We used Fourier transform infrared (FT-IR) spectroscopy to characterize silicon dioxide (SiO(2)) films on a 4H-SiC(0001) Si face. We found that the peak frequency of the transverse optical (TO) phonon in SiO(2) films grown on a 4H-SiC substrate agrees well with that in SiO(2) films grown on a Si substrate, whereas the peak frequency of the longitudinal optical (LO) phonon in SiO(2) films on a 4H-SiC substrate is red-shifted by approximately 50 cm(-1) relative to that in SiO(2) films on a Si substrate. We concluded that this red-shift of the LO phonon is mainly caused by a change in inhomogeneity due to a decrease in density in the SiO(2) films. Furthermore, cathodoluminescence (CL) spectroscopy results indicated that the channel mobility of the SiC metal-oxide-semiconductor field-effect transistor (MOSFET) decreases roughly in proportion to the increase in the intensity of the CL peak at 460 and 490 nm, which is attributed to the increase in the number of oxygen vacancy centers (OVCs). FT-IR and CL spectroscopies provide us with a large amount of data on OVCs in the SiO(2) films on a 4H-SiC substrate.