Homoleptic zirconium amidates: single source precursors for the aerosol-assisted chemical vapour deposition of ZrO2

Abstract

We report the development of a true single source precursor (i.e. without any need for an exogenous source of oxygen) for the growth of zirconia thin films by aerosol-assisted chemical vapour deposition (AACVD) using an original family of zirconium(IV) amidate derivatives, which are easily prepared by protonolysis of [Zr(NMe₂)₄] with the free amide pro-ligands. In all but one case the reactions resulted in the isolation of the corresponding homoleptic eight-coordinate zirconium(IV)tetrakis(amidato) derivatives. Three of these species along with a tris(amidato)dimethylamido zirconium(IV) derivative have been characterised by single crystal X-ray diffraction analysis. The materials potential of the homoleptic compounds was identified through the application of design criteria derived from consideration of the existing knowledge base relating to the pyrolysis of wholly organic amides. In this manner the thermal decomposition of the homoleptic derivatives benefits from facile, molecularly imposed pyrolysis pathways, which provide for the privileged generation of volatile small molecule by-products and the production of contaminant-free solid oxide material. Thermogravimetric analysis, in conjunction with NMR spectroscopic analysis of the volatile products resulting from their thermal decomposition, indicated the potential of the homoleptic species as exquisite single source precursors to ZrO₂ at moderate temperatures. The compound bearing both N- and C-iso-propyl substituents was, thus, applied as a true single source precursor under ambient pressure AACVD conditions. The resultant films, deposited on either SiO₂-coated glass or quartz substrates, are smooth and comprise small and densely packed crystalline particulates that are shown by XRD to be primarily cubic ZrO₂. Compositional analysis by X-ray photoelectron spectroscopy (XPS) revealed that the oxygen delivered, and the decomposition pathway provided, by the amidate ligand structure yields ZrO₂ films which, though slightly sub-stoichiometric (ZrO_1.8–1.9), contain undetectable levels of carbon incorporation.