Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity

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Surfaces, Interfaces, and Applications

Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity Metal Contacts Ghulam Dastgeer, Muhammad Farooq Khan, Janghwan Cha, Amir Muhammad Afzal, Keun Hong Min, Byung Min Ko, Hailiang Liu, Suklyun Hong, and Jonghwa Eom ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20231 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Materials & Interfaces

Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity Metal Contacts Ghulam Dastgeer1, Muhammad Farooq Khan1, Janghwan Cha1, Amir Muhammad Afzal1, Keun Hong Min1, Byung Min Ko1, Hailiang Liu2, Suklyun Hong1, and Jonghwa Eom1*

1Department

of Physics & Astronomy and Graphene Research Institute-Texas

Photonics Center International Research Center (GRI–TPC IRC), Sejong University, Seoul 05006, Korea

2Department

of Electronics and Electrical Engineering, Dankook University, Yongin, 16890, Korea

*E-mail: [email protected]

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract There have been few studies of heterojunctions composed of two-dimensional transition metal dichalcogenides (TMDs) and an oxide layer, but such studies of high-performance electric and optoelectronic devices are essential. Such heterojunctions with low-resistivity metal contacts are needed by the electronics industry to fabricate efficient diodes and photovoltaic devices. Here, a van der Waals heterojunction composed of a p-type black phosphorus (p-BP) and ntype indium–gallium–zinc–oxide (n-IGZO) film with low-resistivity metal contacts is reported, and it demonstrates high rectification. The low off-state leakage current in thick IGZO film accounts for the high rectification ratio in a sharp interface of p-BP/n-IGZO devices. For electrostatic gate control, an ionic liquid is introduced to achieve a high rectification ratio of 9.1 × 104. The photovoltaic measurements of p-BP/n-IGZO show fast rise and decay times, significant open-circuit voltage and short-circuit current, and a high photoresponsivity of 418 mA/W with a substantial external quantum efficiency of 12.1%. The electric and optoelectronic characteristics of TMDs/oxide layer van der Waals heterojunctions are attractive for industrial applications in the near future.

KEYWORDS: Black phosphorus; IGZO; van der Waals p–n diode; Ionic-liquid gatemodulated rectification; Photovoltaic measurements, Fast photoresponse

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INTRODUCTION Beyond graphene1, numerous two-dimensional materials, such as transition metal dichalcogenides (TMDs), have been introduced for the development of efficient electronic and optoelectronic properties. These TMDs are utilized to fabricate field effect transistors (FETs), flexible diodes, photodetectors, and solar cells for effective operations because of their bandgap. Recently, the study of TMD-based heterostructures has been initiated by researchers to fabricate diverse electronic and optoelectronic devices1-4. The heterostructures composed of monolayer or few-layer stacking of TMD materials are known as “van der Waals heterostructures,” which are considered more flexible and efficient in performance than conventional semiconducting devices due to sharp interfaces5. The previously reported heterostructures composed of monolayer and few-layer TMDs, such as p-MoS2/n-MoS26, MoSe2/WSe27, black phosphorus (BP)/WS28, and MoS2/BP9 showed a large diode rectification ratio of as much as 105. Their basic claim for a high rectification ratio was the sharp interface formed between two TMDs, but the Schottky barriers by the metal contacts were neglected in previous reports. So far, TMD-based van der Waals heterojunctions have shown high rectification largely due to the Schottky barrier at the metal contacts. Ohmic contacts are required to explore the intrinsic transport in the TMD channel materials, especially for diode characteristics. The rectification mainly originating from the sharp interface between the TMDs is not evident yet8, 10. Metal–oxide semiconductors are widely used to fabricate metal–oxide–semiconductor field effect transistors11 (FETs), solar cells, and photodiodes12. For example, indium–gallium– zinc–oxide (IGZO) has been adopted as a channel material to obtain a high speed of 200 MHz for backplane driver transistors in flat-panel displays13-14. Recently, IGZO has drawn more attention from researchers seeking materials for high electron mobility with more flexibility 3



