Isotopic enrichment of silicon by high fluence²⁸Si^-ion implantation

Spins in the “semiconductor vacuum” of silicon-28 (²⁸Si) are suitable qubit candidates due to their long coherence times. An isotopically purified substrate of ²⁸Si is required to limit the decoherence pathway caused by magnetic perturbations from surrounding ²⁹Si nuclear spins (I = 1/2), present in natural Si (^natSi) at an abundance of 4.67%. We isotopically enrich surface layers of ^natSi by sputtering using high fluence ²⁸Si⁻ implantation. Phosphorus (P) donors implanted into one such ²⁸Si layer with ∼3000 ppm ²⁹Si, produced by implanting 30 keV ²⁸Si⁻ ions at a fluence of 4 × 10¹⁸ 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 ²⁹Si. A coherence time of T₂ = 285 ± 14 μs is extracted, which is longer than that obtained in ^natSi 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 ²⁸Si⁻ implanted with a fluence of 2.63 × 10¹⁸ cm⁻² into ^natSi. This resulted in an isotopically enriched surface layer ∼100 nm thick, suitable for providing a sufficient volume of ²⁸Si for donor qubits implanted into the near-surface region. We observe a depletion of ²⁹Si to 250 ppm as measured by secondary ion mass spectrometry. The impurity content and the crystallization kinetics via solid phase epitaxy are discussed. The ²⁸Si 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.