Chemical vapor growth of NiGa2O4films: Advantages limitations of a single molecular source

Nanocrystalline NiGa2O4films were deposited on silicon substrates by the CVD of a new heterometal alkoxide, [NiGa2(OtBu)8]. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) showed a single-step decomposition behavior for the molecular precursor at low temperature (240°C), suitable for a CVD process. [NiGa2(OtBu)8] is monomeric in solid state with a tetrahedral Ni2+center coordinated by two monoanionic {Ga(OtBu)4}-moieties. Despite an adequate vapor pressure, the gas-phase transport of [NiGa2(OtBu)8] is susceptible to the distance and geometry of the effective diffusion path. Investigations on CVD deposits obtained using different transport pathways (reservoir –> substrate) show that gas-phase travel through a long (39 cm) and angular reactor tube induces fragmentation of the heterometal compound, which yields minor amounts of Ni, NiO, and Ga2O3(amorphous), besides the target (NiGa2O4) composition. Shortened reservoir –> substrate path length (13 cm) produced stoichiometric NiGa2O4films, apparently due to reduced collision probabilities and a laminar flow. In both cases, the chemical composition was determined using energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS), whilst the structure was evaluated using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). From these, the advantages and limitations of the single molecular source in the growth of NiGa2O4films could be determined.