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Abstract
A self-powered ultraviolet (UV) photodetector (PD) based two-dimensional NiO nanostructure and GaN film was fabricated using a low-temperature aqueous method. The heterostructure exhibited a prominent performance for UV detection response with self-powered properties. The results showed the NiO nanosheet/GaN heterojunction structure can simultaneously enhance the surface area and promote the charge transport. Furthermore, it provides a facile route for the growth of a NiO nanosheet/GaN heterostructure for low cost UV detector applications at remarkably low temperatures.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recently years, GaN-based wide bandgap semiconductors have attracted much attention due to their excellent optical properties [1–10]. However, high quality p-GaN is hard to obtained yet, which restricts the fabrication of GaN-based homojunction devices to some extent.
Meanwhile, for the GaN-based UV detector, the main problem is the photoinduced increase in the conductivity persists for a long time after the light is off. It was called persistent photoconductivity (PPC) effects [11,12]. The origin of the PPC is usually attributed the metastability of intrinsic defects, such as Ga vacancies and the defect [13,14]. In recent year, there are many research results showed that nanostructures based detectors have excellent responsivity [15,16]. However, these photodetectors usually require an external bias. Meanwhile, the device often has slow response time [17–19].
As an important natural p-type material, NiO has been utilized as a hole transport layer material because of its high hole mobility [20–22]. Compared to most of NiO nanostructures which are usually prepared by high temperature methods, low temperature growth way has been developed recently through solution processes [23–25]. Meanwhile, the fabricated NiO-based heterojunction photodetectors display poor device performance with large leakage currents. It was caused by Ni diffusion into the substrate during the high temperature growth process. One possible way to overcome these problems is to use low temperature method. Till now, the p-n photodetector based on p-NiO and n-GaN is less reported by using the low temperature aqueous method [26].
In this work, simple solution method was adopted to fabricate UV detector based on the heterojunction consisting of p-type NiO nanosheets and n-type GaN film. The device shows self-powered properties very fast temporal response, high sensitivity and ultra-fast photoresponse speed under UV light illumination.
2. Experiment
The n-GaN substrate was prepared by metal–organic chemical vapor deposition (MOCVD) on c-plane sapphire. 2 μm-thick n-type GaN (n = 5.0 × 1018cm−3) layer which was obtained by using low-temperature GaN buffer layer. The NiO/GaN heterostructure was grown using two step aqueous method, the similar growth method has been reported previously [27–29]. Firstly, deposition of the NiO seed layer by spin-coating method. The seeding solution contained nickel acetate solution with alcohols as solvent. After spin-coating, the sample then annealed at 200°C for 30 min . For growth of NiO nanostructures, an equimolar (0.03M) aqueous solution of nickel acetate (Ni(CH3COO)2) and hexamethyltetramine (C6H12N4) was used to act as reaction source with deionized water as solvent. The seeded n-GaN/sapphire substrate was put into the Ni(CH3COO)2 and C6H12N4 reaction solution. After 5h hours aqueous growth at 90°C, the sample was fully cleaned and then dried in an air atmosphere. For the fabrication of the NiO nanostructures layer, the sample was annealed at 400 °C for 1 h to improve the quality.
The structural and morphology characteristics of as-grown NiO nanostructures were investigated using X-ray diffraction (XRD), scanning electron microscopy (FE-SEM, Hitachi S-4800). The devices was fabricated using ITO/glass as contacting electrode, the similar way has been reported previously [30,31].The UV photoresponse measurement was performed without bias voltage under UV lamp (λ = 365 nm, 30 mW/cm2) illumination. For the photoresponsivity characterization, a Xe-arc lamp combing with monochromator was used as the light source. And all the photoresponse characteristics of the NiO nanosheets/GaN photodetector were measured using a Keithley 2400 voltage source instrument.
3. Results and discussion
Figure 1 displayed the XRD spectra of NiO nanostructures grown on the n-GaN, except for the peaks from the n-GaN/sapphire template. The observed peaks corresponding to (111), (200), (220), and (311) planes, it confirmed the structure of cubic-NiO (JCPDS cards No. 47-1049). In addition, the peaks arise from Ni(OH)2 were also found, indicating that Ni(OH)2 was not completely decomposed at 400°C for 1 h.