and surface stability14. One of the advantages of IGZO is its low power consumption, because IGZO FETs retain a very low off-state current, which improves the data retention of pixels11. In this research, p-BP/n-IGZO van der Waals heterojunctions were fabricated on a SiO2/Si substrate to study the electric and optoelectronic properties. Most BP sheets exhibit high mobility up to 10,000 cm2/Vs with a high ON/OFF current ratio, whereas they are p-type in the natural state. Therefore, additional doping is not necessary if one uses BP as a p-type semiconductor. It is useful if additional doping is avoidable, because the doping techniques used for TMDs, such as chemical doping15-16 and electrostatic doping17, are effective only for a short time or cause degradation of mobility8, 18. In this study, however, a drop of the dielectric ionic

liquid

DEME-TFSI

IL

(diethylmethyl(2-methoxyethyl)ammonium

bis(trifluoromethylsulfonyl)imide ionic liquid)19-20 was sprayed with a micro pipette over the p-BP/n-IGZO heterojunction for electrostatic gating in devices. Using the ionic liquid gating technique, a high rectification ratio of 9.1 × 104 was obtained. This rectification is truly to the result of the interface at p-BP/n-IGZO, because ohmic and low-resistivity metal contacts were used in the devices; these have seldom been discussed in previous research. In addition, the pBP/n-IGZO p–n diode showed a fast photoresponse, high responsivity, and external quantum efficiency (EQE). The gating-controlled rectification and photovoltaic effects in these devices can contribute to IGZO industrial applications in the near future.

EXPERIMENTAL SECTION Fabrication of BP-IGZO van der Waals diode At the first stage of the fabrication, IGZO (In–Ga–Zn = 1:1:1, Lesker) was sputtered on SiO2/Si substrates with 3% oxygen at 0.5 Pa in ambient argon and then annealed at 400°C for 1 h to improve the crystal structure and stability of the IGZO film14 (Figure S1 in the Supporting 4


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Information). For ohmic contacts21, a trilayer chemical vapor deposition (CVD) graphene film was transferred to make a tunneling-layer between aluminum (Al) contacts and IGZO film (Figure S2a in the Supporting Information). The 50-nm-thick Al film was deposited, and the remaining graphene over the IGZO channel was etched using O2 plasma. In the second stage, a few-layer (5-nm-thick) p-BP sheet was transferred over the n-IGZO by a dry transfer technique, and Au/Ti (50 nm/1 nm) contacts were deposited over p-BP for their ohmic contribution22-23. For gate dielectric and oxidation prevention, the ionic liquid DEME-TFSI IL19-20 was dropped carefully over the final device under a microscope by using a micropipette. The schematic and optical images of the final device are shown in Figures 1a and b. The material characterizations were performed by Raman spectroscopy and X-ray diffraction (XRD) of p-BP and n-IGZO, respectively. In the Raman spectrum of p-BP, the peak 𝐴1𝑔 was observed at 361.7 cm-1, showing the out-of-plane vibration mode, whereas the peaks 𝐵2𝑔, 𝐴2𝑔 were observed at 438.4 and 466.4 cm–1, respectively, showing the in-plane vibration modes8, 24-25, as shown in Figure 2a. These modes of vibration indicate the high quality of the p-BP flakes without any oxidation26-29. The structural characterizations of the n-IGZO film after annealing at different temperatures were examined by XRD, as shown in Figure 2b. The scanning electron microscopy (SEM) images are presented in Figure S1 in the Supporting Information. Figures 2c and d show atomic force microscopy (AFM) images with a height profile of the heterostructure consisting of p-BP/n-IGZO.

