Interface structure in Cu/Ta2O5/Pt resistance switch: a first-principles study

Hydrogen-triggered metal filament rupture in Cu-based resistance switches

Cation-based resistance switches have been considered as promising candidates for memory cells and other novel devices. So far, the most accepted switching processes of such devices are based on the formation/rupture of metallic filaments between two electrodes. Although many recent studies have identified the existence of H2O (and resulting -OH groups) in such devices, their effects on the switching process are still unclear. In the present work, by taking Cu/Ta2O5/Pt device as an example, we have theoretically revealed that H ions may dissociate from -OH groups and accumulate onto the Cu filament in amorphous Ta2O5. After that, the adsorbed H ions will induce a series of changes, such as the elongation of the adjacent Cu-Cu bonds, the weakening of the Cu-Cu bonds, the increase of charge on Cu cations, and the enhancement of diffusivities of Cu cations, all of which eventually lead to the rupture of the Cu filament. Interestingly, our proposed 'H-triggered metal filament rupture' model is similar to the widely studied 'hydrogen embrittlement phenomenon'. The crucial point of this model is the high catalytic activity of Cu towards the splitting of -OH group. Consequently, it is expected that this model could be applicable to other Cu-cation based resistance switches.

Effect of Doping Different Cu Valence States in HfO2 on Resistive Switching Properties of RRAM

Metal dopants are important for HfO2-based resistive switching mechanisms in resistive random-access memory (RRAM) because they can improve the performance of RRAM devices. Although Cu ions have been widely explored as metal dopants, Cu dopants with different valence states have received little attention. Using the first principles method and the Vienna ab initio simulation package (VASP), the effect of electron gain or loss in different doped Cu states in hafnium oxide (HfO2) was investigated. The electron affinity, defect formation energy, and charge density difference suggest that Cu doping results in a loss of electrons, thereby stabilizing the system. The population, the isosurface of partial charge density, and the migration barrier of the Cu-doped systems with different ionic valence states (+2 and 0) were calculated. Furthermore, the impact of doping ions on the formation of conductive filaments and the stability of the system were investigated in this study. The results indicate that the average population of the Cu-doped (+2) system is smaller than that of the Cu (0) system, and the Cu-O bond length increases in the Cu-doped (+2) system. At the same isosurface level, the electronic local clusters in the Cu (+2) system are stable; however, by increasing the isosurface level, the conductive filament of the Cu (0) system breaks first. At the same starting and ending positions, the migration barrier of the Cu (+2) system was much lower. In the transition state of the Cu (+2) system, the number of atoms whose atomic structure changes by more than 0.1 Å is lower than that in the Cu (0) system, which has a relatively small displacement deviation. This study, which indicates that the Cu (+2) system helps to form conductive channels upon applying current or voltage, can provide theoretical guidance for preparing RRAM and improving its performance.

Effects of dopants on the adhesion and electronic structure of a SnO2/Cu interface: a first-principles study

Doping has been adopted as a versatile approach for tuning the adhesion of metal oxide/metal interfaces. Understanding the mechanism of doping at the interface adhesion on the atomic and electronic scale is crucial for the rational design and optimization of metal oxide/metal composites. In this work, we have investigated the effects of dopants on the adhesion of SnO2/Cu interfaces through first-principles calculations. Firstly, O-terminated a SnO2(110)/Cu(111) interface (denoted as I) was considered and the work of separation values of the interfaces with various dopants (Mo, Sb, Ti, Zn and Cu) were calculated to evaluate the interface adhesion strength. It was demonstrated that low-valence dopants (Zn2+ and Cu2+) enhance the adhesion strength of interface I, while high-valence dopants (Mo6+ and Sb5+) play the opposite role. Secondly, the strengthening effects of low-valence dopants were further verified in four candidate interfacial models with different atomic structures (denoted as II-V). The work of separation values indicated that the adhesion of all of the interfaces involved could be enhanced by low-valence doping. The electronic structure of the interface was demonstrated through density of states, charge density and charge density difference analyses. The results revealed that upon low-valence doping, the holes facilitate charge transfer between Cu and SnO2, which generates strong covalent bonds across the interface and thus significantly enhances the interface adhesion. This work not only provides insight into rational doping to enhance the adhesion of SnO2/Cu composites but can also be expanded upon for the design of other metal oxide/metal composites with strong interface adhesion.

