Structure and properties of functional oxide thin films: insights from electronic-structure calculations

The confluence of state-of-the-art electronic-structure computations and modern synthetic materials growth techniques is proving indispensable in the search for and discovery of new functionalities in oxide thin films and heterostructures. Here, we review the recent contributions of electronic-structure calculations to predicting, understanding, and discovering new materials physics in thin-film perovskite oxides. We show that such calculations can accurately predict both structure and properties in advance of film synthesis, thereby guiding the search for materials combinations with specific targeted functionalities. In addition, because they can isolate and decouple the effects of various parameters which unavoidably occur simultaneously in an experiment-such as epitaxial strain, interfacial chemistry and defect profiles-they are able to provide new fundamental knowledge about the underlying physics. We conclude by outlining the limitations of current computational techniques, as well as some important open questions that we hope will motivate further methodological developments in the field.     

Thin-film processing of high-T c superconductors

This review of high-T _c superconducting thin-film processing focuses on the developments in thin-film deposition technologies since 1987. The common deposition processes are described with reference to their effects on superconductor film performance. A comparative evaluation of the potential of the technologies is also given. The development of multilayers and heterostructures is an important requirement for future device applications and is also described. The latest results of the deposition of novel superconducting materials and deposition on uncommon substrates are discussed. The outlook on some imminent topics of future development in process technologies for high-T _c superconducting thin films is discussed.     

Thin-film processing of high-Tc superconductors

This review of high-T _c superconducting thin-film processing focuses on the developments in thin-film deposition technologies since 1987. The common deposition processes are described with reference to their effects on superconductor film performance. A comparative evaluation of the potential of the technologies is also given. The development of multilayers and heterostructures is an important requirement for future device applications and is also described. The latest results of the deposition of novel superconducting materials and deposition on uncommon substrates are discussed. The outlook on some imminent topics of future development in process technologies for high-T _c superconducting thin films is discussed.     

Growth of flat SrRuO3 (111) thin films suitable as bottom electrodes in heterostructures

Thin film growth of ferroelectric or multiferroic materials on SrTiO₃(111) with a buffer electrode has been hampered by the difficulty of growing flat electrodes on this polar orientation. We report on the growth and characterization of SrRuO₃ thin films deposited by pulsed laser deposition on SrTiO₃(111). We show that our SrRuO₃(111) films are epitaxial and display magnetic bulk-like properties. Films presenting a thickness between 20 and 30 nm are found to be very flat and therefore suitable as bottom electrodes in heterostructures.     

Physical properties of lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanochemically synthesized powders

Modern electronics expect functional materials that are eco-friendly and are obtained with lower energy consumption technological processes. The multiferroic lead-free BaFe_1/2Nb_1/2O₃ (BFN) ceramic powder has been prepared by mechanochemical synthesis from simple oxides at room temperature. The development of the synthesis has been monitored by XRD and SEM investigations, after different milling periods. The obtained powders contain large agglomerates built by crystals with an estimated size about 12–20 nm depending on the period of milling. From this powder, the multiferroic BFN ceramic samples have been prepared by uniaxial pressing and subsequent sintering pressureless method. The morphology of the BFN ceramic samples strongly depends on high-energy milling duration. The properties of the ceramic samples have been investigated by dielectric spectroscopy, in broad temperature and frequency ranges. The high-energy milling of the powders has strongly affected the dielectric permittivity and dielectric loss of the BaFe_1/2Nb_1/2O₃ ceramic samples. The usage of the mechanochemical synthesis to obtain the multiferroic lead-free BFN materials reduces the required thermal treatment and simultaneously improves the parameters of the BFN ceramics.     

Effect of Ba(Cu1/3Nb2/3)O3 content on multiferroic properties in BiFeO3 ceramics

The crystal structure, microstructural evolution, and ferroelectric and magnetic properties of BiFeO₃ (BFO) ceramic materials, widely known as room temperature multiferroic materials, were investigated with various contents of Ba(Cu_1/3Nb_2/3)O₃ (BCN). The ceramics were synthesized through the solid state reaction and pressureless sintering in air. By adding tetragonal structured BCN phase to the rhombohedral structured BFO phase, a solid solution was formed. The rhombohedral crystal structure and lattice parameter, densification behavior, and microstructure of BFO were noticeably changed upon the addition of BCB. The electrical properties of BFO were slightly enhanced by the addition of BCN. However, the magnetic properties of BFO which is the most critical issue in BFO multiferroics were drastically changed and the 20 mol% BCN added BFO ceramics showed extremely enhanced magnetization characteristics compared to the other BFO–BCN ceramics. It is expected that BFO–BCN ceramics with this composition could be one of the most promising multiferroic materials for future multiferroic applications.     

