POSITRON STATES IN Si1-xGex ALLOYS: DEVIATION FROM VEGARD'S LAW

Electron-positron momentum densities along different crystallographic directions and positron bulk lifetime in Si 1-x Ge x alloys have been investigated within the pseudopotential formalism employing the independent particle model. Special attention has been given to the effect of the deviation of the alloy lattice parameters from Vegard's rule on the studied quantities. It is found that using Vegard's law leads to an underestimation of the total positron annihilation rate indicating therefore an overestimation of the positron bulk lifetime. This result could not be checked using the Siethoff relation (H. Siethoff, Phys. Stat. Sol.B205, R3 (1998)). Moreover, this relation predicts a monotonic dependence of the positron bulk lifetime on the alloy composition which disagrees with the positron lifetime measurement.