QUANTUM CONFINEMENT OF EXCITONS AND THEIR RELAXATION PROCESSES IN CUCL MICROCRYSTALS

Semiconductor microcrystals of CuCl with different mean sizes of an order of nm are grown inside transparent matrices of NaCl. Since the effective Bohr radius of excitons in CuCl is about 0.7 nm, the excitons in such microcrystals keep their bulk-like character even under the strong influence of confinement effect and the size-quantization occurs mainly in their translational motion. The blue shift and the splitting of the exciton levels are found with a decrease of the crystal size. Although there exists some size distribution of the microcrystals, exciton luminescence under the size-selective excitation with picosecond laser light and the analysis including strong reabsorption effects makes possible the determination of the quantum efficiency of exciton radiative decay. The size-dependent radiative decay time of excitons is found to be inversely proportional to the volume of the microcrystals as long as the energy separation between the size-quantized sublevels is larger than the thermal energy. This fact is quantitatively interpreted as the result of the change in the coherence volume of excitons whose motion is restricted by the microcrystal size. © 1990.