p-Cu2O-shell/n-TiO2-nanowire-core heterostucture photodiodes

  • Tsung-Ying Tsai1,

    Affiliated with

    • Shoou-Jinn Chang1,

      Affiliated with

      • Ting-Jen Hsueh2Email author,

        Affiliated with

        • Han-Ting Hsueh2,

          Affiliated with

          • Wen-Yin Weng1,

            Affiliated with

            • Cheng-Liang Hsu3 and

              Affiliated with

              • Bau-Tong Dai2

                Affiliated with

                Contributed equally
                Nanoscale Research Letters20116:575

                DOI: 10.1186/1556-276X-6-575

                Received: 8 September 2011

                Accepted: 31 October 2011

                Published: 31 October 2011

                Abstract

                This study reports the deposition of cuprous oxide [Cu2O] onto titanium dioxide [TiO2] nanowires [NWs] prepared on TiO2/glass templates. The average length and average diameter of these thermally oxidized and evaporated TiO2 NWs are 0.1 to 0.4 μm and 30 to 100 nm, respectively. The deposited Cu2O fills gaps between the TiO2 NWs with good step coverage to form nanoshells surrounding the TiO2 cores. The p-Cu2O/n-TiO2 NW heterostructure exhibits a rectifying behavior with a sharp turn-on at approximately 0.9 V. Furthermore, the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratios and fast responses.

                Introduction

                UV photodetectors are important devices that have a range of commercial, research, and military applications. They can be used for space communication, ozone layer monitoring, and flame detection [1]. In recent years, high-performance GaN-based (including AlGaN and AlInN) [25], ZnO-based [6], and ZnSe-based [7] photodetectors have all been demonstrated. However, high-quality GaN-based UV photodetectors could only be prepared on a sapphire substrate, which is much more expensive as compared with a glass substrate. On the other hand, the photocurrent-to-dark-current contrast ratio of ZnO-based UV photodetectors is still low. Titanium dioxide [TiO2] is a potentially useful wide direct-bandgap material (3.2 eV for anatase and 3.0 eV for rutile) for UV photodetectors, solar cells, and gas sensors due to its outstanding physical, chemical, and optical properties [810]. TiO2 is a nontoxic naturally n-type semiconductor material which has a high-temperature stability and low-production costs.

                For two-dimensional [2D] films, TiO2 UV photodetectors such as metal-semiconductor-metal detectors and Schottky barrier diodes have been demonstrated [11, 12]. It is difficult to produce p- and n-type materials simultaneously, which is necessary for certain device applications. Zhang et al. reported the formation of a 2D TiO2/Cu2O composite film for a photocatalyst application using the metal ion-implantation method [1315]. Cuprous oxide [Cu2O] is naturally a p-type direct-bandgap semiconductor with a cubic crystal structure and a room-temperature bandgap energy of 2.17 eV [16], which makes it ideal for TiO2-based p-n heterojunctions. Cu2O can be deposited using methods such as thermal oxidation, anodic oxidation, sputtering, solution growth, sol-gel, and electro-deposition [1724]. Among these methods, sputtering is commonly used in the semiconductor industry. By carefully controlling the growth parameters, high-quality 2D Cu2O films can be produced by direct-current [DC] sputtering [18].

                Recently, one-dimensional oxide semiconducting materials have attracted a lot of attention for potential application in optoelectronic devices due to their large surface-area-to-volume ratio [25]. Wu et al. reported the growth of TiO2 nanowires [NWs] on glass substrates by the thermal oxidation-evaporation method [26, 27]. They produced single-crystalline TiO2 NWs, whose size and density were controlled by adjusting the growth parameters. However, no report on the fabrication of p-Cu2O-shell/n-TiO2-nanowire-core heterojunction UV photodetectors could be found in the literature, to our knowledge. The present study reports the deposition of p-Cu2O film onto n-TiO2 NWs by DC sputtering and the fabrication of radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes. The physical, electrical, and optical properties of the fabricated radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes are discussed.

