Pair spectra, edge emission, and the shallow acceptors in melt-grown ZnSe

The nature of edge emission bands in the photoluminescence spectra of as-grown, annealed, and electron-irradiated melt-grown ZnSe crystals at 4.2°K has been investigated. Five different edge emission bands associated with donor-acceptor pair recombination are identified. These bands have their zero-phonon peaks at 4566, 4587, 4595, 4606, and 4625 Å. A high-energy series, free-to-bound recombination band, is observed at 77°K in association with the band at 4625 Å. The bands at 4566, 4587, and 4595 Å are seen only in annealed crystals; the band at 4625 Å is seen only in as-grown crystals; whereas the band at 4606 Å is observed in annealed and as-grown crystals. Discrete pair lines have been resolved only for the bands at 4566 and 4587 Å. It is recognized that the band at 4606 Å is the previously studied group-III-donor-${\mathrm{Li}}_{\mathrm{Zn}}$ recombination band and that the band at 4587 Å is the as yet unidentified complex pair band reported by Dean and Merz. A study of the bound exciton lines occurring with the edge emission indicates that the band at 4625 Å arises from group-III-donor-${\mathrm{Na}}_{\mathrm{Zn}}$ recombination. The acceptor ionization energy, $E_A$, of ${\mathrm{Na}}_{\mathrm{Zn}}$ is deduced to be 122 meV. From the analysis of the discrete pair lines associated with the bands at 4566 and 4587 Å and from the results of electron irradiation and annealing experiments it is suggested that these two bands involve the same donor and acceptor and that the shorter-wavelength band arises from preferential pairing and the longer-wavelength band from random pairing of the donor and acceptor centers. The donor and the acceptor are tentatively assigned to be the doubly ionized zinc interstitial, ${\mathrm{Zn}}_i$, and the singly ionized complex (${\mathrm{Na}}_{\mathrm{Zn}}{V}_{\mathrm{Se}}$), respectively. The ionization energy of the complex center, (${\mathrm{Na}}_{\mathrm{Zn}}{V}_{\mathrm{Se}}$), is estimated from the corresponding bound exciton line to be 80-100 meV. The pair band at 4595 Å is tentatively assigned to a group-III-donor-(${\mathrm{Li}}_{\mathrm{Zn}}{V}_{\mathrm{Se}}$) transition, and this acceptor has $E_A=110-130\mathrm{}$ meV, estimated from the corresponding bound exciton line.