Carlos Rosario1 2 Bo Thöner3 Alexander Schönhals2 Stephan Menzel4 Matthias Wuttig3 Rainer Waser2 4 Nikolai Sobolev1 5 Dirk Wouters2

1, University of Aveiro, Aveiro, , Portugal
2, RWTH Aachen University, Aachen, , Germany
3, RWTH Aachen University, Aachen, , Germany
4, Forschungszentrum Jülich GmbH, Jülich, , Germany
5, National University of Science and Technology “MISiS”, Moscow, , Russian Federation

Redox-based resistive random access memories (ReRAM) are one of the key technologies for revolutionizing the memory device market in the near future. The performance of ReRAM devices position them as ideal candidates for Storage Class Memories, whose purpose is to fill the cost-performance gap between the DRAM and NAND flash technologies. Tantalum oxide (TaOx) is one of the most promising materials for the integration as an insulator in ReRAM devices, showing high endurance and high switching speed. The resistive switching in TaOx-based devices is commonly explained by the filamentary valence change mechanism, in which the formation / dissolution of a conductive filament plays a key role in switching between different resistance states. However, the filament structure, its exact composition and its impact on the performance of the devices are still a matter of debate. We targeted these questions by performing a detailed study of the electronic transport through conductive filaments in TaOx-based ReRAM devices in the low-resistance state (LRS or ON state) at temperatures from 300 K down to 2 K. To further understand the origin of the transport properties of the filaments, we also measured, in the van der Pauw (vdP) geometry, substoichiometric TaOx thin films which, for certain compositions, exhibit the same transport behavior as conductive filaments in the ReRAM devices. Thus, by studying the substoichiometric TaOx films, it was possible to learn more about the conduction mechanisms in the filaments.
The conductance of the TaOx ReRAM devices shows two different regimes in the measured temperature range. At higher temperatures, the conductance exhibits a T 1/2 dependence, while at lower temperatures, an exp(–T 1/2) dependence is observed. The temperature range where the transition between the two regimes takes place varies slightly from device to device. A positive magnetoresistance is observed in the low temperature regime, and it decays exponentially with increasing temperature. These features were also observed in the substoichiometric TaOx films with x ∼ 0.5, both for conductance and for magnetoresistance. For these films, Hall measurements give a very high carrier concentration of the order of 1022 cm-3 and a low Hall mobility of 0.1 cm2V-1s-1. The carrier concentration is of the same order of magnitude as the one measured in a reference Ta film. Furthermore, this Ta film shows the same distinctive T 1/2 behavior corresponding to the aforementioned high temperature regime of the conductance in the TaOx devices and vdP samples. This T 1/2 behavior is commonly reported for disordered metals where quantum corrections to the conductivity dominate the transport. Based on these experimental findings we propose a model where the transport properties of both TaOx ReRAM devices and TaOx vdP samples are determined by a percolation path of disordered Ta granules.