Behavior of lithium donors in bulk single-crystal isotopically pure $^{28}$Si$_{1-x}$ $^{72}$Ge$_{x}$ alloys

The behavior of lithium donors in bulk single-crystal isotopically pure Si$_{1-x}$Ge$_{x}$ alloys ($x$ = 0.0039–0.05) enriched in spinless $^{28}$Si and $^{72}$Ge isotopes (99.998 and 99.984%, respectively) is investigated by electron spin resonance. The fine structure of the spectrum of electrons localized at Li donors is studied at temperatures of $T$ = 3.5–30 K in the expectation that, much like with pure silicon, enriching Si$_{1-x}$Ge$_{x}$ alloys in spinless isotopes ($^{28}$Si$^{\operatorname{iso}}$Ge, where $\operatorname{iso}$ = 70, 72, 74, 76) makes it possible to attain a higher resolution in the spectra of electron spin resonance. The results demonstrate that, despite the irregular arrangement of Ge atoms in the lattice of the $^{28}$Si$_{1-x}$ $^{72}$Ge$_{x}$ alloy, the presence of local distortions in their vicinity, and the broadening of the electron-spin-resonance lines of donor electrons due to random strains, the lines of Li donors observed in the spectra of isotopically pure $^{28}$Si$_{1-x}$ $^{72}$Ge$_{x}$ single crystals with $x$ = 0.39, 1.2, and 2.9 at.% are narrower than the corresponding lines in the spectra of similar crystals with the natural composition of silicon and germanium isotopes. Similarly to the case of silicon crystals, this allowed us to study for the first time the angular dependences of the line positions in the electron-spin-resonance spectra of lithium in Si$_{1-x}$Ge$_{x}$ for different values of $x$.