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Materials

Doping at the Nanoscale

Oxygen vacancies are nanoengineered and imaged with atomic precision

by Bethany Halford
August 9, 2004 | A version of this story appeared in Volume 82, Issue 32

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Credit: COURTESY OF DAVID A. MULLER
LATTICEWORK This image shows the abrupt junction between layers of undoped (bottom) and doped (top) SrTiO3. In the latter, clusters of oxygen vacancies look like bright orange blobs.
Credit: COURTESY OF DAVID A. MULLER
LATTICEWORK This image shows the abrupt junction between layers of undoped (bottom) and doped (top) SrTiO3. In the latter, clusters of oxygen vacancies look like bright orange blobs.

By nanoengineering the deposition of strontium titanate (SrTiO3)--a material that can be nudged from electrically insulating to conducting with extremely small amounts of dopants--and then imaging the material with a new supersensitive technique, scientists have shown that it is possible to control the electronic properties of transition-metal oxides with atomic precision [Nature, 430, 657 (2004)].

The dual developments--masterminded by David A. Muller, an applied and engineering physics associate professor at Cornell University; Harold Y. Hwang, a materials science professor at the University of Tokyo; and coworkers--have applications in both semiconductor electronics and the information industry.

VACANCY BLUES
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Credit: COURTESY OF JOCHEN MANNHART
Ripping a few oxygen atoms out of SrTiO3's crystalline lattice transforms the diamond-like insulator into a deep blue conductive crystal.
Credit: COURTESY OF JOCHEN MANNHART
Ripping a few oxygen atoms out of SrTiO3's crystalline lattice transforms the diamond-like insulator into a deep blue conductive crystal.

In a commentary accompanying the report, Jochen Mannhart, a physics professor at Germany's University of Augsburg, and Darrell G. Schlom, a materials science and engineering professor at Pennsylvania State University, describe the work as "an unexpected double breakthrough" that "greatly broadens the options available for manipulating the electronic properties of oxides, and probably ionic materials of all sorts, at the nanometer scale."

To create their nanoengineered thin films of SrTiO3, the researchers take a "less is more" approach to doping the material. Rather than adding chemical impurities to dope the oxide, Muller and Hwang's team strip a few of the oxygen atoms out of the crystalline lattice. Titanium becomes reduced wherever oxygen atoms are removed, so that in the bo ttom line of electron accounting, two electrons are added back into the SrTiO3 matrix. In essence, these oxygen vacancies function as electron-donating dopants.

The thin films are grown in Hwang's lab, where he and his students use a laser to vaporize SrTiO3, which then condenses in crystalline layers on a substrate. By adjusting the oxygen pressure in the reaction chamber, the researchers can make either doped or undoped SrTiO3. The technique is so precise that the alternating layers of doped and undoped SrTiO3 can be made so that each layer is only three unit cells thick. Muller notes that the growth kinetics of the process prevent the oxygen atoms in the undoped layers from diffusing into the vacancies of the doped layers.

To visualize the oxygen vacancies in the doped material, Muller developed a scanning transmission electron microscope technique that images the crystalline lattice with unrivaled precision. When there's an oxygen vacancy in the crystalline lattice, he explains, the remaining atoms will lean this way and that, adjusting to take up some of the extra space. As they do, the atoms move out of their crystalline alignment and put strain on the lattice. Muller says it's this strain that he detects and images. Although other groups have imaged areas with a large number of atomic vacancies, the new technique is sensitive enough to reveal individual vacancies.

Muller tells C&EN that the research was born out of a 25-cent bet that he and Hwang made several years ago when they shared a lab at Bell Laboratories in Murray Hill, N.J. Hwang bet Muller that certain types of oxide devices could be controlled as well as conventional semiconductors. Although these results don't resolve the bet, Muller says they do seem to indicate that Hwang will be 25 cents richer in the future.

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