Luminescence studies of the excitonic system in germanium: Phase properties and transport

This thesis presents the results of three studies of the excitonic system in Ge. These studies employ near-infrared spectroscopy and imaging techniques to examine the phase and transport properties of the low temperature exciton-plasma system. The first study, presented in Chapter II, examines the properties of the electron-hole liquid (EHL) phase as a function of -uniaxial stress. The application of stress along a axis greatly reduces the degeneracy of the conduction and valence bands, thereby reducing the stability of the EHL. This thesis reports the first observation of the EHL in the high-stress limit (u > 70 kgf/mm2). The observation of the EHL in this high-stress regime serves as an important confirmation of theoretical calculations of EHL properties. The success of this experiment can be attributed to the use of the nonuniform Hertzian stress geometry. The second study, presented in Chapter III, uses time-resolved spectroscopy to study the phase properties of the exciton gas, EHL, and electron-hole plasma (EHP) in unstressed Ge. A detailed spectroscopic analysis of spectra taken near and above the liquid-gas critical temperature is presented. By employing an analysis using three components (excitons, excitonic complexes, and EHP), evidence is found for a second exciton-plasma condensation at temperatures above the EHL critical temperature. A phase diagram for the excitonic system is proposed that includes the second condensed phase. Uniaxial stress is used to study the electron-hole droplet (EHD) cloud in Chapter IV. By applying stress along a axis, the two conduction band valleys perpendicular to the stress axis can be emptied of electrons in the EHL phase. This alteration of the electron Fermi surface causes remarkable changes to occur in the anisotropic EHD cloud structure. The changes in the cloud structure illustrate the important role that the anisotropic Fermi surface plays in giving the cloud its shape.U of I Onlydissertatio