- Nano Express
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Metal-Semiconductor-Metal Near-Ultraviolet (~380 nm) Photodetectors by Selective Area Growth of ZnO Nanorods and SiO2 Passivation
© The Author(s). 2016
Received: 12 February 2016
Accepted: 5 July 2016
Published: 15 July 2016
Metal-semiconductor-metal near-ultraviolet (NUV) photodetectors (PDs) based on zinc oxide (ZnO) nanorods (NRs), operating at λ ~ 380 nm, were fabricated using conventional photolithography and hydrothermal synthesis processes. The vertically aligned ZnO NRs were selectively grown in the channel area of PDs. The performance of ZnO NR-based NUV PDs was optimized by varying the solution concentration and active channel width (W ch). For the fabricated samples, their electrical and photoresponse properties were investigated under the dark state and the illumination at wavelength of ~380 nm, respectively. For the device (W ch = 30 μm) with ZnO NRs at 25 mM, the highest photocurrent of 0.63 mA was obtained with the on/off ratio of 1720 at the bias of 5 V. The silicon dioxide passivation was also carried out to improve the photoresponse properties of PDs. The passivated devices exhibited faster rise and reset times rather than those of the unpassivated devices.
Zinc oxide (ZnO) nanostructures (NSs, i.e., thin films, nanowires, and nanorods) have been extensively investigated in various applications including optoelectronics, transparent electrodes, and sensors due to their high transparency in the visible range, low cost, radiation hardness, and thermal/chemical stabilities [1–12]. Particularly, ZnO is a promising metal oxide semiconductor material for short wavelength (ultraviolet (UV) and blue) applications including photodetectors (PDs), lasers, and light-emitting diodes due to its excellent optical properties such as wide direct bandgap of 3.37 eV and large exciton binding energy of 60 meV at room temperature. In addition, the ZnO NSs have larger surface-to-volume ratio and higher quantum efficiency compared to the bulk ZnO, which makes themselves as a promising candidate for UV PDs [1, 2, 5–9]. However, there is very little information on ZnO homojunction-based UV PDs because undoped ZnO generally exhibits a nature of n-type conductivity while i- or p-type ZnO with high quality and reproducibility is difficult to be obtained [1, 3]. Meanwhile, in most reports on ZnO-based UV PDs, metal-semiconductor-metal (MSM) structures which provide an easy control, stable operation, simple fabrication process, and compatibility with various materials have been employed [1–6].
However, there are still practical limitations to grow ZnO NSs on selected areas of MSM structures. In order to define the selective areas, a conventional photolithography process is required, causing complex fabrication processes with potential damages. Although there has been much effort to grow ZnO NSs efficiently by various methods, these issues still need to be discussed. Meanwhile, hydrothermal synthesis has been considered as a promising method due to its relatively simple process, low working temperature, and short growth time [2, 13–15]. In this work, we investigated the synthesis and characteristics of one-dimensional ZnO nanorods (NRs) hydrothermally formed on the Al-doped ZnO (AZO) layer in MSM structures by the selective area growth and their applications for near-ultraviolet (NUV) PDs. To understand the passivation effect on the photoresponse property, comparative studies between the devices with/without silicon dioxide (SiO2) passivation were also carried out.
Results and Discussion
Figure 2b shows the (i) EDX spectrum of the selectively grown ZnO NRs on the active channel and their corresponding (ii) zinc (Zn), (iii) oxygen (O), and (iv) silicon (Si) elemental mapping images. From the EDX spectrum, the amounts of zinc and oxygen atoms were quantified to be ~14.3 and 20.5 %, respectively, indicating a 1:1 ratio of Zn and O for ZnO NRs. The amount of oxygen atoms was a little higher than that of zinc atoms because the oxygen in SiO2 layer on Si substrate was also counted. The elemental mapping images exhibited that the zinc and oxygen atoms found at high concentrations were uniformly distributed over the active channel region while the silicon was rarely observed. This indicates that the ZnO NRs were densely grown on the active channel without any significant defects or vacancies.