Experimental evidence of enhanced ferroelectricity in Ca doped BiFeO3

Calcium (Ca)-doped bismuth ferrite (BiFeO₃) thin films prepared by using the polymeric precursor method (PPM) were characterized by X-ray diffraction (XRD), field emission gun scanning electron microscopy (FEG-SEM), transmission electron microscopy (TEM), polarization and piezoelectric measurements. Structural studies by XRD and TEM reveal the co-existence of distorted rhombohedral and tetragonal phases in the highest doped BiFeO₃ where enhanced ferroelectric and piezoelectric properties are produced by internal strain. Resistive switching is observed in BFO and Ca-doped BFO which are affected by the barrier contact and work function of multiferroic materials and Pt electrodes. A high coercive field in the hysteresis loop is observed for the BiFeO₃ film. Piezoelectric properties are improved in the highest Ca-doped sample due to changes in the crystal structure of BFO for a primitive cubic perovskite lattice with four-fold symmetry and a large tetragonal distortion within the crystal domain. This observation introduces magnetoelectronics at room temperature by combining electronic conduction with electric and magnetic degrees of freedom which are already present in the multiferroic BiFeO₃.     

Enhanced magnetic properties of chemical solution deposited BiFeO3 thin film with ZnO buffer layer

Magnetic properties of BiFeO₃ films deposited on Si substrates with and without ZnO buffer layer have been studied in this work. We adopted the chemical solution deposition method for the deposition of BiFeO₃ as well as ZnO films. The x-ray diffraction measurements on the deposited films confirm the formation of crystalline phase of BiFeO₃ and ZnO films, while our electron microscopy measurements help to understand the morphology of few micrometers thick films. It is found that the deposited ZnO film exhibit a hexagonal particulate surface morphology, whereas BiFeO₃ film fully covers the ZnO surface. Our magnetic measurements reveal that the magnetization of BiFeO₃ has increased by more than ten times in BiFeO₃/ZnO/Si film compared to BiFeO₃/Si film, indicating the major role played by ZnO buffer layer in enhancing the magnetic properties of BiFeO₃, a technologically important multiferroic material.     

Application of bismuth ferrite,BiFeO3, in heterostructures for HEMTs and high-T c materials

Heterostructures based on multiferroic bismuth ferrite, BiFeO₃, are proposed for use in micro-wave HEMTs and for the synthesis of high-T _c materials with a high superconducting transition temperature T _c.     

Realization of Large Electric Polarization and Strong Magnetoelectric Coupling in BiMn3Cr4O12

Magnetoelectric multiferroics have received much attention in the past decade due to their interesting physics and promising multifunctional performance. For practical applications, simultaneous large ferroelectric polarization and strong magnetoelectric coupling are preferred. However, these two properties have not been found to be compatible in the single‐phase multiferroic materials discovered as yet. Here, it is shown that superior multiferroic properties exist in the A‐site ordered perovskite BiMn₃Cr₄O₁₂ synthesized under high‐pressure and high‐temperature conditions. The compound experiences a ferroelectric phase transition ascribed to the 6s² lone‐pair effects of Bi³⁺ at around 135 K, and a long‐range antiferromagnetic order related to the Cr³⁺ spins around 125 K, leading to the presence of a type‐I multiferroic phase with huge electric polarization. On further cooling to 48 K, a type‐II multiferroic phase induced by the special spin structure composed of both Mn‐ and Cr‐sublattices emerges, accompanied by considerable magnetoelectric coupling. BiMn₃Cr₄O₁₂ thus provides a rare example of joint multiferroicity, where two different types of multiferroic phases develop subsequently so that both large polarization and significant magnetoelectric effect are achieved in a single‐phase multiferroic material.     

Epitaxial Bi2FeCrO6 Multiferroic Thin Film as a New Visible Light Absorbing Photocathode Material

Ferroelectric materials have been studied increasingly for solar energy conversion technologies due to the efficient charge separation driven by the polarization induced internal electric field. However, their insufficient conversion efficiency is still a major challenge. Here, a photocathode material of epitaxial double perovskite Bi₂FeCrO₆ multiferroic thin film is reported with a suitable conduction band position and small bandgap (1.9–2.1 eV), for visible‐light‐driven reduction of water to hydrogen. Photoelectrochemical measurements show that the highest photocurrent density up to ?1.02 mA cm^?2 at a potential of ?0.97 V versus reversible hydrogen electrode is obtained in p‐type Bi₂FeCrO₆ thin film photocathode grown on SrTiO₃ substrate under AM 1.5G simulated sunlight. In addition, a twofold enhancement of photocurrent density is obtained after negatively poling the Bi₂FeCrO₆ thin film, as a result of modulation of the band structure by suitable control of the internal electric field gradient originating from the ferroelectric polarization in the Bi₂FeCrO₆ films. The findings validate the use of multiferroic Bi₂FeCrO₆ thin films as photocathode materials, and also prove that the manipulation of internal fields through polarization in ferroelectric materials is a promising strategy for the design of improved photoelectrodes and smart devices for solar energy conversion.     

