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 ${\mathrm{cm}}^{-1\mathrm{}}$. Absorption spectra were measured in many samples over the following range of impurity concentrations. Net impurity concentrations, expressed in atoms ${\mathrm{cm}}^{-3\mathrm{}}$, ranged from 5×${10}^{15}$ to 9.5×${10}^{16}$ in $p$-type samples, and from 2×${10}^{15}$ to 3×${10}^{17}$ 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 ${\sigma }_p=8.65\mathrm{}\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ per hole and ${\sigma }_n=0.23\mathrm{}\times {10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ per electron (at 9 μ). Whereas the free hole absorption coefficient is roughly independent of wavelength, the free electron absorption ${\sigma }_n$ varies as ${\lambda }^2$ and agrees well with the classical Zener-Drude model.

The main absorption edge at both temperatures may be extended to lower absorption coefficients $\alpha$ by subtracting the extrapolated free carrier absorption coefficients ${\alpha }_c$. The resultant band edge $\ln (\alpha -{\alpha }_c)$ values when plotted against the photoenergy ($\hslash \omega$) 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.

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