                Experimental section

                Before the growth of TiO2 NWs, a Corning 1737 glass substrate (Corning Display Technologies Taiwan Co., Ltd., Taipei City, Taiwan) was wet-cleaned with acetone and deionized water. The glass substrate was subsequently baked at 100°C for 10 min to evacuate moisture. A 400-nm-thick titanium [Ti] film layer was then deposited onto the glass substrate by electron-beam evaporation. Finally, the samples were annealed in a furnace at 700°C for 3 h to synthesize TiO2 NWs in argon [Ar] ambiance. The crystal quality of the as-grown NWs was then characterized by an X-ray diffractometer [XRD] (MXP 18, MAC Science Co., Tokyo, Japan). The surface morphology of the samples and the size distribution of the NWs were characterized by a field-emission scanning electron microscope [FE-SEM] (JEOL JSM-7000F, JEOL Ltd., Tokyo, Japan).

                To investigate the deposition of Cu2O, glass was used as the substrate. The target used to deposit Cu2O was a 4-N pure copper block mounted on the cathode. The distance between the target and the sample was fixed at 60 mm. A rotating magnet fixed on the backside of the cathode was used to enhance the plasma bombardment effect. During sputtering, the Ar flow rate, deposition time, base pressure, and chamber pressure were kept at 15 sccm, 10 min, 2 × 10-6 Torr, and 6 mTorr, respectively, and the DC power, O2 flow rate, and substrate temperature were 200 W, 4 sccm, and 25°C, respectively. The crystallography and structure of the deposited Cu2O and the Cu2O/TiO2 NWs were evaluated by XRD and FE-SEM.

                Prior to the fabrication of p-Cu2O-shell/n-TiO2-nanowire-core photodiodes, a small piece of glass was used to cover the TiO2 NWs to prevent the deposition of Cu2O in these regions. A 200-nm-thick Cu2O layer was subsequently deposited onto the TiO2 NWs. A 500-nm-thick silver layer was then sputtered onto the Cu2O layer and TiO2 NWs to serve as the p-electrode and n-electrode with a shadow mask. Figure 1 schematically shows the structure of the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes. A picoammeter (HP-4145B semiconductor parameter analyzer, Agilent Technologies, Sta. Clara, CA, USA), connected via a GPIB controller to a computer, was then used to measure the current-voltage [I-V] characteristics of the fabricated diodes under darkness. The photo responses of the devices were also measured. During photo-response measurements, a 4-W mercury vapor lamp emitting at 365 nm was used as the excitation source.
                http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Fig1_HTML.jpg
                Figure 1

                Schematic diagram of fabricated p-Cu 2 O-shell/n-TiO 2 -nanowire-core for photodiode measurements.

                Results and discussion

                Figure 2a shows a cross-sectional FE-SEM image of the TiO2 NWs prepared on a Ti/glass template. It can be clearly seen that high-density TiO2 NWs of various lengths were grown on the Ti/glass template. As shown in Figure 2a, it can be seen that the average length, diameter, and density of these TiO2 NWs were 0.3 μm, 50 nm, and 60 wires/μm2, respectively. Figure 2b shows a cross-sectional FE-SEM image of the sample with Cu2O deposited on TiO2 NWs. As shown, the deposited Cu2O filled the gaps between the TiO2 NWs with good step coverage to form radial Cu2O/TiO2 NWs. It was also found that the deposited Cu2O formed at the sample surface after filling the gaps. In order to investigate the coating performance of Cu2O, the deposited sample was scraped with tweezers into an alcohol solution, which was then ultrasonically treated for 20 min to disperse the NWs. The solution was dropped on carbon tape which was then placed on a hot plate to evacuate the alcohol. Figure 3a shows a SEM image of a single NW. Figures 3b and 3c show energy-dispersive X-ray [EDX] spectroscopic mapping images of Cu and Ti, respectively. These figures correspond to the SEM image shown in Figure 3a. After the deposition of Cu2O, Cu and Ti atoms were distributed over the entire NW. These results suggest that the sputtered Cu2O not only forms the head portion of the nanoclubs, but also forms nanoshells surrounding the TiO2 cores in the nanowire portion of the nanoclubs. The formation of such p-Cu2O-shell/n-TiO2-nanowire-core heterostructure should be able to provide us with a large junction area, which is important for the application of photodetectors.
                http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Fig2_HTML.jpg
                Figure 2

                Cross-sectional FE-SEM images. (a) Pure TiO2 nanowires and (b) the sample with Cu2O deposited on TiO2 nanowires at 300°C with 200 W DC power, 6 mTorr chamber pressure, and 3 sccm O2 flow rate.

                http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Fig3_HTML.jpg
                Figure 3

                FE-SEM and EDX images. (a) An FE-SEM image of a single radial nanowire. EDX spectroscopic mapping images of (b) Cu and (c) Ti which were corresponded to (a).

