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arXiv:2509.03243

Isotopically Selected Single Antimony Molecule Doping

Mason Adshead, Maddison Coke, Evan Tillotson, Kexue Li, Sam Sullivan–Allsop, Ricardo Egoavil, William Thornley, Yi Cui, C.M. Gourlay, Katie L. Moore, Sarah J. Haigh, Richard J. Curry

Published Sep 3, 2025• 0 citations
arXivHTMLPDF

Abstract

A reliable route to the deterministic fabrication of impurity ion donors in silicon is required to advance quantum computing architectures based upon such systems. This paper reports the ability to dope isotopically-defined unique (${}^{121}\mathrm{Sb}{}^{123}\mathrm{Sb}$) clusters into silicon with measured detection efficiencies of 94% being obtained. Atomically resolved imaging of the doped clusters reveals a Sb-to-Sb separation of ~2 nm post-implantation, thus indicating suitability to form coupled qudit systems. The method used is fully compatible with integration into processing that includes pre-enrichment of the silicon host to < 3ppm ${}^{29}\mathrm{Si}$ levels. As such, we present a potential pathway to the creation of scaled qudit arrays within silicon platforms for quantum computing.

DOI
10.48550/arxiv.2509.03243

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A reliable route to the deterministic fabrication of impurity ion donors in silicon is required to advance quantum computing architectures based upon such systems. This paper reports the ability to dope isotopically-defined unique (121Sb123Sb{}^{121}\mathrm{Sb}{}^{123}\mathrm{Sb}121Sb123Sb) clusters into silicon with measured detection efficiencies of 94% being obtained. Atomically resolved imaging of the doped clusters reveals a Sb-to-Sb separation of ~2 nm post-implantation, thus indicating suitability to form coupled qudit systems. The method used is fully compatible with integration into processing that includes pre-enrichment of the silicon host to < 3ppm 29Si{}^{29}\mathrm{Si}29Si levels. As such, we present a potential pathway to the creation of scaled qudit arrays within silicon platforms for quantum computing.
ABSTRACT A reliable route to the deterministic fabrication of impurity ion donors in silicon is required to advance quantum computing architectures based upon such systems. This paper reports the ability to dope isotopically‐defined unique ( 121 Sb 123 Sb) 2+ molecular ions into silicon with measured detection efficiencies of 94% being obtained. Atomically resolved imaging of the doped Sb ions reveals substitutionally incorporated atoms with a Sb‐to‐Sb separation of ≈2 nm post‐implantation, thus indicating suitability to form coupled qudit systems. Molecular dynamics simulations support the preference for doped Sb atoms to occupy lattice sites, driven by fast (≈1s) re‐crystallization of localized ion implantation induced damage at 300 K. The Sb doping method used is fully compatible with integration into processing that includes pre‐enrichment of the silicon host to sub‐3 ppm 29 Si levels. As such, we present a potential pathway to the creation of scaled qudit arrays within silicon platforms for quantum computing.
Spawned by the finding of efficient quantum algorithms, the development of a scalable quantum computer has emerged as a premiere challenge for nanoscience and nanotechnology in the last years. Spins of electrons and nuclei in P31 atoms embedded in silicon are promising quantum bit (qubit) candidates. In this article we describe single atom doping strategies and the status of our development of single atom qubit arrays integrated with control gates and readout structures in a “top down” approach. We discuss requirements for P31 qubit array formation by single ion implantation, and integration with semiconductor processing.
Novel isotopically enriched and aluminum (Al) delta-doped silicon crystals with exceptional material properties are proposed and developed, in an effort to bridge superconducting quantum information with silicon-based quantum devices for a new generation of solid-state quantum computing. Quantum computing platforms based on semiconducting and superconducting qubits are two powerful candidates. For semiconductor spin qubits, long coherence times can be achieved when using isotopically enriched 28Si as a host material. However, qubit coupling in semiconductor qubits is difficult to achieve due to the nanometer scale of the devices. For superconductor qubit devices, millimeter scale with long spatial coherence length makes them relatively easy to couple multiple qubits, but losses at the material interface are limiting the device performance. Our ultimate objective is to create a hybrid system where both Si spin qubits and superconductor qubits are coupled in a nuclear spin-free and interface-free material. To achieve this, a superconducting semiconductor with monocrystalline structure is proposed. One possible candidate is through Al delta-doped 28Si, as highly Al doped Si is expected to have a 10× higher critical temperature (Tc) than highly boron (B) doped Si (Tc ≈ 0.6 K). The focus of this thesis is to attack two materials limitations for realizing a monocrystalline, super-semiconducting hybrid architecture: 1) substantially reducing 29Si that limits coherence of semiconducting spin qubits and 2) exploring supersaturated, Al-doped silicon as a system for localized superconductivity within silicon with a potential Tc > 1K.In the first part of this thesis, we demonstrate the advancements