Determining the concentration of free charge carriers in PbTe and Pb$_{0,8}$ Sn$_{0,2}$Te from the reflection spectrum in the far-ir region

A contactless nondestructive method of determining the concentration of free charge carriers in PbTe and Pb$_{0,8}$ Sn$_{0,2}$Te from the IR reflection spectrum in the wavelength range 20-120 microns is proposed. Theoretical calibration curves are constructed, allowing the charge-carrier concentration in PbTe at T = 300 and 80 К and in Pb$_{0,8}$ Sn$_{0,2}$Te at T = 80К to be determined from the position of the point of inflection of the spectral dependence of the reflection coefficient on the light wavelength. The error of the metod is estimated, and shown to be no more than ±30%.