Carrier Engineering in Polarization-Sensitive Black Phosphorus van

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Carrier Engineering in Polarization-Sensitive Black Phosphorus van der Waals Junctions Bao-Wang Su, Xiao-Kuan Li, Xiao-Qiang Jiang, Wei Xin, Kai-Xuan Huang, De-Kang Li, Hao-Wei Guo, Zhibo Liu, and Jian-Guo Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12814 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Carrier Engineering in Polarization-Sensitive Black Phosphorus van der Waals Junctions Bao-Wang Su, Xiao-Kuan Li, Xiao-Qiang Jiang, Wei Xin, Kai-Xuan Huang, De-Kang Li, Hao-Wei Guo, Zhi-Bo Liu*, Jian-Guo Tian The Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, TEDA Institute of Applied Physics and School of Physics, Nankai University, Tianjin 300071, China

ABSTRACT Van der Waals p-n heterostructures based on p-type black phosphorus (BP) integrated with other two-dimensional (2D) layered materials have shown potential applications in electronic and optoelectronic devices, including logic rectifiers, and polarization-sensitive photodetectors. However the engineering of carriers transport anisotropy, which is related to the linear dichroism, have not yet been investigated. Here, we demonstrate a novel van der Waals device of orientation-perpendicular BP homojunction based on the anisotropic band structures between the armchair and zigzag directions. The structure exhibits good gate-tunable diode-like rectification characteristics caused by the barrier between the two perpendicular crystal orientations. Moreover, we demonstrate that the unique mechanisms of the polarization-sensitivity properties of this junction are involved with the linear dichroism and the anisotropic carriers transport engineering. These results were verified by the scanning photocurrent images experiments. This work paves the way for 2D anisotropic layered materials for next-generation electronic and optoelectronic devices.

Keywords: black phosphorus, anisotropy, van der Waals junctions, diode-like, polarization, carrier engineering,

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INTRODUCTION Over the past decade, two-dimensional (2D) materials, owing to their intrinsic properties, have attracted considerable interest for nanoelectronic applications,1-6 including graphene,7,8 TMDCs,9 black phosphorus (BP), and etc. Among of them, BP as a semiconductor with thickness-dependent direct bandgap ranging from ~2eV (monolayer) to ~0.3eV (bulk)10-12 and high carrier mobility, especially its strong intrinsic in-plane anisotropic electrical and optical properties, which are attributed to the anisotropic effective carrier masses and band structures in its armchair (AC) and zigzag (ZZ) crystal directions.13-16 Such unique characteristics make it promising for realization of high performance transistors, novel polarization-sensitive photodetectors17 and other optoelectronic devices.18-23 It’s worth to note that for the polarization-sensitive photodetectors, similar work based on other 2D anisotropic layered materials have been reported, such as ReSe2,24 GSe25 and quasi-1D TiS326 and so on. On the other hand, van der Waals p-n heterostructures based on p-type BP integrated with other 2D layered materials have been reported as logic rectifiers (refs 27), photodetectors28-31 and other devices, exhibiting outstanding potential in electronic and optoelectronic.32-35 But the band alignments between the AC and ZZ directions, the polarization-dependent sensitivity caused by the anisotropic optical properties and the carriers transport engineering have not been mentioned.14,15 Homojunctions with diode-like rectification characteristics caused by chemical doping based on two-dimensional layered materials have been reported, such as MoSe2,37 MoS238,39 and BP.40 Here, we mainly demonstrate a novel van der Waals device of orientation-perpendicular BP homojunction based on the anisotropic band structures between the AC and ZZ directions36, whose principles of the diode-like rectification characteristics are different from those 2D layered materials homojunctions as we mentioned above. This anisotropic effective carrier masses and Fermi levels along the two perpendicular crystal orientations will give rise to the band-offsets and band-barriers at the junction formed by the van der Waals interactions, verified by our scanning photocurrent images (SPI) experiments, which leading to different carriers transport engineering and then the gate-tunable diode-like rectification emerging. Besides, the orientation-perpendicular BP homojunction shows localized photoresponse and remarkable polarization-sensitive optoelectronic properties in the visible range. These results arise from the linear dichroism and different dynamics of carriers transport with different polarizations at various bias voltages. The unique 2


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mechanisms of the current rectification and polarization-sensitive photodetection based on in-plane anisotropic BP may provide new functionalities with anisotropic 2D materials in nanoelectronic and optoelectronic applications.

