Research on Growth Mechanism of Laterally Bridged ZnO Nanostructure and their Optoelectronic Applications

• Main Authors: Ming-YuehChuang, 莊明岳
• Other Authors: Yan-Kuin Su
• Format: Others
• Language: en_US
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/55019379069231382664

博士 === 國立成功大學 === 微電子工程研究所 === 103 === In this dissertation, the laterally bridged ZnO nanorods were grown onto the Au intermediate layer by hydrothermal growth method and applied to the resistive random access memory and ultraviolet photodetector applications, respectively. Hence, the dissertation is divided into two parts, one is the investigation of laterally bridged ZnO nanorods-based resistive random access memory, and the other is that of laterally bridged ZnO nanorods-based ultraviolet photodetectors. In the beginning of this dissertation, a novel memory device based on laterally bridged ZnO nanorods in opposite direction was fabricated by hydrothermal growth method and characterized. The electrodes were defined by a simple photolithography method. This method has lower cost, simpler process, and higher reliability than the traditional focused ion beam lithography method. For the first time, the negative differential resistance and bistable unipolar resistive switching behavior in the current–voltage curve was observed at room temperature. The memory device is stable and rewritable; it has an ultra-low current level of about 10^−13 A in the high resistance state; and it is nonvolatile with an on–off current ratio of up to 1.56 × 10^6. Moreover, its peak-to-valley current ratio of negative differential resistance behavior is greater than 1.76 × 10^2. The negative differential resistance and resistive switching behavior of this device may be related to the boundaries between the opposite bridged ZnO nanorods. Specifically, the resistive switching behavior found in ZnO nanorod devices with a remarkable isolated boundary at the nanorod/nanorod interface was discussed for the first time. The memory mechanism of laterally bridged ZnO nanorod-based devices has not been discussed in the literature yet. In this work, results show that laterally bridged ZnO nanorod-based devices may have next-generation resistive memories and nano-electronic applications. In the second part of this dissertation, we developed a seedless, density-controlled, and pressurized growth method for laterally bridged ZnO nanorods from Au electrode for use in metal–semiconductor–metal photodetector fabrication. This part divided into three sections. (1) The laterally bridged ZnO microrods grown from Au electrode applied to metal–semiconductor–metal photodetector was fabricated. Interlaced ZnO microrods with approximate single-crystalline structure can be grown from Au electrode fingers. The dark-current was 5.00 × 10^−5 A with an applied voltage of 1 V. Highly dense lateral ZnO micro-rod-based photodetectors produce remarkable responsivity of 1.93 × 10^5 A/W. Moreover, an extremely high internal photoconductive gain of 6.28 × 10^5 exists in the fabricated photodetectors. For a given bandwidth of 10 kHz and 1 V applied bias, the noise equivalent power of photodetectors were estimated to be 1.86 × 10^−13 W, and correspond to normalized detectivity of 1.12 × 10^12 cm•Hz^0.5W^−1. This result may be attributed to internal photoconductive gain mechanism and high-density bridged ZnO microrods. Our approach provides a simple and seed-layer-free method to fabricate high-performance ultraviolet photodetectors. (2) The effect of pre-annealing process on suppressing vertical ZnO nanorods is systematically investigated by atomic force microscopy and scanning electron microscopy. The pre-annealing process is demonstrated to have direct influence on controlling vertical/lateral ZnO nanorod density and morphology. Interlaced and density-controlled ZnO nanorods with approximate single-crystalline structure can be directly grown from the side wall of pre-annealed Au electrode fingers without seed-layer. Through pre-annealing process, dark-current can be decreased from 4.99 × 10^−4 to 7.28 × 10^−7 A with an applied voltage of 1 V. Highly dense lateral ZnO nanorod-based photodetectors produce remarkable responsivity of 7.01 × 10^3 A/W and UV/visible rejection ratio of 281.21. Moreover, a high internal photoconductive gain (10^4–10^5) exists in the fabricated photodetectors. For a given bandwidth of 10 k Hz and 1 V applied bias, the noise equivalent power of photodetectors with 0, 10, and 20 min pre-annealing periods are estimated to be 3.58 × 10^−13, 6.78 × 10^−13, and 4.86 × 10^−13 W, and correspond to normalized detectivity of 1.85 × 10^12, 1.17 × 10^12, and 1.99 × 10^12 cm•Hz^0.5W^−1, respectively. This result may be attributed to internal photoconductive gain mechanism and high-density bridged ZnO nanorods. Our approach provides a simple and controllable method to fabricate high-performance ultraviolet photodetectors. (3) This study develops a pressurized and seedless growth method for laterally bridged ZnO nanorods from Au electrode for use in metal–semiconductor–metal photodetector fabrication. The effect of hydrothermal growth pressure on the morphology and crystal quality of ZnO nanorods is systematically investigated by scanning electron microscopy and photoluminescence spectroscopy, respectively. The saturated growth pressure is demonstrated to have direct influence on controlling ZnO nanorod density and morphology. Interlaced and density-controlled ZnO nanorods with approximate single-crystalline structure can be directly grown from the Au electrode fingers without seed-layer. Through increasing the steady-state growth pressure, the dark-current can be decreased from 5.06 × 10^−6 to 5.39 × 10^−8 A with an applied voltage of 0.2 V. Highly dense lateral ZnO nanorod-based photodetectors produce remarkable responsivity of 1.25 × 10^4 A/W and UV/visible rejection ratio of 1113.92. Moreover, a high internal photoconductive gain (10^4–10^5) exists in the fabricated photodetectors. This result may be attributed to internal photoconductive gain mechanism and high-density bridged ZnO nanorods. Our approach provides a simple and controllable method to fabricate high-performance ultraviolet photodetectors.