Quantum transport through Ge-vacancy defects in silicon for application in quantum technologies

Simona Achilli
University of Milan

Special points in silicon have been envisioned as suitable components for quantum information technology. Conventional dopants, as phosphorus, have been already adopted to realize qubits, although their shallow energy levels limit their operating temperature to few Kelvin [1]. Identifying new defects with deep energy levels in the silicon gap would allow qubit operations up to room temperature, by preventing their ionization. Furthermore, the achievement of a spatial control of the impurities, the characterization of the conditions that would enable their activation and the definition of a protocol for their initialization and redout are stepping stones for the application in quantum technology. We demonstrate, through a theoretical analysis performed in strict connection with the experiments, that the implantation of Ge atoms in silicon, followed by thermal annealing, produces stable Ge-vacancy centers [2]. These hybrid complexes combine the properties of the silicon vacancy, i.e. deep states in the bandgap, with the accurate spatial controllability of the defect obtainable through state of the art single-ion implantation techniques. Density functional theory calculations, performed with hybrid functionals or LDA-1/2 approach are both able to reproduce the experimental charge transition levels. By mapping the ab initio calculation into an extended Hubbard Hamiltonian we characterize the conductance of a one dimensional array of such defects in a realistic setup, i.e. considering many defects in the array with a spacing of the order of some nanometers and including positional disorder. We demonstrate that these impurities exhibit strong electronic correlation and temperature activated quantum transport [3]. The theoretical model supports the interpretation of the available conductance measurements, showing the differences with respect to arrays formed by conventional dopants and allowing the definition of the optimal setup for future device fabrication. Moreover, the theoretical analysis reveals the efficiency of a multi-scale theoretical approach that combines ab-initio results and ad-hoc model Hamiltonians, leading to a reliable description of the system, that would not be achieved with the semplified hydrogenic models usually adopted for shallow dopants. This study is part of a joined theoretical and experimental project funded by the Horizon 2020 European Funding Programme [4].


[1]. He, S. K. Gorman, D. Keith, L. Kranz, J. G. Keizer, M. Y. Simmons, A two-qubit gate between
phosphorus donorelectrons in silicon, Nature 571, 371 (2019)
[2] S. Achilli, N. Manini, G. Onida, T. Shinada, T. Tanii, E. Prati, GeVn complexes for silicon-based
room-temperature single-atom nanoelectronics, Sci. Rep. 8, 18054 (2018).
[3] S. Achilli, N. H. Le, G. Fratesi, N. Manini, G. Onida, M. Turchetti, G. Ferrari, T. Shinada, T. Tanii, E.
Prati, Position-controlled functionalization of vacancies in silicon by single- ion implanted germanium
atoms, Adv. Funct. Mater. in press, (https://arxiv.org/abs/2102.01390.)
[4] Nanoscale Foundries and Fine Analysis projects (nffa.eu)