Orientador: Lisandro Pavie Cardoso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-06T02:28:50Z (GMT). No. of bitstreams: 1 DosSantos_AdenilsonOliveira_D.pdf: 4430176 bytes, checksum: 30089a1aff00bfd64635f23601dd3039 (MD5) Previous issue date: 2006 / Resumo: Como primeira contribuição deste trabalho, as varreduras Renninger (VR) da difração múltipla de raios-X foram empregadas no estudo da transição de fase estrutural do Sal de Rochelle (monoclínica-ortorrômbica) induzida por temperatura. Devido ao caráter tridimensional e sensibilidade dessa técnica acompanhamos as deformações na célula unitária com a temperatura, através do deslocamento dos picos secundários com maior sensibilidade. Foi possível determinar os parâmetros de rede e simular cada VR com o programa UMWEG. Os picos secundários de 4-feixes ( 0 0 0 ) ( 0 0 10 )( 1 0 9 )(1 0 1 ) e (0 0 0 )( 0 0 10 )( 2 3 0 )(2 3 10 ) da fase ortorrômbica foram medidos na VR para 24ºC (Tc). Os coeficientes de expansão térmica do sal de Rochelle também foram obtidos, e estão em bom acordo com a literatura. Como segunda contribuição, usamos a VR na determinação precisa dos parâmetros de rede dos materiais magnetocalóricos PrAl2, NdAl2e PrNi5. Implementamos uma rotina baseada na simulação da VR com o programa UMWEG, através da qual escolhe-se o comprimento de onda adequado para a medida de picos secundários muito sensíveis à distorção na célula unitária. Na aplicação no caso do PrAl2 usamos o caso de 4-feixes ( 0 0 0 )( 6 0 0 )( 1 3 7 )( 7 3 7 ) com 2.... =6.663(3)º e obtivemos com grande precisão a=8,03332(7)Å. O efeito de polimento mecânico na superfície dessa amostra foi analisado pelo mapeamento das reflexões de superfície (BSD) e o comportamento mosaico do cristal foi evidenciado. Nas outras medidas, como utilizamos a mesma geometria da estação XRD1, não foi possível obter a melhor condição para a rotina apresentada, mas foram obtidos para o NdAl2 (a=7,9972(5)Å) e PrNi5 (a=4,9590(8)Å e c=3,9794(5)Å) com boa precisão. Outra contribuição foi o estudo do efeito de campo elétrico no cristal orgânico MBANP através do monitoramento por curvas de rocking da reflexão ( 10 0 0 ). Observou-se uma histerese "asa de borboleta", ainda não observada em cristais orgânicos, e sem modelo para cristais monoclínicos. Cálculos usando mecânica quântica para moléculas isoladas de MBANP mostram que as principais características da forma de histerese podem ser explicadas em termos de mudanças induzidas pelo campo elétrico nos perfis de carga e na geometria de moléculas isoladas de MBANP / Abstract: As the first contribution of this work, the Renninger scan (RS) of the X-ray multiple diffraction were used in the study of the Rochelle salt structural phase transition (monoclinic-orthorhombic) induced by temperature. Due to its three-dimensional feature and sensitivity of the technique was possible to follow the unit cell deformations with temperature, through the angular shifts of the most sensitive secondary peaks. We were able to determine the lattice parameters as well as to simulate each RS by using the UMWEG program. The ( 0 0 0 )( 0 0 10 )( 1 0 9 )(1 0 1 ) and ( 0 0 0 )( 0 0 10 )( 2 3 0 )(2 3 10 ) 4-beam peaks for the orthorhombic phase were measured in the RS at 24ºC (Tc). Rochelle salt thermal expansion coefficients were also obtained in good agreement with literature values. As the second contribution, we have also used the RS in the precise lattice parameter determination of PrAl2 , NdAl2 and PrNi5 magnetocaloric materials. We have implemented a routine based on the RS simulation (UMWEG program), through which, one can choose the adequate wavelength to measure the most sensitive secondary peaks to the unit cell variations. The application of thid method in the case of PrAl 2, has allowed to measure the ( 0 0 0 )( 6 0 0 )( 1 3 7 )( 7 3 7 ) 4- beam case that presents 2.... =6.663(3)º and hence, to determine a=8.03332(7)Å, with high precision. PrAl2surface polishing effect was also analyzed by the secondary surface peak (BSD) mapping, through which, the crystal mosaic behavior was exhibited. For the other crystals, the same LNLS geometry was used and the best condition to applying the routine could not be obtained, however good precision lattice parameters were obtained for NdAl2 (a=7.9972(5)Å) and PrNi5 (a= 4.9590(8)Å and c= 3.9794(5)Å). Another contribution to the study of the electric field application in the MBANP organic crystal through the monitoring of ( 10 0 0 ) rocking curves was performed. It was observed a (butterfly wing) hysteresis, not yet observed for organic crystals and with no model suggested for monoclinic crystals. Quantum mechanical calculations on isolated MBANP molecules show that the main features of the hysteresis shape can be explained in terms of field-induced changes in the charge profiles and geometry of isolated MBANP molecules / Doutorado / Física da Matéria Condensada / Doutor em Ciências

