Nucleation and growth of ZnO on (1 1̄ 2 0) sapphire substrates using molecular beam epitaxy

It has been recently demonstrated that it is possible to eliminate 30° rotation twins in c-oriented epitaxial ZnO films by growing on the (1 1̄ 2 0) face of sapphire despite the apparent symmetry mismatch between the (6-fold) epilayer and the (2-fold) substrate. To further elucidate the details of the growth process, we have investigated the initial stages of growth of ZnO on (1 1̄ 2 0) sapphire using molecular beam epitaxy. Films of approximately 3, 5, 8, 10, 20, 100, and 600 nm thickness were investigated by in situ reflection high-energy diffraction, high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and extended X-ray absorption (XAFS). XAFS indicates that there is no discernible change in nearest-neighbor distance with film thickness. On the other hand, HRXRD measurements indicate an ∼1% increase in the c-lattice constant with decreasing film thickness. XAFS measurements of the second nearest-neighbor structural disorder were observed to increase sharply for the thinnest films implying that distortion of bond angles is responsible for the increase in the c-axis observed by HRXRD. HRXRD reciprocal area maps of the symmetric ZnO (0 0 0 2) reflection indicate the onset of diffuse scattering from 20 nm, while AFM indicates a concurrent change in surface morphology suggesting a change in the growth process for t > ∼20 nm.