Multifunctional Homogeneous Lateral Black Phosphorus Junction

Feb 28, 2017 - Furthermore, at high positive gate voltages, the interband tunneling enables the homogeneous lateral p+–n+ junction transistors to pr...
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Multifunctional Homogeneous Lateral Black Phosphorus Junction Devices Jingyuan Jia,† Jiao Xu,† Jin-Hong Park,‡ Byoung Hun Lee,§ Euyheon Hwang,*,† and Sungjoo Lee*,†,‡ †

SKKU Advanced Institute of Nanotechnology (SAINT) and ‡School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon 440-746, Korea § School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea S Supporting Information *

ABSTRACT: We demonstrate a controllable doping technique of few-layer black phosphorus (BP) via surface charge transfer using an ionic liquid mixture of EMIM(C6H11N2+):TFSI(C2F6NO4S2−) [EMIM:TFSI, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide]. A wide range of hole carrier densities, from 1011 cm−2 (nondegenerate) to 1013 cm−2 (degenerate), can be obtained by controlling the weight percentage of the ionic liquid mixture. The doping method we proposed in this paper can be applied to make a multifunctional homogeneous lateral p−n junction device. By doping a fraction of the BP sample and by applying a gate voltage to the other fraction of the BP, we obtain homogeneous lateral p+−p, p+−n, p+−n+ junction diodes in a single BP channel. The homogeneous lateral BP p+−p and p+−n junctions display ideal rectifying behavior and a much stronger photoresponse due to the built-in potential. Furthermore, at high positive gate voltages, the interband tunneling enables the homogeneous lateral p+−n+ junction transistors to provide both a negative differential resistance (NDR) and a negative transconductance (NTC) in the current−voltage characteristics at room temperature. On the basis of our results, it is possible to build novel devices utilizing the large NDR and NTC in BP such as amplifiers, oscillators, and multivalued logic systems.

B

these approaches are limited to the light doping regime. Broad control over charge carriers (from nondegenerate to degenerate region) is a key issue for the development of electronic functions in BP applications. Previously, the charge carrier doping of BP combined with static gating were reported,16,17 where the doping level was controlled through electric field. In spite of its practical importance, the wide range of doping on atomically thin BP has not yet been well explored. In this work, we demonstrate an air-stable chemical doping method of BP which is controllable in producing a wide range of carrier densities from nondegenerate to degenerate regions. In the method, we control the doping level by adjusting the mixing percentage of two different ionic liquids: 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMIM:TFSI) as the surface charge-transfer acceptor. The chemical doping method we proposed in this work can be applied to make a multifunctional homogeneous lateral p−n junction diode that was formed in a single BP channel. Even though several studies have been reported on both vertical and lateral junctions (especially p−n junction) based on graphene and TMDCs materials in the literature,7,18−23 there are very few studies on the p−n diode based on single-channel BP. Even the

lack phosphorus (BP) was recently rediscovered as being a layered two-dimensional (2D) semiconductor material that is potentially useful as the active material in future nanoscale devices.1−6 The recent rapid growing interest in BP has mainly focused on ultrathin or 2D form. The big advantage of 2D black phosphorus over graphene is that it has a natural bandgap so that one can make various electronic devices, such as transistors. The electronic properties of atomically thin BP can be controlled by external gating or by chemical doping. Controlled chemical doping that is stable under ambient air conditions is essential for modulating the carrier concentrations and electronic properties of semiconductor materials. A variety of homojunctions, heterojunctions, and complex devices based on 2D materials have been prepared using a variety of chemical doping processes.7,8 Surface charge doping, which is a simple approach without introducing significant defects, has been widely used to modulate the chemical and electrical properties of 2D materials.9−11 In transition-metal dichalcogenides (TMDCs), such as MoS2 and WSe2, the chemical doping with potassium, benzyl viologen, chlorine, NO2, and DNA has been widely used to produce charge carriers in the 2D materials.10−14 Relatively few studies have described successful BP doping methods. Among these, surface-adsorbed cesium carbonate (Cs2CO3) and molybdenum trioxide (MoO3) have been introduced on the surface of BP as donors and acceptors, respectively, to achieve controllable doping effects.15 However, © 2017 American Chemical Society

