Ferroelectric material feels the squeeze

Low-power, high-efficiency electronic memory could be the long-term result of collaborative research led by Cornell materials scientist Professor Darrell Schlom.

The research, published today (17 April) in the journal Science, involves taking a well-known oxide, strontium titanate, and depositing it on silicon in such a way that the silicon squeezes it into a ferroelectric state and presenting the possibility of transistors that can hold their state when power is removed.

Although ferroelectric memory cells are used in some devices, for more than half a century, scientists have wanted to use ferroelectric materials in transistors, which could lead to instant-on computing – no more rebooting the operating system or accessing memory slowly from the hard drive. No one has yet achieved a ferroelectric transistor that works.

“Adding new functionality to transistors can lead to improved computing and devices that are lower power, higher speed and more convenient to use,” said Schlom, professor of materials science and engineering. “Several hybrid transistors have been proposed specifically with ferroelectrics in mind. By creating a ferroelectric directly on silicon, we are bringing this possibility closer to realisation.”

Ordinarily, strontium titanate in its relaxed state is not ferroelectric at any temperature. The researchers have demonstrated, however, that extremely thin films of the oxide – just a few atoms thick – become ferroelectric when squeezed atom by atom to match the spacing between the atoms of underlying silicon.

“Changing the spacing between atoms by about 1.7 per cent drastically alters the properties of strontium titanate and turns it into a material with useful memory properties,” said Long-Qing Chen, professor of materials science and engineering at Pennsylvania State University, a member of the research team whose calculations predicted the observed behaviour five years ago.

Schlom added: “From various predictions, some dating back nearly a decade, we knew exactly what we were after, but it took our team years to achieve and demonstrate the predicted effect.”

The researchers described successfully growing the strontium titanate on top of silicon – the semiconductor found in virtually all electronic devices – using molecular-beam epitaxy, a technique akin to atomic spray painting.

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