One third of world’s total energy is used in production of electricity and one fifth of the total electricity produced in the world is used in lighting. Hence, the materials that have high potential in the field of photovoltaic’s and photoluminescence have recently drawn special attention to meet the ever increasing energy demands. In this thesis, we have studied a few materials that hold tremendous promises in fabricating photovoltaics and photoluminescent devices. Any ferroelectric material is an efficient solar energy converter as it contains an the intrinsic dipolar field which can effectively separate the photo excited electron and hole. We have developed a few materials which possess inherent polarization efficiently absorb over a wide portion of the solar spectrum and hence can find application in the field of photovoltaics. Secondly, we also dealt with semiconductor nonmaterial’s which are technologically very important owing to their improved photoluminescence properties. We tried to improve their light emitting efficiency by engineering crystal structure in nanometer length scales. The thesis deals with such advanced energy materials and is divided in seven chapters. Chapter 1 provides a brief introduction to the fundamental concepts that are relevant in the subsequent chapters. The chapter is started with a brief scenario of current status of energy production and its usage. Next, we have discussed the prospects of ferroelectric materials in photovoltaic devices. This is followed by a brief background on ferroelectricity and related properties which we have studied subsequently. At the end of this chapter a brief overview of photoluminescence properties in semiconductor nonmaterial’s is presented. In this section we have addressed the particular issues that need to be taken care of in order to improve their light emission properties. Chapter 2 describes different experimental and theoretical methods that have been employed to carry out different studies presented in the thesis. Chapter 3 addresses the possibility of employing BaTiO3 (BTO) based composite perovskite oxides as a potent photovoltaic material. It is known that BTO can produce photocurrent upon excitation with suitable light source. However, inability of BTO to absorb sufficient sunlight owing to its near UV band gap prevents to make use of this material in photovoltaic devices. In order to reduce the band gap we have tried to tune the electronic structure at the band edge by doping non-d0 transition metal ions at Ti site. As it is known in the literature an isovalent substitution of Ti4+ stabilizes non-polar phase of BTO we employed a co-doping strategy to substitute tetravalent Ti with equal percentage of a trivalent and a pentavalent metal ion. Keeping in mind off-centering of Ti4+ is primary reason behind the large ferroelectric polarization of BTO, a judicious choice of co-dopant was necessary to minimize reduction of polarization due to replacement of Ti. We have found at least two pairs of co-dopants, namely Mn3+-Nb5+ and Fe3+-Nb5+ which at low doping concentration ( < 10%) effectively reduces the band gap of BTO without affecting its polarization to a large extent. We systematically increase the doping concentration of both the pair of dopants and found Mn3+-Nb5+ pair is more efficient over Fe3+-Nb5+ both in terms of reducing band gap and retaining the polarization of BTO. We have characterized the ferroelectric nature of all the doped compositions with the help of dielectric, polarization and pyroelectric measurements. We have also performed first principle density functional theory (DFT) calculations for an equivalent doped composition and addressed the nature of modulations of electronic structure at the band edges which is responsible for such large reduction of band gap. Chapter 4 deals with composite perovskite materials which posses large tetragonal distortions with reduced optical band gaps. Here we have exploited