Sputtering temperature dependent growth kinetics and CO2 sensing properties of ZnO deposited over porous silicon

We report the growth kinetics and sensing properties of ZnO deposited over macro-porous silicon substrates at 400 and 600 degrees C using magnetron-sputtering technique. Scanning electron microscopy was employed to investigate the morphology and the particle size of the ZnO nanoparticles (NPs). The grain growth kinetics was analyzed with the help of the phenomenological equation r(n) = k(0)texp(-Q/RT) finding an activation energy Q = 13.92 kJ/mol. The grain growth exponent (n = 2.85) for the growth at 400 degrees C corresponds to an Ostwald ripening process, while the growth at 600 degrees C is described by n = 1.66 implying a higher growth rate attributed to a high surface diffusion of add-atoms contributing to the formation of larger grains. The sensing response of the complete structure has been tested at different temperatures. The highest sensitivity, S similar to 10, was obtained at a sensor temperature of 300 degrees C on the ZnO NPs sputtered on to the porous silicon substrate at 400 degrees C. The high response is attributed to the infiltration, uniform and homogenous distribution of the ZnO NPs into the pores. ZnO NPs sputtered at 400 degrees C are found to be smaller than those grown at 600 degrees C, exhibiting a larger surface-area/volume ratio and hence increasing the oxygen adsorption resulting in an enhanced CO2 sensitivity. (C) 2016 Elsevier Ltd. All rights reserved.