It is well known that elements from Group V of the periodic table behave similarly to hydrogen atoms when incorporated into a single crystal of silicon or germanium, and can easily donate electrons to the material. This property has allowed scientists to control the conductivity of these important semiconductors, and was responsible for kick-starting the semiconductor revolution in the later half of the 20th century. A new application is now emerging for this silicon-donor material system in the 21st Century in the shape of quantum computing.
In a recent paper published in Nature, researchers from the FOM institute for Plasma Physics in the Netherlands, the University of Surrey, and the London Centre for Nanotechnology showed that quantum state of phosphorus donors in silicon could be precisely controlled using far-infrared laser light. This was a significant breakthrough for silicon-based quantum computing.
The energy-level structure of phosphorus donors in silicon is defined by the reduced Coulomb attraction of the donor electron to the phosphorus atom, and the dielectric permittivity of the surrounding crystal environment. This leads to a series of very sharp absorption lines, whose energy signature is precisely defined by the physical properties of the silicon host crystal. We have shown that the incorporation of a small amount of germanium in the silicon matrix allows us to engineer the energy level structure of the phosphorus donors without any detrimental effect on the lifetime dynamics of the phosphorus donor, and hence on its potential quantum applications.
This work has been published in Physical Review B (S. A. Lynch et al, 82(24), pp. 245206, 15 December, 2010.
Journal link: http://link.aps.org/doi/10.1103/PhysRevB.82.245206
Figure: Far-infrared absorption spectra of Si and SiGe doped with phosphorus donors. The curves are colour coded to show different temperatures.