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 × 10 19 atoms cm 3 ) 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 Δh b 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 ¦λ 1 − λ 2¦ 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.