PâN Junction Diode Using Plasma Boron-Doped Black Phosphorus

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P–N Junction Diode using Plasma Boron-Doped Black Phosphorus for High-Performance Photovoltaic Devices Dae-Kyoung Kim, Seok-Bo Hong, Kwangsik Jeong, Changmin Lee, Hyoungsub Kim, and Mann-Ho Cho ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07730 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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P–N Junction Diode using Plasma Boron-Doped Black Phosphorus for High-Performance Photovoltaic Devices Dae-Kyoung Kim,† Seok-Bo Hong,† Kwangsik Jeong,† Changmin Lee,‡ Hyoungsub Kim,‡,§ and Mann-Ho Cho*,† † Department ‡

of Physics, Yonsei University, Seoul 03722, Republic of Korea

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon

16419, Republic of Korea §

Sungkyunkwan Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University,

Suwon 16419, Republic of Korea

ABSTRACT: This study used a spatially controlled boron doping technique that enables a p–n junction diode to be realized within a single two-dimensional (2D) black phosphorus (BP) nanosheet for high performance photovoltaic application. The reliability of the BP surface and state-of-the-art 2D p–n heterostructure’s gated junctions was obtained using the controllable pulsed plasma process technique. Chemical and structural analyses of the boron-doped BP were performed using X-ray photoelectron spectroscopy, transmission electron microscopy, and firstprinciples density functional theory (DFT) calculations, and the electrical characteristics of a

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field-effect transistor based on the p–n heterostructure were determined. The incorporated boron generated high electron density at the BP surface. The electron mobility of BP was significantly enhanced to ~265 cm2/V·s for the top gating mode, indicating greatly improved electron transport behavior. Ultraviolet photoelectron spectroscopy and DFT characterizations revealed the occurrence of significant surface charge transfer in the BP. Moreover, the pulsed plasma boron-doped BP p–n junction devices exhibited high efficiency photodetection behavior (rise time: 1.2 ms and responsivity: 11.3 mA/W at Vg = 0 V). This study’s findings on the tunable nature of the surface transfer doping scheme reveal that BP is a promising candidate for optoelectronic device and advanced complementary logic electronics.

KEYWORDS: black phosphorus, p–n heterostructure, plasma doping process, electron transport, photovoltaic devices

Two-dimensional (2D) layered materials have shown great promise for application in flexible electronics and optoelectronics because of their superior mobility and ultimate flexibility due to the presence of 2D atomic crystals.1-3 Among many 2D materials, transition metal dichalcogenides (TMDCs) semiconductors are considered promising alternatives to graphene because of their high off-current levels (small on/off current ratios). In particular, molybdenum disulfide (MoS2)

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and tungsten diselenide (WSe2)8-11 have larger band gaps compared to

graphene and typical n-type and p-type (or p-type dominant ambipolar) devices, and 2D crystals show the potential to substitute materials with regard to applications in flexible electronics and optoelectronics due to their high mobility and flexibility.12-14 Nevertheless, MoS2, WSe2, and other TMDCs have a strong inherent tendency toward unipolar characteristics for field effect


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transistor (FET) operation, which makes it difficult to fulfill the stable superior mobility in both p-type and n-type conduction in the same material. For example, MoS2 FET devices predominantly show the strong propensity toward n-type semiconducting behavior due to intrinsic atomic defects such as sulfur vacancies and low-lying valence band.15,16 WSe2 is another 2D TMDC that shows ambipolar transport characteristics, although making good electrical contact for both the electron and hole injection remains difficult.17 On the other hand, the logic gate, solar cells, and photodiodes based on practical semiconductor devices require both p-type and n-type unipolar operation in the same parent semiconducting material. Many 2D materials have been actively researched, although modulating the carrier type in 2D materials by work function engineering or separated charge conversion are of limited practical relevance. Recently, black phosphorus (BP), in forms ranging from a monolayer to bulk BP crystal, is an actively researched material because of its physical properties, wherein each phosphorus atom is covalently bonded to three neighboring atoms to form a puckered orthorhombic structure.18,19 Furthermore, BP has a tunable optical band gap of value ranging from ~2 eV for monolayers to ~0.3 eV for bulk crystals, and a direct bandgap for all number of layers.20-22 This property enables applications of BP-based optoelectronic devices.23,24 Phosphorene, the second known monotypic van der Waals 2D material, shows clear ambipolar transport behavior in FET, and excellent field-effect characteristics with an on/off ratio of ~105 and hole mobility of up to ~1,000 cm2/V·s at room temperature compared to those of other TMDCs, making this material suitable for use in transistors.19,25 On the other hand, the ambipolar characteristics of BP FETs exhibit strong asymmetry behavior between hole and electron transport,26 which indicates that the electron mobility and concentration are orders of magnitude lower than those of the hole. Consequentially, the difference in transport characteristics makes it difficult to create


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complementary logic devices on a single BP crystal. Thus, it is very important to effectively tune the n- or p-type doping level of BP to improve control and enhance the performance of BP-based complementary devices. This study investigated surface transfer electron doping to effectively tune the ambipolar characteristics of a few-layer BP FET device via a pulsed plasma process using boron with rapid processing time (30 s under ~13 nm BP thickness) at room temperature. The ionized boron atoms in BP showed strongly n-type doped characteristics. The electron mobility of BP increased significantly to ~265 cm2/V·s in a half (p-type) and half (n-type) device with top gating mode, indicating greatly improved electron transport behavior in the BP FET device. Ultraviolet photoelectron spectroscopy (UPS) and first-principles density functional theory (DFT) characterization confirmed the charge transfer between BP and boron dopant. This doping technique can modulate p–n junctions formed in a spatially controlled and same parent conducting material (i.e., BP flakes), thereby enhancing the photoresponse time and responsivity of BP-based photodetectors. That is, the p–n junction diode is not only sensitive to external electrostatics in BP conductivity but is also efficient at separating the photoexcited electron–hole pairs.