RESULTS AND DISCUSSION BP thin flakes were mechanically exfoliated using adhesive tape from a bulk BP crystal on a 285 nm SiO2/p+-doped Si substrate. And then, we measured the crystal orientation of the selected BP thin flake as shown in Figure S1. The crystal orientation of BP flake was measured using the anisotropic optical contrast spectra in the visible regime41. Figure S1a shows the schematic diagram of our home-made polarized optical microscope measurement system (POMMS) to facilitate the rapid determination of the orientations of BPs36. Next, other BP flakes were mechanically exfoliated with the same way onto a PDMS supporting substrate. Also, we measured the crystal orientation of the selected BP thin flake as shown in Figure S1. Then, the selected BP thin flake on the PDMS stamp was dry-transferred onto the selected BP thin flake on the SiO2/Si substrate, where their AC directions were perpendicular to each other as demonstrated in Figure S1. Due to van der Waals interactions, orientation-perpendicular BP homojunction can be formed at the overlapped regions. Because both of the bottom and top BPs are uniform and no doped. And the van der Waals junction formed at the overlap was caused by the same p-type BP semiconductor materials. So we can call that BP-homojunction. Finally, 5/50 nm of Ti/Au were patterned using photolithography and magnetron sputtering as metal electrodes. And the direction of the lateral electric field was parallel to the AC direction of the bottom BP and perpendicular to the AC direction of the top BP. Figure 1a depicts the schematic of the device. The bottom and top BPs are respectively marked in pink and blue, whereas the pink and blue dashed arrows indicate the crystal orientations of the bottom and top BPs, respectively. Figure 1b shows the optical microscope image of the junction device. Figure 1c demonstrates the orientations of the bottom and top BPs. The thickness of the bottom and top BPs are almost the same, which were determined based on the atomic force microscopy measurements, as shown in Figure 1d, and read ~5.3 and ~5.4 nm, respectively. Figure 1e presents the Raman spectra of the bottom BP, top BP, and the overlapped region. The observed Raman-active modes of the bottom BP (pink line) and the top BP (blue line) are consistent with previously reported work.14,42 The positions of the peaks (green line) from the overlapped 3



region can be observed are also consistent with the bottom and top BPs, which indicates good film quality in the junction region after exfoliation and dry-transfer. Next, we investigated the electrical characteristics of the device structure. All the electrical measurements were performed at room temperature and under ambient conditions. The schematic of the device is presented in Figure 2a. Figure 2b shows the linear (red) and log (blue) plots of I-V characteristics of the junction at zero back gate voltage, which are similar to those of conventional p-n junction diodes. Figure 2c demonstrates the gate-tunable current-rectifying characteristics of the junction structure. The rectification ratio of ~37 is obtained at Vds = -2/+2 V, as shown in Figure S2. These results can be explained by an energy band model describing the current transport of the junction. Figure 2d presents the band alignment in the AC and ZZ directions at different biases, i.e. the reverse bias, zero bias, and forward bias.36 As we known, black phosphorus is a p-type semiconductor, the holes are dominant. For few-layer BP, the effective masses for holes (majority carriers) in AC and ZZ orientation are different.15 Considering of the same thickness of bottom and top BP and the same charge carrier densities in a same device, the energy band difference between the AC and ZZ orientations (∆E = ∆EAC−∆EZZ) can be estimated by analyzing the difference in effective mass.43 Thermodynamically, ∆E can be obtained using ∆E = kB T ⋅ ln (m*AC/m*ZZ), where kB = 8.6 × 10-5 eV K-1 (Boltzman’s constant), T = 300 K (kelvin temperature) and m* is the effective mass of the holes36. Considering the first principle calculation results and other studies, we define m*AC and *m*ZZ here as 0.14 mo and 0.89 mo,15, 36 respectively. Here, mo is the mass of the electron. As a result, ∆E was calculated to be 0.05 eV.36 So that different effective masses for holes (majority carriers) in AC and ZZ orientation will lead to different band offset between valence band edge and Fermi level of AC and ZZ orientation. As a result, orientation induced barrier forms at the junction after reposition of band energy36. Under reverse bias, the potential barrier blocks the majority carriers (holes) in the ZZ direction of the top BP (blue) being injected to the AC direction of the bottom BP (pink), only some minority carriers (electrons) in the AC direction are extracted to the ZZ direction. When the junction is under zero bias, electron-hole pairs are dissociated at the overlapped region to form a very small current, driven by the built-in electric field. Under forward bias, the majority carriers (holes) in the AC direction overcome the barrier to cross the junction, so the current increases with the increase of forward bias. Although the junction can form a current under reverse bias, the concentrations of the minority carriers are 4


