Charge Sensing of Precisely Positioned P Donors in Si

Abstract

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ∼ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.

Spin readout and addressability of phosphorus-donor clusters in silicon

“…1f, correspond to three charge transitions of D 2 . A total of seven charge transitions could be identified for D 2 in a larger-area stability map 10 , but we saw no evidence of completely depleting the donor cluster within our available gate space.…”

“…1a,b). Exploiting the atomicscale precision of STM hydrogen lithography, this planar architecture was fabricated and previously demonstrated to exhibit high contrast charge-sensing characteristics 10 . To review the device layout, Fig.…”

“…1b shows a filled-state STM image, where the yellow features correspond to the device template, generated by desorbing the hydrogen resist monolayer (blue) with the STM tip. Subsequent exposure of the Si surface to PH 3 gas and annealing incorporate P donors, only within the device pattern 10 . A closeup of the central part of the device, after the lithography step, is seen in Fig.…”

Quantum dot spectroscopy using a single phosphorus donor

“…Electrostatic control gates (G 1 , G 2 ) are fabricated along the axis between the dot and the donor, ~50 nm away from the SET dot. After dosing with phosphine gas which only adsorbs to the exposed parts of the Si surface, phosphorus atoms are incorporated into the silicon crystal using a 60 s anneal to 330° C. The whole device is then encapsulated with ~30 nm of epitaxial silicon at 250° C and a growth rate of ~0.15 nm/min [17]. To ensure the deterministic incorporation of a single P atom, we desorb three adjacent Si dimers [1] (see inset of Fig.…”

“…From the height and the width of the Coulomb peaks at zero bias we can also estimate [28] the tunnel rate resistances between source and SET dot, Γ S1 , as ~1 GHz and between the SET dot and drain, Γ 1D , as ~10 MHz, respectively. Finally, for the tunnel resistance between quantum dot and the donor, Γ m , we experimentally derive a lower boundary of ~2 MHz using the SET for time-resolved charge sensing of the single donor [17].…”