Details of below band-gap uniaxial dielectric function of SiC polytypes studied by spectroscopic ellipsometry and polarized light transmission spectroscopy

摘要

The optical properties of the below band-gap uniaxial dielectric function are studied as a function of the degree of hexagonality. The polytypes investigated were lightly doped 4H, 6H, 15R, and 21R silicon carbide. The ordinary component of the dielectric function is extracted from phase modulated spectroscopic ellipsometric measurements including mathematical removal of the overlayer. Accurate spectroscopic birefringence measurements are performed using a θ-2θ ellipsometric setup. The resulting birefringence varies as a third order polynomial as a function of the degree of hexagonality, with a turning point at 50 hexagonality. Furthermore, birefringence resulting from the intraconduction band transitions is studied both directly from the variation in the measured birefringence, and through Kramers-Kronig analysis of the extinction coefficient measured by p and s polarized transmission spectroscopy at 70° incidence. The second derivative of the dielectric function of the strongest below band-gap transition in the extraordinary component fit a one-dimensional critical point, while its energy position appears to be a linear function of the degree of hexagonality of the polytypes under investigation.