Chemical Vapor Deposition of MgAl₂O₄ Thin Films Using Different Mg−Al Alkoxides: Role of Precursor Chemistry

Abstract

We have studied the chemical and material aspects of molecular precursor-derived materials taking the example of the spinel MgAl2O4. Three Mg−Al alkoxides, [MgAl2(OPri)8], [MgAl2(OBut)8] and [MgAl2(OBut)4H4] were used as single molecular precursors in the gas-phase synthesis of the MgAl2O4 films. A comparative evaluation of the growth rates, morphology, microstructure, average particle size, consistency of elemental ratio, and carbon contamination in the films shows that material properties of the CVD deposits are a function of the chemical design of the precursor molecule. The intrinsic precursor properties (physical state, vapor pressure, decomposition temperature, etc.) can be tuned by a judicious choice of ligand(s) or their combination. For instance, [MgAl2(OPri)8] based on isopropoxide ligands displays a potential to oligomerize upon aging due to the presence of an unsaturated metal center (Mg) in the precursor framework. Nevertheless, the liquid state of [MgAl2(OPri)8] provides adequate vapor pressure for growing high-quality spinel films. In contrast, the bulkier tert-butoxide groups in [MgAl2(OBut)8] make it thermally and structurally more stable, however causing a lower vapor pressure and higher decomposition temperature. [MgAl2(OBut)4H4] exhibits substantially high vapor pressure but the films obtained contain small amounts of residual organics, although the combination of hydride and tert-butoxide ligands in [MgAl2(OBut)4H4] induces a designed ligand elimination, based on the β-hydride elimination. Despite the fact that microstructured MgAl2O4 films with sufficient crystallinity and a columnar microstructure could be obtained by tuning the growth parameters of the three Mg−Al compounds, this study underscores the importance of precursor chemistry in designing an efficient CVD process.