Transport of Free Excitons and Electron-Hole Droplets in Silicon and Germanium.

This thesis presents the results of a wide ranging study of the transport properties of photo-excited free excitons (FE) and electron-hole droplets (EHD). The mobilities of both species are largely controlled by interaction with lattice phonons. Non-equilibrium phonon fluxes are capable of moving EHD macroscopic distances in ultrapure crystals of silicon and germanium. Both the FE and the EHD shed momentum acquired from external forces by interaction with thermal phonons. This work places on a quantitative basis the rates at which these species lose momentum as a function of both temperature and applied force. A unique method employing calibrated crystal strain gradients is developed and used to measure the mobility and other key transport parameters of the electrically neutral excitons and electron -hole droplets. With the knowledge of these parameters, the EHD is then used as a quantitative probe of nonequilibrium phonon fluxes. In addition, an unusual three-component phase transition within the electron-hole liquid is examined.