Researchers squeeze atomic arrangements to create superior memory material
The key was in finding the perfect mismatch. Materials Science and Engineering Professor Chang-Beom Eom and his team grew a material of slightly larger atomic structure upon a slightly smaller substrate. Eom engineered strain into barium titanate (BaTiO3 ) thin films giving the material ferroelectric properties that could lead to cleaner, smaller, faster and more efficient memory and electro-optic devices. The team's work is featured in the November 5 issue of the journal Science.
The ability of ferroelectric material to store information resides in its arrangement of atoms, with each structure holding a bit of information. This information changes every time the material receives a pulse of electricity, basically switching the arrangement of atoms. These materials have great appeal to the semiconductor industry because, as memory devices, they can be rewritten more than 1,000 trillion times. Flash memory, such as that used in digital cameras and cell phones, can be rewritten about 100,000 times.
Ferroelectric memory is already in use in thin "contactless" tickets containing tiny wireless transmitters. Commuters wave these pieces of plastic at a card reader before boarding trains or entering buildings. Ultimately, ferroelectric materials might be used to create computers that don't need to "boot up" when restarted because unlike the memory technology used in desktop computers, ferroelectrics have built-in electronic memory does not disappear when the power is shut off.
Eom's new strain-engineered BaTiO3 is significant because unlike other widely pursued ferroelectric candidates, BaTiO3 contains no lead or bismuth that would complicate its introduction into semiconductor fabrication facilities or pose environmental toxicity issues. But even more impressive are the dramatic improvements in transient temperature and remanent polarization.
Transient temperature is the temperature at which a material loses its ferroelectric properties. "Once it disappears you lose all of the information stored in ferroelectric memory," says Eom. "That is about 130 degrees Celsius for unstrained barium titanate. Most devices have a thermal budget for memory of about 85 degrees Celsius, which is too close to the transient temperature. So the device can lose 100 percent of its ferroelectric memory if you go above that temperature. With strain we increased the transient temperature to 650 degrees Celsius. That is a more than 500 degree jump."
Remanent polarization is a measure of how much polarization remains after an applied electric field is removed. To make ever smaller electronic devices requires storing more information in smaller places. Improvements in remanent polarization do just that. Eom's strained barium titanate has remanent polarization at least 250 percent higher than unstrained material.
The team stressed BaTiO3 thin films by growing it upon a material with a similar single-crystal substrate but with a slightly smaller distance between atoms. Slightly, in this case, means 0.06 angstroms or about 1.7 percent smaller. Growing the crystals under these conditions is the key. Eom says it is essentially the same concept one would find in squeezing rubber. The squeezing, however, must be done through the controlled growth of crystals. Mechanically squeezing barium titanate laterally would break the crystals.
"What happens is the substrate tries to squeeze the top layer to adjust the registered growth or so-called coherent epitaxy," says Eom. "So it is squeezed laterally to make the atomic distance of the ferroelectric material the same as the underlying substrate. At the same time, vertically there is no constraint and so the material expands in that direction. We call this biaxial strain."
Until recently, researchers struggled to find a substrate that could strike the proper balance, but with the team's single crystal substrate, Eom not only manages stress, his team engineers it.