Characterization of Inhomogeneity in SiO_2 Films on 4H-SiC Epitaxial Substrate by a Combination of Fourier Transform Infrared Spectroscopy and Cathodoluminescence Spectroscopy

摘要

We measured Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of SiO_2 films with various thicknesses, grown on 4H-SiC substrates. The appearance of broad phonon modes at ～1150-1250 cm~(-1) in p-polarized light and their disappearance in s-polarized light confirmed that the phonon modes at ～1150-1250 cm~(-1) originated from surface polaritons (SPPs). For the thin SiO_2 film (8-nm thick), the peak frequency of the transverse optical (TO) phonon in the SiO_2 film on the 4H-SiC substrate was observed at ～1080 cm~(-1) and was higher than that in SiO_2 films on the Si substrate (1074 cm~(-1)). This suggested that the thin SiO_2 film (8-nm thick) is under compressive stresses at the interface between the SiO_2 film and SiC substrate. On the other hand, for the thick SiO_2 films (85 and 130-nm thick), the TO phonon peak frequency tended to shift toward lower frequencies with increasing oxide layer thickness. The CL measurement indicated that the CL peak intensity at ～640 nm, attributed to non-bridging oxidation hole centers (NBOHCs), became stronger with increasing oxide layer thickness, relative to that of the CL peaks at ～460 and 490 nm due to oxygen vacancy centers (OVCs). By comparing the FT-IR and CL measurements, we concluded that the TO phonon red-shift with increasing oxide layer thickness can mainly be attributed to an increase in inhomogeneity with increasing oxide layer thickness for the thick SiO_2 films.