Direct imaging of straggled ions for top-down fabrication of Si:P qubits using atom probe tomography

(1) Centre for Quantum Computation and Communication Technology, School of Physics, University of Melbourne, AUSTRALIA
(2) Atom Probe Research Group, Department of Materials, University of Oxford, UK

Single 31P ions can be deterministically implanted (“top-down”) into isotopically pure 28Si substrates to build devices in which information can be encoded in the quantum states of the 31P donor electron spin or nuclear spin.  In the case of the nuclear spin, coherent states can be sustained with remarkably long T2 times in excess of 30 s.  Exploiting these results to build a large-scale device requires careful architecture design consistent with the precision limitations of the deterministic implantation method.  Donor atom positioning no deeper than 20 nm below the gate oxide and ion straggling constrains the implantation energy to below 14 keV and preferably below 10 keV.  Models for the straggling process, for example the Stopping and Range of Ions in Matter (SRIM) and crystal-Transport of Ions In Matter (TRIM), allow for constraints imposed by various effects to be evaluated.  However, we have employed Atom Probe Tomography (APT), to facilitate the measurement of low-fluence 14 keV 31P ions implanted into Si with atomic scale accuracy. APT is based on the highly controlled field-evaporation of individual ions from the surface of a very-sharp needle-shaped specimen.  It provides a 3D reconstruction of the distribution of implanted ions, typically within a volume of 80 x 80  x 200 nm3 , which can be imaged to a spatial precision better than 1 nm and a chemical sensitivity of less than 0.01 at.%. We observe that the experimental depth profile has a FWHM of 20 nm with a depth consistent with the SRIM simulations.  Further work will refine these measurements and allow us to apply these experimental constraints to the design of a large scale device fabricated with the top-down deterministic ion implantation method.