The memories are based on tantalum oxide, a common insulator in electronics. 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 by the Rice lab of chemist James Tour could allow for crossbar array memories that store up to 162 gigabits, much higher than other oxide-based memory systems under investigation by scientists. (Eight bits equal one byte; a 162-gigabit unit would store about 20 gigabytes of information.)
Nonvolatile memories hold their data even when the power is off, unlike volatile random-access computer memories that lose their contents when the machine is shut down.
Modern memory chips have many requirements: They have to read and write data at high speed and hold as much as possible. They must also be durable and show good retention of that data while using minimal power.
Tour said Rice’s new design, which requires 100 times less energy than present devices, has the potential to hit all the marks.
“This tantalum memory is based on two-terminal systems, so it’s all set for 3-D memory stacks,” he said. “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.”
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 in other devices. Courtesy of the Tour Group
A schematic shows the layered structure of tantalum oxide, multilayer graphene and platinum used for a new type of memory developed at Rice University. Courtesy of the Tour Group
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.
The researchers determined three related factors give the memories their unique 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 graphene does double duty as a barrier that keeps platinum from migrating into the tantalum oxide and causing a short circuit.
Tour said tantalum oxide memories can be fabricated at room temperature. He noted the control voltage that writes and rewrites the bits is adjustable, which allows a wide range of switching characteristics.
Wang said 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.
Wang is an assistant professor at the Korea University-Korea Institute of Science and Technology’s Graduate School of Converging Science and Technology. Co-authors are former Rice research scientist Jae-Hwang Lee, an assistant professor of mechanical and industrial engineering at the University of Massachusetts, Amherst; and Rice postdoctoral researchers Yang Yang, Gedeng Ruan, Nam Dong Kim and Yongsung Ji.
Oxide-based resistive memory systems have high near-term promise for use in nonvolatile memory. Here we introduce a memory system employing a three-dimensional (3D) networked nanoporous (NP) Ta2O5–x structure and graphene for ultrahigh density storage. The devices exhibit a self-embedded highly nonlinear I–V switching behavior with an extremely low leakage current (on the order of pA) and good endurance. Calculations indicated that this memory architecture could be scaled up to a ∼162 Gbit crossbar array without the need for selectors or diodes normally used in crossbar arrays. In addition, we demonstrate that the voltage point for a minimum current is systematically controlled by the applied set voltage, thereby offering a broad range of switching characteristics. The potential switching mechanism is suggested based upon the transformation from Schottky to Ohmic-like contacts, and vice versa, depending on the movement of oxygen vacancies at the interfaces induced by the voltage polarity, and the formation of oxygen ions in the pores by the electric field.
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