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Understanding the stress effect of TiN top electrode on ferroelectricity in Hf0.5Zr0.5O2 thin films
Journal of Applied Physics
Abstract
We conducted a comprehensive investigation on the influence of TiN thickness and stress on the ferroelectric properties of Hf0.5Zr0.5O2 thin films. TiN top electrode layers with varying thicknesses of 2, 5, 10, 30, 50, 75, and 100 nm were deposited and analyzed. It was observed that the in-plane tensile stress in TiN films increased with the thickness of the TiN top electrode. This is expected to elevate the tensile stress in the Hf0.5Zr0.5O2 film, consequently leading to an enhancement in ferroelectric polarization. However, the effect of stress on the ferroelectric behavior of Hf0.5Zr0.5O2 films exhibited distinct stages: improvement, saturation, and degradation. Our study presents novel findings revealing a saturation and degradation phenomenon of in-plane tensile stress on the ferroelectric properties of polycrystalline Hf0.5Zr0.5O2 films, thereby partially resolving the discrepancies between experimental observations and theoretical predictions. The observed phase transformation induced by tensile stress in Hf0.5Zr0.5O2 films played a crucial role in these effects. Furthermore, we found that the impact of the TiN top electrode thickness on other factors influencing ferroelectricity, such as grain size and oxygen vacancies, was negligible. These comprehensive results offer valuable insights into the influence of stress and TiN top electrode thickness on the ferroelectric behavior of Hf0.5Zr0.5O2 films.
... 1(a) and 1(b), and the corresponding crystal structure changes from orthorhombic to tetragonal. The antiferroelectric/ferroelectric properties of Zr-doped HfO 2 are closely influenced or even modulated by electrode layers [15][16][17] or/and fabricating process. 14,18,19 The commonly used electrode materials include TiN, Pt, Pd, W, Al, Ta, Ru, and RuO 2 , 20-23 and the effects of electrodes on ferroelectric properties of Zr-doped HfO 2 have been studied in detail. ...
... 14,18,19 The commonly used electrode materials include TiN, Pt, Pd, W, Al, Ta, Ru, and RuO 2 , 20-23 and the effects of electrodes on ferroelectric properties of Zr-doped HfO 2 have been studied in detail. 17,21,24 However, less attention has been paid to the effects of electrodes on the antiferroelectric properties of Zr-doped HfO 2 . Thus, in this work, the metal-antiferroelectric-metal capacitors of TiNjHf 0.2 Zr 0.8 O 2 jTiN and WjHf 0.2 Zr 0.8 O 2 jW were fabricated to study the effects of the electrodes on antiferroelectric properties of Hf 0.2 Zr 0.8 O 2 . ...
The influence of electrodes on antiferroelectricity and fatigue endurance of 15 nm thick Hf0.2Zr0.8O2 thin films has been studied by a metal–antiferroelectric–metal capacitor structure using TiN and W as electrodes. The W|Hf0.2Zr0.8O2|W capacitor shows significantly enhanced antiferroelectricity and better endurance compared to the capacitor using TiN as the electrode. Assisted by grazing incidence x-ray diffraction and scanning transmission electron microscopy, the different electrical properties are discussed based on the contents of different phases and the diffusion of oxygen from the thin film into electrodes.
... However, an excessively thick top electrode could degrade the polarization. The electrode thickness was based on previous reports [19,20]. To fortify structural integrity, the specimen underwent N 2 annealing at 500 • C for 60 s using rapid thermal processing (RTP). ...
... However, an excessively thick top electrode could degrade the polarization. The electrode thickness was based on previous reports [19,20]. To fortify structural integrity, the specimen underwent N2 annealing at 500 °C for 60 s using rapid thermal processing (RTP). ...
Flexible Si-based Hf0.5Zr0.5O2 (HZO) ferroelectric devices exhibit numerous advantages in the internet of things (IoT) and edge computing due to their low-power operation, superior scalability, excellent CMOS compatibility, and light weight. However, limited by the brittleness of Si, defects are easily induced in ferroelectric thin films, leading to ferroelectricity degradation and a decrease in bending limit. Thus, a solution involving the addition of an ultra-thin Al buffer layer on the back of the device is proposed to enhance the bending limit and preserve ferroelectric performance. The device equipped with an Al buffer layer exhibits a 2Pr value of 29.5 μC/cm² (25.1 μC/cm²) at an outward (inward) bending radius of 5 mm, and it experiences a decrease to 22.1 μC/cm² (16.8 μC/cm²), even after 6000 bending cycles at a 12 mm outward (inward) radius. This outstanding performance can be attributed to the additional stress generated by the dense Al buffer layer, which is transmitted to the Si substrate and reduces the bending stress on the Si substrate. Notably, the diminished bending stress leads to a reduced crack growth in ferroelectric devices. This work will be beneficial for the development of flexible Si-based ferroelectric devices with high durability, fatigue resistance, and functional mobility.
... [7][8][9][10] Ferroelectricity in HfO 2 -based materials is believed to be contributed by the space group Pca2 1 , which is a polar orthorhombic phase (O-phase) that coexists in the films with a tetragonal phase (T-phase) and monoclinic phases (M-phase). 11,12 The ferroelectric O-phase is a metastable state formed through a complicated process involving modifications to doping, 13 thickness, [14][15][16][17] stress, [18][19][20][21][22] annealing, 23,24 and other factors. Stress plays a crucial role in the creation of ferroelectrics in this process. ...