Figure 2(a)(b) showed the typical SEM images of the as-grown NiO nanostructures with under low and high magnification. From the Fig. 2(a), it can be obviously seen that the NiO nanostructures show sheet-like structures. Meanwhile, the GaN surface is uniformly covered by NiO sheet-like morphology which is very similar to graphene-like two-dimensional sheets. Furthermore, the nanosheets are interconnected and densely packed. It can be seen that the thickness of these sheets varies from 30 to 50 nm according to the Fig. 2(b). Figure 2(c) displayed the cross-sectional image of the NiO/GaN structure, the thickness of the NiO layer is around 1.5μm.
Fig. 2 (a)(b) FE-SEM images of the as-grown NiO/GaN heterojunction samples with different magnifications. (c) The cross-sectional image.
Figure 3 illustrated the schematic structure of the NiO nanosheets/GaN photodetector. For the device structure, the top layer is NiO sheet-like nanostructure with random distributions. In order to maximize contact area between electrode and NiO nanostructure. ITO/glass was adopted to act as electrode, it can enhance the contacting area with the NiO nanostructures. The tip of NiO/GaN was directly contacted with the ITO/glass substrate and simply fixed by wood clip to avoid damaging the NiO nanostructure.
The current-voltage characteristic of the photodetector based on NiO nanosheets /GaN sample under dark and UV illumination (365 nm, 30mW/cm2) was shown in Fig. 4. The I-V curve was performed under bias from −3 to 3 V . It can be seen from the graph that the dark current is very low which indicates the high resistivity of the sample. For the device structure, the top layer is NiO sheet-like nanostructure with random distributions. In order to maximize contact area between electrode and NiO nanostructure, ITO/glass electrode was adopted to enhance the contacting area and form conductive circuit. For the GaN layer, Ag paste was adopted to act as electrode, which will form a relatively high barrier. When the NiO/GaN heterostructure under bias voltage, the energy band of GaN upward bend, and the Ec is enough to accumulate electrons at the GaN side. Meanwhile, under a relatively low driving voltage, the holes in the valence band of the NiO layer could be injected into the GaN side. When the NiO/GaN heterostructure was illuminated with UV light of 365 nm, the photocurrent presented significant increase, it indicated that more photons will be absorbed UV region. It can compensate for the effect of the barrier height. Moreover, the measured current of the NiO nanosheets/GaN heterostructure was increased from 3.3 × 10−8 to 1.2 × 10−5 under UV illumination without applied voltage. The sensitivity (S) of the device to the UV light can be calculated using the following formula:
Fig. 4 Current-voltage characteristics of NiO/GaN heterojunction under dark and illuminated conditions.
The sensitivity of NiO nanosheets/GaN photodetector was found to be 2.7 × 103 without bias voltage.
Figure 5 displayed the photoresponse spectra of the NiO nanosheets/GaN photoconductor at room temperature. It can be obviously seen that the device based on NiO nanosheets/GaN has excellent UV light detection selectivity, which indicated the device can be used for UV detection application. The maximum of photoresponsivity peaked at 365 nm. Moreover, the device showed the maximum of photoresponsivity about 0.5 mA/W without bias voltage.
Figure 6(a) showed the photoresponse of the UV detector as a function of time when the UV illumination was turned on and off periodically without bias voltages. The on and off time duration are both 10s which was controlled by the shutter. It can be seen that the UV detector can work as a self-powered device. Moreover, the heterojunction exhibited a nearly identical and repeatable response after multiple illumination cycles. The ratio of photocurrent to dark current is about 3.4 × 103. Figure 6(b) displayed single period of the device response process. The photocurrent increases very rapidly upon exposure to UV radiation, stays essentially constant during the UV exposure, and decays very fast when UV is turned off. The rectangle response profile indicated the rapid characteristic of the self-powered device. Figure 6(c)(d) showed the enlarged current-time curve of the UV device without bias of with 365 nm UV light illumination. The fast photoresponse performance is very stable and reproducible. No degeneration effect and persistent photoconductivity behavior found during the UV light on/off interval process. The rise time (tr) is ≈40 ms, which was defined to the photocurrent to reach from 10% to 90% of its maximum value And the decay time (td) for the photocurrent to decrease from 90% to 10% of the peak value is ≈40 ms.
Fig. 6 (a) The current-time curves of the photodetector under on/off switch illumination without bias voltages. (b) Single period cycle of the NiO/GaN photoconductor (c, d) Enlarged view of a single on/off cycle of the NiO/GaN photoconductor.