RESULTS AND DISCUSSIONS Thickness-dependent I–V measurements of p-BP/n-IGZO

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A gate voltage (Vilg) is applied by an indium wire dipped into the ionic-liquid droplet to modulate the carrier densities of n-IGZO and p-BP. The carrier density of n-IGZO is calculated by relation n = q ―1𝐶𝑖𝑙𝑔|𝑉𝑡ℎ – 𝑉𝑖𝑙𝑔|, where q is electron charge, Cilg is gate capacitance, and Vth is the threshold voltage5, 8, 30. At Vilg = 0 V, a carrier density n = 7.2 × 1012/cm2 for n-IGZO was extracted, showing better performance with a good interface (Figure S2b in the Supporting Information). The Ids–Vds characteristics of p-BP/n-IGZO diodes with various IGZO film thicknesses of 10 nm, 15 nm and 20 nm at Vilg = 0 were examined. Figure 3a demonstrates the linear Ids– Vds curves of n-IGZO/p-BP with various thicknesses of n-IGZO film. The logarithmic plot of the Ids–Vds curves in Figure 3b discloses that a diode with 10-nm-thick IGZO film has a large forward current as well as large reverse current compared with diodes composed of 15- and 20-nm-thick IGZO film14. When the reverse-biased voltage is increased in devices composed of thin IGZO film of 10 nm, the difference between the Fermi levels of n-IGZO and p-BP increases significantly, making a thin potential barrier. The tunneling of the charge carriers through this barrier increases, causing an increase in corresponding reverse leakage current at high reverse biasing voltage31. A similar mechanism has been explained recently in n-ZnO/pMoS2 diodes31. The poor rectification of p-BP/n-IGZO devices is also verified by the large value of the ideality factor (ɳ). The ideality factor varies from 3.7 to 2.5 at zero gate voltage, as the thickness is increased from 10 to 20 nm. In the case of an ideal diode, the ideality factor must be closer to unity (ɳ = 1). Increasing the thickness of the IGZO film causes an increment of film resistance that limits the forward current, as well as the reverse current, and a large rectification of 4.3 × 104 is achieved at Vilg = 0 V with 20-nm-thick IGZO film under p-BP, as shown in Figure 3c. The rectification ratio was obtained by comparing the forward and reverse current at Vds = ±3 V. By increasing the thickness of the IGZO film, the current density drops, 6


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and corresponding resistance of the film rises. The sharp interface and large series resistance of the devices from source contact to drain contact are responsible for improving the rectification ratio in thick IGZO devices. The high rectification in thin TMD-based diodes can be achieved using dissimilar metal contacts having different work functions, as reported previously10, 14.

Gate-dependent I–V curves with high rectification The diode performance of p-BP/n-IGZO was investigated under the influence of ionic liquid gating, as shown in the schematic diagram in Figure 1a. The advantage of an ionic liquid dielectric over a SiO2 dielectric is that it operates more efficiently, with a 10-times-lower gate voltage. The rectifying output Ids–Vds curves show that the large forward current can be attributed to the increasing interlayer recombination, which can be modulated by electrostatic gating. The gating controls the carrier densities, which limits the forward as well as reverse leakage current through the diodes. The carrier densities of p-BP and n-IGZO were effectively modulated by gating. When the gate voltage was modulated from zero to +2 V, the forward current increased because of the transport of electrons from n-IGZO to p-BP, but, at the same time, the leakage current upsurged, yielding to a poor rectification ratio8, 32. The current through the p-BP/n-IGZO interface can be extracted from the following relation32.

[ ( ) ― 1]

𝐼 = 𝐼𝑜 exp

𝑞𝑉 ɳ𝑘𝑇

(1) Here, Io is the reverse saturation current, k = 1.3807 × 10-23 J/K is the Boltzmann constant, 𝑞

𝑑𝑉

ɳ = (k𝑇)(𝑑ln𝐼)

is the ideality factor, T is temperature, and q = 1.602 × 10-19 C is the electron

charge. The ideality factor is extracted from Figure 4a after fitting the Ids–Vds curves in a small 7