                Figure 4 shows the crystallographic characteristics obtained from XRD measurements. For the pure TiO2 NWs used for adhesion, the peaks were attributed to the rutile-TiO2 (110) phase (JCPDS Card No. 88-1175). For the p-Cu2O/n-TiO2 NWs, the peaks were attributed to the (110) and (111) phases of the Cu2O phase (JCPDS Card No. 78-2076). No Ti-related signal was found, indicating that the Ti film changed into a TiO2 film after the annealing process.
                http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Fig4_HTML.jpg
                Figure 4

                XRD measurements of pure TiO 2 nanowires and p-Cu 2 O/n-TiO 2 nanowires obtained by DC sputtering.

                Figure 5 shows the dark I-V characteristics measured from the fabricated radial Cu2O/TiO2 NWs. The rectifying behavior indicates that a p-n junction formed in the Cu2O/TiO2 NWs. The operation of the photodiode detector involves three steps (1) the generation of electron-hole [e-h] pairs by the absorption of incident light, whose photon energy exceeds the bandgap of the materials in the device; (2) the separation and transport of the e-h pairs by the internal electric field; and (3) the interaction of current with the external circuit to generate an output signal. Hence, the I-V characteristics of a photodiode in a dark environment are similar to those of a normal rectifying diode. If the p-n junction does not form, the generated e-h pairs will exhibit an ohmic character in the I-V curve and change the resistance. When a photodiode with a p-n junction is illuminated with optical radiation, the I-V characteristics shift according to the photocurrent and reverse current. The measured current in the photodiode, I m, is:
                http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Fig5_HTML.jpg
                Figure 5

                Dark I-V characteristic measured from the fabricated radial p-Cu 2 O/n-TiO 2 nanowires.

                I m = I d - I ph http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Equa_HTML.gif

                where I d is the dark current and I ph is the photocurrent. The presence of a reverse current indicates that the photo response is due to the p-n junction, not the TiO2 NWs or the Cu2O. In the process of measurement under illumination, UV light passes through the TiO2 and illuminates the array of the radial p-Cu2O/n-TiO2 NWs; e-h pairs are produced in the radial NWs when the energy of the UV light is absorbed. The e-h pairs are separated by the internal electric field, and a photocurrent is simultaneously generated. Under forward bias, the turn-on occurred at approximately 0.9 V. With a +5-V applied bias, the forward current of the device was 1.53 × 10-7 A, and with a -5-V applied bias, the reverse leakage current was 7.74 × 10-9 A.

                Figure 6 shows the dynamic photo response measured from the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiode. With a +10-V applied bias, the dark reverse leakage current of the diode was only around 3.37 × 10-8 A. However, the reverse leakage current increased rapidly to 1.15 × 10-6 A upon UV illumination. When the UV lamp was turned off, the reverse leakage current rapidly decreased to its original value. The reasonably large photocurrent-to-dark-current contrast ratio and the fast responses suggest that the radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes proposed in this study are potentially useful for UV detector applications.
                http://static-content.springer.com/image/art%3A10.1186%2F1556-276X-6-575/MediaObjects/11671_2011_Article_319_Fig6_HTML.jpg
                Figure 6

                Dynamic photo response measured from the fabricated p-Cu 2 O-shell/n-TiO 2 -nanowire-core photodiode.

                Conclusions

                The deposition of Cu2O onto well-aligned TiO2 NWs by DC sputtering was reported. With the proper sputtering parameters, the deposited Cu2O filled the gaps between the TiO2 NWs with good step coverage to form radial p-Cu2O/n-TiO2 NWs that exhibited rectifying I-V characteristics. The fabricated radial p-Cu2O-shell/n-TiO2-nanowire-core photodiodes had a reasonably large photocurrent-to-dark-current contrast ratio and fast responses.

