Raman scattering studies of chemicalhyphen;vaporhyphen;deposited cubic SiC films of (100) Si

摘要

Raman backscattering studies for a series of cubic SiC (3Chyphen;SiC) singlehyphen;crystal films grown on (100)Si by way of chemical vapor deposition (CVD) with SiC film thicknessesdSiCfrom 600 Aring; to 17 mgr;m are performed. Raman spectra of samples withdSiC4 mgr;m show a sharp and strong feature which obeys the selection rule for the 3Chyphen;SiC LO(Ggr;) phonon line. The Raman signals from the SiC film and the Si substrate show the same polarization behavior which confirms that the crystalline orientations of the Si substrate and 3Chyphen;SiC film are the same. Although there is a big lattice mismatch of 20percnt; between 3Chyphen;SiC and Si the observed Raman shifts between 3Chyphen;SiC/Si and free 3Chyphen;SiC films are le;2 cmminus;1and reflect strains of 0.1percnt;ndash;0.2percnt; in films thicker than 4 mgr;m. Some interesting enhancements of Si and 3Chyphen;SiC Raman signals are reported. The Si 522 cmminus;1phonon from a Si wafer is enhanced in intensity by a factor of 2ndash;3 due to a CVD overlayer of cubic SiC. Furthermore, the 3Chyphen;SiC longitudinal optical phonon at the Ggr; point, LO(Ggr;), from SiC/Si samples is enhanced by a factor of 2 or 3 following the removal of the Si substrate. The strict selection rules are no longer obeyed in this case. The former is possibly due to the electrichyphen;fieldhyphen;induced inelastic scattering from the SiC/Si heterojunction. The latter is explained by the multiple reflection in free 3Chyphen;SiC films. Our theoretical analysis shows that if the cross section for forward scattering is about one order of magnitude larger than that for backscattering, this enhancement will appear. The forward scattering has different selection rules from the back scattering and thus could lead to the appearance of the forbidden transverse optical phonon and the depolarization of the 3Chyphen;SiC Raman phonons from the SiC free films. The variation of the Raman spectrum with incident power has also been studied. The Raman cross section for 3Chyphen;SiC is estimated by a new method. The wavelength shifts of 3Chyphen;SiC LO(Ggr;) phonons taken from SiC/Si or free films are measured and explained.