RESULTS AND DISCUSSION Figure 1a shows the chemical bonding characteristics of the boron, using the pulsed plasma process with increasing pulse time (details of the plasma process method appear in Figure S1 in Supporting Information). The XPS spectra of the samples were obtained at the sequential pulsed time (interval: 5 s) of the plasma process to determine the reaction with BP surface species. The chemical bonding states extracted from the XPS results show the relationship of boron atoms


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with BP (O-B-P, B-B, and B-P binding states) in the plasma boron-doped BP surface region (detection angle = 45).27-29 A Shirley background subtraction was performed for the curve fitting of the XPS data, and all spectra were charge-compensated relative to the binding energy of the C-C bond (284.5 eV). The results of the XPS curve fitting clearly showed that the concentrations of the hybridized B-P species in the pulsed plasma boron-doped film increased with increasing process time of up to 30 s. On the other hand, the oxidized peak from O-B-P increased by a relatively large amount after the process time was extended to 35 s, compared with those of a process time of up to 30 s. In addition, to confirm the doping effect and plasma damage with the BP surface, fitting for the P 2p core level spectra was performed, as shown in Figure 1b. Pristine BP shows a core-level spectrum containing only the characteristic doublet representing the P 2p3/2 and 2p1/2 orbitals with peaks at 130.23 and 131.08 eV, respectively.30 The spin splitting of 0.85 eV remained in all subsequent peak fittings of the bulk BP components. After doping the boron to the BP, the P 2p3/2 peak shifted to the higher binding energy, from 130.23 to 130.34 eV (an increase of 0.11 eV). This demonstrates a value of downward band bending, which is consistent with the observed vacuum level shift created with the downward band bending in BP and the interface dipole formed at the boron-doped BP surface.31 On the other hand, the lower binding energy shift for the plasma process time of 35 s is due to the oxidized surface formed by the damaged surface resulting from the excessive plasma process.32 Other chemical bonding states of BP were generated in the P 2p core level spectra as the plasma process time increased (up to 35 s), as shown by an additional broad peak between 136 and 133 eV in the P 2p spectra of the BP flake. In contrast, a small broad peak was observed for a process time of up to 30 s. The broad peak in the P 2p spectra can be caused by the oxidized states of O with BP, resulting in amorphization of the BP surface.30,33 After 35 s of processing


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time, the increase in the intensity of the broad peak at ~134.65 eV indicated the generation of a few oxide components related to phosphorus oxide species (PxOy). That is, the PxOy peaks of the oxidized phosphorus with large full widths at half maximum (FWHM) suggested the formation of multiple PxOy and O-B-P bonding states caused by the reaction between phosphorus and residual oxygen in the chamber. According to previous reports, PxOy formation in BP crystals can be promoted in an oxidizing environment, which can degrade the performance of the device.34,35 Therefore, it is very important to prevent oxidation during the doping process. On the other hand, the doping effect between plasma processing methods for continuous and pulsed processes using the same total process time of 30 s was compared, as shown in Figure S2. The results of these two methods showed a clear difference; that is, many oxidation states were produced, and the incorporation of boron into BP was retarded with the continuous process, as shown by the results of the XPS analysis. While using the pulsed process, the generation of oxidation states was hindered, and boron was effectively incorporated. As a result, the pulsed process method was more stable than the continuous process for boron incorporation into BP. Furthermore, to accurately observe the influence of the surface damage during the pulsed plasma doping process, we performed AFM, Raman spectroscopy, and TEM measurements to analyze the surface morphology and optical properties of BP before and after the pulsed plasma boron doping process, as shown in Figure S3. After the pulsed plasma boron doping process (interval: 5 s, total process time: 30 s), compared with the pristine BP flake, no perceptible change were observed in the RMS roughness distributions in AFM; A1g, B2g and A2g mode peaks in Raman spectroscopy; and TEM images in the BP flake. The results indicate that the surface morphology and crystallinity of the BP were maintained during the pulsed plasma process, which indicated that stable interaction without surface damage can be ensured in the surface-sensitive BP system


