OPTICAL-ABSORPTION BY FREE HOLES IN HEAVILY DOPED GAAS

Optical absorption in p-type GaAs with hole concentrations between 10(19) and 10(20) cm-3 has been measured for wavelengths between 2 and 20-mu-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 (alpha almost-equal-to 20 000 cm -1 for N(A) = 5 X 10(19) cm-3) is attractive for long-wavelength infrared-detector applications.