Optical Properties of Free Electrons in CdS

Abstract

The contribution of free electrons to the refractive index and extinction coefficient of Ga‐doped CdS has been measured in a series of samples with carrier concentrations ranging from about 10¹⁷ to 2×10¹⁹ electrons/cm³. The effective mass m* for electrons near the bottom of the conduction band was calculated from the free electron contribution to the refractive index and from the carrier concentration as determined from the Hall coefficient. The result, m*=(0.22±0.01)m_e is in satisfactory agreement with previous studies by other workers. The magnitude and wavelength variation of the absorption coefficient observed with about 10¹⁷ carriers/cm³ are in fair agreement with theoretical results calculated for a polar‐mode lattice scattering mechanism. Although similar data for 2×10¹⁸ carriers/cm³ are in good quantitative agreement with the impurity scattering theory at room temperature, the absorption for high carrier concentration is observed to decrease with temperature, in contradiction with the theory for a nondegenerate population. A theory for the degenerate concentration range is needed. Reflection and transmission of radiation polarized parallel and perpendicular to the crystal C axis were studied for one sample. These data show (m_⊥*/m_∥*)=1.08±0.05.