LETTER pubs.acs.org/NanoLett
Photostable Dynamic Rectification of One-Dimensional Schottky Diode Circuits with a ZnO Nanowire Doped by H during Passivation Boram Ryu,†,§ Young Tack Lee,†,§ Kwang H. Lee,† Ryong Ha,‡ Ji Hoon Park,† Heon-Jin Choi,‡ and Seongil Im*,† †
Institute of Physics and Applied Physics and ‡Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea
bS Supporting Information ABSTRACT: For the first time, we demonstrated photostable and dynamic rectification in ZnO nanowire (NW) Schottky diode circuits where two diodes are face-to-face connected in the same ZnO wire. With their properties improved by H-doping from atomic layer deposited Al2O3 passivation, our ZnO NW diode circuits stably operated at a maximum frequency of 100 Hz displaying a good rectification even under the lights. We thus conclude that our results promisingly appoached one-dimensional nanoelectronics. KEYWORDS: ZnO nanowire, Schottky diode, dynamic rectification, face-to-face connection, surface passivation, photostable
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ntil recent years, nanowire (NW) devices such as diodes and field-effect transistors have been extensively studied based on NW forms of Si, Ge, and compound semiconductors.1 6 ZnO-based NW Schottky diode is a device that is a fundamental component for realizing one-dimensional nanoelectronics. Rectifying contacts on ZnO NWs were thus reported along with their Schottky metals, displaying typical current voltage (I V) diode curves.4,7 12 These Schottkytype devices were claimed to be useful for chemical or ultraviolet (UV) sensors along with typical ohmic-type ZnO NW detectors, since the surface state or surface potential changes of ZnO NWs in the Schottky diode take place when targeted molecules are attached on or detached from the ZnO NW surface.4,10 19 These sensing applications may not require fast dynamics.4,12,13,15 19 However, a moderate frequency dynamics is an important issue for electrical and photoelectric applications. In other words, if one would like to use NW Schottky diodes for an electrical switch or a rectifier, the acquisition of proper dynamic rectification properties from the diode should be essential. Despite this fact, interestingly few dynamic rectifications have been reported even with single NW devices, probably due to their low on-state current level. Moreover, it is certain that the single NW devices are not very practical by themselves and thus must be connected to other components for realizing an integrated device unit in the same wire. In the present study, we thus suggested and implemented two important approaches to achieve a dynamic rectifying behavior of NW Schottky diode and a practical integration of diode circuits using the same wire: surface passivation of ZnO NW and face-toface connection between two Schottky diodes. We found that the surface passivation of the ZnO Schottky diode by organic r 2011 American Chemical Society
polymer or hydrogen-containing oxide layer improves the basic conductivity level of ZnO NW enabling the Schottky devices to dynamically operate for rectification. The face-to-face connection circuits operated well in the same way of rectifying dynamics, displaying output voltage (Vout) signals, since the first ZnO NW Schottky diode plays as a rectifier while the opposite-direction connected diode works as a resistor. We also found that this type of two diode circuits that went through the passivation stably operate under visible lights and even under lowenergy UV while a remarkable photosensing was only observed from high-energy UV. Above all, a substrate of 200 nm thick SiO2/p+-Si was applied for fabricating ZnO NW/Ni Schottky diodes. The substrate was ultrasonically cleaned with acetone, methyl alcohol, and deionized water for 15 min, in that order, followed by 1 min O2 plasma treatment. The 200 nm thin ZnO NWs synthesized by carbothermal reduction method were then transferred from a sapphire substrate to the surface treated SiO2/p+-Si substrate;30,31 during the process the NWs were dispersed on the SiO2/p+-Si substrate by using a drop-and-dry method.32 We made patterns for ohmic and Schottky contact electrodes using photolithography processes. First, we coated the lift-off layer (LOL: Micro Chemical 1165) to facilitate removal of the photoresist (PR) prior to PR spin-coating, and we baked LOL coated on the substrate at 115 °C for 2 min. Then, we coated the PR (SPR 3612: Micro Chem) layer and baked SPR-coated (as a second layer) substrate at 95 °C for Received: July 2, 2011 Revised: August 16, 2011 Published: August 29, 2011 4246
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Figure 1. (a) SEM image of a ZnO NW/Ni Schottky contact. Scale bar: 200 nm. (b) SEM image of ZnO NW/Ni Schottky diode with a current path (5 μm). Scale bar: 3 μm. (c) Current voltage curves of ZnO/Ni Schottky diodes with and without passivation layer.
