Tag Archives: I. Valov

New design directions to increase variety, efficiency, selectivity and reliability for memristive devices

A May 11, 2020 news item on ScienceDaily provides a description of the current ‘memristor scene’ along with an announcement about a piece of recent research,

Scientists around the world are intensively working on memristive devices, which are capable in extremely low power operation and behave similarly to neurons in the brain. Researchers from the Jülich Aachen Research Alliance (JARA) and the German technology group Heraeus have now discovered how to systematically control the functional behaviour of these elements. The smallest differences in material composition are found crucial: differences so small that until now experts had failed to notice them. The researchers’ design directions could help to increase variety, efficiency, selectivity and reliability for memristive technology-based applications, for example for energy-efficient, non-volatile storage devices or neuro-inspired computers.

Memristors are considered a highly promising alternative to conventional nanoelectronic elements in computer Chips [sic]. Because of the advantageous functionalities, their development is being eagerly pursued by many companies and research institutions around the world. The Japanese corporation NEC installed already the first prototypes in space satellites back in 2017. Many other leading companies such as Hewlett Packard, Intel, IBM, and Samsung are working to bring innovative types of computer and storage devices based on memristive elements to market.

Fundamentally, memristors are simply “resistors with memory,” in which high resistance can be switched to low resistance and back again. This means in principle that the devices are adaptive, similar to a synapse in a biological nervous system. “Memristive elements are considered ideal candidates for neuro-inspired computers modelled on the brain, which are attracting a great deal of interest in connection with deep learning and artificial intelligence,” says Dr. Ilia Valov of the Peter Grünberg Institute (PGI-7) at Forschungszentrum Jülich.

In the latest issue of the open access journal Science Advances, he and his team describe how the switching and neuromorphic behaviour of memristive elements can be selectively controlled. According to their findings, the crucial factor is the purity of the switching oxide layer. “Depending on whether you use a material that is 99.999999 % pure, and whether you introduce one foreign atom into ten million atoms of pure material or into one hundred atoms, the properties of the memristive elements vary substantially” says Valov.

A May 11, 2020 Forschungszentrum Juelich press release (also on EurekAlert), which originated the news item, delves into the theme of increasing control over memristive systems,

This effect had so far been overlooked by experts. It can be used very specifically for designing memristive systems, in a similar way to doping semiconductors in information technology. “The introduction of foreign atoms allows us to control the solubility and transport properties of the thin oxide layers,” explains Dr. Christian Neumann of the technology group Heraeus. He has been contributing his materials expertise to the project ever since the initial idea was conceived in 2015.

“In recent years there has been remarkable progress in the development and use of memristive devices, however that progress has often been achieved on a purely empirical basis,” according to Valov. Using the insights that his team has gained, manufacturers could now methodically develop memristive elements selecting the functions they need. The higher the doping concentration, the slower the resistance of the elements changes as the number of incoming voltage pulses increases and decreases, and the more stable the resistance remains. “This means that we have found a way for designing types of artificial synapses with differing excitability,” explains Valov.

Design specification for artificial synapses

The brain’s ability to learn and retain information can largely be attributed to the fact that the connections between neurons are strengthened when they are frequently used. Memristive devices, of which there are different types such as electrochemical metallization cells (ECMs) or valence change memory cells (VCMs), behave similarly. When these components are used, the conductivity increases as the number of incoming voltage pulses increases. The changes can also be reversed by applying voltage pulses of the opposite polarity.

The JARA researchers conducted their systematic experiments on ECMs, which consist of a copper electrode, a platinum electrode, and a layer of silicon dioxide between them. Thanks to the cooperation with Heraeus researchers, the JARA scientists had access to different types of silicon dioxide: one with a purity of 99.999999 % – also called 8N silicon dioxide – and others containing 100 to 10,000 ppm (parts per million) of foreign atoms. The precisely doped glass used in their experiments was specially developed and manufactured by quartz glass specialist Heraeus Conamic, which also holds the patent for the procedure. Copper and protons acted as mobile doping agents, while aluminium and gallium were used as non-volatile doping.

