pn homo-junction by efficient carrier type modulation

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Functional Nanostructured Materials (including low-D carbon) 2

Implementing lateral MoSe p-n homojunction by efficient carrier type modulation Shuangqing Fan, Wanfu Shen, Chunhua An, Zhaoyang Sun, Sen Wu, Linyan Xu, Dong Sun, Xiaodong Hu, Daihua Zhang, and Jing Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08422 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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ACS Applied Materials & Interfaces

Implementing Lateral MoSe2 P-N Homo-junction by Efficient Carrier Type Modulation 1

Shuangqing Fan, 1Wanfu Shen, 1Chunhua An, 1Zhaoyang Sun, 1Sen Wu, 1Linyan Xu, 2Dong Sun, 1Xiaodong Hu, 1Daihua Zhang and 1Jing Liu* 1

State Key Laboratory of Precision Measurement Technology and Instruments, School of Precision Instruments and Opto-electronics Engineering Tianjin University, No. 92 Weijin Road, Tianjin, 300072, China

2

International Center for Quantum Materials, School of Physics, Peiking University, No. 5 Yiheyuan Road, Beijing 100871, China

Corresponding author e-mail: [email protected]

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Abstract High performance p-n junctions based on atomically thin two-dimensional (2D) materials are the fundamental building blocks for many nano-scale functional devices that are ideally for future electronic and optoelectronic applications. The lateral p-n homo-junctions with conveniently tunable band offset outperform vertically stacked ones, however, the realization of lateral p-n homo-junctions usually require efficient carrier type modulation in a single 2D material flake, which remains a tech challenge. In this work, we have realized effective carrier type modulation in a single MoSe2 flake, and thus, a lateral MoSe2 p-n homo-junction is achieved by sequential treatment of air rapid thermal annealing and triphenylphosphine (PPh3) solution coating. The rapid thermal annealing modulates MoSe2 flakes from naturally n-type doping to degenerated p-type doping and improves the hole mobility of the MoSe2 field effect transistors from 0.2 cm2·V−1·s−1 to 71.5 cm2·V−1·s−1. Meanwhile, the n-doping of MoSe2 is increased by drop coating PPh3 solution on MoSe2 surface with increased electron mobility from 78.6 cm2·V−1·s−1 to 412.8 cm2·V−1·s−1. The as-fabricated lateral MoSe2 p-n homojunction presents high rectification ratio of 104, ideality factor of 1.2 and enhanced photo-response of 1.3 A·W-1 to visible light. This efficient carrier type modulation within a single MoSe2 flake enables its superb potentials in various functional devices.

Keywords: MoSe2, lateral p-n homo-junction, rapid thermal annealing, triphenylphosphine coating, photocurrent


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1. Introduction Functional electronic and optoelectronic devices fabricated by atomically thin semiconducting materials potentially possess enhanced performances, further scaled-down device dimensions and the capability to confer flexibility to wearable electronics. The two-dimensional (2D) transition metal dichalcogenides (TMDs) family has rich species with varied parameters in terms of band structure and intrinsic doping intensity to allow the implementation of numerous nano-electronic and optoelectronics devices.1 As compared to the extensively studied MoS2, recent studies on MoSe2 reveals that it possesses many superior properties, such as large atomic radius of selenide, high electrical conductivity (1 × 10−3 S·m−1 for selenium vs. 5 × 10−28 S·m−1 for sulfur)2 and large range tunable indirect to direct bandgap from 1.33 eV to 1.72 eV3. These properties make it highly desirable for plentiful applications including lithium/sodium-ion batteries,4,5 hydrogen evolution reaction (HER) catalyst,6,7 photoelectrochemical and photo-voltaic devices.8 One major challenge in developing nano-scale MoSe2 functional devices is to efficiently modulate the carrier type and concentration of few-layer MoSe2 flakes. So far, diverse doping strategies have been developed for atomically thin 2D materials, including electrostatic injection, chemical treatment and atom substitution, etc.9-16 Among these methods, the substitution of either transition metal, e.g. Mo1-xWxSe2,12,13 Nb-doped MoSe2 (p-MoSe2),14 or isoelectronic chalcogen, e.g. MoS2xSe2(1–x),15,16 have been successfully applied to p-dope the naturally n-type MoSe2. Based on these methods, many vertically stacked MoSe2 van der Waals p-n homo-junctions have been successfully fabricated. However, the properties of van der Waals p-n junctions are strongly affected by the stacking orientation and interlayer coupling,16,17 which is unfavorable for large scale high-performance industrial level production. In contrast, the lateral p-n homo-junction will be more attractive than the van der Waals one, because of the easier band offset tuning. Therefore, the implementation of controllable p- and n-type doping in a single MoSe2 flake, and thus, lateral MoSe2 p-n homojunctions is highly desirable to develop MoSe2 functional devices. In this work, we have successfully fabricated a lateral MoSe2 p-n homojunction by combining air rapid thermal annealing (RTA) and triphenylphosphine (PPh3) coating for p- and n-type



