Raman Scattering from Electronic Excitations inn-Type Silicon Carbide

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Abstract
We have performed Raman measurements at 8 K on the 6H polytype of SiC nominally doped with 4×1018 and 6×1019 nitrogen donors per cm3. The 4×1018 samples showed electronic transitions of E2 symmetry at 13.0, 60.3, and 62.6 meV that we interpret as 1s(A1) to 1s(E) valleyorbit transitions at the three inequivalent donor sites having symmetries that are, respectively, hexagonal, cubic, and cubic. To check the assignment of the electronic transitions to the inequivalent sites we studied the Raman spectrum of the 15R polytype of SiC doped with nitrogen. This polytype has five inequivalent sites: two hexagonal and three cubic. We observed two transitions at roughly 7.7 and 11.6 meV, which we assign to the hexagonal sites, and two levels at 50.6 and 54.9 meV and possibly a level at 46.0 meV, which we assign to the cubic sites. These large site-dependent differences in energy within the donor ground state point to a breakdown of effective-mass theory. The E2 symmetry of these transitions in the 6H polytype was used to show that the conduction-band minima must lie along the line ML at the edge of the Brillouin zone. An interference was observed between the 13-meV electronic transition and an E2 phonon of the pure crystal (6H) at 18.6 meV. We have proposed a phenomenological theory and obtained the electron-phonon coupling constant from a fit to theory. The 6×1019 sample revealed a Raman continuum with E2 symmetry extending to about 65 meV that interfered strongly with the 18.5-meV phonon and also with a 33.1-meV E2 phonon. The E2 symmetry and the interference effect suggest that the continuum retains some properties of localized levels even at concentrations greater than 2×1019, where the metallic transition occurs. This sample also exhibited LO-phonon-plasmon coupling. The high-frequency mode (L+) was observed and was highly asymmetric with ωPτ∼13. These parameters were found by assuming the line shape to be proportional to Imε−1, where ε is the total dielectric constant. Due to the large damping the low-frequency mode (L−) was broadened and very small compared to the high-frequency mode. Finally an E2 vibrational mode in the gap region of the SiC phonon spectrum was observed at 78.8 and 79.6 meV for nitrogen concentrations of 4×1018 and 6×