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.
The purpose of this study was to identify the demographic and clinical characteristics of suicide attempts in adolescents who visit the emergency department compared to those of adults.
This study ...included 149 children under the age of 18, and 1427 people in the age of 19-65 who came to the emergency department with suicide attempt from 2009 to 2015. We compare sociodemographic, clinical, and suicide attempt-related characteristics through Chi-square test and logistic regression analysis to evaluate the difference between two groups.
In adolescents, suicide attempters had more number of previous suicide attempt history than adults. Adolescents used more non-lethal method such as poisoning of over the counter drugs and had about 5 times higher odds ratio in suicide attempts with analgesics. The motivation of suicide attempt among adolescents was more related with interpersonal problems but less with financial or illness-related problems. The intention of suicide attempt in adolescents was less serious and lethal compared to adults.
Suicide attempts among adolescents had showed different from adults in method, motivation and intention. Considering the characteristics of suicide attempt among adolescent, it is necessary to keep close attention to adolescent's suicide attempters and develop the customized intervention program to prevent the suicide attempt in this groups.
The promising energy storage properties of new lead‐free antiferroelectric HfxZr1‐xO2 (x = 0.1–0.4) films with high energy storage density are reported. The energy storage density of the Hf0.3Zr0.7O2 ...capacitor does not decrease with the increase in temperature up to 175 °C, and it decreases by only ≈4.5% after field cycling 109 times.
Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase ...in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.
Abstract
Atomic-resolution Cs-corrected scanning transmission electron microscopy revealed local shifting of two oxygen positions (O
I
and O
II
) within the unit cells of a ferroelectric (Hf
0.5
Zr
...0.5
)O
2
thin film. A reversible transition between the polar
Pbc
2
1
and antipolar
Pbca
phases, where the crystal structures of the 180° domain wall of the
Pbc
2
1
phase and the unit cell structure of the
Pbca
phase were identical, was induced by applying appropriate cycling voltages. The critical field strength that determined whether the film would be woken up or fatigued was ~0.8 MV/cm, above or below which wake-up or fatigue was observed, respectively. Repeated cycling with sufficiently high voltages led to development of the interfacial nonpolar
P
4
2
/
nmc
phase, which induced fatigue through the depolarizing field effect. The fatigued film could be rejuvenated by applying a slightly higher voltage, indicating that these transitions were reversible. These mechanisms are radically different from those of conventional ferroelectrics.
Hf0.5Zr0.5O2 thin films are one of the most appealing HfO2-based ferroelectric thin films, which have been researched extensively for their applications in ferroelectric memory devices. In this work, ...a 1 mol % La-doped Hf0.5Zr0.5O2 thin film was grown by plasma-assisted atomic layer deposition and annealed at temperatures of 450 and 500 °C to crystallize the film into the desired orthorhombic phase. Despite the use of a lower temperature than that used in previous reports, the film showed highly promising ferroelectric propertiesa remnant polarization of ∼30 μC/cm2 and switching cycle endurance up to 4 × 1010. The performance was much better than that of undoped Hf0.5Zr0.5O2 thin films, demonstrating the positive influence of La doping. Such improvements were mainly attributed to the decreased coercive field (by ∼30% compared to the undoped film), which allowed for the use of a lower applied field to drive the cycling tests while maintaining a high polarization value. La doping also decreased the leakage current by ∼3 orders of magnitude compared to the undoped film, which also contributed to the strongly improved endurance. Nonetheless, the La-doped film required a larger number of wake-up cycles (∼106 cycles) to reach a saturated remnant polarization value. This behavior might be explained by the increased generation of oxygen vacancies and slower migration of these vacancies from the interface to the bulk region. However, the maximum number of wake-up cycles was less than 0.01% of the total possible cycles, and therefore, initializing the film to the maximum performance state would not be a serious burden.
PDF
