Epitaxial growth of ZnO films on patterned c-plane GaN layer using hydrothermal method

The growth of epitaxial ZnO film with a c-axis orientation on a patterned GaN/sapphire substrate using a two-step hydrothermal (HT) process is demonstrated. Honeycomb arrays of etched holes were formed on the GaN epilayer to unveil m-plane for the synthesis of ZnO crystallites with a lattice mismatch of 0.4% along c-axis (as compared with that of 1.88% on the c-plane). Effects of the etched pattern and HT processing parameters on the coalescence of hexagonal ZnO prisms are investigated. Experimental results reveal that etched holes with a 5-μm diameter, 3-μm spacing, and an off angle of 30° between the close-packing direction of the honeycomb array and the a-axis (i.e., the [11-20] direction) of GaN yield ZnO films might be most efficient for the ZnO epitaxy. The successful growth of ZnO single-crystalline films can be attributed to the m-plane of GaN providing nucleation sites with a lower lattice mismatch for the hetero-epitaxy of ZnO crystallites and the propagation of homo-epitaxial lateral growth over c- and m-plane. Material analysis results confirm that the continuous ZnO films grown on the GaN epilayer have high quality and a single-crystalline wurtzite structure, suggesting a high potential for use in the fabrication of ZnO substrates and devices.