2 min. Then LOL- and SPR-coated substrate were exposed to UV for 7 s with our electrode-patterned photomask. Exposed substrate was subsequently developed. After making patterns, we first deposited a 30 nm thin Ti and then 90 nm thick Ni double layers (Ni/Ti) for an isolated ohmic contact electrode using the electron beam evaporation system, which was followed by lift-off process. While lift-off process, we use acetone, methyl alcohol, and LOL remover. Likewise, using the same lift-off technique, we this time deposited 90 nm thin Ni layers on the ZnO NW surface for an isolated Ni Schottky electrode. For the top passivation layers, we applied two kinds of materials: ALD Al2O3 (20 nm) and CYTOP (Asahi glass CTX-809A) (170 nm thick film by spin coating and curing at 180 o C for 2 h). All the static electrical measurements of our devices were performed with an Agilent 4155C semiconductor parameter analyzer in the dark at RT. Electrical dynamic rectifying characteristics of our ZnO NW/Ni Schottky diode under various alternating current (AC) conditions were measured by using a function generator (Tektronix AFG 310) and an oscilloscope (Tektronix TDS 2014B). Light illumination was performed on our ZnO NW/Ni Schottky diode or face-to-face connected Schottky circuits with a light source setup consisting of a 500 W Hg (Xe) arc lamp, monochromator covering the wavelength in the range of 300 700 nm, and an optical fiber to deliver the monochromatic lights onto our device. The optical power densities from visible and UV illuminations were measured right below the optical fiber by using a Si-based optical power meter (Newport Inc.). Scanning electron microscope was also used for the imaging and size measurements of our ZnO NW Schottky diodes. Figure 1a,b displays scanning electron microscopy (SEM) images of our ZnO NW covered by Schottky/or ohmic metal electrodes. The dashed circle in Figure 1b indicates the metal/ NW contact region shown in Figure 1a. The approximate size of NW was ∼150 nm in diameter. According to the I V curves
Figure 2. ALD oxide-passivated Schottky diode circuit with properly selected commercial resistor (200 KΩ) and capacitor (1 μF) connected to the Ni/Ti ohmic contact electrode. (a) Three-dimensional device schemes. (b) Optical photographic plan view. (c e) Sine wave rectification dynamics. (f) Square wave type of dynamic rectification.
in Figure 1c, our NW Schottky diode without any passivation layer shows very low current in both on- and off-states. However, when these ZnO NW Schottky diodes are properly passivated by either an organic polymer (here we used poly(perfluorobutenylvinylether),commercially named CYTOP20,21) or atomic layer deposited (ALD) Al2O3 their electrical properties were much improved; the former case showed the on- and offcurrent increase to result in ∼250 as an on/off ratio while the latter displayed a little smaller on/off ratio (∼150) but with an order of magnitude higher on-current than that of former case. It is also worth to note that the on-current swing was much improved by ALD passivation on the Schottky diode showing a good ideality factor, η, of ∼1.25 which is compared to those (η = ∼3.5) of original or CYTOP-passivated diode. (This improved η means that more ideal Schottky junction near the interface between Ni and ZnO NW was formed by the ALD passivation.) Exposed to an air ambient, our NW ZnO surface would adsorb O2 molecules that might trap oxygen-vacancy-related mobile electrons making the surface-to-depth region depleted and electrically resistive.12 16,22,23 In contrast, the passivation layer deposition would prevent such molecule adsorption and may provide hydrogen atoms during the passivation that should go through a thermal process at 100 200 o C. Particularly, it is wellknown that the ALD process provides high density H atoms through the dissociation of chemical precursors24 26 and that the H atoms play as electron donor if diffused into ZnO to be located at interstitial sites.27,28 Therefore, it is now understandable why the on- and off-state current level increases by orders of 4247
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Figure 4. Photoresponse properties of our Schottky diode and its circuits passivated by ALD Al2O3. (a) Measurement scheme. (b) Photoinduced I V properties. (c) Dynamic properties of photoinduced current. (d) Dynamic photoinduced output in voltage.
