Black PhosphorusâZinc Oxide Nanomaterial Heterojunction for pân

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Black Phosphorus-Zinc Oxide Nanomaterial Heterojunction for P-N Diode and Junction Field Effect Transistor Pyo Jin Jeon, Young Tack Lee, June Yeong Lim, Jin Sung Kim, Do Kyung Hwang, and Seongil Im Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04664 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 19, 2016

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Black Phosphorus-Zinc Oxide Nanomaterial Heterojunction for P-N Diode and Junction Field Effect Transistor Pyo Jin Jeon,†,§ Young Tack Lee,‡,§ June Yeong Lim,† Jin Sung Kim,† Do Kyung Hwang,*,‡,# and Seongil Im*,† †

Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-gu,

Seoul 03722, Korea. ‡

Center for Opto-Electronic Materials and Devices Post-Silicon Semiconductor Institute, Korea

Institute of Science and Technology (KIST), Hwarangno 14 gil 5, Seongbuk-gu, Seoul 02792, Korea. #

Department of Nanomaterials and Nano Science, Korea University of Science and Technology

(KUST), Daejun 34113, Korea.

ACS Paragon Plus Environment

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TABLE OF CONTENTS

ABSTRACT: Black phosphorous (BP) nanosheet is two dimensional (2D) semiconductor with distinct band gap and attracting recent attention from researches, since it has some similarity to gapless 2D semiconductor graphene in two aspects: single element (P) for its composition, and quite high mobilities depending on its fabrication conditions. Apart from several electronic applications reported with BP nanosheet, here we, for the first time, report BP nanosheet-ZnO nanowire 2D-1D heterojunction applications for p-n diodes and BP-gated junction field effect transistors (JFETs) with n-ZnO channel on glass. For these nano-devices, we take advantages of the mechanical flexibility of p-type conducting of BP and van der Waals (vdW) junction interface between BP and ZnO. As a result, our BP-ZnO nano-dimension p-n diode displays a high ON/OFF ratio of ~104 in static rectification and shows kilohertz dynamic rectification as well while ZnO nanowire channel JFET operations are nicely demonstrated by BP gate switching in both electrostatics and kilohertz dynamics.

KEYWORDS: Black phosphorus, Zinc oxide nanowire, Heterojunction, P-N diode, Junction field effect transistor


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Beyond graphene,1-4 several other promising two dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs) 2-10 and black phosphorus (BP or phosphorene)11-34 have recently attracted much attention from researchers in respects of its relatively high mobility and distinct energy band gap. In particular, BP nanosheet11-34 has some similarity to graphene in two aspects: single element (P) for its composition, and quite high mobilities depending on its fabrication conditions. Its bandgap is reported to range from 0.3 to 1 eV, depending on its thickness.11-18 Besides, there have been exciting reports on its crystal structure,12-20 mechanical flexibility,20-21 ambient stability improvements by encapsulation,22-25 high anisotropic mobilities in field effect transistors (FETs),26-27 and even heterojunction p-n diodes,28-29 which are very few though. Heterojunctions involved with 2D semiconductors are mostly van der Waals junctions whether they are formed by 2D-2D interaction3-5,28-29 or between 2D layer and oxide thin films,6 and such van der Waals interface has played an important role in electronic devices: p-n diode or metal semiconductor field effect transistor (MESFET). In the present study, we report BP-ZnO nanomaterial heterojunction p-n diodes and junction field effect transistors (JFETs) with n-ZnO nanowire channel, taking advantages of the mechanical flexibility20-21 of p-type conducting17,21-25,27-34 17 nm-thin BP nanosheet that is attached on 85 nm-thick ZnO nanowire by direct imprinting4-5,33-34 to form van der Waals heterojunction between BP and ZnO. Such type of one dimension-two dimension (1D-2D) hybrid junction has been rare and unique for p-n diode,35 and never been reported for JFET, to the best of our limited knowledge. Here, our BP-ZnO nanomaterial p-n diode demonstrates good static rectification characteristics with high ON/OFF ratio of ~104 and a kilohertz fast dynamic rectification under a few volts. Moreover, our heterojunction between ZnO and BP successfully


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provides the formation of ZnO channel JFET, which demonstrates kilohertz switching dynamics. We regard that our BP-ZnO junction devices are quite useful toward future nanoelectronics.