forward-bias regime8, 33 (Figures S3a-c in the Supporting Information). The diode behaves like an ideal diode at negative gate voltage with an ideality factor of 1.3, showing much better performance than the previously reported TMD-based heterostructures diodes8, 12, 34-35. The ideality factor tends to rise (ƞ = 2.5) as Vilg is increased from -2 to +2 V. The relative rise in ideality factor is attributed to carrier recombination at the interface because of the increase in the electric field12, 31. As demonstrated in Figure 4b, a large rectification of 9.1 × 104 is achieved at Vilg = -2 V. The reason for the large rectification is that the leakage current through n-IGZO is small at a negative gate voltage14. The high rectification ratio is also attributed to the sharp interface among the p-BP and n-IGZO. Moreover, the diode performance may improve by controlling the charge impurities on the Si/SiO2 substrate, reducing the water and polymer residues at the p-BP/n-IGZO interfaces36-38. Furthermore, the real application of p-BP/n-IGZO diode as a rectifier was examined by applying a sinusoidal AC voltage with an amplitude Vin = ±2 V of 1-kHz frequency as an input signal to the p-BP/n-IGZO diode using a 1-MΩ external resistor, as shown in the inset in Figure 4c. The one-half cycle of the input signal was rectified and converted into a DC output signal (Vout), while the other half cycle diminished, as shown in Figure 4c. To understand the operational mechanism and electrical transportation through the pBP/n-IGZO in detail, an energy band diagram at zero gate voltage is shown in Figure 5. The electron affinity of n-IGZO was qχ = 4.2 eV, and the energy gap Eg = Ec - Ev = 3.1 eV 14, 39-40, whereas the electron affinity of p-BP was extracted as qχ = 4.7 eV with an energy gap of Eg = 0.3 eV41. The Fermi level of n-IGZO was near the conduction band maximum (CBM), so it behaved as an n-type semiconductor (see the transfer curve of n-IGZO in Figure S2b), whereas the Fermi level of p-BP was closer to its valence band maximum (VBM), and it acted as a pure 8


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p-type semiconductor. This band alignment was modulated by Vilg, which controls the carrier densities of holes in p-BP and electrons in the n-IGZO film (see the carrier density calculations in the Supporting Information). The electrostatic tuneability of the Fermi level caused the variation in rectification ratio. At a negative gate voltage, the Fermi level of n-IGZO shifted up, and the reverse bias current decreased to a small value because of a large potential barrier at the interface of p-BP/n-IGZO, which yielded to a high rectification ratio.

Photovoltaic electric measurements Photovoltaic measurements were performed using a p-BP/n-IGZO heterojunction under illumination having a wavelength (λ) of 420 nm at zero gate voltage. The output curves of the p-BP/n-IGZO heterojunction are presented in Figure 6a under different illumination powers (P) of the source. As the illumination power increased, both forward and reverse bias currents drastically increased, which is consistent with a mechanism of photocurrent generation31, 42. The electron–hole pair separation was caused by the built-in potential. The highest value of the open-circuit voltage (Voc) and short-circuit current (Isc) was obtained with the maximum power intensity of 500 mW/cm2 in the experiment. Although a significant variation in the short-circuit current Isc was observed, there was not much variation in the opencircuit voltage Voc for p-BP/n-IGZO diodes for different illumination powers, as shown in Figure 6b. The low variation of Voc is attributed to the small bandgap (0.3 eV) of p-BP22-23. When the illumination power was increased, Isc increased significantly, which was caused by the photo-induced electron–hole pair generation8, 43. Higher illumination power caused more absorption and photo-generation of the carriers in the p-BP/n-IGZO interface, and it increased the Isc as well as Voc23, 31. 9



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To characterize the sensitivity of the p-BP/n-IGZO, the photoresponse of p-BP/n-IGZO was examined under light with a wavelength (λ) of 420 nm with a power intensity of 500 𝑡

mW/cm2.

Fitting the following

𝐼𝑝ℎ = 𝐼𝑑𝑎𝑟𝑘 + 𝐵𝑒

(―

equations44-46

𝐼𝑝ℎ = 𝐼𝑑𝑎𝑟𝑘 + 𝐴𝑒

(𝜆 ) 1

for the light-on state and

𝑡

𝜆2)

for the light-off state in Figure 6c, it was possible to extract the rise time

(𝜆1) and decay time (𝜆2), respectively (Figures S4a and b in the Supporting Information). The p-BP/n-IGZO diode showed two-times-better photoresponse than p-MoS2/n-ZnO31 and pBP/n-WS28 with a fast rise time of 49 ms and decay time of 180 ms. The photoresponsivity (R) ℎ𝑐