Figure 3. Face-to-face connected Schottky diode circuit, which is passivated by ALD-oxide. (a) Three dimensional device schemes. (b) Optical photographic plan view. (c e) Sine wave rectification dynamics. (f) Square wave type of dynamic rectification.
magnitude through the ALD passivation processes. CYTOP passivation would prevent molecule adsorption on ZnO surface but can hardly provide H diffusion because the polymer mainly consists of C C and C F bonds. Curing process at 180 o C might improve source/drain ohmic contact, leading to somewhat improved I V characteristics. Figure 2a,b displays a schematic three-dimensional (3D) and photographic plan view of our ALD oxide-passivated Schottky diode circuit, respectively, where properly selected commercial resistor (200 KΩ) and capacitor (1 μF) were connected to the Ni/Ti ohmic electrode by way of separate cables. Ni Schottky contact pad shows different contrast from that of Ni/Ti pad in view of optical microscopy photo of Figure 2b, since the photo was deliberately taken prior to Ni deposition and lift-off process, to distinguish the two contact areas. According to the sine wave rectification dynamics of Figure 2c e, the rectification showing ∼2 V as peak Vout is clear for all the frequencies although the Vout signals decrease to 1.5 V as the frequency increases to 100 Hz. With parallel-connected 1 μF capacitor, our diode showed ∼1 V DC Vout (Figure 2c). Square wave type of dynamic rectification in Figure 2f displays maximum RC delay of ∼10 ms for turn-on but off-speed was 10 times faster. Similar dynamic rectification was observed from a face-to-face connected Schottky diode circuit that has serially connected opposite-direction Schottky diode as a substitute for a resistor, as shown in Figure 3a f. We implemented this experimentation based on the facts that the first diode plays as a conductor as the second diode does as a resistor under a positive (+) bias or vice versa under a negative ( ) bias; it means that the resistance of
our resistor (the second diode) is variable depending on the bias polarity, while the reverse current of our diode would result in somewhat much higher resistance (∼100 MΩ, see Figure 1c) than that of the unit resistor in Figure 2a. According to Figure 3c e, it is likely that the higher resistance was obtained from the reverse-biased Schottky diode (second diode), and as a result a higher voltage Vout of ∼4 V was measured at 10 Hz although the voltage decreased to 2.7 V at 100 Hz, where slightly unrectified Vout in input voltage (Vin) periods is also observed along with some phase shift. These mean that our face-to-face connected Schottky diode circuits are promising with high Vout signals but also with a frequency limit. The square wave rectification dynamics shows 30 and 7 ms for onand off-state RC delays, which may be attributed to higher resistance of our reverse-biased diode resistors. At higher frequencies over 100 Hz, our diode circuit probably become a coupled-capacitor like circuit. Regardless of such a rectification limit in high frequency regimes, our face-to-face connected Schottky circuit may be worthy of note because any dynamic rectification of NW diodes in one-dimensional circuit form has not been reported, to the best of our limited knowledge. Organic polymer-passivated Schottky diodes supported our points of success, showing inferior rectification dynamics with either commercial or face-to-face connected diode resistor as shown in Supporting Information Figures S1 and S2, according to which dynamic rectification with a peak voltage of 0.15 V was barely achieved at only 10 Hz sine wave although at the same frequency of square wave input the Schottky diode circuit just displayed a capacitor-like behavior. Since the organic polymerpassivated Schottky diode showed an order of magnitude lower conductivity than that of ALD oxide passivated one, it seems that highly improved diode with good forward current over a few microamperes is required to achieve dynamic rectifications and was possible with ZnO NW doped by H atoms through ALD passivation process. Our statistical data for the Schottky diode yield were not bad, particularly if the ZnO nanowires have ALD passivation. Along one NW, we could achieve 4248
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Figure 5. Dynamic rectifications of our ALD oxide-passivated Schottky circuits as observed under the lights. (a c) Sine wave rectification dynamics. (d) Square wave type of dynamic rectification. Our Schottky diode circuits appear quite photostable under red visible and low-energy UV lights.
