Uniaxial locked epitaxy of ZnO on the a face of sapphire

Abstract

High-quality, $c$ -oriented ZnO epitaxial films have been grown on the $a$ surface using molecular-beam epitaxy. The use of $a$ -oriented sapphire eliminates rotational domains and related structural defects which have limited the use of ZnO in electronic applications. The ZnO epitaxial layers are uniquely oriented with the ZnO/sapphire orientational relationship $\text{[0001]‖[112̄0]}$ and $\text{〈112̄0〉‖[0001].}$ This unique orientation is a consequence of the anisotropy of the $a$ -sapphire surface in conjunction with a strong correlation along a single direction leading to the term uniaxial locked epitaxy. High-resolution x-ray diffraction measurements show an increase in x-ray lateral coherence length from several tens of nanometers to $\text{>0.7\hspace{0.05em}μ}\mathrm{m}$ for growth of $c$ -oriented ZnO on the $a$ surface as opposed to the $c$ surface of sapphire.