The hydrothermal epitaxy of ZnO films on a patterned GaN layer with a honeycomb etching hole array is demonstrated. Through m-planes of the GaN layer exposed on the vertical walls of the etching holes, highly crystalline ZnO films via multiple lateral growth stages can be realized. It is found that higher concentrations of zinc nitrate hexahydrate ( ZNH) and hexamethylenetetramine (HMT) in hydrothermal solution yield a larger number of ZnO molecules to speed up ZnO growth during the initial stage of hydrothermal growth, also create secondary crystals and initialize further lateral growth stages to bridge neighboring ZnO prisms after smooth surfaces formed on the m-plane of a ZnO prism. A successive lateral growth mechanism that strongly depends on ZNH and HMT concentrations in the hydrothermal solution is proposed and discussed. (C) 2017 Elsevier B.V. All rights reserved.