Cu-Nb and Cu-Ta pair which upon complete substitution of Ti of BTO leads to composite perovskites with enhanced tetragonal distortion of the perovskite lattice. For two resultant compositions, namely BaCu1/3Nb2/3O3 and BaCu 1/3Ta2/3O3 we have characterized the optical and ferroelectric properties. We found though these material possess small band gap (∼ 2 eV), these are not ferroelectric in nature. Results of second harmonic generation measurements and refinement of powder X-ray diffraction both establish Centro symmetric nature of these materials. We infer from these results that presence of large tetragonal distortion is a result of symmetric Jahn-Teller type distortion of Cu2+ and not due to off-centering of any of the metal ions in their MO6 octahedral geometries. In Chapter 5, we have considered the material SrTiO3 (STO) which is stable in cubic paraelectric phase at room temperature. But at the same time this material is considered as an incipient ferroelectric due to presence of an active polar vibrational mode which does not become completely soft even at temperature close to 0 K. While this polar vibrational mode can easily be frozen by making substitution at Sr site, a similar attempt by making substitution at Ti site failed earlier. Keeping in mind Ti is easier to substitute than Sr we employed same co-doping strategy that we have considered in Chapter 3. We found Mn- Nb and Mn-Ta co-dopants at low doping concentration are extremely useful in transforming incipient ferroelectric STO into a dipolar glass. We have characterized the glassy dipolar property of doped STO with the help of tem-perature dependent dielectric response of these material. At the same time we found these co-doped STO possess enhanced static dielectric constant at room temperature with favourable dielectric loss values in comparison to pure STO. We have also ad-dressed the origin of a glassy dipolar state with the help of DFT calculation performed on equivalent doped composition that we have considered for our experiments. In Chapter 6, we have considered another incipient ferroelectric material TiO2 in rutile phase which also possess polar vibrational mode at temperature close to 0 K. A lattice strain along the polar vibrational mode make symmetric non-polar structure unstable with respect to the distorted polar structure. In this context, we found two particular compositions FeTiTaO6 and CrTiTaO6 that are also stable in rutile phases at room temperature but possess similar strain due to presence of larger Fe or Cr and Ta in rutile lattice. Considering the fact these two composite rutile oxides are relaxer ferroelectric in nature, we critically evaluated the effect of the particular kind of strain that these materials introduce in rutile lattice. We also characterized relaxor ferroelectric property and optical band gap of these materials and commented on the potential of these materials in exploiting them in photovoltaic devices. Chapter 7 presents a unique strategy of making use of crystal defects in improving photoluminescent properties of semiconductor nanocrystals. We have shown defects when introduced in nanocrystals in a controlled protected manner efficiently overcome the problem of self absorption which is known to reduce quantum efficiency of emit-ted light. Controlling synthesis conditions we separately prepared CdS nanocrystals with and without intergrowth defects. We characterized the presence of the intergrowth defect with the help of high resolution transmission electron microscope (HRTEM) image. We have also characterized Stokes’ shifted PL emission and ultrafast charge carrier dynamics of these NCs with intergrowth defects. To support these experimental findings we have computed the electronic structures of model nanoclusters possessing similar intergrowth defects that has been observed in HRTEM images. We find that the presence of defects in a nanocluster particularly affect the position of the band edge. However our joint density of state calculation shows that contribution of these defect states to an absorption spectra is negligible. Thus presence of defect states at band edge ensures a Stokes’ shifted emission without affecting the position of absorption. In a separate section of this chapter we have shown apart from intergrowth defects presence of twin boundary also provide similar mid-gap states that can alter its’ optical proper-ties to large extent. In summary, we have