Kwang Soo Yoo

Sensing characteristics of dc reactive sputtered WO3 thin films as an NOx gas sensor

In order to apply WO3 thin films to the NOx gas sensor, WO3 thin films (3000 A) were fabricated by using dc reactive sputtering method on alumina substrate and assembled as a unit of an NOx gas sensor by adopting a patterned heater on the back side of substrate. The deposition temperatures of WO3 thin film were changed from 200°C to 500°C, and then post-annealed for the crystallization for 4 h at 600°C. There were no WO3 phases at the substrate temperature of 200°C, but the crystalline phases of WO3 thin film were appeared with the increase of substrate temperature from 200°C to 500°C. The post-annealing of as-deposited WO3 thin films at 600°C resulted in the enhancements of crystallinity, but it was observed that the quality of the final phases severely depends on the initial formation of phase during deposition. From the SEM images, crack free morphologies were found, which was different from the room temperature growth films. The sensitivity (Rgas/Rair) of as-deposited thin films was ranged from 4 to 10 for the 5 ppm NO test gas at the measuring temperature of 200°C. However, after post-annealing process at the temperature of 600°C, the sensitivities were increased around the values of 70–180 at the same test condition. These results show the WO3 thin films need to be processed at least at the temperature of 600°C for the well-improved sensitivity against NOx gas. It was also observed that the recovery rate of a sensing signal after measuring sensitivity was faster in the in-situ sputtered films than in the evaporated films or room temperature sputtered films.

Evaluation of the AC response of Li-electrolytic perovskites Li0.5(LnxLa0.5-x)TiO3 (Ln = Nd, Gd) in conjunction with their crystallographic and microstructural characteristics

Abstract We performed an in-depth electrochemical study on the ion-blocking electrochemical cells Pt/Li 1/2 (Ln x La 1/2− x )TiO 3 /Pt (Ln=Nd, Gd) over a wide frequency, impedance magnitude, and temperature range, and proposed an equivalent circuit which is original in its low frequency interpretation. The electrical properties of the bulk and the grain boundary were analyzed closely in relation to the crystallography and the characteristic microstructure of the system.

Surface morphology and gas-sensing characteristics of SnO2−x thin films oxidized from Sn films

Abstract SnO 2−x gas sensors have been fabricated by thermal oxidation of Sn thin films. Sn thin films approximately 15 000A˚thick for commercial applications are deposited on a polished alumina substrate by using a high-vacuum resistance-heating evaporator. These films are oxidized at 500°C in various oxygen partial pressures in order to control the x value in SnO 2−x . The surface morphology and quantitative compositional analysis of SnO 2−x thin films as functions of oxygen partial pressures are systematically investigated by SEM, RBS and XPS. Gas sensitivities and response times of Pd-doped SnO 2−x thin films to H 2 , CO, C 3 H 8 and i-C 4 H 10 gases are measured at 300°C. The relationship between gas sensitivities and the x values in SnO 2−x thin films is discussed.

NO2 gas sensing characteristics of Pt–Wo3–Si3N4–SiO2–Si–Al capacitor

Abstract A new gas-sensitive capacitor utilizing Pt–WO 3 –Si 3 N 4 –SiO 2 –Si–Al MIS structure has been explored. The incorporation of Pt as the catalytic gate and WO 3 as the gas adsorptive oxide with the silicon MIS capacitor enhances the detection of NO 2 at a relatively low temperature. Device performance in terms of sensitivity is presented. A new detection model based on a change in concentration of ions at Pt–WO 3 interface is proposed for the sensing mechanism. Detection mechanism for NO 2 is attributed to the chemisorption at extrinsic surface states on the adsorptive oxide surface in a form of negatively charged nitrogen dioxide ions. The model is confirmed by the reaction kinetic analysis at the interface.

Chemiresistive Sensor Array Based on Semiconducting Metal Oxides for Environmental Monitoring

We present gas sensing performance based on 2uf0b42 sensor array with four different elements (TiO2, SnO2, WO3 and In2O3 thin films) fabricated by rf sputter. Each thin film was deposited onto the selected SiO2/Si substrate with Pt interdigitated electrodes (IDEs) of 5 uf06dm spacing which were fabricated on a SiO2/Si substrate using photolithography and dry etching. For 5 ppm NO2 and 50 ppm CO, each thin film sensor has a different response to offers the distinguishable response pattern for different gas molecules. Compared with the conventional micro-fabrication technology, 2uf0b42 sensor array with such remarkable response pattern will be open a new foundation for monolithic integration of high-performance chemoresistive sensors with simplicity in fabrication, low cost, high reliablity, and multi- functional smart sensors for environmental monitoring.

Temperature vs. Resistance Characteristics by Dopants of VO 2 Thick-Film Critical Temperature Sensors

For various additives doped- critical temperature sensors using the nature of semiconductor to metal transition, the crystallinity, microstructure, and temperature vs. resistance characteristics were systematically investigated. As a starting material of sensor, vanadium pentoxide () powders were used, and CaO, SrO, , , and PbO dopants were used, respectively. The powders with dopants were mixed with a vehicle to form paste. This paste was silk screen-printed on substrates and then -based thick films were heat-treated at for 2 hours in gas atmosphere for the reduction to . From X-ray diffraction analysis, phases for pure , and CaO and SrO-doped thick films were confirmed and their grain sizes were 0.57 to . The on/off resistance ratio of the sensor in phase transition temperature range was and that of the 0.5 wt.% CaO-doped sensor was . The presented critical temperature sensors could be commercialized for fire-protection and control systems.