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 MgA1(2)O(4). Three Mg-A1 alkoxides, [MgA1(2)(OPri)(8)], [MgA1(2)(OBut)(8)] and [MgA1(2)(OBut)(4)H-4] were used as single molecular precursors in the gas-phase synthesis of the MgA1(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, [MgA1(2)(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 [MgA1(2)(OPri)(8)] provides adequate vapor pressure for growing high-quality spinel films. In contrast, the bulkier tert-butoxide groups in [MgA1(2)(OBut)(8)] make it thermally and structurally more stable, however causing a lower vapor pressure and higher decomposition temperature. [MgA1(2)(OBut)(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 [MgA1(2)(OBUt)(4)H-4] induces a designed ligand elimination, based on the P-hydride elimination. Despite the fact that microstructured MgA1(2)O(4) films with sufficient crystallinity and a columnar microstructure could be obtained by tuning the growth parameters of the three Mg-A1 compounds, this study underscores the importance of precursor chemistry in designing an efficient CVD process.