Photoionization and Excitation of Free Excitons in Germanium by Submillimeter Radiation

Abstract

Investigations of the far infrared absorption spectra of excited germanium revealed for the first time the photoionization of free excitons in semiconductors by submillimeter radiation [76], The measurements were carried out on selected samples of undoped germanium which had sufficiently long indirect-exciton lifetimes, as judged by the intensity of the free-exciton luminescence line (714.2 meV). The exciton lifetime in these samples reached several microseconds at liquid helium temperatures and the residual impurity concentration did not exceed 1 × 10¹⁴ cm^-3. The method of excitation of samples of about 1 cm² area and 0.5 mm thick and the spectroscopic measurement technique were both described in Chap. II. The quantity measured directly was the same as in studies of resonance absorption (Chap. III): we measured the ratio of the transmission of an excited sample to its “dark” transmission, which was then transformed into the product αd representing the induced optical absorption.