In this work, the piezoelectric properties of chemical solution deposition derived ferroelectric HfO2–CeO2 thin films deposited on platinized silicon substrates are investigated. Large-signal strain-field measurements show an effective piezoelectric coefficient of approximately d33,eff=12.7pm/V for 17 mol. % cerium under bipolar excitation and d33,eff=8pm/V under unipolar excitation. Progressive bipolar electric field cycling leads to a reduction in the overall field induced strain although no fatigue with regards to the polarization is observed. To explain this, we propose a model explanation based on changes in the polarization reversal pathway from a primarily ferroelastic, i.e., 90° domain wall mediated switching, to a 180° type switching. Furthermore, unipolar strain-field measurements reveal a negative intrinsic piezoelectric coefficient in the absence of any ferroelastic contribution, confirming theoretical predictions. The results suggest that the ferroelastic contribution to the field-induced strain needs to be stabilized in Hafnia-based ferroelectric materials to make them more feasible for micro-electromechanical systems.

Since the discovery of its ferroelectricity, hafnium oxide is widely used for applications in ferroelectric field‐effect transistors and ferroelectric tunnel junctions. is especially favored for its robust ferroelectricity and high remanent polarization at low thicknesses. In addition, is well established as amorphous or crystalline oxide layer in resistive switching devices. Herein, ferroelectric switching is found coexisting with high on/off ratio resistive switching in sub‐10 nm epitaxial . The resistive switching shows typical characteristics of a filamentary‐type valence change memory (VCM), clearly contradicting the polarization charges as the origin of different resistance states. In contrast to previous observations, no electroforming step is required to initiate VCM switching. The bottom electrode enables a RESET to the virgin state, allowing subsequent ferroelectric hysteresis measurements. It is possible to change between both switching schemes repeatedly without impacting the ferroelectric performance. This indicates that ferroelectric switching and oxygen vacancy movement do not interfere with each other, and both switching phenomena can exist independently. This finding opens up ways to unite the different strengths of both switching mechanisms in the same stack. It becomes possible to assign the two operating principles to artificial neural network training and inference according to their respective advantages.

Since the discovery of ferroelectricity in doped HfO2 and ZrO2 thin films over a decade ago, fluorite-structured ferroelectric thin films have attracted much research attention due to their excellent scalability and complementary metal-oxide semiconductor compatibility compared to conventional perovskite ferroelectric materials. Although various factors influencing the formation of the ferroelectric properties are identified, a clear understanding of the causes of the phase formation have been difficult to determine. In this work, ZrO2 films deposited by atomic layer deposition and chemical solution deposition have resulted in films with completely different structural properties. Regardless of these differences, a general relationship between strain and phase formation is established, leading to a more unified understanding of ferroelectric phase formation in undoped ZrO2 films, which can be applied to other fluorite-structured films.

In this work, various stabilization factors for the ferroelectric phase of undoped hafnium oxide prepared by physical vapour deposition were investigated. The capping of the thin films with platinum top electrodes before annealing, as well as the amount of oxygen during sputter deposition and subsequent annealing was shown to have a significant influence on the resulting ferroelectric properties and phase of the HfO x layer. When the prepared films were not capped, only one specific set of process parameters was found to lead to the formation of the ferroelectric phase. We conclude that capping is a crucial condition for stabilizing the ferroelectric phase. Furthermore, it is shown that the amount of oxygen supply during all fabrication processes determines the resulting ferroelectric phase fraction for capped samples. Increasing the oxygen flow during sputtering and annealing results in a larger monoclinic phase fraction and thus a decrease of the remanent polarization.

While the influence of various fabrication parameters during deposition on the ferroelectricity of hafnium oxide has been extensively studied, the effect of different atmospheres on the actual switching process has not yet been investigated. In this work, we characterized the ferroelectric properties of undoped hafnium oxide prepared by reactive sputtering under three different atmospheres: dry oxygen/nitrogen, wet nitrogen, and vacuum conditions. We found a significant correlation between dry and wet atmospheres and resulting polarization. Specifically, we observed a direct effect on ferroelectric switching when the film was exposed to dry atmospheres and vacuum, resulting in a higher electric field necessary to initialize the wake-up effect due to an initial imprint effect. Increasing the amount of wet nitrogen during switching decreased the imprint and lowered the necessary voltage required for the wake up. We present a simple model that explains and discusses the incorporation of moisture and its resulting consequences on the ferroelectric properties of hafnium oxide. Additionally, kinetic Monte Carlo simulations showed that the addition of protons to the oxide thin film leads to a lowering of the potential and to a redistribution of protons and oxygen vacancies, which reduces the initial imprint.

During the past decade, valence change memory (VCM) has been extensively studied due to its promising features, such as a high endurance and fast switching times. The information is stored in a high resistive state (HRS) and a low resistive state (LRS). It can also be operated in two different writing schemes, namely a unipolar switching mode (LRS and HRS are written at the same voltage polarity) and a bipolar switching mode (LRS and HRS are written at opposite voltage polarities). VCM, however, still suffers from a large variability during writing operations and also faults occur, which are not yet fully understood and, therefore, require a better understanding of the underlying fault mechanisms. In this study, a new intrinsic failure mechanism is identified, which prohibits RESET times (transition from LRS to HRS) faster than 400 ps and possibly also limits the endurance. We demonstrate this RESET speed limitation by measuring the RESET kinetics of two valence change memory devices (namely Pt/TaOx/Ta and Pt/ZrOx/Ta) in the time regime from 50 ns to 50 ps, corresponding to the fastest writing time reported for VCM. Faster RESET times were achieved by increasing the applied pulse voltage. Above a voltage threshold it was, however, no longer possible to reset both devices. Instead a unipolar SET (transition from HRS to LRS) event occurred, preventing faster RESET times. The occurrence of the unipolar SET is attributed to an oxygen exchange at the interface to the Pt electrode, which can be suppressed by introducing an oxygen blocking layer at this interface, which also allowed for 50 ps fast RESET times.

Functional thin films are commonly integrated in electronic devices as part of a multi-layer architecture. Metal/oxide/metal structures e.g. in resistive switching memory and piezoelectric microelectrochemical devices are relevant applications. The films are mostly fabricated from the vapour phase or by solution deposition. Processing conditions with a limited thermal budget typically yield nanocrystalline or amorphous layers. For these aperiodic materials, the structure is described in terms of the local atomic order on the length scale of a few chemical bonds up to several nanometres. Previous structural studies of the short-range order in thin films have addressed the simple case of single coatings on amorphous substrates. By contrast, this work demonstrates how to probe the local structure of two stacked functional layers by means of grazing incidence total X-ray scattering and pair distribution function (PDF) analysis. The key to separating the contributions of the individual thin films is the variation of the incidence angle below the critical angle of total external reflection, In this way, structural information was obtained for functional oxides on textured electrodes, i.e. PbZr0.53O0.47O3 on Pt[111] and HfO2 on TiN, as well as HfO2-TiOx bilayers. For these systems, the transformations from disordered phases into periodic structures via thermal teatment are described. These examples highlight the opportunity to develop a detailed understanding of structural evolution during the fabrication of real thin film devices using the PDF technique.