Optical Absorption in Pure Single Crystal InSb at 298° and 78°K

The absorption spectra of single crystal homogeneous InSb were measured in the spectral range 5 to 10 microns at temperatures of 78^∘{}K and 298^∘{}K. Primary emphasis was placed on the precise determination of absorption coefficients less than 400 cm^-1. Absorption spectra were measured in many samples over the following range of impurity concentrations. Net impurity concentrations, expressed in atoms cm^-3, ranged from 5×{}10¹⁵ to 9.5×{}10¹⁶ in p-type samples, and from 2×{}10¹⁵ to 3×{}10¹⁷ in n-type samples, as determined from Hall coefficients measured at 78^∘{}K.In general, the spectral range covered included regions where the absorption was dominated by either free-carrier absorption or valence-conduction band transitions. Free-carrier absorption in p-type InSb indicates a simple valence-band structure about k=0, consisting of light and heavy hole bands. Free carrier cross sections at 298^∘{}K are σₚ=8.65×10^-16 cm² per hole and σₙ=0.23×10^-16 cm² per electron (at 9 {μ}). Whereas the free hole absorption coefficient is roughly independent of wavelength, the free electron absorption σₙ varies as λ² and agrees well with the classical Zener-Drude model.The main absorption edge at both temperatures may be extended to lower absorption coefficients α by subtracting the extrapolated free carrier absorption coefficients αc. The resultant band edge ln(α-αc) values when plotted against the photoenergy (ω) fits a straight line. The slopes of these band edges increase at the lower temperature and decrease (either at 78^∘{} or 298^∘{}K) as the acceptor concentration in the optical sample increases. Various models previously proposed are compared with the experimental results.