Epitaxial growth and characterization of magnesium gallate (MgGa2O4) thin films by pulsed laser deposition

Abstract

In this work, crystalline MgGa2O4 thin films were grown on c-plane sapphire substrates using the pulsed laser deposition (PLD) technique in a broad range of temperatures and oxygen pressures. The parameters for the crystalline growth were within the temperature range of 300 ℃ to 700 ℃ and the pressure range of 1 × 10-1 to 1 × 10-3 Torr. The acquired XRD patterns confirmed the film growth along the [111] preferential crystal orientation in the lattice and the rocking curve measurement of the prominent (222) plane showed an increasing trend of the crystallinity with growth temperature and pressure. Furthermore, the XRD phi (φ) scan demonstrated the six-fold rotational symmetry of the MgGa2O4 films and the epitaxial relationship of 30° between the film and sapphire substrate. The XPS spectra confirmed the presence of +2 and +3 oxidation state for Mg and Ga respectively, in the films and vacancies that increase with decreasing oxygen partial pressure. The direct bandgap of MgGa2O4 films was obtained to be ∼5.27 ± 0.03 eV by analyzing the UV-Vis absorbance spectra. The film surface exhibited a granular morphology with an increasing trend in grain size as the temperature increased. The refractive index and thickness of these films were in the range of ∼1.90 ± 0.02 and ∼70 ± 2.0 nm, respectively determined from the fitted spectroscopic ellipsometry data. The surface roughness was obtained from AFM images and found to be in good agreement with those obtained from the analysis of the ellipsometry data.