Origin of the dielectric dead layer in nanoscale capacitors
TL;DR: The existence of a dielectric dead layer is demonstrated by calculating the dielectrics profile across the interface and its origin is analysed by extracting the ionic and electronic contributions to the electrostatic screening.
Abstract: New theoretical work nails down the microscopic origin of 'dead layers' in nanometre-scale capacitors and demonstrates that it is an intrinsic effect. The results provide practical guidelines for minimizing the deleterious effects of the dielectric dead layer, for example regarding the choice of electrode. Capacitors are a mainstay of electronic integrated circuits and devices, where they perform essential functions such as storing electrical charge, and blocking direct current while allowing alternating currents to propagate. Because they are often the largest components in circuits, extensive efforts are directed at reducing their size through the use of high-permittivity insulators such as perovskite-structure SrTiO3 (refs 1, 2), which should provide more capacitance per unit area of device. Unfortunately, most experiments on thin-film SrTiO3 capacitors have yielded capacitance values that are orders of magnitude smaller than expected3. The microscopic origin of this reduced capacitance, which is often discussed in terms of a low-permittivity interfacial ‘dead layer’4, is not well understood. Whether such a dead layer exists at all, and if so, whether it is an intrinsic property of an ideal metal–insulator interface or a result of processing issues such as defects and strains, are controversial questions. Here we present fully ab initio calculations of the dielectric properties of realistic SrRuO3/SrTiO3/SrRuO3 nanocapacitors, and show that the observed dramatic capacitance reduction is indeed an intrinsic effect. We demonstrate the existence of a dielectric dead layer by calculating the dielectric profile across the interface and analyse its origin by extracting the ionic and electronic contributions to the electrostatic screening. We establish a correspondence between the dead layer and the hardening of the collective SrTiO3 zone-centre polar modes, and determine the influence of the electrode by repeating our calculations for Pt/SrTiO3/Pt capacitors. Our results provide practical guidelines for minimizing the deleterious effects of the dielectric dead layer in nanoscale devices.
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TL;DR: Novel device paradigms based on magnetoelectric coupling are discussed, the key scientific challenges in the field are outlined, and high-quality thin-film multiferroics are reviewed.
Abstract: Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.
3,472 citations
TL;DR: In this paper, the authors present a survey of the use of Wannier functions in the context of electronic-structure theory, including their applications in analyzing the nature of chemical bonding, or as a local probe of phenomena related to electric polarization and orbital magnetization.
Abstract: The electronic ground state of a periodic system is usually described in terms of extended Bloch orbitals, but an alternative representation in terms of localized "Wannier functions" was introduced by Gregory Wannier in 1937. The connection between the Bloch and Wannier representations is realized by families of transformations in a continuous space of unitary matrices, carrying a large degree of arbitrariness. Since 1997, methods have been developed that allow one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set of maximally localized Wannier functions, accomplishing the solid-state equivalent of constructing localized molecular orbitals, or "Boys orbitals" as previously known from the chemistry literature. These developments are reviewed here, and a survey of the applications of these methods is presented. This latter includes a description of their use in analyzing the nature of chemical bonding, or as a local probe of phenomena related to electric polarization and orbital magnetization. Wannier interpolation schemes are also reviewed, by which quantities computed on a coarse reciprocal-space mesh can be used to interpolate onto much finer meshes at low cost, and applications in which Wannier functions are used as efficient basis functions are discussed. Finally the construction and use of Wannier functions outside the context of electronic-structure theory is presented, for cases that include phonon excitations, photonic crystals, and cold-atom optical lattices.
2,217 citations
Cites background from "Origin of the dielectric dead layer..."
...In particular, Stengel and Spaldin (2006a) showed how to modify the above expressions in a way that renders the spread functional strictly invariant under BZ folding....
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...For extended bulk systems, this convergence problem can be ameliorated significantly by calculating the position operator using real-space integrals (Lee, 2006; Lee et al., 2005; Stengel and Spaldin, 2006a)....
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...…or applied electric fields along a specific spatial direction (Giustino and Pasquarello, 2005; Giustino et al., 2003; Murray and Vanderbilt, 2009; Stengel and Spaldin, 2006a; Wu et al., 2006) or for analyzing aspects of topological insulators (Coh and Vanderbilt, 2009; Soluyanov and Vanderbilt,…...
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...This slow convergence was noted by Marzari and Vanderbilt (1997) when commenting on the convergence properties of Ω with respect to the spacing of the Monkhorst-Pack mesh, and has been studied in detail by others (Stengel and Spaldin, 2006a; Umari and Pasquarello, 2003)....
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TL;DR: In this paper, a review of magnetoelectric domain walls is presented, focusing on magneto-electrics and multiferroics but making comparisons where possible with magnetic domains and domain walls.
Abstract: Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.
1,022 citations
Additional excerpts
...…explanations for the worsening of the dielectric constant of thin films, although the exact nature, thickness, and even location of the dead layer, which might be inside the electrode, is still a subject of debate (Sinnamon, Bowman, and Gregg, 2001; Stengel and Spaldin, 2006; Chang et al., 2009)....