maximum 6 diodes and 2 sets of face-to-face Schottky circuit that worked right. As a next step to exploit the face-to-face connected diode circuits and ALD oxide/or organic-polymer passivations, the dynamic photoresponse properties of the Schottky diode and its circuit were also tested as shown in the measurement scheme of Figure 4a. Figure 4b d displays the photoinduced current and Vout results in the case of ALD Al2O3 passivation. For these measurements, we used three monochromatic lights of red (640 nm, 0.3 mW cm 2), low-energy UV (364 nm, 3 mW cm 2), and high energy UV (300 nm, 1 mW cm 2). According to Figure 4b, our Schottky diode showed a high photocurrent only under 300 nm UV while 640 nm red and 364 nm UV lights resulted in negligible photoresponses. It is understandable that the photoresponse from the red light was small, since ZnO semiconductor has 3.3 eV as its band gap. Such small photo signal may come from the small density of trapped electron charges located at the Al2O3/ZnO NW interface.12 16,22,23,27,28 In contrast, a small amount of photoresponse from 364 nm UV with good optical power density (∼3 times higher than that of 300 nm UV) is beyond usual expectation because the photon energy (364 nm, 3.4 eV) is quite higher than the band gap of ZnO. However, it could be also understandable in view of well-known UV sensing mechanism in ZnO NW devices, which originates from the NW surface states that trap or couple surface electron charges and ambient O2 molecules but release those electrons and attached molecules by way of UV admission (e.g., by surface electron hole recombination).12 16,22,23,27,28 The surface passivation would change the O2-adsorption related surface to Al2O3/ZnO NW interface, removing the surface states that were an important source for the UV photosensitivity. Under the low energy UV, photocurrent is now only caused by either the interface trapped electrons or band-to-band generated electron hole pairs that are easily recombined if without strong electric field. Somewhat visible detection sensitivity comes from such a high energy UV over 4.1 eV (∼300 nm), where the electron hole generation is intense enough to supply an effective number of photocarriers.23,29
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Figure 4c again shows such photoinduced current behavior in time domain plots, where quite clear on/off signals were only made by the high energy UV under a fixed input voltage of 10 V. These time domain on/off current signals were accordingly reflected in Figure 4d by voltage signals when we used the face-to-face diode circuits of Figure 4a. (Nonetheless, the 300 nm UV-induced Vout difference (ΔVout) was only 0.15 V between on and off states.) Equivalent dynamic photoresponse measurements were carried out for the Schottky diode circuits with CYTOP passivation as shown in Supporting Information Figure S3b d, where slightly better but less stable responses under the red and low-energy UV are observed; the surface passivation effects by this organic polymer are less effective than by the ALD oxide. With the low photosensitivity in the visible and low-energy UV lights, our face-to-face connected and ALD-passivated Schottky diode circuits exhibited stable dynamic rectifications even under the lights as shown in Figure 5a d. According to the figures, our diode circuit under the red and low-energy UV photons follow an almost ideal rectification behavior with 3.7 V peak at 10 Hz and 3.2 V at 100 Hz; the behavior was even more desirable than that of the dark state (Figure 3c f), due to the aid of photoinduced conductivity. Under high-energy UV of 300 nm our diode circuits showed a little deviation from desirable rectifying behavior in reverse Vin periods particularly at the higher frequency regimes (Figure 5c) but displayed a higher peak voltage of 4.8 V (at 10 Hz, Figure 5a). The dynamic rectification by square-pulsed Vin displayed even less than 1 ms as rising and falling times under the red and low-energy UV lights (Figure 5d). These results indicate that our properly passivated ZnO NW Schottky diode circuits are quite stable under ambient lights and are useful for one-dimensional NW electronics. In summary, we have fabricated ZnO NW Schottky diode circuits to demonstrate their dynamic rectification in the dark and under light ambient. With the Schottky effects obtained by Ni, our face-to-face connected Schottky diode circuits exhibited stable dynamic rectifying operation in the dark although rectification dynamics has their frequency limit at 100 Hz. These performances of our NW Schottky diode circuits are mainly attributed to the surface passivation by ALD Al2O3 layer which appeared to improve the conductivity of ZnO NW by H doping. Since the ALD passivation effectively reduces the UV sensitivity as well, our Schottky diode circuits displayed a photostable operation in rectification dynamics even under low-energy UV. We thus conclude that our NW Schottky diode with ALD oxide passivation layer could be promising for ZnO NW-based future electronics.
’ ASSOCIATED CONTENT
bS
Supporting Information. Electrical dynamic rectifying characteristics of CYTOP passivated Schottky diode circuits with properly selected commercial resistor or face-to-face connected diodes, photoresponse properties of CYTOP passivated Schottky diode circuit. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: 82-2-2123-2842. Fax: 82-2-392-1592. Address: Eelctron Device Laboratory, 4249
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Nano Letters Science Building, Room 240, Yonsei University, Seoul, 120749, Korea. Author Contributions §
Authors with equal contribution.
