Epitaxial growth of Co3O4 thin films using Co(dpm)3 by MOCVD

Abstract

We report on the growth of epitaxial Co3O4 (1 1 1) thin films by close injection showerhead (CIS)-MOCVD on sapphire substrates (c-, a- and r- planes). The deposition was carried out by using tri-dipivaloylmethanato-cobalt (Co(dpm)3) as precursor for Co and pure O2 or H2O vapor as oxidant. The effects of growth conditions, including Co(dpm)3 vaporization temperature, substrate temperature, O2/H2O flow rates, growth pressure and substrate orientation on the growth of the Co3O4 thin films were investigated. The crystallinity, phase purity and surface morphology of the films were studied by XRD, Raman and UV-visible spectroscopies, RHEED, SEM, and AFM. The as-grown films were found to be pure (1 1 1) oriented normal spinel type epitaxial Co3O4 thin films. The morphology of the films was strongly dependent on the identity of oxygen source used. Films grown using pure oxygen showed strongly faceted structures while those grown using H2O were dominated by porous interwoven and nanostructured films. The size and density of the faceted surface are strongly dependent on the growth rate of the films, controlled by adjustment of growth parameters. High growth rates resulted in small and dense grains. The growth of Co3O4 (1 1 1) was only successful on the a- and c-plane sapphire substrates, with no indication of nucleation and growth on r-plane sapphire substrates. Under otherwise identical growth conditions, the growth rate of films grown using H2O vapor was found to be 5 times higher than when using pure oxygen. These results demonstrate that Co(dpm)3 is a viable source for Co3O4 thin film growth by MOCVD, with resultant film morphology being highly dependent on the oxygen source and growth parameters.