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 nano-meter 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 TiO2 and Al2O3 has drawn attention of researches worldwide. TiO2 belongs to the materials integrated into ReRAM devices since the early start in the beginning of this millennium. However, most of the single-layer TiO2 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 Al2O3 barrier layer into the TiO2 ReRAM device. So far, in the scientific literature, there is no clear consensus if this type of Al2O3/TiO2 bilayer cells reveal a standard VCM-type filamentary switching or an area-dependent switching behavior. The present study aims at a clarification of different phenomena associated with the bipolar resistive switching in bilayer ReRAM devices built from Al2O3 and TiO2 layers. In order to cope with device sizes, which are close to the industrial scale, nano-crossbar ReRAM cells were fabricated with an electrode area of (60 nm)2 to (100 nm)2 and oxide layer thickness below 10 nm. Nanometer-thin, dense oxide layers of reproducible quality were grown by means of ALD. Here a Pt bottom electrode was used as the Schottky electrode. In contrast, for the Pt/oxide/metal structures the metal top electrode was varied between Ti and TiN. A systematic study was performed regarding the effect of the resistive switching oxide comparing single-layers of Al2O3 and TiO2 and bilayers with different stack sequence, this means, Al2O3/TiO2 and TiO2/Al2O3. Study of the electroforming behavior in the various device stacks Pt/Al2O3/Pt cells reveals the identical breakdown strength as observed for other reported Al2O3 single-layer devices. In contrast, Pt/Al2O3/Ti devices show a linear dependence of the electroforming voltage for the Al2O3 thickness of 2 to 5 nm. Pt/TiO2/Ti devices are conductive in their initial state for TiO2 thickness below 10 nm. In bilayer stacks the electroforming voltage is dominated by the thickness of the Al2O3 layer, but the additional TiO2 layer is not negligible. According to the different oxidation enthalpies, the use of a Ti electrode results in a more reproducible and stable switching compared to TiN. Pt/metal oxide/Ti nano-crossbar devices with a thin Al2O3 layer show filamentary VCM-type counter-eightwise (c8w) bipolar resistive switching after successful electroforming and first RESET step. The resistance ratio is controlled by choosing values of current compliance and RESET stop voltage between high and low resistance state, respectively. Deep RESET behavior is obtained for the Al2O3 film thicker than 4 nm. However, effects appear which are attributed to a filling of trap states in the Al2O3 layer adjacent to the Pt Schottky electrode. The charge transport behavior of the different cells was systematically analyzed considering the voltage and temperature dependence of the initial state, the high (HRS) and low resistance state (LRS). The current transport in the insulating devices, i.e. Al2O3 and the bilayers with Al2O3, was successfully simulated by the Simmons' equation for tunneling through a trapezoidal barrier. This also holds for the HRS state with differences in tunneling barrier and tunneling area. The area of 100 nm2 attributed to the HRS fits well to the physical diameter of the filament of about 10 nm that was determined from the crystallized regime appearing in the cross-section of a switched device via transmission electron microscopy. For all devices the LRS exhibits an almost metallic-type conduction characteristic. Pulse switching analysis leads to a SET kinetic, which is well described by the ion hopping model utilizing Mott-Gurney law for oxygen vacancy drift.Pt/TiO2/Ti nano-crossbar devices show an extraordinary behavior of the coexistence of standard filamentary counter-eightwise (c8w) and stable eightwise (8w) switching at significantly reduced currents. The two switching modes with opposite polarity share a common state, this is, the c8w HRS equals the 8w LRS*. A model is proposed which describes this coexistence as a competition between oxygen vacancy drift/diffusion and oxygen incorporation/extraction at the Pt/TiO2-x interface. The reduced/increased amount of oxygen vacancies in the regime of the conductive filament's disc leads to a band bending and a change of the parabolic shaped tunneling barrier at the switching interface. In the Pt/TiO2/Ti nano-devices the 8w-switching process occurs at switching voltages of about 2 V, but at a significantly reduced current level with resistance values of about Mega- and Giga-Ohm in LRS* and HRS*, respectively. The deeper understanding of switching phenomena and conduction behavior in the various Al2O3 and TiO2 single-layer and bilayer nano-crossbar devices can be utilized for improvement of existing switching models and for future cell design addressing particular applications
