Physicists have uncovered a way to access physical information that had been hidden to science for 140 years, according to a recent paper.
Back in 1879, physicist Edwin Hall discovered that electrical currents bend when placed in a magnetic field, producing a voltage and a new electrical field perpendicular to the current. Scientists have since exploited this phenomenon, known as the Hall effect, to study the properties of materials like semiconductors that make up microchips—but, frustratingly, the Hall effect prevents scientists from making certain measurements simultaneously. Researchers at IBM, the Korea Advanced Institute of Science and Technology, the Korea Research Institute of Chemical Technology, and Duke University have now devised a one-shot technique to extract this information, called the carrier-resolved photo-Hall measurement technique. It could be especially useful for developing future solar cells and other materials.
“This could create an exciting advance to understand semiconductors in greater detail,” Oki Gunawan, the study’s first author and researcher at the IBM T. J. Watson Research Center, told Gizmodo. “We hope it will bring advances in the near future.”
Electrical charges move through semiconductors as discrete units called charge carriers: negatively charged electrons and positively charged “holes,” electron voids in the material that can move the same way that electrons can. Scientists use the Hall effect to figure out the properties of the charge carriers in a material, like how fast they move and how densely packed they are. More recently, they used the Hall effect to understand the effect of light on the materials they were studying, as light striking certain materials will produce electrons and holes. But techniques based on the Hall effect can only measure the properties of the more numerous charge carrier, called majority charge carrier, rather than the properties of both the minority and majority charge carrier simultaneously. Basically, if there are more electrons, then Hall effect measurements can only reveal information on the electrons; if there are more holes, they can only reveal information about the holes.
Using a thought experiment, Gunawan was able to find a way to extract the minority charge carrier information at the same time as the majority charge carrier information. He imagined two systems, each with the same majority charge carrier at the same density and traveling at the same speed, but with different minority charge carrier speeds. Without any added energy, the two systems would behave the same. But add more energy from light pulses, and they begin to behave slightly differently due to the effects from the minority charge carrier. From this thought experiment, he and his team devised an equation to describe both the minority and majority charge carriers simultaneously, according to the paper published last week in Nature.
But the technique requires a way to reduce noise, in the case that the material only weakly experiences the Hall effect or that there are other potential confounding signals. IBM researchers had previously developed a new kind of system called a parallel dipole line, a pair of cylindrical magnets that, acting together, create something like a magnetic field trap. They put two samples, one silicon and another a light-sensitive material called a perovskite, into the trap, and used their new equation to extract information on both the majority and the minority charge carriers.
This might seem rather in-the-weeds, but measuring these properties is important when trying to determine whether a material would be useful in a solar cell, Gunawan explained. Plus, it’s a fundamental physics result linking magnetic fields, electricity, and light.
There are limitations, of course. Gunawan explained that the experimental method can falter on materials with high charge carrier densities—they’d require high-energy lasers to study, which could melt the material.
Still, this is exciting stuff. It’s not often you hear about a new fundamental physics result that changes the way we understand something that’s taught in basic physics classes.