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.
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