Received: January 17, 2017 Revised: February 28, 2017 Published: February 28, 2017 3143

DOI: 10.1021/acs.chemmater.7b00210 Chem. Mater. 2017, 29, 3143−3151

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Chemistry of Materials

Figure 1. (a) Chemical structures of the EMIM:TFSI dopant molecules adsorbed onto black phosphorus. (b) Schematic diagram of the controlled doping process at different ionic liquid (EMIM:TFSI) concentrations. (c) Schematic diagram showing the lateral BP junction devices and the corresponding energy band diagram. (d) OM image of the junction structure in a single BP film. (e) Raman spectra of the undoped BP and EMIM:TFSI-doped BP region. (f) A2g peak center position Raman mapping image of the BP junction, collected from the area denoted by the white rectangle in Figure 1d. (g) Kelvin probe force microscopy (KPFM) mapping image of the BP junction. (h) X-ray photoelectron spectroscopy (XPS) spectra of the undoped BP and EMIM:TFSI-doped BP.

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RESULTS AND DISCUSSION Doping BP with EMIM:TFSI and Homogeneous Lateral Junction Formation. We first describe the doping processes for BP. Atomically thin BP was prepared by the mechanical exfoliation of bulk BP. The exfoliated BP flakes were transferred onto a 285 nm SiO2/p++ Si wafer and submitted to soft Ar plasma treatment, by which we reduced the BP layer thickness and cleaned the BP surface, as reported in previous work.24 Ionic liquid EMIM:TFSI (Figure 1a) solutions prepared with different weight percentages, ranging from 12.5% to 100%, were then drop-cast onto the BP surfaces at room temperature. Figures 1a,b show a schematic illustration of the process by which the BP was controllably doped using different weight percentages of the ionic liquid EMIM:TFSI. EMIM:TFSI, an ionic liquid mixture of positive ions (EMIM, C6H11N2+) and negative ions (TFSI, C2F6NO4S2−), has a low molecular weight and a high polarizability that could impose a wide range of charge induction effects on the underlying solid material. The ionic liquid adsorbed onto the BP surface forms an electric double layer and/or trapping centers at the interface between the EMIM:TFSI layer and BP, which induces electron accumulation at the interface and introduces p-doping into the underlying BP surface. As discussed in Figure 2, the BP doping level could be modulated by tuning the ion concentrations in the EMIM:TFSI solution. Figure 1c shows homogeneous lateral junctions formed on a single BP layer. To

reported BP diodes operate only when a gate voltage is applied and immediately lose its functionality in the absence of a gate voltage.1 For the practical electronic devices based on BP a stable homogeneous lateral junction diode is required, but the stable BP p−n diode has not been successfully fabricated yet. Our doping approach permitted not only realization of stable junctions but also distinct various functionalities: p+−p (M−S junction), p+−n (PN junction), and p+−n+ (which provides technically important negative differential resistance, NDR, in I−V characteristics). We show that the p+−p and p+−n junctions yielded a strong photoresponse in a wide range of wavelengths of light from infrared to visible regions, indicating that the internal electric field displayed unique open-circuit voltage (VOC) and short-circuit current (ISC) properties with a high external quantum efficiency (EQE) and responsivity. The p+−n+ junction devices showed NDR behavior in the ID−VD characteristics even at room temperature. These characteristics resulted from interband electron tunneling as a result of broken-gap band alignment at lateral BP junctions, yielding a new class of 2D tunneling FETs. In addition, the homogeneous lateral BP junction FET showed negative transconductance (NTC) characteristics in the ID−VG curves obtained from the lateral BP junction FET. The proposed junction diodes with these electrical characteristics may provide the possibility to extend application areas of future nanoelectronic devices, as a result, to overcome the limits of conventional device structures. 3144

