Characterization of gas sensing HfO2 coatings synthesized by spray pyrolysis technique

Hafnium oxide (HfO2) films were deposited using the ultrasonic spray pyrolysis deposition technique. The films were prepared using hafnium oxychloride as a raw material and deposited on Corning glass substrates at temperatures ranging from 300 °C to 500 °C. Their crystalline structure was dependent on the deposition temperature. At substrate temperatures below 350 °C the deposited films were almost amorphous, while at substrate temperatures higher than 400 °C the films became the monoclinic phase of HfO2. Scanning electron microscopy and electron energy dispersion analysis showed a very rough surface with spherical particles of nearly stoichiometric HfO2. Such films were assessed as active layers for humidity and propane sensors. Furthermore, their response characteristics calculated on the basis of the real and imaginary parts of the impedance and also on the total impedance value were determined as a function of the measurement frequency. The dynamic response to different concentrations of propane was obtained. The response and recovery times were also determined. Finally, a brief discussion on the possible sensing mechanisms was also presented.