Tuning the Performance of Pt/HfO2/Ti/Pt ReRAM Devices Obtained from Plasma-Enhanced Atomic Layer Deposition for HfO2 Thin Films

Abstract

The market of portable electronics and Green IT solutions demands for non-volatile memory circuits with high speed data access, high reliability, and low power consumption. Resistive switching random access memories (ReRAM) based on the valence change mechanism (VCM) represent a promising concept. VCM-type memristive devices are generally based on transition metal oxide thin films sandwiched between metal electrodes that differ in respect of work function and oxidation enthalpy. ReRAM cells built from insulating metal oxides with a high band gap, like e.g. TiO2, HfO2, Al2O3, typically require an electroforming step to enable resistive switching. Decisive for a stable device operation are a homogeneous microstructure and a controlled defect density in the oxide films of a few nm in thickness. Atomic layer deposition (ALD) can fulfill these requirements by its unique surface-reaction controlled self-limiting growth enabling a control of the film defect structure even for thicknesses in the nm-regime and for low growth temperatures. During the electroforming process, an electronically conducting filament is formed a significantly increased density of oxygen vacancies. The stable VCM-type resistive switching is related to drift/diffusion processes of oxygen vacancies (or oxygen ions, respectively) which depend on the oxidation/reduction enthalpies of the materials involved in the switching event. In this presentation we are going to discuss possibilities to tune the resistive switching performance of nanostructured ReRAM devices by means of a variation of metal oxide bilayer structures with respect to the layer thickness and the stack sequence. This study was performed on various bilayers based on combinations of TiO2/Al2O3 [1] and TiO2/HfO2 [2] with a total thickness < 10 nm integrated into nano-crossbar structures with a size of 100 x 100 nm2. Addressing the ReRAM device performance we discuss effects on the electroforming voltage, necessary SET- and RESET-voltages, achievable resistance ratios, switching stability and variability. In addition, changes in the resistive switching polarity, i.e. from bipolar to complementary-type, which have been observed for certain stacks, are discussed. This work was supported in part by the Deutsche Forschungsgemeinschaft (SFB917).