Intrinsic Absorption "Edge" in II-VI Semiconducting Compounds with the Wurtzite Structure

The intrinsic optical absorption due to longitudinal optical phonon-assisted creation of "direct" excitons is calculated for semiconductors having the wurtzite structure, and consequently anisotropic energy bands. The results are compared with the absorption-"edge" data for CdS. There is good agreement between these data and the results calculated using only experimentally determined parameters. Satisfactory accord between the ZnO data and the calculated absorption is achieved for a range of reasonable hole effective mass tensors (the only undetermined parameter). From a study of the behavior of the measured absorption for $\mathcal{E}\perp C$ around the one-phonon threshold, it is concluded that Thomas's original identification of the "$A$" reflectivity structure as a manifestation of an intrinsic exciton is correct and, concomittantly, that there is presently no compelling reason for disputing his valence assignments. This is contrary to the suggestion based on an interpretation of the "$A$" structure as an effect due to a crystal imperfection. The absorption coefficients for CdSe, ZnS, and ZnSe, the other II-VI compounds which have the wurtzite structure, have been calculated for a range of values of the presently undetermined parameters $m_{h_{\parallel }}$.