The structure of planar defects in ZnO films on (0112) r-plane sapphire was studied by the combination of high-resolution electron microscopy and computer simulations. The dominant defects present in these films were identified as a type I-1 intrinsic-stacking fault. with a displacement vector of (a/6) [2023] and a density of similar to 6 x 10(4) cm(-1), lying in the (0001) basal plane. We found the formation of these stacking faults to be growth related, corresponding to the face-centered-cubic stacking sequence in wurtzite ZnO. (c) 2006 American Vacuum Society.