Aug 24, 2010
Electrically 'tunable' memristors
A memristor (memory resistor) is a device that remembers how much current has flown through it, resulting in different resistance states. These fascinating devices have been highlighted for new applications not only for passive circuit elements, but also for next-generation non-volatile memory devices called resistive random access memory (ReRAM) and logic circuit devices.
Generally the memristor device has a metal-insulator-metal capacitor structure. The insulator layer is composed of two parts, one being the switching layer where the actual resistance change occurs and the other being the non-switching layer, which is essential for memristive behaviour, even though no resistance change occurs in the layer itself. These two layers are derived by using different materials or by using the same material with different chemical status, such as TiO2–x in TiO2.
One of the most well known of such structures is a Ti/TiO2–x/TiO2/Pt structure where the Ti/TiO2–x interface acts as the non-switching layer and Pt/TiO2 interface acts as the switching layer. In a memristor, opposite bias polarity is required for the writing and erasing process. This switching polarity depends on the location of the switching layer.
In previous work, led by Hewlett Packard, this structure had been defined by the materials deposition process so that once the device is fabricated the switching polarity cannot be reversed or controlled. Recently, a research group from the Department of Materials Science and Engineering, Seoul National University, Republic of Korea, has developed a completely different type of memristor that is not dependent on the fabrication process.
First, the researchers stack a Pt/TiO2/Pt structure, which initially does not have the switching layer. Then, using electrical methods, they can form a switching layer at any given location in the TiO2 film, spanning from near the top electrode to the vicinity of the bottom electrode. In other words, a memristor with post-fabrication switching polarity tunability has been invented.
New path for memristor applications
In the future, more complex and effective devices will be developed using the memristor. This will raise the demand for various switching polarity memristors in one circuit. If this was to be met by the conventional fabrication process, all memristors would need to be fabricated individually, which would be a hurdle for memristor applications. The group's result, however, provides a new method of deriving the switching polarity of memristors electrically, which will suggest a new path for the application of memristors.
Full details can be found in the journal Nanotechnology.
About the author
The study was conducted by research teams of the Dielectric Thin Film Laboratory (DTFL) in the Department of Materials Science and Engineering, Seoul National University, Republic of Korea. Dr Kyung Min Kim, who is a postdoctoral researcher, performed the experiments and developed all of the methods. Prof. Cheol Seong Hwang is the head of the DTFL and led and guided these experiments. Dr Byung Joon Choi received his PhD degree in 2009 in this team and is now a postdoctoral researcher at the University of Pennsylvania, US. Dr Min Hwan Lee received his PhD degree at Stanford University, US, in 2008 and is a postdoctoral researcher in this team. Prof. Seungwu Han, who is a professor in the Department of Materials Science and Engineering at Seoul National University, gave many helpful ideas and discussions for these results.