No description available.

This work deals with the structural and dielectric properties of Bariumtitanate (BTO) / Strontiumtitanate (STO) superlattices. The investigations were carried during the research for a doctoral thesis on the IFW Dresden, Institute for Metallic Materials (under supervision of Prof. Schulz). These multilayers have been prepared on single crystalline STO of (100) and (111) orientated substrates. All films where grown in an epitaxial mode. Additional superlattices and Bariumstrontiumtitanate (BSTO) thin films on silicon substrates with platinum bottom electrodes have been prepared. Thereby, (111) fibre-textured polycrystalline superlattices were produced. According to our knowledge this result was achieved for the first time (is unique in the world at the moment). According to high resolution TEM investigations of (001) oriented superlattices multilayers with atomically thin interfaces without noticeable interdiffusion have been prepared. XRD pattern of a multilayer consisting of BTO and STO monolayers that have only a thickness adequate one unit cell of BTO respective STO confirm this assumption. Multilayers on (111) oriented STO substrates show a much higher interface roughness than (001) orientated films. Regarding to the examinations in this thesis it is suggested that the roughness is correlated with the reduction of internal stresses by deformation of the stack and not with interdiffusion between the monolayers. For electrical measurements the film thickness has been varied from 30 nm to 300 nm and the periodicity in the range from 0.8 nm to 20 nm. Additionally, BSTO films of equivalent thickness and integral chemical composition were produced. Dielectical measurements were carried out in the temperature range from 20 K to 600 K and hysteresis measurements were done. It has to be pointed out, that multilayers have always lower dielectrical performances then BSTO films. In all cases the dielectric constant (DC) decreases with decreasing film thickness. Multilayers of a small periodicity show the highest DC?s, decreasing with increasing monolayer thickness in all cases. The maximum of DC shifted with decreasing film thickness to higher temperatures thus correlating with an increase of the out of plane lattice parameter. In this paper the mismatch between the stack respectivly the BSTO layers and the substrate has widely been discussed. In the case of BSTO the dielectric data can be qualitatively explained with the theory of strained films, developed mainly by Pertsev, under the assumption of a strain gradient in the thin film. Strain effects do also play an important role in ferroelectric multilayers as well as size and coupling effects between the monolayers. An adequate theory for the description of the dielectric behaviour of the ferroelectric superlattice produced during this research does yet not exist. Some thesis where pointed out, which effects have to be essentially included in to a consistent theory of ferroelectric multilayer. Some practical tips are also given, how to prepare monolayers and superlattices with very high DC and exellent hysteretic behaviour. / Es wurden (001) und (111) orientierte symmetrische BTO/STO-Multilagen auf niobdotierten STO-Einkristallen abgeschieden. Hierbei wurde sowohl die Gesamtschichtdicke, als auch deren Periodizität variiert. Zum Vergleich wurden weiterhin Ba0.5Sr0.5TiO3-Mischschichten unterschiedlicher Dicke präpariert. Aus den HRTEM und XRD Untersuchungen kann geschlossen werden, dass alle erhaltenen Schichten sowohl phasenrein als auch perfekt biaxial texturiert sind. Im Falle der (001) orientierten Multilagen konnten atomar scharfe Grenzflächen zwischen Einzellagen erhalten werden, wobei sich die Einzellagendicke bis auf eine Monolage (0.4 nm) reduzieren lässt. Aus der Schichtdickenabhängigkeit von d(001), dem mittleren out-of-plane Gitterparameter der Schicht, wird geschlossen, dass die Schichten auf den STO-Einkristallen Spannungsgradienten in den Schicht-normalen besitzen und an der Grenzfläche zum Substrat am stärksten verspannt sind. Die (111) orientierten Multilagen auf den STO-Einkristallen zeigen gegenüber den Schichten auf den (100) orientierten STO-Einkristallen eine deutlich erhöhte Interfacerauhigkeit. Vermutet wird, dass dies einerseits durch die andere kristallographische Orientierung der Wachstumsnormalen bedingt ist, weil damit jeweils keine geschlossenen SrO- bzw. BaO- und TiO3-Lagen ausgebildet werden. Andererseits zeigen die TEM-Aufnahmen eine deutliche Zunahme der Welligkeit der Einzellagen mit wachsendem Abstand vom Substrat, die rein mechanischen Effekten zugeschrieben wird. Die Verwölbung der Einzellagen könnte damit der Reduzierung der mechanischen Energie innerhalb des Systems dienen, wobei die Netzebenen dem Verlauf der Einzellagen folgen. Auf platinbeschichteten Siliziumsubstraten konnten erstmals phasenreine (111) fasertexturierte Mischschichten und BTO/STO-Multilagen abgeschieden werden. Grundlage hierfür war die Optimierung des Pt/Ti/SiO2/Si Schichtsystems hinsichtlich seiner thermischen Stabilität bis zu 800°C. Die Textur der Schichten wird von der Platingrundelektrode übernommen und deren Rauhigkeit teilweise verstärkt. Eine mechanische Verwölbung der Einzellagen konnte hier nicht beobachtet werden. Für die elektrischen Messungen wurden auf allen Schichten etwa 50 nm dicke Platinelektroden durch eine Hartmaske mittels Elektronenstrahlverdampfung im Hochvakuum bei etwa 300°C aufgebracht. Anschließend wurden die Schichten an Luft getempert, um das Sauerstoffdefizit, dass sich bei der Elektrodenabscheidung einstellt, auszugleichen. Die elektrischen Messungen zeichnen sich durch den sehr großen untersuchten Temperaturbereich aus. Temperaturabhängige Messungen im Bereich von 30-600 K finden sich für ferroelektrische Dünnschichten sehr selten in der Literatur und stellen für BTO/STO-Multilagen ein Novum dar. Auch die biasabhängige und teilweise auch temperaturabhängige Messung der Kapazität der Multilagen (C-V-Messungen) ist bisher einmalig. Durch die temperaturabhängigen Hysteresemessungen wurden Einblicke in den elektrischen Polungszustand der Schichten erhalten. Dadurch wird eine sinnvolle Interpretation der ε(T)-Kurven erst möglich. Der Vorteil der Integration des Polarisationsstromes unter Verwendung einer Dreieckspannung als Messsignal besteht in der direkten physikalischen Aussage der Strom-Spannungskurven über die Schaltspannung der Schichten.