studied a few bulk and nano-materials which can show improved photovoltaic and photoluminescence property. We investigated effect of external dopant ions on a classical ferroelectric material BaTiO3 and two incipient ferroelectric materials SrTiO3 and rutile TiO2. We have also shown that efficient defect engineering could be extremely useful in improving photoluminescent property of CdS nanocrystals which is a prototype of II-VI semiconductor nanomaterials. In a separate Appendix Chapter, we have shown an easy and efficient way to suppress coffee ring effect which takes place universally when a drop of colloidal suspension is dried on a solid substrate. We have shown temporary modification of hydropho-bicity of a glass substrate not only can suppress the coffee ring effect but also leaves the particle in a highly ordered self-assembled phase after completion of drying process

In recent years bismuth-based, layer-structured perovskites such as SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT) have been investigated extensively, because of their potential use in ferroelectric random access memories (FeRAMs). In comparison with non-layered perovskite ferroelectrics such as Pb(Zr,Ti)O3 (PZT), these offer several advantages such as fatigue free, lead free, low operating voltages and most importantly their ferroelectric properties are independent of film thickness in the 90 to 500 nm range. For FeRAM device applications, large remnant polarization (Pr), low coercive field (Ec) accompanied by high Curie temperature (Tc) are required for better performance and reliable operation. Much effort has been made to improve the ferroelectric properties of SBN and SBT ceramics by doping on A or B sites. It was known in the literature that partial substitution of Sr2+ by Bi3+ ions in SBN and SBT would increase the Curie temperature and improve the dielectric properties. The focus of the investigations that were taken up was to improve the electrical, dielectric and ferroelectric characteristics of SrBi2Nb2O9 ceramics. It was reported that the ferroelectric and nonlinear optical properties of LiNbO3 and LiTaO3 could be improved when vanadium, the lightest element in group V of the periodic table is substituted for Nb or Ta along with Li and three oxygens. It is with this background the investigations have been taken up to see whether one can extend the same argument to the Aurivillius family of oxides. Therefore, the central theme of the present investigations aimed at substituting Nb5+ by a smaller cation V5+ in SBN and study its influence on the formation temperature, sinterability, structural and microstructural characteristics apart from its physical properties. Recently the optical properties of this material have been recognized to be important from the optical device point of view. Unfortunately single crystal growth of vanadium doped SBN was hampered because of the bismuth and vanadium loss (high volatility) observed in the process of growth. One of the routes that attracted our attention has been the glass-ceramic. It would be interesting to visualize the behavior of crystallites of nano/micrometer size embedded in a glass matrix as these crystals were known to give rise to exotic properties. One of the crucial steps in the process of fabrication of a glass nanocrystalcomposite system in which crystalline phases have symmetries that would eventually give rise to basic non - centrosymmetric properties such as piezoelectric, pyroelectric and Pockels effects, has been to choose a compatible matrix material associated with easy glass forming capability and the ability to evenly disperse dipolar defects within itself. Recent investigations into strontium borate SrB4O7 (SBO), lithium borate Li2B4O7 (LBO) glasses indicated that LBO by virtue of its favorable structure, thermal and optical properties would form a suitable host glass matrix for dispersing layer structured ferroelectric oxides belonging to the Aurivillius family of oxides. Since lithium borate has wide transmission window, it was worth making an attempt to fabricate optical composite of Li2B4O7 (LBO) and vanadium doped SrBi2Nb2O9 (SBVN) and to study its structural, dielectric, pyroelectric, ferroelectric and optical properties. Therefore the present thesis reports detailed investigations into the effect of vanadium doping on the structural and various physical properties of an n = 2 member of the Aurivillius family of oxides