and EQE31 were estimated by the relation R = Iph/PA43 and 𝐸𝑄𝐸 = 𝑅𝑒𝜆. Here Iph is the photocurrent, the difference of Ilihgt and Idark, and plotted as a function of time in Figure 6c, P is the power intensity of the incident light, and A is the active area of the device. The estimated value of R is 418 mA/W, which is three times larger than that for WSe2/SnS243 (108.7 mA/W) and 40 times larger than for MoS2/WSe247 (11 mA/W). A noteworthy EQE of 12.1% was estimated for the p-BP/n-IGZO diode with incident power of 500 mW/cm2, which is 10 times higher than that for MoS2/WSe247 (1.5%) and 40 times higher than for BP/MoS248 (0.3%). The high responsivity and EQE are attributed to low-resistivity metal contacts and a large built-in potential8,

23, 31, 43.

The p-BP/n-IGZO diode demonstrated great potential for rectifying

characteristics, along with a good photovoltaic effect.

CONCLUSIONS In summary, a van der Waals heterojunction consisting of a p-type BP and an n-type IGZO was fabricated, where ion-gel was used as the gating dielectric. It was demonstrated that the IGZO film thickness affects the rectifying characteristics of the p-BP/n-IGZO 10


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heterostructure. A large rectification was obtained with 20-nm-thick IGZO film. The electrostatic gate control of the p-BP/n-IGZO device resulted in a high rectification of 9.1 × 104 with ohmic metal contacts. Because of the ohmic contacts, this high rectification can be truly attributed to the sharp interface of p-BP/n-IGZO. In photovoltaic measurements, a large photocurrent was observed, with an illumination power intensity of 500 mW/cm2. A fast photoresponse with a rise time of 49 ms and decay time of 180 ms was achieved. Large values of short-circuit current and open-circuit voltage were measured in the p-BP/n-IGZO diode. Comparatively, a high photoresponsivity of 418 mA/W was obtained with a noteworthy EQE. The combination between TMDs and oxide materials shows great potential for energy harvesting and optoelectronic devices in the future.

Acknowledgments This work was supported by the Priority Research Center Program (2010-0020207), the Basic Science Research Program (2016R1D1A1A09917762), and the Global Research and Development Center Program (2018K1A4A3A01064272) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education and the Ministry of Science and ICT. GD thanks Ghazanfar Nazir and Sikandar Aftab for the discussion on the device fabrication technique.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

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the direct growth of an n-IGZO film over Si/SiO2 substrates with SEM and EDX analysis, schematic diagram along with electrical characteristics of n-IGZO devices after patterning low-resistivity metal contacts, the output Ids–Vds curves of p-BP/n-IGZO heterojunction p–n diode at different gate voltages, density of state of p-BP/n-IGZO heterojunction under different external electric fields, and photoresponse of p-BP/n-IGZO van der Waals heterojunction.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jonghwa Eom: 0000-0003-4031-4744

Notes The authors declare no competing financial interest.

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(22) Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Van Der Zant, H. S. J.; Castellanos-Gomez, A. Fast and Broadband Photoresponse of Few-layer Black Phosphorus Field-effect Transistors. Nano Letters 2014, 14 (6), 3347-3352. (23) Buscema, M.; Groenendijk, D. J.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Photovoltaic Effect in Few-layer Black Phosphorus PN Junctions Defined by Local Electrostatic Gating. Nature Communications 2014, 5, 4651. (24) Fan, S.; Shen, W.; Liu, J.; Hei, H.; Hu, R.; Hu, C.; Zhang, D.; Hu, X.; Sun, D.; Chen, J.-H.; Ji, W.; Liu, J. Solution-Based Property Tuning of Black Phosphorus. ACS Applied Materials & Interfaces 2018, 10 (46), 39890-39897. (25) Song, M.-K.; Namgung, S. D.; Sung, T.; Cho, A.-J.; Lee, J.; Ju, M.; Nam, K. T.; Lee, Y.-S.; Kwon, J.-Y. Physically Transient Field-Effect Transistors Based on Black Phosphorus. ACS Applied Materials & Interfaces 2018, 10 (49), 42630-42636. (26) Sun, X.; Wang, Z. Sodium Adsorption and Diffusion on Monolayer Black Phosphorus with Intrinsic

Defects.