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Figure 2. Schematic structural diagrams and electronic characteristics of the BP transistors doped with EMIM:TFSI at different concentrations structures. (a) Schematic diagram of a back-gated transistor fabricated on a BP doped with different concentrations of EMIM:TFSI. (b) Transfer characteristics of a 7 nm doped BP device as a function of the dopant weight percentage. (c), (d) Threshold voltage and carrier concentration (Hall measurement) of the same device, as a function of the dopant weight percentage.

2p peaks obtained from the undoped and doped BP films. The P 2p peaks obtained from the doped BP were shifted toward lower values compared to the peaks obtained from the undoped BP. Specifically, P 2p1/2 was shifted from 130.3 to 130.1 eV and P 2p3/2 was shifted from 129.3 to 129.1 eV. The downshift in the XPS peaks was attributed to the doping effects, which shifted the Fermi level toward the valence band edge. Similar results were reported in the case of graphene 28 and TMDCs.8,29,30 The crystal structure of doped BP is confirmed from transmission electron microscopy (TEM) images. The selected area electron diffraction (SAED) patterns (Figures S1a and S1b insets of the Supporting Information) confirmed that the doped product was high-quality single crystalline in structure with an orthorhombic crystalline character and without noticeable impurities, in good agreement with the properties of the undoped BP film (Figure S1a). High-resolution TEM images corroborated this result (Figures S1a and S1b). The lattice parameters acquired by HRTEM from both films were indistinguishable, indicating that the ionic liquid-doped BP films maintained their unique crystalline properties so that a homogeneous lateral BP junction could be formed. Controllable p-Doping Effects of the BP FET. The doping level of BP doped with EMIM:TFSI could be controlled by varying the ionic concentrations in the ionic liquid. The ionic liquid was diluted with PMMA (liquid phase at room temperature) to obtain different weight percentages: 12.5%, 25%, 50%, 75%, and 100%. On the basis of the doping technique with control, we fabricated and characterized backgated FET devices composed of few-layer (∼3 nm) BP doped with different doping levels, as shown in Figure 2a. Figure 2b presents the evolution of the transfer characteristics (VG sweep from −50 to 50 V) of a BP transistor for various EMIM:TFSI ionic concentrations. All the measurements were conducted on the same BP device. Between measurements, the BP device was

make the junction, we first covered a part of BP with poly(methyl methacrylate) (PMMA) and patterned the PMMA using e-beam lithography so that BP beneath the PMMA layer remained undoped, and the uncovered BP region was doped with EMIM:TFSI. The undoped BP region was electrostatically controlled by the back gate and formed different types of functional junction diodes. Figure 1d shows an OM image of the homogeneous lateral BP junction. The black dotted line indicates the boundary between the undoped and the doped BP regions. Raman spectra were collected from the undoped and doped regions of the BP samples (Figure 1e). The doped BP was prepared using 100% EMIM:TFSI. The spectrum of the undoped BP region displayed a single peak at 366 cm−1 (A1g) corresponding to out-of-plane vibrations and two conventional peaks at 443 and 471 cm−1 (B2g and A2g, respectively) corresponding to in-plane vibrations. The BP doped with 100% EMIM:TFSI displayed red shifts (1.3 cm−1) in the B2g and A2g peaks due to an increase in the hole carrier density, which is consistent with previous reports of the Raman peak changes observed in doped two-dimensional materials.25−27 No shift in A1g was observed. Figure 1f shows a Raman mapping of the A2g peak center positions, collected from the same partially doped BP sample (white rectangle in Figure 1d). The color contrast between the mapping images of the undoped and doped BP areas clearly indicated that a homogeneous lateral junction formed on the BP layer. Kelvin probe force microscopy (KPFM) measurements (Figure 1g) and X-ray photoelectron spectroscopy (XPS) analysis (Figure 1h) were also performed on the partially doped BP films. Surface potential (ϕs) mapping images obtained from both regions displayed clear differences. The value of Δϕs across the two regions was estimated to be 50 meV, and work functions of 4.28 and 4.33 eV were obtained from the undoped and doped BP, respectively. Figure 1h reveals the phosphorus 3145