doping, respectively. These two doping methods are highly compatible with each other, and thus, the lateral MoSe2 p-n homo-junction can be fabricated by sequentially annealing few-layer MoSe2 flake in air at 360 oC (p-type doping) and drop coating PPh3 on the annealed and partially covered (by h-BN) MoSe2 flake (n-type doping). The MoSe2 flake becomes p-doped during air RTA as oxygen atoms occupy Se vacancies and/or further oxidize MoSe2 at elevated temperature, while PPh3 can n-dope the MoSe2 flake as a result of charge transferring between PPh3 and MoSe2. We further investigated the optimum conditions for pand n-type doping methods for MoSe2, respectively, which were extensively characterized by Raman spectra and X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM). The as-fabricated lateral MoSe2 p-n homojunction showed a high rectification ratio of 104 and ideality factor of 1.2 with enhanced photo-response of 1.3 A·W-1 to visible light at 0 V gate and source-drain bias. 2. Results and Discussion 2.1 P-doping MoSe2 by air RTA Figure 1 shows the effectiveness of air RTA treatment to p-dope MoSe2. The pristine MoSe2 FET was prepared by sequentially mechanical exfoliating a few-layer MoSe2 flake onto the SiO2/Si substrate, and depositing Cr (20 nm)/Au (30 nm) electrodes on the surface by E-beam evaporation (the optical microscope image of the fabricated device is shown in Supporting Information Fig. S1). Figure 1a displays the transfer characteristic (ID-VG) of the pristine MoSe2 FET (Electrode 2 and 3 were used as the source and drain electrodes, respectively) with a source-drain bias of 1 V. Compared to the value at threshold gate bias, the drain current (ID) increases when gate bias either increases or decreases. However, the drain current of the MoSe2 FET at +60 V gate bias is three orders of magnitude higher than the value at -60 V gate bias, indicating an electron-dominated ambipolar characteristic. The on-off current ratio is ~106, which is at the same level as the previous reports.18-19 The electron and hole mobilities are calculated to be 78.6 cm2·V−1·s−1 and 0.2 cm2·V−1·s−1, respectively, according to the transfer curve. After being annealed under 360 oC for 2 min, transfer curve of the device shifts from n-type dominant to almost symmetric ambipolar and the hole current increases by two orders of magnitude (Fig. 1b) under -60 V gate bias. This is consistent with a positive


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shift of the threshold voltage from -20 V to 0 V and a sharp decrease of the on-off current ratio to 103. As the annealing duration increased to 4 min, the transfer curves evolve to be p-type, reaching an on-off current ratio of 105 (Fig. 1c). Figure 1d-f depict the atomic configurations of the carrier type transition process of MoSe2 under air RTA annealing for 0 min (pristine), 2 min and 4 min, respectively. The pristine MoSe2 device is usually n-doped, similar to several other TMDs (e.g. MoS2, MoTe2),

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because of the abundant selenium vacancies existed in MoSe2 flakes (as shown in Fig. 1d). 23,24

