Infrared Absorption inP-Type Germanium

Infrared absorption in $p$-type germanium was investigated at various temperatures down to 5°K. The absorption coefficient is proportional to the hole concentration and shows band structure, a strong band beyond 10 microns and two weaker bands at shorter wavelengths. The strong band is shown to be caused by excitations within the filled band because of the presence of holes. The relative strengths of the two weaker bands change with temperature, only one being observable at low temperatures.