Hot Excitons in ZnSe Quantum Wells

Abstract

In a semiconductor, an incident photon with suitable energy can excite an electron from the valence band to the conduction band, leaving a hole in the valence band. The Coulomb attraction between the electron and the hole may lead to the formation of a hydrogen-like bound state, called exciton. Beside the relative motion of the electron and the hole within the exciton, the exciton as a whole has its center-of-mass momentum and kinetic energy. In the case that the electron-hole pair is excited with some excess energy, the exciton can be formed with a significant kinetic energy and momentum. The hot exciton then travels in the semiconductor according to its momentum, and relaxes to its band minimum by giving its excess energy to the lattice through phonon scattering processes. Relaxed excitons, also called cold excitons, have an average excess energy comparable to the thermal energy of the lattice. At this stage, a quasi-equilibrium state of exciton and phonon system is reached. Eventually, the relaxed exciton recombines radiatively, resulting in photoluminescence (PL), or nonradiatively, converting the energy into heat. At high temperatures, thermal dissociation processes can break the excitons into electron-hole pairs. When the temperature is low enough that the phonon population is not sufficient for these processes, the exciton is very stable and dominates many optical processes of semiconductors.