Figure 2c shows the current-voltage (I-V) curves of the ZnO NR-based NUV PDs at (i) 15 mM, (ii) 25 mM, and (iii) 50 mM. Herein, the W ch was fixed to be 30 μm. At the bias of 5 V, very low dark currents were observed from all the devices while the photocurrents were measured to be ~0.35, 0.63, and 0.46 mA for the solution concentrations of 15, 25, and 50 mM, respectively. It is well known that the magnitude of photocurrent is closely related to the surface area and crystallinity of ZnO NRs. The ZnO NRs grown at 25 mM provided a relatively large surface area. At 50 mM, on the other hand, the photocurrent decreased due to the reduced surface area and low crystallinity of large ZnO NRs [2, 16, 17]. The inset of Fig. 2c shows the corresponding cross-sectional FE-SEM images of the ZnO NRs grown at 15, 25, and 50 mM using the hydrothermal synthesis. The diameter and height of ZnO NRs were increased as the solution concentration was increased. These values were estimated to be 40 ± 5 nm (diameter)/385 ± 15 nm (height), 65 ± 10 nm/1000 ± 50 nm, and 205 ± 20 nm/2600 ± 300 nm, indicating the aspect ratio of 9.6, 15.4, and 12.7, for the solution concentrations of 15, 25, and 50 mM, respectively. This can be explained by the fact that the diffusion of Zn2+ ions to nuclei is more enhanced at higher solution concentration. At 50 mM, however, the aspect ratio was slightly decreased because the excessive Zn2+ ions cause the higher growth rate on the overall facets of ZnO, i.e., the rapid isotropic growth of ZnO NRs [2, 18, 19].
Figure 3c shows the spectral responsivity (R λ ) of the unpassivated and passivated ZnO NR-based NUV PDs grown at 25 mM with the W ch of 30 μm as a function of wavelength of incident light. The SiO2 passivation was performed at different deposition times of 120, 300, and 600 s to reduce the surface defects of ZnO NRs and environmental effects. The R λ indicates the ratio of photocurrent to light intensity at certain wavelength. For measurements, the monochromatic light source was used from 350 to 450 nm with an interval of 5 nm. At the bias of 5 V, for all devices, the R λ exhibited relatively high values below 390 nm while it was gradually decreased above 400 nm, which exhibits similar tendency with typical absorption spectra of ZnO NRs. Though the passivation on ZnO NRs with dielectric or organic materials enables enhanced photoconduction and efficiency by reducing the probability of surface recombination [29–31], for the ZnO NR-based NUV PDs, the R λ was dramatically dropped down with increasing the SiO2 deposition time for passivation. This is mainly attributed to the high resistivity of SiO2 . The optical losses by SiO2 passivation are not considered because it is highly transparent (i.e., transmittance > 90 %) in the NUV and visible ranges. At the wavelength of 380 nm, the highest R λ values were obtained to be 101.68, 59.29, 46.80, and 29.62 mA/W for the devices passivated with SiO2 deposited at the deposition times of 0, 120, 300, and 600 s, respectively. The inset of Fig. 3c shows the external quantum efficiency (EQE) of the unpassivated and passivated ZnO NR-based NUV PDs grown at 25 mM with the W ch of 30 μm as a function of wavelength of incident light. The EQE of ZnO NR-based NUV PDs was calculated using the equation of EQE = 1240 × R λ × λ − 1. At 380 nm, as shown in the inset of Fig. 3c, the maximum EQE values were observed to be 33.2, 19.4, 15.3, and 9.7 % for the devices passivated with SiO2 at 0, 120, 300, and 600 s, respectively.
Comparison of device properties between this and other ZnO-based UV PDs
Rise/fall time (s)
Bias voltage (V)
2.0 × 107
ZnO NPs-graphene core-shell
640 × 103
13.28 × 103
ZnO NRs between asymmetry Au
ZnO NRs on AZO
The ZnO NR-based NUV PDs in the type of MSM structure were successfully fabricated by the conventional photolithography and hydrothermal synthesis processes. The device characteristics were analyzed and optimized with various solution concentrations for different W ch. The optimized device performance was achieved from the ZnO NRs grown at 25 mM with the W ch of 30 μm, indicating the photocurrent of 0.63 mA and on/off ratio of 1720. The SiO2 passivation enhanced the photoresponse properties (i.e., reduced rise and reset times), but it made the photocurrent reduced. These results may be helpful for the facile fabrication and optimization of device performance in ZnO NR-based UV PD applications.
This work was supported by the National Research Foundation of Korea (NRF) grant and funded by the Korea government (MSIP) (NRF-2013R1A1A2010037).
