Optical properties of ZnSe doped with Ag and Au

We present bound-exciton (BE) and donor-acceptor-pair (DAP) spectra of ZnSe grown by liquid-phase epitaxy and doped with the transition metals (TM) Ag and Au. Luminescence, luminescence excitation, and time decay spectra establish the assignments of the spectral features and show that Ag forms a medium deep acceptor, ${\left(E_A\right)}_{\mathrm{Ag}}=431\mathrm{}\pm 2$ meV, consistent with the activation energy for thermal quenching of the DAP spectra. This thermal technique, together with the less precise spectral measurements available for the more-strongly-phonon-coupled Au acceptor indicate that ${\left(E_A\right)}_{\mathrm{Au}}\sim 550$ meV, appreciably less than the probable value for Cu, ∼650 meV. Peculiarities in the BE properties within this TM sequence are discussed with reference to the influence of their $d$-state characteristics. Strong BE luminescence with no-phonon energy near 2.747 eV is attributed to a neutral ${\mathrm{Ag}}_{\mathrm{Zn}}$-${\mathrm{Ag}}_{\mathrm{I}}$ associate, possibly a split interstitial. Reasons for its absence in ZnSe: Au are discussed. Isotope effects in this spectrum and that of the Li neutral acceptor BE are contrasted. The latter provides proof that ${\mathrm{Li}}_{\mathrm{Zn}}$ is the persistent shallow $T_d$ site acceptor in ZnSe. Further associate BE luminescence is tentatively identified for ZnSe: Ag and ZnSe: Au.