ELECTRON-POSITRON MOMENTUM DISTRIBUTIONS AND POSITRON LIFETIME IN SEMICONDUCTORS IN THE GENERALIZED GRADIENT APPROXIMATION

The positron annihilation characteristics have been calculated taking the electron-positron correlation in the generalized gradient approxim ation (GGA). The calculated electron-positron momentum distributions i n Si along the [110] direction in the GGA scheme agree very well with the experiment. The comparison of anisotropies of the momentum distrib utions along different crystal directions with the theory shows that o nly the GGA scheme gives the exact values. The enhancement factor for the valence electrons in the electron-positron momentum density is fou nd to be weakly dependent on the momentum. The positron lifetimes in g roup TV, m-V, and II-VI semiconductors agree very well with the previo us calculations and the experiment.