Optical and dynamic nuclear polarization of 29Si nuclei via photoexcited triplet states of oxygen-vacancy complexes in isotopically controlled silicon

Optical and dynamic nuclear polarization of 29Si nuclei under saturation of the electron paramagnetic resonance (EPR) transitions of the photoexcited spin S = 1 states (Si-SL1 centers) of the oxygen+vacancy complexes (A-centers) have been investigated using silicon single crystals with various 29Si nuclear abundances. The optical nuclear polarization alone led to the 29Si nuclear polarization enhancement by a factor of ∼500 in the magnetic fields below 50 mT, and such enhancement was found to decrease significantly in the stronger fields up to 300 mT. On the other hand, saturation of the EPR lines of Si-SL1 centers having the electron spin polarization of 70-80% was transferred successfully to surrounding 29Si nuclear spins to achieve the overall 29Si nuclear spin polarization of ∼6.4%, and a method to increase up to 13-15% is proposed. The optimum condition to achieve such a high degree of nuclear polarization was found by systematic investigations of the dynamic nuclear polarization mechanisms.