As the first step of our device fabrications, the chemical vapor deposition with ZnO/C mixture source and Au catalyst was used to synthesize single crystalline ZnO nanowires on a sapphire substrate.36,39-40 The nanowire growth was carried out at 1037 K for 30 min under the flow of Ar working as carrier gas in a tube furnace. Our grown ZnO nanowires display several tens of µm in length and ~100 nm in diameter. As-grown ZnO nanowires were transferred from a sapphire substrate to 285 nm-thick SiO2/p+-Si wafer and glass (Corning, Eagle XG) substrates by using a drop-and-dry method with IPA (isopropyl alcohol) solvent.37-40 After dispersing ZnO nanowires onto substrates, the spin-coating process was carried out to stack the lift-off resist (LOR 3A, Micro Chem) and photoresist (AZ GXR-601, AZ electronic materials), followed by curing at 130 °C for 2 min. A 50/50 nm-thick Au/Ti electrode37-39 patterning was performed by a UV photo-lithography, direct current (DC) magnetron sputtering, and lift-off processes, in that order. A 50 nm-thick Ti contact metal was deposited prior to 50 nm-thick Au capping metal in a high-vacuum chamber of ~10-7 Torr. Next, a BP nanosheet was mechanically exfoliated from single crystal bulk BP by using sticky and transparent polydimethylsiloxane (PDMS) stamp.4-5,3334

The exfoliated BP nanosheet as attached beneath PDMS was then transferred onto already-

prepared ZnO nanowire by controlling the motion of the micro-aligner stage, being monitored by CCD (charge coupled device) camera image. The BP nanosheet was precisely stacked on a ZnO nanowire by van der Waals (vdW) force. As the final step of device fabrication, a 50/50 nm-thick Au/Ti metal24,27,31-34 was patterned on a BP nanosheet by photo-lithography and lift-off processes.


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For basic materials characterizations, both Raman spectra and photoluminescence (PL) were measured with Horiba LabRam Aramis Raman Microscope using two different incident laser sources at excitation wavelengths of 532 nm with 500 µW power for Raman shift and 325 nm with 300 µW for PL, respectively. Highly-resolved cross section images of nanomaterials heterojunction were obtained by scanning transmission electron microscope (STEM: model FEI Titan Themis). All electrical measurements were performed in the dark with a semiconductor parameter analyzer (Agilent 4155C), function generator (Tektronix AFG3022B), and oscilloscope (Tektronix TDS2014B). Figure 1a~c display the fabrication of BP-ZnO nanowire junction devices in a step-bystep manner. A 85 nm-thick ZnO nanowire initially had two terminals with Au/Ti ohmic electrode.37-39 Then, 17 nm-thin BP nanosheet is transferred to directly covers the center of ZnO nanowire (Figure 1b) and to form a van der Waals junction interface as aforementioned. Since the length of BP sheet was long enough, our BP has two terminals with Au/Ti24,27,31-34 ohmic metal as seen in the optical microscopy image of Figure 1c, where BP nanosheet is vertically located on ZnO nanowire as a result of fabrication processes. Photographic snapshot and schematic 3D view of our device are respectively prepared in Figure 1d and e. Scanning transmission electron microscopy image was also prepared and displayed in Figure 1f for the cross sectional image of our BP-ZnO junction.13,40 Fourier transformation patterns for the reciprocal lattices of thin film BP and ZnO wire are noted as insets, where the line spots evidence that BP is 2D. Further materials analysis on our 2D BP and ZnO nanowire, such as Raman spectroscopy19,30 or photoluminescence14,36 is found in Figure S1a and S1b of Supporting Information. Raman spectra in Figure S1a were taken as an evidence for our channel materials, probing the BP nanosheet, ZnO nanowire, and BP sheet on ZnO wire with the light source. Three