Figure 7 showed the time-resolve photocurrent response under the illumination of 365 nm light with various light intensities. It can be obviously seen that the device based on the NiO/GaN structures is very sensitive to the UV light intensities. The measured absolute current increased significantly with light illumination, which indicated that the device can be used to detect the UV light intensity.
Fig. 7 Photocurrent of NiO/GaN heterostructure device measured as a function of the excitation intensity without applied bias.
Meanwhile, the relationship between the photocurrent and light intensity was shown in the Fig. 8. The relationship between the UV light intensity and photocurrent obeyed the power law function Ilight∝Pk, the power law factor k is 0.7 for NiO/GaN structure. The nonunity power law factor can be attributed to process of electron-hole generation, hole-trapping and recombination [32]. It can be found that NiO/GaN heterostructure possesses high UV response performance, especially for current on/off ratio and response speed compare with other results [33].
The fast-response mechanism is due to the p–n junction formation between NiO and GaN and the sheet-like structure of NiO. The energy band diagram is illustrated in Fig. 9. The electron affinity potentials of GaN and NiO are assumed to 4.2 and 1.8ev, respectively [34]. The ΔEc can be calculated by using ΔEc = χGaN-χNiO = 2.4eV. ΔEV can be calculated by using ΔEv = (χGaN + EgGaN)-(χNiO + EgNiO) = 2.1eV. Where χGaN and χNiO are the electron affinity potentials of GaN and NiO, EgGaN(3.40eV) and EgNiO(3.69eV) are the band gaps of GaN and NiO, respectively [26]. When NiO/GaN heterojunction is formed, space charge region will be developed near the interface due to the flow of electrons. Built-in electric field will be formed to balance the electron drift. Due to the well-aligned energy-band structure of the NiO/GaN heterojunction, it will cause photovoltaic effect. Meanwhile, the NiO/GaN heterostructure can confine electron in the n-GaN film layer. Similarly, the holes in the valence band of p-NiO nanostructures layer could be injected into the n-GaN layer without driving voltage. The NiO/GaN heterojunction structure could simultaneously enhance the surface area and promote the charge transport. As efficient internal driving force, built-in electric field can separate the photogenerated electron-hole pairs under UV light illumination, which will cause increase of the photocurrent. Moreover, the high UV response can be attributed to the NiO sheet-like structure which can increase the surface area, and enhance the number of active sites on the surface and high absorption in the UV region. Furthermore, it can also contribute to the fast response speed due to the high charge mobility due to the unique sheet-like NiO nanostructures .
4. Conclusion
In summary, self-powered UV detector based on NiO/GaN heterostructure was obtained by simple way. The detector exhibited a prominent performance for UV detection response. The NiO/GaN heterojunction structure could simultaneously enhance the surface area and promote the charge transport. These results may offer great potential in developing large area growth NiO/GaN heterostructure for low cost UV detector applications.
Funding
National Natural Science Foundation of China (Grant No. 11474045, 51102036, 61474031); Fundamental Research Funds for the Central Universities (DC201502080201,WD01144); Program for Liaoning Excellent Scientific and Technological Talents in University (LR2015017); Liaoning Provincial Natural Science Foundation of China (201602202); Guangxi Key Laboratory of Precision Navigation Technology and Application, Guilin University of Electronic Technology (No.DH201813).
