Spins in the “semiconductor vacuum” of silicon-28 (${}^{28}\mathrm{Si}$) are suitable qubit candidates due to their long coherence times. An isotopically purified substrate or epilayer of ${}^{28}\mathrm{Si}$ is required to limit the decoherence pathway caused by magnetic perturbations from surrounding ${}^{29}\mathrm{Si}$ nuclear spins ($I=1/2$), present in natural Si (${}^{\text{nat}}\mathrm{Si}$) at an abundance of 4.67%. We isotopically enrich surface layers of ${}^{\text{nat}}\mathrm{Si}$ by sputtering using high fluence ${}^{28}{\text{Si}}^{-}$ implantation. Phosphorus (P) donors implanted into one such ${}^{28}\mathrm{Si}$ layer with $\sim 3000$ ppm ${}^{29}\mathrm{Si}$, produced by implanting 30 keV ${}^{28}{\text{Si}}^{-}$ ions at a fluence of $4\times {10}^{18}{\mathrm{cm}}^{-2}$, were measured with pulsed electron spin resonance, confirming successful donor activation upon annealing. The monoexponential decay of the Hahn echo signal indicates a depletion of ${}^{29}\mathrm{Si}$. A coherence time of $T_2=285\pm 14\mu \mathrm{s}$ is extracted, which is longer than that obtained in ${}^{\text{nat}}\text{Si}$ for similar doping concentrations and can be increased by reducing the P concentration in the future. Guided by simulations, the isotopic enrichment was improved by employing one-for-one ion sputtering using 45 keV ${}^{28}{\text{Si}}^{-}$ implanted with a fluence of $2.63\times {10}^{18}{\mathrm{cm}}^{-2}$ into ${}^{\text{nat}}\text{Si}$. This resulted in an isotopically enriched surface layer $\sim 100$ nm thick, suitable for providing a sufficient volume of ${}^{28}\mathrm{Si}$ for donor qubits implanted into the near-surface region. We observe a depletion of ${}^{29}\mathrm{Si}$ to 250 ppm as measured by secondary ion mass spectrometry. The impurity content and the crystallization kinetics via solid phase epitaxy are discussed. The ${}^{28}\mathrm{Si}$ layer is confirmed to be a single crystal using transmission electron microscopy. This method of Si isotopic enrichment shows promise for incorporation into the fabrication process flow of Si spin-qubit devices.

• Received 18 September 2020
• Accepted 17 December 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.5.014601