Piezospectroscopy of isolated lithium donors and lithium-oxygen donor complexes in silicon

Abstract

The effect of uniaxial stress on the excitation spectra of interstitial lithium donors (Li) and of lithium-oxygen donor complexes (Li-O) in silicon is studied under a high resolution. For a compressive force, $\overrightarrow{\mathrm{F}}$, along [001], each $1s\to \mathrm{np}$ transition in Si(Li) splits into three components, the central component occurring at the zero-stress position even at 2 kbar, the highest stress used. The intensity of the high-energy component decreases dramatically while that of the low-energy component increases. At the highest stress only the central and the low-energy components survive and the position of the low-energy component reaches a constant value as the stress increases. Interstitial lithium donors have an anomalous, "inverted" ground state with the fivefold $1s(E+T_2)$ state close to the effective-mass position, its site symmetry being $T_d$; the totally symmetric $1s\left(A_1\right)$ state lies ${6\Delta }_c=1.76\mathrm{}\pm 0.04$ meV above it. The inverted ground state, the small value of ${6\Delta }_c$, the stress dependence of the ground-state wave functions, and a shear-deformation-potential constant ${\Xi }_u$ of 8.77 ± 0.07 eV characterizing both the ground and the excited states account for its striking piezospectroscopic behavior. Our studies in Li-O donor centers show that they have a group-V-like ground state with $1s\left(A_1\right)$ lying below $1s\left(E\right)\mathrm{}$ and $1s\left(T_2\right)$. One of the donor species has a symmetry lower than $T_d$ with a symmetry axis along $〈100〉$, showing the effects of orientational degeneracy in its piezospectroscopic behavior.