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Abstract
High performance solar-blind photodetectors have been fabricated from diamond wafers. The peak responsivity is 13.0 A/W at 222 nm with a dark current of 0.93 nA under 60 V bias. The rise and decay times of the photodetector are about 1.3 µs and 203 µs, respectively. The responsivity and response time of the device are both among the best values ever reported for diamond-based photodetectors. A solar-blind optical communication system has been constructed by employing the diamond photodetector as a signal receiver for the first time. Benefiting from the high spectral selectivity of the diamond photodetector, the communication system has excellent anti-interference ability. The results reported in this paper may pave the way for the future application of diamond-based solar-blind photodetectors in confidential communications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Wireless communications play a vital role in modern military and civilian applications [1], which are usually achieved through radio frequency (RF) and optical communication systems using electromagnetic radiation to convey information. The RF communication usually need high power and are susceptible to interception due to the overuse of the available RF spectrum. Optical communication systems depend on optical radiation to convey information with wavelength ranging from infrared to ultraviolet (UV). Compared with RF communication, optical communication shows several potential advantages, such as large bandwidth, low power-consumption, small-size, high power-densities and high resistance to jamming [2–5]. Relatively mature infrared (IR) technology has been applied to communication systems [6,7]. However, both of the solar irradiation and fluorescence noise can caused interferences to IR detectors, which may limit the application of IR system significantly [8]. Due to the strong absorption and scattering of deep UV light by ozone and particles in the atmosphere, the solar irradiation with wavelength shorter than 280 nm cannot reach the earth surface, thus this region is called “solar-blind” waveband [9,10]. Owing to the “black background”, optical communication operating in this spectral range has an extremely low background noise. In recent years, with the rapid development of deep UV solid-state light source and photodetector technology, solar-blind communication has attracted more and more attention [11,12]. Because of the strong scattering and absorption, UV light intensity reduces quickly with the increasing transmission distance, thus the UV communication system is very suitable for a short-distance confidential communication which is hard to be eavesdropped. Photodetector that works as a signal receiver is one of the key components for optical communication systems. Nowadays, more and more solar-blind photodetectors have been developed from wide bandgap semiconductors, such as AlGaN [13,14], ZnMgO [15,16], Ga2O3 [17–19], and diamond [20–22], etc. These photodetectors usually have some advantages including small-size, light-weight, intrinsic solar blindness and high radiation hardness, which are recognized as the potential alternative for photomultipliers and traditional silicon-based devices. Diamond has a wide bandgap of 5.5 eV along with extreme properties such as high thermal conductivity, high carrier mobility and saturation velocity, and high radiation hardness, etc [23]. The above characters make diamond a promising candidate for high-performance solar-blind photodetection. To date, many different kinds of diamond-based photodetectors have been reported. However, none report on solar-blind communication system constructed by diamond-based photodetectors can be found.
In this paper, diamond based solar-blind photodetectors have been fabricated. The photodetectors show a low dark current of 0.93 nA at 60 V bias, ensuring a low noise of the photodetectors. The peak responsivity is 13.0 A/W at 222 nm and the UV/visible rejection ratio is five orders of magnitude with a cut-off wavelength at 225 nm. The device has a fast response speed with a rise time of about 1.3 µs and a decay time of 203 µs, respectively. An optical communication system has been constructed by employing the photodetector as a signal receiver, and the system exhibits excellent anti-interference ability, and information transmission has been realized for the first time.
2. Experimental setup
2.1 Diamond wafers growth
Commercially available synthetic high-pressure high-temperature (HPHT) Ib-type diamonds with (100) orientation have been employed as substrates for the growth of diamond wafers by using a microwave plasma chemical vapor deposition (MPCVD) system. The size of the substrates was about 4×4×1 mm3. During the growth process, the ratio of CH4/H2 was 5% with a total flow rate of 200 sccm and the substrate temperature was maintained at about 950°C, which was monitored using an IR pyrometer. After 100 hours deposition, the substrate was detached by laser-cutting and the grown diamond layer was polished to an 800 µm thick freestanding wafer. After the polishing process, the CVD diamond wafer was placed in aqua regia (HCl: HNO3=3:1 by volume) for 3 hours to remove metallic contaminants. Subsequently, the diamond wafer was ultrasonically cleaned in acetone, ethanol and deionized water for 10 minutes to remove any organic contaminants. To fabricate a diamond-based photodetector, the diamond wafer was loaded into a thermal evaporation chamber, and Au films were deposited onto the surface of the wafer. Eventually, interdigital Au electrodes were fabricated by photolithography and wet etching process. The optical image of the diamond photodetector is shown in the inset of Fig. 2(b). The electrodes were consisted of 12 pairs fingers, which with 5 µm in width, 500 µm in length and 10 µm in space.
