Hyperfine Structure and Nuclear Hyperpolarization Observed in the Bound Exciton Luminescence of Bi Donors in Natural Si

As the deepest group-V donor in Si, Bi has by far the largest hyperfine interaction and also a large $I=9/2$ nuclear spin. At zero field this splits the donor ground state into states having total spin 5 and 4, which are fully resolved in the photoluminescence spectrum of Bi donor bound excitons. Under a magnetic field, the 60 expected allowed transitions cannot be individually resolved, but the effects of the nuclear spin distribution, $-9/2\le I_z\le 9/2$, are clearly observed. A strong hyperpolarization of the nuclear spin towards $I_z=-9/2$ is observed to result from the nonresonant optical excitation. This is very similar to the recently reported optical hyperpolarization of P donors observed by EPR at higher magnetic fields. We introduce a new model to explain this effect, and predict that it may be very fast.

Figure 1

Zeeman level diagrams for Bi $D^0$ and Bi $D^{0}X$ indicating the origin of the PL structure from 0 to 2 T. $J_z$ labels the hole angular momentum projection in $D^{0}X$, while $S_z$ and $I_z$ label electron and nuclear spin projections, respectively, in $D^0$. The energy scale for $D^{0}X$ is compressed by a factor of 4. On the right the allowed transitions at 2 T are labeled from 1 to 6 in order of increasing energy, with each having 10 hyperfine subcomponents.

Figure 2

PL of the Si:Bi NP $D^{0}X$ transitions is shown at zero field, and at 2 T for 3 different temperatures. Vertical lines indicate relative strengths of individual hyperfine components obtained from the fit. The residual error of each fit is shown just below the spectrum.

Figure 3

PL spectra of the ${}^{\mathrm{nat}}\mathrm{Si}$:Bi sample at low $T$ and fields of 0, 3 and 6 T. The excitation conditions and the intensity scale are the same for all three spectra. The inset shows a simple labeling scheme of the Bi and P hyperfine states for a discussion of the origin of the nuclear hyperpolarization.