Dynamic Nuclear Polarisation in P-doped Silicon by Microwave Ultrasonics

We have investigated the dynamic nuclear polarisation (DNP) of 298i nuclei in highly doped silicon single crystals. The saturation of the electronic Zeeman levels in a magnetic field of 3.3 kG at 1.60)( was obtained using microwave ultrasonics of about 9 GHz. The ultrasonic waves were generated in a cavity by means of a transducer consisting of a thin film of CdS evaporated on the polished surface of the silicon. Preliminary results obtained with a silicon-rod of 2 x 2 x 12 mm cut with the long axis parallel to the (no) direction rendered a reduction of the NMR signal to about 1 in agreement with the expected negative sign of the DNP There are two mechanisms for dynamic nuclear polarisation (DNP), namely, 1) DNP caused by relaxation processes (Overhauser effect); 2) DNP originating from the simultaneous flip of two spins when absorbing a quantum from the r.f. -field (solid state effect). Moreover, DNP by relaxation can also occur in solids provided there is a suitable time dependence of the interaction between the two spins. Earlier examples of such an Overhauser effect in solids were the DNP of 7Li in Li metal [1] and of 29Si in highly P-doped silicon [2]; in both cases the DNP results from the interaction of the nuclei with the conduction electrons. The DNP which can be obtained is given by