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https://doi.org/10.1016/j.apsusc.2017.08.199. (27) Yan, J.; Ji, Y.; Kong, L.; Li, Y.; Navlani-García, M.; Liu, S.; Kuwahara, Y.; Mori, K.; Che, M.; Yamashita, H. Black Phosphorus-Based Compound with Few Layers for Photocatalytic Water Oxidation. ChemCatChem 2018, 10 (16), 3424-3428. (28) Xue, Y.; Zhang, Q.; Zhang, T.; Fu, L. Black Phosphorus: Properties, Synthesis, and Applications in Energy Conversion and Storage. ChemNanoMat 2017, 3 (6), 352-361, DOI: doi:10.1002/cnma.201700030.

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(29) Tsai, H.-S.; Liang, J.-H. Production and Potential Applications of Elemental TwoDimensional Materials beyond Graphene. ChemNanoMat 2017, 3 (9), 604-613. (30) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. science 2004, 306 (5696), 666-669. (31) Xue, F.; Chen, L.; Chen, J.; Liu, J.; Wang, L.; Chen, M.; Pang, Y.; Yang, X.; Gao, G.; Zhai, J. p-Type MoS2 and n-Type ZnO Diode and Its Performance Enhancement by the Piezophototronic Effect. Advanced Materials 2016, 28 (17), 3391-3398. (32) Sze, S. M.; Ng, K. K. Physics of semiconductor devices, John wiley & sons: 2006. (33) Schroder, D. K. Semiconductor Material and Device Characterization, John Wiley & Sons: 2006. (34) Jeon, P. J.; Lee, Y. T.; Lim, J. Y.; Kim, J. S.; Hwang, D. K.; Im, S. Black Phosphorus–Zinc Oxide Nanomaterial Heterojunction for p–n Diode and Junction Field-Effect Transistor. Nano Letters 2016, 16 (2), 1293-1298. (35) Jariwala, D.; Sangwan, V. K.; Wu, C.-C.; Prabhumirashi, P. L.; Geier, M. L.; Marks, T. J.; Lauhon, L. J.; Hersam, M. C. Gate-tunable Carbon Nanotube–MoS2 Heterojunction p-n Diode. Proceedings of the National Academy of Sciences 2013, 110 (45), 18076-18080. (36) Wang, F.; Wang, Z.; Xu, K.; Wang, F.; Wang, Q.; Huang, Y.; Yin, L.; He, J. Tunable GaTeMoS2 van der Waals p–n Junctions with Novel Optoelectronic Performance. Nano letters 2015, 15 (11), 7558-7566.

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(37) Breitenstein, O.; Altermatt, P.; Ramspeck, K.; Schenk, A. In The Origin of Ideality Factors n> 2 of Shunts and Surfaces in the Dark IV Curves of Si Solar Cells, Proceedings of the 21st European photovoltaic solar energy conference, Citeseer: 2006; pp 625-628. (38) Shah, J. M.; Li, Y.-L.; Gessmann, T.; Schubert, E. Experimental Analysis and Theoretical Model for Anomalously High Ideality Factors (n ≫ 2.0) in AlGaN/GaN pn Junction Diodes. Journal of Applied Physics 2003, 94 (4), 2627-2630. (39) Münzenrieder, N.; Zysset, C.; Petti, L.; Kinkeldei, T.; Salvatore, G. A.; Tröster, G. Room Temperature Fabricated Flexible NiO/IGZO pn Diode Under Mechanical Strain. Solid-State Electronics 2013, 87, 17-20. (40) Bratkovsky, a. M. Spintronic Effects in Metallic, Semiconductor, Metal–oxide and Metal– semiconductor Heterostructures. Reports on Progress in Physics 2008, 71 (2), 026502-026502. (41) Jeon, P. J.; Lee, Y. T.; Lim, J. Y.; Kim, J. S.; Hwang, D. K.; Im, S. Black Phosphorus-zinc Oxide Nanomaterial Heterojunction for p-n Diode and Junction Field-effect Transistor. Nano Letters 2016, 16 (2), 1293-1298. (42) Sze, S. M.; Ng, K. K. Semiconductor Devices: Physics and Technology, 2006; p 568-568. (43) Yang, T.; Zheng, B.; Wang, Z.; Xu, T.; Pan, C.; Zou, J.; Zhang, X.; Qi, Z.; Liu, H.; Feng, Y. Van der Waals Epitaxial Growth and Optoelectronics of Large-scale WSe2/SnS2 Vertical Bilayer p– n Junctions. Nature communications 2017, 8 (1), 1906. (44) Chitara, B.; Krupanidhi, S. B.; Rao, C. N. R. Solution Processed Reduced Graphene Oxide Ultraviolet Detector. Applied Physics Letters 2011, 99 (11), 113114.

