Degradation of the piezoelectric response of sputtered $c$-axis AlN thin films with traces of non-(0002) x-ray diffraction peaks

We have observed the apparently anomalous behavior of sputtered AlN thin films that exhibit poor piezoelectric response in spite of having (0002) preferred orientation and narrow rocking curve with full width at half maximum below 2°; however, other AlN films with weak 0002 x-ray diffraction peak and rocking curve as wide as 8° provide a good value of the piezoelectric coefficient $d31$ $(2.1pm∕V)$⁠. The critical difference between both types of films is the presence of traces of non-(0002) reflections in the x-ray diffraction patterns of the former. Non-(0002) reflections may be related to defects (probably, inversion domains) that significantly reduce the net piezoelectric field.