Investigation of longitudinal-optical phonon-plasmon coupled modes in SiC epitaxial film using fourier transform infrared reflection

Room-temperature Fourier transform infrared reflection (FTIR) spectroscopy was carried out on heavily and lightly doped 4H-SiC films grown by chemical vapor deposition on conducting, n-type (nitrogen) doped substrates. A model-based curve fit of the experimentally observed reflectance spectra from these samples is performed using a dielectric function that accounts for the phononphoton coupling and plasmon-photon coupling. The value of the longitudinal-optical (LO) phonon frequency estimated from the reflectance spectrum in the range 600–1,200 cm−1 is observed to increase in direct correlation with the electron free-carrier concentration (FCC). This shift reflects phonon-plasmon coupled modes. The shift from the undisturbed ωLO observed, for example, in semi-insulating SiC is exploited to improve the model-based estimation of epi parameters such as thickness and doping in cases where the free-carrier concentration is less than about 2×1018 cm−3.