Layered Film That Stacks Up
Using pulsed laser deposition, ORNL researchers have grown perfect ferroelectric superlattices with surprising properties.
In 2004, Ho Nyung Lee surprised his fellow researchers in the Thin Film and Nanostructured Materials Physics Group in ORNL's Condensed Matter Sciences Division. "Ho Nyung Lee used pulsed laser deposition to fabricate perovskite nanostructures with a degree of perfection we had not seen before," says Hans Christen. Unlike MBE, pulsed laser deposition forms these oxides without the need for separate heat treatment in oxygen after the film is grown.
So he wouldn't be shooting in the dark, Lee used reflection high-energy electron diffraction during film growth. "With the diffraction as his flashlight, he realized the importance of slowing down the process to precisely control the amount of material deposited for each crystalline unit of the film," Christen says. "Now, he can stack atomically smooth layers of different composition to create a perfect ferroelectric superlattice, with hundreds of individually controlled layers that together are about 200 nanometers thick."
Ferroelectric materials store electronic charge at their surfaces because of the asymmetric displacements of ions within their crystalline structure. The displaced ions give each layer positive and negative sides.
Lee, working with Christen, Matt Chisholm, Chris Rouleau, and Doug Lowndes, synthesized and characterized "asymmetric three-component ferroelectric superlattices" with a "strong polarization enhancement" that was the subject of a letter published in the January 27, 2005, issue of Nature magazine.
The superlattices described by the ORNL researchers in the Nature letter consist of dozens of repetitions of barium titanate (BaTiO3), strontium titanate (SrTiO3), and calcium titanate (CaTiO3), stacked with atomic precision on top of conducting, perfectly flat strontium ruthenate (SrRuO3) layers. Each repetition is 3 to 10 nm thick.
Barium titanate, accounting for only a fraction of the total material in the superlattice, is the only ferroelectric compound among the constituents. Yet, partly because "strain" is maintained—that is, the crystalline film is forced to grow in alignment
with the SrTiO3 substrate—the ORNL superlattice has 50% greater polarization than similarly grown pure BaTiO3.
"The purposes of this study were to prove we could make a perfect ferroelectric film using pulsed laser deposition, to determine the mechanisms on an atomic scale that influence the material's properties, and to learn how to control and modify the material's properties," Christen says. "Such information could allow us to engineer future ferroelectric films for specific applications and would ultimately help us understand atomic-scale mechanisms in other oxide nanostructures."
The critical issue is the material's behavior directly at each interface between layers. "Our data revealed that the specific interface structure and local asymmetries played an unexpected role in the polarization enhancement," Christen says.
Researchers are now ready to go beyond tailoring ionic displacements at interfaces to changing the electronic configuration in materials such as lanthanum manganite. Aided by excellent tools and strong collaborations, the big picture is coming into focus, helping ORNL researchers improve the behavior of their thin films.
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