p-Cu2O-shell/n-TiO2-nanowire-core heterostucture photodiodes
- Tsung-Ying Tsai†1,
- Shoou-Jinn Chang†1,
- Ting-Jen Hsueh†2Email author,
- Han-Ting Hsueh†2,
- Wen-Yin Weng†1,
- Cheng-Liang Hsu†3 and
- Bau-Tong Dai†2
© Tsai et al; licensee Springer. 2011
Received: 8 September 2011
Accepted: 31 October 2011
Published: 31 October 2011
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.
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 . In recent years, high-performance GaN-based (including AlGaN and AlInN) [2–5], ZnO-based , and ZnSe-based  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 [8–10]. 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 [13–15]. 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 , 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 [17–24]. 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 .
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 . 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.
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.
Results and discussion
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.
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.
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.
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
- 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.Google Scholar
- 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 ArticleGoogle Scholar
- 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 ArticleGoogle Scholar
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.