The Mn + 1AXn phases: Materials science and thin-film processing

This article is a critical review of the M_n + 1AX_n phases (“MAX phases”, where n = 1, 2, or 3) from a materials science perspective. MAX phases are a class of hexagonal-structure ternary carbides and nitrides (“X”) of a transition metal (“M”) and an A-group element. The most well known are Ti₂AlC, Ti₃SiC₂, and Ti₄AlN₃. There are ~ 60 MAX phases with at least 9 discovered in the last five years alone. What makes the MAX phases fascinating and potentially useful is their remarkable combination of chemical, physical, electrical, and mechanical properties, which in many ways combine the characteristics of metals and ceramics. For example, MAX phases are typically resistant to oxidation and corrosion, elastically stiff, but at the same time they exhibit high thermal and electrical conductivities and are machinable. These properties stem from an inherently nanolaminated crystal structure, with M_n + 1X_n slabs intercalated with pure A-element layers. The research on MAX phases has been accelerated by the introduction of thin-film processing methods. Magnetron sputtering and arc deposition have been employed to synthesize single-crystal material by epitaxial growth, which enables studies of fundamental material properties. However, the surface-initiated decomposition of M_n + 1AX_n thin films into MX compounds at temperatures of 1000-1100 °C is much lower than the decomposition temperatures typically reported for the corresponding bulk material. We also review the prospects for low-temperature synthesis, which is essential for deposition of MAX phases onto technologically important substrates. While deposition of MAX phases from the archetypical Ti-Si-C and Ti-Al-N systems typically requires synthesis temperatures of ~ 800 °C, recent results have demonstrated that V₂GeC and Cr₂AlC can be deposited at ~ 450 °C. Also, thermal spray of Ti₂AlC powder has been used to produce thick coatings. We further treat progress in the use of first-principle calculations for predicting hypothetical MAX phases and their properties. Together with advances in processing and materials analysis, this progress has led to recent discoveries of numerous new MAX phases such as Ti₄SiC₃, Ta₄AlC₃, and Ti₃SnC₂. Finally, important future research directions are discussed. These include charting the unknown regions in phase diagrams to discover new equilibrium and metastable phases, as well as research challenges in understanding their physical properties, such as the effects of anisotropy, impurities, and vacancies on the electrical properties, and unexplored properties such as superconductivity, magnetism, and optics.     

Silicon nitride: the engineering material of the future

The purpose of this review is to present the recent developments in silicon nitride (Si₃N₄) ceramics and to examine the achievements regarding our understanding of the relationship between processing conditions, chemical composition, microstructure and mechanical properties of Si₃N₄. Si₃N₄ is one of the most important structural ceramics because it possesses a combination of advanced properties such as good wear and corrosive resistance, high flexural strength, good fracture resistance, good creep resistance and relatively high hardness. These properties are obtained through the processing method involving liquid phase sintering in which a tailored microstructure, with high aspect ratio grains and chemistry of intergranular phase, triggers the toughening and strengthening mechanisms leading to the development of high fracture toughness and fracture strength. However, despite high fracture toughness and strength, Si₃N₄ ceramic materials still break catastrophically, and the fracture behaviour of this ceramic is considered to be the major obstacle for its wider use as a structural material. In addition to the macrostructure–mechanical properties relationship, this paper also reviews new designs involving laminates possessing no plane of weakness and some theoretical developments involving crack opening displacement. Proposals of how to improve the fracture resistance were also discussed.     

Anisotropic electronic structure in single crystalline orthorhombic TbMnO3 thin films

We have deposited the c-axis-oriented orthorhombic TbMnO₃ (o-TMO) films with well-aligned in-plane orientations on NdGaO₃ (001) substrates by using pulsed laser deposition. The distinctive orientation alignments between the film and substrate allow the study of the X-ray absorption spectroscopy (XAS) with the electric field along three major crystallographic directions, respectively. Polarization-dependent XAS spectra show significant anisotropy in the electronic structure of o-TMO. The correlation between the electronic structure, the bonding anisotropy, and the magnetoelectric effect in the multiferroic materials is revealed.     

