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THE CONDUCTION BANDS 6H AND 15R SILICON CARBIDE - II ABSORPTION MEASUREMENTS.
ROYAL AIRCRAFT ESTABLISHMENT FARNBOROUGH (ENGLAND)
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It is shown, that the free carrier absorption in a uniaxial semi-conductor is characterized by two independent absorption coefficients. For a general multivalley model these coefficients are complicated functions of the effective mass and relaxation time tensors but are simplified if the energy 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 absorption coefficients if these are measured both at high frequencies and also in the low frequency region in the same specimen. At long wavelengths approx. 20 microns the absorption coefficients approach limiting values as predicted by the theoretical expressions for low frequencies. From these results the effective mass in the direction of symmetry axis was found to be 1.5 plus or minus 0.2 mo in the 6H polytype and approx. 0.53 mo 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.
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