Physical Review B

NMR study of optically hyperpolarized phosphorus donor nuclei in silicon

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We use above-band-gap optical excitation, via a 1047-nm laser, to hyperpolarize the P31 spins in low-doped (ND=6×1015cm-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 Si28-enriched silicon crystal. We show that the broadening is consistent with previous electron-nuclear double-resonance 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 T1 and the buildup time for the optically induced P31 hyperpolarization in the natural abundance silicon sample were observed to be 178±47 and 69±6 s, respectively, significantly shorter than the values previously measured in Si28-enriched samples under the same conditions. We measured the T1 and hyperpolarization buildup time for the P31 signal in natural abundance silicon at 9.4 T to be 54±31 and 13±2 s, respectively. The shorter buildup and nuclear spin T1 times at high field are likely due to the shorter electron spin T1, which drives nuclear spin relaxation via nonsecular hyperfine interactions. At 6.7 T, the phosphorus nuclear spin T2 was 16.7±1.6 ms at 4.2 K, a factor of 4 shorter than in Si28-enriched crystals. This was observed to shorten to 1.9±0.4 ms in the presence of the infrared laser.