Far-Infrared Studies of Excitons in Germanium and Silicon

Abstract

<p>This thesis examines three aspects of exciton physics by using far-infrared (FIR) spectroscopy. First, the energy level structure of free excitons was investigated in germanium under the application of a uniaxial, compressive stress along the [lll] direction. At high stress, the exciton absorption spectra display a nearly hydrogenic-like structure with strong, isolated absorption lines. A theoretical model was developed to predict the positions and strengths of these absorption lines. Excellent agreement was obtained between experiment and theory. This is in contrast to the situation at zero stress where the exciton spectrum is not understood very well.</p> <p>Then, the first measurement of the FIR absorption spectrum of a bound exciton is reported. This spectrum provides information on the odd parity excited states of the bound exciton which cannot be observed using conventional near-infrared spectroscopy. The bound exciton absorption spectrum was found to be in excellent agreement with those of acceptors in silicon, giving the best confirmation of the model of the energy level structure of an isoelectronic bound exciton.</p> <p>Finally, the thermodynamic equilibrium between free excitons and carriers is investigated in germanium. Their absolute concentrations were obtained from their FIR absorptions. Measurements of the exciton and carrier concentrations allowed a direct determination of the equilibrium constant for any given temperature and carrier generation rate. The experimental equilibrium constant was found to be in excellent agreement with the classical equilibrium constant of an ideal gas.</p>