Ferroelectrics are promising for nonvolatile memories. However, the diffi culty of fabricating ferroelectric layers and integrating them into complementary metal oxide semiconductor (CMOS) devices has hindered rapid scaling. Hafnium oxide is a standard material available in CMOS processes. Ferroelectricity in Si-doped hafnia was first reported in 2011, and this has revived interest in using ferroelectric memories for various applications. Ferroelectric hafnia with matured atomic layer deposition techniques is compatible with three-dimensional capacitors and can solve the scaling limitations in 1-transistor-1-capacitor (1T-1C) ferroelectric random-access memories (FeRAMs). For ferroelectric field-effect-transistors (FeFETs), the low permittivity and high coercive field Ec of hafnia ferroelectrics are beneficial. The much higher Ec of ferroelectric hafnia, however, makes high endurance a challenge. This article summarizes the current status of ferroelectricity in hafnia and explains how major issues of 1T-1C FeRAMs and FeFETs can be solved using this material system.

The polarization reversal in ferroelectric HfO₂ is adopted to store information in three distinct device classes - ferroelectric field effect transistors (FeFET), ferroelectric capacitors (FeCAP) and ferroelectric tunnel junctions (FTJ). Common to all three concepts is the adoption of a ferroelectric layer stack that acts either as gate dielectric in the FeFET or as the capacitor dielectric and tunneling barrier in the FeCAP or FTJ, respectively. A composite structure including an inevitably or purposefully formed dielectric layer is frequently adopted. In this work we report on the co-existence of all three memory concepts within one device structure and propose a 2T1C ferroelectric memory cell that allows the operation and comparative characterization of the trinity of ferroelectric memory devices.

With the discovery of the ferroelectric (FE) properties within HfO₂, the scaling gap between state-of-the-art technology nodes and non-volatile memories based on FE materials can be bridged. In addition to non-volatility, new memory concepts should guarantee sufficient endurance and operation stability. However, in contrast to optimized perovskite based FEs, binary oxide based FE memories still show changes in the memory window (MW) followed by either hard breakdown or closure of the MW. Recently, we have shown that anti-FE (AFE) materials exhibit very stable and significantly higher endurance with respect to the FE counterparts. Inspired by the robustness and remarkable cycling performance of the AFE materials, we analyze the remaining reliability aspects of these devices. By characterizing the pure film properties of capacitor stacks and switching performance when integrated into devices, we compare and investigate temperature stability, imprint, retention, and variability of both FE and AFE memories. We investigate if the lower energetic barrier to be overcome together with partial switching and lower switching induced stress are responsible for the higher endurance of the AFE with respect to the FE based memories. By utilizing charge trapping and charge pumping tests together with leakage current spectroscopy in combination with comprehensive modeling we check that assumption. Moreover, we identify the interfacial buffer layer as the weakest link of these devices.

Visualization of domain structure evolution under an electrical bias has been carried out in ferroelectric La:HfO2 capacitors by a combination of Piezoresponse Force Microscopy (PFM) and pulse switching techniques to study the nanoscopic mechanism of polarization reversal and the wake-up process. It has been directly shown that the main mechanism behind the transformation of the polarization hysteretic behavior and an increase in the remanent polarization value upon the alternating current cycling is electrically induced domain de-pinning. PFM imaging and local spectroscopy revealed asymmetric switching in the La:HfO2 capacitors due to a significant imprint likely caused by the different boundary conditions at the top and bottom interfaces. Domain switching kinetics can be well-described by the nucleation limited switching model characterized by a broad distribution of the local switching times. It has been found that the domain velocity varies significantly throughout the switching process indicating strong interaction with structural defects.

The electric-field-driven and reversible polarization switching in ferroelectric materials provides a promising approach for nonvolatile information storage. With the advent of ferroelectricity in hafnium oxide, it has become possible to fabricate ultrathin ferroelectric films suitable for nanoscale electronic devices. Among them, ferroelectric field-effect transistors (FeFETs) emerge as attractive memory elements. While the binary switching between the two logic states, accomplished through a single voltage pulse, is mainly being investigated in FeFETs, additional and unusual switching mechanisms remain largely unexplored. In this work, we report the natural property of ferroelectric hafnium oxide, embedded within a nanoscale FeFET, to accumulate electrical excitation, followed by a sudden and complete switching. The accumulation is attributed to the progressive polarization reversal through localized ferroelectric nucleation. The electrical experiments reveal a strong field and time dependence of the phenomenon. These results not only offer novel insights that could prove critical for memory applications but also might inspire to exploit FeFETs for unconventional computing.

