Crystal Structure of Monoclinic Hafnia and Comparison with Monoclinic Zirconia
01 Mar 1970-Journal of the American Ceramic Society (Blackwell Publishing Ltd)-Vol. 53, Iss: 3, pp 126-129
TL;DR: In this article, the crystal structure of monoclinic HfO2 was determined using Weissenberg techniques using a needle-like single crystal which was grown from a lithium molybdate melt.
Abstract: The crystal structure of monoclinic HfO2 was determined using Weissenberg techniques. Data were obtained from a needlelike single crystal which was grown from a lithium molybdate melt. The crystal was rotated about the α axis, and reflection data were obtained for nine levels. A three-dimensional least-squares refinement confirms that monoclinic hafnia is isomorphous with monoclinic zirconia. The atomic coordinates in the structures agree within one standard deviation. Thus, the fractional coordinates for the metal atom differ by less than 0.0005 and those for the oxygen atoms by no more than 0.01. Whereas three of the seven Hf–O distances are larger than the corresponding Zr–O distances, the average value is approximately 0.01 A smaller. The average metal-metal distance is approximately 0.02 A less in HfO2 than in ZrO2.
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TL;DR: A structural investigation revealed the orthorhombic phase to be of space group Pbc2(1), whose noncentrosymmetric nature is deemed responsible for the spontaneous polarization in this novel, nanoscale ferroelectrics.
Abstract: The transition metal oxides ZrO2 and HfO2 as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO2 thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO2–ZrO2 mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc21, whose noncentrosymmetric nature is deemed responsible for the spontaneous...
1,161 citations
TL;DR: In this paper, the structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2 and Hf0.5O2 are investigated with carefully validated density functional computations.
Abstract: 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.
528 citations
TL;DR: In this article, a review of the science and technology of HfO2 and hafnium-based materials in terms of processing, phase transformation, microstructure, and mechanical properties is presented.
Abstract: Hafnia (HfO2) and hafnium-based materials are traditionally regarded as technologically important materials in the nuclear industry, a consequence of their exceptionally high neutron absorption coefficient. Following the discovery of transformation toughening in the mid 1970s, a considerable research effort has been devoted to zirconia (ZrO2)-toughened ceramics (ZTCs). They are considered to be potentially useful materials for structural applications at low and intermediate temperatures (T 1000 °C) is related to the low temperature of the tetragonal to monoclinic transformation in ZrO2. On the basis that HfO2 exhibits a similar crystal structure and in particular that its tetragonal to monoclinic transformation temperature (∼1700 °C) is approximately 700 °C higher than that for ZrO2, it has been suggested that high-temperature transformation toughening could be possible in HfO2-toughened ceramics (HTCs). Although the concepts behind this suggestion are universally appreciated, only a limited success has been made of the fabrication and the microstructural and mechanical property evaluation of these materials. The fracture toughness values obtained so far in HfO2 toughened ceramics are, in fact, considerably lower than those obtained in their ZrO2 counterparts. A great deal of further research work is therefore required in order to understand fully and to exploit toughened ceramics in the HfO2-based and HfO2-containing systems. This review covers the science and technology of HfO2 and HfO2-toughened ceramics in terms of processing, phase transformation, microstructure, and mechanical properties.
476 citations
TL;DR: In this paper, the authors review how metal oxide-based gate dielectrics emerged from all likely candidates to become the new gold standard in the microelectronics industry, its different phases, reported electrical properties, and materials processing techniques, including carrier scattering, interface state passivation, phonon engineering, and nano-scale patterning.
Abstract: The move to implement metal oxide based gate dielectrics in a metal-oxide-semiconductor field effect transistor is considered one of the most dramatic advances in materials science since the invention of silicon based transistors. Metal oxides are superior to SiO 2 in terms of their higher dielectric constants that enable the required continuous down-scaling of the electrical thickness of the dielectric layer while providing a physically thicker layer to suppress the quantum mechanical tunneling through the dielectric layer. Over the last decade, hafnium based materials have emerged as the designated dielectrics for future generation of nano-electronics with a gate length less than 45 nm, though there exists no consensus on the exact composition of these materials, as evolving device architectures dictate different considerations when optimizing a gate dielectric material. In addition, the implementation of a non-silicon based gate dielectric means a paradigm shift from diffusion based thermal processes to atomic layer deposition processes. In this report, we review how HfO 2 emerges from all likely candidates to become the new gold standard in the microelectronics industry, its different phases, reported electrical properties, and materials processing techniques. Then we use specific examples to discuss the evolution in designing hafnium based materials, from binary to complex oxides and to non-oxide forms as gate dielectric, metal gates and diffusion barriers. To address the impact of these hafnium based materials, their interfaces with silicon as well as a variety of semiconductors are discussed. Finally, the integration issues are highlighted, including carrier scattering, interface state passivation, phonon engineering, and nano-scale patterning, which are essential to realize future generations of devices using hafnium-based high- k materials.
450 citations
TL;DR: In this article, the structural, thermal, and dielectric properties of the ferroelectric phase of HfO$_2, ZrO_2$ and Hf$_{0.5}$ Zr$_{1-\chi}$ O$-2$ (HZO) are investigated with carefully validated density functional computations.
Abstract: The structural, thermal, and dielectric properties of the ferroelectric phase of HfO$_2$, ZrO$_2$ and Hf$_{0.5}$ Zr$_{0.5}$ O$_2$ (HZO) are investigated with carefully validated density functional computations. We find, that the free bulk energy of the ferroelectric orthorhombic Pca2$_{1}$ phase is unfavorable compared to the monoclinic P2$_{1}$/c and the orthorhombic Pbca phase for all investigated stoichiometries in the Hf$_{\chi}$Zr$_{1-\chi}$O$_2$ 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 HfO$_2$ 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 ZrO$_2$ from the P4$_2$/nmc phase as 1 MV/cm in agreement with experimental data, explaining the mechanism of field induced phase transformation.
432 citations
References
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1,249 citations
TL;DR: In this paper, the crystal structure of dicyandiamide, the dimer of cyanamide, was investigated, with the results described in Section 2.2.1.
Abstract: In an earlier paper the crystal structure of dicyandiamide, the dimer of cyanamide, was reported. I have now completed a similar investigation of melamine, or cyanuric triamide, the trimer of cyanamide, with the results described below.
319 citations
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TL;DR: In this article, the absolute values of |F (h)| 2 can be deduced to a close approximation from the relative intensity data without any further experimental work, and the derivation of the equations needed in this deduction in the case of a linear crystal is shown below.
Abstract: IN the 'absolute measurements' of X-ray intensities of reflexions from a crystal, a measurement must be made of the monochromatic radiation incident upon the crystal in one second1, which, though it can be done fairly conveniently by the ionization chamber method2, requires a complicated design of apparatus in the use of the rotation photographic method. I have considered the problem in the light of a new synthesis of X-ray data3 and found that the absolute values of |F (h)| 2 may be deduced to a close approximation from the relative intensity data without any further experimental work. The derivation of the equations needed in this deduction in the case of a linear crystal is shown below.
180 citations
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