Resistive switching phenomena in stacks of binary transition metal oxides grown by atomic layer deposition

Information technology is approaching the era of artificial intelligence. New computing architectures are required to cope with the huge amount of data that has to be processed in all types of cognitive applications. This requires dedicated energy efficient solutions on the level of the computing hardware. The new concepts of neuromorphic computing(NC), like artificial neural networks (ANNs) and computation in memory (CIM), aim to overcome the limitations of classical computers based on von Neumann architecture. Redox-type resistive random access memory (ReRAM) devices are intensively investigated for NC applications due to their non-volatility and energy efficiency, process compatibility with standard complementary metal oxide semiconductor (CMOS) technology, and the ability for device scaling and three-dimensional (3D) integration. The variety of applications requests for different desired properties of the ReRAM devices ranging from an analog-type programmable multilevel behavior to a binary-type switching at high resistance ratio and with linear resistance states. ReRAM research today focuses on devices built of metal oxide layers with nanometer thickness sandwiched between a chemically inert electrode like Pt or TiN and a chemically reactive electrode. The precise thickness control is achieved by vapor phase deposition techniques, in particular, atomic layer deposition (ALD). However, some basic issues like switching stability and resistance variability are still obstacles on the way towards massive integration. One of the efforts to improve the device performance is the use of combinations of two metal oxides layers, so called bilayer oxide stacks. The two different metal oxide layers are selected regarding their insulation resistance and oxidation enthalpy. Here, especially the bilayer ReRAM stack of TiO$_{2}$ and Al$_{2}$O$_{3}$ has drawn attention of researches worldwide. TiO$_{2}$ belongs to the materials integrated into ReRAM devices since the early start in the beginning of this millennium. However, most of the single-layer TiO$_{2}$ devices lack stability in the standard valence change mechanism (VCM)-type filamentary switching behavior and suffer from a too high residual leakage current. One approach for improvement is the addition of an Al$_{2}$O$_{3}$ barrier layer into the TiO$_{2}$ ReRAM device. So far, in the scientific literature, there is no clear consensus if this type of Al$_{2}$O$_{3}$/TiO$_{2}$ bilayer cells reveal a standard VCM-type filamentary switching or an area-dependent switching behavior. [...