Design of Electrodeposited Bilayer Structures for Reliable Resistive Switching with Self-Compliance

Programmable memory characteristics of electrodeposited CuO_x-based resistive random access memory (ReRAM) can be significantly improved by adopting a bilayer structure with a built-in current limiter. To control the on-current and enhance the device uniformity, the bilayer structure of Pt/CuO_x (switching layer)/CuO_x (current limiter)/Pt is proposed. This structure is synthesized by controlling solution pH during electrochemical deposition (ECD). The bilayer structure of Pt/CuO_x (synthesized at pH 9)/CuO_x (synthesized at pH 11.5)/Pt exhibits reliable and uniform self-compliant resistive switching behavior. The origin of resistive switching is attributed to formation and rupture of conductive filaments in the CuO_x (pH 9) layer. However, the CuO_x (pH 11.5) layer acts as the resistor without resistive switching to control the overall resistance in ReRAM. Reversible "on" and "off" switching occurs with a switching time of 100 ns. Devices based on the bilayer structure showed long data retention and good endurance. This simple use of ECD to improve the memory characteristics of electrodeposited ReRAM offers the opportunity to realize reliable and self-compliant memory devices with low-cost solution processes.

Atomic Insight into the Origin of Various Operation Voltages of Cation-Based Resistance Switches

Low operation voltage (V_SET), which means low power consumption and good stability, is one of the most important factors in designing the resistance switches with high performance. However, the atomic details for the various V_SET values of such devices are still lacking, which hinders their further improvement. In the present study, by taking Ag/Ta₂O₅/Pt (V_SET = 0.6 V) and Cu/Ta₂O₅/Pt (V_SET = 2.0 V) as the examples, we have examined the switching mechanisms of these two cation-based devices by using first principle simulation. Several possible reasons have been addressed to explain the much lower V_SET of Ag/Ta₂O₅/Pt than that of Cu/Ta₂O₅/Pt: (i) the faster diffusion of Ag ions in Ta₂O₅ compared to Cu ions; (ii) the more preferable nucleation process of Ag ions at Pt/Ta₂O₅ interface compared to Cu ions; (iii) the lower Schottky barrier height (SBH) of Ag/Ta₂O₅/Pt than that of Cu/Ta₂O₅/Pt. On the basis of these results, several key factors have been suggested to design the cation-based resistance switches (oxidizable-metal/Ta₂O₅/inert-metal) with low V_SET values: (i) the weak interaction strength between oxidizable metal ions and Ta₂O₅ surface; (ii) the low formation energy of oxidizable metal ions on inert electrode; (iii) the low SBH, which could be controlled by tuning the ambient water pressure during the device fabrication process.

Bottom-up synthesis of ordered metal/oxide/metal nanodots on substrates for nanoscale resistive switching memory

The bottom-up approach using self-assembled materials/processes is thought to be a promising solution for next-generation device fabrication, but it is often found to be not feasible for use in real device fabrication. Here, we report a feasible and versatile way to fabricate high-density, nanoscale memory devices by direct bottom-up filling of memory elements. An ordered array of metal/oxide/metal (copper/copper oxide/copper) nanodots was synthesized with a uniform size and thickness defined by self-organized nanotemplate mask by sequential electrochemical deposition (ECD) of each layer. The fabricated memory devices showed bipolar resistive switching behaviors confirmed by conductive atomic force microscopy. This study demonstrates that ECD with bottom-up growth has great potential to fabricate high-density nanoelectronic devices beyond the scaling limit of top-down device fabrication processes.