Use of quantum effects as potential qualifying metrics for “quantum grade silicon”

Across solid state quantum information, material 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 material metrics, e.g., enrichment, are needed, while existing classical metrics such as mobility might be relaxed compared to conventional electronics. In this work, we examine locally grown silicon that is superior in enrichment, but inferior in chemical purity compared to commercial-silicon, as part of an effort to underpin the material standards needed for quantum grade silicon and establish a standard approach for the intercomparison of these materials. We use a custom, mass-selected ion beam deposition technique, which has produced isotopic enrichment levels up to 99.999 98% 28Si, to isotopically enrich 28Si, but with chemical purity >99.97% due to the molecular beam epitaxy techniques used. From this epitaxial silicon, we fabricate top-gated Hall bar devices simultaneously on 28Si and on the adjacent natural abundance Si substrate for intercomparison. Using standard-methods, we measure maximum mobilities of ≈(1740 ± 2) cm2/(V s) at an electron density of (2.7 × 1012 ± 3 × 108) cm−2 and ≈(6040 ± 3) cm2/(V s) at an electron density of (1.2 × 1012 ± 5 × 108) 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 oscillations in the longitudinal magnetoresistance. This provides the transport characteristics of isotopically enriched 28Si and will serve as a benchmark for the classical transport of 28Si at its current state and low temperature, epitaxially grown Si for quantum devices more generally.