Hafnium-based ferroelectric materials have attracted a lot of attention, but the distributions of the materials need to be tuned for commercialization, including phase distribution and polarization orientation distribution. The orientation of ferroelectric materials plays a significant role in memory device performance; however, there have been no reliable methods to control this orientation until now. This paper investigates the control of ferroelectric phase polarization orientation in 3D cylindrical capacitors. This optimization is made possible because the 3D cylindrical capacitor allows stress application in the tangential, axial, and radial directions, offering a wider range of adjustment options than 2D planar structures. The study focuses on how stress distribution affects ferroelectric phase orientation in hafnium-based materials when the diameters of cylindrical capacitors are varied. First, stress simulations are conducted to analyze the stress distribution in cylindrical capacitors with different diameters. The results indicate that the hafnium oxide layer experiences increased radical stress as the capacitor diameter decreases. Subsequently, capacitors with two different diameters are fabricated, significantly improving the polarization orientation in the thinner one. It is found that the capacitor diameter and the polarization orientation are strongly related through the correlation analysis. Finally, we demonstrate the improvement in the polarization orientation of ferroelectric thin films by radical stress through first-principles calculations. This study provides valuable insights into how stress distribution influences polarization orientation in hafnium-based ferroelectric films and is crucial for advancing the use of ferroelectric materials in future technologies.
... 2 The origin of ferroelectricity in hafnium-based materials is a polar orthorhombic phase (O phase) called Pca2 1 , which is a metastable phase in HfO 2 . 3,4 The formation of ferroelectric hafnium oxide is a complex process that requires adjustments in doping, [5][6][7][8] thickness, 9,10 stress, [11][12][13][14] and annealing. [15][16][17] Stress, in particular, is a critical factor in the formation of hafnium-based ferroelectric materials. ...
To meet commercialization requirements, the distributions of materials in hafnium-based ferroelectric devices—including their phase and orientation—need to be controlled. This article presents a method for improving the ferroelectric phase ratio and orientation by adjusting the stress distribution of the annealing structure in a three-dimensional capacitor. In such a structure, stress can be applied in three directions: tangential, axial, and radial; there are, thus, more ways to regulate stress in three-dimensional structures than in two-dimensional structures. This work sought to clarify the role of the stress direction on the proportions and orientations of ferroelectric phases. The results of stress simulations show that a structure with an internal TiN electrode, but no filling provides greater axial and tangential stresses in the hafnium-oxide layer. In comparison with the case of the hole being filled with tungsten, the proportion of the O phase is increased by approximately 20%, and in experiments, the projection of the polarization direction onto the normal was found to be increased by 5%. Axial and tangential stresses are regarded to be beneficial for the formation of the O phase and for improving the orientation of the polarization direction. This work provides a theoretical basis and guidance for the three-dimensional integration of hafnium-based ferroelectric materials.
Ferroelectric hafnium zirconium oxide (HZO) holds promise for nextgeneration memory and transistors due to its superior scalability and seamless integration with complementary metal‐oxide‐semiconductor processing. A major challenge in developing this emerging ferroelectric material is the metastable nature of the non‐centrosymmetric polar phase responsible for ferroelectricity, resulting in a coexistence of both polar and non‐polar phases with uneven grain sizes and random orientations. Due to the structural similarity between the multiple phases and the nanoscale dimensions of the thin film devices, accurate measurement of phase‐specific information remains challenging. Here, the application of 4D scanning transmission electron microscopy is demonstrated with automated electron diffraction pattern indexing to analyze multiphase polycrystalline HZO thin films, enabling the characterization of crystallographic phase and orientation across large working areas on the order of hundreds of nanometers. This approach offers a powerful characterization framework to produce a quantitative and statistically robust analysis of the intricate structure of HZO films by uncovering phase composition, polarization axis alignment, and unique phase distribution within the HZO film. This study introduces a novel approach for analyzing ferroelectric HZO, facilitating reliable characterization of process‐structure‐property relationships imperative to accelerating the growth optimization, performance, and successful implementation of ferroelectric HZO in devices.
Literature is rich on the study of different strategies to tailor ferroelectric properties of HfO2. Among them, chemical doping is the most studied. La doped HfO2 films have attracted interest because they show very low leakage current and high endurance. On the other hand, stress controlled by substrate selection has shown to induce ferroelectric properties variations in Hf0.5Zr0.5O2 films. Here, we investigate stress effects in La-doped epitaxial HfO2 films. Interestingly, ferroelectricity is measured in films grown on substrates having a broad range of lattice parameter from 3.71 to 4.21 Å. While comparing the obtained results with those obtained in epitaxial Hf0.5Zr0.5O2, it is observed that La doped HfO2 shows always larger remanent polarization (Pr) if the same substrate is used. Films grown on substrates with large lattice parameter (TbScO3 and GdScO3) show very large values of remanent polarization (29 μC/cm²), but it is also noticeable that the films on substrates with small parameter (YAlO3) show remanent polarization above 5 μC/cm², whereas negligible Pr was detected in equivalent Hf0.5Zr0.5O2 films. Therefore, chemical doping and epitaxial stress do not compete and can be both used to synergetically tailor ferroelectric properties and eventually improve them.
In this work, the effect of rapid thermal annealing (RTA) temperature on the ferroelectric polarization in zirconium-doped hafnium oxide (HZO) was studied. To maximize remnant polarization (2P r ), in-plane tensile stress was induced by tungsten electrodes under optimal RTA temperatures. We observed an increase in 2P r with RTA temperature, likely due to an increased proportion of the polar ferroelectric phase in HZO. The HZO capacitors annealed at 400°C did not exhibit any ferroelectric behavior, whereas the HZO capacitors annealed at 800°C became highly leaky and shorted for voltages above 1 V. On the other hand, annealing at 700 °C produced HZO capacitors with a record-high 2P r of ∼ 64 μ C cm ⁻² at a relatively high frequency of 111 kHz. These ferroelectric capacitors have also demonstrated impressive endurance and retention characteristics, which will greatly benefit neuromorphic computing applications.
