Temporal and spatial evolution of nuclear polarization in optically pumped InP

The electron-nuclear interaction in optically pumped NMR of semiconductors manifests itself through changes in spectral features (resonance shifts, linewidths, signal amplitudes) and through the magnitude of the nuclear-spin polarization. We show that these spectral features can provide a measure of the parameters that govern the optical pumping process: electron-nuclear cross-relaxation rate, Bohr radius and fractional occupancy of the optically relevant defect (ORD), and electron polarization at the ORD. Applying a model of the spatial and temporal evolution of the nuclear spins under optical pumping to super(31)P in semi-insulating InP we find an ORD Bohr radius of 6 nm, independent of the electron polarization used to fit the data, confirming the ORD is a shallow donor. For an electron polarization of -0.15, the ORD fractional occupancy is 0.02, leading to an electron-nuclear cross-relaxation time of 0.20 s and a hyperfine frequency shift of 8.1 kHz for super-bandgap irradiation. Allowing the electron polarization to vary in the model constrained to the hyperfine shift data, we find the fractional occupancy and electron-nuclear cross-relaxation rate to be approximately inversely proportional to the electron polarization. From the long-time evolution of the nuclear polarization we calculate an ORD density of 5 x 10 super(15) cm super(-3).