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using the pulsed plasma process. According to previous reports, the higher plasma density in the plasma process can hinder the reaction with the sample surface, and surface damage can also be induced during the process. In addition, high plasma density primarily generates electron shading effects; the difference in the distribution of ions and electrons leads to a net charge buildup on the sample surface.36 Therefore, high plasma density in the continuous process critically restricts the incorporation of ionized boron as well as induces surface damage of BP, thus producing oxidation states and hindering boron incorporation. In contrast, on/off pulse modulation in the pulsed plasma process substantially changes the critical parameters (plasma density) for electron shading. Consequently, the pulsed plasma process can be controlled to ensure the incorporation of ionized boron without surface damage. That is, both controllable doping concentration and a low oxidation state can be successfully achieved using the conventional doping process. In addition, based on these effective doping characteristics of the pulsed plasma process, a detailed quantitative analysis was conducted to incorporate boron into BP using changed chemical bonding states along the depth direction via additional angle-dependent XPS (ADXPS) measurements for each pulsed plasma process time. Figure S4b shows the quantitative depth distribution of chemical bonding characteristics (O-B-P, B-B, B-P) from the calculated peak areas of the results of ADXPS in Figure S4a. The peak areas corresponding to the O-B-P chemical bonding for the process time of 35 s dramatically increased throughout the film depth in the direction of the surface, compared with those for the process time of 30 s. This finding is consistent with the results of the XPS analysis (detection angle = 45) seen in Figure 1a. Also, incorporation of boron into BP, corresponding to B-P chemical bonding, increased with depth, away from the film surface, for a process time of 30 s. Non-chemical transformation of B-B for the plasma boron doping process towards the surface (detection angle = 20) showed a


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considerable decrease for a process time of 30 s, compared with the corresponding values for 20 s and 25 s. These changes in the quantitative depth distribution of boron ratio and concentrations of oxidation species indicate that the formation of the defective chemical bond states from the BP surface and downwards was closely related to the limitation and optimization processing time characteristic of the pulsed plasma process. Based on the results of the ADXPS analysis, the quantitative characteristics of peak area transition ratio at a detection angle of 90 were extracted. The time for maximum incorporation of boron at a lower oxidation state was observed to be 30 s, as shown in Figure 1c. Furthermore, the results of UPS combined with the XPS measurements showed a difference in the value of the work function between the pristine BP and pulsed plasma boron-doped BP. The onset energy directly yielded the work function because the secondary electron cutoff, known as the onset of the secondary photoelectrons, corresponded to the vacuum level of the sample with respect to the Fermi energy, as shown in Figure 1d. In particular, the UPS spectra in Figure 1d showed a decrease in the work function by ΔE = 0.2 eV in the boron-doped BP sample, which was attributed to the electron doping of the BP surface by the pulsed plasma process. Several recent works on surface charge transfer for n-doping of BP have reported successful procedures. For example, Cu adatoms were employed for controllable electron doping of few-layer BP.37 Cs2CO3 film on the BP surface was deposited for n-doping of few-layer BP.38 Capping a thin BP flake with a layer of cross-linked polymethyl methacrylate (PMMA) could also effectively modify the conductivity type of BP in the surface electron transfer process.39 However, the surface charge may suffer from disadvantages of instability due to volatility and incompatibility with complementary semiconductor routes, rendering the application impractical. Stable electron doping ensuring depth distribution uniformity and a nonvolatile method still remain to be


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discovered for BP optoelectronic and logic electronic devices. Thus, the pulsed boron plasma process can be a promising method as it does not pose the disadvantages of instability and incompatibility. To further characterize the structure stabilization and atomic depth distribution of pulsedplasma boron-doped BP crystals, transmission electron microscopy (TEM) analysis and TEMenergy-dispersive X-ray spectroscopy (EDX) were performed. The EDX technique allows one to obtain the element signal at different energies, providing a depth profile of the detected elements. Figure 2a shows the relative intensity versus sample depth for C, O, B, and P elements, obtained from the carbon capping layer on the boron-doped exfoliated few-layer BP for a pulse process time of 30 s. The intensity of the boron signal was clearly observed for a penetration depth of ~10 nm from the surface in the boron-doped BP. Moreover, even after the doping process using pulsed plasma was completed, TEM measurement confirmed that the majority of doped boron existed along the depth direction under the surface of the exfoliated BP, and no crystal structure deformation was observed, as seen in Figure 2b. Moreover, to confirm the quantitative depth distribution of O, B, and P elements during the pulsed process times, additional line profile EDX measurements were performed, as shown in Figure S5. Comparing the results of the EDX depth profile showed that, compared with the 30 s sample, it is notable that the boron in the depth direction for process times of 20 s and 25 s was densely located in the BP surface region. Furthermore, for the pulsed process time of 35 s, the depth profile data showed an overlap of densely located boron at the BP surface and depth distributions for the oxygen element of oxidized BP related with (PxOy) and O-B-P species in the surface region. Therefore, based on the results of the chemical and quantitative depth distributions of incorporated boron using XPS (Figure S4) and EDX (Figure S5), it was concluded that the pulsed plasma process was suitable