SHL fabricated the ZnO NRA-based NUV photodetectors by selective area growth and SiO2 passivation, and measured the device characteristics. SHK assisted the fabrication and measurements. JSY supervised the conceptual framework and drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- Zhang H, Hu Y, Wang Z, Fang Z, Peng LM (2016) Performance boosting of flexible ZnO UV sensors with rational designed absorbing antireflection layer and humectant encapsulation. ACS Appl Mater Interfaces 8:381View ArticleGoogle Scholar
- Ko YH, Nagaraju G, Yu JS (2015) Fabrication and optimization of vertically aligned ZnO nanorod array-based UV photodetectors via selective hydrothermal synthesis. Nanoscale Res Lett 10:323View ArticleGoogle Scholar
- Chen HY, Liu KW, Chen X, Zhang ZZ, Fan MM, Jiang MM, Xie XH, Zhao HF, Shen DZ (2014) Realization of a self-powered ZnO MSM UV photodetector with high responsivity using an asymmetric pair of Au electrodes. J Mater Chem C 2:9689View ArticleGoogle Scholar
- Tian C, Jiang D, Zhao Y, Liu Q, Hou J, Zhao J, Liang Q, Gao S, Qin J (2014) Effects of continuous annealing on the performance of ZnO based metal-semiconductor-metal ultraviolet photodetectors. Mater Sci Eng B-Adv Funct Solid-State Mater 184:67View ArticleGoogle Scholar
- Wang T, Jiao Z, Chen T, Li Y, Ren W, Lin S, Lu G, Ye J, Bi Y (2013) Vertically aligned ZnO nanowire arrays tip-grafted with silver nanoparticles for photoelectrochemical applications. Nanoscale 5:7552View ArticleGoogle Scholar
- Wu J, Lin LY (2015) A flexible nanocrystal photovoltaic ultraviolet photodetector on a plant membrane. Adv Opt Mater 3:1530View ArticleGoogle Scholar
- Cheng W, Tang L, Xiang J, Ji R, Zhao J (2016) An extreme high-performance ultraviolet photovoltaic detector based on a ZnO nanorods/phenanthrene heterojunction. RSC Adv 6:12076View ArticleGoogle Scholar
- Perng DC, Lin HP, Hong MH (2015) High-performance ultraviolet detection and visible-blind photodetector based on Cu2O/ZnO nanorods with poly-(N-vinylcarbazole) intermediate layer. Appl Phys Lett 107:241113View ArticleGoogle Scholar
- Chen H, Liu H, Zhang Z, Hu K, Fang X (2016) Nanostructured photodetectors: from ultraviolet to terahertz. Adv Mater 28:403View ArticleGoogle Scholar
- Lee JH, Ko KH, Park BO (2003) Electrical and optical properties of ZnO transparent conducting films by the sol–gel method. J Cryst Growth 247:119View ArticleGoogle Scholar
- Behera B, Chandra S (2015) A MEMS based acetone sensor incorporating ZnO nanowires synthesized by wet oxidation of Zn film. J Micromech Microeng 25:015007View ArticleGoogle Scholar
- Pan X, Zhao X, Chen J, Bermak A, Fan Z (2015) A fast-response/recovery ZnO hierarchical nanostructure based gas sensor with ultra-high room-temperature output response. Sens Actuator B-Chem 206:764View ArticleGoogle Scholar
- Greene LE, Law M, Goldberger J, Kim F, Johnson JC, Zhang Y, Saykally RJ, Yang P (2003) Low-temperature wafer-scale production of ZnO nanowire arrays. Angew Chem-Int Edit 42:3031View ArticleGoogle Scholar
- Pal U, Santiago P (2005) Controlling the morphology of ZnO nanostructures in a low-temperature hydrothermal process. J Phys Chem B 109:15317View ArticleGoogle Scholar
- Guo M, Diao P, Wang X, Cai S (2005) The effect of hydrothermal growth temperature on preparation and photoelectrochemical performance of ZnO nanorod array films. J Solid State Chem 178:3210View ArticleGoogle Scholar
- Ko YH, Yu JS (2010) Structural and antireflective properties of ZnO nanorods synthesized using the sputtered ZnO seed layer for solar cell applications. J Nanosci Nanotechnol 10:8095View ArticleGoogle Scholar
- Ahn KS, Shet S, Deutsch T, Jiang CS, Yan Y, Al-Jassim M, Turner J (2008) Enhancement of photoelectrochemical response by aligned nanorods in ZnO thin films. J Power Sources 176:387View ArticleGoogle Scholar
- Baruah S, Dutta J (2009) Hydrothermal growth of ZnO nanostructures. Sci Technol Adv Mater 10:013001View ArticleGoogle Scholar
