Luminescent Time Decay of Excitons Bound to Zn–O Complexes in Gap

Abstract

Time-decay measurements of the photoexcited red luminescence near room temperature in a series of GaP (Zn, O) samples with varying zinc concentrations and varying thermal histories (annealing conditions) are presented. A phenomenological analysis of the electron recombination processes in GaP (Zn, O) based on semiconductor statistics and a simple kinetic model is then discussed which takes the (accurately measurable) time decays as independent variables and predicts the values of less accurately known quantities. It is demonstrated that such elusive but important parameters as the internal quantum efficiency, and the relative strengths of radiative and nonradiative centers, can be quantitatively predicted using time-decay and free-hole-concentration measurements. Curves of internal quantum efficiency are presented as functions of the experimentally measurable time decays and free-hole concentrations; as is the ratio of the capture time of the nonradiative centers to the capture time of the Zn–O complexes. These curves lead to an explanation of the dependence of the luminescent time decay of a sample on its previous thermal history. The explanation depends on (1) thermalization of electrons captured by Zn–O complexes back into the conduction band, (2) the presence of nonradiative recombination centers, and (3) a change in the relative concentrations of radiative and nonradiative recombination centers with annealing. From an analysis of the luminescent time decays it is predicted that the energy level of an electron on a Zn–O complex is 0.24±0.02 eV below the conduction-band edge at room temperature. The temperature dependences of the luminescent decay and the external quantum efficiency (measured in the range 300°–450°K using both above and below bandgap photoexcitation) can also be understood on the basis of the model. When the intrinsic radiative excitonic lifetime is allowed to vary as a function of exciton binding-energy agreement between predicted and measured photoluminescent quantum efficiencies results.