Writing for IEEE Spectrum, Alexander Hellemans notes that researchers have thus far produced two commercially promising types of memristors: electrochemical metallization memory (EMC) cells and valence change mechanism memory (VCM) cells.
“In EMC cells, which have a copper electrode (called the active electrode), the copper atoms are oxidized—stripped of an electron—by the ‘write’ voltage,” he explained. “The resulting copper ions migrate through a solid electrolyte towards a platinum electrode. When they reach the platinum they get an electron back. Other copper ions arrive and pile on, eventually forming a pure metallic filament linking both electrodes, thus lowering of the resistance of the device.”
Meanwhile, both negatively charged oxygen ions and positively charged metal ions result from the “write” voltage in VCM cells. Theoretically, says Hellemans, the oxygen ions are taken out of the solid electrolyte – contributing to a filament consisting of semiconducting material that builds up between the electrodes.
Recently, an international research team led by Ilia Valov at the Peter Grünberg Institute in Jülich, Germany, announced it was investigating memristors with both a tantalum oxide electrolyte and an active tantalum electrode – which would effectively erase many of the differences between EMC and VCM cells.
“We show[ed], using scanning tunnelling microscopy and supported by potentiodynamic current–voltage measurements, that in three typical valence change memory materials (TaOx, HfOx and TiOx) the host metal cations are mobile in films of 2 nm thickness,” Valov explained in a Nature Nanotechnology article abstract. “The cations can form metallic filaments and participate in the resistive switching process, illustrating that there is a bridge between the electrochemical metallization mechanism and the valence change mechanism. Reset/Set operations are, we suggest, driven by oxidation (passivation) and reduction reactions.”
For the Ta/Ta2O5 system, says Valov, a rutile-type TaO2 film is believed to mediate switching, with devices [that] can be switched from a valence change mode to an electrochemical metallization mode by introducing an intermediate layer of amorphous carbon.
“[Essentially], our studies show that these two types of switching mechanisms in fact can be bridged,” Valov told IEEE Spectrum. “We don’t have a purely oxygen type of switching as was believed, but also positive [metal] ions, originating from the active electrode, [which] are mobile.”
As we’ve previously discussed on Rambus Press, ReRAM is ideally suited to improve the power, performance and reliability requirements in military, aerospace and commercial memory applications. Indeed, it fills the gap between what DRAM and Flash can provide – while being both highly reliable and high speed.
It should be noted that Rambus recently confirmed a RRAM partnership with researchers at Tsinghua University in Beijing, China. According to Rambus Labs VP Gary Bronner, RRAM could potentially capture a lucrative position between flash memory and DRAM.
“Many semiconductor companies have expressed a strong interest in ReRAM,” said Bronner. “As such, our collaborative goal with Tsinghua researchers is to explore how to further improve the non-volatile memory so that it is ultimately suitable for consumer devices as well as more demanding platforms and environments.”