Luminescence studies of photoexcited electron-hole plasmas in silicon and germanium

This thesis presents the results of three studies of the photoluminescent properties of Si and Ge at low temperature. The first study is presented in Chapter 2 and deals with the transport of photoexcited electron-hole plasma (Ef:JP} in Si following pulsed laser excitation at temperatures below 50 K. The goal here was to determine the rate at which the photoexcited carriers expand fran the excitation surface into the crystal and. to determine the primary motive force. We found that for laser excitation levels up to the threshold for visible surface damage, the carriers moved away from the excitation region at velocities near or below the speed of sound. By imaging the expansion process, we found that under intense excitation levels the carriers were driven from the surface as a shell of plasma. Both of these facts point to a phonon-wind driven transport; phonons created in the hot-carrier thermalization process near the excitation region move ballistically into the crystal at low temperatures and drag the carriers with them i.e., the carriers are driven into the crystal by a "phonon wind." The second study, which is presented in Chapter 3, examines the equilibrium. properties of electron-hole liquid (EHL) in Ge as a function of uniaxial stress. This condensed phase of photoexcited carriers that forms in Ge and. Si has a high stability due to the degeneracies present in the band structure of the material. By applying uniaxial stress to a Ge crystal, many of the degeneracies are lifted -altering the energetics of the system. We employed. both uniaxial-stress and strain-confinement geanetries to examine the changes in EHL density and. lifetime as a function of stress, and the results are compared to the results of many-body theoretical calculations. The third study is presented in Appendix A and consists of a detailed examination of the phase diagram of EHL in stressed Ge. When EHL condenses from a gas of free excitons (FE), t:NJ phase changes occur: the gas-liquid transition and an insulator-metal transition. By using a strain-confinement technique, we were able to observe the phase changes both spectrally and spatially. We fomxl evidence that the 1:N:I types of phase transition occurred separately within a small temperature range.U of I OnlyThesi