Use of quantum effects as potential qualifying metrics for "quantum grade silicon"

Across solid state quantum information, materials deficiencies limit performance through enhanced relaxation, charge defect motion or isotopic spin noise. While classical measurements of device performance provide cursory guidance, specific qualifying metrics and measurements applicable to quantum devices are needed. For quantum applications, new materials metrics, e.g., enrichment, are needed, while existing, classical metrics like mobility might be relaxed compared to conventional electronics. In this work, we examine locally grown silicon superior in enrichment, but inferior in chemical purity compared to commercial-silicon, as part of an effort to underpin the materials standards needed for quantum grade silicon and establish a standard approach for intercomparison of these materials. We use a custom, mass-selected ion beam deposition technique, which has produced isotopic enrichment levels up to 99.99998 % ²⁸Si, to isotopically enrich ²⁸Si, but with chemical purity > 99.97% due the MBE techniques used. From this epitaxial silicon, we fabricate top-gated Hall bar devices simultaneously on the ²⁸Si and on the adjacent natural abundance Si substrate for intercomparison. Using standard-methods, we measure maximum mobilities of $\approx (1740\pm 2){\text{cm}}^2/(\text{V}\cdot \text{s})$ at an electron density of $\left(2.7\times {10}^{12}\pm 3\times {10}^8\right)$ cm^-2 and $\approx (6040\pm 3){\text{cm}}^2/(\text{V}\cdot \text{s})$ at an electron density of $\left(1.2\times {10}^{12}\pm 5\times {10}^8\right)$ cm^-2 at $T=1.9$ K for devices fabricated on ²⁸Si and ^natSi, respectively. For magnetic fields $B>2$ T, both devices demonstrate well developed Shubnikov-de Haas (SdH) oscillations in the longitudinal magnetoresistance. This provides transport characteristics of isotopically enriched ²⁸Si, and will serve as a benchmark for classical transport of ²⁸Si at its current state, and low temperature, epitaxially grown Si for quantum devices more generally.