Optical absorption by free holes in heavily doped GaAs

Optical absorption in p-type GaAs with hole concentrations between ${10}^{19}$ and ${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$ has been measured for wavelengths between 2 and 20 μm and compared with results of theoretical calculations. In contrast to previous measurements at lower doping levels, the occupied hole states are far from the zone center, where the heavy- and light-hole bands become parallel. This gives rise to a large joint density of states for optical transitions. It is found that the overall magnitude of the observed absorption is explained correctly by the theory, with both free-carrier (indirect) and inter-valence-band (direct) transitions contributing significantly to the total absorption. The strength of the absorption (α≊20 000 ${\mathrm{cm}}^{\mathrm{-}1}$ for ${\mathit{N}}_{\mathit{A}}$=5×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$) is attractive for long-wavelength infrared-detector applications.