Resisitive RAM (ReRAM), sometimes called a memristor, is now being researched extensively as a potential replacement for flash memory which is not expected not to scale in plan much below 20-nm minimum dimensions. Some further memory capacity scaling may be achieved by stacking flash memory cells vertically but it is thought that resistive memory could displace flash memory if it can offer planar as well as z-directions scaling. Silicon oxide is now being researched as a potential alternative to metal-oxide types of ReRAM.

The UCL team has developed a silicon oxide memory, described in a recent paper in the Journal of Applied Physics, which they claim performs the switch in resistance much more efficiently than has been previously achieved.

The paper, entitled Resistive switching in silicon suboxide films, reports that the UCL team have worked with silcon-rich silicon dioxide. This differs from the work of Professor I-Wei Chen at the University of Pennsylvania who has reported the use of inclusions of atomically dispersed platinum in silicon oxide material. The UCL work appears to be closer to that of a team under James Tour at Rice University (see Making memory out of silicon oxide).

"The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. In contrast to other work in the literature, switching occurs in ambient conditions, and is not limited to the surface of the active material," the researchers state in the abstract to their paper.

They go on to propose a switch that is operated by field-driven formation and current-driven destruction of filamentary conductive pathways and report on/off resistence ratios of 10^4:1 and higher. The conductive pathways are 10-nanometers in diameter or smaller and programming currents can be as low as 2-microamps, with transition times on the order of nanoseconds.

"Our ReRAM memory chips need just a thousandth of the energy and are around a hundred times faster than standard flash memory chips," said Tony Kenyon, one of the researchers at UCL's department of electronic and electrical engineering, in statement. The UCL devices also display a continuously variable resistance that depends on the last voltage that was applied. This is an important property that allows the device to mimic how neurons in the brain function.



UCL ReRAM test circuits on broken wafer. Source: UCL/Adnan Mehonic.


The behavior was discovered by accident as researchers at UCL were working on silicon-based light emitting diodes but noticed that devices appeared to be unstable. UCL PhD student, Adnan Mehonic, was asked to look specifically at the material's electrical properties and he discovered that the material was flipping between conducting and non-conducting states.

The technology has potential application beyond memory storage. The team is also exploring the use of silcon-rich silica as a logic switch for use in processors.

UCL team has been backed by UCLB, the technology transfer company of UCL, and has recently filed a patent. UCL said UCLB is in discussions with a number of semiconductor companies.


Abstract to UCL's Journal of Applied Physics paper