Monoisotopic <sup>28</sup>Si in Spin Resonance Spectroscopy of Electrons Localized on Shallow Donors

Abstract

The inverted structure of the 1s ground state of lithium in silicon provides a unique opportunity to study inter-valley spin-orbit interactions of donor electrons. A study of lithium doped silicon enriched in the 28Si isotope with a low oxygen content (N 21014 cm3) has demonstrated at low temperatures (T = 3.8 K) a family of electron spin resonance (ESR) spectra with anisotropic g factors associated with Li donor centres. The spectra were investigated without and with application of external stress to the sample and their g factors were found to be less then 2.000. The analysis of experimental data and numerical simulation of the spectra and their angular dependencies in the second order perturbation theory assuming the splitting of states due to internal strains in the crystal is larger than the Zeeman and spin-orbit splitting have shown that the spectrum, having g tensor components corresponding to the tetragonal symmetry, consists of two lines belonging to the triplet state T2, and the other two lines in the spectrum have an angular dependence behaviour of the doublet states E. The ratio of the inter-valley spin-orbit coupling λ and λ' to the Δ parameters, characterizing the splitting of the states under internal strains have been defined. From the dependencies of the triplet ESR lines intensity on the compressive stress of the crystal along the [11 it was obtained the value of the internal strains, which allowed to determine the parameters of the spin-orbit coupling λ and λ'. Their values were found to be three orders of magnitude smaller than were obtained earlier for Li spectra with g > 2. Since experimentally observed Δ value was of the order of the Zeeman splitting parameter the spectra were analyzed using the full matrix of the spin Hamiltonian for the fivefold degenerate ground state. We found that angular dependencies of the spectra observed for the triplet and doublet states with g < 2 are well described by the solutions of the spin Hamiltonian with parameters λ and λ' obtained from our experimental data. At the same time, we are not able to find solutions that satisfy the data obtained for the spectra with g > 2.000 in previous studies.