XAFS observations of initial growth of (0001) ZnO on {110} sapphire substrates

We have investigated the initial stages of growth and relaxation of ZnO grown on {110} (a-sapphire) films ranging from 2.5 to 100 nm in thickness. A significant feature of this process is the change in symmetry in going from the twofold symmetry of the a-sapphire surface to the six-fold symmetry of ZnO. Asgrown films were investigated by atomic force microscopy (AFM), high-resolution x-ray diffraction, and polarized XAFS. AFM measurements indicated planar growth even from the earliest stages of epitaxy reflective of the long surface diffusion length of Zn. High-resolution x-ray diffraction measurements demonstrated resolution-limited (0002) rocking curves for films up to 30 nm thick (early growth) upon which the onset of diffuse scattering was observed, eventually coming to completely dominate for films thicker than 100 nm (late-growth). Polarized XANES and EXAFS measurements were performed on the Zn edge to elucidate structural changes from early to late-growth. Analysis of EXAFS data indicated an anisotropic accomodation of misfit with large in-plane bond rotations and with only a slight outward relaxation of the ZnO lattice along the [0001] direction. Electrical measurements indicate that the electron concentration in this initial layer is degenerate.