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(45) Kumar, M.; Bhat, T. N.; Rajpalke, M. K.; Roul, B.; Kalghatgi, A. T.; Krupanidhi, S. B. Transport and Infrared Photoresponse Properties of InN Nanorods/Si Heterojunction. Nanoscale research letters 2011, 6 (1), 609-609. (46) Yuga, M.; Daiki, Y.; Kuniharu, T.; Takayuki, A.; Seiji, A. Effect of Buffer Layer on Photoresponse of MoS 2 Phototransistor. Japanese Journal of Applied Physics 2018, 57 (6S1), 06HB01. (47) Furchi, M. M.; Pospischil, A.; Libisch, F.; Burgdörfer, J.; Mueller, T. Photovoltaic Effect in an Electrically Tunable van der Waals Heterojunction. Nano Letters 2014, 14 (8), 4785-4791. (48) Peng, C.; Jianyong, X.; Hua, Y.; Jing, z.; Guibai, X.; Shuang, W.; Xiaobo, L.; Guole, W.; Jing, Z.; Fusheng, W.; Zhongyuan, L.; Rong, Y.; Dongxia, S.; Guangyu, Z. Gate tunable MoS 2 –black Phosphorus Heterojunction Devices. 2D Materials 2015, 2 (3), 034009.

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Figure 1. Schematic and optical image of p-BP/n-IGZO device: (a) schematic diagram displaying the van der Waals heterostructure consisting of p-BP and n-IGZO with metal contacts — the Al/graphene contacts are fabricated over n-IGZO, while Au/Ti contacts are attached with p-BP, and (b) optical image of the final p-BP and n-IGZO device after metal contact deposition

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Page 21 of 26

(a)

(b)

2

Ag 2

Bg 1

Ag

250

300

350

400

450

200°C 300°C 400°C

Intensity (a.u.)

Intensity (a.u.)

Raman spectra of p-BP

10

500

20

30

40

50

60

2 (deg)

-1

Raman shift (cm ) 14

(d)

12

3nm BP

Height (nm)

10 8

10 nm IGZO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 2 0 -2 8

9

10

11

12

Distance (m)

Figure 2. Materials characterization using Raman spectroscopy and XRD: (a) Raman spectra of p-BP show three major peaks —peak A1g is due to out-of-plane vibrations and peaks B2g and A2g are due to in-plane vibrations, (b) XRD of IGZO film, after annealing at different temperatures, (c) AFM image of the heterostructure consisting of p-BP/n-IGZO films, and (d) height profile shows the thickness of p-BP and n-IGZO films

21



-4

50

10

(a)

(b)

-5

10

-6

10

40

-7

30

Ids (A)

Ids (A)

20 10

10

-8

10

-9

10

10 nm 15 nm 20 nm

-10

10

10 nm 15 nm 20 nm

-11

10

0

-12

10 -3

-2

-1

0

1

2

-3

3

-2

-1

0

1

2

3

Vds (V)

Vds (V)

(c)

4

9.0x10

Rectification

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

6.0x10

4

3.0x10

0.0

Rectification Ratio

9

12

15

18

21

Thickness (nm)

