A schematic shows the layered structure of tantalum oxide, multilayer graphene and platinum used for a new type of memory developed at Rice University. The memory device overcomes crosstalk problems that cause read errors in other devices. Credit: Tour Group/Rice University
The memories are based on tantalum oxide, a common insulator in electronics. Applying voltage to a 250-nm-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 Rice lab chemist James Tour could allow for crossbar array memories that store up to 162 gigabits, much higher than other oxide-based memory systems. Like the Tour lab’s previous discovery of silicon oxide memories, the new devices require only 2 electrodes/ circuit, making them simpler than present-day flash memories that use 3. “But this is a new way to make ultradense, nonvolatile computer memory,” Tour said.
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. Rice’s new design, which requires 100X less energy than present devices, has the potential to hit all the marks.
The layered structure consists of tantalum, nanoporous tantalum oxide and multilayer graphene between 2 platinum electrodes.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.
>> 3 related factors give the memories their unique switching ability.
1. 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 1 way) contact and back.
2. The boundary’s location can change based on oxygen vacancies ie “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”
3. 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.
The graphene does double duty as a barrier that keeps platinum from migrating into the tantalum oxide and causing a short circuit.
Tantalum oxide memories can be fabricated at room temperature and 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.
http://news.rice.edu/2015/08/10/tantalizing-discovery-may-boost-memory-technology/
2 movies show the partially interconnected and randomly distributed internal pores in new memory devices created at Rice University.
https://youtu.be/rwCJQPOluF8Â
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