Une des méthodes utilisées pour étudier la ferroélectricité à l'échelle nanométrique dans les couches minces est la technique appelée « Piezoresponse Force Microscopy » (PFM). La PFM est un mode dérivé de l’AFM (Atomic Force Microscopy) en mode contact. Cette technique est basée sur l’effet piézoélectrique inverse : lorsqu’on applique un champ électrique sur un matériau piézoélectrique celui-ci se déforme. La pointe est posée sur la surface et mesure donc une déformation locale due à la tension appliquée. Les résultats obtenus par PFM sur des couches minces deviennent difficiles à interpréter dès lors que des charges d’origine non ferroélectriques (différentes de la charge de polarisation) entrent en jeu : charges électroniques piégées dans l’oxyde après l’injection de courant dues aux courants de fuite, charges déjà présentes dans la couche, les charges de surface, ainsi que les différents phénomènes électrochimiques due à la présence de la couche d’eau sous la pointe lors des mesures sous atmosphère ambiante. Le but de ce travail de thèse est de montrer que dans le cas de couches très minces les courants de fuite et les phénomènes électrochimiques peuvent conduire à l’interprétation de résultat PFM erroné. Des mesures PFM ont été réalisées sur des couches minces de PbZrTiO3, BaTiO3 et des nanostructures de BiFeO3 ferroélectriques. Les paramètres de mesure utilisés en PFM sont discutés avec une attention particulière sur la première résonance de contact qui permet d’amplifier le signal PFM. L’impact des phénomènes électrochimiques sur le contraste en PFM est discuté et mis en évidence d’un point de vue expérimentale. Des images PFM sur des couches minces non-ferroélectriques sont obtenues semblable à celle obtenues lors de l’utilisation d’une procédure standard sur des échantillons ferroélectriques. Ces images sont réalisées sur des couches minces d’aluminate de lanthane (LaAlO3), d'oxyde de Gadolinium (Gd2O3) et d’oxyde de Silicium (SiO2). Les motifs obtenus sur le LaAlO3 et le Gd2O3, similaires à des domaines de polarisation opposés, tiennent dans le temps sous atmosphère ambiante. Ces mesures sont comparées avec des résultats obtenus sur des couches minces de BaTiO3 préparées par MBE (Molecular Beam Epitaxy). Différentes méthodes de caractérisation électriques à l’échelle macroscopique sont présentées afin de confirmer la ferroélectricité des couches minces étudiées dans cette thèse. L'objectif est de disposer d'une procédure permettant d'affirmer qu'un échantillon dont on ne sait rien est ou n'est pas ferroélectrique. / Piezoresponse Force Microscopy (PFM) is a powerful tool for the characterization of ferroelectric materials thanks to its ability to map and control in a non destructive way domain structures in ferroelectric films. Most of the time, the ferroelectric behaviour of a film is tested by writing domains of opposite polarization with the Atomic Force Microscope (AFM) tip and/or by performing hysteresis loops with the AFM tip as a top electrode. A given sample is declared ferroelectric when domains of opposite direction have been detected; corresponding to zones of distinct contrast on the PFM image, or when an open hysteresis loop is obtained. More prudently in certain cases, the ferroelectricity is at last attested only when the contrast is stable within several hours. But as the thickness of the films studied by PFM decrease, data become difficult to interpret. In particular, charges trapped after current injection due to leakage currents and electrochemical phenomena due to the water layer under the tip may contribute in a non-negligible way to the final contrast of PFM images. In this thesis, some PFM measurements are performed on ferroelectric PbZrTiO3, BaTiO3 thin films and BiFeO3 nanostructures. Different parameters used in PFM measurements are discussed with special attention on the buckling first harmonic PFM measurements which allow the amplification of the PFM signal. The impact of electrochemical effects on the PFM contrast are discussed and are shown experimentally. Then, the standard procedure which is used in order to show the ferroelectricity of a film is applied to a non-ferroelectric sample with apparently the same results. To do so, we use a LaAlO3, Gd2O3 and SiO2 amorphous dielectric films and apply similar voltages as for artificially written ferroelectric domains. The resulting pattern is imaged by PFM and exhibit zones of distinct PFM contrasts, stable with time, similar to the one obtained with ferroelectric samples. These results are explained and is compared with results obtained on BaTiO3 thin films prepared by Molecular Beam Epitaxy which are supposed to be ferroelectric. In order to confirm the ferroelectricity of our thin films, several macroscopic electrical techniques are introduced. The aim of this study is to establish a reliable procedure which would remove any ambiguity in the characterization of the ferroelectric nature of such samples.