Figure 3. Thickness-dependent rectification: (a) linear Ids–Vds curves of p-BP/n-IGZO van der Waals heterojunction devices with the IGZO film thickness of 10, 15, and 20 nm, (b) logarithmic plot of Ids–Vds curves of p-BP/n-IGZO van der Waals heterojunction with different thickness of IGZO film showing the clear variation in reverse as well as forward currents, and, (c) at zero gate voltage, rectification of p-BP/n-IGZO plotted with error bars as a function of IGZO film thickness — the rectification rises with an increase in its thickness

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10

40

-6

Ids (A)

10

5

1.0x10 20

-7

10

-8

10

0 -3

-2

-1

1

0

3

2

Vilg= +2 V Vilg= +1 V Vilg= 0 V Vilg= -1 V Vilg= -2 V

-9

10

-10

10

-11

10

-12

10

(a) -4

(b)

Gate-dependent rectification

4

Rectification Ratio

-5

60

Ids (A)

-4

10

-3

-2

-1

0

1

2

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

0.0

3

-2

-1

Vin (V)

1

2

Vilg (V)

Vds (V) 1.0

0

(c)

1.0

0.5

0.5

0.0

0.0

-0.5

-0.5 AC-input DC output

-1.0 0

2

4

6

Vout(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.0 8

10 12 14 16 18 20

Time (ms)

Figure 4. Gate-dependent electrical characterizations of p-BP/n-IGZO: (a) log-scale of Ids–Vds curves of p-BP/n-IGZO van der Waals heterojunction at different gate voltages showing the variation in reverse as well as forward currents with ionic liquid gating — inset figure is the linear Ids–Vds plot of the same device, (b) rectification ratio of p-BP/n-IGZO plotted as a function of gate voltage —maximum current is rectified at negative gate voltage Vilg = -2V, and (c) rectifying behavior of p-BP/n-IGZO van der Waals heterostructure observed by applying a sinusoidal AC signal (f = 1 kHz) as Vin shown on the left y-axis —DC signal as Vout shown on 23



the right y-axis was obtained using an external resistance of 1 MΩ, as shown in inset circuit diagram

Figure 5. Energy band diagram of p-BP/n-IGZO van der Waals heterostructure showing the Fermi level and band alignment at zero gate voltage — as the energy gap (Eg) and electron affinity (qXn-IGZO) of n-IGZO are known as 3.1 and 4.2 eV, respectively, the electron affinity (qXp-BP) can be extracted as 4.7 eV with an energy gap of 0.3 eV in the case of few-layer black phosphorus (BP)

24


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Page 25 of 26

400

240

200

210

100

180

Isc(nA)

Ids (nA)

300

270

(a)

0

Voc

-100 -200 -300 -400

0.30

0.25

150 120

dark 32mW 50mW 70mW

Isc

0.35

(b)

0.20

90

Isc Voc

60

0.15

30

-0.6

-0.4

-0.2

0.0

0.2

0.4

Voc(V)

100

0.6

200

300

400

500

2

Power (mW/cm )

Vds (V) 300

(c)

250

Photocurrent

100

Light ON

150

Light OFF

Light ON

200

Iph (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 0 0

5

10

15

20

25

30

35

Time (s) Figure 6. Photovoltaic measurements and photoresponse of p-BP/n-IGZO van der Waals heterostructure: (a) I–V curves under different incident powers having a constant wavelength of 420 nm showing variation in Isc and Voc, (b) short-circuit current and open-circuit voltage plotted as a function of incident power intensity — black line shows the variation in shortcircuit current, while the blue line shows the variation in open-circuit voltage on the right yaxis, and (c) time-dependent photoresponse of p-BP/n-IGZO van der Waals heterostructure showing fast rise and decay times

25



Table of Contents

1.0

0.5

0.5

0.0

0.0

-0.5

-0.5

Vin (V)

1.0

AC-input DC output

-1.0 0

2

4

6

-1.0 8

10 12 14 16 18 20

Time (ms)

26


Vout(V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity

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