Step-flow growth of ZnO(0Â 0Â 0Â 1) on GaN(0Â 0Â 0Â 1) by metalorganic chemical vapor epitaxy

We studied the properties of 500-900 nm thick ZnO(0 0 0 1) layers grown by metalorganic chemical vapor deposition on GaN(0 0 0 1) templates. A two-step growth process was employed where a buffer layer was first grown at 350-400∘C. The temperature was then ramped up to 750-950∘C for the growth of the epitaxial layer. Atomic force micrographs over a 20×20μm2 area revealed locally straight steps, indicating step-flow growth. The root-mean-square roughness was less than 2.6 nm. Hexagonal pits were also observed with a pit density of 7.5×105-2.1×108cm-2. The full width at half maximums of ω-scans across the symmetric (0 0 0 2) and the (202¯1) reflections were as narrow as 0.091∘ and 0.159∘, respectively. Reciprocal space maps showed that the ZnO films were fully relaxed. Transmission electron microscopy revealed that the defects were mainly of mixed or pure edge dislocation type. No pure screw dislocations or stacking faults were observed.