Oct 13 2016
MIT researchers have found that interactions between electrons and heat-carrying particles called phonons can play a significant role in preventing heat dissipation in microelectronic devices. (Credit: MIT)
Technological development in the near future will see more transistors being placed into much smaller areas within computer chips. MIT engineers state cell phones, laptops, and other electronic gadgets are likely to face a greater risk of overheating, resulting in interactions between electrons and heat-carrying particles known as phonons.
According to the researchers these previously underestimated interactions could be crucial in stopping heat dissipation in microelectronic devices. Their findings are published in the Nature Communications journal.
The team conducted a few experiments where they used precisely timed laser pulses to calculate the interactions between phonons and electrons in a very thin silicon wafer. As the concentration of electrons in the silicon grew, the more the electrons dispersed phonons and stopped them from carrying heat away.
When your computer is running, it generates heat, and you want this heat to dissipate, to be carried out by phonons. If phonons are scattered by electrons, they’re not as good as we thought they were in carrying heat out. This will create a problem that we have to solve as chips become smaller.
Bolin Liao, Former Graduate Student, MIT
In contrast, Liao states this same effect could be advantageous to thermoelectric generators, which transform heat directly into electrical energy. In such devices, dispersing phonons and decreasing heat leakage, would greatly optimize their performance.
“Now we know this effect can be significant when the concentration of electrons is high,” Liao says. “We now have to think of how to engineer the electron-phonon interaction in more sophisticated ways to benefit both thermoelectric and microelectronic devices.”
Liao’s co-authors include Gang Chen, the Carl Richard Soderberg Professor in Power Engineering and the head of the Department of Mechanical Engineering; Alexei Maznev, a senior research scientist in the Department of Chemistry; and Keith Nelson, the Haslam and Dewey Professor of Chemistry.
Blocking Flow
When transistors are built using semiconductor materials such as silicon, and electrical cables manufactured from metals, electrons become the main agents responsible for transmitting electricity via a material. One of the key reasons behind such materials having a finite electrical resistance is the presence of definite roadblocks to the flow of electrons - specifically, interactions with the heat-carrying phonons, which can bump into electrons, moving them off their electricity-conducting paths.
For long scientists have analyzed the effect of such electron-phonon interactions on electrons themselves, but how these same interactions impact phonons - and the ability of a material to transmit heat - is not understood much.
People hardly studied the effect on phonons because they used to think this effect was not important. But as we know from Newton’s third law, every action has a reaction. We just didn’t know under what circumstances this effect can become significant.
Bolin Liao, Former Graduate Student, MIT
Scatter and Decay
Liao and his colleagues had previously calculated that in silicon, when the concentration of electrons is over 1019 per cm3, the interactions between phonons and electrons would robustly disperse phonons. They would also decrease the ability of the material to scatter heat by nearly 50% when the concentration is at 1021 per cm3.
“That’s a really significant effect, but people were skeptical,” Liao says. This was mainly due to earlier experiments on materials with high electron concentrations where they guessed that the drop in heat dissipation was not because of electron-phonon interaction but due to defects in materials.
These defects occur from the process of doping, where extra elements such as boron and phosphorous are blended into silicon to boost its electron concentration.
“So the challenge to verify our idea was, we had to separate the contributions from electrons and defects by somehow controlling the electron concentration inside the material, without introducing any defects,” Liao says.
The team developed a method known as three-pulse photo-acoustic spectroscopy to precisely increase the number of electrons in a thin wafer of silicon by optical techniques, and measure any effect on the phonons of the material.
The method builds on a traditional two-pulse photo-acoustic spectroscopy method, where scientists shine two precisely timed and tuned lasers on a material. The first laser produces a phonon pulse in the material, and the second laser measures the function of the phonon pulse as it disperses, or decays.
Liao incorporated a third laser, which when shone on silicon precisely raised the concentration of electrons of the material without causing any defects. When he calculated the phonon pulse after introducing the third laser, he discovered that it deformed a lot faster, signifying that the increased concentration of electrons served to disperse phonons and diminish their activity.
Very happily, we found the experimental result agrees very well with our previous calculation, and we can now say this effect can be truly significant and we proved it in experiments. This is among the first experiments to directly probe electron-phonon interactions’ effect on phonons.
Bolin Liao, Former Graduate Student, MIT
The researchers initially began noticing this effect in silicon that was loaded with 1019 electrons per cm3- comparable or even less in concentration than some existing transistors.
“From our study, we show that this is going to be a really serious problem when the scale of circuits becomes smaller,” Liao says. “Even now, with transistor size being a few nanometers, I think this effect will start to appear, and we really need to seriously consider this effect and think of how to use or avoid it in real devices.”
This study was supported by S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy’s Office of Basic Energy Sciences.