Targeted enrichment of 28Si thin films for quantum computing.

We report on the growth of isotopically enriched 28Si epitaxial films with precisely controlled enrichment levels, ranging from natural abundance ratio of 92.2% all the way to 99.99987% (0.83 × 10-6 mol mol-1 29Si). Isotopically enriched 28Si is regarded as an ideal host material for semiconducting quantum computing due to the lack of 29Si nuclear spins. However, the detailed mechanisms for quantum decoherence and the exact level of enrichment needed for quantum computing remain unknown. Here we use hyperthermal energy ion beam deposition with silane gas to deposit epitaxial 28Si. We switch the mass selective magnetic field periodically to control the 29Si concentration. We develop a model to predict the residual 29Si isotope fraction based on deposition parameters and measure the deposited film using secondary ion mass spectrometry (SIMS). The measured 29Si concentrations show excellent agreement with the prediction, deviating on average by only 10%.

A compact, ultra-high vacuum ion source for isotopically enriching and depositing 28Si thin films.

An ultrahigh vacuum (UHV) compatible Penning ion source for growing pure, highly enriched 28Si epitaxial thin films is presented. Enriched 28Si is a critical material for quantum information due to the elimination of nuclear spins. In some cases, the material must be grown by low temperature molecular beam epitaxy, e.g., scanning tunneling microscopy hydrogen lithography-based devices. Traditional high-purity physical vapor methods typically deliver a very small fraction of source material onto the target substrate, making the cost for use with highly enriched source materials very high. Thus, directed beam sources provide an efficient alternative. This UHV Penning source uses all metal or ceramic parts and a removable electromagnet to allow bake-out. The source gas is a commercial (natural isotope abundance) silane gas (SiH4), an inexpensive source material. High enrichment levels up to 99.999 87% (8.32 × 10-7 mol/mol 29Si) and high chemical purity of 99.965% are shown without postprocessing. We present and discuss the discharge properties of this new source, the ion mass spectrum when coupled to our mass filter, and the secondary ion mass spectroscopy of the grown films.

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 ≈(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 (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.

Temperature dependent 29Si incorporation during deposition of highly enriched 28Si films.

In this study, we examine the mechanisms leading to 29Si incorporation into highly enriched 28Si films deposited by hyperthermal ion beams at elevated temperatures in the dilute presence of natural abundance silane (SiH4) gas. Enriched 28Si is a critical material in the development of quantum information devices because 28Si is free of nuclear spins that cause decoherence in a quantum system. We deposit epitaxial thin films of 28Si enriched in situ beyond 99.99998 % 28Si onto Si(100) using an ion beam deposition system and seek to develop the ability to systematically vary the enrichment and measure the impact on quantum coherence. We use secondary ion mass spectrometry to measure the residual 29Si isotope fraction in enriched samples deposited from ≈ 250 °C up to 800 °C. The 29Si isotope fraction is found to increase from < 1 × 10-6 at the lower temperatures, up to > 4 × 10-6 at around 800 °C. From these data, we estimate the temperature dependence of the incorporation fraction, s, of SiH4, which increases sharply from about 2.9 × 10-4 at 500 °C to 2.3 × 10-2 at 800 °C. We determine an activation energy of 1.00(8) eV associated with the abrupt increase in incorporation and conclude that below 500 °C, a temperature independent mechanism such as activation from ion collisions with adsorbed SiH4 molecules is the primary incorporation mechanism. Direct incorporation from the adsorbed state is found to be minimal.

A particulate isotopic standard of uranium and plutonium in an aluminosilicate matrix.

Mixed-actinide microstandard particles have been produced for calibration and performance testing of isotope-ratio mass spectrometers and ion and electron microprobe instruments. The spherical micrometer-size particles consist of an aluminosilicate matrix loaded with 2.2% by weight of isotopically certified uranium and 0.11% by weight of isotopically certified plutonium. The uranium and plutonium isotopic compositions have been verified by both thermal ionization mass spectrometry and secondary ionization mass spectrometry (SIMS). The elemental composition of the microspheres has been determined by both electron microprobe and SIMS analysis.

Molecular ion imaging and dynamic secondary ion mass spectrometry of organic compounds.

An ion microscope equipped with a resistive anode encoder imaging system has been used to acquire molecular secondary ion images, with lateral resolution on the order of 1 microns, from several quaternary ammonium salts, an amino acid, and a polynuclear aromatic hydrocarbon which were deposited onto copper transmission electron microscope grids. All images were generated by using the secondary ion signal of the parent molecular species. The variation of parent and fragment molecular ion signals with primary ion dose indicates that, for many bulk organic compounds, bombardment-induced fragmentation of parent molecules saturates at primary ion doses of (1-8) X 10(14) ions/cm2. Subsequent ion impacts cause little further accumulation of damage in the sample, and intact parent molecular ions are sputtered even after prolonged ion bombardment (i.e. primary ion doses greater than 1 X 10(16) ions/cm2). This saturation process allows molecular images to be obtained at high primary ion doses and allows depth profiles to be obtained from simple molecular solid/metal test structures.