Phonons and Electrons in Chalcopyrite Semiconductors

In recent years the phonons and the electron phonon interaction of binary tetrahedral semiconductors have been profusely investigated by ab initio techniques and compared with experimental results. Of particular interest have been binary compounds in which the cations contain semi-core d-electrons (CuCl, CuI, AgI) which display anomalies related to the semi-core d-states ((CuCl)-Cu-3d, (AgI)-Ag-4d). Here we present the corresponding data and anomalies which have been observed in ternary compounds of chalcopyrite structure (e.g. CuGaS2, AgGaX2 (X = S, Se, Te)). We present new ab initio calculations of the phonon dispersion relations of AgGaS2 and compare them with available Raman and IR data. Anomalies in the temperature dependence of the electronic gaps, which have been found in the binary chalcogenides, are also hinted at by the results for the ternary compounds with chalcopyrite structure. In view of the large number of atomic combinations possible for these materials (AgGaS2, AgGaSe2, CuGaTe2, ...) we believe that a detailed investigation of the whole family of chalcopyrites should provide a clear picture of their properties and lattice anomalies.