DOI: 10.1021/acs.chemmater.7b00210 Chem. Mater. 2017, 29, 3143−3151

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Figure 3. (a) Structure and energy band diagram of the BP p+−p and p+−n junction FETs at equilibrium. (b) ID−VD curves of 7 nm BP junction FETs with a VG between 0 and +30 V. (c) If/Ir at different VG values. Photoresponse of the BP junction device upon illumination at 655 nm, Iphoto− VD were measured before and after EMIM:TFSI partial doping at (d) VG = 0 V (e) and 30 V. (f) ID as a function of VD and VG (inset) for the BP p+−p and p+−n junction FETs.

increased from 0 to 100%, indicating that the BP doping level could be readily controlled via EMIM:TFSI doping. These results enabled calculation of the Fermi level position EF in the BP layers prepared at different doping levels. EF was calculated with a hole band effective mass 0.2me (where me is the electron mass in free space) and found to be 1 meV for pristine BP, and this value gradually shifted, reaching 250 meV for the 100% doping. Characteristics of the Homogeneous Lateral BP Junction Devices. As described in Figure 1c, homogeneous lateral BP junctions were fabricated by covering part of the BP channel using patterned PMMA, thereby preserving the semiconducting properties beneath the PMMA layer, while permitting chemical doping on the uncovered parts of BP. To achieve various functional diodes, we doped the uncovered part with 100% EMMI:FSI ionic liquid (i.e., p+ type). This structure formed a lateral junction diode. The undoped regions of the BP were electrostatically controlled by the back gate voltage. Therefore, it is possible to make a variety of functional junction devices, such as p+−p, p+−n, or p+−n+ junctions. We measured I−V characteristics of the diodes by applying a drain voltage VD to the p+-doped side (Figure 3a). For VD > 0, holes drifted from the p+ region to the undoped region. Figure 3b depicts the ID− VD characteristics for p+−p and p+−n junction devices. As the gate voltage increased from 0 to 30 V, both the forward (If) and reverse (Ir) currents decreased. The significant decrease in Ir is attributed to a stronger built-in potential, which limited Ir. The

cleaned by using Isopropyl Alcohol (IPA) solution, followed by dipping in acetone solution for 5 min. The transfer curve obtained from an undoped BP device displayed p-type dominant transport behavior with a current minimum at 8 V. For the sample with 12.5% dopant weight percentage, the minimum current position shifted toward positive values, beyond 50 V, indicating marked p-type doping on the BP film. The transfer curves for higher percentage represented typical ptype behavior. The threshold voltage also continued to shift positively from −5 V (0%) to 150 V (100%) as EMIM:TFSI percentage increased. For the device doped with 100% ionic liquid, the current in the BP channel is almost constant, showing typical metallic properties, indicating that controllable doping was achieved from the nondegenerate to the degenerate level (p+ doping). The hole concentration induced by the pristine BP was estimated to be 7 × 1010 cm−2 from the transfer curve, and the carrier concentration progressively increased to 2 × 1013 cm−2 (300 times) as the EMIM:TFSI weight percentage was increased (Figure S2). Hall measurements were performed using two opposing contacts oriented perpendicular to the drain−source current path (see the schematic diagram shown in the inset of Figure 2d). The carrier density could be calculated using the expression nH = IB/eVH, where nH is the carrier concentration, B is the vertical magnetic field, VH is the horizontal voltage, and I is the current through the BP channel. As shown in Figure 2d, the hole concentration increased from 1011 to 1013 cm−2 as the EMIM:TFSI weight percentage 3146

