Optical properties of (AlxGa1-x)2O3 on sapphire

The (AlxGa1-x)2O3 and Ga2O3 films are epitaxially grown on sapphire by pulsed laser deposition (PLD). From X-ray photoelectron spectroscopy (XPS) and X-ray diffraction measurements, the (AlxGa1-x)2O3 films with Al compositions of 0.39, 0.49 and up to 0.53 are all single crystal and there is an out-of-plane tensile strain in (AlxGa1-x)2O3 films within the range from 0.164% to 0.345%. The optical properties are investigated by Spectral Ellipsometry (SE) together with the optical transmission method. The spectral dependence of the refractive index (n) by SE is in accordance with the reported experiment results. The thicknesses of the Ga2O3 and (AlxGa1-x)2O3 films obtained by SE fitting are 201, 116.8, 40 and 84.61 nm, respectively, which is consistent with the field emission scanning electron microscopy (FESEM) measurement results. In addition, with the Al composition increasing, the bandgaps of the (AlxGa1-x)2O3 films determined from the SE are both increase from 4.95 to 5.49, 5.7 and 5.75 eV, almost identical to the values determined by the transmittance spectra, which is larger than some extent compared to reference [13] for the compressive strain in the (AlxGa1-x)2O3 films.