When the MoSe2 flake is annealed in controlled air environment at an elevated

temperature (360 oC), the Se vacancy sites can be occupied by oxygen atoms that are dissociated from oxygen molecules under high temperature (as shown in Fig. 1e).25 As oxygen atoms fully occupy the Se vacancy sites and form Mo-O bonds, the impurity states in the band structure induced by Se vacancies disappear, and the band structure evolves to a state that is similar to the original state of a defect-free MoSe2.25 Consequently, the device is doping free and exhibits symmetric ambipolar transfer characteristic. If the annealing duration is further extended, oxygen atoms may further oxidize MoSe2 and form MoOx (x>2). The formation of MoOx shifts the Fermi level downward to the valence band through the band alignment.26 Meanwhile, extra O2 molecules can be adsorbed on MoSe2 flake under the ambient environment (Fig. 2f), which act as electron acceptors and cause further increase in hole doping.27 This is evidenced by 3 orders of magnitude decrease of the off-state current after placing the annealed MoSe2 FET in air at room temperature for 2 months (on-state current does not change much, see Supporting Information Fig. S2).



Figure 1. (a-c) ID-VG characteristics (black line: linear scale; red line: semi-log scale) of the same device with VD = 1 V before (a), and after rapid thermal annealing at 360 ℃ for 2 min (b) and 4 min (c), respectively. (d-f) The atomic configurations of pristine n-dominated ambipolar (d), air RTA-treated symmetric ambipolar (e), and air RTA-treated unipolar p-type (f) MoSe2, respectively. Se vacancies are marked in (d).

Next, we further explored the optimum annealing temperature and duration to p-dope MoSe2. Figure 2a shows a linear scale ID-VG curves of few-layer MoSe2 FETs before and after being annealed in air under 270 oC, 300 oC, 330 oC, 360 oC, and 390 oC for 4 min, respectively (see Supporting Information Fig. S3 for semi-log-scale plot of ID-VG curves). The MoSe2 FET annealed at 360 oC (green line) presents the maximum on-state current of 12 mA and on-off current ratio of 106，respectively, while annealing at other temperatures (either lower or higher than 360 oC) leads to significant reduction on both on-state current and on-off current ratio. This optimized annealing temperature for MoSe2 is higher than that used for MoTe2 reported by D. Qu and co-workers. 22 The reason is possibly due to that a higher energy barrier is required for O atom to be chemibsorbed on MoSe2 than on MoTe2. Thus, we chose the annealing temperature of 360 oC for the following MoSe2 device fabrication. Figure 2b and c investigate the effect of annealing time on the doping level of MoSe2. Figure 2b shows the transfer characteristic evolution of the MoSe2 device in a semi-log scale at 360 o

C as annealing time increases from 0 min (pristine) to 8 min. The increase of the channel

current and hole mobility (to 71.5 cm2·V−1·s−1) at -60 V gate bias (see Figure 2c), as well as degraded gate tunability of channel current, indicates the progressively increased p-type


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doping up to degenerated level with doping density of 6.8×1012 cm-2. This result suggests that the precise control of the p-doping concentration of MoSe2 can be achieved by varying the annealing time of air RTA. We further study the thickness effect of MoSe2 on the annealing induced p-doing in Supporting Information Fig. S4, which indicates that this method is generally applicable to samples with thickness equal to or thicker than 4.2 nm.

Figure 2. (a) Linear scale ID-VG curves of MoSe2 FETs before and after air RTA treatment at various temperatures. (b) Evolution of device transfer characteristics from n-type doped to degenerated p-type doped after being annealed in an air chamber at 360 ℃ for various durations. (c) Hole mobility µh and channel current ID at VD = 1 V as a function of annealing time. 2.2 Characterization of p-doped MoSe2 To inspect the doping process and mechanism, Raman and X-ray photoelectron spectroscopy (XPS) measurements were performed on pristine and annealed MoSe2 samples, respectively. The pristine samples were mechanically exfoliated from bulk material and transferred onto h-BN/SiO2/Si substrates for Raman tests (adding a h-BN flake between the MoSe2 and SiO2/Si substrate was to improve the device yield and performance after air RTA treatment). Figure 3a shows the Raman spectra of a MoSe2 flake before and after annealing. Two characteristic peaks of A1g and E12g modes are observed for the pristine MoSe2 samples.