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very low. Therefore, the reverse current is much smaller than the forward current, resulting in the current-rectifying characteristics of the junction. To further investigate the reason of rectification characterizations and the spatial distribution of the photo-induced charge carriers, the scanning photocurrent images (SPI) experiments were performed with a focused 532 nm and ~1.5 µm size laser spot. To get more accurate results, we assure that the size of the overlap is bigger than the light spot (~1.5µm) used in the SPI experiments. Figure 2e shows the results of our SPI experiments of the device under different bias while at zero gate voltage. Apparent photocurrent signals can be observed at the interface between the bottom and top BPs under a reverse bias at Vds = -1 V, which indicates strong electron-hole pairs separation at the junction, as shown in Figure 2e (i). It proved that there was barrier at the interface. Figure 2e (ii) presents the scanning photocurrent image at Vds = 0 V. When the junction is excited by a 532 nm laser, the photogenerated electron-hole pairs will separate into free charges at the overlap and the interface between bottom and top BPs and form a photocurrent driven by the built-in electric field, namely the photovoltaic effect. As demonstrated in Figure 2e (iii), when the device under a forward bias at Vds = 1 V, the strongest photoresponse occur at the overlap and the bottom BP, which means more photons absorption and photo-induced carriers generation at the overlap and the bottom BP. We will discuss this in detail later. Through analyzing the results of the SPI experiments, we can verify that the rectification characteristics were caused by the anisotropic band structures. Figure 3a shows the I-V characterizations of the BP device under illumination by 532 nm laser with various incident powers while at zero gate voltage. The photocurrent increases with the increase of incident light powers. Figure 3b plots the photoresponsivity (R) and external quantum efficiency (EQE) of the device under various incident laser powers while Vds = -1 V. The photoresponsivity R, defined as Iph/Plaser, where Iph is defined as Iillumination-Idark, and Iillumination and Idark are the ID with and without illumination, Plaser is the power of the incident laser. The R ~0.9 mA/W at P = 25 µW. While the external quantum efficiency (EQE) is defined as Iphhc/eλP, where h is the Planck constant, c is the speed of light, e is the elementary charge, λ is the wavelength of incident light. The EQE ~0.21% at P = 25 µW. The polarization-dependent I-V characteristics of the device under illumination by 532 nm laser with incident light power at P = 0.7 mW were shown in Figure 3c. The polarization angle of 0o and 90o corresponds to the AC direction of the bottom and top BPs, respectively. And the polarization-dependent photocurrent at Vds = ±1 V were plotted in Fig5



ure 3d. We can observe that the photocurrent decrease gradually when the incident light polarization angle varies from 0o to 90o. And the photocurrent anisotropy ratios, σ = (Iphmax-Iphmin) / ( Iphmax+Iphmin),

were estimated to be ~0.14 and 0.20 at -1 and +1 V bias voltages, respectively.

To figure out the spatial distribution of the photo-induced charges and the polarized photocurrent generation, we performed the SPI experiments under different incident light polarizations. Figure 4 shows the polarization-dependent SPI results of the device at Vds = -1 V while at zero back gate voltage. The optical microscope image of the BP device was shown in Figure 4a. The pink and blue dotted arrows represent the crystal orientations of the bottom and top BPs, respectively. Figure 4b- h shows the results of SPI under different polarization angles, respectively. The black solid arrows indicate the polarization of the incident light. It should be noted that, the polarization angles of 0o and 90o respectively correspond to the x crystal axis of the bottom and top BPs. We can also observe that the intensities of the photoresponse decrease gradually when the incident light polarization angle varies from 0o to 90o, indicating that the polarized absorption and the resulting linear dichroic photocurrent generation.23 It’s worth to note that the location of the photoresponse is fixed, which is concentrated at the interface between the bottom and top BPs because of the barrier. The similar results can be obtained when the device under zero and forward biases, as shown in Figure S3 and Figure S4. And these results are consistent with the SPI results as we mentioned above in Figure 2e. We can explain these simply by the band diagrams of the junction under illumination, more details in Figure S5. When the incident light polarization is parallel to the x crystal axis of the bottom BP (pink) and is parallel to the direction of the lateral electric field. Then, the photons will be mainly absorbed along the AC direction of the bottom BP rather than along the ZZ direction of the top BP (blue), and thus, producing more electron-hole pairs. These photo-induced carriers are separated and recombine at the interface (Figure 4), overlap area and the interface (Figure S3), transmitting along the direction of the lateral electric field to form a remarkable photocurrent. While, when the incident light polarization is parallel the x crystal axis of the top BP (blue) and is perpendicular to the lateral electric field. Although the photons are mainly absorbed and induce the electron-hole pairs along the AC direction of the top BP rather than along the ZZ direction of the bottom BP (pink), these photogenerated charges are separated and recombine at the interface and the overlap, transmitting perpendicular to the direction of the lateral electric field. As a consequence, when the incident light polarization angle varies from 0o to 90o, the 6


