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Atomic switches pave way for new computing architectures

Posted: 31 Mar 2011 ?? ?Print Version ?Bookmark and Share

Keywords:nanodevices? atomic switches? computing?

An overview of nanodevices and computing architectures that use cationic-based atomic switches has been published by researchers at the WPI Center for Materials Nanoarchitectonics at Japan's National Institute for Materials Science (NIMS) in the March 2011 issue of the journal Science and Technology of Advanced Materials.

The report discusses the fundamental mechanisms governing the operation of nanoionic atomic switches. The researchers, predicting a future for integrating atomic switches with conventional silicon devices via ionic conductive materials, detail their own three terminal devices.

Mechanical atomic switches are operated by manipulating atoms between a conducting surface and the tip of a scanning tunneling microscope (STM). First reported in the early 1990s, mechanical switches created interest in the development of electrically controlled atomic switches, produced by the movement of cationic ions in solid electrochemical reactions, where the operation of cationic atomic switches is governed by the formation of a conducting channel either in or on an ionic conductor.

In its simplest configuration, the operation of a nanoionic atomic switch consists of the formation and disintegration of nanometer sized metallic wires via a solid electrochemical reaction, which leads to major changes in the resistance between electrodesthe 'on' and 'off' states. The challenge for such switches is the fabrication of nanoionic device structures that can be integrated with conventional metal oxide silicon semiconductor devices.

In the report, Takami Hino and his co-researchers describe the control of silver ions in the ionic conductor silver sulphide using an STM tip to inject electrons to produce silver protrusions on the surface of silver sulphide, and their shrinkage by applying an appropriate bias voltage between the STM tip and electrode. Importantly, the application of a positive bias between a silver sulphide tip and a platinum surface leads to the growth of silver wires and a negative bias led their shrinkage. This bipolar control is important for practical device applications.

A second type of atomic switch is the gap-type atomic switch. This is a fundamental building block for bipolar nanoionic devices. In the report, the researchers give a detailed account of bipolar switching using silver sulphide STM tips and platinum electrodes based on their own experiments on 'crossbar' device structures with a 1nm gap between silver sulphide and platinum, with emphasis on the physical mechanism governing high speed switching at 1MHz, and the finding that switching time decreases exponentially with increasing bias voltage. The authors stress that the development of a reproducible method for fabricating 'crossbar' devices was a major breakthrough, which enabled the first demonstration of nanoionic circuits such as logic gates.

The authors gave examples of advanced atomic switches including gapless-type devices consisting of metal/ionic conductor/metal structures, where one of the metals is electrochemically active and the other inert. Notably, recent reports on the use of metal oxides as ionic conductors have added further momentum for device commercialization.

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