Picosecond spectroscopy of CdSe at high excitation densities

We report time-resolved spectra of CdSe luminescence at high excitation densities and superfluid-helium temperatures. Measurements are taken that separate various spurious effects, which tend to distort such data. Density inhomogeneity effects and excitation-induced absorption and gain are shown to be important in interpreting the data. Time-resolved measurements are extended to much later time delays than previously reported. At short time delays, i.e., immediately following the pulsed excitation, the luminescence is shown to derive from an electron-hole plasma. Fitting the time-resolved spectra to a plasma line-shape function shows that the plasma, when over-pumped, expands into the crystal in times that are short compared to 20 ps. These fittings also give $E_B=E_x-E\left(n_0\right)=3\mathrm{}$ meV, where $E_x$ is the free-exciton energy and $E\left(n_0\right)$ is the minimum of the ground-state energy per pair curve of the electron-hole plasma. The 3-meV value agrees fairly well with theory. With increasing time delay, the plasma emission is shown to go over to $M$-band emission and finally to bound-exciton emission. Polarization measurements indicate that the $M$ band derives from $I_2$-band luminescence from the deep regions of the crystal, which has been distorted by absorption while passing through an electron-hole plasma lying close to the surface of the crystal. We also show that spontaneous $P$ bands do not exist and that stimulated $P$ bands derive from an electron-hole plasma. We propose an explanation for why the energy of the $P$ band scales with the free-exciton binding energy in different II-VI semiconducting compounds.