Route to high-precision positioning of single ion-implanted impurities in silicon

The only quantum technology (QT) fabrication technology that can readily leverage microelectronic fabrication processes with the existing ability of large scale-up, enabling big enough qubit arrays for error correction, or that can potentially repeatably manufacture large numbers of identical devices, is the incorporation of single impurity qubits through implantation. However, unless fully deterministic implantation of single ions (ISI) is developed, the advantages of impurity-based QT for scale-up will not be realized. Quantum computing based on ion traps, superconducting circuits and semiconductor quantum dots using a small number of qubits are well advanced, but very large-scale reproduction constitutes a major challenge for each. Small numbers of impurity qubits in silicon can also be made with high quality using hydrogen lithography, which is based on scanning probe techniques, that have enabled atomic-scale precision leading to such ground-breaking achievements as the single-atom transistor (However, it is slow and does not provide an easily scalable route to the millions of qubits needed for manufacturable quantum computers. Implantation in silicon of single impurity qubit atoms offers a solution, but most of the research in this area centres on samples with stochastic incorporation of impurities with some limited control over the placement through masks or with focussed beams. The challenge here is therefore the opposite compared with ion traps etc - large scale repetition is easy, but the positioning (and consequent error rate) of each qubit is poorer and must be improved. The placement precision is limited by the focusing of the implanted ion and the movement of the ion after it enters the target material, known as the impact straggle. Implantation also causes undesirable damage to the crystal host, as the energetic ion ricochets through channels in the crystal. This is the challenge we seek to address, using a speculative idea that will not only repair this impact damage cloud but also, and most importantly, allow much higher precision positioning of the implanted impurity.
We propose a solution based on lateral solid phase epitaxial regrowth (L-SPER). Simply put, the target area is pre-amorphised (implanting silicon ions into silicon breaks bonds but does not introduce impurities and can even improve isotopic purity) by a focussed ion beam or through broad area lithography and ion implantation. Following implantation of a single ion, a low-temperature anneal restores the crystal through epitaxial regrowth, which is seeded by the surrounding crystalline material. Full pre-amorphisation is well known to result in higher crystallinity following annealing, compared to the partial amorphisation caused solely by the implantation process. The nature of this proposal is to consider what effect L-SPER has on an individual implanted atom. There is every reason to expect that, as the amorphised region shrinks during regrowth, the impurity atom is slowly pushed to the centre as the crystal reforms. If we can demonstrate this, then the precision of the final placement of the atom may be affected more strongly by the central positioning of the pre-amorphised regions rather than limited by the focusing uncertainty and straggle of the implanted ion, where the former can be of the order of a nanometer giving an order of magnitude improvement in the final positioning.