Dynamic nuclear polarisation in P-Doped silicon by microwave ultrasonics

Abstract

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 ultra­ sonics 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 (1) where ~ is a coupling parameter; in the case of the scalar coupling of the conduction electrons with the 29Si nuclei in our case ~ = -1. The leakage factor 0 < j < 1 can be set j= 1 as a very good approximation in our case. The saturation parameter 0 < S < 1 measures the necessary deviation of the population of the electronic Zeeman levels from the Boltzman equili­ brium. This deviation is usually obtained by saturating the ESR transitions between these levels which requires in a magnetic field of several kG a frequency in the microwave region. In all measurements with highly doped Silicon made so far, and even more so in metals, very small particles with a diameter d < ill had to be used because of the skin effect. The aim of our experiment is to investigate Single crystals with dimen­ sions of the order of several mm and to saturate the electronic Zeeman levels by microwave ultrasonics in order to avoid the skin effect. The pos­ sibility of generating microwave ultrasonics has received increasing atten-