Second-harmonic generation and birefringence of some ternary pnictide semiconductors

A first-principles study of the birefringence and the frequency-dependent s econd-harmonic generation (SNG) coefficients of the ternary pnictide semico nductors with formula ABC(2) (A=Zn, Cd; B=Si Ge; C = As, P) with the chalco pyrite structures was carried out. The zero-frequency limits of chi(123)((2 )) were found to be in reasonable agreement with available experimental dat a for all the considered materials. We found that substitution of P by As, Si by Ge, and Zn by Cd is favorable to get a higher value of chi((2))(0). A n analysis of the different contributions shows that the anomalously high v alue of the zero-frequency SHG in CdGeAs2, appears as a result of a very sm all interband term in the zero-frequency limit which, contrary to most of t he other materials of this class, does not compensate the large intraband c ontribution. Simple inverse power scaling laws between gaps and chi((2)) va lues are not supported by our results. We find that the (001) oriented 1+1 superlattice structure has significantly lower gaps than the chalcopyrite a nd correspondingly higher chi((2)) However, this smaller gap structure is c haracterized by a large alternatingly compressive and tensile lateral strai n in the layers, which makes it unfavorable. The calculated values off the birefringence for ZnGeP2 and CdGeAs2 are in fair agreement (discrepancies b eing rather constant and of the order of 10%) with experiment in the freque ncy range corresponding to the middle of the gap. [S0163-1829(99) 14503-X].