Plasma-assisted molecular beam epitaxy was used to grow ZnO(0001) layers on GaN(0001)/Al2O3 templates and GaN/4H-SiC(0001) layers. The GaN(0001)/Al2O3 template surfaces were subjected to various pre-treatment procedures (Zn, Ga or N pre-exposure or none) prior to the ZnO growth. We studied the impact of these pre-treatment procedures on the initial growth conditions of ZnO(0001). These layers were compared to ZnO layers deposited on 4H-SiC utilizing a GaN( 0001) buffer layer that was grown in situ on the 4H-SiC substrate and immediately before the growth of ZnO. The GaN buffer layers were not pre-treated or exposed to ambient. Atomic force and scanning electron microscopy as well as secondary ion mass spectroscopy revealed that the pre-treatment procedures resulted in a very high density of islands. The islands coalesced into films as the growth progressed. In contrast, no ZnO growth occurred on the untreated GaN( 0001)/Al2O3 template surfaces. Our main finding is that Ga,03, sub-oxides residing on the surface of the as-received GaN-templates, drastically reduced the ZnO nucleation rate and completely inhibited subsequent coalescence and growth. Our various surface pre-treatment procedures aimed at removing the sub-oxides were necessary for achieving ZnO growth on the GaN-templates. No surface pre-treatment was needed to enable ZnO growth on the in situ grown GaN(0001)14H-SiC layers. (c) 2014 The Authors. Published by Elsevier B.V.