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photoresponse decrease gradually no matter what the bias is. Namely, the bottom BP is the major factor for polarization-dependent photoresponse, because its AC orientation is parallel to the direction of the lateral electric field. And that is why the strongest photoresponse signals are concentrated at the overlap and the bottom BP at Vds = 1 V, as shown in Figure S4. As we discussed above, the AC direction of the bottom BP is parallel to the lateral electric field, so that the bottom BP is dominant for polarization-dependent photoresponse. To further verify these explanations, another orientation-perpendicular BP device was fabricated with the similar thickness. The difference is that the direction of the lateral electric field was parallel to the AC direction of the top BP and perpendicular to the AC direction of the bottom BP. And the results of the second orientation-perpendicular BP device were demonstrated in Figure 5. Figure 5a shows the schematic of the device. The blue and pink dashed arrows respectively indicate the crystal orientations of the bottom and top BPs, where their AC directions are perpendicular to each other. The optical microscope image of the junction was presented in Figure 5b. As demonstrated in Figure 5a, the bottom BP and its orientation direction are marked as blue, and the top BP and its orientation direction are marked as pink. Figure 5c exhibits the orientations characterizations of the bottom and top BPs. The gate tunable I-V characteristics and the rectification ratio at different gate voltage were plotted in Figure 5d. The rectification ratio ~22 can be obtained at zero gate voltage. Figure 5e shows the I-V characterizations of the BP device under illumination by 532 nm laser with various incident powers at zero gate voltage. The photocurrent increases with the increase of incident light powers, as same as shown in Figure 3a. The polarization-dependent SPI results of the device at Vds = -1 V and Vds = 1 V were demonstrated in Figure 5f and Figure 5g, respectively. The polarization angle of 0o and 90o respectively correspond to the x crystal axis of the top and bottom BPs. Similar results can be observed, the strongest photocurrent signals were concentrated at the interface between the bottom and top BPs when the device under reverse bias, while the location of the strongest photoresponse changed when the device under forward bias, i. e. at the overlap and the top BP. And the photoresponse decrease gradually with the polarization angle varying from 0o to 90o. In summary, the polarization-dependent photoresponse mainly depend on the directions of the electric field, the AC orientations of the BPs, the related linear dichroism and the carriers transport engineering between the anisotropic band structures, and have nothing with the order of the two orientation-perpendicular BPs. 7



To further analyze the effect of the barrier that caused by the anisotropic band structure, we have designed another type of orientation-induced BP homojunction with the similar thickness, where the crystal orientation of the bottom BP is parallel to that of the top BP. Moreover, the direction of the lateral electric field is parallel to their AC directions. Figure 6a shows the schematic illustration of the BP device. The pink and blue dashed arrows respectively indicate the crystal orientations of the bottom and top BPs. Figure 6b exhibits the optical microscope image of the device. As demonstrated in Figure 6a, the bottom BP and its orientation direction are marked as pink, and the top BP and its orientation direction are marked as blue. The orientations characterizations of the bottom and top BPs were shown in Figure 6c. Figure 6d plots the gate tunable I-V characteristics. The insignificant current-rectifying characteristics are due to the absence of significant potential barrier at the interface between the bottom and top BPs. The band alignment in the AC directions of the bottom and top BPs reveals a negligible orientation-induced barrier without any evidence for the band offset according to the band theory as we analyzed above. Figure 6e shows the I-V characterizations of the BP device under illumination by 532 nm laser with various incident powers at zero gate voltage. Similar results can be observed, the photocurrent increases with the increase of incident light powers. Figure S6 shows the polarization-dependent photocurrent of the orientation-parallel BP device. And the photocurrent anisotropy ratios (σ), was estimated to be ~0.19 and 0.20 at -1 and +1 V bias voltage, respectively. The polarization-dependent SPI of the device at Vds = 1 V were demonstrated in Figure 6f. The black solid arrows indicate the polarization of the incident light. The polarization angles of 0o and 90o respectively correspond to the x crystal axis and the y crystal axis of the bottom BP. Figure S7 and S8 shows the SPI results of the device under zero and reverse bias, respectively. As we expected, the photoresponse decrease gradually with the polarization angle varying from 0o to 90o. Considering of the orientations of the two BP sample are the same, so we can explain these results by treating the two samples as one single BP photodetector. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material.23 It is worth to notice that no matter what the bias is, the strong photoresponse were concentrated at the interface between the two BPs, which is may attributed to the non-uniform contact at the interface. Meanwhile, another orientation-parallel BP device was also fabricated, where the crystal orientation of the bottom BP is parallel to that of the top BP. The difference is that the direction of the lateral electric 8


