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A solid-state quantum microscope for wavefunction control of an atom-based quantum dot device in silicon

Nature Electronics volume 6, pages 409–416 (2023)Cite this article

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Abstract

Quantum states of atomic systems can be directly addressed using quantum optical microscopes. However, solid-state microscopy techniques cannot typically achieve both local measurements and control of the state due to their measurement mechanism or the presence of a globally conductive substrate that impedes local gate control. Here we report a solid-state quantum microscope that can control and locally probe the wavefunctions of atomic quantum dots in silicon. Our microscope consists of a scanning tunnelling microscope tip, source and gate electrodes defined on an insulating silicon substrate by subsurface antimony implantation and phosphorus dopants incorporated with atomic precision. In contrast to conventional semiconductor qubit devices, the macroscopic electrodes are fabricated before patterning the nanoscale elements. A light-assisted method is designed to make the substrate conductive to stabilize the microscope tip close to the quantum dots, before reversing to an insulator for local gating and spectroscopy. We show that the microscope can be used to tune and map the charge states of single and double quantum dots, as well as control the relative electrochemical potential between two dots.

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Fig. 1: Quantum microscopy of an atomic device in silicon.
Fig. 2: Spatially and spectroscopically resolving an atomically engineered QD.
Fig. 3: Tuning the strength of the STM probe.
Fig. 4: Aligning two QDs in space and energy.

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Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We acknowledge support from the ARC Centre of Excellence for Quantum Computation and Communication Technology (CE170100012), an ARC Discovery Project (DP180102620), Silicon Quantum Computing Pty Ltd, the US Army Research Office (W911NF-17-1-0202) and from the NSW and ACT Nodes of the Australian National Fabrication Facility. J.S. acknowledges support from an ARC DECRA fellowship (DE160101490). B.C.J. and J.C.M. acknowledge the AFAiiR node of the NCRIS Heavy Ion Capability for access to the ion implantation facilities.

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Authors and Affiliations

  1. Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, UNSW Sydney, Kensington, New South Wales, Australia

    B. Voisin, J. Salfi, D. D. St Médar, M. Y. Simmons & S. Rogge

  2. Silicon Quantum Computing Pty Ltd, UNSW Sydney, Kensington, New South Wales, Australia

    B. Voisin, D. D. St Médar & M. Y. Simmons

  3. Stewart Blusson Quantum Matter Institute, and Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, British Columbia, Canada

    J. Salfi

  4. Centre of Excellence for Quantum Computation and Communication Technology, School of Physics, The University of Melbourne, Parkville, Victoria, Australia

    B. C. Johnson & J. C. McCallum

  5. Centre of Excellence for Quantum Computation and Communication Technology, School of Engineering, RMIT University, Melbourne, Victoria, Australia

    B. C. Johnson

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  1. B. Voisin
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Contributions

B.V., J.S. and S.R. designed the project. B.C.J. and J.C.M. designed and performed the stopping and range of ions in matter simulations, antimony implantation and Rutherford backscattering measurements. B.V. performed the atomic fabrication, low-temperature measurements and data analysis, with J.S., M.Y.S. and S.R. providing inputs. D.D.S.M. performed the electrostatic simulation of the device, with J.S., B.V. and S.R. providing inputs. B.V. wrote the manuscript with contribution of all authors.

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Correspondence to B. Voisin or S. Rogge.

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Competing interests

M.Y.S. is a director of the company Silicon Quantum Computing Pty Ltd.

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Nature Electronics thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Implantation procedure and simulation.

a, Implantation process. The implantation energy is chosen to overcome a 22.5 nm sacrificial oxide barrier. The sample presented here was implanted with a double dose of 60 keV and 100 keV with respective doses of 4 × 1014 and 1 × 1015 cm2. These energies are less than that of the sample presented in the manuscript (70 and 110 keV) to compensate for a thinner oxide (22.5 vs 27 nm). b, Stopping and Range of ions in matter (SRIM) simulations of the implantation profile, for 60 keV (black), 100 keV (red) and total (blue). The concentration peaks around 40 nm in the silicon substrate, and is largely above the metal/insulator transition at the SiO2/Si interface. The lateral range of these implants is less than 20 nm while the diffusion upon flash annealing is less than 25 nm. This is sufficient to ensure Sb is confined to the electrodes and does not diffuse into the regions where the STM is performed.

Extended Data Fig. 2 Implantation profile analysis.

a, Comparison of the SRIM simulation and (black) and of Rutherford back-scattering (RBS) measurements of a sample annealed at 660∘C for 15h. The depth of the SRIM data was offset to match the onset of the RBS signal, as the sacrificial oxide was etched for the RBS samples. The random data (red) correspond to the total amount of Sb, the channeled data (blue) correspond to interstitial and unactivated Sb atoms. b, Random RBS data for different flash anneal conditions: 660∘C for 15h (red), followed by a 1100∘C/20 s (dark red) or a 1200∘C/20 s flash (brown). The 1100∘C used for the device leaves a high concentration of antimony atoms in the top few nanometers of the silicon substrate. c, Sb sheet density, determined as the area of the random RBS signal, as a function of the flash anneal conditions. The density remains above the metal-insulator transition despite a reduction due to deep diffusion in the substrate. d, Sb activated fraction, determined as 1 − Ac/Ar where Ar and Ac are the area under the random and channeled RBS signals, respectively. The first point (no annealing) is close to zero as annealing is necessary to activate the Sb atoms, it is slightly negative due to a statistical difference between the random and channeled measurements. The activated fraction is above 80% once the sample is annealed, the error bars increase with annealing temperature due to lower Sb counts in the channeled data. The mean values and error bars shown for the sheet density and activated fraction are obtained from least-square Gaussian fits to the RBS measurements, with details given in Supplementary Information Section 1.

