Dielectric constant of unusual metal oxide much higher than thought

A team led by researchers at the University of Minnesota Twin Cities has solved a longstanding mystery surrounding strontium titanate, an unusual metal oxide that can be an insulator, a semiconductor or a metal. This research, reported in a paper in the Proceedings of the National Academy of Sciences, provides insight for future applications of strontium titanate in electronic devices and data storage.

When an insulator like strontium titanate is placed between oppositely charged metal plates, the electric field between the plates causes the negatively charged electrons and the positive nuclei in the insulator to line up in the direction of the field. This orderly lining up of electrons and nuclei is resisted by thermal vibrations, and the degree of order is measured by a fundamental quantity called the dielectric constant. At low temperatures, where the thermal vibrations are weak, the dielectric constant is larger.

In semiconductors, the dielectric constant plays an important role by providing effective ‘screening’, or protection, of the conducting electrons from other charged defects in the material. For applications in electronic devices, it’s critical for semiconductors to have a large dielectric constant.

High quality centimeter-size samples of strontium titanate exhibit a measured low-temperature dielectric constant of 22,000, which is quite large and useful for practical applications. But most applications in computers and other devices call for thin films. Despite an enormous effort by many researchers to grow thin films using diverse methods, only a modest dielectric constant of 100–1000 has so far been achieved in thin films of strontium titanate.

In these thin films, which can be just a few atomic layers thick, the interface between the film and the substrate – or the film and the next layer up – can play an important role.

Bharat Jalan, professor in the University of Minnesota’s Department of Chemical Engineering and Materials Science and senior author of the paper, theorized that these ‘buried’ interfaces might be masking the true dielectric constant of strontium titanate. By carefully accounting for this masking effect, Jalan and his students discovered that the true dielectric constant of their strontium titanate films exceeds 25,000 – the highest ever measured for this material.

The findings by Jalan and his students and collaborators provide critical insight into the role of the interface between an insulator and a metal, even when both the metal and the insulator are derived from the same material. Such interfaces are found in the capacitor structures that are ubiquitous in modern technology.

"Semiconductors are among the most important materials used in modern technology,” Jalan said. “While much is known about conventional semiconductors such as silicon and gallium arsenide, there are several unsolved mysteries surrounding oxide semiconductors like strontium titanate.”

With this work, the researchers resolved a longstanding issue concerning the low dielectric constants of strontium titanate films.

“These results build on a remarkable record of success for the method of film growth known as hybrid Molecular Beam Epitaxy, discovered by Jalan,” said Richard James, a professor in the Department of Aerospace Engineering and Mechanics, and a co-author of the paper. “The quality of the films from Jalan’s group is truly exceptional.”

The student leading the growth effort was Zhifei Yang, a graduate student in the School of Physics and Astronomy at the University of Minnesota, who was supervised by Jalan. “It was quite rewarding to see that an interface only a few atomic layers thick can have a tremendous impact on the measured value,” Yang said.

This story is adapted from material from the University of Minnesota, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.