Article pubs.acs.org/JPCC
Transport Property Tuned by Gate Irradiation in ZnO Nanotetrapod Devices Wenhua Wang,† Junjie Qi,† Zi Qin,† Qinyu Wang,† Xu Sun,† and Yue Zhang*,†,‡ †
State Key Laboratory for Advanced Metals and Materials, School of Materials Science and Engineering, and ‡Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ABSTRACT: The transport properties tuned by gate electron-beam irradiation was investigated for ohmic- and Schotty-contact-type semiconductor optoelectronic devices based on a single zinc oxide (ZnO) nanotetrapod. Measurements of the I−V characteristics and time-dependent current were conducted. The results indicate that, for both ohmic and Schottky contact devices, the electrical transport properties can be readily tuned by electron-beam irradiation at the gate leg of the tetrapod, with favorable repeatability and reversibility. The response for the Schottky-contact-type device was obviously greater than that for the ohmic-contact-type device, and the closer the irradiated position approached the crystal nucleus, the larger the current response became. A probable mechanism is proposed and discussed. The ZnO nanotetrapod could potentially be used as a detector in irradiation environments.
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INTRODUCTION Because of its wide band gap of 3.37 eV and large excitation energy of 60 meV at room temperature, zinc oxide (ZnO) has been intensively studied in recent years. Compared with other wide-band-gap semiconductors, ZnO has several advantages such as excellent electrical conductivity, unusual thermal conductivity, chemical stability, and biological compatibility. A variety of ZnO crystal morphologies1 have been synthesized, such as nanowires, nanobelts, nanorod arrays, and nanotetrapods.2 A ZnO nanotetrapod consists of four needle-shaped legs with the [0001] wurtzite structure bonded at the ZnO central core along the ⟨111⟩ axis of the zinc blende structure.3 The angle between two adjacent legs is about 109° in the threedimensional tetrahedral structure. Because of its specific structure, the tetrapod can be used in an extensive range of applications.4 So far, ZnO nanotetrapods have been widely used as field-effect transistors (FETs),5 gas sensors,6,7 UV detectors,7 p−n junction diodes,8 logic switches,9 and Schottky photodiodes.10 Gu et al.5 measured the electrical transport characteristics of a single ZnO tetrapod by in situ nanoprobes. Zhang et al.11 investigated the photoluminescence and waveguide behaviors of a ZnO tetrapod. Zhang et al.6 also fabricated ZnO tetrapod-based sensors to distinguish false signals and increase detection sensitivity. Despite all of these reports, to the best of our knowledge, no study has been conducted on the response of a tetrapod to electron-beam irradiation at its gate leg. Thus, it was necessary to do some research into this aspect. In this work, two types of semiconductor optoelectronic devices were constructed based on a single ZnO nanotetrapod with ohmic and Schottky contact characteristics. The effect of electron-beam irradiation at the gate leg of the tetrapod was investigated. The electrical transport properties of the tetrapod can be tuned by the gate irradiation, and the response to the gate irradiation exhibits excellent reversibility as well as © XXXX American Chemical Society
repeatability. A possible mechanism for this phenomenon is also proposed and discussed.
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EXPERIMENTAL METHODS ZnO nanotetrapods were synthesized by chemical vapor deposition12 and released from the substrate by ultrasonication treatment in alcohol. Two types of single ZnO nanotetrapodbased semiconductor optoelectronic devices were constructed, one with ohmic and one with Schottky contact characteristics. The building process of the devices was as follows: For the ohmic contact devices, a ZnO nanotetrapod dispersion in alcohol was cast onto a silicon wafer covered with an insulating oxide layer of 500-nm thickness, and then conducting silver glue was used to fix two adjacent legs among the three legs which were lying approximately parallel to the substrate, as illustrated in Figure 1a. The Schottky contact device was constructed by casting the tetrapod dispersion in alcohol onto predeposited brick-like Pt electrodes patterned by photolithography, as shown in Figure 1b. Figure 1c shows a schematic diagram of the system setup. For both ohmic and Schottky contact devices, one of the three legs parallel to the substrate served as the source, and the other two served as the drain and gate. The transport characteristics were studied by introducing electron-beam irradiation at the gate leg. The electrical property measurements were carried out in a vacuum inside a scanning electron microscope (JSM-6490, JEOL, Tokyo, Japan) equipped with a detachable Zyvex S100 manipulator system. Tungsten probes of 50-nm radius at the tip were precleaned in isopropyl alcohol (IPA) to remove the surface oxide layer to ensure good electrical contacts with the Received: November 14, 2011 Revised: May 18, 2012
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dx.doi.org/10.1021/jp210934f | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. Scanning electron microscopy images of the ZnO tetrapod (a) in ohmic contact with two Ag electrodes and (b) in Schottky contact with Pt electrodes. (c) Schematic diagram of the system setup.
electrodes. The accelerating voltage of the scanning electron microscope was 20 kV when the gate leg of the tetrapod was irradiated by the electron beam, and a magnification of 10000× could be obtained at this voltage. A semiconductor characterization system (Keithley 4200) equipped with a preamplifier was employed to apply voltage and measure the corresponding current of the system.
