X-ray diffraction measurement of residual stress in epitaxial ZnO/α-Al2O3 thin film

Piezoelectric thin films on high acoustic velocity non piezoelectric substrates, such as ZnO and AlN, deposited on diamond or sapphire substrates, are attractive for high frequency and low-loss surface acoustic wave devices.In this work, ZnO films were epitaxialy grown on R-Al2O3 and C-Al2O3 substrates using plasma-assisted molecular beam epitaxy (MBE). The surface roughness of the ZnO layers is about 0.2 nm for a 2 μm × 2 μm AFM scan area. The absolute epitaxial relationships between ZnO films and α-plane sapphire are investigated by X-ray diffraction techniques. The resulting relationships for ZnO films on R-Al2O3 and C-Al2O3 substrates are found to be (1 1 2¯ 0)[0 0 0 1]ZnO//(0 1 1¯ 2)[0 1¯ 1 1]R-Al2O3 and (0 0 0 1)[1 0 1¯ 0]ZnO//(0 0 0 1)[2 1¯ 1¯ 0]C-Al2O3, respectively. An analytical technique to determine residual strain in epitaxial ZnO films by X-ray diffraction (XRD) was studied. Based on piezoelectric constitutive equations and Bragg law, we applied an extended model, in which the elastic, piezoelectric and dielectric constants are introduced, to evaluate the residual stress in these films.