Fluorescent Decay Times of Excitons Bound to Isoelectronic Traps in GaP and ZnTe

An investigation of the radiative decay times of bound excitons in GaP: N, GaP: Bi, and ZnTe: O is reported. New experimental techniques involving the use of a pulsed Van de Graaff generator for exciting the samples in liquid refrigerants are described. In each material, decay to the ground state of the electronic complex can occur through either an allowed ($A$) or forbidden ($B$) transition. A study of the temperature dependence of the kinetics permits the individual decay components, ${\tau }_A$ and ${\tau }_B$, to be separated. For each material, these quantities are used to calculate the oscillator strengths of the transitions, and a value for $\frac{N}{{\alpha }_0\mathrm{}\left(A\right)\mathrm{}}$, where $N$ is the density of impurities, and ${\alpha }_0\mathrm{}\left(A\right)\mathrm{}$ the value of the absorption coefficient at the $A$ line. Agreement is found with independent measurements of $\frac{N}{{\alpha }_0\mathrm{}\left(A\right)\mathrm{}}$ in GaP: N and GaP: Bi. The values of $\frac{{\tau }_B}{{\tau }_A}$ substantiate a proposed classification of the impurities into isoelectronic donors and acceptors. In ZnTe: O, a mechanism is proposed to account for the approximately constant fluorescent efficiency and anomalous increase in decay time with increasing temperature in the range 30 to 300°K. In each material, the luminescence quenches at higher temperatures, and the simultaneous decrease in decay time enables activation energies and capture cross sections to be determined.