Epitaxial growth and interface strain coupling effects in manganite film/piezoelectric-crystal multiferroic heterostructures

We constructed multiferroic structures by epitaxially growing colossal magnetoresistive La_0.7Sr_0.3MnO₃ (LSMO) thin films on piezoelectric single-crystal substrates of composition 0.67Pb(Mg_1/3Nb_2/3)O₃–0.33PbTiO₃ (PMN-PT). Due to the efficient elastic coupling at the interface, the electric-field-induced piezoelectric strain (?_piezo) in the PMN-PT substrate is effectively transferred to the LSMO film, giving rise to a remarkable modulation of the lattice strain, resistivity, and Curie temperature T_C of the LSMO film. Particularly, it was found that the magnetic field has an opposite effect on the strain-tunability of resistivity above and below T_C. Moreover, we found that the resistivity of the film is most sensitive to ?_piezo near T_C and becomes less sensitive to ?_piezo when the temperature is lower or higher than T_C. These, together with the well fitted resistivity data into a phenomenological model based on coexisting phases, demonstrate that the phase separation is crucial to understand the strain-mediated multiferroic properties in manganite film/PMN-PT structures.     

Substitution of transparent conducting oxide thin films for indium tin oxide transparent electrode applications

The present status and prospects for further development of reduced or indium-free transparent conducting oxide (TCO) materials for use in practical thin-film transparent electrode applications such as liquid crystal displays are presented in this paper: reduced-indium TCO materials such as ZnO-In₂O₃, In₂O₃-SnO₂ and Zn-In-Sn-O multicomponent oxides and indium-free materials such as Al- and Ga-doped ZnO (AZO and GZO). In particular, AZO thin films, with source materials that are inexpensive and non-toxic, are the best candidates. The current problems associated with substituting AZO or GZO for ITO, besides their stability in oxidizing environments as well as the non-uniform distribution of resistivity resulting from dc magnetron sputtering deposition, can be resolved. Current developments associated with overcoming the remaining problems are also presented: newly developed AZO thin-film deposition techniques that reduce resistivity as well as improve the resistivity distribution uniformity using high-rate dc magnetron sputtering depositions incorporating radio frequency power. In addition, stability tests of resistivity in TCO thin films evaluated in air at 90% relative humidity and 60 °C have demonstrated that sufficiently moisture-resistant AZO thin films can be produced at a substrate temperature below 200 °C when the film thickness was approximately 200 nm. However, improving the stability of AZO and GZO films with a thickness below 100 nm remains a problem.     

High temperature superconducting ceramics

We review some of the recent developments in our empirical understanding of the materials science of high T_c materials. First, the distinguishing phenomenological Ginzburg-Landau features of these materials are identified, and the possible technological implications discussed. We then focus on two key issues for processing the 1-2-3 materials: oxygen stoichiometry and grain boundary phases. Finally, we briefly examine the impact of the recent discoveries of bismuth-based and thallium-based materials on the field.     

Magnetic and electrical properties of multiferroic BiFeO3, its synthesis and applications

Major ways to improve the magnetic and electrical properties of promising multiferroic BiFeO₃ and optimize its synthesis have been studied, and its applications in spintronics, photonics, and magnonics have been discussed.     

Bi4Ti3O12–(Ni0.5Zn0.5)Fe2O4 dielectric–ferromagnetic ceramic composites synthesized with nanopowders

Bi₄Ti₃O₁₂ and (Ni_0.5Zn_0.5)Fe₂O₄ nanopowders were prepared by co-precipitation and hydrothermal methods, respectively. It was found that ethanolamine is effective as a precipitating agent in the synthesis of Bi₄Ti₃O₁₂ nanopowders via co-precipitation, and it also plays an important role in the synthesis of (Ni_0.5Zn_0.5)Fe₂O₄ nanopowders. Bi₄Ti₃O₁₂–(Ni_0.5Zn_0.5)Fe₂O₄ multiferroic ceramics were obtained by sintering the as-prepared nanopowders. Lower sintering temperatures (800–900 °C) were available when compared with the traditional solid state method and ceramic composites with higher density and limited interfacial reaction were obtained. The ceramics also showed lower dielectric loss and higher magnetic moments. Both the ferroelectric and magnetic phases preserve their individual properties in bulk composite form and thus, these types of composite ceramics appear to be good candidate multiferroic materials.