Valley-orbit splitting of lithium-related donors in silicon

The energy spectra of lithium-related donors in silicon are revisited by temperature-dependent infrared absorption spectroscopy. We determine the valley-orbit splitting of an interstitial isolated lithium donor and its complex with residual oxygen in float-zone grown crystals doped from the melt. Lithium-oxygen donors exhibit a temperature evolution of their absorption spectra similar to that of substitutional single-electron group-V donors, while its valley-orbit splitting is corrected upwards to +9.865(5) meV. For the lithium donor, the previously reported inverted valley-orbit splitting is not confirmed since the components of different symmetry are not spectrally resolved in our study. Instead, the chemical shift of the ground state of the donor, having a symmetry different from that of group-V donors, was corrected to a slightly larger value of 1.82(2) meV. Intracenter transitions from the split-off ground state of the Li donor exhibit an anomalous broadening. In addition, several Rydberg-like high excited states, up to 7h +/-, of lithium-related centers were determined from absorption spectra of moderately doped samples.