                Declarations

                Acknowledgements

                The authors would like to thank the National Science Council and Bureau of Energy, Ministry of Economic Affairs of Taiwan, Republic of China for the financial support under contract nos. 100-2221-E-006-040-MY2 and 100-D0204-6 and the LED Lighting Research Center of NCKU for the assistance on device characterization.

                Authors’ Affiliations

                (1)
                Institute of Microelectronics and Department of Electrical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University
                (2)
                National Nano Device Laboratories
                (3)
                Department of Electronic Engineering, National University of Tainan

                References

                1. Monroy E, Calle F, Munoz E, Omnes F, Beaumont B, Gibart P: Visible-blindness in photoconductive and photovoltaic algan ultraviolet detectors. J Electron Mater 1999, 28: 240–245. 10.1007/s11664-999-0021-2View Article
                2. Chang SJ, Ko TK, Su YK, Chiou YZ, Chang CS, Shei SC, Sheu JK, Lai WC, Lin YC, Chen WS, Shen CF: GaN-based p-i-n sensors with ITO contacts. IEEE Sens J 2006, 6: 406–411.View Article
                3. Zhang J, Zhao H, Tansu N: Large optical gain AlGaN-delta-GaN quantum wells laser active regions in mid- and deep-ultraviolet spectral regimes. Appl Phys Lett 2011, 98: 171111. 10.1063/1.3583442View Article
                4. Zhang J, Tong H, Liu G, Herbsommer JA, Huang G, Tansu N: Characterizations of Seebeck coefficients and thermoelectric figures of merit for AlInN alloys with various In-contents. J Appl Phys 2011, 109: 053706. 10.1063/1.3553880View Article
                5. Zhang J, Kutlu S, Liu G, Tansu N: High-temperature characteristics of Seebeck coefficients for AlInN alloys grown by metalorganic vapor phase epitaxy. J Appl Phys 2011, 110: 043710. 10.1063/1.3624761View Article
                6. Weng WY, Chang SJ, Hsu CL, Hsueh TJ: A ZnO nanowire phototransistor prepared on glass substrates. ACS Appl Mat Interfaces 2011, 3: 162–166. 10.1021/am100746cView Article
                7. Lin TK, Chang SJ, Su YK, Chiou YZ, Wang CK, Chang CM, Huang BR: ZnSe homoepitaxial MSM photodetectors with transparent ITO contact electrodes. IEEE Trans Electron Devices 2005, 52: 121–123. 10.1109/TED.2004.841288View Article
                8. Chiba Y, Islam A, Komiya R, Koide N, Han L: Conversion efficiency of 10.8% by a dye-sensitized solar cell using a TiO 2 electrode with high haze. Appl Phys Lett 2006, 88: 223505–223507. 10.1063/1.2208920View Article
                9. Kopidakis N, Neale NR, Zhu K, Lagemaat JVD, Frank AJ: Spatial location of transport-limiting traps in TiO 2 nanoparticle films in dye-sensitized solar cells. Appl Phys Lett 2005, 87: 202106–202108. 10.1063/1.2130723View Article
                10. Shen L, Zhu G, Guo W, Tao C, Zhang X, Liu C, Chen W, Ruan S, Zhong Z: Performance improvement of TiO 2 /P3HT solar cells using CuPc as a sensitizer. Appl Phys Lett 2008, 92: 073307–073309. 10.1063/1.2884270View Article
                11. Xue H, Kong X, Liu Z, Liu C, Zhou J, Chen W, Ruan S, Xu Q: TiO 2 based metal-semiconductor-metal ultraviolet photodetectors. Appl Phys Lett 2007, 90: 201118–201120. 10.1063/1.2741128View Article
                12. Kong X, Liu C, Dong W, Zhang X, Tao C, Shen L, Zhou J, Fei Y, Ruan S: Metal-semiconductor-metal TiO 2 ultraviolet detectors with Ni electrodes. Appl Phys Lett 2009, 94: 123502–123504. 10.1063/1.3103288View Article