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peak positions were observed near 361, 437, 465 cm-1 which correspond to the A1g, B2g, and A2g vibration modes in 2D BP, respectively.19,30,34 On the other hand, a ZnO nanowire did not show any peaks in this range (300 ~ 500 cm-1). A ZnO nanowire appears to have a sharp PL peak at 378 nm which indicates its optical bandgap energy of 3.28 eV.36 A 17 nm-thick BP did not show any PL peak in 350 ~ 700 nm range because such thick BP should have low energy gap of ~0.3 eV.14 BP-ZnO overlapped area shows attenuated PL intensity, since the ZnO nanowire was covered by a 17 nm-thick BP nanosheet which would reduce the intensity of incident light. In Figure 2a the two terminal current-voltage (I-V) measurements on ZnO nanowire and on BP nanosheet (see inset photo) evidence that either ZnO nanowire or BP nanosheet keeps ohmic behavior with Au/Ti contact, while the conductance (100 µS) of BP nanosheet appears two~three orders higher than that (0.6 µS) of ZnO nanowire. Another two terminal I-V measurements between BP nanosheet and ZnO nanowire were also conducted and their results are shown in Figure 2b as diode-like rectifying behavior. (In Figure S2 linear scale I-V curves for our diode are also shown.) It is regarded that BP nanosheet/ZnO nanowire system is certainly a heterojunction p-n diode. According to Figure S3a and b, BP nanosheet17,21-25,27-34 and ZnO nanowire37-40 were proven to be p- and n-type semiconductors, respectively when field effect transistors with the two nanomaterial channels were tested on a universal back gate: 285 nmthick SiO2 gate dielectric/p+-Si. The inset photo of Figure 2b includes the measurement setup for left and right diodes. The two diodes show almost the same properties each other in respects of ON/OFF ratio (~104) and ideality factor (~1.3). Dynamic rectification was also observed from our p-n diode as seen in Figure 2c which is the time domain current plot obtained under 1 kHz square AC pulse (VA = ±5 V). The dynamic current at 5 V appears to be ~10 µA, which is the same as the static one observed in Figure 2b. Another rectification dynamics (Figure 2d) was


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taken with 1 kHz sinusoidal AC pulses; this time a circuit with external resistance (REXT = 100 kΩ; see the inset of Figure 2d) was set up for output voltage (VOUT) measurement. According to Figure 2c and d, no time delay is observed, which implies that even higher frequency rectification is possible in our p-n diodes. Certainly, the BP-ZnO nanostructure heterojunction pn diode is regarded to have promising properties. On the one hand, it would be interesting to note that ZnO and BP nanomaterials in the former two terminal measurements (Figure 2a) independently show linear conductance regardless of counterpart nanomaterial (for instance ZnO nanowire lies across and beneath BP nanosheet with overlapped area). These results imply that one nanochannel does hardly influence the other unless rectifying connection such as Figure 2b is involved. We could thus assume that ZnO and BP form van der Waals (vdW) junction interface which affords a few angstrom gap between the two materials. Moreover, since the high conductance and thickness of our p-type BP nanosheet indicate that our BP flake contains a high hole concentration and low energy gap of ~0.3 eV as well,11-18 we could make two other important progresses based on above facts and assumption: 1) approximation of BP-ZnO nanomaterial heterojunction band diagram and 2) realization of BP gate JFET with ZnO channel. An approximate BP-ZnO nanomaterial band diagram could be drawn after completing one more experimentation, which is heterojunction energy barrier measurement and it is conceptually the same or close to the Schottky barrier measurement assuming thick BP as a metallic conductor. For such barrier analysis, we implemented reverse bias I-V measurements at various elevated temperatures as shown in Figure 3a based on the following Schottky barrier equation (1),41

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(1)


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, where A, A*, T, q, kB and qΦB are the contact area, Richardson constant, temperature, electonic charge, Boltzmann constant, and Schottky barrier height, respectively. VA is applied reverse bias voltage. In the plot of Figure 3b, the slope is taken as qΦB and it is estimated to be ~0.7 eV here. As a result, we could finally obtain an approximate band diagram of BP-ZnO heterojunction in Figure 3c, according to which the vdW gap is depicted and BP is regarded as strong p-type so that its Fermi energy (Ef) may be almost close to valence band maximum (VBM). We regard that this heterojuction band digram is basically important to understand the operation of our heterojunction p-n diode after all. The vdW interfaces make possible two types of JFET operation in the present work: BP nanosheet-channel and ZnO nanowire-channel JFET operations. Figure 4a displays the drain current-gate voltage (ID-VGS) transfer characteristic and gate leakage (IG) behavior obtained from BP channel JFET (its circuit configuration is indicated in inset photo). As shown, ID of ~0.1 mA never changes with VGS while IG maintains a low level of 10 nA (but of course it increases to 1 µA at VGS of -2 V or below, which drives a forward bias action for ZnO-BP p-n diode). It is thus quite evident that our BP contains high density hole carriers effectively screening the gate ZnOinduced electric field. So, BP-channel JFET was not quite possible. In contrast, the other JFET with ZnO channel and BP gate appears well operating with ON/OFF ID ratio of ~2×103 according to its transfer characteristics of Figure 4b, which also shows low IG of a few nA (circuit configuration is shown in inset photo) and OFF ID of ~1 nA at -1 V gate voltage. Output characteristics (ID-VDS) of the ZnO-channel JFET show good ID saturation in Figure 4c and the saturation clearly starts from 2 V drain voltage. Transconductance plots in Figure 4d display 1.7 µS as maximum at 1 V gate voltage and the inset drawings illustrate nanowire device cross sections for ON and OFF states. We attribute this successful JFET operation to the vdW