2.2 Characterization
The absorption spectra of the diamonds were measured in an ultraviolet-visible spectrophotometer (Hitachi, UH4150). The X-ray diffraction (XRD) patterns of the samples were recorded in the Bragg-Bretano geometry with a X’Pert Pro diffractometer (PANalytical) using Cu Kα radiation (λ=1.5418 Å). Raman measurement was carried out by LabRAM HR high resolution spectrometer (Horiba Jobin Yvon) working in a confocal mode with a 532 nm laser as the excitation source. The photoluminescence (PL) spectra were measured using a SOL instrument (Confotec MR520) spectrometer with 532 nm and 325 nm lasers as the excitation sources. The current-voltage (I-V) properties of the photodetectors were measured using a semiconductor characterization system (Keithely 4200-SCS). And the photocurrent and current-time (I-t) curves were collected under illumination of a laser-driven light source (Energetiq, EQ-99XFC) with wavelength from 190 nm-2100 nm. The response spectra of the photodetectors were recorded using a 150W Xe lamp, a monochromator, an optical chopper, and a lock-in amplifier in a synchronous detection scheme. The transient photoresponse was recorded by an oscilloscope (Tektronix DPO 2024B) under the excitation of a 213 nm pulse laser. A laser-driven light source (Energetiq, EQ-99XFC) with the output filtered by a 220 nm bandpass filter was used as the light source of the communication system.
3. Results and discussion
The XRD pattern of the CVD diamond wafer is shown in Fig. 1(a). Only a peak at 119.8° is visible from the pattern, which can be assigned to the (400) facet of diamond, revealing the highly orientation of the diamond wafer. The Raman spectrum of the CVD diamond wafer is shown in Fig. 1(b). There is only one sharp peak at around 1332 cm−1, corresponding to the sp3 band configuration of carbon atoms. The full width at half maximum (FWHM) of the Raman peak is 2.10 cm-1, indicating a high-quality diamond wafer. To investigate the optical quality of the diamond wafer, optical absorption spectrum was measured. As shown in Fig. 1(c), the CVD diamond wafer shows a strong absorption to deep UV light but almost transparent to visible light. There is a sharp absorption edge at around 225 nm, which correspond to the bandgap of diamond. The above results indicate that the CVD diamond wafer can be used to fabricate solar-blind photodetectors with relatively high UV/visible rejection ratio. Figure 1(d) shows the photoluminescence spectra of the CVD diamond excited by 532 nm and 325 nm lasers. The spectra show no NV (575 nm and 638 nm) and SiV(738 nm) centers, indicating high purity of the CVD diamond. Thus, the CVD diamond is ideal for high performance optoelectronic devices.
Fig. 1. (a) XRD pattern of the CVD diamond. (b) Raman spectrum of the CVD diamond. (c) Absorption spectrum of the CVD diamond. (d) PL spectra of the diamond employing 532 nm and 325 nm laser lines as excitation source.
