An international team of physicists and chemists based at the University of California at Berkeley has, for the first time, recorded the action of silicon electrons becoming freed from their atomic shells using attosecond pulses of soft X-ray light lasting only a few billionths of a billionth of a second.These mobile electrons make the semiconductor material conductive so that an applied voltage results in a flowing current.
While earlier femtosecond lasers were unable to resolve the jump from the valence shell of the silicon atom across the band-gap into the conduction electron region, this research shows that the transition takes less than 450 attoseconds.
“Though this excitation step is too fast for traditional experiments, our novel technique allowed us to record individual snapshots that can be composed into a ‘movie’ revealing the timing sequence of the process,” explains Stephen Leone, UC Berkeley professor of chemistry and physics.
Leone, his UC Berkeley colleagues and collaborators from the Ludwig-Maximilians Universität in Munich, Germany, the University of Tsukuba, Japan, and the Molecular Foundry at the Department of Energy’s Lawrence Berkeley National Laboratory report their achievement in the journal, Science.
In semiconducting materials, electrons are initially localized around the individual atoms forming the crystal and thus cannot move or contribute to electrical currents. When light hits these materials or a voltage is applied, some of the electrons absorb energy and get excited into mobile states in which the electrons can move through the material. The localized electrons take a quantum leap into the conduction band, tunneling through the barrier that normally keeps them bound to atoms.
They say that the unprecedented resolution of this attosecond technology will allow scientists to resolve extremely brief electronic processes in solids that, to date, seemed too fast to be approached experimentally.