The presence of the top electrode on hafnium oxide‐based thin films during processing has been shown to drive an increase in the amount of metastable ferroelectric orthorhombic phase and polarization performance. This “Clamping Effect,” also referred to as the Capping or Confinement Effect, is attributed to the mechanical stress and confinement from the top electrode layer. However, other contributions to orthorhombic phase stabilization have been experimentally reported, which may also be affected by the presence of a top electrode. In this study, it is shown that the presence of the top electrode during thermal processing results in larger tensile biaxial stress magnitudes and concomitant increases in ferroelectric phase fraction and polarization response, whereas film chemistry, microstructure, and crystallization temperature are not affected. Through etching experiments and measurement of stress evolution for each processing step, it is shown that the top electrode locally inhibits out‐of‐plane expansion in the HZO during crystallization, which prevents equilibrium monoclinic phase formation and stabilizes the orthorhombic phase. This study provides a mechanistic understanding of the clamping effect and orthorhombic phase formation in ferroelectric hafnium oxide‐based thin films, which informs the future design of these materials to maximize ferroelectric phase purity and corresponding polarization behavior. Through comparison between the microstructures, crystallization temperatures, and biaxial stresses of hafnium zirconium oxide thin films processed with and without top TaN electrode layers, the origin of the clamping effect in HfO2‐based ferroelectrics is investigated. It is shown that local inhibition in out‐of‐plane expansion during crystallization stabilizes the ferroelectric orthorhombic phase, which drives an increase in film tensile biaxial stress.
Fluorite‐structure ferroelectric thin films have been extensively studied as promising candidates for next‐generation non‐volatile memory. However, these ferroelectric thin films have fatal issues such as the irregular formation of the ferroelectric phase, low cycling endurance, and wake‐up and fatigue during endurance cycling tests. These problems are reportedly caused by oxygen vacancies, which form due to the interface reaction between the thin films and bottom electrodes during deposition and the post‐annealing process. Therefore, in this work, the enhanced ferroelectric characteristics of Hf1‐xZrxO2 thin films that control the oxygen vacancies in thin films through interfacial pretreatment are investigated. Interfacial treatment using an oxygen source can reduce oxygen vacancies and improve crystallinity through intentional oxidation of the bottom electrode. As a result, the remanent polarization value is increased by ≈1.6 times by applying the optimized pretreatment condition, and the measured 2Pr is a very high value of 73 µC cm−2. Furthermore, it exhibits very stable ferroelectric properties without a wake‐up effect or significant fatigue, up to 108 cycles even under a severe electric field of 3.5 MV cm−1. This simple strategy provides a new avenue to effectively improve the performance and cycling endurance of devices with ferroelectric thin films. The superior stable ferroelectric characteristics of HZO‐TiN‐based capacitors are demonstrated via ozone pretreatment before thin film deposition. An excellent ferroelectric characteristic with a 2Pr of 73 µC cm−2 and constant Pr value up to 108 cycles through ozone pretreatment is successfully achieved. This simple process represents a simple and novel strategy to effectively improve the performance and cycling endurance of devices using ferroelectric thin films.
Ferroelectric memories based on hafnium oxide are an attractive alternative to conventional memory technologies due to their scalability and energy efficiency. However, there are still many open questions regarding the optimal material stack and processing conditions for reliable device performance. Here, we report on the impact of the sputtering process conditions of the commonly used TiN top electrode on the ferroelectric properties of Hf1-xZr
x
O2. By manipulating the deposition pressure and chemistry, we control the preferential orientation of the TiN grains between (111) and (002). We observe that (111) textured TiN is superior to (002) texturing for achieving high remanent polarization (Pr). Furthermore, we find that additional nitrogen supply during TiN deposition leads to >5× greater endurance, possibly by limiting the scavenging of oxygen from the Hf1-xZr
x
O2 film. These results help explain the large Pr variation reported in the literature for Hf1-xZr
x
O2/TiN and highlights the necessity of tuning the top electrode of the ferroelectric stack for successful device implementation.
The discovery of ferroelectric properties in hafnium oxide has brought back the interest in the ferroelectric non-volatile memory as a possible alternative for low power consumption electronic memories. As far as real hafnium oxide-based materials have defects like oxygen vacancies, their presence might affect the ferroelectric properties due to oxygen atom movements during repolarization processes. In this work, the transport experiments are combined with the modeling to study evolution of the oxygen vacancy concentration during the endurance and to determine the optimal defect density for a higher residual polarization in lanthanum-doped hafnium oxide.
In this paper, we propose a method to improve the performance of TiN/Hf0.5Zr0.5O2 (HZO)/TiN Nano-capacitors used in memory devices. Instead of direct fabrication of the TiN/HZO/TiN device, our method involves an intermediate step in which W metal is used as a capping material to induce a large in-plane tensile strain during rapid thermal annealing, resulting in a total suppression of the monoclinic phase and the appearance of the ferroelectric phase. Consequently, after removing the W capping electrode through an etching process and the post-deposition of a TiN top electrode at room temperature, a high remnant polarization of approximately 40 μC cm-2 and a 65% increase of coercive field were obtained. Moreover, the leakage current was reduced by an order of magnitude compared to the normal TiN/HZO/TiN capacitor; this result is attributed to the presence (absence) of the W/HZO (TiN/HZO) top interface during thermal annealing. The formation of a TiO x interfacial layer at elevated temperatures, which pulls oxygen from the HZO layer, resulting in the formation of oxygen vacancies, is the main cause of the high leakage current through the TiN/HZO/TiN stacks. It was confirmed that the re-capped TiN/HZO/TiN capacitor has a comparable endurance to a normal capacitor. Our results offer the re-capping process as a promising approach to fabricating HfO2-based ferroelectric memory devices with various electrode materials.
Memory devices with high speed and high density are highly desired to address the ‘memory wall’ issue. Here we demonstrated a highly scalable, three-dimensional stackable ferroelectric diode, with its rectifying polarity modulated by the polarization reversal of Hf0.5Zr0.5O2 films. By visualizing the hafnium/zirconium lattice order and oxygen lattice order with atomic-resolution spherical aberration-corrected STEM, we revealed the correlation between the spontaneous polarization of Hf0.5Zr0.5O2 film and the displacement of oxygen atom, thus unambiguously identified the non-centrosymmetric Pca21 orthorhombic phase in Hf0.5Zr0.5O2 film. We further implemented this ferroelectric diode in an 8 layers 3D array. Operation speed as high as 20 ns and robust endurance of more than 10⁹ were demonstrated. The built-in nonlinearity of more than 100 guarantees its self-selective property that eliminates the need for external selectors to suppress the leakage current in large array. This work opens up new opportunities for future memory hierarchy evolution.
