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

Abstract

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 ${I}_{1}^{d}$ 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 ${I}_{3}$ lines are higher than those of the ${I}_{2}$ lines. The origin of ${I}_{3}$, 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 ${I}_{2}$ and ${I}_{3}$ 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 ${I}_{2}$ exist in the PLE spectra for ${I}_{3}$ in addition to the other peaks related to the excited states of ${I}_{3}$, and that the spectra for ${I}_{3s}$ is similar to that for ${I}_{3w}$.