Cw spectroscopy of ultrafast relaxation in semiconductor heterostructures

Steady state hot luminescence measurements can be used to determine hot carrier distributions and relaxation rates for extremely fast processes with low injected carrier densities. Luminescence gives a direct measure of the distribution of hot carriers in a semiconductor. We discuss experiments which probe the distribution of electrons high in the conduction band. These measurements have been used in GaAs to determine the LO phonon emission time by a hot electron(100 fs), the F to L intervalley scattering rate (250 fs for an electron initially about 70 meV above the bottom of the L-valleys), and the r to X intervalley scattering rate (30 fs for an electron about 85 meV above the bottom of the X-valleys). Recent experiments which measure the relaxation of hot electrons in the presence of a high density of cold electrons in bulk GaAs and GaAs quantum wells are presented.