Dynamic nuclear polarization of S 29 I nuclei in isotopically controlled phosphorus doped silicon

Dynamic nuclear polarization (DNP) of S 29 i nuclei in isotopically controlled silicon single crystals with the S 29 i isotope abundance f29Si varied from 1.2% to 99.2% is reported. It was found that both the DNP enhancement and S 29 i nuclear spin-lattice relaxation time under saturation of the electron paramagnetic resonance transitions of phosphorus donors increase with the decrease in the S 29 i abundance. A remarkably large steady-state DNP enhancement, Ess =2680 which is comparable to the theoretical upper limit of 3310, has been achieved through the "resolved" solid effect that has been identified clearly in the f29Si =1.2% sample. The DNP enhancement depends not only on the S 29 i abundance but also on the electron spin-lattice relaxation time that can be controlled by temperature and/or illumination. The linewidth of S 29 i NMR spectra after DNP shows a linear dependence on f29Si for f29Si ≤10% and changes to a square-root dependence for f29Si 50%. Comparison of experimentally determined nuclear polarization time with nuclear spin diffusion coefficients indicates that the rate of DNP is limited by the polarization transfer rather than by spin diffusion.