Validation of Raman spectroscopy as a tool for mapping transport parameters in inhomogeneous N-doped 4H-SiC

We assess Raman spectroscopy as a tool for fast and non-invasive mapping of charge carrier density and carrier mobility in inhomogeneously doped 4H-SiC. For this purpose, we compare values of these transport parameters obtained by magneto-transport and Raman measurements of N-doped 4H-SiC. The comparison is not straightforward. The effective charge density and mobility, which are obtained from resistivity and Hall measurements by employing the commonly used effective one-band model, deviate from the values extracted by applying the established line-shape models for describing the longitudinal optical phonon coupled (LOPC) modes in the Raman spectra. Differentiating between free and localized carriers in the framework of a three-band transport model confirms that only the free charge carriers in the conduction band of N-doped 4H-SiC contribute to the LOPC Raman signal and their density agrees well with that obtained by the line shape analysis. The agreement of the mobility values is good keeping in mind that different frequencies of the applied electric fields are used in the two approaches, i.e., dc-transport measurements at 0 Hz and ac-fields of the exciting laser at about 500 THz. Moreover, the excitation of electrons into the conduction band by the laser, which is inherent to the Raman experiment, causes differences in the temperature dependence of the carrier density compared with the electrical transport data.