PREFERRED ORIENTATION AND PIEZOELECTRICITY IN SPUTTERED ZNO FILMS

Thin ZnO films were deposited on various types of substrates with rf m agnetron reactive sputtering of a Zn target in pure O-2 atmosphere. Th e layers were characterized by x-ray diffraction and electrical measur ements. The piezoelectric strain constant d(31) Of the films was deter mined via optical interferometric measurements on the piezoelectricall y forced vibration of silicon cantilevers. Because of the high resisti vity of the ZnO samples (higher than 10(9) Omega cm), piezoelectric ex citation down to frequencies of 100 Hz was found to be feasible. The r elation between the piezoelectric strain constant and the c-axis orien tation 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 th e experimental effective piezoelectric strain constants are, at most, 60% of the value predicted by the theoretical calculations. It is thou ght that this discrepancy is due to the cancellation of piezoelectric activity by ZnO grains of opposing polarity. The latter effect and oth er structural properties of the films ate discussed in relation to nuc leation and growth of sputtered ZnO films. (C) 1998 American Institute of Physics.