The systems that make up the electronics we use every day seem unreal. With the touch of a finger, we have access to a whole world of information, and most people hardly ever think about the materials that make this possible.
Through innovative research, Chang-Beom Eom, UW-Madison professor of materials science and engineering, and his team have been able to show evidence of a complementary set of two-dimensional electronic systems only tens of atoms thick that make a whole new type of oxide material electronics possible.
All different types of electronics need conductors to work — these conductors use both positive and negative charge carriers that are completely complementary, each set moving like particles in gas. Their characteristics are opposite, but both are necessary. Semiconductors use this type of system and are extremely useful because they are easily controlled. This oxide materials system also uses the complementary charges expressed through a 2D electron gas (the negative counterpart) and a 2D hole gas (the positive counterpart) that together have extremely unique properties.
A little over a decade ago, the electron gas was demonstrated through the perfect matching of extremely thin layers of atoms, allowing for a large amount of very conductive electrons to form. These electrons have superconductivity and magnetism that are not seen in the electrons of a semiconductor.
Eom explained that “the 2D electron gas was discovered, but the 2D hole gas has not been demonstrated.”
Based on the discovery of the electron gas, researchers predicted that the 2D hole gas could be shown by depositing layers of atoms with specific arrangements. For over a decade, researchers have been attempting to demonstrate this phenomenon, but no one has really been able to, until now.
After working with the 2D electron gas for years and using, “interesting devices and demonstrations,” Eom and his team became curious about the absence of the 2D hole gas.
One of their hypotheses was that the 2D hole gas actually does form somewhere but is killed by a tiny defect that exists in the atomic layering and sequencing of the system. They proposed the idea that this defect was a lack of oxygen.
After performing some theoretical calculations, the team determined that with oxygen vacancies, the 2D hole gas cannot exist, validating their hypothesis. According to Eom, a “very small amount of oxygen vacancies” kill the 2D hole gas.
So, Eom and his team began to take a unique approach: minimize the oxygen vacancies in the p-type interface (the positive counterpart’s interface, the 2D hole gas’s interface) and create an “atomically sharp” p-type interface with specific layering.
Instead of depositing and spraying one monolayer of atoms, as other researchers have attempted before, Eom and his team team stacked materials using an inverted structure. Basically, the top sequence of layers forms an atomic sequence where the hole gas can form and the bottom sequence of layers forms an atomic arrangement where the electron gas can form.
Obtaining the correct atomic stacking was just one aspect, though. The defects also had to be removed. By incorporating oxygen at a very high oxygen partial pressure during the growth of the layers, Eom was able to minimize the oxygen vacancies. Then, through a complicated method of fabrication and electrical-transport measurement in magnetic fields, the team measured both the top interface and bottom interface to find that one had a positive slope, and the other had a negative slope. This demonstrated the opposite charges that they hoped they could see. “That is a direct demonstration of evidence of electron gas and hole gas," said Eom.
Now, they truly believed their hypothesis to be correct. After filling the oxygen vacancies and using a unique stacking method, Eom and his team were able to visualize these layers at an atomic scale through a holography technique that the hole gas was being expressed in the top interface and the electron gas was being expressed in the top interface.
This realization of the 2D hole gas along with the 2D electron gas means a great deal for future generations of electronics. With the complementary charges and characteristics of the coexisting 2D hole gas and the 2D electron gas, electronics with oxide materials are possible. This powerful duo can work perfectly to achieve a level of superconductivity and magnetism that no semiconductors have been able to achieve before.
This discovery not only makes new devices possible, but also new physical phenomena. This is not just about speeding up computers or cell phones — this also about the basic scientific understanding of the expression of these gases, and the envisioning and realization of devices with entirely new functionalities.
Eom’s collaborators include UW-Madison physics professor Mark Rzchowski and scientists from Ohio State University, the University of Nebraska at Lincoln, Argonne National Laboratory, Sungkyunkwan University and Pohang University of Science and Technology in Korea.