in the polycrystalline form and novel glass composites comprising nano/microcrystallites of this phase. Chapter 1 comprises a brief introduction to the dielectric, pyroelectric, ferroelectric and nonlinear optical properties of materials. In addition to the principles and phenomena, the material aspects of these important branches of physics are discussed. It also forms a preamble to the glasses, criteria for glass formation, glass – ceramics and subsequently ferroelectric and nonlinear optical effects that were observed in glasses and glass - ceramics. Chapter 2 describes the material fabrication techniques adopted to prepare polycrystalline and grain – oriented ceramics, glasses and glass nanocrystalcomposites. The details of various structural, dielectric, pyroelectric, ferroelectric and optical measurement techniques employed to characterize these materials are also included. Chapter 3 discloses the fabrication of strontium bismuth niobate ceramics and their characterization for dielectric and impedance properties. The dielectric properties of strontium bismuth niobate ceramics have been modeled based on Jonscher’s Universal formalism. The coefficients of the Jonscher’s expression, exponent n(T) undergoes a minimum and A(T) exhibits a peak at the Curie temperature, Tc (723K). A strong low frequency dielectric dispersion (LFDD) associated with an impedance relaxation has been found to exist in these ceramics in the temperature range 573 - 823K. The Z′′ of the AC complex impedance showed two distinct slopes in the frequency range 100Hz-1MHz suggesting the existence of two dispersion mechanisms. The exponents m and n were obtained from the curve fitting. The exponent n was found to exhibit a minimum at the Curie temperature, Tc (723K) whereas the m was temperature independent. Chapter 4 deals with the fabrication of vanadium doped SrBi2Nb2O9 ceramics and their characterization for microstructural, dielectric, pyroelectric and ferroelectric properties. The average grain size of the SrBi2Nb2O9 (SBN) ceramic containing V2O5 was found to increase with increase in V2O5 content. The dielectric constant (εr) as well as the dielectric loss (D) increased with increase in grain size (6µm-17µm). The pyroelectric coefficient was found to be positive at 300K and showed an increasing trend with increasing grain size. Interestingly, the SrBi2(Nb0.7V0.3)2O9-δ ceramics consisting of 17µm sized grains showed higher remnant polarization (Pr) and lower coercive field (Ec) than those with grains of 7µm. Chapter 5 deals with the dielectric properties which were studied in detail in the 100Hz to 1MHz frequency range at various temperatures (300 – 823 K) for undoped and vanadium (10 mol%) doped SrBi2Nb2O9 (SBVN10) ferroelectric ceramics. A strong low frequency dielectric dispersion was encountered in these ceramics in the 573 – 823 K temperature range. The dielectric constants measured in the wide frequency and temperature ranges for both the samples were found to fit well to the Jonscher’s dielectric dispersion relations. The dielectric behavior of SBN and SBVN10 ceramics was rationalized using the impedance and modulus data. The electrical conductivity studies of layered SrBi2(Nb1-xVx)2O9-δ ceramics with x lying in the range 0 to 0.3 (30 mol%) were centered in the 573 – 823K temperature range as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D0) and the mean free path (a) were calculated using Rice and Roth formalism. The conductivity parameters such as ion hopping rate (ωp) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The afore mentioned microscopic parameters were found to be V2O5 content dependent in SrBi2(Nb1-xVx)2O9-δceramics. Chapter 6 describes the fabrication of partially grain – oriented SrBi2(Nb1-xVx)2O9-δ (0 ≤x≤3.0 in molar ratio) ceramics and characterization for their structural, microstructural, dielectric, pyroelectric and ferroelectric properties. The grain – orientation factor and the microstructural features were studied by XRD and scanning electron microscopy as a fuction of sintering temperature and V2O5 content. The dielectric constant measured along the direction parallel and perpendicular to the pressing axis has shown a significant anisotropy. The pyroelectric and ferroelectric properties were superior in the direction perpendicular to the pressing axis (polar) to that in the parallel direction. The fabrication and characterization details of (100 – x) (Li2B4O7) – x (SrO - Bi2O3 - 0.7 Nb2O5 – 0.3 V2O5) (10 ≤ x ≤ 60, in molar ratio) glasses and glass nanocrystal composites are dealt within Chapter 7. The nanocrystallization of strontium bismuth niobate doped with vanadium (SrBi2(Nb0.7V0.3)2O9-δ(SBVN)) has been demonstrated in Li2B4O7 glasses. The glassy nature of the as – quenched samples was established by differential thermal analyses (DTA). The amorphous nature of the as – quenched glasses and crystallinity of glass nanocrystal composites were confirmed by X – ray powder diffraction studies. High resolution transmission electron microscopy (HRTEM) of the glass nanocrystal composites (heat – treated at 783K/6h) confirm the presence of nano rods of SBVN embedded in Li2B4O7 glass matrix. Chapter 8 presents the physical properties of the glasses and glass nanocrystal composites. Dielectric constant of both the as – quenched and glass nanocrystal composites was found to increase with increase in the composition, whereas the loss was observed to decrease with increasing SBVN composition. Different dielectric mixture formulae were employed to analyze the dielectric properties of the glass nanocrystal composite. The electrical behaviour of the glasses and glass nanocrystal composites was rationalized using impedance spectroscopy. The observed pyroelectric response and ferroelectric hysteresis of these composites confirmed the polar nature. Various optical parameters such as optical band gap (Eopt), Urbach energy (∆E), refractive index (n), optical dielectric constant (ε′∞) and ratio of carrier concentration to the effective mass (N/m*) were determined. The effects of composition of the glasses and glass nanocrystal composites on these parameters were studied. Transparent glasses embedded with nanocrystallites of SBVN exhibited intense second harmonic signals in transmission mode when exposed to IR laser light at λ = 1064 nm. The thesis ends with a summary of the important findings and conclusions.

Ferroelectric HfO₂ attracts a huge amount of attention not only for memory and negative capacitance, but also for programmable logic including memory-in-logic and neuromorphic applications. However, the understanding of material fundamentals still needs to be improved. This paper gives material fundamentals and new insights to this ferroelectric material for future device applications. In particular, the key role of dopants, effects of the interface on the ferroelectric phase, and a detailed discussion of the switching kinetics are of central focus. Based on material properties newly obtained, we discuss opportunities of ferroelectric HfO₂ for device applications.

To overcome the fundamental limit of the transistor subthreshold swing of 60 mV/dec at room temperature, the use of negative capacitance (NC) in ferroelectric materials was proposed [1]. Due to the recent discovery of ferroelectricity in CMOS compatible HfO₂ and ZrO₂ based thin films [2], [3], the promise of ultra-low power steep-slope devices seems within reach. However, concerns have been raised about switching-speed limitations and unavoidable hysteresis in NC devices [4], [5]. Nevertheless, it was shown that NC effects without hysteresis can be observed in fast pulsed voltage measurements on ferroelectric/dielectric capacitors [6], which was recently confirmed using ferroelectric Hf₀.₅ Zr₀.₅ O₂[7], [8]. While in these works only the integrated charge after each pulse was studied, here we investigate for the first time if the transient voltage and charge characteristics are also hysteresis-free.

The aim of the work was a processing of ceramic material based on BST for ferroelectric application. Wet chemical techniques based on precipitations and sol-gel methods with ultrasound, hydrothermal or mechanochemical treatment supporting deaglomeration and reducing particle growth were used for BST ceramic powder synthesis. Suitable powders were selected by the evaluation of particle morphology, size and agglomeration, from these powders BST bulk ceramic with defined phase composition and morphology applicable for ferroelectric applications was prepared. It was found, that the most suitable method for preparation of phase pure and nanosized BST powder was sol-gel synthesis with solvothermal treatment (200 °C/48 h). Ceramic with relative density of 85 % TD and with the average grain size of about 1, 22 µm was prepared by pressing and sintering of the powder synthesized by the sol-gel method.