References
1. H. Kind, H. Q. Yan, B. Messer, and P. Yang, “Nanowire ultraviolet photodetectors and optical switches,” Adv. Mater. 14(2), 158–160 (2002). [CrossRef]
2. K. Liu, M. Sakurai, and M. Aono, “ZnO-based ultraviolet photodetectors,” Sensors (Basel) 10(9), 8604–8634 (2010). [CrossRef] [PubMed]
3. M. Wang, Y. Hu, J. Han, R. Guo, H. X. Xiong, and Y. D. Yin, “TiO2/NiO hybrid shells:P-n junction photocatalysts with enhanced activity under visible light,” J. Mater. Chem. A Mater. Energy Sustain. 3(41), 20727–20735 (2015). [CrossRef]
4. Y. Z. Chiou and J. J. Tang, “GaN photodetectors with transparent indium tin oxide electrodes,” Japn, J. Appl. Phys. 43(7), 4146–4169 (2004). [CrossRef]
5. J. H. Ha, S. M. Kang, S. H. Park, H. S. Kim, Y. H. Cho, J. H. Lee, N. H. Lee, J. B. Kim, and Y. K. Kim, “Annealing effect of the 6H-SiC semiconductor detector for alpha particles,” Radiat. Meas. 43(2–6), 1140–1143 (2008). [CrossRef]
6. L. V. Schalkwyk, W. E. Meyer, J. M. Nel, F. D. Auret, and P. N. M. Ngoepe, “Implementation of an AlGaN-based solar-blind UV four-quadrant detector,” Physica B 439, 93–96 (2014). [CrossRef]
7. D. B. Li, K. Jiang, X. J. Sun, and C. L. Guo, “AlGaN Photonics:Recent Advances in Materials and Ultraviolet Devices,” Adv. Opt. Photonics 10(1), 43–110 (2018). [CrossRef]
8. D. B. Li, X. J. Sun, Y. P. Jia, M. I. Stockman, H. P. Paudel, H. Song, H. Jiang, and Z.-M. Li, “Direct observation of localized surface plasmon field enhancement by Kelvin probe force microscopy,” Light Sci. Appl. 6(8), e17038 (2017). [CrossRef] [PubMed]
9. X. Sun, D. Li, Z. Li, H. Song, H. Jiang, Y. Chen, G. Miao, and Z. Zhang, “High spectral response of self-driven GaN-based detectors by controlling the contact barrier height,” Sci Rep-UK. 5(1), 16819 (2015). [CrossRef]
10. D. B. Li, X. Sun, H. Song, Z. Li, H. Jiang, Y. Chen, G. Miao, and B. Shen, “Effect of asymmetric Schottky barrier on GaN-based metal- semiconductor-metal ultraviolet detector,” Appl. Phys. Lett. 99(26), 261102 (2011). [CrossRef]
11. M. De Vittorio, B. Potì, M. T. Todaro, M. C. Frassanito, A. Pomarico, A. Passaseo, M. Lomascolo, and R. Cingolani, “High temperature characterization of GaN-based photodetectors,” Sens. Actuators A Phys. 113(3), 329–333 (2004). [CrossRef]
12. P. Calvani, M. Girolami, S. Carta, M. C. Rossi, and G. Conte, “Optoelectronic performance of GaN-based UV photodetectors,” Nucl. Instrum. Meth. A. 610(1), 311–313 (2009). [CrossRef]
13. X. Y. Guo, T. L. Williamson, and P. W. Bohn, “Enhanced ultraviolet photoconductivity in porous GaN prepared by metal-assisted electroless etching,” Solid State Commun. 140(3–4), 159–162 (2006). [CrossRef]
14. N. Prakash, G. Kumar, A. Barvat, K. Anand, B. Choursia, P. Pal, and S. P. Khanna, “Exploration of trap levels in GaN and Al0.2Ga0.8N layers by temperature-dependent photoconductivity measurement,” Materials Today: Proceedings. 5(1), 2132–2138 (2018). [CrossRef]
15. Y. Liu, X. Zhang, J. Su, H. Li, Q. Zhang, and Y. Gao, “Ag nanoparticles@ZnO nanowire composite arrays: an absorption enhanced UV photodetector,” Opt. Express 22(24), 30148–30155 (2014). [CrossRef] [PubMed]
16. Y. Li, C. Cheng, X. Dong, and J. S. Gao, “Facile fabrication of UV photodetectors based on ZnO nanorod networks across trenched electrodes,” J. Semicond. 30(6), 38–41 (2009).
