Submillimeter Absorption Spectra and Phase Transition of Indirect Excitons in Germanium

The results are presented of the detailed studies on the exciton absorption spectra in ultrapure Ge in the spectrum regions 0.67-1.1 and 1.7-3.7 meV with a backward-wave tube radiation. It is shown that the measured spectrum corresponds to the exciton´s transitions from the ground to excited states. As a result of temperature measurements at 1.6-4.2 K the phase transition of excitons into the condensed state has been observed without the formation of excitonic molecules.