Isotopic enrichment of silicon by high fluence <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msup><mml:mrow/><mml:mn>28</mml:mn></mml:msup><mml:msup><mml:mtext>Si</mml:mtext><mml:mo>−</mml:mo></mml:msup></mml:mrow></mml:math> ion implantation
D. Holmes, B. C. Johnson, C. Chua, B. Voisin, S. Kocsis, S. Rubanov, S. G. Robson, J. C. McCallum, D. R. McCamey, S. Rogge, D. N. Jamieson
Abstract
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 ${}^{28}{\text{Si}}^{\ensuremath{-}}$ implantation. Phosphorus (P) donors implanted into one such $^{28}\mathrm{Si}$ layer with $\ensuremath{\sim}3000$ ppm $^{29}\mathrm{Si}$, produced by implanting 30 keV ${}^{28}{\text{Si}}^{\ensuremath{-}}$ ions at a fluence of $4\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, 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 ${T}_{2}=285\ifmmode\pm\else\textpm\fi{}14\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ 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 ${}^{28}{\text{Si}}^{\ensuremath{-}}$ implanted with a fluence of $2.63\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ into ${}^{\text{nat}}\text{Si}$. This resulted in an isotopically enriched surface layer $\ensuremath{\sim}100$ 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.