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Electron spins in silicon

We aim to control single electron spins in silicon where a long spin lifetime is expected due to the low spin-orbit coupling and also a low density of nuclear spins. In our research, the electrons are either confined in the atomic-like potential of a single dopant or in a quantum dot. The research is funded though grants from EPSRC (EP/H016872/1 & EP/G062331/1) and the Leverhulme Trust.


Dopants

Using nanoscale field effect transistors it is possible to measure the electrical transport through individual dopants, the fingerprint of this appears as conduction peaks below the threshold of the FET. The magnetic field dependence allows the electron spin state to be identified. A more interesting problem is to investigate the nuclear spin state, as this identifies the dopant species. In addition, access to the nuclear spin is important as this is a long-lived spin state of great interest for quantum computation. We are combining electrical transport, sensitive charge detection and local electron spin resonance techniques to measure the electron and nuclear spins.

silicon with dopants
(a) A silicon single electron transistor next to a short circuited microwave waveguide. (b) The SET is on a lightly doped silicon substrate to enable single dopants to be probed. (c) Typical measurement of one of the tunnel barriers showing resonant states from single dopants. This a rf-reflectometry measurement and the colour scale shows the amplitude of the reflected signal.


Quantum dots

While the deep confinement potential of dopants are attractive, there is also interest in confining electrons within a man-made potential, a quantum dot. In this case there is more electrostatic control over the system, sample fabrication is more controlable and the spin-lifetimes are also expected to be long. We use intrinsic silicon, and silicon on insulator, substrates to make electrostatically tunable quantum dots (Angus et al.). On the fabrication, we collaborate closely with the Nano group at the University of Southampton. At Cambridge we have extensive low temperature measurement facilities for highly sensitive electrical measurements. Using the radio-frequency single electron transistor allows us to perform high-bandwidth charge detection on the silicon quantum dots (Anguset al., Poddet al.). We are currently working to take these samples into the few electron limit.

Silicon Quantum Dot
(a) Scanning electron micrograph of  a silicon quantum dot. (b) Schematic showing the device structure including the electrostatic gates, ohmic contacts and intrinsic substrate. (c) Coulomb diamonds measured for a single quantum dot device at mK temperatures.