- Song J, Baek S, Lee J, Lim S (2008) Role of OH− in the low temperature hydrothermal synthesis of ZnO nanorods. J Chem Technol Biotechnol 83:345View ArticleGoogle Scholar
- Lin JC, Huang MC, Wang T, Wu JN, Tseng YT, Peng KC (2015) Structure and characterization of the sputtered ZnO, Al-doped ZnO, Ti-doped ZnO and Ti, Al-co-doped ZnO thin films. Mater Express 5:153Google Scholar
- Suwanboon S, Amornpitoksuk P, Haidoux A, Tedenac JC (2008) Structural and optical properties of undoped and aluminium doped zinc oxide nanoparticles via precipitation method at low temperature. J Alloy Compd 462:335View ArticleGoogle Scholar
- Oba F, Nishitani SR, Isotani S, Adachi H, Tanaka I (2001) Energetics of native defects in ZnO. J Appl Phys. 90:824Google Scholar
- Xu CX, Sun XW, Zhang XH, Ke L, Chua SJ (2004) Photoluminescent properties of copper-doped zinc oxide nanowires. Nanotechnology 15:856View ArticleGoogle Scholar
- Chen J, Sun Y, Lv X, Li D, Fang L, Wang H, Sun X, Huang C, Yu H, Feng P (2014) Preparation and characterization of high-transmittance AZO films using RF magnetron sputtering at room temperature. Appl Surf Sci 317:1000View ArticleGoogle Scholar
- Ratana T, Amornpitoksuk P, Ratana T, Suwanboon S (2009) The wide band gap of highly oriented nanocrystalline Al doped ZnO thin films from sol–gel dip coating. J Alloy Compd 470:408View ArticleGoogle Scholar
- Ko H, Tai WP, Kim KC, Kim SH, Suh SJ, Kim YS (2005) Growth of Al-doped ZnO thin films by pulsed DC magnetron sputtering. J Cryst Growth 277:352View ArticleGoogle Scholar
- Banerjee P, Lee WJ, Bae KR, Lee SB, Rubloff GW (2010) Structural, electrical, and optical properties of atomic layer deposition Al-doped ZnO films. J Appl Phys 108:043504View ArticleGoogle Scholar
- Koleilat GI, Wang X, Labelle AJ, Ip AH, Carey GH, Fischer A, Levina L, Brzozowski L, Sargent EH (2011) A donor-supply electrode (DSE) for colloidal quantum dot photovoltaics. Nano Lett 11:5173View ArticleGoogle Scholar
- Rostami A, Dolatyari M, Amini E, Rasooli H, Baghban H, Miri S (2013) Sensitive, fast, solution-processed ultraviolet detectors based on passivated zinc oxide nanorods. Chem Phys Chem 14:554Google Scholar
- Chen Q, Ding H, Wu Y, Sui M, Lu W, Wang B, Su W, Cuie Z, Chen L (2013) Passivation of surface states in the ZnO nanowire with thermally evaporated copper phthalocyanine for hybrid photodetectors. Nanoscale 5:4162View ArticleGoogle Scholar
- Moazzami K, Murphy TE, Phillips JD, Cheung MC-K, Cartwright AN (2006) Sub-bandgap photoconductivity in ZnO epilayers and extraction of trap density spectra. Semicond Sci Technol 21:717View ArticleGoogle Scholar
- Janotti A, Van de Walle CG (2007) Hydrogen multicentre bonds. Nat Mater 6:44View ArticleGoogle Scholar
- Xue X, Wang T, Jiang X, Jiang J, Pan C, Wu Y (2014) Interaction of hydrogen with defects in ZnO nanoparticles—studied by positron annihilation, Raman and photoluminescence spectroscopy. Crys Eng Comm 16:1207View ArticleGoogle Scholar
- Barnett CJ, Kryvchenkova O, Smith NA, Kelleher L, Maffeis TGG, Cobley RJ (2015) The effects of surface stripping ZnO nanorods with argon bombardment. Nanotechnology 26:415701View ArticleGoogle Scholar
- Van de Walle CG (2000) Hydrogen as a cause of doping in ZnO. Phys Rev Lett 85:1012View ArticleGoogle Scholar
- Bao J, Shalish I, Su Z, Gurwitz R, Capasso F, Wang X, Ren Z (2011) Photoinduced oxygen release and persistent photoconductivity in ZnO nanowires. Nanoscale Res Lett 6:404View ArticleGoogle Scholar
- Dhara S, Giri PK (2011) Enhanced UV photosensitivity from rapid thermal annealed vertically aligned ZnO nanowires. Nanoscale Res Lett 6:504View ArticleGoogle Scholar
- Ahn SE, Ji HJ, Kim K, Kim GT, Bae CH, Park SM, Kim YK, Ha JS (2007) Origin of the slow photoresponse in an individual sol-gel synthesized ZnO nanowire. Appl Phys Lett 90:153106View ArticleGoogle Scholar
- Shao D, Yu M, Sun H, Hu T, Lian J, Sawyera S (2013) High responsivity, fast ultraviolet photodetector fabricated from ZnO nanoparticle–graphene core–shell structures. Nanoscale 5:3664View ArticleGoogle Scholar