The study of ZnO nanowires-based optoelectronics and sensor devices

• Main Authors: Chien-Yuan Lu, 盧建元
• Other Authors: Shoou-Jinn Chang
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/80289670795452494990

博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 97 === The main goal of this dissertation is the achievement of ZnO nanowire-based UV photodetectors, sensors, p-NiO/ ZnO nanowires heterojunction P-N diode and ZnO nanowires/p-GaN light emitting diode. In the study of ZnO nanowire-based photodetectors, four types of UV photodetectors: photodetectors with Au contacts, photodetectors with insulating spin-on-glass (SOG) layer, interlaced ZnO nanowires-based photodetectors and lateral ZnO nanowires-based photodetectors were studied. For the first two above-mentioned photodetectors, the well-aligned vertical ZnO nanowires used in this study were gorwn on ZnO:Ga/glass templates. For the fabrication of photodetectors, Au metal was thermally evaporated onto as-grown ZnO nanowires. It was found that tips of the ZnO nanowires seem to be covered by the subsequently deposited Au. It was also found that head of each individual club contacted closely with those of the neighboring clubs to form continuous Au thin film at the sample surface. Utilizing the matchstick-like nanowires to form a continuous Au thin film as contact, the 3D nanostructure of photodetector can be achieved. With a 2V applied bias, it was found that dark current of this photodetector was 2.198 x 10-4 A. Furthermore, we also reported the characteristics of he oxygen gas sensors. Upon the injection of oxygen gas, it was found that the response of the sensors increased from 20.7% to 136.6% with the increased oxygen gas ratio. Besides, in order to achieve good electrical contact to both ends of the ZnO nanowires and effectively reduce the dark current of the PDs, we coated a thin SOG film onto the as-grown ZnO nanowires. With 2V applied bias, it was found that dark current density of the photodetector with SOG layer was only 3.8 x 10-9 A/cm2. It was also found that the time constant and UV-to-visible rejection ratio of the fabricated photodetectors was around 0.44 mSec and 1000 with a maximum quantum efficiency of 12.6%, respectively. Furthermore, it was also found that noise equivalent power and normalized detectivity of the ZnO nanowire photodetector were 5.73x10-11W and 6.17x109cmHz0.5W-1, respectively. By using the crabwise growth technique, the average length and diameter of lateral ZnO nanowires were around 5μm and 30 nm, respectively. The lateral ZnO nanowires can be bridged across two neighboring fingers to provide electrical paths. With 2V applied bias, it was found that dark current and photo current of the fabricated detector were 4.1x10-9 and 5.0x10-8A, respectively. It was also found that the corresponding time constant of our lateral ZnO nanowire photodetector was around 452 mSec. On the part of the interlaced ZnO nanowire UV photodetectors, the high-density signle crystalline ZnO nanowires were grown on patterned ZnO:Ga/SiO2/Si templates. The ZnO nanowires grown on a sputtered ZnO:Ga layer were vertically aligned while those grown directly on a SiO2 layer were randomly oriented. Interlaced ZnO nanowires will also provide electrical paths on a SiO2 layer. With an incident wavelength of 375 nm, it was found that measured responsivity was 0.055 A/W for the interlaced ZnO nanowires photodetector with a 1V applied bias. The transient time constants measured during the turn-on and turn-off states were τon = 12.72 ms and τoff = 447.66 ms, respectively. Furthermore, the low frequency noise spectra obtained from the UV photodetector were purely due to the 1/f noise. Besides, the noise Vequivalent power (NEP) and normalized detectivity (D*) of the ZnO nanowire photodetector were 2.32x10-9 W and 7.43x109 cmHz0.5W-1, respectively. In addition, we also reported the characteristics of ZnO nanowire-base humidity sensor. It was found that measured resistance of the sensor decreased linearly with the increase of RH. Finally, we reported the depositon of NiO onto ZnO nanowires prepared on ZnO:Ga/glass templates. With the sputtered NiO, the nanowires became mushroom-like. The p-NiO/n-ZnO heterostructure exhibits rectifying behavior with a sharp turn on at ~1V. Furthermore, the current vs. voltage characteristic is dominated by space-charge-limited current (SCLC) at high (1.1V) forward bias. Besides, the p-GaN/ ZnO nanowire heterostructure was also fabricated. The photoluminescence spectrum of p-GaN film exhibited broad bands at 432nm and 583nm and is attributed to shallow donors to deep Mg acceptor level or interface defects or transitions from conduction band. It was also found that EL emission of the LED is dominated by defect-related emission in the p-GaN layer.