Deposition of doped and undoped ZnO thin films for gas sensors

ZnO thin films have been deposited onto a (100)Si substrate by using rf planar magnetron sputtering. ZnO:Al films have also been prepared by co-sputtering by means of two distinct rf sources, matched to Al and ZnO targets, respectively. The structural and morphological properties have been investigated in order to optimize the film growth for gas sensor applications. It is shown that the polycrystalline films exhibit strong c-axis orientation. The full width at half maximum (FWHM) of x-ray rocking curve of (002) plane is obtained less than 2.60 degrees. ZnO films are almost stoichiometric, while the content of Al in doped ZnO films is measured as 1 divided by 3 at. %. The x-ray photoelectron spectroscopy (XPS) depth profile reveals a uniform stoichiometric ratio along the growth axis. The crystallite grain size of the films, measured by scanning electron microscope (SEM), is in the range 100 divided by 130 nm The thin films show good piezoelectric properties and ZnO-on-Si layered structures have been used to implement surface acoustic wave (SAW) delay lines. ZnO:Al layers have been tested as sensitive elements of calorimetric gas sensors based on SAW delay line as thermal probe. Transient responses of ZnO:Al SAW calorimetric sensor towards O-2 are reported.