Figure 2(a) shows the I-V characteristics of the photodetector measured under dark conditions and illumination of a 150 W Xenon lamp, respectively. At 60 V bias, the dark current is only 0.93 nA. The low dark current promises low dark noise and high signal-to-noise ratio of the photodetector. As compared with the dark current, the photocurrent is about four orders of magnitude larger, revealing that the device can be employed for photodetection. In order to estimate the spectral selectivity of the diamond photodetector, the response spectra of the photodetector were measured, as shown in Fig. 2(b). The responsivity rises sharply when the wavelength of the incident light is shorter than 250 nm, and the response peak is located at around 222 nm. The peak responsivity of the photodetector increases with the bias voltage, and it can reach about 13.0 A/W when the bias is 60 V. The photodetector shows a gain of about 7200% under 60 V. The high gain may be caused by the fact that the minority carriers are trapped by surface states at the interface between electrodes and the diamond, which could reduce the carrier recombination probability [24,25]. Figure 2(c) illustrates the plot of the response spectrum under 60 V bias in logarithmic scale. The cutoff wavelength is located at 225 nm, which is consistent with the absorption edge of the diamond wafer shown in Fig. 1(c). The responsivity of the photodetector in the visible and near ultraviolet region is almost negligible. The UV/visible rejection ratio which defined as the ratio of the responsivity at 220 nm and 400 nm (R220/R400) is about 1.1×105, and the solar-blind rejection ratio at 220 nm and 280 nm (R220/R280) is 3.0×104. These results confirm that the photodetector has a relatively high spectral selectivity to solar-blind irradiation. To evaluate the reliability property of the photodetector, time-resolved photocurrent of the device was measured with on/off the incident light repeatedly. Figure 2(d) shows the current-time curves of the photodetector under different working voltage with the turn-on and turn-off time in 5-second intervals. The current can be reversibly modulated by irradiation and the photocurrents and dark currents are reproducible and stable at different working voltage, indicating the photodetector has high stability and repeatability, which is essential for future applications. Response time is one main parameter of photodetectors. The transient photoresponse properties of the device were measured under the under the excitation of a 213 nm pulse laser and the results are demonstrated in Fig. 3. The response time is defined as the time for the photocurrent increase from 10% to 90% of the maximum and recovery time is defined as the time for the photocurrent to decrease from 90% to 10% of the maximum. The response time and recovery time of the photodetector are 1.3 µs and 203 µs, respectively. For comparison, the main parameters are summarized with other pervious reported diamond-based photodetectors in Table 1. The responsivity and response time of the device are both among the best values.
Fig. 2. (a) I-V characteristics of the diamond photodetector in dark and under UV irradiation. (b) Photoresponse spectra of the diamond photodetector at different bias, and the inset is the optical image of the photodetector. (c) The response spectrum at 60 V in a semilogarithmic coordinate. (d) Time-resolved photocurrent of the photodetector with light on and off repeatedly at 10-40 V bias.
Fig. 3. Normalized transient photoresponse characteristics measured under the excitation of 213 nm pulse laser at 20 V bias.
Table 1. Comparison of the main parameters for the reported diamond photodetectors.
To evaluate the solar-blind communication capability of the photodetector, a communication system was constructed employing the diamond-based photodetector as a signal receiver. The schematic illustration of the communication system is shown in Fig. 4(a). The input signal is solar-blind light with wavelength between 215 and 225 nm from a broadband laser-driven light source and filtered by a 220 nm bandpass filter. The light beam can be modulated on and off periodically by a shutter controlled by Transistor-Transistor Logic (TTL) input signals. The TTL signals was generated by a multifunction I/O device (NI PCIe-6363) which was controlled by a tailored LabVIEW program. In order to transmit information, the letters in the message are converted into ASCII codes by the program initially. The I/O device output TTL signals according to the ASCII codes— “1” corresponds “5 V”, “0” corresponds “0 V”. The photodetector was connected in series to a 3 MΩ resistor at a supply voltage of 20 V. As the shutter is opened, light beam passes through the shutter and irradiates onto the photodetector, the currents increase due to the generation of photo-induced carriers and then the voltage of the resistor increases and stay at a high voltage state, which is defined as “1” in the binary version. When the shutter is closed, the light is blocked, thus the current decreases and the voltage obtained from the resistor stay at a low voltage state, which is defined as “0”. So that, the ASCII codes can be obtained by monitoring the output voltage of the resistor. Finally, the transmitted message can be read out through converting ASCII codes to the corresponding letter.
Fig. 4. (a) Schematic illustration of the solar-blind communication system employing the diamond photodetector as signal receiver. (b) The input signals which control the mechanical shutter to modulate the beam. (c) The output signals which are the voltage value across the resistor in series with the photodetector.
To test the communication system, we tried to transfer three letters “PHY” and inserted a “0” between each letter as a separator. The TTL signal input to the shutter and the output voltage signal of the resistor are shown in Figs. 4(b) and 4(c), respectively. The shape of the output signal curve is nearly the same as the input signal. Output voltage signals with different data transfer rates show in Fig. 5(a). The signal shows excellent integrity without obvious distortion when the data transfer rate is up to 10 bps (the max operating frequency of the shutter). Normalized response as a function of input signal frequency from 10 Hz to 4000 Hz is revealed in Fig. 5(b). The -3 dB cutoff frequency can be applied to evaluate the response time of a photodetector, at which the responsivity of the photodetector is half of that at state conditions [33]. The -3 dB cutoff frequency of the photodetector was calculated to be 1970 Hz, which is comparable to previously reported wide bandgap based photodetectors [19,27,34–36].
