The conduction bands in 6H and 15R silicon carbide II. Absorption measurements

Abstract

It is shown, on the basis of classical theory, that the free carrier absorption in a uniaxial semiconductor having an anisotropic effective mass and relaxation time is characterized by two independent absorption coefficients. For a general multivalley model these absorption coefficients are shown to be quite complicated functions of the effective mass and relaxation time tensors; these functions are considerably simplified if the energy band extrema are located on the symmetry axis. In this case the anisotropy of the effective mass and also that of the relaxation time may be obtained from the free carrier absorption coefficients if these are measured at high frequencies (w 2 >r 2 ≫ 1) and also in the low frequency region (w 2 r 2 ≪ 1) in the same specimen. At low frequencies the free carrier absorption coefficients no longer have a near quadratic dependence upon wavelength but tend to limiting values. Results of measurements in the high frequency region are presented for a number of samples of both the 6H and 15R forms of silicon carbide. At wavelengths for which the absorption is due to free carriers the absorption coefficient for radiation with its electric vector (X) perpendicular to the symmetry axis (c axis) is greater than that for X || c by a factor of ˜ 4 in the 15R polytype and by a factor of ˜ 15 in the 6H form. The wavelength dependences of the absorption coefficients in the high-frequency region are discussed in terms of the probable scattering mechanisms. In one specimen of each polytype the m easurem ents were extended to ˜ 20 μm and the absorption coefficients were observed to approach limiting values as predicted by the theoretical expressions for low frequencies. Quite good agreement was found between the magnitudes of the calculated and observed absorption coefficients, and the onset of low-frequency behaviour was found to occur in approximately the wavelength region predicted by classical theory. From these results the effective m ass in the direction of sym m etry axis was found to be 1.5 ± 0*2 m 0 in the 6H polytype and ˜ 0-53 m 0 in the 15R polytype; the relaxation time appropriate to the axial direction was found to be greater than that for the transverse plane by a factor of 2.5 in the 6H form and 1.6 in the 15R form. Additional absorption bands for X ||c were found at 19.9 μm in 6H SiC and a t 17.5 μm in the 15R polytype and were of magnitude ca . 100 cm -1 and ca . 150 cm -1 respectively; their origin is as yet unknown. All the results reported relate to room temperature.