C.S. Prajapati

Tailoring the Microstructural, Optical, and Electrical Properties of Nanocrystalline WO3 Thin Films Using Al Doping

We have studied the influence of Al doping on the microstructural, optical, and electrical properties of spray-deposited WO3 thin films. XRD analyses confirm that all the films are of polycrystalline WO3 in nature, possessing monoclinic structure. EDX profiles of the Al-doped films show aluminum peaks implying incorporation of Al ions into WO3 lattice. On Al doping, the average crystallite size decreases due to increase in the density of nucleation centers at the time of film growth. The observed variation in the lattice parameter values on Al doping is attributed to the incorporation of Al ions into WO3 lattice. Enhancement in the direct optical band gap compared to the undoped film has been observed on Al doping due to decrease in the width of allowed energy states near the conduction band edge. The refractive indices of the films follow the Cauchy relation of normal dispersion. Electrical resistivity compared to the undoped film has been found to increase on Al doping.

Single Chip Gas Sensor Array for Air Quality Monitoring

This paper describes the fabrication of a four element gas sensor array for monitoring air pollutants, namely CO, CO₂, NO₂, and SO₂. The four micro-heaters share a single suspended SiO₂ diaphragm, utilizing thermal proximity to achieve low power consumption (~10 mW for 300 °C). Plasma enhanced chemical vapour deposition SiO₂ diaphragm is demonstrated to give higher yield compared with thermally grown SiO₂. Sensor array elements are fabricated by customizing each element to sense a specific gas, using different sensing materials. Optimized thin films of ZnO, BaTiO₃-CuO doped with 1% Ag, WO₃, and V₂O₅ are used for selective sensing of CO, CO₂, NO₂, and SO₂. Four sensors can be independently controlled to operate at different temperatures to get high selectivity for test gases. The sensor array is packaged on Kovar header, and then characterized for gas sensing. It is demonstrated that the sensors exhibit good sensitivity and selectivity. We report a maximum repeatable response to CO (~78.3% for 4.75 ppm), CO₂ (~65% for 900 ppm), NO₂ (~1948.8% for 0.9 ppm), and SO₂ (~77% for 3 ppm) at operating temperatures of 330 °C, 298 °C, 150 °C, and 405 °C, respectively. [2016-0047]

ppb level detection of NO2 using a WO3 thin film-based sensor: material optimization, device fabrication and packaging

In this study, we have investigated the thickness-dependent nitrogen dioxide (NO2) sensing characteristics of a reactive-ion magnetron sputtered tungsten trioxide (WO3) film, followed by morphological and electrical characterizations. Subsequently, the sensing material was integrated with an MEMS platform to develop a sensor chip to integrate with electronics for portable applications. Sputtered films are studied for their sensing performance under different operating conditions to discover the optimum thickness of the film for integrating it with a CMOS platform. The optimized film thickness of similar to 85 nm shows the 16 ppb lower limit of detection and 39 ppb detection precision at the optimum 150 degrees C operating temperature. The film exhibits an extremely high sensor response (R-g - R-a)/R-a x 100 = 26%] to a low (16 ppb) NO2 concentration, which is a comparatively high response reported to date among reactively sputtered films. Moreover, this optimum film has a longer recovery time than others. Thus, an intentional temperature overshoot is made part of the sensing protocol to desorb the NO2 species from the film surface, resulting in full recovery to the baseline without affecting the sensing material properties. Finally, the optimized film was successfully integrated on the sensor platform, which had a chip size of 1 mm(2), with an inbuilt micro-heater. The minimum power consumption of the microheater is similar to 6.6 mW (similar to 150 degrees C), which is practically acceptable. Later, the sensor device was packaged on a Kovar heater for the detailed electrical and sensing characterizations. This study suggests that optimization of the sensing material and optimum operating temperature help to develop a highly sensitive, selective, stable, and portable gas sensor for indoor or outdoor applications.