The influence of metal-free ZnO nanoseed-layers on the growth of ZnO films on a-plane sapphire substrates by chemical vapor deposition (CVD) is investigated. For epitaxial films with a high density of (1 (1) over bar 01) inclusions we find a twofold orientation of these inclusions with the epitaxial relationships (11 (2) over bar0)(Al2O3) parallel to (1 (1) over bar 01)(ZnO) and (10 (1) over bar4)(Al2O3) parallel to (0001)(ZnO). HRXRD measurements show that these inclusions are tilted by approx. 1.45 degrees towards the sapphire substrate as a result of strain relaxation during heteroepitaxial growth. Numerical simulations with ANSYS confirm this as the energetically favorable situation. Detailed HRXRD and EBSD studies show that the occurrence of (1 (1) over bar 01) oriented nanocrystallites during nucleation is responsible for the formation of dislocations, and that the nucleation process has a pronounced influence on the crystal quality of c-plane ZnO films grown on a-plane sapphire. (C) 2015 Elsevier B.V. All rights reserved