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for boron incorporation under the optimized condition. Although the pulsed plasma process is more stable and effective for boron incorporation because of the rapid processing time (dozens of seconds) at room temperature compared with other reported chemical doping processes for BP, a suitable optimization processing time was required to ensure depth distribution uniformity of the dopant and stable interaction without surface damage in the surface-sensitive BP system. In other words, the densely located boron at the BP surface for the 20 s and 25 s pulsed plasma process can be attributed to the net charge buildup from electron shading effects. The densely charged boron located in the surface region and the on/off pulse modulation in the plasma process contributed to the diffusion of the incorporated surface B into the BP (as indicated by the uniformity of boron bonded with BP) along the depth direction during the 30 s pulsed plasma process. Since the diffusion barrier was changed with the increased amount of sufficiently ionized boron and a radical state, the diffusion could take place after the surface boron had accumulated sufficiently under a pulsed plasma process time of 30 s. On the other hand, although diffusion also occurred during the boron incorporation over the 30 s pulsed plasma process, it was impeded by the charged surface caused by surface oxidation (PxOy or O-B-P species in the surface region). The results of the total boron and oxygen atomic concentrations for each pulsed plasma process in this work are exhibited in Figure 2c. Thus, optimized depth distribution of the pulsed-plasma boron-doped process (30 s), corresponding to a sensitive BP surface, is possible, resulting in a stable and uniformly dispersed dopant along the depth direction. In addition, we also investigated the element distribution of the boron-doped BP along the depth direction in the boron-doped exfoliated few-layer thin BP flake (~4 nm)/SiO2/Si for a pulse process time of 30 s, as shown in Figure S6. The results of the depth element distribution analysis show a clear boron signal for a penetration depth of ~4 nm from the surface in the boron-doped BP. Thus, the results


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strongly suggest that the process is successful at incorporating boron with a uniform dispersion profile along the depth direction regardless of BP thickness due to the on/off pulse modulation optimization. In order to investigate the change in the Fermi level and role of the transport properties of boron-doped BP, first-principles DFT calculations were performed. The band structure of the boron-doped BP and binding energies of various bond structures in the BP system were obtained, as shown in Figure 3a. The effect of two possible structures containing boron impurities on the band structure of few-layer BP was calculated. The first case concerns boron impurity adsorbed on the topmost BP layer (referred to as Bsurface), and the second concerns boron impurity inserted between the first and second layer (Binterface), as shown in the two geometries of Figures 3b and c. The binding energy configurations of both impurities were found to be at local minima, with EB [Bsurface] = -0.523 eV and EB [Binterface] = -2.492 eV, defined as EB = Eboron+phosphorene – Eboron – Ephosphorene. These values of evaluated binding energy were attributed to the strong interaction and hybridization of the boron atom with the lone electron pair in phosphorus atoms at each atomic site, allowing stable incorporation in the phosphorene system, resulting in large outward and inplane atomic displacements of phosphorus atoms around the boron dopant. Since the plasma was very active and reacted easily with the surface, doping of B on the BP surface was preferred. However, the incorporation of B into the BP interface took place after this process was completed. Thus, two dopant positions can be practically available when using the plasma doping process. In addition, the electronic band structures as well as the corresponding density of states (DOS) of the pristine and boron-doped phosphorene were investigated, as shown in Figures 3b and c, respectively. The Fermi level (EF) of the pristine BP was exhibited towards the valence band of the bandgap (as shown in the p-type semiconductor). In contrast, EF in the


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boron-doped BP (Bsurface and Binterface) was shifted upward to the conduction band, indicating that the boron impurity can function as an effective electron donor in the boron–phosphorene system. Recently, a similar example of a bond between boron and phosphorus in boron phosphide was reported. Padavala et al. showed n-type epitaxy of boron phosphide, with the highest electron mobility of 37.8 cm2/V·s and the lowest carrier concentration of 3.15 × 1018 cm−3.40 Ding et al. reported on boron phosphide nanowires with artificially controllable carrier type (n- or p-type) for the fabrication of homojunctions by adjusting the borane/phosphine ratio during the deposition process.41 The donated electron originated from electron transport between the boron atom and the BP. Calculations of the partial charge density, integrated from the energy range EF of -0.5 eV to the EF values of Binterface, indicated that, at most, the boron atom donated ~1.9 electrons to the BP host lattice. To investigate the transport characteristics of the FET with a single boron-doped BP film, and a half (p-type, BP) and half (n-type, boron-doped BP) geometry p–n junction device, top-gated FETs were fabricated on an exfoliated single BP flake (~13 nm) onto pre-patterned alignment marker SiO2 (300 nm)/Si wafers to minimize the processing time and reduce air exposure. Figure 4a shows the comparison of transfer characteristics of boron-doped BP and pristine BP top-gated FETs (Al2O3 ~30 nm with Ti/Au top metal electrode). The measured data show that pristine BP FET exhibits p-type behavior with on/off ratio and threshold voltage (Vth) of ~104 and 4 V, respectively. Moreover, both hole current (Imax = 8.2 μA) and mobility (~167 cm2/V·s) were measured under ambient conditions at 295 K. After the completion of the plasma boron process, the boron-doped BP transistor clearly exhibited n-type conductance with on/off ratio and Vth of ~104 and -4.5 V, respectively, and the electron mobility was enhanced to as high as ~272 cm2/V·s, accompanied by an increase in current in the electron regime. The single boron-doped


