Study of the impurity photoconductivity in $p$-InSb using epitaxial $p^+$ contacts

The optical absorption coefficient $\alpha$ in $p^+$-InSb layers (with hole concentrations of $p\approx$ 1 $\times$ 10$^{17}$–1.2 $\times$ 10$^{19}$ cm$^{-3}$), grown by liquid-phase epitaxy on $p$-InSb substrates, is measured in the spectral range of 5–12 $\mu$m at 90 K, and the impurity photoconductivity is measured (at 60 and 90 K) in $p^+$–$p$ structures. It is found that a in the $p^+$ layers reaches a value of 7000 cm$^{-1}$ (at $p\approx$ 2 $\times$ 10$^{19}$ cm$^{-1}$). It is shown that the measured substrate value of ($\alpha\approx$ 1–3 cm$^{-1}$) is overestimated in comparison with estimates ($\alpha\approx$ 0.1 cm$^{-1}$) based on comparing the photoconductivity data. This discrepancy is explained by the fact that the optical transitions of holes responsible for photoconductivity are obscured by the excitation of electrons to the conduction band. The photoionization cross section for these transitions does not exceed 1 $\times$ 10$^{-15}$ cm$^2$.