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interface, which would protect channel nanomaterials from any mutual (contamination/chemical) influence as discussed in Figure 2a. Resistive-load inverter was also set up with our JFET and 10 MΩ external resistor as shown in the inset circuit of Figure 5a, to extend the usage of our JFET toward further practical application. Figure 5a shows voltage transfer characteristics (VTCs) of our inverter as obtained from the present setup and its voltage gain is displayed in the plots of Figure 5b, and the peak static voltage gain of ~4 is achieved at 5 V of supply voltage (VDD). Then, under this supply voltage inverter switching dynamics were carried out with square pulse AC voltage input of ±0.5 V. Figure 5c and d exhibit 0.1 and 1 kHz switching dynamics, respectively. Maximum and minimum output signals were observed to be 4 and 1.5 V, which are precisely the same as the values in the static VTC curve of Figure 5a (as noted by spots). For this inverter switching, RC delay appeared to be ~250 µs along with an AC gain of ~2.5. Since no apparent overlap capacitance but Schottky junction-induced depletion capacitance exists in our nanowire channel JFET, such delay is attributed to external resistance-coupled RC delay; we used 10 MΩ for VOUT in JFET circuit but only 100 kΩ for rectified VOUT signals in p-n diode circuit (Figure 2d), so that the delay in p-n circuit might be 100 times shorter (~2.5 µs) than that in JFET circuit. For improving the inverter performance in electrostatics and dynamics, property-optimization study is necessary near future. But in the present study it is clearly regarded that the BP nanosheet-ZnO nanowire vdW heterojunction could nicely work supporting the device operation for both p-n diode and JFET. We fabricated some other vdW junction devices composed of the same BP/ZnO materials but with different thicknesses, to make sure of our observation. (Details are seen in Figure S4).


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In summary, we have fabricated BP nanosheet-ZnO nanowire heterojunction p-n diodes and JFETs with n-ZnO channel on glass, taking advantages of the mechanical flexibility and ptype conduction of BP. As a result, our BP-ZnO nano-dimension p-n diode displays a high ON/OFF ratio of ~104 in static rectification and shows kilohertz dynamic rectification as well while ZnO channel JFET operations are nicely demonstrated by BP gate switching in both electrostatics and kilohertz dynamics. The successful JFET operation is attributed to vdW junction interface between ZnO and BP. We now conclude that our BP-ZnO vdW heterojunction is quite useful and promising toward future nanoelectronic devices.


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ASSOCIATED CONTENT Supporting Information Raman and photoluminescence spectra, linear scale I-V curves of p-n diodes, transfer curves of BP and ZnO bottom gate FETs, and another device set of BP/ZnO diodes and JFETs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Seongil Im, E-mail: [email protected]. *Do Kyung Hwang, E-mail: [email protected]. Author Contributions §Pyo Jin Jeon and Young Tack Lee equally contribute to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors acknowledge the financial support from the National Research Foundation of Korea (NRF) (NRL program, Grant No. 2014R1A2A1A01004815; Nano-Materials Technology Development Program, Grant No. 2012M3A7B4034985), the Yonsei University (FutureLeading Research Initiative of 2014, Grant No. 2014-22-0168), KIST Institution Program (Grant No. 2V04010; Grant No. 2E26420) and the Brain Korea 21 Plus Program.


