Effect of Impurities on Free-Hole Infrared Absorption inp-Type Germanium

The free-hole absorption in $p$-type germanium (gallium-doped) is studied as a function of concentration over the range from ${10}^{15}$ to ${10}^{20}$ acceptors/${\mathrm{cm}}^3$. The spectra are modified as the carrier and total impurity concentrations are increased. With increasing concentration the structure in the spectrum becomes less pronounced. The effects observed are consistent with changes in the Fermi level and with nonvertical transitions induced by charged impurity centers.