Inspired by neurobiological systems, Spiking Neural Networks (SNNs) are gaining an increasing interest in the field of bio-inspired machine learning. Neurons, as central processing and short-term memory units of biological neural systems, are thus at the forefront of cutting-edge research approaches. The realization of CMOS circuits replicating neuronal features, namely the integration of action potentials and firing according to the all-or-nothing law, imposes various challenges like large area and power consumption. The non-volatile storage of polarization states and accumulative switching behavior of nanoscale HfO₂ - based Ferroelectric Field-Effect Transistors (FeFETs), promise to circumvent these issues. In this paper, we propose two FeFET-based neuronal circuits emulating the Integrate-and-Fire (I&F) behavior of biological neurons on the basis of SPICE simulations. Additionally, modulating the depolarization of the FeFETs enables the replication of a biology-based concept known as membrane leakage. The presented capacitor-free implementation is crucial for the development of neuromorphic systems that allow more complex features at a given area and power constraint.

No description available.

No description available.

Ferroelectric (FE) HfO₂-based thin films, which are considered as one of the most promising material systems for memory device applications, exhibit an adverse tendency for strong imprint. Manifestation of imprint is a shift of the polarization–voltage (P–V) loops along the voltage axis due to the development of an internal electric bias, which can lead to the failure of the writing and retention functions. Here, to gain insight into the mechanism of the imprint effect in La-doped HfO₂ (La:HfO₂) capacitors, we combine the pulse switching technique with high-resolution domain imaging by means of piezoresponse force microscopy. This approach allows us to establish a correlation between the macroscopic switching characteristics and domain time–voltage-dependent behavior. It has been shown that the La:HfO₂ capacitors exhibit a much more pronounced imprint compared to Pb(Zr,Ti)O₃-based FE capacitors. Also, in addition to conventional imprint, which evolves with time in the poled capacitors, an easily changeable imprint, termed as “fluid imprint”, with a strong dependence on the switching prehistory and measurement conditions, has been observed. Visualization of the domain structure reveals a specific signature of fluid imprint—continuous switching of polarization in the same direction as the previously applied field that continues a long time after the field was turned off. This effect, termed as “inertial switching”, is attributed to charge injection and subsequent trapping at defect sites at the film–electrode interface.

