We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700 to 4000 ${\mathrm{cm}}^{\mathrm{-}1}$ using Fourier-transform infrared spectroscopic ellipsometry. Photon absorption by transverse optical phonons produces a strong reststrahlen band between 797 and 1000 ${\mathrm{cm}}^{\mathrm{-}1}$ with the effects of phonon anisotropy being observed in the region of the longitudinal phonon energy (960 to 100 ${\mathrm{cm}}^{\mathrm{-}1}$). The shape of this band is influenced by plasma oscillations of free electrons, which we describe with a classical Drude equation. For the 6H-SiC samples, we modify the Drude equation to account for the strong effective mass anisotropy. Detailed numerical regression analysis yields the free-electron concentrations, which range from $7\times {10}^{17}$ to ${10}^{19}{\mathrm{cm}}^{\mathrm{-}3},$ in good agreement with electrical and secondary ion mass spectrometry measurements. Finally, we observe the Berreman effect near the longitudinal optical phonon energy in $n-/n+$ homoepitaxial 4H SiC and hydrogen implanted samples, and we are able to determine the thickness of these surface layers.

• Received 16 February 1999

DOI:https://doi.org/10.1103/PhysRevB.60.11464