Development of high-quality gate insulators to improve the performance of MISiC Schottky-diode hydrogen sensors

by Tang, Wing-man

(Uncorrected OCR) Abstract of thesis entitled Development of High-Quality Gate Insulators to Improve the Performance of MISiC Schottky-Diode Hydrogen Sensors submitted by Tang Wing Man for the Degree of Master of Philosophy at The University of Hong Kong in August 2004 Hydrogen is a highly inflammable gas, which needs only a spark to ignite and cause a serious explosion. Because hydrogen is so potentially dangerous, hydrogen sensors are routinely used to detect hydrogen leaks in industrial fabrication processes, medical installations, and hydrogen-fueled vehicles. There has recently been increasing interest in the development of hydrogen sensors that can operate in harsh environments, such as aerospace and automobile applications. Different materials and fabrication methods are used to make the sensors. Metal-Insulator-SiC (MISiC) Schottky diodes are the most commonly- used hydrogen sensors, because they are relatively easy to make and use fairly simple electronic circuitry in the ir operation. The sensitivity and stability of a sensor depend largely on its gate insulator, as the interface between the gate insulator and the substrate can greatly influence the sensor? performance. The development of high-quality gate insulator for sensors has therefore become a very important topic of research. In this study, MISiC Schottky-diode hydrogen sensors with gate insulators grown in various conditions were fabricated, and investigated to determine the optimum ii processing conditions for making sensitive, stable and reliable hydrogen sensors. MISiC Schottky-diode hydrogen sensors with gate insulators grown in three different nitridation gases (NO, N2O and NH3) were fabricated. Current-voltage, voltage-time and current-time characteristics were studied for each sample and compared. Experimental results showed that the passivation effect of NH3 improved sensor performance. The study also found that N2O provided the fastest oxide growth and highest sensitivity of the three nitrided samples. Sensitivity was found to be strongly related to the thickness of the insulator: the thicker the insulator, the higher the sensitivity. Sensors with oxynitride grown at different temperatures and for different durations were also fabricated for investigation. Steady-state and transient responses were measured at various temperatures and hydrogen concentrations with a computer-controlled measurement system. Increasing the nitridation temperature and extending the nitridation duration were both found to improve sensor performance, particularly the former. The effects of metal annealing on sensor performance were also studied. A new technique for growing the insulator layer in O2 and TCE was also developed to improve sensor performance. The hydrogen-sensing characteristics of this novel sensor were fully studied. The effects of hydrogen adsorption on the barrier height and current conduction mechanism of the sensor were investigated. The proposed sensor was compared with another sensor with the insulator grown in NO gas and the control sensor without the insulator, and displayed higher sensitivity, higher reliability, and faster response. With its excellent hydrogen-sensing properties, this sensor is ideal iii for detecting hydrogen leakage in harsh environments, and deserves to be fund a practical application in this field. iv