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film could also effectively induce n-doping unipolar characteristics for the top and back gating measurements (see Figure S7 in Supporting Information). Furthermore, to accurately determine the influence of the surface damage to the SiO2 substrate during the pulsed plasma doping process, we additionally examined the gate leakage current (gate to source), as shown in Figure S8. The I–V curve of the boron-doped BP/SiO2/Si device showed that the gate leakage current had the same low current level (∼3.9 × 10-10 A). The pristine BP and boron-doped BP engaged for the characteristics of p–n homojunction. To create a half (p-type, BP) and half (n-type, borondoped BP) geometry p–n junction device, electron beam lithography was followed by thermal evaporation and liftoff to fabricate source and drain metal electrode Ti and Au (10 and 80 nm, respectively) on an exfoliated single BP flake (~13 nm), and PMMA buffer layer deposition for exposure to the plasma process (inset of Figure 4b) and Al2O3 (~30 nm) deposition with top metal electrode (see detail process sequence for Figure S9 in Supporting Information). The fabricated devices were annealed in situ under high vacuum (< 10−8 Torr) at 375 K for 2 h to reduce hysteresis and remove unintended doping due to ambient conditions and fabrication residues, and device characteristics were measured at 295 K. Figures 4b and c show that the FET characteristics with the half p-type and half n-type device exhibited ambipolar transport behavior. In particular, the drain current–gate voltage (Id–Vg) characteristics for carrier transport showed that the hole mobility of μFE was ~228 cm2/V·s and its electron mobility was ~265 cm2/V·s. Moreover, the gate voltage can control the doping concentrations for both semiconductors, allowing tunability in the potential barrier at the heterointerface of the p–n region. This reflects clear n-doping, which was probably introduced by the field-induced effect, described as follows. The pulsed plasma boron-doped on BP induced an n-channel region through a redistribution of the electrons in the spatially controlled BP area, with electrons


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accumulating on the BP flake and enhancement of the electron current of the device. On the other hand, we observed noteworthy change in the transfer characteristics of the boron-doped p– n junction depending on the short and long channel lengths, as shown in Figure S10. The transfer characteristics of the p–n diode (~13 nm BP thickness, Vsd = 100 mV) using boron-doped BP showed a hump with two valleys, characteristic of asymmetry in electron–hole conduction for the short channel length of ~2 μm (~1 μm of each p and n region). The humped shape behavior of the transfer curve in the lateral BP p–n junction can be commonly observed for a short channel length of each p and n region.31,39,42 Based on our results of the two different channel length conditions (Figures 4c and S10), the humped shape and asymmetry in electron–hole conduction characteristics can be deduced from the dependence on the channel length (p–p, p–n, and n–n regions in the total area), with good control of the doping type and concentration in the BP channel for the lateral p–n junction. The rectifying behavior was evident with the gate tunable output characteristics (Id–Vd) of the BP p–n diode, which indicated the operating mechanism of a p–n homojunction diode, as shown in Figure 4d. The output Id–Vd curve exhibits the typical rectification characteristics of BP p–n diodes under gate voltages ranging from 4 to –4 V with a step of 2 V in the logarithmic scale in Id, as shown in the Figure 4e. The output curves exhibit diode-like current rectifying characteristic. The current rectifying can be effectively modulated by gate voltage under forward and reverse voltage (Vd). The rectification ratio, defined as the ratio of the forward/reverse current, increases as the gate voltage decreases. By using -4 V gate voltage, a high rectification ratio of 2.8 × 104 is obtained at Vd = -2/+2 V, as shown in the added Figure 4f. The built-in electric field was efficiently decreased in BP p–n junction region under the forward bias voltage, which encouraged large enhancement of the current. Furthermore, to confirm the stability in plasma boron-doped FET devices with different


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exposure time and under high on-state current densities, we performed additional cumulative electrical measurements of boron-doped BP and p–n junction FET devices at different exposure times (after 50, 60, and 70 days). The carrier transport characteristics, such as mobility and on/off current ratios, extracted from the Id–Vg characteristics under the different exposure times, were quite consistent with those of the initial measurements of the FET devices, as shown in Figure S11a. Also, the transfer characteristics show that the boron-doped BP exhibited stable ntype behavior with increasing Vds, and its maximum drain current density reached 506 μA at 295 K with Vsd = 4 V and Vg = 50 V, as shown in Figure S11b. In addition, to clarify the influence of the atomic layer deposition (ALD) process on the BP carrier transport characteristics, we performed additional electrical measurements for back gating (BP/SiO2/Si structure) before and after ALD of the Al2O3 in the top surface region. The Id–Vg carrier transport characteristics after Al2O3 deposition were quite consistent with those of the initial measurement of the BP/SiO2/Sistructure FET devices without other interfering carrier transport behavior, whereas degraded transport characteristics were observed in the BP/SiO2/Si exposed for 5 h in air, as shown in the Figure S12. The results for the cumulative electrical measurement before and after ALD deposition of Al2O3 in the top surface region indicated that other interfering factors affecting the carrier transport characteristics can be excluded during the ALD process. The photovoltaic effect is a crucial factor for an efficient and tunable p–n junction. It facilitates the separation of photoexcited electron–hole pairs and photon energy conversion. Thus, the photoinduced characteristics of the boron-doped BP p–n junction were investigated. Figure 5a shows time-resolved photo-switching at an incident power of 100 to 400 μW (785 nm, ~10 Hz, source–drain voltage = 0.1 V, and Vg = 0 V). The signal was well retained, and the diodes showed good reversibility and reliability for hundreds of on–off cycles. Moreover, the high-


