POSITRON-ANNIHILATION IN II-VI COMPOUND SEMICONDUCTORS - THEORY

Positron states and annihilation rates are calculated for nine different II-VI compound semiconductors. Positron annihilation from the delocalized states in the perfect lattice as well as from the localized states at vacancies and divacancies is considered. The calculations are based on the local-density approximation (LDA) for the electron-positron correlation effects. The calculations are performed using non-self-consistent electron densities and electrostatic potentials obtained by atomic superposition and solving for the three-dimensional positron wavefunctions by a relaxation method. For the perfect lattices, also self-consistent electron densities and positron states (linear muffin-tin orbital method within atomic-spheres approximation, LMTO-ASA) are calculated. The results show that positron annihilation with the outer d electrons of the group II metal atoms plays an important role. The positron lifetimes calculated for perfect lattices are a few per cent shorter than the experimental ones, indicating an LDA overestimation of the d-electron enhancement around the positron. These bulk lifetimes can be corrected efficiently by a semiempirical model introduced in this work. In the case of positrons trapped by defects, the present theoretical description is less satisfactory, because atomic relaxations due neither to rearrangements in the electronic structure nor to the localized positron are taken into account. However, the calculated annihilation probabilities with core and valence electrons will serve as an important database for methods in which defects can be identified using the positron angular correlation or Doppler broadening measurements.