Chemical Vapor Growth of NiGa2O4 Films: Advantages and Limitations of a Single Molecular Source

Abstract

Nanocrystalline NiGa 2 O 4 films were deposited on silicon substrates by the CVD of a new heterometal alkoxide, [NiGa 2 (O t Bu) 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. [NiGa 2 (O t Bu) 8 ] is monomeric in solid state with a tetrahedral Ni 2+ center coordinated by two monoanionic (Ga(O t Bu) 4 }-moieties. Despite an adequate vapor pressure, the gas-phase transport of [NiGa 2 (O t Bu) 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 Ga 2 O 3 (amorphous), besides the target (NiGa 2 O 4 ) composition. Shortened reservoir → substrate path length (13 cm) produced stoichiometric NiGa 2 O 4 films, 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 NiGa 2 O 4 films could be determined.