Power/Performance Bits: Aug. 18

Reducing crosstalk with tantalum oxide memories

Scientists at Rice University created a solid-state memory technology that allows for high-density storage with a minimum incidence of crosstalk errors.

The memories are based on tantalum oxide. Applying voltage to a 250-nanometer-thick sandwich of graphene, tantalum, nanoporous tantalum oxide and platinum creates addressable bits where the layers meet. Control voltages that shift oxygen ions and vacancies switch the bits between ones and zeroes.

The discovery could allow for crossbar array memories that store up to 162 gigabits, much higher than other oxide-based memory systems under investigation by scientists.

“This tantalum memory is based on two-terminal systems, so it’s all set for 3-D memory stacks,” said James Tour, a chemist at Rice. “And it doesn’t even need diodes or selectors, making it one of the easiest ultradense memories to construct. This will be a real competitor for the growing memory demands in high-definition video storage and server arrays.”

The layered structure consists of tantalum, nanoporous tantalum oxide and multilayer graphene between two platinum electrodes. In making the material, the researchers found the tantalum oxide gradually loses oxygen ions, changing from an oxygen-rich, nanoporous semiconductor at the top to oxygen-poor at the bottom. Where the oxygen disappears completely, it becomes pure tantalum, a metal.

A layered structure of tantalum oxide, multilayer graphene and platinum is the basis for a new type of memory developed at Rice University. The memory device seen in this electron microscope image overcomes crosstalk problems that cause read errors. (Source: Tour Group/Rice University)

The researchers determined three related factors give the memories their switching ability.

First, the control voltage mediates how electrons pass through a boundary that can flip from an ohmic (current flows in both directions) to a Schottky (current flows one way) contact and back.

Second, the boundary’s location can change based on oxygen vacancies. These are “holes” in atomic arrays where oxygen ions should exist, but don’t. The voltage-controlled movement of oxygen vacancies shifts the boundary from the tantalum/tantalum oxide interface to the tantalum oxide/graphene interface. “The exchange of contact barriers causes the bipolar switching,” said Gunuk Wang, lead author of the study and a former postdoctoral researcher at Rice.

Third, the flow of current draws oxygen ions from the tantalum oxide nanopores and stabilizes them. These negatively charged ions produce an electric field that effectively serves as a diode to hinder error-causing crosstalk. While researchers already knew the potential value of tantalum oxide for memories, such arrays have been limited to about a kilobyte because denser memories suffer from crosstalk that allows bits to be misread.

The remaining hurdles to commercialization include the fabrication of a dense enough crossbar device to address individual bits and a way to control the size of the nanopores.

Giving batteries a stretchy shell

Researchers at the U.S. Department of Energy’s Argonne National Laboratory are developing lithium-ion batteries containing silicon-based materials. The most commonly used commercial lithium-ion batteries are graphite-based, but scientists are becoming increasingly interested in silicon because it can store roughly 10 times more lithium than graphite.

“When we talk about batteries, we talk in terms of the amount of energy that can be stored,” said Daniel Abraham, materials scientist in Argonne’s Chemical Sciences and Engineering Division. “Silicon-based batteries could double or even triple the energy stored in conventional batteries, which would greatly benefit the consumer electronics market and the automotive industry.”

However current batteries based on silicon materials don’t last long. The problem lies in the battery’s chemistry. The electrolyte inside the battery transports lithium ions back and forth between positive and negative electrodes as the battery charges and discharges. The positive electrode contains a lithium-bearing compound, while the negative electrode contains materials such as graphite or silicon.

Lithium ions react with the negative electrode to form a new compound, causing the electrode to expand, while the electrolyte produces a protective coating called the solid electrolyte interphase. But the coating also needs to expand and contract with the electrode, or else it will crack and the battery won’t work.

Ilya Shkrob, a chemist in the Chemical Sciences and Engineering Division, compared this phenomenon to an imaginary experiment – pumping air into an egg. The egg starts to expand and the shell cracks. The shell cracks again when the air is released and the egg returns to its original size.

Lithium ions react with silicon to form a new compound, which causes the electrode to expand. Researchers found that flouroethylene carbonate molecules produce a rubber-like protective layer that can accommodate the electrode expansion. (Source: Sana Sandler/Sarah Schlieder)

“When the protective layer cracks, the electrode surface reacts and consumes the electrolyte,” Shkrob said. “If the electrolyte is completely consumed, then the battery won’t work.”

In today’s graphite-based lithium-ion batteries, the electrode expands about 10 percent – a small enough change that cracks in the coating aren’t an issue.

But the electrode in a silicon-based lithium-ion battery expands up to 300 percent. These batteries need a different electrolyte in order to produce an elastic shell.

The researchers found that when fluorine is added to ethylene carbonate, the resulting electrolyte forms a coating that can stretch and accommodate the volume changes in the electrode.

However, Abraham noted that the team has not completely solved the electrolyte problem. “There may be other compounds that produce a rubber-like coating more efficiently than the fluoroethylene carbonate molecules,” he added.

“It’s the idea that’s important – an egg shell versus rubber,” Abraham said. “Solving the electrolyte problem is an important step towards commercializing silicon-based lithium-ion batteries, which will vastly increase the amounts of energy we can store.”