and implanted (p-type) MoS2 transistors, extraction methodology for hole field effect mobilities and sheet concentrations from IDS−VGS data and calculation of theoretical limit for degenerate hole doping in MoS2, the output characteristics of unimplanted and implanted MoS2 transistors showing ohmic nature of contacts to n and p-type MoS 2 respectively, impact of thermal annealing and air exposure on electrical characteristics of diodes and transistors, DFT analysis of ionic binding energies for various experimental scenarios of P incorporation in the MoS2 system and their correlation with observed XPS binding energy shifts, and DFT analysis of the impact of interlayer separation on p-type doping of MoS2 (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the Confederation of Indian Industry (CII) and the Department of Science and Technology (DST), 2136

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