17. Y. Q. Bie, Z. M. Liao, H. Z. Zhang, G. R. Li, Y. Ye, Y. B. Zhou, J. Xu, Z. X. Qin, L. Dai, and D. P. Yu, “Self-powered, ultrafast, visible-blind UV detection and optical logical operation based on ZnO/GaN nanoscale p-n junctions,” Adv. Mater. 23(5), 649–653 (2011). [CrossRef] [PubMed]
18. J. Qi, X. Hu, Z. Wang, X. Li, W. Liu, and Y. Zhang, “A self-powered ultraviolet detector based on a single ZnO microwire/p-Si film with double heterojunctions,” Nanoscale 6(11), 6025–6029 (2014). [CrossRef] [PubMed]
19. X. W. Fu, Z. M. Liao, Y. B. Zhou, H. C. Wu, Y. Q. Bie, J. Xu, and D. P. Yu, “Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector,” Appl. Phys. Lett. 100(22), 223114 (2012). [CrossRef]
20. B. Parida, S. Kim, M. Oh, S. Jung, M. Baek, J. H. Ryou, and H. Kim, “Nanostructured-NiO/Si heterojunction photodetector,” Mater. Sci. Semicond. Process. 71(15), 29–34 (2017). [CrossRef]
21. S. Anitha, M. Suganya, D. Prabha, J. Srivind, S. Balamurugan, and A. R. Balu, “Synthesis and characterization of NiO-CdO composite materials towards photoconductive and antibacterial applications,” Mater. Chem. Phys. 211(1), 88–96 (2018). [CrossRef]
22. J. Y. Chen, Y. C. Chen, C. M. Wei, and Y. F. Chen, “Magnetic field modulation of photonic bandgap on FeCo/NiO half-shell array,” Opt. Lett. 36(13), 2563–2565 (2011). [CrossRef] [PubMed]
23. X. Wang, H. Pu, D. Hu, Y. Zang, J. Hu, Y. Yang, and C. Chen, “Preparation of p-NiO/n-SiC heterojunction on 4H-SiC substrate,” Mater. Lett. 227(15), 315–317 (2018). [CrossRef]
24. A. A. Ahmed, M. Devarajan, and N. Afzal, “Fabrication and characterization of high performance MSM UV photodetector based on NiO film,” Sensor. Actuat. A-Phys 262(1), 78–86 (2017).
25. N. P. Klochko, V. R. Kopach, I. I. Tyukhov, D. O. Zhadan, K. S. Klepikova, G. S. Khrypunov, S. I. Petrushenko, V. M. Lyubov, M. V. Kirichenko, S. V. Dukarov, and A. L. Khrypunova, “Metal oxide heterojunction (NiO/ZnO) prepared by low temperature solution growth for UV-photodetector and semi-transparent solar cell,” Sol. Energy 164(4), 149–159 (2018). [CrossRef]
26. L. Li, Z. Liu, L. Wang, B. zhang, Y. Liu, and J.-P. Ao, “Self-powered GaN ultraviolet photodetectors with p-NiO electrode grown by thermal oxidation,” Mater. Sci. Semicond. Process. 76(15), 61–64 (2018). [CrossRef]
27. Y. Xiang, N. Yu, J. Liu, and L. Cao, “Simple fabrication of ZnO nanosheets/p-GaN heterostructure and ultraviolet detection,” Physica E 102, 29–32 (2018). [CrossRef]
28. N. S. Yu, B. Dong, W. W. Yu, B. Y. Hu, Y. Q. Zhang, and Y. Cong, “Investigations of ZnO nanostructures grown on patterned sapphire using different precursors in aqueous solutions,” Appl. Surf. Sci. 258(15), 5729–5732 (2012). [CrossRef]
29. S. Huang, N. Yu, T. Wang, and J. Li, “Simple fabrication of UV photo-detector based on NiO/ZnO structure grown by hydrothermal process,” Funct. Mate. Lett. 11(02), 1850045 (2018). [CrossRef]
30. M. Dong, Y. Wang, Z. Li, Z. Weng, and N. Yu, “Simple Fabrication of Homogeneous ZnO Core/Shell Nanorod Arrays for Ultraviolet Photodetectors,” J. Nanosci. Nanotechnol. 18(8), 5686–5691 (2018). [CrossRef] [PubMed]
31. H. Li, Y. Li, G. Xiao, X. Gao, Q. Li, Y. Chen, T. Fu, T. Sun, F. Zhang, and N. Yu, “Simple Fabrication ZnO/β-Ga2O3 core/shell nanorod arrays and their photoresponse properties,” Opt. Mater. Express 8(4), 794 (2018). [CrossRef]
32. L. Li, Z. Lou, and G. Shen, “Hierarchical CdS nanowires based rigid and flexible photodetectors with ultrahigh sensitivity,” ACS Appl. Mater. Interfaces 7(42), 23507–23514 (2015). [CrossRef] [PubMed]
33. K. S. Gour, B. Bhattacharyya, O. P. Singh, A. K. Yadav, S. Husale, and V. N. Singh, “Nanostructured Cu2ZnSnS4 (CZTS) thin film for self-powered broadband photodetection,” J. Alloys Compd. 735, 285–290 (2018). [CrossRef]
34. L. Li, W. Wang, L. He, X. Zhang, Z. Wu, and Y. Liu, “Determination of band offsets between p-NiO gate electrode and unintentionally doped GaN for normally-off GaN power device,” J. Alloys Compd. 728, 400–403 (2017). [CrossRef]