DOI: 10.1021/acs.chemmater.7b00210 Chem. Mater. 2017, 29, 3143−3151

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Figure 4. Time-dependent photocurrent measured at VD = 0.1 V upon illumination at 655 nm at a power density of 62.5 W/cm2 (device area is 50 μm2), before and after partial EMIM:TFSI doping, at (a) VG = 0 V and (b) 30 V. Photoresponse of the BP junction diodes upon illumination at 1064 nm, Iphoto−VD were measured before and after partial EMIM:TFSI doping at (c) VG = 0 V and (d) 30 V.

obtained If/Ir ratio increased from 3.66 to 33 as the back gate voltage varied from 0 to 30 V (Figure 3c). An ideality factor of 1.16 was obtained, indicating insignificant carrier recombination in the homogeneous lateral junction BP diodes. One possible application of the p+−p and p+−n diodes is photoelectronic devices. Upon the light illumination, we measured the photocurrents of two junction devices (p+−p at VG = 0 and p+−n at VG = 30 V), and these values were compared with those obtained from pristine BP channel devices (without a junction). Light of wavelength of 655 and 1064 nm was used in this experiment. Red light illumination at a power of 62.5 mW/ cm2 revealed that Iphoto for the junction device was much higher than that obtained from the pristine BP FET. The photocurrents reached 10 and 4 μA for junction devices at 0 or 30 V gate voltages, respectively, and these values were 10 times higher than the value obtained from the pristine channel. The enhanced photocurrent arises from the built-in potential in the junction device effectively separated and drove the photogeneration of e−h pairs. Although built-in potential of the p+−n junction was stronger than the p+−p junction, Iphoto for the p+− n junction was slightly less than that of the p+−p junction because the carrier density in the p+−p junction was much higher than that in the p+−n junction. Under red laser illumination, the open-circuit voltage (Voc) and the short-circuit current (Isc) were increased with laser power density (Figure S3) and reached 0.1 V, 80 nA for the p+−p junction and 0.15 V, 30 nA for the p+−n junction at 62.5 mW/cm2. These trends were not observed in the pristine BP channel due to the absence of a built-in potential.31,32 Figure 4a,b shows the timedependent photoresponse characteristics of the pristine BP (black line) and junction BP photodetectors (p+−p and p+−n junctions, red lines) at VD = 0.1 V and VG = 0 and 30 V, respectively. An apparent photocurrent enhancement, com-

pared to the photocurrent measured in the pristine channel, was identified for both p+−p and p+−n junctions. The observed rise time (0.03 s) and fall time (0.05 s) in the lateral BP junction devices were 1 order of magnitude faster than the corresponding times measured from the pristine BP devices (0.2 and 0.15 s). This significant improvement in the response times is attributed to a higher carrier density and the built-in potential generated in the junction devices. The BP p+−p and p+−n junctions also showed enhanced photoresponses in the IR spectrum (1064 nm), as shown in Figure 4c,d, which plot the photocurrent vs voltage curves of the pristine and junction BP photodetectors at VG = 0 V (Figure 4c) and 30 V (Figure 4d). The output characteristics under illumination suggested that BP would be useful in broad band optical applications, particularly IR (or mid-IR) optoelectronic applications. Very few 2D layer semiconductor materials (for example, TMDC materials) have been identified as useful in IR optoelectronics. The photocurrents measured under IR illumination also suggested that the p+−p and p+−n junctions were advantageous over the pristine BP channel materials, and photocurrents of 3.2 and 2.5 μA could be achieved for the p+−p and p+−n junctions at VD = 0.5 V, as observed in the red light illumination case. We evaluated the optical performances of BP p+−p and p+−n junctions by calculating the external quantum efficiency (EQE) and photoresponsivity (R). An EQE value of 24000% was obtained at VD = 1 V, VG = +30 V (p+−n), and R reached 120 A/W, nearly 3 times the value obtained from an intrinsic fewlayer BP phototransistor and much higher than the values reported recently for BP p−n junctions.1 Previous studies of photoresistors obtained a high EQE or R by ensuring a very high VG and VD,33,34 which induced the generation of additional thermionic charges that drove a greater number of photogenerated carriers. The built-in potentials of our p+−p and p+− 3147