28

After annealing at 360 oC for 2 min and 4 min, the A1g and E12g peaks of the same MoSe2 sample showed no observable changes in terms of intensity or position (Fig. 3b), indicating that the lattice integrity of MoSe2 flakes remained intact after air RTA. Supporting Information Fig. S5a and b show the high resolution transmission electron microscope (HR-TEM) image and fast Fourier transform (FFT) pattern of a MoSe2 flake before and after annealing, which also indicates that the highly crystalline hexagonal MoSe2 phase remains after annealing. Figure 3b and c show the high-resolution XPS spectra of Mo 3d and Se 3d



core levels, respectively, of pristine and annealed MoSe2. In the XPS spectra of pristine MoSe2 sample (bottom lines in Fig. 3b and c), the Mo 3d core level has the (+4) 3 d3/2 and (+4) 3d5/2 doublet at 229.2 eV and 232.3 eV, respectively, which are characteristic peaks of crystalline MoSe2.5,29 Minor sub-bands of Mo6+ corresponding to oxidized MoSe2 are also apparent at 233.0 eV and 235.9 eV, since the samples were exposed to atmosphere before XPS analysis.30 After the sample is annealed, the intensity ratio of Mo6+/Mo4+ increases with increased annealing time, indicating the progressing adsorption of oxygen atoms on MoSe2 and formation of MoOX during air RTA. After MoSe2 is annealed for 4 min, the Mo 3d and Se 3d peaks both downshift by 0.3 eV as compared to pristine sample, indicating that the Fermi level of MoSe2 moves toward the valence band edge. XPS survey of annealed MoSe2 are presented in Supporting Information Fig. S6, in which no obvious peaks corresponding to new chemical bonds are observed after annealing, indicating limited effect of moisture and carbon species in ambient air on MoSe2 annealing.

Figure 3. (a) Raman spectra of MoSe2 on h-BN/SiO2/Si substrate treated by air RTA with various durations. Raman peaks were normalized and calibrated by silicon peak. (b, c) XPS spectra of core-level Mo 3d (b) and Se 3d (c) binding energy (BE) peaks of MoSe2 with increased air RTA treatment duration. 2.3 N-doping MoSe2 by PPh3 The n-branch performance of the MoSe2 FET can be enhanced by drop-coating the n-type dopant PPh3 solution on MoSe2 surface. Figure 4a shows the schematic illustration of the n-doping mechanism. As previously studied, the PPh3 layer coated on the MoSe2 devices will induce negative charge transfer from PPh3 to MoSe2, and thus, shifts the Fermi level of MoSe2 towards conduction band.31 Figure 4b is the transfer characteristics of MoSe2 FETs coated by PPh3 in toluene with various concentrations ranging from 10 mM to 30 mM. As the


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concentration of PPh3 increases, the threshold gate voltage shifts towards the negative value, resulting in increased on-state current and decreased on-off current ratio within ±60 V gate bias. The MoSe2 FET achieves degenerated n-type doping when the concentration of PPh3 is 30 mM. We further calculated the electron mobility µn and doping density ne of the MoSe2 FET coated by PPh3 with different concentrations under VG=60 V bias (see Supporting Information Fig. S7). As the PPh3 concentration increases from 0 mM to 30 mM, µn increases from 78.6 cm2·V−1·s−1 to 412.8 cm2·V−1·s−1 and ne increases from 8.8 × 1011 cm-2 to 9.0 × 1012 cm-2. Figure 4c and d are the output characteristics of pristine and 20 mM PPh3 coated MoSe2 devices, respectively. The output curves of 20 mM PPh3 coated MoSe2 are more linear and symmetric than that of the pristine one, owing to the improved Ohm contact between n-doped MoSe2 and electrode Cr.

Figure 4. (a) Schematic illustration of back-gated (280-nm thick SiO2 gate dielectric) MoSe2 transistor device used for PPh3 doping. (b) Transfer characteristics of the MoSe2 FETs treated by PPh3 solutions with different concentrations. (c, d) ID-VD characteristics of the same MoSe2 FET device before (c) and after (d) 10 mM PPh3 doping.

