Electron-hole plasma expansion in the direct-band-gap semiconductors CdS and CdSe

The properties of the electron-hole plasma in direct-band-gap semiconductors are investigated by excite- and probe-beam techniques with use of CdS and CdSe as examples. By spatially resolved transmission and reflection spectroscopy and by using samples of different thicknesses, the diffusion length and other data of the plasma are determined. A ``slow drift'' is found, i.e., the drift velocity is smaller than the Fermi velocities of electrons and holes. The drift distances are of the order of 10 {μ}m with some significant differences between CdS and CdSe. Various models used in the literature to fit the observed gain spectra are critically discussed.