Epitaxial growth and characterization of copper gallate (CuGa2O4) thin films by pulsed laser deposition

Abstract

High-quality crystalline copper gallate (CuGa2O4) thin films were grown on c-plane sapphire substrates using the pulsed laser deposition (PLD) technique by optimizing the growth parameters (i.e., temperature, oxygen chamber pressure, and laser energy density). The obtained XRD patterns validated that the preferred orientation of the crystalline CuGa2O4 thin film is along the [111] direction, and the crystallinity of the films improved with increasing substrate temperature while maintaining a constant O2 pressure, as assessed by measuring the full width at half maximum (FWHM) of the prominent (222) plane. Reducing the oxygen pressure leads to the emergence of β-Ga2O3 peaks rather than the CuGa2O4 peaks due to the incomplete oxidation of copper (Cu). Subsequent XRD phi (φ) scans revealed a sixfold rotational symmetry of CuGa2O4 and a 30° epitaxial relationship between the film and the sapphire substrate. X-ray photoelectron spectroscopy (XPS) spectra confirmed the presence of Cu1+, Cu2+, and Ga3+ oxidation states in the films. Increasing laser energy density resulted in the complete oxidation of Cu and an increase in the film thickness from 51.8 nm to 183.4 nm. The direct bandgap of CuGa2O4 films was determined to be approximately 4.50 eV by analyzing the UV–Vis absorbance spectra using the Tauc equation. The refractive index was found to be approximately 2.05 ± 0.01 based on the spectroscopic ellipsometry data. While the impact of increasing laser energy density was negligible on the parameters such as crystal structure, rotational symmetry, oxidation states of gallium (Ga), oxygen (O), bandgap, and refractive index of CuGa2O4 thin film, notable alterations in the oxidation state of Cu and film thickness were observed. The surface roughness (Rrms) measured from AFM images showed a strong correlation with the values derived from ellipsometry analysis.