Jan 16, 2012
Anti-serially connected resistance switching structure shows improved reliability
Resistance switching (RS) is derived by various mechanisms. In most RS systems, RS is induced by a type of ionic motion that may suffer from high energy consumption and a relatively uncontrolled switching behaviour. Recently, scientists from the Department of Materials Science and Engineering at Seoul National University, Korea, have reported an electronic RS mechanism following the analysis of TiO2 film, which is usually regarded to have an ionic type RS. In their latest work, the researchers have examined the origin of the degradation of the electronic bipolar RS, and suggest a method that could improve device endurance.
According to their study, the electronic RS is induced by the non-uniform distribution of traps in the oxide and the asymmetric potential barrier at each end of the metal/TiO2 interfaces. When the traps were distributed non-uniformly, particularly when a much higher trap density was formed near one metal-insulator interface in a metal-insulator-metal structure, the barrier for trapping and detrapping becomes asymmetric with respect to bias polarity. Such asymmetric trap distributions invoke bias-polarity dependent hysteretic current-voltage (I-V) curves, which can be regarded as the advent of resistive switching phenomena.
Because such asymmetric I-V curves most likely have an electronic origin, the team expected that the reliability and endurance characteristics of the sample might be superior to other ion-migration related switching cases. However, this assumption proved to be wrong. This was ascribed to the highly asymmetric shape of the I-V curve for the single electronic RS cell; a sudden increase of current for the set switching, but a more gradual decrease of current for the reset switching.
Therefore, the scientists attempted to make the I-V curve of the RS cell more symmetric while maintaining its hysteretic switching characteristics. Their idea was to anti-serially connect two RS cells, where the two RS cells may have similar or different trap densities, as can be seen in the schematic band structure shown above.
Depending on the detailed process conditions, such as the trap distribution and switching voltages, the anti-serially connected RS cell showed an astonishing improvement in endurance from several hundred cycles up to several tens of thousands of cycles. Subsequent work will focus on realizing the same improvement in a single device.
Additional information can be found in the journal Nanotechnology.
About the author
The study was conducted by researcher from the Dielectric Thin Film Laboratory (DTFL) in the Department of Materials Science and Engineering at Seoul National University, Korea. 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 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.