Study of III-V Nitride and ZnO Based Gas Sensors

• Main Authors: Tai-YouChen, 陳泰佑
• Other Authors: Wen-Chau Liu
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/25631813942058261635

博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 101 === In this dissertation, a series of high-performance compound semiconductor based hydrogen sensors, including Schottky diodes, heterostructure field-effect transistors, and resistors, are fabricated and studied. Pd and Pt are used as sensing metals due to their excellent catalytic activity towards hydrogen and ammonia gases, respectively. GaN and AlGaN materials are served as sensing platforms because of their larger band gap than that of Si-based materials. In addition, hydrothermal growth of one-dimensional (1-D) ZnO nanorods-based device possesses higher surface to volume ratio (SV) than thin film-based device. It is beneficial to the adsorption of gas. Moreover, Pd nanoparticles-decorated ZnO nanorods could significant enhance the sensing response. We present the related electric characteristics, detection performance, and dynamic behaviors of these sensors measured under different hydrogen and ammonia concentrations at different temperatures. First, Pd/GaN Schottky diode-type hydrogen sensors with and without plasma surface treatment are studied and demonstrated. The studied device exhibits significant sensing performance, including high sensing response, large Schottky barrier height variation, widespread temperature operation regime, and fast transient response time. A comparative study between forward and reverse biases is presented. A simple detection model is proposed to elucidate the hydrogen sensing behavior of devices with plasma surface treatment. A rougher Pd surface exhibits considerable influences on the hydrogen adsorption properties. Second, ammonia-sensing characteristics of a Pt/AlGaN/GaN-based Schottky diode are studied. The related ammonia-sensing mechanisms, direct dissociation of ammonia gas and triple-point model, are presented to explain the effects of dissociation of ammonia molecules, diffusion of hydrogen atoms, boundaries between NH3, O2, and Pt metal, and formation of dipolar layer, thus reducing the effective Schottky barrier height. The temperature-dependent NH3 sensing behaviors are investigated for the studied device. The studied device is suitable for high temperature operation because the temperature effect would facilitate dissociation and diffusion of gases. Third, field-effect transistors based on AlGaN/GaN heterostructures are fabricated with catalytically active platinum (Pt) gate electrodes to induce the sensitivity of ammonia gas. For the studied device, a 10 nm-thick Pt metal grain is grown. When the target gas is introduced, a polarized layer is formed at Pt/AlGaN interface due to the triple-point contact (Pt, NH3, and O-, O2, or O2-), which leads to the reduction of Schottky barrier height and the increased of saturation current. Comprehensive analysis on the electrical properties at different temperatures is presented. A drastic change of ammonia detection sensitivity is observed in the cut-off region. Ammonia-induced effects on electrical parameters of a field-effect transistor (FET), such as threshold voltage, transconductance, and on-off current ratio are investigated. Fourth, ZnO nanorods (NRs)-based resistance-type ammonia sensor are mentioned and studied. Different growth conditions of ZnO NRs are discussed, such as precursor concentration, thickness of ZnO seedlayer, pH value, and growth temperature. A combination of interdigitated electrodes and 1-D ZnO NRs has been used to make an ammonia sensor with a high degree of sensitivity. We are going to study the effect of varying the electrode spacing on the sensing performance. Finally, the NH3 sensing properties of ZnO NRs decorated with Pd nanoparticles are investigated. The large surface-to-volume ratio of ZnO nanostructures is favorable for the adsorption of analytes and reduction of nanoparticles. This leads to the improved sensor performance. ZnO NRs were synthesized through a hydrothermal route on a sapphire substrate. Pd nanoparticles were reduced on the surface of ZnO NRs using an impregnation approach. It was observed that. the nano-sized Pd particles (~5 nm) were uniformly distributed on the surface of ZnO NRs. A maximum NH3 sensing response of 310.2 was found when the device was exposed to a 1000 ppm NH3/air gas at 250oC. The as-grown ZnO nanorods, however, illustrated the maximum sensing response only of 87. The temperature-dependent NH3 sensing properties of the studied device were also systematically investigated.