Investigation of initial growth and very thin (image) ZnO films by cross-sectional and plan-view transmission electron microscopy

Initial growth and very thin a-plane (image) ZnO films grown on r-plane (image) 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 (image) facets were developed first followed by the (image) facets. Critical thickness was determined to be a value between 2.5 and 3.5 nm. Since surface normal of the (image) Al2O3 is [image], while that of (image) ZnO is [image], two diffraction patterns for Al2O3 [image] and ZnO [image] 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.