Anelastic relaxation in silicon doped with lithium and boron

Abstract

Single crystals of Si doped interstitially with Li and substitutionally with B (up to 3.7 × 1019 atoms cm3) exhibit a well-defined internal friction peak at 150°C for a frequency of 1.2 kHz. The peak is governed by a single relaxation time, and has an activation energy Q of 0.83 ± 0.015 eV. The temperature and concentration dependence of the relaxation strength conform to a mass-action equation based on the reaction Li+ + B-ai Li+B-, and indicate that the relaxation is produced by the reorientation of bound Li+B- pairs rather than by unassociated Li+ ions. The binding enthalpy Δhb is found to be 0.28±0.025 eV/pair. From measurements on 〈100〉 and 〈111〉 oriented samples, the symmetry of the Li+B- pair is found to be trigonal. The dipole shape factor |λ1 - λ2| has the value 0.1. The defect symmetry indicates that the pair is composed of a Li+ ion occupying a tetrahedral interstitial site in a nearest neighbor position to a substitutional B- ion. All the available evidence is contrary to Weiser's suggestion that free Li+ ions in Si enter the hexagonally coordinated interstitial sites which lie midway between the tetrahedral sites. © 1970.