Fig. 5. (a) The output signals of the communication system with different data transfer rates. (b) Normalized response as a function of input signal frequency and the -3 dB point is marked with the dash line.
In order to investigate the anti-interference ability of the solar-blind communication system, another input signal channel with a 632 nm laser (2 mW) as light source was introduced to the system. The schematic illustration of the communication system with two input channels is shown in Fig. 6(a). The two light beams were both focused onto the photodetector but modulated by two independent shutters. Different messages were inputted into the two channels simultaneously, as shown in Figs. 6(b) and 6(c). The output signal acquired from the diamond photodetector is illustrated in Fig. 6(d). As compare to the two curves of the input TTL signals (“CAT” in Channel 1 and “DOG” in Channel 2), the curve of the output voltage of the resistor is as the same as “DOG”. No interference signals from channel 1 was observed, indicating an excellent anti-interference ability of the solar-blind communication system, which is resulted from the high spectral selectivity of the diamond photodetector.
Fig. 6. (a) Schematic illustration of the communication system with two input channels using solar-blind light and 632 nm laser as light source, respectively. (b) The input signals of channel 1 which control the mechanical shutter 1. (c) The input signals of channel 2 which control the mechanical shutter 2. (d) The output signals of the communication system acquired from the voltage across the resistor in series with the photodetector.
4. Conclusions
In conclusion, solar-blind communication systems have been constructed by employing a diamond-based photodetector as the signal receiver for the first time. The photodetector was fabricated by a single crystalline CVD diamond wafer with interdigital Au electrodes. The peak responsivity is 13.0 A/W at 222 nm with a low dark current of 0.93 nA under 60 V bias. The UV/visible rejection ratio is about 5 orders of magnitude with a sharp cutoff wavelength at 225 nm. In addition, the device has a fast response speed with rise time shorter than 1.3 µs and the decay time of 203 µs, respectively. The communication system shows high anti-interference ability with a -3 dB cutoff frequency of 1970 Hz. The results reported in this paper reveal that the diamond photodetector is a promising candidate for solar-blind communication system, thus may push forward the future applications of diamond-based photodetectors.
Funding
National Natural Science Foundation of China (61604132, U1604263, U1804155); China National Funds for Distinguished Young Scientists (61425021); National Key R&D Program of China (2018YFB0406500).
References
1. L. Y. Wei, C. W. Hsu, C. W. Chow, and C. H. Yeh, “20.231 Gbit/s tricolor red/green/blue laser diode based bidirectional signal remodulation visible-light communication system,” Photonics Res. 6(5), 422–426 (2018). [CrossRef]
2. S. Hu, L. Mi, T. Zhou, and W. Chen, “35.88 attenuation lengths and 3.32 bits/photon underwater optical wireless communication based on photon-counting receiver with 256-PPM,” Opt. Express 26(17), 21685–21699 (2018). [CrossRef]
3. Z. Xu, G. Chen, F. Abou-Galala, and M. Leonard, “Experimental performance evaluation of non-line-of-sight ultraviolet communication systems-art,” Proc. SPIE 6709, 67090Y (2007). [CrossRef]