Figure 2. (a) I−V characteristics with and without electron-beam irradiation for the device in ohmic contact with two Ag electrodes. The top left inset shows the experimental setup. The bottom right inset shows the current response with intermittent electron-beam irradiation. (b) I−V characteristics with electron-beam irradiation in different regions for the device in ohmic contact with two Ag electrodes. The top left inset shows the experimental setup.
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RESULTS AND DISCUSSION The current−voltage (I−V) performance of the ohmic contact device based on a single ZnO tetrapod was investigated first. Before any measurements were conducted, electrical contact was formed by directly contacting the tungsten tip with the silver paste electrode. Linear I−V characteristics between the drain and source were observed, as shown in Figure 2a, when a voltage ranging from −10 to +10 V was applied, indicating good ohmic contact between the Ag electrodes and the ZnO tetrapod. This result agrees well with the theoretical band structures because the work function of Ag is 4.26 eV and the electron affinity of ZnO is 4.5 eV.13 To investigate the response to gate irradiation, the electron beam was focused at the gate leg of the tetrapod. The I−V behaviors in the same voltage range were measured, as illustrated in Figure 2a, and a current increase upon irradiation with the electron beam can be clearly seen. According to the calculation, the current was estimated to increase by 14%, whereas the resistance of the system decreased from 411 to 361 kΩ. To investigate the reversibility of this device, a time-resolved measurement of the response to electron-beam irradiation was conducted at a fixed voltage of 2 V, as shown in the bottom right inset of Figure 2a. These
results indicate that the response to the gate irradiation was reversible with a favorable repeatability. The influence of irradiation applied at varied positions along the gate leg was also examined, as shown in Figure 2b. The I−V curves between the drain and source indicate that the closer the irradiated position approached the crystal nucleus of the tetrapod, the larger the current flowing through the tetrapod became. As for the device with Schottky contact shown in the inset at the top left corner of Figure 3a, I−V curves between the drain and source were obtained by contacting the tungsten tip tightly with the Pt electrodes. The I−V curve measured in the dark with a voltage from −10 to 10 V revealed double Schottky contact characteristics. Because Pt has a work function of 5.65 eV and ZnO has an electron affinity of 4.5 eV,14 a Schottky barrier is formed at the ZnO/Pt interface, which is consistent with the measured I−V curve. The forward-bias dark I−V data were fitted according to the following equation which is used to B
dx.doi.org/10.1021/jp210934f | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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the tetrapod was subjected to electron-beam irradiation and the other in the dark. The curves indicate that the I−V performances of the device was converted from rectifying to nearly ohmic behavior upon the application of irradiation on the gate leg. According to the calculation, a 54-fold increase in current was observed, which is significantly larger than that of the ohmic contact device. The current response to intermittent electron-beam irradiation was recorded to investigate the irradiation reversibility, as illustrated in the bottom right inset of Figure 3b. The results indicate that the effect of electronbeam irradiation was well restored, with excellent repeatability. These experimental results demonstrate that the effects of gate irradiation can be detected in real time with ease by monitoring the current response between the source and drain. A significant response can be obtained, with favorable repeatability and reversibility. As to the cause, a possible mechanism is suggested in the schematic diagram of Figure 4.
Figure 3. (a) I−V characteristics without electron-beam irradiation for the Pt−ZnO−Pt junction. The top left inset shows the experimental setup, and the bottom right inset shows forward-bias dark I−V curve for the Pt−ZnO−Pt junction, with the current on a logarithmic axis. (b) I−V characteristics with and without electron-beam irradiation for the Pt−ZnO−Pt junction. The top left inset shows the experimental setup. The bottom right inset shows the current response to intermittent electron-beam irradiation.
describe the thermionic emission of charge over the barrier at the ZnO/Pt interface J = J0 [exp(qva /nkT ) − 1]
J0 =
qm*(kT )2 2π 2ℏ3
exp( − e0 ΦB /kT )
Figure 4. Schematic diagrams of the mechanism for the device response to electron-beam irradiation at the gate leg for devices with (a) ohmic and (b) Schottky contact characteristics.
