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^ and 298^. 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 510^15 to 9. 510^16 in p-type samples, and from 210^15 to 310^17 in n-type samples, as determined from Hall coefficients measured at 78^. 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^ are ₏=8. 6510^-16 cm^2 per hole and ₍=0. 2310^-16 cm^2 per electron (at 9). Whereas the free hole absorption coefficient is roughly independent of wavelength, the free electron absorption ₍ varies as ^2 and agrees well with the classical Zener-Drude model. The main absorption edge at both temperatures may be extended to lower absorption coefficients by subtracting the extrapolated free carrier absorption coefficients ₂. The resultant band edge ln (-₂) values when plotted against the photoenergy () fits a straight line. The slopes of these band edges increase at the lower temperature and decrease (either at 78^ or 298^) as the acceptor concentration in the optical sample increases. Various models previously proposed are compared with the experimental results.