  Spins in the `semiconductor vacuum' of silicon-28 ($^{28}$Si) are suitable
qubit candidates due to their long coherence times. An isotopically purified
substrate of $^{28}$Si is required to limit the decoherence pathway caused by
magnetic perturbations from surrounding $^{29}$Si nuclear spins (I=1/2),
present in natural Si (nat Si) at an abundance of 4.67%. We isotopically enrich
surface layers of nat Si by sputtering using high fluence $^{28}$Si$^-$
implantation. Phosphorus (P) donors implanted into one such $^{28}$Si layer
with ~3000 ppm $^{29}$Si, produced by implanting 30 keV $^{28}$Si$^-$ ions at a
fluence of 4x10^18 cm^-2, were measured with pulsed electron spin resonance,
confirming successful donor activation upon annealing. The mono-exponential
decay of the Hahn echo signal indicates a depletion of $^{29}$Si. A coherence
time of T2 = 285 +/- 14 us is extracted, which is longer than that obtained in
nat Si for similar doping concentrations and can be increased by reducing the P
concentration in future. The isotopic enrichment was improved by employing
one-for-one ion sputtering using 45 keV $^{28}$Si$^-$ implantation. A fluence
of 2.63x10^18 cm^-2 $^{28}$Si$^-$ ions were implanted at this energy into nat
Si, resulting in an isotopically enriched surface layer ~100 nm thick; suitable
for providing a sufficient volume of $^{28}$Si for donor qubits implanted into
the near-surface region. We observe a depletion of $^{29}$Si to 250 ppm as
measured by secondary ion mass spectrometry. The impurity content and the
crystallization kinetics via solid phase epitaxy are discussed. The $^{28}$Si
layer is confirmed to be a single crystal using transmission electron
microscopy. This method of Si isotopic enrichment shows promise for
incorporating into the fabrication process flow of Si spin qubit devices.