Extended Data Fig. 3 Measurement protocol.

a, Measurement protocol. The transition between the topography mode (feedback loop and light on, Vb = Vg = − 1.6 V) and the spectroscopy mode is defined as follows. The light and feedback loop are first turned off, and then the bias and gate voltages are swept to the initial values of the spectrum to be acquired, which are closer to zero with respect to topography settings. The tip is then brought closer to the sample. This order, gate and bias voltages first before tip height offset, ensures for the tunnel current not to exceed 1 nA, which could create spurious hydrogen desorption at the surface. The spectroscopy diagram shown here is designed to record the tunnel current as a function of the bias voltage, for a fixed gate voltage and tip position. Once the spectroscopy is completed, a transition is implemented to revert back to topography settings. A similar protocol is implemented for spatially resolved spectroscopy. b, Current Isg between the source and the gate when the light is off. In this configuration, the transimpedance amplifier is connected to the gate electrode and the tip was left floating. Minimal leakage current is observed for a large bias range Vb − Vg from -1.5 to +1.5 V, with a linear fit of the region around Vb − Vg = 0 V (black dotted line) yielding Rsg = 9.45 GΩ, similar to other dopant-based devices.

Extended Data Fig. 4 Evolution of the QD charge distribution upon increasing filling factor.

a-c STM images of first three charge transitions for QD#1, same as main text. d Average line cut of the normalised STM images, taken along the horizontal grey section #1 shown in a. The third transition shows more charge distribution in the bottom left corner of the QD designed area. e Average line cut taken along the horizontal grey section #2 shown in a. The third transition shows more charge distribution around the top right section of thew QD designed area.

Extended Data Fig. 5 Wavefunction and spectroscopy data for QD#2.

a, Stability diagram Isd vs Vb, Vg. This QD shows more resonances than QD#1 indicating that more donors have been incorporated. b, Wavefunction image taken at Vg = − 0.35 V and Vb = − 0.23 V. The dashed rectangle highlights the size of the lithography area. Beside drift correction, an additional Fourier filtering was performed for this image as the tunnel current was close to the noise background. c, Wavefunction image taken at Vg = 0 V and Vb = − 0.8 V. At such large occupation number and large source-drain bias, the wavefunction extends outside the lithography area. d, Spectroscopy line Isd vs Vb and x taken across the reservoir and the QD at Vg = 0 V. e, Same as b taken at Vg = -1 V. centred on the QD. The plateaux in tunnel current vs Vb indicate the addition of electrons on the QD. f Spectroscopy line cuts extracted from b and c, see arrows for colour code. There is no tunnel current within the silicon gap away from the QD and the reservoir. The Coulomb plateaux are visible when the tip is located over the QD. The silicon gap completely closes when the tip is above the source reservoir (black line), which evidences the continuity in the metallic contact between the antimony implanted electrode and its atomic-scale phosphorus extension.

Extended Data Fig. 6 Large scale image of the double dot device.

a, Topography image of the double dot device lithography, same as main text. b, Quantum state image taken at Vb = Vg = − 0.8 V, same scale as the lithography image. As in the main text, we take as reference to align the furthest dot from the source (called LD). The two dots are barely visible at this lower gate and bias voltages, but direct tunnel current over the reservoir can be evidenced. A diffusion of the source lead Δdlead = 2.0 nm towards the quantum dots can be detected in this image, attributed to the high concentration of dopants in this element. c Large scale topography image (Vb = Vg = − 2.1 V) around the DQD (associated topography image to the quantum state image shown in b), where the dangling bonds can be evidenced as bright features. d Small scale topography image (Vb = Vg = − 2.1 V) around the DQD, corresponding to the dashed square in b and c. A difference in the dangling bond brightness can be observed between the topography and the quantum state images. The silicon substrate is illuminated and conductive in topography mode, resulting in all dangling bonds having a similar brightness. On the contrary, only the dangling bonds close to incorporated dopants (DQD or source) are bright in the quantum state image, as the sample is insulating in this mode and transport must occur through conductive elements.

Supplementary information

Supplementary Information

Supplementary Sections 1–3 and Figs. 1–3.

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Voisin, B., Salfi, J., St Médar, D.D. et al. A solid-state quantum microscope for wavefunction control of an atom-based quantum dot device in silicon. Nat Electron 6, 409–416 (2023). https://doi.org/10.1038/s41928-023-00979-z

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