Effect of impurities on the Raman spectra of spray-coated β-Ga2O3 thin films

Here, the incorporation of impurities into doped thin β-Ga2O3 films was studied by Raman spectroscopy, and a simple spring model was employed to estimate the impurity concentration from the impurity-modified frequencies of first-order phonon modes. β-Ga2O3 thin film samples were prepared using the spray-coating technique. As impurities, we used rare earth atoms (Er, Sm, and Gd) as well as Mg, Al, and Zn, with the nominal impurity concentrations varying from 0.5% up to 5.0%. As the impurities are expected to predominantly occupy Ga sites in the β-Ga2O3 lattice, heavier and lighter atoms than Ga should have a pronounced influence on Ga-related lattice vibrations. Therefore, in the Raman spectra of the thin films measured using 325-nm excitation, the impurity-induced shifts of the frequencies of vibrations involving Ga and O atoms were employed to estimate the impurity concentration. In addition, a high-impurity concentration can cause the formation of impurity-related oxides, as it is clearly visible for Zn. Besides, the Raman spectra with Mg as the impurity show that Mg most probably occupies interstitial rather than substitutional sites as the Raman modes do not shift with respect to the impurity concentration.