Time is fast running out for the semiconductor industry as transistors become ever smaller and their insulating layers of silicon dioxide, already only atoms in thickness, reach maximum shrinkage. In addition, the thinner the silicon layer becomes, the greater the amount of chemical dopants that must be used to maintain electrical contact. And the limit here also is close to being reached.
But a Cornell University researcher has caused an information industry buzz with the discovery that it is possible to precisely control the electronic properties of a complex oxide material -- a possible replacement for silicon insulators -- at the atomic level. And this can be done without chemicals. Instead, the dopant is precisely nothing.
In a paper in a recent issue of Nature (Aug. 5, 2004), David Muller, associate professor of applied and engineering physics at Cornell, and his collaborator, Harold Hwang of the University of Tokyo, report that by removing oxygen atoms from layers in thin films of the oxide strontium titanate, they can precisely control the conducting ability of the material by creating empty spaces, or vacancies, that act as electron-donating dopants. And they have used a scanning transmission electron microscope (STEM) to tell exactly where the missing atoms are in the material.
Across the semiconductor industry, such complex oxides are being sought as a replacement for silicon. The roadblock is that all the oxides tested easily lose a few oxygen atoms, making them leaky and defective when exposed to electric fields, typically stronger than those inside a lightning bolt.
"The important parts of the work are actually being able to see vacancies buried inside the material," says Muller. "From a materials analysis point of view, that's very important. The reason is that missing atoms can change the properties of a material very dramatically." He adds, "We have been able to show that we can stop on a dime in controlling where you put these vacancies."
In an accompanying commentary to the Nature article called "The value of seeing nothing," Jochen Mannhart of the University of Augsburg, Germany, and Darrell G. Schlom of Pennsylvania State University, observe that the research by Muller and his colleagues "greatly broadens the options available for manipulating the electronic properties of oxides" at the nanometer scale. A nanometer is the width of three silicon atoms.