NMR study of optically hyperpolarized phosphorus donor nuclei in silicon

Abstract

We use above-bandgap optical excitation, via a 1047 nm laser, to hyperpolarize the ${}^{31}$P spins in low-doped (N${}_D=6\times 10^{15}$ cm${}^{-3}$) natural abundance silicon at 4.2 K and 6.7 T, and inductively detect the resulting NMR signal. The $30$ kHz spectral linewidth observed is dramatically larger than the 600 Hz linewidth observed from a ${}^{28}$Si-enriched silicon crystal. We show that the observed broadening is consistent with previous ENDOR results showing discrete isotope mass effect contributions to the donor hyperfine coupling. A secondary source of broadening is likely due to variations in the local strain, induced by the random distribution of different isotopes in natural silicon. The nuclear spin T${}_1$ and the build-up time for the optically-induced ${}^{31}$P hyperpolarization in the natural abundance silicon sample were observed to be $178\pm 47$ s and $69\pm 6$ s respectively, significantly shorter than the values previously measured in ${}^{28}$Si-enriched samples under the same conditions. We also measured the T${}_1$ and hyperpolarization build-up time for the ${}^{31}$P signal in natural abundance silicon at 9.4 T to be $54\pm 31$ s and $13\pm 2$ s respectively. The shorter build-up and nuclear spin T${}_1$ times at high field are likely due to the shorter electron-spin T${}_1$, which drives nuclear spin relaxation via non-secular hyperfine interactions. At 6.7 T, the phosphorus nuclear spin T${}_2$ was measured to be $16.7\pm 1.6$ ms at 4.2 K, a factor of 4 shorter than in ${}^{28}$Si-enriched crystals. This was observed to further shorten to $1.9\pm 0.4$ ms in the presence of the infra-red laser.

5 pages