Synthesis and characterization of nonstoichiometric NiGa2O4 transparent thin films

Transparent nickel gallate spinel films were synthesized by means of an inter-diffusion reaction between NiO ceramics and (0 0 1) β-Ga2O3 single crystalline substrates at 1300 oC and annealed at various temperatures after removal of NiO pellets. The as-prepared films were characterized by X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, and optical absorption spectroscopy. It is revealed that the transparent reaction layers are single phase crystalline spinel with preferred orientations. Results from the EDX analysis show a homogeneous composition after equilibration, with a Ga/Ni atomic ratio of about 2.4 and 2.7 for samples quenched from 1300 oC and 1400 oC, respectively. Optical absorption spectra of spinel films are dominated by ligand field transitions of Ni2+ ions in octahedral sites with overlapping absorption bands of Ni2+ ions on tetrahedral sites.