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field is parallel to their ZZ directions. Similar results can be obtained in Figure S9.

CONCLUSION In summary, we developed a new van der Waals device of orientation-perpendicular BP homojunction based on the in-plane anisotropic band structures. These homostructures exhibit good gate-tunable current-rectifying characteristics due to the band offsets and band barriers between the armchair and zigzag directions of the junctions, which provide a novel way to design 2D p-n junctions with one kind of layered material. Meanwhile, the localized photoresponse and the spatial distribution of the photogenerated carriers are investigated using the scanning photocurrent images experiment. Through analyzing the SPI results, not only the barrier theory have been verified, but also demonstrate the reason of the polarization-dependent photoresponse, which is due to the intrinsic linear dichroism and the carriers transport engineering between the anisotropic band structures. The orientation-dependent van der Waals junctions might prove a new way for electronic and optoelectronic devices applications based on other 2D anisotropic layered materials (such as ReS2, ReSe2) with the similar unique characteristics, while, the related experiments based on other 2D anisotropic layered materials are still need to be further investigated. METHODS Device Fabrication. First, black phosphorus thin flakes were mechanically exfoliated using adhesive tape (3M Scotch) from bulk BP crystals (XFNANO, Inc) on a clean SiO2/ Si substrate. The Si substrate is p-doped and the thickness of the SiO2 is ~285 nm. As shown in Figure S1, the crystal orientation of the selected BP thin flake was measured, and other BP flakes were mechanically exfoliated following the same way onto a PDMS supporting substrate. In addition, the crystal orientation of the selected BP thin flake was shown in Figure S1. The selected BP thin flake on the PDMS stamp was dry-transferred onto the selected BP thin flake on the SiO2/Si substrate by a micromanipulator using an optical microscope, with a tunable angle between their crystal orientations. Afterwards, 5/50 nm of Ti/Au were patterned through the photolithography with a femtosecond laser (800 nm, 35 fs) direct writing technology and magnetron sputtering (JZCK-465 of Sky Technology Development) as metal electrodes. Finally, vacuum thermal annealing treatment (330 °C for 1 h) was performed to achieve a good contact between the top and bottom BPs, as well as between the BP samples and the electrodes. 9



Electrical Measurements. All the electrical and photoresponse measurements were performed under the ambient conditions and at room temperature. The electrical characteristics were measured using Keithley 4200A semiconductor parameter analyzer, and the photoresponse measurements were performed using Keithley 2400, 2450 digital source-meters.

ASSOCIATED CONTENT Supporting Information Details of the crystal orientation determination of the orientation-perpendicular BP device (S1), I-V characterizations of the orientation-perpendicular BP device (S2), polarization-dependent scanning photocurrent images of the orientation-perpendicular BP device under zero bias and forward bias, respectively (S3) and (S4), schematic and band diagrams of the device under illumination (S5), polarization-dependent photocurrent of the orientation-parallel BP device (S6), polarization-dependent scanning photocurrent images of the orientation-parallel BP device under zero bias and reverse bias, respectively (S7) and (S8), schematic diagrams and characterizations of another orientation-parallel BP device (S9).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was mainly supported by the Natural Science Foundation of China (Grant 11374164), the National Science Foundation of Tianjin (Grant 18JCZDJC30400), and the National Key Research and Development Program of China (Grant 2016YFA0200200, 2016YFA0301102).

