Complex defects in ZnTe created by Cu diffusion

An extensive study of complex defects induced by Cu diffusion in ZnTe is reported. About thirty-five different bound excitons are observed in optical spectra at low temperatures in the range 2.0-2.4 eV, all related to different neutral ("isoelectronic") or acceptor-like Cu-related complexes. The electronic structure of these bound excitons is studied in detail with a combination of optical emission and absorption spectra, together with Zeeman data and FTIR transmission. In most cases the electron is the loosely bound particle in the bound exciton for both acceptor complexes ("pseudo-donor model") and neutral complexes (HTL model). The hole is more localized, presumably due to the hole-attractive Cu_Zn pseudopotential. For acceptor-like complexes usually only one electronic bound exciton line is seen, while other electronic states are lifted above the bandgap energy by the action of the local stress field. Similarly neutral complexes usually have a single bound hole state in the gap, derived from a Cu_Zn central cell. Further, novel electronic satellite spectra in photoluminescence and resonant Raman scattering, seem to be common for Cu-related complex defects at high doping levels. Another interesting effect is a strongly enhanced phonon coupling observed in the absorption spectrum of these complex bound excitons, compared to the coupling strength in the emission case. The possible identities of these Cu-related complex defects are discussed, considering their observed symmetries and the available information on chemical impurities present in the starting material. Several of these complex defects are thermally unstable at room temperature, presumably typical for the cases where an interstitial Cu_i is part of the complex.