Optical absorption by free holes in heavily doped GaAs

Optical absorption in p-type GaAs with hole concentrations between 10¹⁹ and 10²⁰ cm^-3 has been measured for wavelengths between 2 and 20 {μ}m 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 cm^-1 for NA=5×{}10¹⁹ cm^-3) is attractive for long-wavelength infrared-detector applications.