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resolution temporal response was investigated, as shown in Figure 5b, because the response time (τ) of the photodetector is a key specification for a p–n junction photodetector using BP. The 1090% rise and fall times were measured to be ~1.2 and 7.6 ms, respectively. The response times were on par with the values of previous BP-based photodetectors, compared with the reported gate-defined BP p–n junction (response time: 2–3 ms).43 Nevertheless, the p–n semiconductor of the junction device in that study required to be tuned using two split gates, resulting in complicated device operation. In our p–n diode, the results of photovoltaic switching with Vg = 0 V, and small source–drain bias enables one to harvest light energy using a simple method and more efficient photo absorption. For the p–n junction diode, this study’s spatially controlled boron-doped p–n junction device (Id = 0) can further improve the photon energy conversion into multiple electron–hole pairs, leading to enhanced photo response and efficiency.44 According to the output characteristics presented in Figure 5c, under light illumination, the Id–Vd curves did not cross the zero current (voltage) point and shifted towards a more negative current value even when the applied voltage (current) was zero with varying incident power densities. This is the photocurrent generation mechanism for separation of the electron–hole pairs operated by the internal electric field in the p–n junction region. The non-zero open-circuit voltage (Id = 0) and short-circuit current (Vd = 0) values are distinct features of the p–n junction, suggesting that the boron-doped BP homojunction diode is suitable for photovoltaic energy conversion without any external power consumption and input. Moreover, the logarithmic increase in the open-circuit voltage (VOC) and linear increase in the short-circuit current (ISC) with power density were appropriate for a photoexcited device with an ideal solar cell configuration, demonstrating that the photocurrent generation dominated due to the photovoltaic effect, as shown in Figure 5d.45 The responsivity (R) was calculated using R = Iph/Pin, where the photocurrent Iph = Ion - Ioff, and


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Pin is the input power incident on the diode (inset of Figure 5d). The results showed that the responsivity decreased with the increase in excitation laser power density. The maximum measured photoresponsivity was 11.3 mA/W at low incident power. Fitting responsivity versus power density with R = CPdα-1 (C is constant, and α = 0.86) indicated that the high recombination of the photogenerated carriers can be attributed to the trap states and electron/phonon interactions.46 In order to compare the performance of the BP p–n junction photovoltaic device using the boron-doped n-type BP with the reported value, the relevant figures-of-merit reported in the literature for other devices with BP-based p–n junctions and other layered materials are summarized in Table 1. The measured response time and responsivity of the BP-based p–n junction device using the boron-doped n-type BP was more efficient than those of the other p–n junctions based on BP and layered materials. Some materials showed higher responsivity values but slower response times and electron mobility in BP p–n junctions. The p–n junction diode of the boron-doped n-type BP was sensitive to external electrostatics in BP conductivity, as well as efficient for separation of the photoexcited electron–hole pairs. The combination of fast (τrise = 1.2 ms) photoresponse, higher electron mobility (~265 cm2/V·s), and considerable R (= 11.3 mA/W) at small source–drain bias (Vd = 0.1 V) suggests that the boron-doped BP can be very promising for applications in optoelectronics and complementary logic electronics devices.

CONCLUSIONS In summary, surface transfer electron doping was investigated to effectively tune the ambipolar characteristics of few-layer pristine BP FET and boron-doped BP FET, using the pulsed plasma process with boron. The pulsed plasma process successfully incorporated the B into the BP flake, which was confirmed by the cross-sectional boron profile. Moreover, the


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pulsed plasma process exhibited considerable potential toward meeting the majority of scaling challenges or large-area applications, given the rapid processing time and stable 2D nanosheet structure, compared with those of the other chemical deposition and continuous plasma processes. The BP was found to be strongly n-type after doping by ionized boron atoms. The electron mobility of boron-doped BP was significantly increased to ~265 cm2/V·s in the top gating mode, indicating greatly improved electron transport behavior in half (p-type, BP) and half (n-type, boron-doped BP) devices. The UPS and first-principles DFT characterization confirmed the charge transfer between BP and boron dopant. This doping technique could modulate p–n junctions formed in spatially controlled single BP flakes. Moreover, the doped BP devices exhibited largely enhanced photo detection behavior (rise time: 1.2 ms and responsivity: 11.3 mA/W at Vg = 0 V). This study’s findings concerning the tunable nature of the surface transfer doping scheme show promise for applications in optoelectronics devices as well as complementary logic electronics using BP.

