Optical investigations of the undulation spectrum of GaP:N:Zn

The undulation spectrum occurs in the luminescence of GaP when it is doped with the isoelectronic trap, nitrogen, and an acceptor---in the present case Zn. A broad emission band is observed which extends about 20 meV below the nitrogen bound-exciton energy EA and contains regular undulations. We show that the luminescence originates from the decay of excitons bound to N-Zn pairs of up to 50-{} separation, and that the undulatory behavior is a result of averaging the numbers of equivalent sites in the pair-separation shells. The N-Zn interaction is investigated by time-resolved resonant excitation and temperature-dependent measurements. The 1.6-K decay lifetime of the luminescence decreases sharply with luminescence energy from {~} 1 {μ}sec at EA to about 50 nsec at the low-energy limit of the undulation spectrum. The splitting of the exciton energy levels increases up to 5 meV as the N-Zn separation decreases and the dominant interaction is thought to be an internal Stark effect. Spatial energy transfer by tunneling of excitons is important and determines the temperature-dependent nonradiative recombination.