4. L. Guo, D. Meng, K. Liu, X. Mu, W. Feng, and D. Han, “Experimental research on the MRC diversity reception algorithm for UV communication,” Appl. Opt. 54(16), 5050–5056 (2015). [CrossRef]
5. Z. Ghassemlooy, P. A. Haigh, F. Arca, S. F. Tedde, O. Hayden, I. Papakonstantinou, and S. Rajbhandari, “Visible light communications: 375 Mbits/s data rate with a 160 kHz bandwidth organic photodetector and artificial neural network equalization [Invited],” Photonics Res. 1(2), 65–68 (2013). [CrossRef]
6. D. Schall, D. Neumaier, M. Mohsin, B. Chmielak, J. Bolten, C. Porschatis, A. Prinzen, C. Matheisen, W. Kuebart, B. Junginger, W. Templ, A. L. Giesecke, and H. Kurz, “50 GBit/s Photodetectors Based on Wafer-Scale Graphene for Integrated Silicon Photonic Communication Systems,” ACS Photonics 1(9), 781–784 (2014). [CrossRef]
7. T. Mueller, F. Xia, and P. Avouris, “Graphene photodetectors for high-speed optical communications,” Nat. Photonics 4(5), 297–301 (2010). [CrossRef]
8. Z. Xu and B. M. Sadler, “Ultraviolet communications: potential and state-of-the-art,” IEEE Commun. Mag. 46(5), 67–73 (2008). [CrossRef]
9. J. Lu, X. Sheng, G. Tong, Z. Yu, X. Sun, L. Yu, X. Xu, J. Wang, J. Xu, Y. Shi, and K. Chen, “Ultrafast solar-blind ultraviolet detection by inorganic perovskite CsPbX3 quantum dots radial junction architecture,” Adv. Mater. 29(23), 1700400 (2017). [CrossRef]
10. S. Oh, Y. Jung, M. A. Mastro, J. K. Hite, C. R. Eddy Jr., and J. Kim, “Development of solar-blind photodetectors based on Si-implanted β-Ga2O3,” Opt. Express 23(22), 28300–28305 (2015). [CrossRef]
11. J. A. Sanderson, “Optics at the Naval Research Laboratory,” Appl. Opt. 6(12), 2029–2043 (1967). [CrossRef]
12. L. X. Qian, Z. H. Wu, Y. Y. Zhang, P. T. Lai, X. Z. Liu, and Y. Li, “Ultrahigh-responsivity, rapid-recovery, solar-blind photodetector based on highly nonstoichiometric amorphous gallium oxide,” ACS Photonics 4(9), 2203–2211 (2017). [CrossRef]
13. F. Xie, H. Lu, D. Chen, X. Ji, F. Yan, R. Zhang, Y. Zheng, L. Li, and J. Zhou, “Ultra-low dark current AlGaN-based solar-blind metal-semiconductor-metal photodetectors for high-temperature applications,” IEEE Sens. J. 12(6), 2086–2090 (2012). [CrossRef]
14. W. Zhang, J. Xu, W. Ye, Y. Li, Z. Qi, J. Dai, Z. Wu, C. Chen, J. Yin, J. Li, H. Jiang, and Y. Fang, “High-performance AlGaN metal-semiconductor-metal solar-blind ultraviolet photodetectors by localized surface plasmon enhancement,” Appl. Phys. Lett. 106(2), 021112 (2015). [CrossRef]
15. H. Zhu, C. X. Shan, L. K. Wang, J. Zheng, J. Y. Zhang, B. Yao, and D. Z. Shen, “Metal-oxide-semiconductor-structured MgZnO ultraviolet photodetector with high internal gain,” J. Phys. Chem. C 114(15), 7169–7172 (2010). [CrossRef]
16. H. Chen, P. Yu, Z. Zhang, F. Teng, L. Zheng, K. Hu, and X. Fang, “Ultrasensitive self-powered solar-blind deep-ultraviolet photodetector based on all-solid-state polyaniline/MgZnO bilayer,” Small 12(42), 5809–5816 (2016). [CrossRef]
17. Y. C. Chen, Y. J. Lu, Q. Liu, C. N. Lin, J. Guo, J. H. Zang, Y. Z. Tian, and C. X. Shan, “Ga2O3 photodetector arrays for solar-blind imaging,” J. Mater. Chem. C 7(9), 2557–2562 (2019). [CrossRef]
18. M. Chen, J. Ma, P. Li, H. Xu, and Y. Liu, “Zero-biased deep ultraviolet photodetectors based on graphene/cleaved (100) Ga2O3 heterojunction,” Opt. Express 27(6), 8717–8726 (2019). [CrossRef]
19. Y. C. Chen, Y. J. Lu, C. N. Lin, Y. Z. Tian, C. J. Gao, L. Dong, and C. X. Shan, “Self-powered diamond/β-Ga2O3 photodetectors for solar-blind imaging,” J. Mater. Chem. C 6(21), 5727–5732 (2018). [CrossRef]
