Microstructural investigation of ZnO films grown on (1 1 1) Si substrates by plasma-assisted molecular beam epitaxy

Abstract

Microstructural analyses of of ZnO films on (1 1 1) Si substrates grown by plasma-assited molecualr beam epitaxy were performed in this study. Zn pre-deposition and its subsequent oxidation, in which either oxygen gas or oxygen-plasma was used as the oxygen source, were employed before ZnO growth. Both reflection high energy electron diffraction and x-ray pole figure showed the single crystalline features in the ZnO films with both post-oxidation of deposited Zn. Detailed transmission electron microscopy (TEM), however, revealed a locally multi-crystalline feature with 30 degrees-rotated domians at the near-interface regions in the ZnO film with oxidation by oxygen gas. ZnO film with oxidation of pre-deposited Zn by oxygen-plasma was observed to be single crystalline through the whole thickness by TEM. We observed a new epitaxial relationship, (0 0 0 1)ZnO//(1 1 1)Si and [0 1 1¯ 0]ZnO//[1 1¯ 0]Si, with a crystallographic rotation of ZnO with respect to Si by 30 degrees, which is energitically more favorable because of a lower lattice misfit (2.2%). No cracks were observed from the ZnO film with a thickness of 1.5 μm, supporting the mechanical integrity of the film prepared in this study.