We measured the Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of silicon dioxide (SiO2) films grown on 4H-silicon carbide (4H-SiC) substrates and confirmed that the phonon observed at around 1150-1250 cm(-1) originates from the upper branch of the surface phonon polaritons (SPPs) in the SiO2 films and that its frequency is sensitive to the oxide thickness. The relative intensity of the upper branch of SPPs normalized by that of the transverse optical phonon (TO) tended to increase with decreasing channel mobility (CM). A comparison of the FT-IR and CL measurements shows that the relative intensity is correlated with an inhomogeneity in the SiO2-SiC interface and the CM of SiC devices. A combination of FT-IR spectroscopy and CL spectroscopy provides us with a large amount of data on the inhomogeneity, defect, and oxide thickness of SiO2 films on 4H-SiC substrates.