Time-Resolved Low Temperature Luminescence in Cadmium Sulfide: Line Shapes, Surface Effects, Polariton Excitation Transport and Spin-Polarization Effects.
Several aspects of low temperature edge emission of CdS have been investigated using time-resolved luminescence techniques. These include the origin of the line shapes and polarizations of the luminescence, the bulk or surface origin of the luminescence, the mechanism by which incident light activates the luminescence centers, and the effect on the luminescence of strong magnetic fields at low temperatures, which result in highly spin-polarized electrons and holes. The line shape problem was successfully solved by adding to the line broadening due to donor-acceptor pair coulomb energy a broadening component consisting of a series of TO phonon replicas separated by 5.5 meV, whose amplitudes correspond to those of the well known 38.5 meV LO phonon replicas, as well as Lorentz shape broadening arising from the charged neighbors surrounding the active luminescence pair. This latter amounts to a half-width contribution of several meV and varies with sample concentration and compensation. The origin of the polarization of the luminescence was not ascertained, but doubt was raised concerning a previously proposed mechanism involving acceptor impurity states based upon valence band functions corresponding to different valence bands. High resolution spectra on high grade samples which are characterized by narrow line widths showed no observable energy difference (within 0.5 meV) for the luminescence with the different polarizations, contrary to the theoretical expectation. The preponderant contribution of the surface, rather than the bulk, to the luminescence was established by using combinations of sample surface treatments which greatly lower the luminescence efficiency of one or both sample surfaces. In this way, it was not only established that the surface provides the principal luminescence contribution but that a non-optical excitation traverses samples of several mm thicknesses with little attenuation and activates luminescence centers on the rear surface. This is one of our principal results, and we suggest that the common excitation mechanism at both surfaces derives from exciton-polaritons generated by the incident light at the front surface. Experiments of this type which take advantage of the time-resolved mode of detection also indicate that velocity components of the exciton-polariton as low as 10('3) cm/sec are effective in exciting luminescence centers. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI.
- Pub Date:
- Physics: Condensed Matter