Electronic and structural properties of II-VI ternary alloys and superlattices

Using a combination of first-principles molecular dynamics and conventional electronic-structure calculational methods, we examine the pronounced bowing phenomenon, which characterizes the direct energy gap. E_g^Γ in ZnSe_1-xTe_x alloys. The bowing of E_g^Γ is found to be attributable to significant electronegativity difference between the two anions. We also examine the structure of sinusoidally modulated ZnSe_1-xTe_x superlattices whose composition varies in the range of 0.25<x<0.75. We find an elongation of the Zn-Te bond and a contraction of the Zn-Se bond relative to those of the corresponding binary compounds. The elongation of the Zn-Te bond may explain the redshifts of E_g^Γof 0.14 eV observed in the photoluminescence measurements on these supperlattices. We also study the electronic and structural properties of the Zn_1-xBe_xSe and Zn_1-xCd_xSe alloys and find that in the Zn_1-xBe_xSe alloys the Zn-Se bond length decreases slightly, while the Be-Se bond length increases by 10% relative to those of the binary compounds. This result suggests a softening of the Be-Se bond in the Zn_1-xBe_xSe alloys. For Zn_1-xCd_xSe, the Zn-Se and Cd-Se bond lengths deviate from those of the respective binary compounds by not more than 2%. We also find that the direct band gap E_g^Γ as a function of x bows down for both Zn_1-xBe_xSe and Zn_1-xCd_xSe in agreement with experimental data.