METHODS Sample preparation. BP crystals were procured from a commercial supplier (Smart Elements). BP was exfoliated using the standard scotch tape method. Mechanically exfoliated BP was transported onto 300 nm SiO2/Si with shadow-evaporated Au markers. The flakes were identified by optical contrast, atomic force microscopy for flake height, and metal (Ti/Au) electrode-patterned by electron beam lithography. After the samples were exposed to the plasma process, they were stored in a vacuum chamber. The sample properties were evaluated under vacuum conditions (e.g., vacuum XPS, UPS, and TEM measurements) and ambient conditions


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with ALD-Al2O3 layer (e.g., FET and optical device measurements). The Al2O3 films deposited on all BP device samples by ALD are used for top gate oxide and surface protection (air-exposed degradation). ALD was carried out at 120 °C using trimethylaluminum (TMA) as the precursor and deionized H2O vapor as the oxygen source. N2 (99.999%) was used as a purge gas during the film growth with a long purging time. The ALD cycle using the precursor and oxygen source was designed as processes alternating between TMA (1 s)/N2 (180 s) and H2O (1 s)/N2 (180 s), where N2 (180 s) was used for purging process. The initial vacuum level of the growth chamber was ~1 × 10−2 Torr and the process vacuum level was maintained at ~1 Torr during the deposition. Plasma process. The plasma process used for boron doping onto BP was performed with BF3 (dopant gas). The conditions for stable doping were optimized as follows: 100 W plasma power, 4000 V accelerating voltage, 1 × 10-2 Torr processing pressure with various (continuous and pulsed; i.e., interval process time = 5 s) doping times. Measurements of electrical and photoresponse. The current–voltage characteristics of the BP FETs were evaluated using two Keithley 2400 Source Meters. All electrical measurements were conducted at room temperature. The mobility values were calculated using the following equation: 𝐿𝑔𝑑

𝜇𝑒𝑓𝑓 = 𝑊𝐶𝑜𝑥𝑉𝑠𝑑

(1)

where μeff is the field-effect mobility, L is the channel length, gd is the transconductance, W is the channel width, Cox is the oxide capacitance, and Vsd is the source–drain voltage.51 All measurements were performed at room temperature. For photoresponse characterization, light excitation of 785 nm was provided by laser diodes operated in continuous wave mode. The beam was guided through an optical lens and was subsequently made incident onto the device channel


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without overlap focusing. The area of the channel was < 10 μm × 10 μm. Therefore, the photoresponsivity was calculated by normalizing with the incident light intensity that overlapped with the channel area. Measurements of electronic structures and physical properties. The changes in chemical bonding configuration of BP were examined using high-resolution XPS using a monochromatic Al Kα X-ray source (hν = 1486.7 eV) with a pass energy of 20 eV. All the spectra were calibrated to the binding energy of carbon (284.5 eV). To deconvolute the XPS core-level spectra, the background was removed using Shirley-type subtraction, and the FWHM of the constituent peaks were kept constant. Fitting curves were determined using the Gaussian and Lorentzian distributions. TEM and high-angle annular dark-field images were obtained using JEOL 2100F TEM operated at 200 kV, and EDX measurements were performed to construct elemental mappings of cross-sectional boron-doped BP. DFT calculations. DFT calculations were performed to investigate the possible modes of formation of various structures through the reaction of the boron atom with BP by applying super cell models. The atomic structures and energy states were calculated using the Vienna ab-initio simulation package code with the exchange correlation function of the generalized gradient approximation PBESol and the DFT-D3 Van der Waals correction. Geometry optimization of the cell size and atomic position of 3-layered BP was conducted with a vacuum slap. The size of the phosphorene lattice was set as 35 Å to eliminate the inter-cell interaction. To render the k-point spacing to less than 0.25/Å, a 9 × 1 × 5 grid of k points was chosen. Geometrical optimizations were performed using the conjugate gradient algorithm under a cut-off energy value of 500 eV and iterating condition-satisfied grid spacing of less than 0.01 eV Å−1. After the optimization of the 3-layered BP unit structure, a 3 × 3 × 3 super cell structure was built to analyze the bonding


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between the surface and interface of BP. DFT calculations for the geometrical optimization and density of states were carried out. During the geometric optimization of the BP surface and interface with added atoms, a 3 × 3 × 3 k-point grid that satisfied the k-point spacing of less than 0.25 Å−1 and a cut-off energy of 500 eV was used. Only the atomic position was relaxed until the 0.05 eV Å−1 condition was satisfied with the conjugate gradient algorithm.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Schematic diagram of the boron plasma process, Comparison of XPS of the continuous and pulsed plasma processes, Comparison of AFM, Raman, and TEM of the before and after pulsed plasma processes, Angle-dependent XPS spectra as a function of pulsed plasma processing time, EDX elemental spectrum of TEM, Back and top gating transfer characteristics of single pulsed plasma boron-doped BP and pristine BP for FETs, Procedure to fabricate a half (p-type) and half (n-type) geometry device and transfer characteristics (short channel and air exposure time) (PDF)

AUTHOR INFORMATION Corresponding Author * Email: [email protected] ORCID Dae-Kyoung Kim: 0000-0001-5894-8628 Hyoungsub Kim: 0000-0003-3549-4250


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Mann-Ho Cho: 0000-0002-5621-3676 Author Contributions D.-K.K conceived and designed this study. D.-K.K. and S.-B.H. fabricated the samples and prepared the experimental apparatus. D.-K.K. performed the experiments and analyzed the data with contributions from all authors. D.-K.K. and M.-H.C. wrote the manuscript.

ACKNOWLEDGMENT The authors acknowledge the financial support provided by the Yonsei University research fund (Post Doc. researcher supporting program) of 2018 (project no.: 2018-12-0142), and the National Research Foundation of Korea (NRF) grant funded by the korea government (Grant No. 2017R1A5A1014862, SRC program vdWMRC center), and the academy-industry joint-research program between Yonsei University and Samsung Electronics.

