Acoustic and optical phonon assisted formation of biexcitons

Efficient exciton relaxation is required for bounding two cooled excitons to form biexciton. Acoustic and optical phonon scatterings playing key roles in exciton relaxation are responsible for formation of biexcitons at various temperatures. Using ZnO powders, the authors observed a sublinear dependence on excitation power at low temperature, in which the relaxation process involves only emission of acoustic phonons due to the excitons having kinetic energy lower than those of the optical phonons. However, the exponent comes near theoretical value of 2 for participation of optical phonons when the exciton kinetic energy approaches to the energy of the lowest optical phonon.