Electron Spin Resonance of Lithium Related Donor Centers in Bulk Si_1−xGe_x Crystals Enriched in ²⁸Si and ⁷²Ge Isotopes

The lithium donor centers in Si_1–xGe_x (x = 0.0039–0.05) single crystals enriched in spinless ²⁸Si and ⁷²Ge isotopes (99.998 and 99.984%, respectively) are studied by electron spin resonance at temperatures of T = 3.5–30 K and compared with its behavior in Si crystals. It is shown that lithium center with trigonal ([111]) symmetry has a most stable configuration in bulk Si_1–xGe_x for different values of x < 5 at %. Axial symmetry is explained by the distortion of the lithium central position and their neighboring atoms. The spin relaxation rates were studied in temperature range 4–30 K and it was shown that transverse and longitudinal relaxation consist of two components with rates differ by almost an order of magnitude. Together with Raman processes for 1/T₁, a decrease in the exponent below T⁵ is observed. This behavior is explained by cross-relaxation through states of the distorted configuration of lithium, which arise due to modulation of the crystal field potential by random distribution of Ge atoms.