Synapses, the connections between neurons, have the ability to transmit signals with varying degrees of strength when they are excited by a quick succession of electrical impulses. One effect of this repeated activity is to increase the concentration of calcium ions, with the result that more neurotransmitters are emitted. Depending on the activity, other effects cause long-term structural changes, which impact the strength of the transmission for several hours, or potentially even for the rest of the person’s life. Memristive elements allow the strength of the electrical transmission to be changed in a similar way to synaptic connections, by applying a voltage. In electrochemical metallization cells (ECMs), a metallic filament develops between the two metal electrodes, thus increasing conductivity. Applying voltage pulses with reversed polarity causes the filament to shrink again until the cell reaches its initial high resistance state. Copyright: Forschungszentrum Jülich / Tobias Schlößer

Record switching time confirms theory

Based on their series of experiments, the researchers were able to show that the ECMs’ switching times change as the amount of doping atoms changes. If the switching layer is made of 8N silicon dioxide, the memristive component switches in only 1.4 nanoseconds. To date, the fastest value ever measured for ECMs had been around 10 nanoseconds. By doping the oxide layer of the components with up to 10,000 ppm of foreign atoms, the switching time was prolonged into the range of milliseconds. “We can also theoretically explain our results. This is helping us to understand the physico-chemical processes on the nanoscale and apply this knowledge in the practice” says Valov. Based on generally applicable theoretical considerations, supported by experimental results, some also documented in the literature, he is convinced that the doping/impurity effect occurs and can be employed in all types memristive elements.

Top: In memristive elements (ECMs) with an undoped, high-purity switching layer of silicon oxide (SiO2), copper ions can move very fast. A filament of copper atoms forms correspondingly fast on the platinum electrode. This increases the total device conductivity respectively the capacity. Due to the high mobility of the ions, however, this filament is unstable at low forming voltages. Center: Gallium ions (Ga3+), which are introduced into the cell (non-volatile doping), bind copper ions (Cu2+) in the switching layer. The movement of the ions slows down, leading to lower switching times, but the filament, once formed remains longer stable. Bottom: Doping with aluminium ions (Al3+) slows down the process even more, since aluminium ions bind copper ions even stronger than gallium ions. Filament growth is even slower, while at the same time the stability of the filament is further increased. Depending on the chemical properties of the introduced doping elements, memristive cells – the artificial synapses – can be created with tailor-made switching and neuromorphic properties. Copyright: Forschungszentrum Jülich / Tobias Schloesser

Here’s a link to and a citation for the paper,

Design of defect-chemical properties and device performance in memristive systems by M. Lübben, F. Cüppers, J. Mohr, M. von Witzleben, U. Breuer, R. Waser, C. Neumann, and I. Valov. Science Advances 08 May 2020: Vol. 6, no. 19, eaaz9079 DOI: 10.1126/sciadv.aaz9079

This paper is open access.

For anyone curious about the German technology group, Heraeus, there’s a fascinating history in its Wikipedia entry. The technology company was formally founded in 1851 but it can be traced back to the 17th century and the founding family’s apothecary.

Extending memristive theory

This is kind of fascinating. A German research team based at JARA (Jülich Aachen Research Alliance) is suggesting that memristive theory be extended beyond passive components in their paper about Resistive Memory Cells (ReRAM) which was recently published in Nature Communications. From the Apr. 26, 2013 news item on Azonano,

Resistive memory cells (ReRAM) are regarded as a promising solution for future generations of computer memories. They will dramatically reduce the energy consumption of modern IT systems while significantly increasing their performance.

Unlike the building blocks of conventional hard disk drives and memories, these novel memory cells are not purely passive components but must be regarded as tiny batteries. This has been demonstrated by researchers of Jülich Aachen Research Alliance (JARA), whose findings have now been published in the prestigious journal Nature Communications. The new finding radically revises the current theory and opens up possibilities for further applications. The research group has already filed a patent application for their first idea on how to improve data readout with the aid of battery voltage.