Here, J is the current density in forward bias; T is the absolute temperature; k is the Boltzmann constant; ΦB is the effective barrier height; n is the ideality factor; and m* is the effective electron mass, which can be defined as m* = 0.29me, where me is the electron rest mass. The inset at the bottom right in Figure 3a shows the forward-bias dark I−V curve for the Pt−ZnO−Pt junction with the current on a logarithmic axis. The effective barrier height, ΦB, at the ZnO/Pt interface was determined to be 0.47 eV, which was calculated by fitting the data using the above equations and is close to previously reported value of 0.43 eV for Pt−ZnO contact.15 In Figure 3b, the I−V performance of the device under two different conditions are compared: one in which the gate leg of
Upon the irradiation of an electron beam at the tip of the gate leg, electron−hole pairs are generated16,17 at the local regions irradiated by the electron beam. The holes move to the surface and are readily captured at the surface of the ZnO tetrapod,18−21 whereas the electrons migrate along the leg to the crystal nucleus and subsequently to other legs of the ZnO tetrapod under the bias of the external circuit, as shown in Figure 4a. Consequently, the carrier density is increased, and thus, the resistance of the ZnO tetrapod is reduced, leading to a large increase in current. As the irradiation position approaches C
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(11) Zhang, Z. X.; Yuan, H. J.; Gao, Y.; Wang, J. X.; Liu, D. F.; Shen, J.; Liu, L. F.; Zhou, W. Y.; Xie, S. S.; Wang, X.; Zhu, X.; Zhao, Y. C.; Sun, L. F. Appl. Phys. Lett. 2007, 90, 153116. (12) Li, H. F.; Huang, Y. H.; Zhang, Y.; Qi, J. J.; Yan, X. Q.; Zhang, Q.; Wang, J. Cryst. Growth Des. 2009, 9, 1863−1868. (13) Wang, Z. L.; Song, J. H. Science 2006, 312, 242−246. (14) Mead, C. A. Solid-State Electron. 1996, 9, 1023−1033. (15) Newton, M. C.; Firth, S.; Warburton, P. A. Appl. Phys. Let. 2006, 89, 072104. (16) Kamiya, T.; Tajima, K.; Nomura, K.; Yanagi, H.; Hosono, H. Phys. Status Solidi 2008, 205, 1929−1933. (17) Bera, A.; Basak, D. Appl. Phys. Lett. 2008, 93, 053102. (18) Gao, P.; Wang, Z. Z.; Liu, K. H.; Xu, Z.; Wang, W. L.; Bai, X. D.; Wang, E. G. J. Mater. Chem. 2009, 19, 1002−1005. (19) Li, Q. H.; Wan, Q.; Liang, Y. X.; Wang, T. H. Appl. Phys. Lett. 2004, 84, 4556. (20) Kind, H.; Yan, H. Q.; Messer, B. J. M.; Law, M.; Yang, P. D. Adv. Mater. 2002, 14, 158−160. (21) Fan, Z. Y.; Chang, P. C.; Lu, J. G.; Walter, E. C.; Penner, R. M.; Lin, C. H.; Lee, H. P. Appl. Phys. Lett. 2004, 85, 6128.
the crystal nucleus of the tetrapod, the migration distance is shortened, and the chance for recombination of electrons and holes decreases, resulting in a larger response amplitude. Comparatively, the irradiation-induced response for the Schottky-contact-type device is much larger than that for the ohmic-contact-type device, mainly because of a lower dark current rendered by the appearance of a Schottky barrier at the interface between the Pt electrode and the ZnO tetrapod. When the electron-beam irradiation is introduced, the accumulation of the electrons at the tetrapod leads to a larger current response. In addition, to some extent, the reduction of the effective barrier height, as illustrated in Figure 4b, might also contribute to the larger increase of the current response.
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CONCLUSIONS In conclusion, the transport properties of a single nanotetrapod have been studied under electron-beam gate irradiation. For both ohmic and Schottky contact single-nanotetrapod devices, the current response between the source and drain can be tuned by the electron-beam irradiation at the gate leg, and the Schottky-contact-type device is comparatively more sensitive to the gate irradiation than the ohmic-contact-type device. The current can also be affected by the gate irradiation position. The irradiation at the gate leg can tune the electrical signals between the source and drain. Therefore, ZnO nanotetrapod-based devices hold great promise for detection in irradiation environments.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Major Project of International Cooperation and Exchanges (2012DFA50990), the NSFC (51172022), the Research Fund of Co-construction Program from Beijing Municipal Commission of Education, the Fundamental Research Funds for the Central Universities, and the Program for Changjiang Scholars and Innovative Research Team in University.
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REFERENCES
(1) Wang, Z. L. J. Phys.: Condens. Matter 2004, 16, R829−858. (2) Dai, Y.; Zhang, Y.; Li, Q. K.; Nan, C. W. Chem. Phys. Lett. 2002, 358, 83−86. (3) Takeuchi, S.; Iwanaga, H.; Fujii, M. Philos. Mag. 1994, 69, 1125− 1129. (4) Newton, M. C.; Warburton, P. A. Mater. Today 2007, 10, 50−54. (5) Gu, Y. D.; Zhou, J.; Mai, W. J.; Dai, Y.; Bao, G.; Wang, Z. L. Chem. Phys. Lett. 2010, 484, 96−99. (6) Zhang, Z. X.; Sun, L. F.; Zhao, Y. C.; Liu, Z.; Liu, D. F.; Cao, L.; Zou, B. S.; Zhou, W. Y.; Gu, C. Z.; Xie, S. S. Nano Lett. 2008, 8, 652− 655. (7) Chow, L.; Lupan, O.; Chai, G. Y. Phys. Status Solidi B 2010, 247, 1628−1632. (8) Newton, M. C.; Shaikhaidarov, R. Appl. Phys. Lett. 2009, 94, 153112. (9) Sun, K.; Qi, J. J.; Zhang, Q.; Yang, Y.; Zhang, Y. Nanoscale 2011, 3, 2166−2168. (10) Newton, M. C.; Firth, S.; Warburton, P. A. Nanotechnology 2008, 7, 20−23. D
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