Optical properties of complex defects created by Ag diffusion in ZnTe

Several complex defects in ZnTe created by Ag diffusion at a rather high doping level are studied by optical spectroscopy. In addition to the usual substitutional ${\mathrm{Ag}}_{\mathrm{Zn}}$ acceptor bound exciton (BE) at 2.3737 eV, a new prominent BE, $S_1$, with a lowest energy of 2.3149 eV appears. This transition has a strong phonon coupling and corresponds to a neutral isoelectronic defect. Two electronic states at zero field are revealed by transmission data; a doublet at the lowest state (2.3149 eV) and a singlet at slightly higher energy (2.3155 eV). The electronic properties of the $S_1$ BE are revealed by optical data, including the magneto-optical Zeeman spectra. The electronic structure can be understood as a consequence of a strong compressional axial local crystal field in combination with an electron-hole exchange interaction. It is further concluded that both carriers are bound to the complex defect by an attractive central-cell potential. The identity of the $S_1$ defect as a pair of substitutional ${\mathrm{Ag}}_{\mathrm{Zn}}$ and interstitial ${\mathrm{Ag}}_{\mathrm{i}}$ in the 〈111〉 direction is consistent with the trigonal symmetry observed in magneto-optical data. In addition to the $S_1$ defect, several acceptorlike complex defects are created, of which $S_2$ is a BE with its lowest electronic line at 2.3486 eV, and $S_3$ similarly at 2.365 eV. These BE excitations give rise to several electronic levels both in the ground state (the acceptor hole) and in the excited state (the BE state). The complicated electronic structure can be explained by a combination of a low-symmetry crystal field and an exchange interaction. An identification of the defects $S_2$ and $S_3$ as composed of three atoms is suggested.