The Apr. 23, 2013 JARA news release, which originated the news item, provides some background information about data memory before going on to discuss the ReRAMs,

Conventional data memory works on the basis of electrons that are moved around and stored. However, even by atomic standards, electrons are extremely small. It is very difficult to control them, for example by means of relatively thick insulator walls, so that information will not be lost over time. This does not only limit storage density, it also costs a great deal of energy. For this reason, researchers are working feverishly all over the world on nanoelectronic components that make use of ions, i.e. charged atoms, for storing data. Ions are some thousands of times heavier that electrons and are therefore much easier to ‘hold down’. In this way, the individual storage elements can almost be reduced to atomic dimensions, which enormously improves the storage density.

Here’s how the ions behave in ReRAMs (from the news release),

In resistive switching memory cells (ReRAMs), ions behave on the nanometre scale in a similar manner to a battery. The cells have two electrodes, for example made of silver and platinum, at which the ions dissolve and then precipitate again. This changes the electrical resistance, which can be exploited for data storage. Furthermore, the reduction and oxidation processes also have another effect. They generate electric voltage. ReRAM cells are therefore not purely passive systems – they are also active electrochemical components. Consequently, they can be regarded as tiny batteries whose properties provide the key to the correct modelling and development of future data storage.

In complex experiments, the scientists from Forschungszentrum Jülich and RWTH Aachen University determined the battery voltage of typical representatives of ReRAM cells and compared them with theoretical values. This comparison revealed other properties (such as ionic resistance) that were previously neither known nor accessible. “Looking back, the presence of a battery voltage in ReRAMs is self-evident. But during the nine-month review process of the paper now published we had to do a lot of persuading, since the battery voltage in ReRAM cells can have three different basic causes, and the assignment of the correct cause is anything but trivial,” says Dr. Ilia Valov, the electrochemist in Prof. Rainer Waser’s research group.

This discovery could lead to optimizing ReRAMs and exploiting them in new applications (from the news release),

“The new findings will help to solve a central puzzle of international ReRAM research,” says Prof. Rainer Waser, deputy spokesman of the collaborative research centre SFB 917 ‘Nanoswitches’ established in 2011. In recent years, these puzzling aspects include unexplained long-term drift phenomena or systematic parameter deviations, which had been attributed to fabrication methods. “In the light of this new knowledge, it is possible to specifically optimize the design of the ReRAM cells, and it may be possible to discover new ways of exploiting the cells’ battery voltage for completely new applications, which were previously beyond the reach of technical possibilities,” adds Waser, whose group has been collaborating for years with companies such as Intel and Samsung Electronics in the field of ReRAM elements.

The part I found most interesting, given my interest in memristors, is this bit about extending the memristor theory, from the news release,

The new finding is of central significance, in particular, for the theoretical description of the memory components. To date, ReRAM cells have been described with the aid of the concept of memristors – a portmanteau word composed of “memory” and “resistor”. The theoretical concept of memristors can be traced back to Leon Chua in the 1970s. It was first applied to ReRAM cells by the IT company Hewlett-Packard in 2008. It aims at the permanent storage of information by changing the electrical resistance. The memristor theory leads to an important restriction. It is limited to passive components. “The demonstrated internal battery voltage of ReRAM elements clearly violates the mathematical construct of the memristor theory. This theory must be expanded to a whole new theory – to properly describe the ReRAM elements,” says Dr. Eike Linn, the specialist for circuit concepts in the group of authors. [emphases mine] This also places the development of all micro- and nanoelectronic chips on a completely new footing.

Here’s a link to and a citation for the paper,

Nanobatteries in redox-based resistive switches require extension of memristor theory by I. Valov,  E. Linn, S. Tappertzhofen,  S. Schmelzer,  J. van den Hurk,  F. Lentz,  & R. Waser. Nature Communications 4, Article number: 1771 doi:10.1038/ncomms2784 Published 23 April 2013

This paper is open access (as of this writing).

Here’s a list of my 2013 postings on memristors and memristive devices,

2.5M Euros for Ireland’s John Boland and his memristive nanowires (Apr. 4, 2013 posting)

How to use a memristor to create an artificial brain (Feb. 26, 2013 posting)

CeNSE (Central Nervous System of the Earth) and billions of tiny sensors from HP plus a memristor update (Feb. 7, 2013 posting)

For anyone who cares to search the blog, there are several more.