Hafnium oxide (HfO₂)-based ferroelectric field-effect transistor (FeFET) is an attractive device for nonvolatile memory. However, when compared to the well-established flash devices, the memory window (MW) of FeFETs reported so far is rather limited, which might be an obstacle to practical applications. In this article, we report on FeFETs fabricated in the 28-nm high-𝑘 metal gate (HKMG) bulk technology with 90 and 80 nm for the channel length and width, respectively, which show a large MW of nearly 3 V. This is achieved by adopting 20-nm-thick HfO₂ films in the gate stack instead of the usually employed 10-nm-thick films. We show that such a thickness increase leads to only a moderate increase of the switching voltages, and to a significantly improved resilience of the memory characteristics upon the parasitic charge trapping. The devices display a good retention at high temperatures and endure more than 10⁵ bipolar cycles, thus supporting this technology for a future generation of FeFET memories.

Recently, the proposal to use voltage amplification from ferroelectric negative capacitance (NC) to reduce the power dissipation in nanoelectronic devices has attracted significant attention. Homogeneous Landau theory predicts, that by connecting a ferroelectric in series with a dielectric capacitor, a hysteresis-free NC state can be stabilized in the ferroelectric below a critical film thickness. However, there is a strong discrepancy between experimental results and the current theory. Here, we present a comprehensive revision of the theory of NC stabilization with respect to scaling of material and device dimensions based on multi-domain Ginzburg–Landau theory. It is shown that the use of a metal layer in between the ferroelectric and the dielectric will inherently destabilize NC due to domain formation. However, even without this metal layer, domain formation can reduce the critical ferroelectric thickness considerably, limiting not only the range of NC stabilization, but also the maximum amplification attainable. To overcome these obstacles, the downscaling of lateral device dimensions is proposed as a way to prevent domain formation and to enhance the voltage amplification due to NC. These insights will be crucial for future NC device design and scaling towards nanoscale dimensions.

Neuron is the basic computing unit in brain-inspired neural networks. Although a multitude of excellent artificial neurons realized with conventional transistors have been proposed, they might not be energy and area efficient in large-scale networks. The recent discovery of ferroelectricity in hafnium oxide (HfO₂) and the related switching phenomena at the nanoscale might provide a solution. This study employs the newly reported accumulative polarization reversal in nanoscale HfO₂-based ferroelectric field-effect transistors (FeFETs) to implement two key neuronal dynamics: the integration of action potentials and the subsequent firing according to the biologically plausible all-or-nothing law. We show that by carefully shaping electrical excitations based on the particular nucleation-limited switching kinetics of the ferroelectric layer further neuronal behaviors can be emulated, such as firing activity tuning, arbitrary refractory period and the leaky effect. Finally, we discuss the advantages of an FeFET-based neuron, highlighting its transferability to advanced scaling technologies and the beneficial impact it may have in reducing the complexity of neuromorphic circuits.

Spontaneous Raman spectroscopy (SR) is a versatile method for analysis and visualization of ferroelectric crystal structures, including domain walls. Nevertheless, the necessary acquisition time makes SR impractical for in situ analysis and large scale imaging. In this work, we introduce broadband coherent anti-Stokes Raman spectroscopy (B-CARS) as a high-speed alternative to conventional Raman techniques and demonstrate its benefits for ferroelectric domain wall analysis. Using the example of poled lithium niobate, we compare the spectral output of both techniques in terms of domain wall signatures and imaging capabilities. We extract the Raman-like resonant part of the coherent anti-Stokes signal via a Kramers–Kronigbased phase retrieval algorithm and compare the raw and phase-retrieved signals to SR characteristics. Finally, we propose a mechanism for the observed domain wall signal strength that resembles a Cerenkov-like behavior, in close analogy to domain wall signatures obtained by secondharmonic generation imaging.We, thus, lay here the foundations for future investigations on other poled ferroelectric crystals using B-CARS.