Optical and structural characterization of ZnO thin films and fabrication of bulk acoustic wave resonator (BAW) for the realization of gas sensors by stacking ZnO thin layers fabricated by e-beam evaporation and rf magnetron sputtering techniques

High quality zinc oxide thin films have been fabricated by reactive e-beam evaporation in an oxygen environment. The effect of the growth temperature on the optical and structural properties of the e-beam evaporated ZnO is investigated. The hexagonal wurtzite structure is ascertained with X-ray diffraction and Raman spectroscopy. X-ray diffraction measurements have shown that e-beam evaporated ZnO films are highly c-axis oriented. Transmittance measurements showed that the best optical and structural quality of the e-beam evaporated ZnO occurred at 300 °C, correlating with X-ray diffraction and Raman spectroscopy. Finally, thin films of ZnO evaporated by e-beam technique have served to vanish the compressive stress due to the sputtered piezoelectric ZnO, and therefore to ameliorate the quality of the fabricated resonators by stacking these ZnO layers fabricated by electron beam technique and rf magnetron sputtering, respectively. A model for wurzite ZnO resonators based on Mason model was developed in order to simulate its hyperfrequency behavior. The reflection coefficient is derived from simulation and showed good agreement with the experimental characterization by the network analyser. Both experiment and simulation showed a resonant frequency at about 710 MHz.