                13. Zhang KJ, Xu W, Li XJ, Zheng SJ, Xu G, Wang JH: Photocatalytic oxidation activity of titanium dioxide film enhanced by Mn non-uniform doping. Trans Nonferrous Met SOC China 2006, 16: 1069–1075. 10.1016/S1003-6326(06)60379-8View Article
                14. Yasomanee JP, Bandara J: Multi-electron storage of photoenergy using Cu 2 O-TiO 2 thin film photocatalyst. Sol Energy Mat Sol Cells 2008, 92: 348–352. 10.1016/j.solmat.2007.09.016View Article
                15. Zhang YG, Ma LL, Li JL, Yu Y: In situ Fenton reagent generated from TiO 2 /Cu 2 O composite film: a new way to utilize TiO 2 under visible light irradiation. Environ Sci Technol 2007, 41: 6264–6269. 10.1021/es070345iView Article
                16. Siripala W, Ivanovskaya A, Jaramillo TF, Baeck SH, McFarland EW: A Cu 2 O/TiO 2 heterojunction thin film cathode for photoelectrocatalysis. Sol Energy Mat Sol Cells 2003, 77: 229–237. 10.1016/S0927-0248(02)00343-4View Article
                17. Ghijsen J, Tjeng LH, Elp JV, Eskes H, Westerink J, Sawatzky GA, Czyzyk MT: Electronic structure of Cu 2 O and CuO. Phys Rev B 1988, 38: 11322–11330. 10.1103/PhysRevB.38.11322View Article
                18. Ishizuka S, Kato S, Okamoto Y, Sakurai T, Akimoto K, Fujiwara N, Kobayashi H: Passivation of defects in polycrystalline Cu 2 O thin films by hydrogen or cyanide treatment. Appl Surf Sci 2003, 216: 94–97. 10.1016/S0169-4332(03)00485-9View Article
                19. Herion J, Niekisch EA, Scharl G: Investigation of metal oxide/cuprous oxide heterojunction solar cells. Sol Energy Mater 1980, 4: 101–112. 10.1016/0165-1633(80)90022-2View Article
                20. Fortin E, Masson D: Photovoltaic effects in Cu 2 O-Cu solar cells grown by anodic oxidation. Solid State Electron 1982, 25: 281–283. 10.1016/0038-1101(82)90136-8View Article
                21. Fernando CAN, Wetthasinghe SK: Investigation of photoelectrochemical characteristics of n-type Cu 2 O films. Sol Energy Mater Sol Cells 2000, 63: 299–308. 10.1016/S0927-0248(00)00036-2View Article
                22. Armelao L, Barreca D, Bertapelle M, Bottaro Y, Sada C, Tondello E: A sol-gel approach to nanophasic copper oxide thin films. Thin Solid Films 2003, 442: 48–52. 10.1016/S0040-6090(03)00940-4View Article
                23. Golden TD, Shumsky MG, Zhou Y, VanderWerf RA, Leeuwen RAV, Switzer JA: Electrochemical deposition of copper (I) oxide films. Chem Mater 1996, 8: 2499–2504. 10.1021/cm9602095View Article
                24. Mahalingam T, Chitra JSP, Chu JP, Sebastian PJ: Preparation and microstructural studies of electrodeposited Cu 2 O thin films. Mater Lett 2004, 58: 1802–1807. 10.1016/j.matlet.2003.10.055View Article
                25. Hsueh TJ, Chen HY, Tsai TY, Weng WY, Yeh YM, Dai BT, Shieh JM: Si nanowire-based photovoltaic devices prepared at various temperatures. IEEE Electron Dev Lett 2010, 31: 1275–1277.
                26. Wu JM, Shih HC, Wu WT: Electron field emission from single crystalline TiO 2 nanowires prepared by thermal evaporation. Chen Phys Lett 2005, 413: 490–494. 10.1016/j.cplett.2005.07.113View Article
                27. Wu JM, Shih HC, Wu WT: Formation and photoluminescence of single-crystalline rutile TiO 2 nanowires synthesized by thermal evaporation. Nanotechnology 2006, 17: 105–109. 10.1088/0957-4484/17/1/017View Article

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                © Tsai et al; licensee Springer. 2011

                This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://​creativecommons.​org/​licenses/​by/​2.​0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.