Temperature dependance of the anisotropy of the infrared dielectric properties and phonon-plasmon coupling in n-doped 4H-SiC

Abstract

Polarized infrared reflectivity spectroscopy was used to measure the temperature dependence of the anisotropy of the infrared dielectric properties and coupling be- tween phonon and plasmon modes in n-doped 4H-SiC. The advantage of polarized infrared spectroscopy over the commonly used Raman spectroscopy is that it en- ables dielectric, plasmonic, and transport properties to be probed along and per- pendicular to the c-axis, which is impossible with Raman spectroscopy because of its selection rules. In addition, the infrared spectrum, comparatively rare in Raman spectroscopy, can be described at high temperatures using simple classical theory, providing more precise information on the material’s properties. Analysis of the s- and p-polarized infrared reflectivity spectra of an n-doped 4H-SiC substrate shows that the strength and damping of the oscillators, the response of the bound elec- trons to infrared excitation, the effective ion mass, and the transverse frequency and anharmonicity of the phonons of n-doped 4H-SiC are temperature-sensitive and strongly anisotropic. It also shows that the plasmonic properties of n-doped 4H-SiC, namely the relaxation time and collective oscillation frequency of free electrons, are also highly anisotropic and, owing to the long relaxation time of free electrons in the direction parallel to the c-axis, only the axial longitudinal phonon-plasmon modes split into low-frequency and high-frequency modes at temperatures above 700 K.