Spatial Metrology of Dopants in Silicon with Exact Lattice Site Precision
Professor Elisabetta Barberio
Speaker: Dr. Muhammad Usman, Centre for Quantum Computation and Communication Technology, School of Physics
Shallow dopants in silicon are promising candidates for the implementation of spin qubits and quantum logic gates. Excellent progress in the last few years such as minutes-long coherence time , atomically precise fabrication technique , and surface code based architecture scheme  has brought silicon-donor based quantum computers much closer to reality. One of the key challenges in silicon based quantum computing is to find exact dopant positions after the fabrication and overgrowth processes, which would greatly help in the design and optimisation of highly precise quantum logic gates – a key ingredient for quantum error correction and scale up. Here we present an atomically precise metrology based on low temperature STM measurements  in conjunction with a fully quantum, large-volume treatment of the STM-dopant system , which demonstrates the pinpointing of the position of subsurface phosphorous (P) and arsenic (As) dopants in silicon down to individual lattice sites.
The STM based metrology technique requires atomic scale understanding of the measured dopant images with quantitative precision. To solve this challenging problem, we establish a theoretical framework by coupling the Bardeen’s tunnelling theory  with the atomistic tight-binding simulations  of donor wave function in silicon, which reproduce the STM images of subsurface dopant wave functions with an unprecedented accuracy. Based on high-level understanding of STM images and their systematic analysis, we discover unique patterns of image features, which are highly sensitive to the 3D positioning of the donor underneath the Si surface. Quantitative agreement between the measured and computed images provides a clear identification of the exact 3D location of both P and As donors in the Si lattice, down to depths of 5 nm below the Si surface . The established exact donor position metrology will transform our knowledge of devices and dopant physics at the most fundamental scale leading to devices with optimised functionality, both in quantum and classical application.
 K Saeedi et al., Science 342, 830, 2013
 B. Weber et al., Science 335, 64, 2012
 C. Hill et al., Science Adv. 1, e1500707, 2015
 J. Salfi et al., Nature Materials 13, 605, 2014
 M. Usman et al., arXiv:1601.02326, 2016
 J. Bardeen PRL 6, 57, 1966
 M. Usman et al., Phys. Rev. B. 91, 245209, 2015
Venue: Hercus Theater, Physics South.