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.
We present a comprehensive first principles study of doped hafnia in order to understand the for- mation of the ferroelectric orthorhombic[001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal[100] grains can transform into orthorhombic[001] during thermal annealing, when they are laterally confined by other grains. We show that among 0%, 2% and %4 Si doping, 4% doping provides the best conditions for the tetragonal[100] → orthorhombic[001] transformation. This also holds for Al dop- ing. We also show that for HfxZr1−xO2, where we have studied x = 1.00, 0.75, 0.50, 0.25, 0.00, the value x = 0.50 provides the most favorable conditions for the desired transformation. In order for this transformation to be preferred over the tetragonal [100] → monoclinic [100] transformation, out-of-plane confinement also needs to be present, as supplied by a top electrode. Our findings illuminate the mechanism that causes ferroelectricity in hafnia-based films and provide an expla- nation for common experimental observations for the optimal ranges of doping in Si:HfO2, Al:HfO2 and HfxZr1−xO2. We also present model thin film heterostructure computations of Ir/HfO2/Ir stacks in order to isolate the interface effects, which we show to be significant.
Various possibilities have been proposed as the cause of the doped‐ or undoped‐HfO2 thin film materials showing unusual ferroelectricity. These assumptions are based on empirical results, yet finding the origin of the unprecedented ferroelectricity within HfO2 has suffered from a serious gap between its theoretical calculation, mostly based on thermodynamic approach and the actual experimental results. To fill the gap, this study proposes to consider the kinetic energy, providing the evidence of the kinetic energy barrier upon a phase transformation from the tetragonal phase to the monoclinic phase affected by the TiN top electrode (capping layer). 10 nm thick Hf0.5Zr0.5O2 thin films are deposited and annealed with or without the TiN capping layer with subsequent annealing at different time and temperature. Arrhenius plot is constructed to obtain the activation energy for the tetragonal‐to‐monoclinic phase transformation by calculating the amount of the transformed phase using X‐ray diffraction pattern. Johnson–Mehl–Avrami and nucleation‐limited transformation models are utilized to describe the characteristic nucleation and growth time and calculate the activation energy for the monoclinic phase transformation of the Hf0.5Zr0.5O2 thin film. Both models demonstrate that the TiN capping layer provides a kinetic energy barrier for tetragonal‐to‐monoclinic phase transformation and enhances the ferroelectric property.
The discovery of ferroelectricity in hafnium oxide has revived the interest in ferroelectric memories as a viable option for low power non-volatile memories. However, due to the high coercive field of ferroelectric hafnium oxide, instabilities in the field cycling process are commonly observed and explained by the defect movement, defect generation and field induced phase transitions. In this work, the optical and transport experiments are combined with ab-initio simulations and transport modeling to validate that the defects which act as charge traps in ferroelectric active layers are oxygen vacancies. A new oxygen vacancy generation leads to a fast growth of leakage currents and a consequent degradation of the ferroelectric response in Hf0.5Zr0.5O2 films. Two possible pathways of the Hf0.5Zr0.5O2 ferroelectric property degradation are discussed.
In this letter, effects of top electrodes (TEs) on ferroelectric properties of Hf
0.5
Zr
0.5
O
2
(HZO) thin films are examined systematically. The remnant polarization (Pr) of HZO thin films increases by altering TEs with lower thermal expansions coefficient (α). The largest 2P
r
value of 38.72 μC/cm
2
is observed for W TE with α = 4.5 × 10
-6
/K, while the 2P
r
value is only 22.83 μC/cm
2
for Au TE with α = 14.2 × 10
-6
/K. Meanwhile, coercive field (E
c
) shifts along the electric field axis and the offset is found to be dependent on the difference of workfunctions (WFs) between TE and TiN bottom electrode (BE). E
c
shifts toward negative/positive direction, when the WF of TE is larger/smaller (Pt, Pd, Au/W, Al, Ta) than TiN BE. This letter provides an effective way to modulate HfO
2
-based device performance for different requirements in actual application.
The discovery of ferroelectric (FE) properties in binary oxides has enabled CMOS compatible and scalable FE memories. Recently, we reported a simple approach to introduce non-volatility into state-of-the-art dynamic random-access memory (DRAM) stacks that show anti-ferroelectric (AFE) behavior. By employing a pair of electrodes with different work functions, a built-in bias is generated. Consequently, this bias modulates the energy potential of the AFE and enables two stable non-volatile states. Using this approach, a significant endurance improvement compared to hafnia based FE memories can be obtained. In this paper we investigate the possibility to bypass the usage of asymmetric workfunction electrodes. Using the interface-engineering approach, based on fixed charge or dipole formation, we show two additional methods for built-in bias generation within AFE layer stacks. By characterizing the film properties and performance of AFE capacitors, we compare and investigate retention and endurance of both work function-difference-based and interface-based AFE non-volatile memory. Finally, for the first time we present the concept of a binary oxide based anti-ferroelectric tunnel junction that leverages both an interface and work function engineered anti-ferroelectric stack.
We report on atomic layer deposited Hf0.5Zr0.5O2 (HZO)-based capacitors which exhibit excellent ferroelectric (FE) characteristics featuring a large switching polarization (45 μC/cm²) and a low FE saturation voltage (∼1.5 V) as extracted from pulse write/read measurements. The large FE polarization in HZO is achieved by the formation of a non-centrosymmetric orthorhombic phase, which is enabled by the TiN top electrode (TE) having a thickness of at least 90 nm. The TiN films are deposited at room temperature and annealed at 400 °C in an inert environment for at least 1 min in a rapid thermal annealing system. The room-temperature deposited TiN TE acts as a tensile stressor on the HZO film during the annealing process. The stress-inducing TiN TE is shown to inhibit the formation of the monoclinic phase during HZO crystallization, forming an orthorhombic phase that generates a large FE polarization, even at low process temperatures.