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(28) Deng, Y.; Luo, Z.; Conrad, N. J.; Liu, H.; Gong, Y.; Najmaei, S.; Ajayan, P. M.; Lou, J.; Xu, X.; Ye, P. D. Black Phosphorus Monolayer MoS2 van der Waals Heterojunction p-n Diode. ACS Nano. 2014, 8, 8292-8299. (29) Ye, L.; Li, H.; Chen, Z.; Xu, J. Near-Infrared Photodetector Based on MoS2/Black Phosphorus Heterojunction. ACS Photonics 2016, 3, 692-699. (30) Hong, T.; Chamlagain, B.; Wang, T.; Chuang, H. J.; Zhou, Z.; Xu, Y. Q. Anisotropic Photocurrent Response at Black Phosphorus-MoS2 p-n Heterojunctions. Nanoscale 2015, 7, 18537-18541. (31) Ye, L.; Wang, P.; Luo, W. J.; Gong, F.; Liao, L.; Liu, T.; Tong, L.; Zang, J.; Xu, J.; Hu, W. Highly Polarization Sensitive Infrared Photodetector Based on Black Phosphorus-on-WSe2 Photogate Vertical Heterostructure. Nano Energy 2017, 37, 53-60. (32) Geim, A. K.; Grigorieva, I. V. Van der Waals Heterostructures. Nature 2013, 499, 419-425. (33) Li, M. Y.; Chen, C. H.; Shi, Y.; Li, L. J. Heterostructures Based on Two-dimensional Layered Materials and Their Potential Applications. Mater. Today 2016, 19, 322-335. (34) Lee, G. H.; Cui, X.; Kim, P. Atomically Thin p–n Junctions with van der Waals Heterointerfaces. Nat. Nanotechnol. 2014, 9, 676-681. (35) Britnell, L.; Ribeiro, R. M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311-1314. (36) Xin, W.; Li, X. K.; He, X. L.; Su, B. W.; Jiang, X. Q.; Huang, K. X.; Zhou, X. F.; Liu, Z. B.; Tian, J. G. Black-Phosphorus-Based Orientation-Induced Diodes. Adv. Mater 2018, 30, 1704653. (37) Jin, Y.; Keum, D. H.; An, S. J.; Kim, J.; Lee, H. S.; Lee, Y. H. A Van Der Waals Homojunction: Ideal p-n Diode Behavior in MoSe2. Adv Mater 2015, 27, 5534-5540. (38) Min, S. C.; Qu, D.; Lee, D.; Liu, X.; Watanabe, K.; Taniguchi, T.; Yoo, W. J. Lateral MoS2 p-n Junction Formed by Chemical Doping for Use in High-Performance Optoelectronics. ACS Nano. 2014, 8, 9332-9340. (39) Li, H. M.; Lee, D.; Qu, D.; Liu, X.; Ryu, J.; Seabaugh, A.; Yoo, W. J. Ultimate Thin Vertical p–n Junction Composed of Two-Dimensional Layered Molybdenum Disulfide. Nat. Commun. 2015, 6, 6564. (40) Yu, X.; Zhang, S.; Zeng, H.; Wang, Q. J. Lateral Black Phosphorene P–N junctions Formed 13



via Chemical Doping for High Performance Near-Infrared Photodetector. Nano Energy 2016, 25, 34-41. (41) Mao, N.; Tang, J.; Xie, L.; Wu, J.; Han, B.; Lin, J.; Deng, S.; Ji, W.; Xu, H.; Liu, K.; Tong, L.; Zhang, J. Optical Anisotropy of Black Phosphorus in the Visible Regime. J. Am. Chem. Soc. 2016, 138, 300-305. (42) Akahama, Y.; Kobayashi, M.; Kawamura, H. Raman Study of Black Phosphorus up to 13 GPa. Solid State Commun. 1997, 104, 311-315. (43) Wang, F.; Wang, Z.; Xu, K.; Wang, F.; Wang, Q.; Huang, Y.; Yin, L.; He, J. Tunable GaTe-MoS2 van der Waals p-n junctions with Novel Optoelectronic Performance. Nano Letters 2015, 15, 7558-7566.

