HfO2 and ZrO2 have increasingly drawn the interest of researchers as lead-free and silicon technology-compatible materials for ferroelectric, pyroelectric, and piezoelectric applications in thin ...films such as ferroelectric field-effect transistors, ferroelectric random access memories, nanoscale sensors, and energy harvesters. Owing to the environmental regulations against lead-containing electronic components, HfO2 and ZrO2 offer, along with AlN, (K,Na)NbO3- and (Bi0.5Na0.5)TiO3-based materials, an alternative to Pb(Zr x Ti1–x )O3-based materials, which are the overwhelmingly used ceramics in industry. HfO2 and ZrO2 thin films may show field-induced phase transformation from the paraelectric tetragonal to the ferroelectric orthorhombic phase, leading to a change in crystal volume and thus strain. These field-induced strains have already been measured experimentally in pure and doped systems; however, no systematic optimization of the piezoelectric activity was performed, either experimentally or theoretically. In this screening study, we calculate the ultimate size of this effect for 58 dopants depending on the oxygen supply and the defect incorporation type: substitutional or interstitial. The largest piezoelectric strain values are achieved with Yb, Li, and Na in ZrO2 and exceed 40 pm V–1 or 0.8% maximal strain, which exceeds the best experimental findings by a factor of 2. Furthermore, we discovered that Mo, W, and Hg make the polar-orthorhombic phase in the ZrO2 bulk stable under certain circumstances, which would count in favor of these systems for the ceramic crystallization process. Our work guides the development of the performance of a promising material system by rational design of the essential mechanisms so as to apply it to unforeseen applications.
The recent progress in ferroelectricity and antiferroelectricity in HfO2‐based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as ...Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non‐volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si‐compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si‐doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferroelectricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm−2, and their coercive field (≈1–2 MV cm−1) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field‐effect‐transistors and three‐dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid‐state‐cooling, and infrared sensors.
Recent progress in ferroelectricity and antiferroelectricity in HfO2‐based thin films is comprehensively reviewed. The properties of ferroelectric HfO2‐based films, different from those of conventional ferroelectrics, are believed to solve the problems of conventional ferroelectrics in non‐volatile memory. Moreover, the pyroelectricity of antiferroelectric films is expected to be useful for various applications, including energy harvesting and storage, solid‐state cooling, and infrared sensors.
Thin film metal–insulator–metal capacitors with undoped HfO2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and ...annealing temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence X‐ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses in the 10–50 nm range and a negligible non‐ferroelectric monoclinic phase fraction. Sputtering HfO2 with additional oxygen during the deposition decreases the remanent polarization. Overall, the impact of oxygen vacancies and interstitials in the HfO2 film during deposition and annealing is correlated to the phase formation process.
Oxygen vacancies stabilize ferroelectric phase in HfO2. Undoped sputtered HfO2 shows good ferroelectric properties over a wide thickness range. Changing the oxygen ambient during deposition changes the amount of oxygen vacancies and influences the nucleation during deposition. During deposition, the formed tetragonal phase nuclei stabilize after annealing the metastable ferroelectric orthorhombic phase.
The outstanding remanent polarization of 40 µC cm–2 reported for a 10 nm thin La:HfO2 film in 2013 has attracted much attention. However, up to now, no explanation for this large remanent ...polarization has been presented. Density functional theory and X‐ray diffraction are used to shine light onto three major aspects that impact the macroscopically observed remanent polarization: phase fraction, spontaneous polarization, and crystallographic texture. Density functional theory calculations show that the spontaneous polarization (Ps) of La:HfO2 is indeed a bit larger than for other HfO2‐ or ZrO2‐based compounds; however, the Ps is not large enough to explain the observed differences in remanent polarization. While neither phase fractions nor spontaneous polarization nor strain are significantly different from those in other HfO2 films, a prominent 020/002 texture distinguishes La doped from other HfO2‐based ferroelectric films. Angular‐dependent diffraction data provide a pathway to calculate the theoretically expected remanent polarization, which is in agreement with the experimental observations. Finally, an interplay of the in‐plane strain and texture is proposed to impact the formation of the ferroelectric phase during annealing. Further aspects of the special role of La as a dopant are collected and discussed to motivate future research.
