For the first time, scientists have captured the essence of semiconductor computing on film by taking snapshots of the electron transfer from valence to conduction band states. It is this leap that forms the basis for the entire semiconductor industry, digital electronics and modern computing as we know it.
Using attosecond extreme ultraviolet (XUV) spectroscopy much like a stopwatch, the team of physicists and chemists based at UC Berkeley were able to time the step rise at ~450-attoseconds, shedding light on the fundamental speed limit of modern electronic circuitry.
Just how fast is this microscopic event? Consider that an attosecond is equal to one quintillionth of a second. Put another way, an attosecond is to a second what a second is to approximately 31.7 billion years.
As explained by Berkeley science writer Robert Sanders, the age of digital electronics is based on mobile electrons making a semiconductor material conductive so that the application of light or voltage results in a flowing current. In a computer chip, electronic current flowing across transistors facilitates the switch between two binary states, zero and one, giving rise to the fundamental language of computers.
The key event occurs when electrons attached to atoms in the crystal lattice jumps from the valence shell of the silicon atom across the band-gap into the conduction electron region. The previous generation of femtosecond lasers were unable to glimpse this event, which takes place faster than a quadrillionth of a second after laser excitation from the slower lattice motion of the silicon atomic nuclei.
“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,” said Stephen Leone, UC Berkeley professor of chemistry and physics.
The attosecond extreme ultraviolet (XUV) spectroscopy responsible for the breakthrough recording was developed in the Attosecond Physics Laboratory, which is operated by Leone and Daniel Neumark, UC Berkeley professor of chemistry.
The experimental data was supported by supercomputer simulations of the excitation process and the subsequent interaction of X-ray pulses with the silicon crystal. A team from the University of Tsukuba and the Molecular Foundry at the Department of Energy’s Lawrence Berkeley National Laboratory performed the computing using resources provided by Lawrence Berkeley National Laboratory, the National Energy Research Scientific Computing Center (NERSC) and the Institute of Solid State Physics, University of Tokyo. Funding for the project was provided by the US Department of Defense and the Defense Advanced Research Projects Agency’s PULSE program.
The UC Berkeley colleagues together with researchers from Ludwig-Maximilians Universität in Munich, Germany, the University of Tsukuba, Japan, and the Molecular Foundry at Lawrence Berkeley National Laboratory describe their findings in the Dec. 12 issue of the journal Science.