2.4 Characterization of n-doped MoSe2 Figure 5 shows the XPS spectra and Raman spectra of PPh3 doped MoSe2. In Fig. 5a, the total intensity of MoOX peaks of 20 mM PPh3 doped MoSe2 significantly decreases to ~20% of



that of pristine sample. Additionally, a new peak is evolved at 234.7 eV for PPh3 doped sample, which is designated as the Mo5+ MoOX state,29 indicating the reduction of Mo6+ (i.e. electrons transferring to MoSe2 results in a lowered chemical state of Mo). Moreover, after PPh3 is coated on the MoSe2 flakes, the binding energy of Mo 3d and Se 3d shifts by around 0.1 and 0.2 eV to the high energy direction, which can be interpreted by the up-shift of the Fermi level toward the conduction band (see Fig. 5a and b). Since the XPS analysis cannot quantitatively measure the exact value of Fermi level shift, we performed KPFM on a MoSe2 sample transferred on SiO2/Si substrate and partially doped by 30 mM PPh3 (see Fig. 5c). The KPFM image obtained from the partially n-doped MoSe2 flake shows significant difference in surface potential (Φs) between the PPh3 and MoSe2. The surface potential of the PPh3 is 653 mV lower than that of the MoSe2, which causes the electron transfer from PPh3 to MoSe2. Figure 5d shows the Raman spectra of the pristine and PPh3 doped MoSe2 samples. In comparison with pristine sample, the E12g peak of 30 mM PPh3 doped MoSe2 sample shifts 3.5 cm-1 to the lower wavenumber while the intensity of the A1g is weakened. These spectra changes may be interpreted by the reduction of Mo6+ and electron−phonon coupling. The above XPS, KPFM, and Raman measurement results together suggest a negative charge transfer from PPh3 to MoSe2, resulting in the decrease of the oxidation state and the Fermi level up-shift of MoSe2.


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Figure 5. (a, b) The XPS core level spectra of Mo 3d (a) and Se 3d (b) of MoSe2 before and after 20 mM PPh3 doping. (c) KPFM mapping image of the PPh3-doped and undoped regions on the same MoSe2 flake. (d) Raman spectra of MoSe2 before and after 10 mM and 30 mM PPh3 doping at room temperature.

2.5 Lateral MoSe2 p-n homojunction In Fig. 6, we demonstrate a lateral MoSe2 p-n homo-junction fabricated by the aforementioned p- and n-type doping methods. A few-layer MoSe2 flake was mechanical exfoliated and transferred on the h-BN/SiO2/Si substrate. Then, it was annealed in air at 360 o

C for 4 min to achieve p-type doping. After that, the MoSe2 was partially covered by another

h-BN flake to preserve the p-type doping, while the remaining uncovered area was n-doped by 20 mM PPh3. Consequently, the p-n homojunction was formed on a single MoSe2 flake, the optical microscopy image of which is shown in the inset of Fig. 6a. Figure 6a shows the output I-V curves of the MoSe2 p-n junction with various gate bias in semi-log scale, presenting typical rectifying behavior with a decent rectifying ratio of 104. The output curve of the p-n junction at VG=0 V (Fig. 6b) shows an ideality factor of 1.2, implying very low



charge trap density at the homo-junction interface.14 Figure 6c presents the corresponding transfer characteristic of the diode under forward bias of 1 V. Two valleys (at ~ -40 V and ~ +0 V) and a hump (at ≈ 9.5 V) appear in the transfer curve, which has been reported for p-n homo-junctions.32,33 Figure 6d shows the dynamic optical response of the homo-junction diode at 0 V gate and source-drain bias under light illumination with output power density of 0.2 mW·cm-2 and wavelength ranging from 300 nm to 650 nm. The area of the device exposed under light illumination was measured to be approximately 12 µm2. Therefore, at a zero bias voltage, the photoresponsivity to 532 nm light is calculated to be around 1.3 A·W-1, which is almost 100 times larger than the value obtained from a monolayer MoSe2 photoconductor34 and comparable to the MoSe2-MoOX heterojunction device.35 The intensity dependent photocurrent of the device under 532 nm light illumination is shown in Supporting Information Fig. S8, which as expected increases as the light intensity increases.