Figures

Figure 1. Schematic illustration and characterizations of the orientation-perpendicular BP device. (a) Schematic of the device. The pink and blue dashed arrows respectively indicate the crystal orientations of the bottom and top BPs, where their AC directions are perpendicular to each other. The direction of the electric field is parallel to the AC direction of the bottom BP and perpendicular to the AC direction of the top BP. (b) Optical microscope image of the junction. As demonstrated in Figure 1a, the bottom BP and its orientation direction are marked as pink, and the top BP and its orientation direction are marked as blue. Scale bar, 5 µm. (c) The orientations characterizations of the bottom and top BPs. (d) The thickness characterizations of the bottom and top BPs determined by AFM. Scale bar, 5 µm. (e) Raman spectra of the bottom BP, top BP, and the overlapped region.

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Figure 2. I-V characterizations, band diagrams and scanning photocurrent images of the device. (a) Schematic of the orientation-perpendicular BP device. (b) The linear (red) and log (blue) plots of I-V characteristics of the junction device at zero back gate voltage. (c) The gate tunable I-V characteristics. (d) Band alignments of the structure at zero gate voltage. Under reverse bias (i), zero bias (ii) and forward bias (iii). (e) The corresponding scanning photocurrent images under different bias at zero gate voltage as shown in Figure 2d. At Vds = -1 V (i), Vds = 0 V (ii) and Vds = 1 V (iii).

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Figure 3. (a) I-V characterizations of the BP device under illumination by 532 nm laser with various incident powers at zero gate voltage. (b) The photoresponsivity (R) and external quantum efficiency (EQE) of the device under various incident laser powers while Vds = -1 V. The R ~0.9 mA/W at P = 25 µW. (c) The polarization-dependent I-V characteristics under illumination by 532 nm laser with incident light power at P = 0.7 mW. (d) The polarization-dependent photocurrent at Vds = ±1 V, respectively.

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Figure 4. Polarization-dependent scanning photocurrent images of the device under reverse bias at Vds = -1 V. (a) The optical microscope image of the BP device. The pink and blue dotted arrows represent the crystal orientations of the bottom and top BPs, respectively. Scale bar, 5 µm. (b) - (h) SPI under different polarization angles. The black solid arrows indicate that the polarization of the incident light. The polarization angle of 0o and 90o respectively correspond to the x crystal axis of the bottom and top BPs.

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Figure 5. Schematic diagram, characterizations, and photoelectric properties of another orientation-perpendicular BP device. (a) Schematic of the device. The blue and pink dashed arrows respectively indicate the crystal orientations of the bottom and top BPs, where their AC directions are perpendicular to each other. The direction of the electric field is parallel to the AC direction of the top BP and perpendicular to the AC direction of the bottom BP. (b) Optical microscope image of the junction. As demonstrated in Figure 5a, the bottom BP and its orientation direction are marked as blue, and the top BP and its orientation direction are marked as pink. Scale bar, 5 µm. (c) The orientations characterizations of the bottom and top BPs. (d) The gate tunable I-V characteristics. Inset: The rectification ratio at different gate voltages. The rectification ratio ~22 at zero gate. (e) I-V characterizations of the BP device under illumination by 532 nm laser with various incident powers at zero gate voltage. (f) and (g) Polarization-dependent scanning photocurrent images of the device at Vds = -1 V and Vds = 1 V, respectively. The polarization angle of 0o and 90o respectively correspond to the x crystal axis of the top and bottom BPs. The black solid arrows indicate that the polarization of the incident light.

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Figure 6. Schematic diagram, characterizations, and photoelectric properties of the orientation-parallel BP device. (a) Schematic of the device. The pink and blue dashed arrows respectively indicate the crystal orientations of the bottom and top BPs, where their AC directions are parallel to each other. And the direction of the electric field is parallel to their AC direction. (b) Optical microscope image of the junction. As demonstrated in Figure 6a, the bottom BP and its orientation direction are marked as pink, and the top BP and its orientation direction are marked as blue. Scale bar, 5 µm. (c) The orientations characterizations of the bottom and top BPs. (d) The gate tunable I-V characteristics. (e) I-V characterizations of the BP device under illumination by 532 nm laser with various incident powers at zero gate voltage. (f) Polarization-dependent scanning photocurrent images of the device at Vds = 1 V. The black solid arrows indicate the polarization of the incident light. The polarization angles of 0o and 90o respectively correspond to the x crystal axis and the y crystal axis of the bottom BP. Scale bar, 5µm.

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Carrier Engineering in Polarization-Sensitive Black Phosphorus van

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