Thickness effect on stress, structural, electrical and sensing properties of (002) preferentially oriented undoped ZnO thin films

Preferentially oriented (002) ZnO thin films with c-axis-oriented wurtzite structure have been grown on Si (100) and glass substrates using radio frequency magnetron sputtering. The residual stresses have been determined and calculated via the Stoney formalism. The ZnO thin films have been also characterised by X-ray diffraction and scanning electron microscope, and their stoichiometry was verified by Rutherford backscattering spectroscopy. The evolution of the residual stress was studied as a function of film thickness in the 10-1200nm range. A growth scenario is proposed and a possible correlation between the residual stress, film's texture and crystallographic orientation is highlighted. The crystalline quality was found to improve, while the stress values decreased with increasing thickness, and as a ramification the thicker films developed better sensing response to gases. The mechanical (stress) and electrical properties of the films were also investigated as a function of the film thickness, which tended to manifestly improve in dependence on thickness as well. We attribute this to the fact that the thinner films are under vehement misfit stress that declines with increasing the film thickness further.