Figure 6. (a) The ID−VD characteristics of the MoSe2 diode plotted in semi-log scale as gate voltages ranging from −40 to 0 V with increasement step of 10 V. The inset is the optical microscope image of the device, and the scale bar indicates 10 µm. (b) ID – VD curve of the MoSe2 diode in linear (blue line) and log (red) scale at VG = 0 V. (c) ID-VG curve of the MoSe2


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diode at VD = 1 V. (d) Dynamic optical response of the device under illumination of light with wavelength ranging from 300 nm to 650 nm. The light intensities are kept at the same level of 0.2 mW·cm-2.

3. Conclusion In summary, we have accomplished efficient carrier type modulation in a single MoSe2 flake through air RTA and PPh3 doping, respectively. The carrier type of the MoSe2 flake can be accurately tuned, from n-type all the way to degenerated p-type through air RTA, as oxygen atoms initially occupy Se vacancies and further oxidize MoSe2. The n-type doping of MoSe2 is enhanced by drop-coating PPh3 solution on the surface of MoSe2 flake through charge transferring between PPh3 and MoSe2. These two carrier modulation methods are compatible with each other, so that a lateral p-n homojunction is demonstrated in a single MoSe2 flake with decent diode characteristics and improved photo-response of 1.3 A·W-1 at 532 nm without any source-drain biases. The demonstration of efficient carrier type modulation and lateral p-n homo-junction in a single MoSe2 flake may pave the way to develop functional nano devices based on MoSe2. 4. Experimental Section Sample characterization: The Raman spectra were obtained from a Renishaw InVia Raman microscope using a 532 nm laser at ∼1.38mW power. XPS measurements were performed in ultrahigh vacuum in an ESCALAB 250 Xi (Thermo Scientific, USA) with a nominal spot size of 400 µm. KPFM characterization was conducted in PeakForce KPFM mode on a Dimension Icon (Bruker, German) atomic force microscope using Kelvin probe. HR-TEM images were taken by a FEI Talos F200 at an acceleration voltage of 200 kV. HR-TEM sample was prepared by micro-pipetting liquid exfoliated MoSe2 onto a copper grid with carbon mesh. Sample and Device Fabrication: MoSe2 flakes were mechanically exfoliated from bulk MoSe2 and transferred onto 280 nm thick SiO2 thermally oxidized on a p-type doped Si substrate or h-BN substrate (mechanically exfoliated from bulk h-BN). For fabricating homojunction diode, we first exfoliated a few-layer h-BN flake onto Si/SiO2 substrates using blue Nitto tape. Then a MoSe2 flake was transferred onto the h-BN/SiO2 substrate. The Cr/Au (15 nm/30nm) electrodes were deposited on the MoSe2 flake by using standard electron-beam



lithography (EBL) system and an electron beam evaporator. After p-doping at optimum conditions, an h-BN flake was partially stacked onto the p-doped MoSe2 device. The PPh3 chemical dopant was drop-coating on the partially stacked h-BN/MoSe2 device, and dried under 100 oC. All the device electrical measurements were performed using an Agilent B1500A semiconductor parameter analyzer under ambient conditions. Carrier mobility and doping density calculation: The carrier mobility and doping density were calculated by the formula µ=(dID/dVG) (L/WVDCox) and ne= (IDL)/(qWVDµn), respectively, where µ is the carrier mobility (µn is the electron mobility), ID is the drain current, VG is the back gate bias, VD is the drain bias, Cox = 12.1 × 10−9 F cm−2 is the gate capacitance for 285 nm thick SiO2 dielectric, L is the channel length of the transistor, and W is the width. Acknowledgement This work is supported by the National Nature Science Foundation of China (No. 21405109) and Seed Foundation of State Key Laboratory of Precision Measurement Technology and Instruments (Pilt No. 1710). Conflict of interests There are no conflicts to declare.


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pn homo-junction by efficient carrier type modulation

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