Monoisotopic silicon $^{28}$Si in spin resonance spectroscopy of electrons localized at donors

The fine structure of the spectra of a shallow-level Li donor center and a Fe$^0$ deep donor ($S$ = 1), which occupy tetrahedral interstices in a silicon lattice, is studied in monoisotopic silicon $^{28}$Si due to considerable narrowing of the lines of ESR spectra. In the case of the Li donor center, experimental data are found to confirm the role of internal strains in the crystal when observing the ESR spectra of the ground state 1$s$ $T_2$ and state $E$ at $T$ = 3.8–10 K with $g <$ 2.000. The anisotropy in the distribution of strains, which turned out to have a tetragonal type, is investigated using the angular dependences of the line width of the spin resonance corresponding to the triplet state $T_{2z}$ of Li. Similar anisotropy is found in the case of the introduction of Fe$^0$ ions into the initial crystals based on the theory of angular dependences of the width of ESR lines caused by the transitions -1 $\to$ 0 and 0 $\to$ +1 ($\Delta M_s$ = 1) in comparison with the transition -1 $\to$ +1 ($\Delta M_s$ = 2).