A research team probing the properties of a semiconductor combined with a new thin oxide film has observed a surprising new source of conductivity from oxygen atoms trapped inside.
Scott Chambers, materials scientist at the Department of Energy’s Pacific Northwest National Laboratory, reported the team’s discovery at the American Physical Society’s Spring 2022 meeting.
This finding has broad implications for understanding the role of thin oxide films in the future design and fabrication of semiconductors. Specifically, semiconductors used in modern electronics come in two basic flavors – n-type and p-type – depending on the electronic impurity added during crystal growth. Modern electronic devices use both n- and p-type silicon materials. But there is continued interest in the development of other types of semiconductors. Chambers and his team were testing germanium in combination with a specialized thin film crystal of lanthanum-strontium-zirconium-titanium oxide (LSZTO).
“We report a powerful tool for probing the structure and function of semiconductors,” Chambers said. “Hard X-ray photoelectron spectroscopy revealed in this case that oxygen atoms, an impurity of germanium, dominate the properties of the material system when germanium is joined to a particular oxide material. It was a big surprise.
Using the Diamond light source on the Harwell Science and Innovation Campus in Oxfordshire, England, the research team found that they could learn much more about the electronic properties of the germanium/LSZTO system than they could. It was possible to use the usual methods.
“When we tried to probe the material with conventional techniques, the much higher conductivity of germanium essentially caused a short circuit,” Chambers said. “As a result, we were able to learn something about the electronic properties of Ge, which we already know a lot about, but nothing about the properties of the LSZTO film or the interface between the LSZTO film and germanium, which we suspected very interesting and possibly useful for technology.
The so-called “hard” X-rays produced by the Diamond Light Source could penetrate the material and generate information about what was happening at the atomic level.
“Our results were best interpreted in terms of oxygen impurities in germanium being responsible for a very interesting effect,” Chambers said. “Oxygen atoms near the interface donate electrons to the LSZTO film, creating holes, or the absence of electrons, in the germanium in a few atomic layers of the interface. These specialized holes resulted in behavior that totally eclipsed the semiconductor properties of n- and p-type germanium in the various samples we prepared.This too was a big surprise.
The interface, where the thin-film oxide and the base semiconductor come together, is where interesting semiconductor properties often emerge. The challenge, according to Chambers, is to learn how to control the fascinating and potentially useful electric fields that form at these interfaces by altering the electric field at the surface. Ongoing experiments at PNNL are exploring this possibility.
While the samples used in this research likely don’t have the immediate potential for commercial use, the techniques and scientific discoveries made should pay longer-term dividends, Chambers said. New scientific knowledge will help materials scientists and physicists better understand how to design new semiconductor materials systems with useful properties.