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Figure 5. (a) Structure and energy band diagram of a BP p+−n+ junction at different VD. (b) ID−VD curve at VG = 80 V (red curve) and 100 V (black curve). (c) ID−VG curve obtained from the same BP junction FET before and after EMIM:TFSI partial doping.

p+−n+ junction with increasing VD. The NDR properties of the BP lateral p+−n+ junction were observed starting at VG = +80 V and could be clearly observed in the measured output characteristics at VG = +100 V (Figure 5b). No significant hysteretic behavior was observed from the forward (0 → 2 V) and reverse (2 → 0 V) sweeps. This is the first demonstration of tunneling-induced NDR transport in a single BP channel. The obtained peak to valley ratio is ∼2 at room temperature, which is higher than recently reported values from 2D heterojunction structures such as BP/SnSe235 and MoS2/ WSe2.36 The ID−VG characteristics of a homogeneous lateral BP junction FET was studied and compared to that of a pristine BP channel device (Figure 5c). Although both FETs exhibited hole-dominant transport characteristics (high ID at negative values of VG), a convex peak (i.e., local maximum in current) appeared for the BP junction FET (red curve) at positive values of VG (50 V < VG < 95 V, the current increased first and then decreased as VG increased), demonstrating negative transconductance characteristics (NTC, dID/dVG < 0). This NTC was observed even at room temperature. The observed NTC can be understood by combined effects among diffusion currents, a barrier formation at the junction, and Schottky barrier (ϕSC) between BP and the metal contact. Figure S4 shows an energy band diagram that explains the NTC behavior in the ID−VG curves at positive values of VG. For a VG less than 30 V (Figure S4-I, p+−p junction), a typical hole carrierdominant ID curve was observed, indicating that ID decreased as VG increased. After reaching a minimum value at VG = 30 V, the current increased with the gate voltage. We understand that the minimum current arises when the Fermi energy of metal contact matches with the middle of the band gap of n-type BP. The Schottky barrier becomes highest at this point and the contact resistance becomes maximum. For VG values of 30−75 V (Figure S4-II, in which the junction becomes basically p+-n diode), since the Schottky barrier (ϕSC) between the semiconducting BP and the metal contact decreases as VG

n junctions offer exceptional benefits for separating and collecting photocarriers, even at small VD and zero VG, rendering them suitable for low-power photodetector operation. Furthermore, filling factor (FF) and current conversion efficiency (η) were estimated to be 0.2, 4.8% for p+−p junction and 0.4, 5.4% for p+−n junction, respectively. The homogeneous lateral BP junction diode was further studied under a high positive VG to form p+−n+ diode, which showed an anomalous current−voltage characteristic, that is, negative differential resistance (NDR) transport behavior. NDR is characterized by a negative slope in the ID−VD curve (dID/ dVD < 0) and is associated with the quantum tunneling through the junction barrier (depletion region). As shown in Figure 5, ID−VD curve of the p+−n+ diode at room temperature exhibits NDR behavior. Figure 5a illustrates the arrangement of energy levels associated with the conduction, valence band edges, and Fermi level. Under a small forward bias (0 < VD < 0.8 V, Figure 5a-I), electrons drifted from the n+ region by tunneling through the barrier into the empty states of the p+ region through a common energy window (EFN+ − EFP+). The tunneling current increased mainly because the available joint density of states (DOS) for conduction increased with the bias voltage, I ∝ DOSP. DOSN. A peak current is obtained when the joint DOS becomes a maximum. When the forward bias voltage is further increased (0.8 V < VD < 1.5 V, Figure 5a-II), the common energies for both bands (i.e., joint DOS) decreased, which gives rise to the decrease of the tunneling current. The junction current reached a minimum value as the total DOS approached zero. At VD = 1.5 V the bottom of the conduction band of the n+ region is aligned at the top of the valence band of the p+ region. Thus, at this voltage the tunneling is not allowed and no current flow because there are no available DOS for conduction. With further increases in VD (VD > 1.5 V, Figure 5a-III), the normal diffusion current and the thermionic emission current increased with VD because the energy barrier between the conduction bands of the p+ and n+ region decreased. The current, therefore, increased in the lateral BP 3148

