Optical transitions in ultra-high-purity zinc selenide

We present photoluminescence (PL) and PL excitation data for unintentionally doped zinc selenide epilayers grown by molecular-beam epitaxy using ultra-high-purity sources. In particular, we discuss a transition $I_V^0$ at 22380 ${\mathrm{cm}}^{-1\mathrm{}}$ (2.7738 eV) which had been identified as an LO replica of the free excitons in the past. We present data to show that $I_V^0$ is not related to the formation of free excitons even though it is always observed in samples which show strong freeexciton transitions. Evidence is also presented to show that $I_V^0$ may be a result of recombinations involving selenium-site-related effects.