Subnanosecond Time-Resolved Emission Study of Cu sub 2 O and GaAs

摘要

This thesis presents a series of time-resolved luminescence and light scattering experiments which are aimed at studying how nonequilibrium electron-hole populations excited by picosecond laser pulses in semiconductors reach equilibrium. The development of a time-delayed coincidence photon counting system for the time-dependent measurements is described, and this system is tested by measuring the photoexcited free carrier lifetimes in GaAs doped with deep impurities. The bulk of this thesis is devoted to the study of the dynamics of excitons in Cu sub 2 O. We first develop a model to explain quantitatively the lineshape of nonthermalized luminescence spectra in Cu sub 2 O which have been obtained with continuous wave lasers. Then we measure the time-dependence of the ortho-exciton and para-exciton populations in Cu sub 2 O as a function of temperature. We demonstrate conclusively that the orthoexciton lifetime is dominated by its decay into the para-exciton, and furthermore, this decay rate increases with temperature as T/sup 3/2/. As a result, at low temperatures the ortho-excitons and para-excitons are not in thermal equilibrium, but at T> approx. 40 degrees K the two exciton systems reach thermal equilibrium almost instantaneously. We also studied the time-dependence of both one-phonon and two-phonon Raman modes when resonantly enhanced at the ortho-exciton in Cu sub 2 O, and found from their time-dependence that these modes should be more correctly identified as hot luminescence. We conclude that time-dependent light scattering and emission measurement is a technique which is capable of greatly improving our knowledge about dynamic processes occurring not only in condensed matter, but, in principle, also in molecular and biological systems. 17 references. (ERA citation 09:050203)