The Far Infrared Absorption Spectrum of Silicon and Germanium Under High Optical Excitation

Abstract

<p>A study has been conducted, at low temperatures, of the far infrared absorption spectrum of silicon and germanium under high optical excitation. This work presents the first experimental observation of the absorption of both the gas of indirect excitons and the electron-hole drops (EHD) in silicon. Using the strength of the exciton absorption, a study of the coexistence of the exciton gas with the EHD was undertaken. For germanium, the absorption spectra produced by excitons and the behaviour of the exciton transitions in the presence of a small magnetic field were measured. The experimental excitonic absorption spectra in both silicon and germanium are compared with the results of the effective mass Hamiltonian model and it is found that the results of this model account well for the observed structure in both materials. A model for the response of the exciton levels to an externally applied magnetic field was developed. The comparison of the experimental results provided a check on the identification of the observed transitions between lS to 2P exciton states in germanium. The absorption spectrum produced by EHD is silicon is compared with a calculated spectrum using the Mie theory for the absorption of radiation by small particles. The calculation included both interband and intraband transitions for the valence bands. The agreement with the experiment is good and justifies an estimate of the value of the electron-hole density inside the drops in silicon. As a result of the study of the coexistence of the exciton gas with the EHD, estimates could be done for the radius of the drops in silicon.</p>