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Characterization of the EMIM:TFSI Doped BP Films. The Raman spectra of the EMIM:TFSI/BP/SiO2/Si samples were collected and compared with those obtained from a control sample (BP/SiO2/Si) using a laser micro-Raman spectrometer (Kaiser Optical System RAMANRXN1, at an excitation wavelength of 532 nm). The TEM measurements (JEOL JEM 2100F) were conducted by transferring the BP samples onto a copper grid with a carbon mesh. TEM imaging was carried out at an acceleration voltage of 300 kV. The chemical configurations were determined by X-ray photoelectron spectroscopy (XPS, ESCA2000). The XPS measurements were performed using Al Kα and Mg Kα X-ray sources. An AFM-Raman measurement system with a Kelvin probe (NT-MDT 830) was used to estimate the work functions of the BP flake samples with or without doping. FET Fabrication and Measurements. The back-gated BP transistors were fabricated by patterning source/drain electrodes onto the BP/SiO2/Si samples using e-beam lithography, followed by Cr (10 nm) and Au (50 nm) deposition in an e-beam evaporator. Transistors were doped with different concentrations of the EMIM:TFSI solution (12.5%, 25%, 50%, 75%, and 100%). The diode structure was fabricated by partially capping the BP surface with PMMA (BP channels were patterned using e-beam lithography) through a mask to enable patterned EMIM:TFSI doping. The EMIM:TFSI chemical dopant was dropped onto the partially passivated BP channels. All electrical measurements were conducted using a Keithley 4200 parameter analyzer under ambient conditions. The photocurrent was measured using monochromatic light, 655 and 1024 nm with power density of 62.5 mW/cm2. The photoresponsivity (R) and external quantum efficiency (EQE) were calculated from the ID−VD curves: R = (Iphoto × As)/(PLight × A) and EQE = 100 × hcR/eλ, where Iphoto is the generated photocurrent, PLight is the total incident optical power, A is the effective area of detector, As is the illumination area, e is the absolute value of the electron charge, and λ is the laser wavelength. Filling factor (FF) is defined as FF = Pel,max/IscVoc, where Pel,max is the maximum of Pel and Pel is defined as Pel = IDVD. Current conversion efficiency (η) is defined as η = Pel,maxAs/PLightA.37

increases and the main current comes from the electron injection from n-doped BP region to p+ BP region, ID increases with the gate voltage. After ID reached a peak value at VG = 70 V, the current decreased again with the gate voltage. The downturn in ID for VG values exceeding 70 V was mainly attributed to an enhanced barrier (ϕbi) at the boundary region between p+−n+, which suppressed the total ID. The barrier is proportional to the built-in potential at the interface. The lateral BP junction devices displayed both NDR and NTC characteristics, which is significant for further research into a new class of 2D functional devices.

