Preferred orientation and piezoelectricity in sputtered ZnO films

Thin ZnO films were deposited on various types of substrates with rf magnetron reactive sputtering of a Zn target in pure O2 atmosphere. The layers were characterized by x-ray diffraction and electrical measurements. The piezoelectric strain constant d31 of the films was determined via optical interferometric measurements on the piezoelectrically forced vibration of silicon cantilevers. Because of the high resistivity of the ZnO samples (higher than 109 Ω cm), piezoelectric excitation down to frequencies of 100 Hz was found to be feasible. The relation between the piezoelectric strain constant and the c-axis orientation distribution of the film [obtained from the full width at half maximum of the x-ray rocking curves at the ZnO (002) diffraction] was determined both experimentally and theoretically. It was found that the experimental effective piezoelectric strain constants are, at most, 60% of the value predicted by the theoretical calculations. It is thought that this discrepancy is due to the cancellation of piezoelectric activity by ZnO grains of opposing polarity. The latter effect and other structural properties of the films are discussed in relation to nucleation and growth of sputtered ZnO films.