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Nathan Cassidy

My publications


England Jonathan, Cox David, Cassidy Nathan, Mirkhaydarov Bobur, Perez-Fadon Andres (2019) Investigating the formation of isotopically pure layers for quantum computers using ion implantation and layer exchange, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 461 pp. 30-36 Elsevier
Quantum computers have been proposed that exploit entangled quantum states between atoms that are isolated from environmental perturbations in a ?semiconductor vacuum? which can be formed by cryogenically cooling an isotopically pure, defect free crystalline layer consisting of Si, or Ge. In a preliminary investigation of an implant and deposition layer exchange technique to produce such ?vacuums?, a layer of aluminium was implanted with 28Si using a conventional implanter. After annealing and cross sectioning, layer exchange was observed to have produced multiple isolated crystals in a cross sectional TEM image. Further deposited Al layers were implanted with Ge using a SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanter over a range of fluences. After anneals at 250/°C and Al removal, crystals of Ge (which also contained Si) were seen at areal densities that increased with implant fluence.
Cassidy N., Blenkinsopp P., Brown I. Dr., Curry Prof. R. J, Murdin Prof. B.N, Webb Prof R.B, Cox Dr. D.C (2020) Single Ion Implantation of Bismuth, Physica Status Solidi A: Applications and Materials Science Wiley
We present the results from a focused ion beam instrument designed to implant single ions with a view to the fabrication of qubits
for quantum technologies. The difficulty of single ion implantation is accurately counting the ion impacts. This has been achieved
here through the detection of secondary electrons generated upon each ion impact. We report implantation of single bismuth ions
with different charge states into Si, Ge, Cu and Au substrates, and we determine the counting detection efficiency for single ion im-
plants and the factors which affect such detection efficiencies. We found that for 50 keV implants of Bi++ ions into silicon we can
achieve a 89% detection efficiency, the first quantitative detection efficiency measurement for single ion implants into silicon without
implanting through a thick SiO2 film. This level of counting accuracy provides implantation of single impurity ions with a success
rate significantly exceeding that achievable by random (Poissonian) implantation.