Modification of the optical properties of ZnSe by iodine impurification

The optical characterization of a ZnSe crystal highly impurified with iodine reveals very different properties in relation to those expected for the pure material. From the analyses of transmission and reflectivity experiments we determined the index of refraction and the absorption coefficient in the range from 1.1 to 3.1 eV. Employing a model of direct polar transitions between parabolic bands for the absorption coefficient, we obtained a value of Eg = 2.496 eV for the fundamental band gap, much lower than that of the pure semiconductor. Due to the reduction in Eg no blue photoluminescence was observed, but a strong orange-yellow emission centered around 2.1 eV. At energies lower than the band gap we observed absorption with the typical behavior of Urbach tails. The changes in the optical properties are attributed to the presence of iodine in ZnSe. The correlation of the optical properties with the structural properties of the sample suggest that the iodine atoms enter as substitutional impurities in the ZnSe matrix. (Author)