Initial growth and very thin a-plane (1 1 (2) over bar 0) ZnO films grown on r-plane (1 (1) over bar 2 0) Al2O3 substrates by plasma-assisted molecular beam epitaxy (PAMBE) were studied using cross-sectional and plan-view transmission electron microscopy (TEM). The ZnO films were grown via Volmer-Weber growth mode, in which the (1 0 (1) over bar 0) facets were developed first followed by the (1 1 (2) over bar 0) facets. Critical thickness was determined to be a value between 2.5 and 3.5 nm. Since surface normal of the (1 (1) over bar 0 2) Al2O3 is [1 (1) over bar 0: 4025], while that of (1 1 (2) over bar 0) ZnO is [1 1 (2) over bar 0], two diffraction patterns for Al2O3 [2 (2) over bar 0 1] and ZnO [1 1 (2) over bar 0] zone axes were overlapped, which shows very different features in the bright field (BF) micrographs and selected area electron diffraction (SAED) patterns. So, careful analysis and caution are needed in characterizing the structural defects by plan-view TEM. (C) 2009 Elsevier B. V. All rights reserved.