’ ACKNOWLEDGMENT Authors acknowledge the financial support from NRF grant (NRL program: Grant 2011-0000375), and Brain Korea 21 Program. B.R. would like to thank for the LOTTE scholarship. ’ REFERENCES (1) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species. Science 2001, 293, 1289–1292. (2) Xiang, J.; Lu, W.; Hu, Y.; Wu, Y.; Yan, H.; Lieber, C. M. Ge/Si Nanowire Heterostructures as High Performance Field-Effect Transistors. Nature 2006, 441, 489–493. (3) Li, Y.; Qian, F.; Xiang, J; Liber, C. M. Nanowire Electronic and Optoelectronic Devices. Mater. Today 2006, 9, 18–27. (4) Heo, Y. W.; Norton, D. P.; Tien, L. C.; Kwon, Y.; Kang, B. S.; Ren, F.; Pearton, S. J.; LaRoche, J. R. ZnO Nanowire Growth and Devices. Mater. Sci. Eng., R 2004, 47, 1–47. (5) Keem, K.; Jeong, D.-Y.; Lee, M.-S.; Yeo, I.-S.; Chung, U.-I.; Moon, J.-T.; Kim, S. Fabrication and Device Characterization of OmegaShaped-Gate ZnO Nanowire Field-Effect Transistors. Nano Lett. 2006, 6, 1454–1458. (6) Goldberger, J.; Sirbuly, D. J.; Law, M.; Yang, P. ZnO Nanowire Transistors. J. Phys. Chem. B 2005, 109, 9–14. (7) Park, W. I.; Kim, J. W.; Park, S. M.; Yi, G. C. Schottky Nanocontacts on ZnO Nanorod Arrays. Appl. Phys. Lett. 2003, 82, 4358–4360. (8) Liu, J.; Fei, P.; Song, J.; Wang, X.; Lao, C.; Tummala, R.; Wang, Z. L. Carrier Density and Schottky Barrier on the Performance of DC Nanogenerator. Nano Lett. 2008, 8, 328–332. (9) Yang, Q.; Guo, X.; Wang, W.; Zhang, Y.; Xu, S.; Lien, D. H.; Wang, J. L. Enhancing Sensitivity of a Single ZnO Micro-/Nanowire Photodetector by Piezo-phototronic Effect. ACS Nano 2010, 4, 6285–6291. (10) Heo, Y. W.; Tien, L. C.; Norton, D. P.; Kang, B. S.; Ren, F.; LaRoche, J. R.; Pearton, S. J. Pt/ZnO Nanowire Schottky Diodes. Appl. Phys. Lett. 2004, 85, 3107–3109. (11) Kim, J.; Yun, J. H.; Kim, C. H.; Park, Y. C; Woo, J. Y.; Park, J.; Lee, J. -H.; Yi, J.; Han, C. S. ZnO Nanowire-Embedded Schottky Diode for Effective UV Detection by the Barrier Reduction Effect. Nanotechnology 2010, 21, 115205. (12) Zhou, J.; Gu, Y.; Hu, Y.; Mai, W.; Yeh, P. H.; Bao, G.; Sood, A. K.; Polla, D. L.; Wang, Z. L. Gigantic Enhancement in Response and Reset Time of ZnO UV Nanosensor by Utilizing Schottky Contact and Surface Functionalization. Appl. Phys. Lett. 2009, 94, 191103. (13) Kind, H.; Yan, H.; Messer, B.; Law, M.; Yang, P. Nanowire Ultraviolet Photodetectors and Optical Switches. Adv. Mater. 2002, 14, 158–160. (14) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. ZnO Nanowire UV Photodetectors with High Internal Gain. Nano Lett. 2007, 7, 1003–1009. (15) Lupan, O.; Chow, L.; Chai, G.; Chernyak, L.; Lopatiuk-Tirpak, O.; Heinrich, H. Focused-Ion-Beam Fabrication of ZnO NanorodBased UV Photodetector Using the In-Situ Lift-Out Technique. Phys. Status Solidi A 2008, 205, 2673–2678. (16) Kumar, S.; Gupta, V.; Sreenivas, K. Synthesis of Photoconducting ZnO Nano-Needles Using an Unbalanced Magnetron Sputtered ZnO/ Zn/ZnO Multilayer Structure. Nanotechnology 2005, 16, 1167–1171. (17) Wang, H. T.; Kang, B. S.; Ren, F.; Tien, L. C.; Sadik, P. W.; Norton, D. P.; Lin, J.; Pearton, S. J. Hydrogen-Selective Sensing at Room Temperature with ZnO Nanorods. Appl. Phys. Lett. 2005, 86, 243503.
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