“When your computer is running, it generates heat, and you want this heat to dissipate, to be carried out by phonons,” explained former MIT graduate student Bolin Liao. “If phonons are scattered by electrons, the phonons not as good as we thought they were in carrying heat out.”
“From our study, we show that this is going to be a serious problem when the scale of circuits becomes smaller. Even now, with transistor size being a few nanometres, I think this interaction effect will start to appear, and we need to consider this effect and think of how to use or avoid it,” she continued.
On the other hand, Liao said this same effect may benefit thermoelectric generators, which convert heat directly into electrical energy. In such devices, scattering phonons, and thereby reducing heat leakage, would improve their performance.
The team has developed a technique called three-pulse photoacoustic spectroscopy to precisely increase the number of electrons in a thin wafer of silicon by optical methods, and measure any effect on the material’s phonons. The technique expands on a conventional two-pulse photoacoustic spectroscopy technique, in which scientists shine two lasers on a material. The first laser generates a phonon pulse in the material, while the second measures the activity of the phonon pulse as it scatters, or decays.
Liao added a third laser, which when shone on silicon precisely increased the material’s concentration of electrons, without creating defects. When he measured the phonon pulse after introducing the third laser, he found that it decayed much faster, indicating that the increased concentration of electrons acted to scatter phonons and dampen their activity.
“This is among the first experiments to directly probe electron-phonon interactions’ effect on phonons,” said Liao. “Now we know this effect can be significant when the concentration of electrons is high. We have to think of how to engineer the electron-phonon interaction in more sophisticated ways to benefit both thermoelectric and microelectronic devices.”