Abstract
Nuclear spins of phosphorus [P] donor atoms in crystalline silicon are among the most coherent qubits found in nature. For their utilization in scalable quantum computers, distinct donor electron wavefunctions must be controlled and probed through electrical coupling by application of either highly localized electric fields or spin-selective currents. Due to the strong modulation of the P-donor wavefunction by the silicon lattice, such electrical coupling requires atomic spatial accuracy. Here, the spatially controlled application of electrical current through individual pairs of phosphorus donor electron states in crystalline silicon and silicon dangling bond states at the crystalline silicon (100) surface is demonstrated using a high‐resolution scanning probe microscope operated under ultra‐high vacuum and at a temperature of 4.3K. The observed pairs of electron states display qualitatively reproducible current-voltage characteristics with a monotonous increase and intermediate current plateaus.
Highlights
The (100) oriented silicon substrate is doped with phosphorus with a dark resistivity of 0.08–0.01 ohm-cm at room temperature and a lower, but still significant dark conductance at 4.3 K due to wave function overlap of the neutral P donor states caused by high dopant concentrations ([P] ∼ 1017 cm−3 to 1018 cm−3, see sample preparation in the methods section)
We have calculated the maximal detectable tunneling distance from the probe tip to phosphorus states in the sample to be approximately three Bohr radii. We find that both the diameter (FWHM) of these patches, as well as their observed areal density (∼ 1011 cm−2 corresponding to ∼ 1017 cm−3 with ∼ 3 Bohr radii accessible probe depth), indicate that the bright regions in the image are likely be caused by the randomly distributed P donor atoms near the silicon surface
In order to corroborate the findings presented above and in particular the hypothesis that the charge percolation for the observed conduction atomic force microscopy (AFM) images is caused by P-dangling bonds (DBs) transitions, we have repeated the imaging and identified hundreds of locations with highly localized current maxima
Summary
A positive DC bias is applied to the tip with respect to the back contact of the substrate. The tip is brought in proximity of the sample surface while the interaction between the Pt probe and the surface is measured by observation of Δ f. The height feedback controller for the qPlus sensor uses the measured gap-dependent frequency shift to control the height of the probe (constant frequency shift) during imaging, with the assumption that each point of the surface provides an equal interaction with the probe tip. Local carrier conduction occurs from sample to Pt tip via tunneling. The tip and sample are in thermal contact with a liquid He4 reservoir maintaining a stable temperature of 4.3K
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