in isotopically enriched 28Si in terms of chemical purity, enrichment, and electrical properties. In this work, a new, compact UHV version of the hyperthermal Penning ion source was designed and built. An improvement in the chemical purity from 98.47 % to 99.97 %, while maintaining a 28Si enrichment of > 99.999987 % (0.832 ppm 29Si) has been achieved. We broaden the material variety of 28Si at different levels in the quantum information community by demonstrating the ability to grow isotopically enriched 28Si epitaxial films with precisely controlled (≈ 90 % accuracy) enrichment levels, ranging from natural abundance to < 1 ppm 29Si. In addition, to better assess the quality of our 28Si material, we have successfully fabricated and measured 28Si MOSFET devices, and compared to those from natural abundance Si on the same substrate. The charge carrier mobility on isotopically enriched 28Si is found to be approximately a factor of 3 lower compared to the natural abundance Si, a result of the short-range scattering (impurity scattering). In the second part of this thesis, we report on the material growth and characterization of super-saturated Al delta layers in Si and explore the possibility for superconductivity. To reach a critical density needed for superconducting transition, the first step is to study the saturation density of this dopant and a way to confine it in 3D. Using a combination of different characterization tools, the maximum 2D atom density of one atomic layer of Al on Si(100) surface before cluster formation is found to be 3.4 × 1014 cm-2. We also studied the effects of different material growth methods on electrical conduction and the possibility of reaching higher 3D density of Al in this Si-Al-Si heterostructure. We found that Al delta doping in Si behaves differently compared to other dopants: the incorporation anneal does not change the dopant activation efficiency. Standard molecular beam epitaxy (MBE) and locking layer (LL) growth on Al layer is not successful and Al dopant activation is found to be < 50 %, most likely due to the tendency of Al atoms to move toward the surface and the cluster states been developed from the thermal anneals. The electrical conduction of this delta layer at low temperature is also studied and modeled using a temperature dependent two-carrier type model, which is the first reported conducting Al delta layer in Si. We believe that reaching the superconducting transition using an Al delta layer as a dopant in Si is possible, but this requires further studies both experimentally and theoretically to minimize the Al segregation in order to achieve a high enough 3D dopant density.
We discuss progress towards the fabrication and demonstration of a prototype silicon-based quantum computer. The devices are based on a precise array of 31P dopants embedded in 28Si. Fabrication is being pursued via two complementary pathways – a ‘top-down’ approach for near-term production of few-qubit demonstration devices and a ‘bottom-up’ approach for large-scale qubit arrays. The ‘top-down’ approach employs ion implantation through a multi-layer resist structure which serves to accurately register the donors to metal control gates and single-electron transistor (SET) read-out devices. In contrast the ‘bottom-up’ approach uses STM lithography and epitaxial silicon overgrowth to construct devices at an atomic scale. Techniques for qubit read-out, which utilise coincidence measurements on novel twin-SET devices, are also presented.
Scalable silicon quantum computers will require a material perfection that has never been attempted. Ground state wavefunctions for conduction electrons orbiting individual phosphorous donors must be polarized electronically and coupled to nearest neighbors with great precision. Elimination of all randomizing influences can be achieved only with a fully epitaxial structure; and we believe that output circuitry must also be integrated into the qubit arrays in order to achieve the uniformity needed for large-scale integration. A process that could potentially accomplish this will be outlined, based on scanning tunneling microscope (STM) removal of individual hydrogen atoms from the H-terminated silicon surface followed by phosphine dosing and ultra-low-temperature overgrowth. Self-ordering of PH3 molecules onto extended areas of bare silicon should permit patterning of planar single-electron transistors along with P-donor qubits in the same lithographic step. Initial plans for an experiment to characterize exchange coupling under gate control will be described.
We propose an architecture for quantum computing involving substitutional donors in photonic-crystal silicon cavities and the optical initialization, manipulation, and detection processes already demonstrated in ion traps and other atomic systems. Our scheme leads to easily achievable requirements on the positioning of the donors and considerably simplifies the implementation of the building blocks required for the operation of silicon-based quantum computing devices, including realization of one- and two-qubit gates, initialization, and readout of the qubits. Detailed consideration of the processes involved, using state-of-the-art values for the relevant parameters, indicates that this architecture leads to errors per gate compatible with fault-tolerant quantum computation and should be useful for quantum simulations and quantum optics applications.