20. M. Liao and Y. Koide, “High-performance metal-semiconductor-metal deep-ultraviolet photodetectors based on homoepitaxial diamond thin film,” Appl. Phys. Lett. 89(11), 113509 (2006). [CrossRef]
21. A. Saravanan, B. R. Huang, J. C. Lin, G. Keiser, and I. N. Lin, “Fast photoresponse and long lifetime UV photodetectors and field emitters based on ZnO/ultrananocrystalline diamond films,” Chem. - Eur. J. 21(45), 16017–16026 (2015). [CrossRef]
22. R. D. Mckeag and R. B. Jackman, “Diamond UV photodetectors: Sensitivity and speed for visible blind applications,” Diamond Relat. Mater. 7(2-5), 513–518 (1998). [CrossRef]
23. Y. J. Lu, C. N. Lin, and C. X. Shan, “Optoelectronic diamond: growth, properties, and photodetection applications,” Adv. Opt. Mater. 6(20), 1800359 (2018). [CrossRef]
24. L. X. Wang, X. K. Chen, G. Wu, W. T. Guo, S. Z. Cao, K. W. Shang, and W. H. Han, “The mechanism of persistent photoconductivity induced by minority carrier trapping effect in ultraviolet photo-detector made of polycrystalline diamond film,” Thin Solid Films 520(2), 752–755 (2011). [CrossRef]
25. J. S. Liu, C. X. Shan, B. H. Li, Z. Z. Zhang, C. L. Yang, D. Z. Shen, and X. W. Fan, “High responsivity ultraviolet photodetector realized via a carrier-trapping process,” Appl. Phys. Lett. 97(25), 251102 (2010). [CrossRef]
26. Y. Koide, M. Liao, and J. Alvarez, “Thermally stable solar-blind diamond UV photodetector,” Diamond Relat. Mater. 15(11-12), 1962–1966 (2006). [CrossRef]
27. M. Liao, Y. Koide, and J. Alvarez, “Single Schottky-barrier photodiode with interdigitated-finger geometry: Application to diamond,” Appl. Phys. Lett. 90(12), 123507 (2007). [CrossRef]
28. Z. Liu, J. P. Ao, F. Li, W. Wei, J. Wang, J. Zhang, and H. X. Wang, “Fabrication of three dimensional diamond ultraviolet photodetector through down-top method,” Appl. Phys. Lett. 109(15), 153507 (2016). [CrossRef]
29. C. N. Lin, Y. J. Lu, X. Yang, Y. Z. Tian, C. J. Gao, J. L. Sun, L. Dong, F. Zhong, W. D. Hu, and C. X. Shan, “Diamond-based all-carbon photodetectors for solar-blind imaging,” Adv. Opt. Mater. 6(15), 1800068 (2018). [CrossRef]
30. L. Wang, X. Chen, G. Wu, W. Guo, Y. Wang, S. Cao, K. Shang, and W. Han, “Study on trapping center and trapping effect in MSM ultraviolet photo-detector on microcrystalline diamond film,” Phys. Status Solidi A 207(2), 468–473 (2010). [CrossRef]
31. X. Chang, Y. Wang, X. Zhang, Z. Liu, J. Fu, S. Fan, R. Bu, J. Zhang, W. Wang, H. Wang, and J. Wang, “UV-photodetector based on NiO/diamond film,” Appl. Phys. Lett. 112(3), 032103 (2018). [CrossRef]
32. Z. Liu, F. Li, S. Li, C. Hu, W. Wang, F. Wang, F. Lin, and H. Wang, “Fabrication of UV Photodetector on TiO2/Diamond Film,” Sci. Rep. 5(1), 14420 (2015). [CrossRef]
33. F. P. G. D. Arquer, A. Armin, P. Meredith, and E. H. Sargent, “Corrigendum: Solution-processed semiconductors for next-generation photodetectors,” Nat. Rev. Mater. 2(3), 16100 (2017). [CrossRef]
34. Z. Liu, J. P. Ao, F. Li, W. Wang, J. Wang, J. Zhang, and H. X. Wang, “Photoelectrical characteristics of ultra thin TiO2/diamond photodetector,” Mater. Lett. 188, 52–54 (2017). [CrossRef]
35. Y. Li, F. D. Valle, M. Simonnet, I. Yamada, and J. J. Delaunay, “Competitive surface effects of oxygen and water on UV photoresponse of ZnO nanowires,” Appl. Phys. Lett. 94(2), 023110 (2009). [CrossRef]
36. K. Liu, M. Sakurai, M. Liao, and M. Aono, “Giant improvement of the performance of ZnO nanowire photodetectors by Au nanoparticles,” J. Phys. Chem. C 114(46), 19835–19839 (2010). [CrossRef]
