Tuning electrical resistivity anisotropy of ZnO thin films for resistive sensor applications

This work reports on the preparation and characterization of Zinc Oxide (ZnO) thin films, exploring their anisotropic electrical resistivity behaviour evolution as a function of the different preparation conditions. Results show that by increasing the oxygen content in the work gas flow, the electrical resistivity increases from about 2.4 x 10(-2) Omega.cm (common in fairly conductive-like materials) to values in order of M Omega.cm, which is typical of materials that vary from semiconducting to roughly insulating type. Regarding the thin films structural behaviour, a relatively clear evolution of the preferential growth becomes clear when preparing the films within the three different regimes. The X-ray diffraction (XRD) patterns revealed the development of a polycrystalline hexagonal-like structure, confirmed by the presence of dominant and very intense ZnO (002) and ZnO (101) diffraction peaks. A change in preferential growth was noticed with the change in the preparation conditions. By in-air annealing the samples from room temperature (RT) up to 200 degrees C and cooling down to RT again, the differences between the resistivity values along the xx and yy directions become significant, a clear sign of the anisotropic nature of the films. The anisotropy obtained during the annealing cycles of the films increased from 0.98 to 1.10 and, consequently, the Temperature Coefficient of Resistance, TCR, increased in both xx and yy directions, from TCRxx (and yy) similar to 9.0x10(-4) to TCRxx = 6.82 x 10(-3) and TCRyy = 5.97 x 10(-3) degrees C-1. Important also to note is that the increase of the grain size of the films showed to have a significant effect in the effective anisotropy, revealing to be a major factor to take into account.