Ferroelectric field effect transistor (FeFET) memories based on a new type of ferroelectric material (silicon doped hafnium oxide) were studied within the scope of the present work. Utilisation of silicon doped hafnium oxide (Si:HfO2) thin films instead of conventional perovskite ferroelectrics as a functional layer in FeFETs provides compatibility to the CMOS process as well as improved device scalability. The influence of different process parameters on the properties of Si:HfO2 thin films was analysed in order to gain better insight into the occurrence of ferroelectricity in this system. A subsequent examination of the potential of this material as well as its possible limitations with the respect to the application in non-volatile memories followed. The Si:HfO2-based ferroelectric transistors that were fully integrated into the state-of-the-art high-k metal gate CMOS technology were studied in this work for the first time. The memory performance of these devices scaled down to 28 nm gate length was investigated. Special attention was paid to the charge trapping phenomenon shown to significantly affect the device behaviour.

Ferroelectric field effect transistor (FeFET) memories based on a new type of ferroelectric material (silicon doped hafnium oxide) were studied within the scope of the present work. Utilisation of silicon doped hafnium oxide (Si:HfO2) thin films instead of conventional perovskite ferroelectrics as a functional layer in FeFETs provides compatibility to the CMOS process as well as improved device scalability. The influence of different process parameters on the properties of Si:HfO2 thin films was analysed in order to gain better insight into the occurrence of ferroelectricity in this system. A subsequent examination of the potential of this material as well as its possible limitations with the respect to the application in non-volatile memories followed. The Si:HfO2-based ferroelectric transistors that were fully integrated into the state-of-the-art high-k metal gate CMOS technology were studied in this work for the first time. The memory performance of these devices scaled down to 28 nm gate length was investigated. Special attention was paid to the charge trapping phenomenon shown to significantly affect the device behaviour.:1 Introduction 2 Fundamentals 2.1 Non-volatile semiconductor memories 2.2 Emerging memory concepts 2.3 Ferroelectric memories 3 Characterisation methods 3.1 Memory characterisation tests 3.2 Ferroelectric memory specific characterisation tests 3.3 Trapping characterisation methods 3.4 Microstructural analyses 4 Sample description 4.1 Metal-insulator-metal capacitors 4.2 Ferroelectric field effect transistors 5 Stabilisation of the ferroelectric properties in Si:HfO2 thin films 5.1 Impact of the silicon doping 5.2 Impact of the post-metallisation anneal 5.3 Impact of the film thickness 5.4 Summary 6 Electrical properties of the ferroelectric Si:HfO2 thin films 6.1 Field cycling effect 6.2 Switching kinetics 6.3 Fatigue behaviour 6.4 Summary 7 Ferroelectric field effect transistors based on Si:HfO2 films 7.1 Effect of the silicon doping 7.2 Program and erase operation 7.3 Retention behaviour 7.4 Endurance properties 7.5 Impact of scaling on the device performance 7.6 Summary 8 Trapping effects in Si:HfO2-based FeFETs 8.1 Trapping kinetics of the bulk Si:HfO2 traps 8.2 Detrapping kinetics of the bulk Si:HfO2 traps 8.3 Impact of trapping on the FeFET performance 8.4 Modified approach for erase operation 8.5 Summary 9 Summary and Outlook

A 2TnC ferroelectric memory gain cell consisting of two transistors and two or more ferroelectric capacitors (FeCAP) is proposed. While a pre-charge transistor allows to access the single cell in an array, the read transistor amplifies the small read signals from small-scaled FeCAPs that can be operated either in FeRAM mode by sensing the polarization reversal current, or in ferroelectric tunnel junction (FTJ) mode by sensing the polarization dependent leakage current. The simultaneous read or write operation of multiple FeCAPs is used to realize compute-in-memory (CiM) algorithms that enable processing of data being represented by both, non-volatilely internally stored data and externally applied data. The internal gain of the cell mitigates the need for 3D integration of the FeCAPs, thus making the concept very attractive especially for embedded memories. Here we discuss design constraints of the 2TnC cell and present the proof-of-concept for realizing versatile (CiM) approaches by means of electrical characterization results.