Excitons and the Absorption Edge in ZnSe

The optical absorption threshold at about 2.7 eV, near the lowest-energy fundamental edge, was investigated for cubic ZnSe single crystals. Absorption spectra are reported for temperatures between 2 and 200°K, and absorption coefficients between 4 and 600 ${\mathrm{cm}}^{-1\mathrm{}}$. Although contributions associated with defects dominated the results in most crystals, intrinsic absorption could be observed for $T\gtrsim 60$°K in the purest available crystals. This intrinsic absorption results from longitudinal-optical-phonon-assisted creation of excitons, as is shown by the good agreement between the observed magnitude, the temperature, and photon-energy dependence, and the absorption calculated for this mechanism. The relevant exciton states involve electrons and holes from band extrema near the center of the Brillouin zone; when created without phonon assistance, excitons from these same extrema also give the relatively very intense "direct transition" absorption lines. No evidence is found that any band gap is smaller than that at the zone center.