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(22) Na, J.; Lee, Y. T.; Lim, J. A.; Hwang, D. K.; Kim, G.-T.; Choi, W. K.; Song, Y.-W. FewLayer Black Phosphorus Field-Effect Transistors with Reduced Current Fluctuation. ACS Nano 2014, 8, 11753-11762. (23) Avsar, A.; Vera-Marun, I. J.; Tan, J. Y.; Watanabe, K.; Taniguchi, T.; Castro Neto, A. H.; Özyilmaz, B. Air-Stable Transport in Graphene-Contacted, Fully Encapsulated Ultrathin Black Phosphorus-Based Field-Effect Transistors. ACS Nano, 2015, 9, 4138-4145. (24) Wood, J. D.; Wells, S. A.; Jariwala, D.; Chen, K.-S.; Cho, E.; Sangwan, V. K.; Liu, X.; Lauhon, L. J.; Marks T. J.; Hersam, M. C. Effective Passivation of Exfoliated Black Phosphorus Transistors against Ambient Degradation. Nano Lett. 2014, 14, 6964-6970. (25) Liu, H.; Neal, A. T.; Si, M.; Du, Y.; Ye, P. D. The Effect of Dielectric Capping on FewLayer Phosphorene Transistors: Tuning the Schottky Barrier Heights. IEEE Electron Device Lett. 2014, 35, 795-797. (26) Fei, R.; Yang, L. Strain-Engineering the Anisotropic Electrical Conductance of Few-Layer Black Phosphorus. Nano Lett. 2014, 14, 2884-2889. (27) Xia, F.; Wang, H.; Jia, Y. Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics. Nat. Commun. 2014, 5, 4458. (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) Gehring, P.; Urcuyo, R.; Duong, D. L.; Burghard, M.; Kern, K. Thin-layer black phosphorus/GaAs heterojunction p-n diodes. Appl. Phys. Lett. 2015, 106, 233110.


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(30) Du, Y.; Liu, H.; Deng, Y.; Ye, P. D. Device Perspective for Black Phosphorus Field-Effect Transistors: Contact Resistance, Ambipolar Behavior, and Scaling. ACS Nano 2014, 8, 10035-10042. (31) 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 Lett. 2014, 14, 3347-3352. (32) Koenig, S. P.; Doganov, R. A.; Schmidt, H.; Castro Neto, A. H.; Özyilmaz, B. Electric field effect in ultrathin black phosphorus. Appl. Phys. Lett. 2014, 104, 103106. (33) Lee, Y. T.; Kwon, H.; Kim, J. S.; Kim, H.-H.; Lee, Y. J.; Lim, J. A.; Song, Y.-W.; Yi, Y.; Choi, W.-K.; Hwang, D. K.; S. Im. Nonvolatile Ferroelectric Memory Circuit Using Black Phosphorus Nanosheet-Based Field-Effect Transistors with P(VDF-TrFE) Polymer. ACS Nano 2015, 9, 10394-10401. (34) Kim, J. S.; Jeon, P. J.; Lee. J.; Choi, K.; Lee, H. S.; Cho. Y.; Lee, Y. T.; Hwang, D. K.; Im, S. Dual Gate Black Phosphorus Field Effect Transistors on Glass for NOR Logic and Organic Light Emitting Diode Switching. Nano Lett. 2015, 15, 5778-5783. (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. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 18076-18080. (36) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H.-J. Controlled Growth of ZnO Nanowires and Their Optical Properties. Adv. Funct. Mater. 2002, 12, 323-331.


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(37) Raza, S. R. A.; Shokouh, S. H. H.; Lee, Y. T.; Ha, R.; Choi, H.-J.; Im, S. NiOx SchottkyGated ZnO Nanowire Metal-Semiconductor Field Effect Transistor: Fast Logic Inverter and Photo-Detector. J. Mater. Chem. C 2014, 2, 4428-4435. (38) Jeon, P. J.; Lee, S.; Lee, Y. T.; Lee, H. S.; Oh, K.; Im, S. Manipulating ZnO nanowires for field-effect device integration by optical-fiber grip coated with thermoplastic copolymer. J. Mater. Chem. C 2013, 1, 7303-7307. (39) Shokouh, S. H. H.; Pezeshki, A.; Raza, S. R. A.; Lee, H. S.; Min, S.-W.; Jeon, P. J.; Shin, J. M.; Im, S. High-Gain Subnanowatt Power Consumption Hybrid Complementary Logic Inverter with WSe2 Nanosheet and ZnO Nanowire Transistors on Glass. Adv. Mater. 2015, 27, 150-156. (40) Lee, Y. T.; Im, S.; Ha, R.; Choi, H.-J. ZnO nanowire and mesowire for logic inverter fabrication. Appl. Phys. Lett. 2010, 97, 123506. (41) Yang, H.; Heo, J.; Park, S.; Song, H. J.; Seo, D. H.; Byun, K.-E.; Kim, P.; Yoo, I.; Chung, H.-J.; Kim, K. Graphene Barristor, a Triode Device with a Gate-Controlled Schottky Barrier. Science 2012, 336, 1140-1143.


