Jul 19, 2011
SNU team details electronic bipolar resistance switching in Pt/TiO2/Pt
Resistance switching is derived by various mechanisms. In most of these, the process is induced by a type of ionic motion that can be described as valence change, thermochemical, electrochemical or phase change, according to the categorization by Waser et al. However, ionic motion tends to be slower and more energy consuming than electronic motion, which might limit the switching speed and power efficiency of these systems. Electronic-type resistance switching on the other hand is dominated by the change in the electronic states of thin insulating layers, derived by the trapping and detrapping of electronic carriers. This change of electronic state can be achieved not only by an electric field, but also by other energy sources such as light and heat. This suggests that this system has the potential to become a new type of memory, possibly one that does not use electrical signals in the future. However, electronic-type resistance switching has received less attention relative to other systems because there are fewer groups of materials that can be exploited for such use.
Recently, a research group from the Department of Materials Science and Engineering, Seoul National University, Republic of Korea, has reported an electronic resistance switching mechanism for a TiO2 film that is usually regarded as an ionic-type switching material. According to their study, the electronic resistance switching is induced by the non-uniform distribution of traps in the oxide and asymmetric potential barrier at interfaces.
When the traps are distributed non-uniformly, particularly when a much higher trap density is formed near one metal-insulator interface in a metal-insulator-metal structure, the barrier for trapping and detrapping becomes asymmetric with respect to the bias polarity. Figure (a) shows the schematic energy band structure considering the Coulombic interactions between the free carriers and trap centres (blue dot-dashed line) and the image force effect (red dashed line).
The team estimated the trap density and trap layer thickness and then discussed the change in switching behaviour when these values were changed. Figure (b) shows the difference in the asymmetric barrier height as a function of trap distance or trap density.
This study not only describes the electronic-type switching mechanism in detail, but also discusses the advantage of electronic-type resistance switching. The correlation between the ionic type unipolar resistance switching and electronic charge carrier driven resistance switching is also elucidated.
The researchers presented their results in a special issue of 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, Korea Republic. Dr Kyung Min Kim, who performed the experiments, was a postdoctoral researcher in DTFL and is now a senior researcher at the Samsung Advanced Institute of Technology. Prof. Cheol Seong Hwang is leader of the DTFL and guided these experiments. Dr Byung Joon Choi received his PhD degree in 2009 in the same group and is now a postdoctoral researcher at the University of Pennsylvania, US. Dr Min Hwan Lee received his PhD degree at Stanford University in 2008 and is currently a postdoctoral researcher in the same group. Dr Seungwu Han, who is a professor at the Department of Materials Science and Engineering, Seoul National University, gave many helpful ideas and discussions for these results. Gun Hwan Kim, Seul Ji Song, Jun Yeong Seok and Jeong Ho Yoon are student members of the DTFL, and made contributions to the experimental procedures and sample fabrication.