Characterization of Inhomogeneity in Thermal Oxide SiO₂ Films on 4H-SiC Epitaxial Substrates by a Combination of Fourier Transform Infrared Spectroscopy and Cathodoluminescence Spectroscopy

Abstract

We measured Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of SiO2 films with a various thickness, grown on 4H-SiC substrates. The peak frequency of the transverse optical (TO) phonon mode was blue-shifted by about 5 cm−1 as the oxide-layer thickness decreased from 50-60 nm to 10 nm. The blue shift of the TO mode is considerd to be caused by interfacial compressive stresses in the oxide-layer. On the other hand, the TO phonon mode was found to dramatically decrease as the oxide-layer thickness decreased from 10 nm to 1.7 nm. The CL measurement indicates that the intensity of the CL peaks at about 460 and 490 nm attributed to oxygen vacancy centers (OVCs) for No.2 become stronger than that for No.1. From a comparison between FT-IR and CL measurements, we concluded that the red-shift of the TO phonon with decreasing the oxide-layer thickness can mainly be attributed to an increase in inhomogeneity at the SiO2/SiC interface with decreasing oxide-layer thickness.

Stress Characterization of the Interface Between Thermal Oxide and the 4H-SiC Epitaxial Layer Using Near-Field Optical Raman Microscopy

“…Hirai and Kita 25 measured FT-IR spectra for thermal oxides on 4H-SiC (0001) Si and C faces and reported that the peak frequency of TO phonons remains nearly constant for thicker oxides (5–30 nm thick); this result is only slightly different from that reported by the authors, which agrees well with that reported by Seki et al. 26 In an extant study, 6 using a two-plate model comprising a thin SiO 2 layer on a 4H-SiC epitaxial layer, the authors suggested that the observed blueshift could be ascribed to a compressive stress of approximately 0.44 GPa induced across the SiO 2 –SiC interface.

Figure 6.Change in TO-phonon frequency for samples 1 and 2 as a function of oxide-layer thickness.

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“…Our model is supported by the increase of CL intensity, ascribed to OVCs in Yoshikawa et al. 6 We believe that the tensile stress in the SiO 2 /4H-SiC interface, greater than that in the SiO 2 /Si interface, is one of factors that D it and Q eff in the SiO 2 /4H-SiC interface are one to two orders of magnitude higher than those in the SiO 2 /Si interface.…”

“…(3) The TO-phonon frequency in FT-IR spectra demonstrates a drastic redshift along with an increase SiO 2 inhomogeneity (SiO 2–x formation), 6 thereby resulting in dramatic reduction of the strong compressive stress induced within the oxide layer. A large redshift of the TO phonon in FT-IR spectra can be observed near the SiO 2 –Si interface.…”

“…In this study, epitaxial layers measuring nearly 10 μm-thick were grown via chemical vapor deposition (CVD) on a 4° off-axis 4H-SiC (0001) Si face. 6 After being cleansed in accordance with the cleaning process of the Radio Corporation of America (RCA), the said layers were thermally oxidized in dry O 2 at 1280 ℃ for 30 min. Using an ellipsometer, the oxide-layer thickness was measured to be in the range of 50–60 nm.…”

“…1,2 Fourier transform infrared (FT-IR) spectroscopy is a powerful technique for studying chemical-bonding structures within silicon-oxide films. [3][4][5] In a prior study, 6 FT-IR measurements were measured along with cathodoluminescence (CL) spectra of thermal oxide (SiO 2 ) films on 4H-SiC substrates. Findings of the said study demonstrated TO phonon to be blueshifted by approximately 5 cm À1 owing to reduction in oxide-layer thickness from 65 nm to 10 nm.…”