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Silicon spintronics warms up

Pushing electron charge around a silicon chip drives modern electronics, from supercomputers to mobile phones. Charge distributions have been used for decades to encode digital information, because they are easily tunable as well as surprisingly robust and predictable. Moreover, charges respond rapidly to changes in the voltage of a device, yet quickly relax to the equilibrium configuration required by such voltages. These advantageous properties, however, also mean that a fundamental minimum energy is required to switch a device from an ‘on’ to an ‘off ’ voltage. As modern chips approach this fundamental limit, the prospect of replacing electron charge with electron spin has become increasingly attractive. On page 491 of this issue, Dash et al. describe a dramatic advance in the field of spin-transport electronics (spintronics): the demonstration in silicon at room temperature of the electrical injection and detection of spin-polarized electrons — electrons that all have the same spin orientation. A decade ago, the demonstration of longlived coherent spin precession (the rotation of an electron’s spin orientation about an axis) in semiconductors at room temperature galvanized research into the use of spintronics for semiconductors. To precess coherently, the spins cannot be in thermodynamic equilibrium, and that first experiment showed that the time for which spins can remain out of equilibrium — the spin coherence time — is easily a million times longer than is possible for charge distributions. Switching the spin polarization (Fig. 1), for example from spin ‘up’ to spin ‘down’, on the basis of spins out of thermal equilibrium, might then be possible, but would require electrical injection, transport and detection of these coherently precessing spins. Silicon, the best semiconductor material for charge-based electronics, also seemed a promising choice for spintronics, as the coupling between the electron spin and direction of motion (spin–orbit interaction) in silicon is weak compared with that in many other semiconductors. And as this spin–orbit inter action limits spin coherence times, silicon should have a long coherence time. Successful demonstrations of electrical injection, transport and detection of spins in the semiconductor gallium arsenide were closely followed by those in silicon, although at low temperatures. Dash et al. now achieve spin injection and detection in silicon at room temperature, and control the injected spins by means of a weak magnetic field. The efficiency of the experimental pro cedure used by Dash and colleagues is considerably higher than that of previous attempts using ‘doped’ silicon (possessing a background of unpolarized charge carriers in thermal equilibrium). The authors obtained an electron spin polarization of a few per cent, compared with much less than one per cent in previous work with doped silicon; higher percentages have been achieved with undoped silicon. The electrical detection of spin polarization involves measuring the electrical potential at a ferromagnetic contact. The strength of this potential depends on the spin polarization and doping in the semiconductor beneath the contact; the larger spin polarization obtained by the authors in doped silicon led to a much larger detection voltage (millivolts instead of microvolts). Their approach to electrical injection and detection relied on a single electrical contact, which consisted of a ferromagnetic metal electrode and an aluminium oxide interface (barrier) between the silicon and the electrode, for both injection and detection. This interface increases the efficiency of the electrical injection of spin-polarized electrons from the ferromagnet into the silicon. Keeping the E