The origin of the large remanent polarization in La:HfO2 thin films is investigated. Density functional theory calculations suggest a larger spontaneous polarization for the incorporation of La compared to other dopants. In‐depth X‐ray diffraction analysis together with Rietveld refinement allows for the derivation of the stress state of the film and the role of crystallographic texture.
Fluorite-structured hafnium and zirconia require different, complementary characterization methods to identify the numerous metastable phases. This is because of the many possible positions of the ...oxygen ions, which are difficult to observe directly.
Ab initio
simulations are useful to probe the corresponding XRD, Raman, and infrared spectra for fingerprints. However, the predictive power of theoretical methods is limited both by model errors and by boundary conditions such as defects, stresses, and morphology that are difficult to detect. We first consider the calculation of Raman and infrared spectra of the most interesting undoped phases of HfO
2
and ZrO
2
, compare the results with known results, and discuss the uncertainties. Next, we consider the possibilities of classifying the phases using X-ray diffraction. To this end, we introduce the effects of doping, which increases the uncertainty due to structural disorder. For illustration, we examine a large data set of doped structures obtained with
ab initio
calculations. To make an unbiased assignment of phases, we use machine learning methods with clusters. The limits of X-ray diffraction spectroscopy are reached when phase mixtures are present. Resolution of single-phase polycrystalline samples may only be possible here if these three characterization methods are used.
The phase composition of HZO thin films is critical for the ferroelectric and electrical properties of the films and the devices they are integrated into. Optimization is a major challenge since the ...phase formation depends significantly on many influencing variables that are only partially understood so far. The Curie temperature is identified as an important parameter for understanding the behavior, since it depends sensitively on Zr content, the density of oxygen‐related defects, layer thickness, and external stress. A two‐step process, phase formation by pure kinetic transformation followed by nucleation, is proposed for phase formation. This is necessary because nucleation theory alone cannot explain the experimentally observed dependence on oxygen content. The classical nucleation model is modified at two crucial points. First, the polycrystalline structure is incorporated which allows the size effect to be implemented. Furthermore, the interface energies between the child and parent phase, which result from static ab initio calculations, are rescaled from dynamical effects. The resulting model is used to calculate the phase fractions during thermal processing. The results for the most important influencing variables are discussed and compared with experimental results. The causes of the undesired monoclinic phase are further analyzed.
The new phase formation model contains two steps that allow phase transformation: In the first step, nuclei in the amorphous deposit can transform kinetically without interfacial effects. In the second step, grains can transform according to a modified nucleation model depending on the interfacial energy. This results in a Curie‐temperature depending on grain size, composition, and oxygen content of the film, coherent with experimental results.
The discovery of ferroelectric properties of binary oxides revitalized the interest in ferroelectrics and bridged the scaling gap between the state-of-the-art semiconductor technology and ...ferroelectric memories. However, before hitting the markets, the origin of ferroelectricity and in-depth studies of device characteristics are needed. Establishing a correlation between the performance of the device and underlying physical mechanisms is the first step toward understanding the device and engineering guidelines for a novel, superior device. Therefore, in this paper a holistic modeling approaches which lead to a better understanding of ferroelectric memories based on hafnium and zirconium oxide is addressed. Starting from describing the stabilization of the ferroelectric phase within the binary oxides via physical modeling the physical mechanisms of the ferroelectric devices are reviewed. Besides, limitations and modeling of the multilevel operation and switching kinetics of ultimately scaled devices as well as the necessity for Landau–Khalatnikov approach are discussed. Furthermore, a device-level model of ferroelectric memory devices that can be used to study the array implementation and their operational schemes are addressed. Finally, a circuit model of the ferroelectric memory device is presented and potential further applications of ferroelectric devices are outlined.
From first principles, the energy and volume of metastable phases of ZrO2 and HfO2 are calculated. At low energy, two inequivalent, nonpolar, orthorhombic phases of the same space group Pbca number ...P61 are found.
From first‐principles investigation, two inequivalent, nonpolar, orthorhombic phases of the same space group Pbca (no. 61) are found to exist. Their energy is below the ferroelectric Pca21 (no. 29) and they may reduce the remanent polarization of HZO thin films.
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