  Single crystal silicon is an excellent system in which to explore dynamic
nuclear polarization (DNP), as it exhibits a continuum of properties from
metallic to insulating as a function of doping concentration and temperature.
At low doping concentrations DNP has been observed to occur via the solid
effect, while at very high doping concentrations an Overhauser mechanism is
responsible. Here we report the hyperpolarization of 29Si in n-doped silicon
crystals, with doping concentrations in the range of 1-3 x 10^17 /cc. In this
regime exchange interactions between donors become extremely important. The
sign of the enhancement in our experiments and its frequency dependence suggest
that the 29Si spins are directly polarized by donor electrons via an Overhauser
mechanism within exchange-coupled donor clusters. The exchange interaction
between donors only needs to be larger than the silicon hyperfine interaction
(typically much smaller than the donor hyperfine coupling) to enable this
Overhauser mechanism. Nuclear polarization enhancement is observed for a range
of donor clusters in which the exchange energy is comparable to the donor
hyperfine interaction. The DNP dynamics are characterized by a single
exponential time constant that depends on the microwave power, indicating that
the Overhauser mechanism is the rate-limiting step. Since only about 2 % of the
silicon nuclei are located within one Bohr radius of the donor electron,
nuclear spin diffusion is important in transferring the polarization to all the
spins. However, the spin-diffusion time is much shorter than the Overhauser
time due to the relatively weak silicon hyperfine coupling strength. In a 2.35
T magnetic field at 1.1 K, we observed a DNP enhancement of $244 +/- 84
resulting in a silicon polarization of $10.4 +/- 3.4 % following two hours of
microwave irradiation.
