Excitation Spectra of Lithium Donors in Silicon and Germanium

The excitation spectra of lithium in silicon and germanium have been measured. These spectra exhibit excitation lines which indicate that the excited states of lithium are similar to those of the Group-V impurities. For oxygen-free silicon and germanium, the lithium excitations observed occur at energies close to those of the Group-V transitions which originate from the upper ground states of these impurities. This indicates that the chemical splitting of the ground state of lithium in silicon and germanium may be very small. The effect of uniaxial stress on the excitation spectrum of lithium in silicon has been examined. The experimental results for lithium in oxygen-free silicon are interpreted as follows: The site symmetry of the isolated lithium is tetrahedral; the ground state is an "inverted" Group-V-like ground state, the $1s\left(A_1\right)$ state lying 1.8±0.1 meV above the $1s(E+T_1)$ state. For lithium in oxygen-containing silicon, zero-stress and uniaxialstress measurements indicate that the ground state is Group-V-like with the $1s(E+T_1)$ state lying 7.7±0.1 meV above the $1s\left(A_1\right)$ state. The spectra of lithium in germanium and oxygen-containing silicon exhibit lines which do not fit the Group-V pattern.