Probing the arrangement of subsurface dopants in a silicon quantum device platform

High-density structures of atomically sharp and narrow sheets of phosphorous dopants placed inside the bulk silicon have garnered interest as a silicon-based quantum computer platform. The potential usage of these Si:P “δ-layer” structures cannot be understated, ranging from quantum dots and tunnel barriers to metallic interconnects to other key components required for quantum device engineering. Despite their esteemed electronic properties and technological applications, the exact in-plane arrangement of the sub-surface P dopants has remained an unsettled debate. Said arrangement is expected to massively affect the electronic properties of the δ-layer with respect to the energy (valley) splitting of the confined quantum well states. Until now, a reliable experimental study has remained elusive, thus inhibiting the full understanding, characterization, and advancement of Si-based quantum device technologies.

In this work, the chemical specificity of X-ray photoelectron diffraction (XPD) was exploited for obtaining the precise structural configuration of dopants placed in sub-surface Si:P δ-layers. Exploiting the high brightness and fully automated angle-resolved core level measurements available at the SuperESCA beamline (Fig. 1a), the researchers disentangled the signatures inherent to the sub-surface P atoms of the δ-layer to demonstrate their concentration and coordination with sub-nanoscale precision.

Covering a range of different dopant concentrations and established sample preparation schemes, the researchers unambiguously determined that in-plane δ-layer doping is predominantly substitutional, i.e., with P atoms replacing Si atoms and assuming the bulk-like Si structure (Fig. 1b). Importantly, no signatures of dimerization between adjacent P atoms within a δ-layer (Fig. 1c), being detrimental to the desired charge doping and concomitant free-electron-like energy states, could be observed even for the highest doping concentrations.

Figure 1: Angle-dependent X-ray photoelectron spectroscopy and diffraction of encapsulated phosphorous dopants in silicon. a): High-resolution core level (i.e., XPS) measurements of the P 2p signal recorded from δ-layer doped Si. The P1 components can be assigned to the sub-surface dopant atoms residing within the d-layer. b): A sketch of substitutional doping (top), where the P atoms can assume one of the nine inequivalent positions of the fcc unit cell. Simulations of the X-ray photoelectron diffraction (XPD) expected from in-plane substitutional doping with P and Si as nearest neighbours (bottom, grey) lead to the best agreement – i.e., lowest factor R, with the measured XPD (bottom, orange) from the sub-surface dopant components (P1). c): A sketch of in-plane dimerization between adjacent P atoms within a δ-layer (top). The agreement (R) between the measured diffraction pattern from the dopants in the experiment (bottom, orange) and the expected diffraction pattern from in-plane P—P dimers (bottom, grey) is poor.

These findings not only settle – once and for all, the long-lasting debate about the P dopant arrangements in δ-layers, but furthermore provide details that should significantly advance the understanding, modelling, and implementation of their derived, Si-based, nanoscale quantum devices.

This research was conducted by the following research team:

Håkon I. Røst^1,2, Ezequiel Tosi^3,4, Frode S. Strand¹, Anna Cecilie Åsland¹, Paolo Lacovig³, Silvano Lizzit³ and Justin W. Wells^1,5
1 Center for Quantum Spintronics, Department of Physics, Norwegian University of Science and Technology (NTNU), Trondheim, Norway
2 Department of Physics and Technology, University of Bergen (UiB), Bergen, Norway
3 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
4 Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC), Madrid, Spain
5 Department of Physics and Centre for Materials Science and Nanotechnology, University of Oslo, Oslo, Norway

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