Carrier concentration and lattice absorption in bulk and epitaxial silicon carbide determined using infrared ellipsometry

We have measured the dielectric function of bulk nitrogen-doped 4H and 6H SiC substrates from 700 to 4000 cm(-1) using Fourier-transform infrared spectroscopic ellipsometry. Photon absorption by transverse optical phonons produces a strong reststrahlen band between 797 and 1000 cm(-1) with the effects of phonon anisotropy being observed in the region of the longitudinal phonon energy (960 to 100 cm(-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 x 10(17) to 10(19) cm(-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. [S0163-1829(99)00240-4].