Magnetic-Field-Dependent Phonon Scattering by Li Ions in Si

Abstract

The Li ion acts as a shallow donor in Si and its ground state has been studied by far infra-red, [1], EPR, [2], APR, [3], ultrasonics, [4]and thermal conductivity [5,6]. It appears to be in an interstitial site and its levels are inverted with respect to those of substitutional group V impurities. The lowest levels are the approximately degenerate 2E and 2T2 states and the 2A1 level lies at an energy Δ ~ 440GHz. This appears to be the smallest chemical shift for shallow donors in silicon and makes it of potential value for frequency crossing experiments. The sizes of the spin-orbit coupling constants λ, λ′ within 2E + 2T2 are not known but expected to be small. If λ, λ′ → 0, APR attenuation should be Kramers forbiiden and so very weak. The rather strong APR signals observed by WIGMORE [3] at g ~ 2 therefore suggest significant spin-orbit mixing. This should lead to fine structure around g ~ 2 [7] from which λ, λ′ and the E-T2 splitting should be obtainable if not smeared out by strain. No such structure was seen by WIGMORE although he did observe additional lines corresponding to g ∿ 1.38 and 0.97 which are not explicable by this model. No anisotropy measurements were made. Evidence that about 20% of the Li ions experienced large strain splittings ( ~ 50 GHz) in the samples used for thermal conductivity experiments [5,6] comes from the measurements below IK. Above IK, the scattering at Δ = 440 GHz is dominant but at lower temperatures, where the effect of this becomes weak, the strong scattering observed is attributable to processes within the 2E + 2T2 states.