Photo-Excited States in Germanium at Liquid Helium Temperatures.

A wide variety of experimental work dealing with the basic properties of photoexcited states in Ge at liquid helium temperatures is presented. The primary emphasis is on the electron-hole liquid (EHL) and the free exciton (FE). The EHL is composed of two interpenetrating Fermi liquids, one of electrons and one of holes, each with its own Fermi level. The FE dealt with here is a mobile, loosely bound state of an electron and a hole. We report the first absolute measurement of the density dependence of the enhancement factor g(,eh)(O) for the EHL in Ge. This factor is a measure of the electron -hole spatial correlation function, and provides a valuable and sensitive test for the predictions of various many -body-theory approximations. An EHL droplet - FE gas system confined to a strain induced potential well was used. The measurement approach relies on only a few simple and verifiable assumptions. The results presented here agree quantitatively with the predictions of two different many-body-theory approximations. A byproduct of this work was the measurement as a function of stress of: the electron and hole Fermi levels, the EHL density, the condensation energy of a FE relative to the EHL, and the binding energy of a FE relative to free carriers (FC). The EHL decay was also characterized. The first direct measurement of the FE diffusivity D(,x) is reported. The evolution in time of spatial profiles of FE luminescence were measured. From these FE density profiles, D(,x)(4.2K) = 300('2)cm s('-1), the surface recombination velocity S = 3000 cm s('-1), and the FE lifetime (tau)(,x) = 27 (mu)s with surface effects excluded were determined. It is observed that surface recombination may have dramatically effected previous measurements of (tau)(,x). An electron -phonon scattering time (tau)(,p) = 2 x 10('-10)s is inferred. We have studied the hysteresis of the EHL no-phonon luminscence in order to explore several hypotheses concerning nucleation sites for EHL droplets. We find evidence for droplets being bound on several qualitatively different crystal defects. The decay of a FC-FE system confined to a strain well is studied.