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FIGURES

Figure 1. (a)~(c) Fabrication steps of black phosphorus (BP)-ZnO nanowire hetrojunction device. (d) Photographic snapshot and (e) schematic illustration of our BP-ZnO heterojunction device as fabricated on glass substrate. (f) Cross sectional STEM image of BP-ZnO junction along with Fourier transformation patterns for the reciprocal lattice of BP and ZnO.


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Figure 2. (a) Two terminal current-voltage (I-V) curves of BP nanosheet and ZnO nanowire channels shown in the inset photo. (b) Two terminal I-V curves of left and right p-n diodes shown in the measurement setup of the inset photo. (c) Rectified output current of our p-n diode under applied AC voltage of square waveform (VA = ± 5 V) at 1 kHz. (d) Rectified output voltage obtained from the inset diode circuit with external resistor (REXT = 100 kΩ) under applied AC voltage of sine waveform (VA = -5 ~ 5 V) at 1 kHz.


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(a) 0

(b) -30

-5 363 K

ln(I/T )

-10

2

Current (nA)

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373 K

-32

-34

-15 383 K

-20 -3

∆T = 10 K

-2 -1 Applied Voltage (V)

(c)

0

ZnO nanowire

-36

qΦB= 0.70 eV

30

32

34 36 e/(kBT)

38

BP nanosheet

EVAC qΧZnO=4.3 eV CBM

qΧBP~4.7 eV qΦB= 0.7 eV

Ef

Eg.BP=0.3 eV

Eg.ZnO=3.3 eV

VBM vdW gap, ~Å

Figure 3. (a) Temperature-dependent current-voltage (I-V) curves of our p-n diode as obtained under reverse bias condition of VA = -5 ~ 0 V by elevating temperature from 323 to 383 K. (b) Richardson plot of ln(I/T2) versus e/(kBT). The potential barrier qΦB, ~0.7 eV is obtained from the slope of the linear fit. (c) Energy band diagram of n-type ZnO nanowire/ p-type BP nanosheet heterojunction with van der Waals (vdW) gap. Since the electron affinity (qXZnO) and Eg values of ZnO are known as 4.3 and 3.3 eV, respectively, the electron affinity (qXBP) can be extracted out, to be 4.7 eV.


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Figure 4. (a) Drain current-gate voltage (ID-VGS) transfer curve of BP nanosheet JFET under ZnO nanowire gating at VDS = -1 V and (b) the transfer curves of ZnO nanowire channel JFET under BP nanosheet gating at VDS = 2 ~ 5 V. (See inset photos and circuits) (c) Drain currentdrain voltage (ID-VDS) output curves of ZnO nanowire channel JFET under VGS which increases from -1 to 0.4 V with 0.2 V step. Saturation clearly starts from 2 V. (d) Transconductance (gm = dID/dVGS) curves of our ZnO channel JFET as a function of gate voltage. Inset cross section views illustrate ON and OFF states of BP-gated JFET with nanowire channel.


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Figure 5. (a) Voltage transfer characteristics obtained from a load-resistive inverter setup comprised of our ZnO channel JFET and external resistor of 10 MΩ (inset circuit) under supply voltages of VDDs = 1 ~ 5 V. (b) Voltage gain (-dVOUT/dVIN) curves as a function of input voltages. Dynamic switching behavior of our resistive-load inverter under AC square waves at (c) 100 Hz and (d) 1 kHz. AC gain of 2.5 and RC delay of 250 µs were obtained from this inverter setup.


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