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

We have studied the chemical and material aspects of molecular precursor-derived materials taking the example of the spinel MgAl 2 O 4 . Three Mg−Al alkoxides, [MgAl 2 (OPr i ) 8 ], [MgAl 2 (OBu t ) 8 ] and [MgAl 2 (OBu t ) 4 H 4 ] were used as single molecular precursors in the gas-phase synthesis of the MgAl 2 O 4 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, [MgAl 2 (OPr i ) 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 [MgAl 2 (OPr i ) 8 ] provides adequate vapor pressure for growing high-quality spinel films. In contrast, the bulkier tert -butoxide groups in [MgAl 2 (OBu t ) 8 ] make it thermally and structurally more stable, however causing a lower vapor pressure and higher decomposition temperature. [MgAl 2 (OBu t ) 4 H 4 ] 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 [MgAl 2 (OBu t ) 4 H 4 ] induces a designed ligand elimination, based on the β-hydride elimination. Despite the fact that microstructured MgAl 2 O 4 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.