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...…this depolarization field and, although the screening is never perfect (Batra and Silverman, 1972; Dawber, Jung, and Scott, 2003; Dawber et al., 2003; Stengel and Spaldin, 2006), good electrodes can stabilize ferroelectricity down to films just a few unit cells thick (Junquera and Ghosez, 2003)....
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TL;DR: The progress made in the properties of dielectric nanosheets is reviewed, highlighting emerging functionalities in electronic applications and a perspective on the advantages offered by this class of materials for future nanoelectronics.
Abstract: Two-dimensional (2D) nanosheets, which possess atomic or molecular thickness and infinite planar lengths, are regarded as the thinnest functional nanomaterials. The recent development of methods for manipulating graphene (carbon nanosheet) has provided new possibilities and applications for 2D systems; many amazing functionalities such as high electron mobility and quantum Hall effects have been discovered. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, capacitors, and gate dielectrics. Along with graphene, inorganic nanosheets have thus increasingly attracted fundamental research interest because they have the potential to be used as dielectric alternatives in next-generation nanoelectronics. Here, we review the progress made in the properties of dielectric nanosheets, highlighting emerging functionalities in electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanoelectronics.
958 citations
TL;DR: Room-temperature ferroelectricity in 2D CuInP2S6 (CIPS) with a transition temperature of ∼320 K is reported and switchable polarization is observed in thin CIPS of ∼4 nm.
Abstract: Two-dimensional (2D) materials have emerged as promising candidates for various optoelectronic applications based on their diverse electronic properties, ranging from insulating to superconducting. However, cooperative phenomena such as ferroelectricity in the 2D limit have not been well explored. Here, we report room-temperature ferroelectricity in 2D CuInP2S6 (CIPS) with a transition temperature of ∼320 K. Switchable polarization is observed in thin CIPS of ∼4 nm. To demonstrate the potential of this 2D ferroelectric material, we prepare a van der Waals (vdW) ferroelectric diode formed by CIPS/Si heterostructure, which shows good memory behaviour with on/off ratio of ∼100. The addition of ferroelectricity to the 2D family opens up possibilities for numerous novel applications, including sensors, actuators, non-volatile memory devices, and various vdW heterostructures based on 2D ferroelectricity. Two dimensional materials are promising for electronic applications, which await the exploration of cooperative phenomena. Here, Liu et al. report switchable ferroelectric polarization in thin CuInP2S6film at room temperature, demonstrating good memory behaviour with on/off ratio of ∼100 based on two-dimensional ferroelectricity.
559 citations
References
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IBM1
TL;DR: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way and can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function.
Abstract: An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
61,450 citations
TL;DR: In this paper, the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method, is reviewed.
Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.
6,917 citations
TL;DR: It is shown that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferro electric properties below a critical thickness of about six unit cells, suggesting the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.
Abstract: The integration of ferroelectric oxide films into microelectronic devices, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 A (ten unit cells), ref. 3. Recent atomistic simulations have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric-electrode interface. We show that, contrary to current thought, BaTiO3 thin films between two metallic SrRuO3 electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (approximately 24 A). A depolarizing electrostatic field, caused by dipoles at the ferroelectric-metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.
1,355 citations
TL;DR: In this paper, a synchrotron x-ray study of lead titanate as a function of temperature and film thickness for films as thin as a single unit cell was performed.
Abstract: Understanding the suppression of ferroelectricity in perovskite thin films is a fundamental issue that has remained unresolved for decades. We report a synchrotron x-ray study of lead titanate as a function of temperature and film thickness for films as thin as a single unit cell. At room temperature, the ferroelectric phase is stable for thicknesses down to 3 unit cells (1.2 nanometers). Our results imply that no thickness limit is imposed on practical devices by an intrinsic ferroelectric size effect.
1,055 citations
TL;DR: The theory of metal-ceramic interfaces is a collection of approaches which are complementary as mentioned in this paper, ranging from thermodynamic modelling based on empirical correlations, through the image model of adhesion, semi-empirical tight-binding calculations, to first-principles calculations based on applying the density functional theory or Hartree - Fock theory.
Abstract: The theory of metal - ceramic interfaces is a collection of approaches which are complementary. They range from thermodynamic modelling based on empirical correlations, through the image model of adhesion, semi-empirical tight-binding calculations, to first-principles calculations based on applying the density functional theory or Hartree - Fock theory. This article reviews the present state of theoretical calculations, with particular reference to electronic structure and adhesion. A section on the thermodynamic background clarifies the concept of work of adhesion which is the goal of many calculations. Cluster models and periodic slabs have been considered, both self-consistent and non-self-consistent. The most sophisticated and complete calculations have been made for metals on MgO and alumina. There a consistent picture of the nature of the bonding has emerged, although there are still significant unexplained discrepancies in numerical values.
533 citations