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CONCLUSION We used an ionic liquid mixture of EMIM(C6H11N2+):TFSI(C2F6NO4S2−) to control the doping level of few-layer black phosphorus. The doping technique allows one to vary the hole carrier density from 1011 cm−2 (nondegenerate) to 1013 cm−2 (degenerate) by simply changing the weight percentage of the ionic liquid mixture. The doping method we proposed in this work can be applied to make a multifunctional homogeneous lateral p−n junction diode. By doping part of a BP sample and by applying gate voltage to the other part of the sample, we obtained homogeneous lateral p+−p, p+−n, and p+−n+ junction diodes in a single BP, yielding an electrically tunable p−n junction. The environmental stability issues were not detected from our devices during the characterization period of several weeks. The homogeneous lateral p+−p and p+−n junctions displayed ideal rectifying behavior and a much stronger photoresponse due to the built-in potential, which can be controlled by tuning the gate voltage. The open-circuit voltage and short-circuit current measurements suggested that the lateral BP p+−n diode may be applicable to photovoltaic cells. Furthermore, at high positive gate voltages the diode becomes p+−n+ diodes, and in this case the interband tunneling enabled the homogeneous lateral p+−n+ junction transistors to provide both a negative differential resistance (NDR) and a negative transconductance (NTC) in the current−voltage characteristics at room temperature. The obtained peak to valley ratio in NDR characteristic ID−VD curves reached ∼2 at room temperature. Our results provide an effective approach to build multifunctional lateral BP junctions (p+−p, p+−n, and p+−n+) based on a single BP FET which is prepared by partially doping a sample. The observation of NDR and NTC properties in BP p+−n+ junction FETs opens up new opportunities for BP-based devices in future applications of low-power high-speed electronic device applications. Finally, we emphasize that in addition to the multifunctional junction diode proposed in this paper, we can extend the device scheme for other purposes simply by adjusting the doping level of the BP layer.

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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/acs.chemmater.7b00210. Atomic structure obtained from TEM of BP before and after 100% EMIM:TFSI doping; linear ID−VG curve obtained from a single BP device prepared with different doping levels; carrier concentration evaluation from ID− VG transfer curve with different doping levels; power dependence of Voc and Isc for p+−n junction; energy band diagrams of the BP FETs under various applied gate biases (PDF)

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AUTHOR INFORMATION

Corresponding Authors

EXPERIMENTAL SECTION

*E-mail: [email protected] (E.H.). *E-mail: [email protected] (S.L.).

Preparing the BP Films and the Ionic Liquid in Different Weight Percentages. Few-layer BP was obtained by cleaving commercially available bulk BP crystals with blue Nitto tape. After exfoliation, for further thickness control, an inductively coupled plasma (ICP) was used to treat the BP flakes. Ar gas was used to maintain the pressure at 30 mTorr and 350 W rf power delivered at 13.56 MHz was applied to a four-turn spiral coil to discharge the high-density plasma. The ionic liquid was prepared in different weight percentages by adding different amounts of PMMA (495, A5; liquid phase at RT; 0, 4, 12, 36, and 84 mg) to 12 mg ionic liquid to yield ionic liquid weight percentages of 12.5%, 25%, 50%, 75%, and 100%. During the doping process, the ionic liquids were dropped directly onto the BP films.

ORCID

Sungjoo Lee: 0000-0003-1284-3593 Author Contributions

J. Jia and S. Lee conceived and designed the experiments. J. Xu, J.-H. Park, and B. H. Lee contributed to the experimental design and device fabrication. E. Hwang analyzed the data and performed the theoretical calculations. S. Lee supervised the research. All authors discussed the results and commented on the manuscript. 3149

DOI: 10.1021/acs.chemmater.7b00210 Chem. Mater. 2017, 29, 3143−3151

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Chemistry of Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873) of the National Research Foundation of Korea (NRF) and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2014M3C1A3053024), and Basic Science Research Program through the National Research Foundation of Korea funded by the Korean government (MSIP) (Grants: 2015R1D1A1A09057297 and 2015M3A7B7045496).

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DOI: 10.1021/acs.chemmater.7b00210 Chem. Mater. 2017, 29, 3143−3151

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DOI: 10.1021/acs.chemmater.7b00210 Chem. Mater. 2017, 29, 3143−3151