Topical area B:
Kinetics and switching speed

Topical area B

The speed of the transition between the states involved is a crucial point for the application of chalcogenides in information technology. We have to keep in mind that a significant voltage-time dilemma needs to be overcome. One needs a mechanism which changes the state of the memory by a write voltage pulse of (preferably less than) 10 ns, while it must withstand read voltages for up to 10 years, i.e. approx. 3x108 s. Nevertheless, the read voltage may not be less than approximately a tenth of the write voltage. Obviously, there must be switching kinetics involved with an extremely high degree of non-linearity. Within one order of magnitude on the voltage scale, the switching time needs to change by not less than 15 (!) orders of magnitude. In topical area B, we are aiming to understand the microscopic mechanism underlying the switching kinetics and how it can be influenced by the material's composition and its defect structure as well as by the cell design.

The thermally and/or electrochemically driven kinetics in the case of VC material systems are strongly connected with the question of which type of ions diffuse and which are the dominant diffusion paths for the different oxide material configurations, e.g. special types of extended defects or even bulk diffusion as in the case of GaOx. According to switching speeds in the sub-10 ns regime, observed in most filamentary switching materials, it is likely that the redox-process or the formation of new phases is restricted to a nanoscopic region in a conducting filament. In order to check this assumption, sophisticated experiments have to be performed and accompanied with suitable modelling techniques.

Measuring switching processes on a timescale of a few nanoseconds and length scale of nanometres poses an analytical challenge, which can only be met with the development of dedicated experimental facilities. We have set up the facilities for ultra-fast and integral electrical characterisation of resistive switching cells and developed a cell design tailored to the specific requirements. Furthermore, dedicated tools have been developed within topical area B to image changes of the atomic and electronic structure in the different resistive states. We are moving towards imaging switching cells during fast operation by a variety of different analytical techniques and will correlate it to the observed switching kinetics. In particular, utilising Synchrontron-based spectroscopy will offer us the exciting possibility to study the switching kinetics with pico-second time-resolution by performing pump-probe experiments.

For the VC materials, we are aiming to correlate the switching kinetics with the ionic movement by per-forming tracer diffusion experiments for the different ionic species. Combining diffusion experiments with atomistic simulations, continuum-level simulations, microstructural and spectroscopic characterisation and studies of the switching kinetics will permit new insights into the microscopic switching and failure mechanisms of VCM devices.

back | top | projects