Strontium titanate (STO) is a wide-gap insulator with a perovskite structure that is routinely used as a substrate for growing high-temperature-superconducting cuprates, colossal magnetoresistive manganites and multiferroics, among other materials. More elaborate structures, such as titanium oxide (TiO2)-terminated STO substrates are used to fabricate complex oxide heterostructures with atomic-scale flat interfaces through controlled film growth methods, such as pulsed laser deposition.

Researchers have also recently discovered a number of interesting properties, such as quasi-two-dimensional electron gas behaviour, magnetism, resistance switching, the giant thermoelectric effect and colossal ionic conductivity in various oxide heterostructures based on STO substrates. Such characteristics mean that the materials might be used to make oxide electronics, thermoelectrics and solid oxide fuel cells.

The interface structures made by Yunzhong Chen of the Technical University of Denmark and colleagues consist of TiO2-terminated STO substrates and various insulating top films. The researchers deposited the films by pulsed laser deposition techniques at room temperature. By performing electron transport measurements and X-ray photoelectron spectroscopy on the samples, they found that when the deposited films are made up of lanthanum aluminate (LAO), STO or yttria-stabilized zirconia (YSZ), a metallic, conducting, interface is produced. However, when the overlayer is La7/8Sr1/8MnO3 (LSMO), the interface remains insulating.

Oxygen vacancies
According to the team, which includes scientists from the University of Twente and the Beijing National Laboratory for Condensed Matter Physics, oxygen vacancies near the interface are responsible for the conductivity in the LAO/STO, STO/STO, YSZ/STO samples. Chen says that redox reactions on the surface of the STO crystal might be playing an important role here. "The plasma species involved in growing LAO, STO and YSZ films may show higher oxygen affinity with respect to the TiO2-terminated STO surface," he told nanotechweb.org. "And, when the two systems meet face to face, reduction on the STO substrate side occurs."

The result could be important for understanding the origin of the metallic interface, which is essentially a quasi-2D electron gas between two insulating oxides (one of which is STO). "That our new type of heterostructure has a conducting interface is unexpected since it includes a non-crystalline overlayer," explained Chen. "It is exciting that this interface has similar characteristics to those of its crystalline counterpart, notably similar carrier density, electron mobility and critical film thickness."

The team is now busy looking at its amorphous STO-based oxide heterostructures in more detail. "These studies should help us find out whether the conducting interface is also superconducting or magnetic – as observed in the crystalline LAO/STO heterointerface," said Chen.

The present work was detailed in Nano Letters.