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TOC / Abstract Graphics


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FIGURE CAPTIONS

Figure 1. Fitted X-ray photoelectron spectroscopy (XPS) (a) B 1s and (b) P 2p core level spectra of boron-doped BP as a function of pulsed process time (each process time = 5 s). (c) Peak area transition for chemical states (B-B, B-P, and O-B-P) for each pulsed process time. The area was calculated from the ADXPS spectra at θ = 90. The optimized process time satisfying the maximum B-P states and the minimum oxidized BP states can be obtained for the pulsed plasma time of 30 s. UPS spectra of (d) pristine BP and boron-doped BP using the pulsed plasma process. The dashed lines show the deviations of the secondary electron cutoff. The results show a decrease in the work function by ΔE = 0.2 eV in the boron-doped BP, which is attributed to the electron doping of the BP surface by the pulsed plasma process.


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Figure 2. (a) EDX elemental depth profile (bottom) and the scanning transmission electron microscopy image scanned along the depth direction (top image) of BP for a pulsed plasma process time of 30 s. The boron signals were clearly observed at the penetration depth of ~10 nm from the surface in boron-doped BP. High resolution cross-sectional TEM image (b) and the boron and oxygen atomic concentration (c) for each pulsed process time.


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Figure 3. Calculated atomic structures, band structures, and total density of states for (a) pristine BP, (b) surface-adsorbed B on BP (Bsurface), and (c) intercalated B into BP layers (Binterface). The dashed line indicates the Fermi level for various bond structures in the BP system. Both impurities are found to be at local minima with binding energies of EB [Bsurface] = -0.523 eV and EB [Binterface] = -2.492 eV, and the EF in boron-doped phosphorene shifts upward to the conduction band, indicating that the boron atom can act as an effective electron donor in the boron-phosphorene system.


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Figure 4. Electrical characteristics of the BP p–n homojunction device. (a) Transfer characteristics (Id–Vg) for the pristine (red line) and boron-doped BP FETs (blue line) at a source–drain voltage of 100 mV. The inset depicts the same data on a logarithmic scale. (b) and (c) show the transfer curves (Id–Vg) of the p–n homojunction device of the half pristine BP (ptype) and half boron-doped BP (n-type) in the linear and logarithmic scale, respectively. The inset shows the optical microscopy image (PMMA buffer layer for the plasma process) and the illustration of the bandgap diagram for the p–n junction. The half p-type and half n-type device exhibited ambipolar transport behavior, and the electron mobility of μFE was 265 cm2/V·s, and the hole mobility of μFE was 228 cm2/V·s at room temperature. Gate tunable output characteristics (Id–Vd) of the BP p–n diode device for the (d) linear and (e) logarithmic scale show a high rectification ratio (up to 2.8 × 104). (f) Diode rectification ratio as a function of applied gate bias.


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Figure 5. (a) Time-resolved photo-switching under modulated laser illumination power from 100 to 400 μW at bias voltage Vd = 0.1 V and gate voltage Vg = 0 V. (b) Rise and fall time of the diode were estimated to be ~1.2 and ~7.6 ms, respectively. (c) I–V characteristics in p–n configuration under illumination with different power densities (200 and 400 μW) for the device. The Id–Vd curves did not cross the zero current (voltage) point even when the applied voltage (current) was 0 V, and this short-circuit current (Vd = 0) was a feature of the p–n homojunction diode, which is capable of photovoltaic energy conversion without any external power input and consumption. (d) Dependence of open-circuit voltage (VOC) and short-circuit current (ISC) on illumination density. The inset depicts the results of photoresponsivity. By fitting the responsivity versus power density, it was possible to obtain α = 0.86, indicating the high recombination of the photogenerated carriers.


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TABLE

Table 1. Comparison of figures-of-merit for the photovoltaic device comprising a BP-based p–n junction and other 2D materials

Material

Measurement conditions

Boron-doped (Plasma process) Electron mobility: 265 cm2/V·s, Vds = 0.1 V, Vg = 0 V, λ = 785 nm, Pd = 400 μW BP (p–n) Cs2CO3/BP (p–n)

(Vacuum deposition) Electron mobility: 27 cm2/V·s, Vg = 30 V, λ = 405 nm, P = 10 mW

Al-doped BP (Vacuum deposition) Electron mobility: 105 cm2/V·s, Vds = 0.1 V, Vg = 0 V, λ = 1550 nm, P = 860 nW (p–n)

Response Responsivity Ref. time (ms) (mA/W) 1.2

11.3

This work

Not applicable

1.88 × 103

36

3

6.2

43

MoS2

Vds = 1 V, Vg = −2 V, λ = 633 nm, P = 50 mW

>103

110

47

WS2

Vds = 30 V, Vg = N.A., λ = 458 nm, P = 2 mW

5.3

21.4 × 10-3

48

In2Se3

Vds = 5 V, Vg = 0 V, λ = 300 nm, P = 2.08 W

18

395 × 103

49

GaSe

Vds = 5 V, Vg = 0 V, λ = 254 nm, P = 1 mW

300

2.8 × 103

50


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PâN Junction Diode Using Plasma Boron-Doped Black Phosphorus

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