Intraband relaxation time in highly excited semiconductors

Effects of Carrier Confinement and Intervalley Scattering on Photoexcited Electron Plasma in Silicon

The ultrafast reflectivity of silicon, excited and probed with femtosecond laser pulses, is studied for different wavelengths and energy densities. The confinement of carriers in a thin surface layer delimited by a nanoscale Si-layered system buried in a Si heavily-doped wafer reduces the critical density of carriers necessary to create the electron plasma by a factor of ten. We performed two types of reflectivity measurements, using either a single beam or two beams. The plasma strongly depends on the photon energy density because of the intervalley scattering of the electrons revealed by two different mechanisms assisted by the electron-phonon interaction. One mechanism leads to a negative differential reflectivity that can be attributed to an induced absorption in X valleys. The other mechanism occurs, when the carrier population is thermalizing and gives rise to a positive differential reflectivity corresponding to Pauli-blocked intervalley gamma to X scattering. These results are important for improving the efficiency of Si light-to-electricity converters, in which there is a possibility of multiplying carriers by nanostructurization of Si.

Long-wavelength limit of the dynamical local-field factor and dynamical conductivity of a two-component plasma

A systematic approach to the optical conductivity is given within a dielectric function formalism. The response function as well as the dynamical local-field factor G(k-->,omega) of an electron-ion plasma can be expressed in terms of determinants of equilibrium correlation functions which allow for a perturbative treatment. The dynamical collision frequency nu(omega)=-iomega(2)(pl)G(0,omega)/omega for fully ionized weakly coupled plasmas is evaluated in the low-density limit. A renormalization function is given to describe the effects of higher moments of the distribution function, thus the Spitzer formula is reproduced in the static limit. The existence of the third moment sum rule can be shown analytically. Numerical calculations are presented for the dynamical conductivity of hydrogen plasmas at solar core conditions.