Electrocrystallization of epitaxial zinc oxide onto gallium nitride

We have studied in detail the structure and the nucleation and growth process of electrodeposited epitaxial hexagonal ZnO on GaN(0001). The crystallites grow with the c-axis perpendicular to substrate and the in-plane relationship is ZnO[100] parallel to GaN[100]. We show that the deposit aspect progressively changes with the deposition time from isolated dispersed dots toward fully covering flat zinc oxide single crystals. A simple model is presented which explains these morphological changes. The nucleation is instantaneous and the growth rate is controlled by the crystal surface reactions. The velocity of crystal growth is found dependent on the crystallographic face of the grain: it is approximately five times faster on the {0001} planes than on the {1010} and {0110} families of planes. We show that nucleation problems are encountered with high-quality GaN presenting a low surface defect density. The difficulty has been overcome by chemically treating the substrate with an aqueous ammonium hydroxide solution prior to the deposition. This treatment allows the nucleation of zinc oxide but has a detrimental effect on the deposit "mosaicity" which is better in the case of GaN with high defect density.