The unexpected ferroelectric properties in nanoscale hafnia-zirconia are considered to be promising for a wealth of applications including ferroelectric memory, field effect transistors, and energy-related applications. However, the reason why the unexpected ferroelectric Pca21 phase can be stabilized has not been clearly understood although numerous extensive theoretical and experimental results have been reported recently. The ferroelectric orthorhombic phase is not a stable phase at processing condition from the view point of bulk free energy. Although the possible stabilization of the ferroelectric phase due to the surface energy effect has been theoretically suggested, such theoretical model has not been systematically compared with the actual experimental results. In this study, the experimental observations on polymorphism in nanoscale HfO2-ZrO2 solid solution thin films, in a wide range of film composition and thickness, are comprehensively related to the theoretical predictions based on the thermodynamic surface energy model. The theoretical model can semi-quantitatively explain the experimental results on the phase-evolution, but there was non-negligible discrepancies between the two results. To understand these discrepancies, various factors such as film stress, role of a TiN capping layer, and the kinetics of crystallization are systematically studied. The work also reports on the evolution of electrical properties of the film, i.e. dielectric, ferroelectric, anti-ferroelectric, and morphotropic phase change, as a function of the film composition and the thickness. The in-depth analyses of the phase change are expected to provide an important guideline for subsequent studies.
To investigate the impact of mechanical stress on their ferroelectric properties, polycrystalline (Hf0.5Zr0.5)O2
thin films were deposited on (111)Pt-coated SiO2, Si, and CaF2 substrates with thermal expansion coefficients of 0.47, 4.5, and 22 × 10−6/ °C, respectively. In-plane X-ray diffraction measurements revealed that the (Hf0.5Zr0.5)O2
thin films
deposited on SiO2 and Si substrates were under in-plane tensile strain and that their volume fraction of monoclinic phase decreased as this strain increased. In contrast, films
deposited on CaF2 substrates were under in-plane compressive strain, and their volume fraction of monoclinic phase was the largest among the three kinds of substrates. The maximum remanent polarization of 9.3 μC/cm2 was observed for Pt/(Hf0.5Zr0.5)O2/Pt/TiO2/SiO2, while ferroelectricity was barely observable for Pt/(Hf0.5Zr0.5)O2/Pt/TiO2/SiO2/CaF2. This result suggests that the in-plane tensile strain effectively enhanced the ferroelectricity of the (Hf0.5Zr0.5)O2
thin films.
The ferroelectric
properties and crystal structure of doped HfO2
thin films were investigated for different thicknesses, electrode
materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a Pr of up to 35 μC/cm2. This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO2
films.
Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO2.
The structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2, and Hf0.5Zr0.5O2 (HZO) are investigated with carefully validated density functional computations. We find that the free bulk energy of the ferroelectric orthorhombic Pca21 phase is unfavorable compared to the monoclinic P21/c and the orthorhombic Pbca phase for all investigated stoichiometries in the Hf1−xZrxO2 system. To explain the existence of the ferroelectric phase in nanoscale thin films, we explore the Gibbs/Helmholtz free energies as a function of stress and film strain and find them unlikely to become minimal in HZO films for technological relevant conditions. To assess the contribution of surface energy to the phase stability, we parameterize a model, interpolating between existing data, and find the Helmholtz free energy of ferroelectric grains minimal for a range of size and stoichiometry. From the model, we predict undoped HfO2 to be ferroelectric for a grain size of about 4 nm and epitaxial HZO below 5 nm. Furthermore, we calculate the strength of an applied electric field necessary to cause the antiferroelectric phase transformation in ZrO2 from the P42/nmc phase as 1 MV/cm in agreement with experimental data, explaining the mechanism of field induced phase transformation.
We report that crystalline phases with ferroelectric behavior can be formed in thin films of SiO2 doped hafnium oxide. Films with a thickness of 10 nm and with less than 4 mol. % of SiO2 crystallize in a monoclinic/tetragonal phase mixture. We observed that the formation of the monoclinic phase is inhibited if crystallization occurs under mechanical encapsulation and an orthorhombic phase is obtained. This phase shows a distinct piezoelectric response, while polarization measurements exhibit a remanent polarization above 10 μC/cm2 at a coercive field of 1 MV/cm, suggesting that this phase is ferroelectric. Ferroelectric hafnium oxide is ideally suited for ferroelectric field effect transistors and capacitors due to its excellent compatibility to silicon technology.
The effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf0.5Zr0.5O2 films were examined. The Hf0.5Zr0.5O2 films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases.
Binary oxides of Hf
0.5
Zr
0.5
O
2
(HZO) have attracted considerable attention of the ferroelectric research community, owing to their excellent ferroelectric properties and CMOS compatibility. In particular, HZO films of a relatively high thickness (>10 nm) are studied widely for sensor and display applications. However, one of the major constraints of HZO materials is the formation of monoclinic phases (m-phase) with increasing film thickness resulting in the degradation of its remanent polarization (
). Herein, we present a stress engineering method to achieve high ferroelectricity in thick hafnia using an interlayer. In our work, we attempted to address the aforesaid limitation of HZO by inserting a dielectric interlayer and elucidated the influence of interlayer on the relatively thick HZO films. high resolution TEM (HRTEM) analysis revealed that the presence of interlayer allows the growth of the top and bottom HZO layer in an independent direction thereby preventing the loss of ferroelectricity in HZO films with higher thickness by controlling its grain size. Similarly, grain angle incidence X-ray diffraction (GIXRD) and residual stress measurements suggest that the interlayer affects the o-phase formation from the t-phase owing to the tensile stress applied to the HZO films because of the coefficient of thermal expansion (CTE) mismatch between the HZO and interlayer. In our study, an improved
value of 30.2
/cm
2
was achieved by inserting a TiO
2
dielectric interlayer in a relatively thicker HZO film. We believe that this approach can be adopted in various applications such as sensors, displays, and memory devices.
