Emissions related to donor-bound excitons in highly purified zinc selenide single crystals

Photoluminescence (PL), selective photoluminescence, and photoluminescence excitation (PLE) spectra have been measured on high-purity ZnSe single crystals at 4.2 K. Zn-dip treatment at 973 K for a week makes the I₁ᵈ line disappear and the effective temperature of free-exciton gas decrease from 8.4 to 5.6 K through an increase in the lifetime of the free excitons. Accurate agreement is found for the transition energies of donor-bound excitons and their excited states with the values reported by Dean et al. Accurate values are estimated for the effective-mass donor binding energy and the static dielectric constant. The notable spectral feature in the PL spectra is that the emission intensities of the I₃ lines are higher than those of the I₂ lines. The origin of I₃, the recombination of the excitons bound to ionized donors, is considered to be unreliable, judging from the donor-concentration dependence of the intensity ratio between the I₂ and I₃ emission lines and the spectral change under a cw dye-laser excitation with the energy below the band gap. PLE spectra show that the peaks related to excited states of I₂ exist in the PLE spectra for I₃ in addition to the other peaks related to the excited states of I₃, and that the spectra for I₃ₛ is similar to that for I3w.