Optical properties of Ag-related centers in bulk ZnSe

A detailed optical study of defects induced by Ag diffusion into bulk ZnSe is presented. Two dominating Ag-related centers are found, a deep acceptor ${\mathrm{Ag}}_{\mathrm{Zn}}$ with binding energy $E_A$=430±2 meV and a neutral complex with a bound exciton (BE) transition at 2.747 eV. These results are in good agreement with recent data for liquid-phase-epitaxial Ag-doped ZnSe, but differ from previous work on bulk ZnSe, where several additional Ag-related defects were reported from photoluminescence data. From dye-laser-excited selective photoluminescence and excitation spectra both S- and P-like ‘‘hydrogenic’’ excited states of the ${\mathrm{Ag}}_{\mathrm{Zn}}$ acceptor are observed, in a sequence quite similar to more shallow substitutional acceptors in ZnSe. Thus the ${\mathrm{Ag}}_{\mathrm{Zn}}$ acceptor is a conventional acceptor with a 4$d^{10}$5$s^1$ configuration, i.e., no $d^9$ hole state is observed in the band gap. The temperature dependence of the binding energy of this acceptor level is obtained from a detailed treatment of the phonon coupling in the experimental optical cross sections ${\sigma }_n^0$(hν). The 2.747-eV BE state is concluded to be a neutral Ag-related complex with an electronic structure compatible with the Hopfield-Thomas-Lynch model, with a hole-attractive central cell, and a shallow donor-like electron state. No splitting of this BE line is observed, and the thermal activation energy for the hole is 52±5 meV, in agreement with this model. The identity for this dominating Ag-related complex is suggested to be the ${\mathrm{Ag}}_{\mathrm{Zn}}$-${\mathrm{Ag}}_{\mathrm{i}}$ pair (where ${\mathrm{Ag}}_{\mathrm{i}}$ is a silver atom in an interstitial position), probably in the simplest trigonal configuration. The thermal broadening and quenching of the 2.747-eV electronic line is consistent with a linear coupling of low-energy phonon modes (both acoustic lattice modes and quasilocalized modes of energy ∼2 meV) with an effective Debye temperature, ${\Theta }_D$,eff∼60 K, indicating a strong enhancement of these low-energy modes in the phonon coupling, compared to the lattice phonon density of states in ZnSe.