Three-dimensional NAND architecture (3-D NAND) based on ferroelectric field-effect transistors (FeFETs) is explored for in-memory computing. In ferroelectric Hafnia-based polycrystalline thin film, which is deposited on the gate of the FeFETs, the monoclinic (M), and orthorhombic (O) phases coexist. These two phases of positional distribution introduce a read-out current variation in the 3-D nand of FeFETs. Herein, we employ TCAD simulations to quantify such variation and optimize bias conditions for improving the accuracy of in-memory computing. Furthermore, the array-level impact of the phase variation on vector-matrix multiplication has been investigated using a 3-D netlist with SPICE simulations, indicating sufficient read-out accuracy possible for analog-to-digital conversion.
In recent years, several experimental approaches have been adopted to study and understand the mechanism and improve the ferroelectricity of fluorite-type hafnia-based ferroelectric materials. In this regard, significant efforts have been made to elucidate the role of top electrode and bottom electrode (TE and BE) materials in defining the ferroelectricity in such systems, especially in terms of induced mechanical tensile stress by these materials during the process of annealing. However, the effect of the electrode material was not investigated both at TE and BE, and despite numerous efforts, there is still a lack of accurate and systematic understanding. In this report, we have carried out a systematic investigation on the effect of TE and BE materials having different coefficient of thermal expansion (CTE), by changing the electrode material one at a time, both at the top and bottom. The influence of the TE was confirmed using [TE/Hf
0.5
Zr
0.5
O
2
(HZO)/TiN] structure in which the BE was fixed as TiN, and the influence of the BE was confirmed using [TiN/HZO/BE] structure by fixing TiN as the TE. As revealed by polarization versus electric field and residual stress analysis, smaller CTE of the electrode was found to result in higher tensile stress in the HZO films during the annealing process, facilitating the formation of higher ferroelectric o-phase and thereby resulting in greater ferroelectricity. Although the influence of TE and BE on the ferroelectric property of HZO films was found to show similar trends according to the CTE value of the electrodes, the influence of TE on the ferroelectric property of the HZO capacitors is found to be mainly due to the variation in the induced mechanical tensile stress; pulse switching measurement and X-ray photoelectron spectrometer (XPS) analysis suggest that in case of BE, both the induced mechanical tensile stress and the interfacial dead layer were found to play a significant part. As a result, BE was found to have a greater influence on ferroelectricity of the HZO capacitors when compared with that of TE. The highest remnant polarization of 48.2 and
/cm
2
was obtained for W with the lowest of CTE of
in both the configurations. The results obtained in this article are expected to provide a new way out to optimize the interface quality and ferroelectricity in HZO-based capacitors.
We report on 4.5-nm-thick Hf0.5Zr0.5O2 (HZO) thin-film-based ferroelectric tunnel junctions (FTJs) with a tungsten (W) bottom electrode. The HZO on the W electrode exhibits stable ferroelectricity with a remanent polarization of 14 μC/cm², an enhanced tunneling electroresistance of 16, and excellent synaptic properties. We found that a large tensile stress was induced on a HZO thin film, owing to a low thermal expansion coefficient of the W bottom electrode. The low thermal expansion coefficient results in the effective formation of an orthorhombic phase, even in an ultra-thin HZO film. This was verified by a comparative study of the electrical characteristics, grazing-angle incidence x-ray diffraction, and residual stress measurement of the HZO film on various bottom electrodes with different thermal expansion coefficients. In addition, this study demonstrates the suitable functions of the FTJ for electronic synapses, such as analog-like resistance transition under various pulse schemes. The fabricated stress-engineered FTJ exhibits an appropriate conductance ratio, linearly modulated long-term potentiation and depression characteristics, and excellent reliability. These characteristics render FTJs ideal electronic devices for neuromorphic computing systems.
Electrical and reliability characteristics of hafnia ferroelectric capacitor are influenced by a capping electrode layer which controls the type of stress and the amount of oxygen vacancy inside hafnia. Here, we present the impact of metal nitride and metal oxide electrode on the ferroelectricity of a Hf
0.5
Zr
0.5
O
2
(HZO) capacitor. For comparison, we employed two different top electrodes (RuO
2
and TiN) with hafnia ferroelectric layer, forming RuO
2
/HZO/TiN and TiN/HZO/TiN capacitors. The RuO
2
top electrode provides additional oxygen to the HZO film, lowering the amount of oxygen vacancies in the film. From material analysis, we found that the top RuO
2
/HZO interface exhibits less oxygen vacancy in comparison to the top TiN/HZO interface. In addition, for RuO
2
/HZO/TiN, due to different thermal expansion coefficient between top and bottom electrodes, the HZO film experiences significant tensile stress, resulting in the high o-phase formation and remnant polarization (
/cm
2
) as compared with that of TiN/HZO/TiN capacitor (
/cm
2
). This article suggests an efficient solution to reduce the interfacial defects and oxygen vacancies as well as to enhance o-phase formation and ferroelectricity.
Ferroelectric hafnia-based thin films are promising candidates for emerging high-density embedded nonvolatile memory technologies, thanks to their compatibility with silicon technology and the possibility of 3D integration. The electrode–ferroelectric interface and the crystallization annealing temperature may play an important role in such memory cells. The top interface in a TiN/Hf0.5Zr0.5O2/TiN metal–ferroelectric–metal stack annealed at different temperatures was investigated with X-ray photoelectron spectroscopy. The uniformity and continuity of the 2 nm TiN top electrode was verified by photoemission electron microscopy and conductive atomic force microscopy. Partial oxidation of the electrode at the interface is identified. Hf is reduced near the top interface due to oxygen scavenging by the top electrode. The oxygen vacancy (VO) profile showed a maximum at the top interface (0.71%) and a sharp decrease into the film, giving rise to an internal field. Annealing at higher temperatures did not affect the VO concentration at the top interface but causes the generation of additional VO in the film, leading to a decrease of the Schottky Barrier Height for electrons. The interface chemistry and n-type film doping are believed to be at the origin of several phenomena, including wake-up, imprint, and fatigue. Our results give insights into the physical chemistry of the top interface with the accumulation of defective charges acting as electronic traps, causing a local imprint effect. This may explain the wake-up behavior as well and also can be a possible reason of the weaker endurance observed in these systems when increasing the annealing temperature.