Hafnium oxide is widely used for resistive switching devices, and recently it has been discovered that ferroelectricity can be established in (un-)doped hafnium oxide as well. Previous studies showed that both switching mechanisms are influenced by oxygen vacancies. For resistive switching, typically amorphous oxide layers with an asymmetric electrode configuration are used to create a gradient of oxygen vacancies. On the other hand, ferroelectric switching is performed by having symmetric electrodes and requires crystalline structures. The coexistence of both effects has recently been demonstrated. In this work, a detailed analysis of the reversible interplay of both switching mechanisms within a single capacitor cell is investigated. First, ferroelectric switching cycles were applied in order to drive the sample into the fatigued stage characterized by increased concentration of oxygen vacancies in the oxide layer. Afterwards, a forming step that is typical for the resistive switching devices was utilized to achieve a soft breakdown. In the next step, twofold alternation between the high and low resistance state is applied to demonstrate the resistive switching behavior of the device. Having the sample in the high resistance state with a ruptured filament, ferroelectric switching behavior is again shown within the same stack. Interestingly, the same endurance as before was observed without a hard breakdown of the device. Therefore, an effective sequence of ferroelectric—resistive—ferroelectric switching is realized. Additionally, the dependence of the forming, set, and reset voltage on the ferroelectric cycling stage (pristine, woken-up and fatigued) is analyzed giving insight into the physical device operation.

Transient negative capacitance effects in epitaxial ferroelectric Pb(Zr₀.₂Ti₀.₈)O₃ capacitors are investigated with a focus on the dynamical switching behavior governed by domain nucleation and growth. Voltage pulses are applied to a series connection of the ferroelectric capacitor and a resistor to directly measure the ferroelectric negative capacitance during switching. A time-dependent Ginzburg-Landau approach is used to investigate the underlying domain dynamics. The transient negative capacitance is shown to originate from reverse domain nucleation and unrestricted domain growth. However, with the onset of domain coalescence, the capacitance becomes positive again. The persistence of the negative capacitance state is therefore limited by the speed of domain wall motion. By changing the applied electric field, capacitor area or external resistance, this domain wall velocity can be varied predictably over several orders of magnitude. Additionally, detailed insights into the intrinsic material properties of the ferroelectric are obtainable through these measurements. A new method for reliable extraction of the average negative capacitance of the ferroelectric is presented. Furthermore, a simple analytical model is developed, which accurately describes the negative capacitance transient time as a function of the material properties and the experimental boundary conditions.

The properties of ferroelectric materials, which were discovered almost a century ago¹ , have led to a huge range of applications, such as digital information storage² , pyroelectric energy conversion³ and neuromorphic computing⁴⁻⁵ . Recently, it was shown that ferroelectrics can have negative capacitance⁶⁻¹¹, which could improve the energy efficiency of conventional electronics beyond fundamental limits¹²⁻¹⁴. In Landau–Ginzburg–Devonshire theory¹⁵⁻¹⁷, this negative capacitance is directly related to the doublewell shape of the ferroelectric polarization–energy landscape, which was thought for more than 70 years to be inaccessible to experiments¹⁸. Here we report electrical measurements of the intrinsic double-well energy landscape in a thin layer of ferroelectric Hf₀.₅Zr₀.₅O₂. To achieve this, we integrated the ferroelectric into a heterostructure capacitor with a second dielectric layer to prevent immediate screening of polarization charges during switching. These results show that negative capacitance has its origin in the energy barrier in a double-well landscape. Furthermore, we demonstrate that ferroelectric negative capacitance can be fast and hysteresis-free, which is important for prospective applications¹⁹. In addition, the Hf₀.₅Zr₀.₅O₂ used in this work is currently the most industry-relevant ferroelectric material, because both HfO₂ and ZrO₂ thin films are already used in everyday electronics²⁰. This could lead to fast adoption of negative capacitance effects in future products with markedly improved energy efficiency.

Manifold research has been done to understand the detailed mechanisms behind the performance instabilities of ferroelectric capacitors based on hafnia. The wake-up together with the imprint might be the most controversially discussed phenomena so far. Among crystallographic phase change contributions and oxygen vacancy diffusion, electron trapping as the origin has been discussed recently. In this publication, we provide evidence that the imprint is indeed caused by electron trapping into deep states at oxygen vacancies. This impedes the ferroelectric switching and causes a shift of the hysteresis. Moreover, we show that the wake-up mechanism can be caused by a local imprint of the domains in the pristine state by the very same root cause. The various domain orientations together with an electron trapping can cause a constriction of the hysteresis and an internal bias field in the pristine state. Additionally, we show that this local imprint can even cause almost anti-ferroelectric like behavior in ferroelectric films.