Epitaxial Growth of ZnO Nanowires on a- And C-Plane Sapphire

Epitaxial ZnO nanowires were grown on a- and c-plane sapphire substrates by metalorganic chemical vapor deposition without metal catalysts or templates. Nanowires with monodisperse diameters grow in dense arrays perpendicular to a-plane sapphire and with in-plane rotational alignment due to ZnO∥sapphire, View the MathML source epitaxy. On c-plane sapphire, multiple possible epitaxial relations give a mixture of nanowire orientations. The majority of the nanowires grow in one of the three directions all at an angle of 51.8° off the substrate plane with ZnO∥sapphire, View the MathML sourceZnO∥sapphire epitaxy. A small fraction of the nanowires grow perpendicular to the substrate with ZnO∥sapphire.

Optical properties in undoped-ZnO epilayers grown by the laser-molecular-beam epitaxy method on lattice-matched ScAlMgO4 substrates were investigated. The absorption spectrum at 5 K has two sharp peaks, both of which are attributed to resonances of A and B excitons, which reflect a small nonradiative damping constant of excitons as well as high film crystallinity accomplished by the virtue of lattice matching. The coupling strengths of exciton-acoustic phonon and of exciton–longitudinal-optical phonon were directly determined from the temperature dependence of exciton absorption spectra independently for A and B excitons, which are close in energy and obey the same selection rule for each other.


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