The newly discovered hafnium oxide (HfO2)-based ferroelectric film shows many advantages over the traditional perovskite films in the application of information storage. However, the mechanism of ferroelectric phase transition of the HfO2-based film is still confusing to the researchers. Here, the effects of oxygen vacancies and the complex defects formed by the combination of oxygen vacancies and typical impurity elements on ferroelectric phase transition and polarization performance of HfO2 were systematically investigated by first-principle calculation. Due to the ambiguous effects of electrode/ferroelectric interfaces on the ferroelectricity of HfO2-based film, the influence of oxygen vacancies at the TiN/HfO2 interface was also studied. It was found that the oxygen vacancies, and impurities N and La, which would form defect dipoles by combining with oxygen vacancies and induce a local build-in bias, would promote the ferroelectric phase transition in the bulk of HfO2-based film. Additionally, oxygen vacancies, which are inclined to migrate to the interface, would cause the transition of the interfacial tetragonal phase to the ferroelectric phase, and then to the monoclinic phase. This result may be helpful for the understanding of the origin of ferroelectricity as well as the mechanisms of wake-up and fatigue effects of HfO2-based ferroelectric film.
HfO2-based ferroelectric film has shown great potential for the application of ferroelectric memory due to its specific advantages as compared to the traditional ferroelectrics. However, the origin of the ferroelectricity of the HfO2-based ferroelectric film is still under debate. In this work, by performing the ab initial molecular dynamics calculation, the phase stability and the polarization switching behavior of HfO2 were systematically studied to illuminate the intrinsic origin its ferroelectricity. Results show that, under different in-plane constraints and room temperature, the out-of-plane polarized orthorhombic ferroelectric phase Pca21 is always the metastable phase of HfO2. As driven by the out-of-plane electric field, HfO2 exhibits linear dielectric behavior or antiferroelectric behavior with the ferroelectric-tetragonal-ferroelectric phase transformation under the in-plane compression. While with the tensile strain condition, the non-ferroelectric HfO2 could be transformed to be the out-of-plane polarized orthorhombic ferroelectric phase, which shows good ferroelectricity under the periodic electric field. The triggered phase transformation and ferroelectricity as modulated by the epitaxial constraint as found in this work were verified by the recent experiment and should be intrinsic origin of the ferroelectricity of the HfO2-based films.
The critical impact of epitaxial stress on the stabilization of the ferroelectric orthorhombic phase of hafnia is proved. Epitaxial bilayers of Hf0.5Zr0.5O2 (HZO) and La0.67Sr0.33MnO3 (LSMO) electrodes were grown on a set of single crystalline oxide (001)-oriented (cubic or pseudocubic setting) substrates with lattice parameter in the 3.71 – 4.21 Å range. The lattice strain of the LSMO electrode, determined by the lattice mismatch with the substrate, is critical in the stabilization of the orthorhombic phase of HZO. On LSMO electrodes tensile strained most of the HZO film is orthorhombic, whereas the monoclinic phase is favored when LSMO is relaxed or compressively strained. Therefore, the HZO films on TbScO3 and GdScO3 substrates present substantially enhanced ferroelectric polarization in comparison to films on other substrates, including the commonly used SrTiO3. The capability of having epitaxial doped HfO2 films with controlled phase and polarization is of major interest for a better understanding of the ferroelectric properties and paves the way for fabrication of ferroelectric devices based on nanometric HfO2 films.
The discovery of ferroelectricity in both pure and doped HfO2-based thin films have revitalized interest in using ferroelectrics for nanoscale device applications. To take advantage of this silicon-compatible ferroelectric, fundamental questions such as the origin of ferroelectricity and better approach to controlled realization of ferroelectricity at the nanoscale need to be addressed. The emergence of robust polarization in HfO2-based thin films is considered as the cumulative effect of various extrinsic factors such as finite-size effects and surface/interface effects of small grains, compressive stress, dopants, oxygen vacancies, and electric fields. The kinetic effects of phase transitions and their potential impacts on the emergence of ferroelectricity in HfO2 at the nanoscale are not well understood. In this paper, we construct the transition paths between different polymorphs of hafnia with density-functional-theory calculations and variable-cell nudged elastic band technique. We find that the transition barriers depend strongly on the mechanical boundary conditions and the transition from the tetragonal phase to the polar orthorhombic phase is a fast process kinetically under clamping. The effects of growth orientations and epitaxial strains on the relative stability of different phases of HfO2 are investigated. The two orthorhombic phases, polar Pca21 and nonpolar Pbca, become thermodynamically stable in (111)-oriented thin films over a wide range of epitaxial strain conditions. This paper suggests a potential avenue to better stabilize the ferroelectric phase in HfO2 thin films through substrate orientation engineering.
Hf
0.5
Zr
0.5
O
2
(HfZrO) thin-film ferroelectric materials have recently drawn considerable attention due to their attractive properties such as large bandgap (>5 eV), extreme thin thickness (≤10 nm), and good Si-compatibility. However, high crystallization temperature (600 °C-1000 °C) and relatively low remanent polarization (P
r
) compared to conventional perovskite ferroelectric materials are not suitable for flexible energy related devices, and are insufficient to overcome the barriers of conventional ferroelectric memory. In this paper, we investigated the effect of high-pressure postmetallization annealing (HPPMA) on the ferroelectric properties of the HfZrO metal-ferroelectric-metal (MFM) devices. HfZrO MFM capacitors annealed at 450 °C/50 bar shows a Pr value of over 20 μC/cm
2
which is very advanced Pr value. Based on the short-pulse switching technique, we quantitatively and systematically examined the improved characteristics of HfZrO prepared by HPPMA in terms of coercive field (E
c
) and possibly involved interfacial capacitance (C
i
).
The surprising ferroelectricity displayed by hafnia thin films has been attributed to a metastable polar orthorhombic (Pca21) phase. Nevertheless, the conditions under which this (or another competing) ferroelectric phase may be stabilized remain unresolved. It has been hypothesized that a variety of factors, including strain, grain size, electric field, impurities and dopants, may contribute to the observed ferroelectricity. Here, we use first-principles computations to examine the influence of mechanical and electrical boundary conditions (i.e., strain and electric field), on the relative stability of a variety of relevant non-polar and polar phases of hafnia. We find that although strain or electric field, independently, do not lead to a ferroelectric phase, the combined influence of in-plane equibiaxial deformation and electric field results in the emergence of the polar Pca21 structure as the equilibrium phase. The results provide insights for better controlling the ferroelectric characteristics of hafnia thin films by adjusting the growth conditions and electrical history.
In this study, we control the oxidant dose to promote ferroelectricity in dopant-free ALD hafnium oxide films. By lowering the oxidant dose during growth, we show that we can achieve near total suppression of the monoclinic phase in sub-10 nm hafnium oxide films with no major impurity doping. Using metal-insulator-metal structures, we demonstrate that lowering the oxidant dose can give rise to a six-fold improvement in remanent polarization. Using this technique, we observe a remanent polarization of 13.5 μC/cm² in a 6.9 nm-thick hafnium oxide film and show that some ferroelectricity can persist in pure hafnium oxide films as thick as 13.9 nm. Using a trap-assisted tunneling model, we show the relationship between the oxidant dose and oxygen vacancy concentration in the films, suggesting a possible mechanism for the suppression of the monoclinic phase.
Since 2011, ferroelectric HfO2 has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake-up and fatigue effects alter the shape of the polarization hysteresis loop during field cycling. Such changes are problematic for application of HfO2 to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale film structure, which modifies the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in films at different cycling stages, enabling development of a structural model to explain the wake-up and fatigue phenomena. The wake-up regime arises due to changes in bulk and interfacial structuring: the bulk undergoes a phase transformation from monoclinic to orthorhombic grains, and the interfaces show changes in and diminishment of a nonuniform, defect rich, tetragonal HfO2 layer near the electrodes. The evolution of these aspects of structuring contributes to the increase in Pr and the opening of the constricted P–V hysteresis that are known to occur with wake-up. The onset of the fatigue regime is correlated to an increasing concentration of bulk defects, which are proposed to pin domain walls.
Novel HfO2-based ferroelectrics reveal full scalability and CMOS integratability compared to perovskite-based ferroelectrics, that are currently used in non-volatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake-up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake-up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis and TEM study with compact and TCAD modeling, it was found out that during the wake-up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion was identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation resulted in an understanding of the main mechanisms behind the evolution of the ferroelectric response.
Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO2
thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO2
thin films.
The structural and electrical properties of Hf0.5Zr0.5O2 thin films after post deposition and post metallization annealing were investigated. The possibility of the annealing without top electrodes for attainment of ferroelectric properties in Hf0.5Zr0.5O2 thin films was demonstrated. It was, however, shown, that the annealing in presence of top electrode leads to the more preferable crystallization to polar o-phase and the increase in the remanent polarization value. The influence of the film thickness on the structural and ferroelectric properties was studied both after post metallization and post deposition annealing. The optimal Hf0.5Zr0.5O2 thickness was found to be equal to ∼19 nm for electrode free annealing, which is promising value for modern nanoscale ferroelectric devices.
Structural and electrical evidence for a ferroelectric phase in yttrium doped hafnium oxide thin films is presented. A doping series ranging from 2.3 to 12.3 mol% YO1.5 in HfO2 was deposited by a thermal atomic layer deposition process. Grazing incidence X-ray diffraction of the 10 nm thick films revealed an orthorhombic phase close to the stability region of the cubic phase. The potential ferroelectricity of this orthorhombic phase was confirmed by polarization hysteresis measurements on titanium nitride based metal-insulator-metal capacitors. For 5.2 mol% YO1.5 admixture the remanent polarization peaked at 24 μC/cm2 with a coercive field of about 1.2 MV/cm. Considering the availability of conformal deposition processes and CMOS-compatibility, ferroelectric Y:HfO2 implies high scaling potential for future, ferroelectric memories.
Pb ( Zr 0.52 Ti 0.48)0.96 Nb 0.04 O 3 (PZTN) thin films were deposited on BaPbO 3 (BPO) electrodes by rf-magnetron sputtering. 34, 68, 135, and 270 nm thick BPOs were adopted in this study. The preferred orientation changes from slightly (100)/(001) to slightly (101)/(110) as the BPO thickness increased. The mean grain sizes obtained by Williamson–Hall plots are 81 nm, 120 nm, 146 nm, and 90 nm, respectively. The same tendency was observed by atomic force microscopy method. In residual stress analysis, tensile stress was observed in every film. The stress magnitude is the maximum in the film with 135 nm thick BPO. Through a simple calculation, we suggest that the tensile stress in our films mainly originates from the phase transformation stresses. We also note that the ferroelectric and dielectrics properties are improved with the raise of tensile stresses. A possible reason is that the tensile stress benefits the tetragonal–monoclinic phase transition in the PZTN films with composition near morphotropic phase boundary. The other possible reason is that the raise of the tensile stress is consistent with the increasing of grain size, which decreases the grain boundary density and facilitate domain mobility.
Nitride-based coatings are nowadays widely studied both from fundamental and technological point of views due to their unique physical and mechanical properties. Among the binary nitrides, TiN is the most stable thermodynamically and has been widely used due to the combination of its covalent and metal-like characteristics. Coatings produced by Physical Vapor Deposition (PVD) techniques generally exhibit a crystallographic texture, which in turn may strongly affect their properties, such as hardness, wear resistance, or diffusion barrier properties in microelectronic devices. Therefore great efforts have been made in recent years to understand the underlying mechanisms governing texture development in nitride thin films. In particular, the issue of stress build-up during PVD growth and its possible interplay with film preferred orientation is essential to address.