Predictions and measurements of the effect of biaxial strain on the properties of epitaxial ferroelectric thin films and superlattices are reviewed. Results for single-layer ferroelectric films of biaxially strained SrTiO3, BaTiO3, and PbTiO3 as well as PbTiO3/SrTiO3 and BaTiO3/SrTiO3 superlattices are described. Theoretical ap- proaches, including first principles, thermodynamic analysis, and phase-field models, are applied to these biaxially strained materials, the assumptions and limitations of each technique are explained, and the predictions are compared. Measurements of the effect of biax- ial strain on the paraelectric-to-ferroelectric transition temperature (TC) are shown, demonstrating the ability of percent-level strains to shift TC by hundreds of degrees in agreement with the predic- tions that predated such experiments. Along the way, important ex- perimental techniques for characterizing the properties of strained ferroelectric thin films and superlattices, as well as appropriate sub- strates on which to grow them, are mentioned.

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Strain Tuning of Ferroelectric Thin Films

The broad spectrum of electronic and optical properties exhibited by oxides offers tremendous opportunities for microelectronic devices, especially when a combination of properties in a single device is desired Here we describe the use of reactive molecular-beam epitaxy and pulsed-laser deposition to synthesize functional oxides, including ferroelectrics, ferromagnets, and materials that are both at the same time Owing to the dependence of properties on direction, it is often optimal to grow functional oxides in particular directions to maximize their properties for a specific application But these thin film techniques offer more than orientation control; customization of the film structure down to the atomic-layer level is possible Numerous examples of the controlled epitaxial growth of oxides with perovskite and perovskite-related structures, including superlattices and metastable phases, are shown In addition to integrating functional oxides with conventional semiconductors, standard semiconductor practices involving epitaxial strain, confined thickness, and modulation doping can also be applied to oxide thin films Results of fundamental scientific importance as well as results revealing the tremendous potential of utilizing functional oxide thin films to create devices with enhanced performance are described

/pdf/a-thin-film-approach-to-engineering-functionality-into-51t9902h5j.pdf

A Thin Film Approach to Engineering Functionality into Oxides

Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769414000013

Elastic strain engineering of ferroic oxides

A theoretical model analysis for a recently developed scanning evanescent microwave microscope has been performed. The result enables a quantitative microscopy of local complex dielectric constant profiles for dielectric materials. Various experiments were performed and found to be in good agreement with the theoretical results. The estimation of intrinsic resolution of the microscope suggests that nanometer spatial resolution is achievable. System analysis gives a limiting sensitivity of about δe/e∼1×10−5.

Quantitative microwave near-field microscopy of dielectric properties

Substrate selection presents particular challenges for the production of high‐quality high‐temperature superconducting (HTS) thin films suitable for applications. Because the substrate is generally a passive component, it is often ignored and assumed to have a negligible effect on the structure residing on top of it. There is also a technological motivation to use substrates that conventional wisdom would argue are unlikely to support high‐quality HTS films. These facts have led to rediscovery of many of the fundamental issues governing the role of the substrate in determining the properties of the thin film(s) it supports. For this reason, the study of issues in substrate selection for HTS materials presents a microcosm for substrate selection more generally. We consider the major issues governing the role of the substrate in HTS thin‐film technology and discuss many of the material classes and specific materials that have been studied for their suitability as substrates for HTS films.

Substrate selection for high‐temperature superconducting thin films

We measure the relative permittivitye
r and dielectric loss tangent tanδ of substrate materials for high-T
c superconducting films at 18–300 K and at 5–10 GHz using the cavity-resonator method. The materials measured are single crystals of MgO, LaAlO3, YAlO3, LaSrGaO4, NdGaO3, sapphire, and ZrO2 ceramic. Thee
r values are 10–30 and become almost constant below about 50 K. The tanδ values decrease with decreasing temperature and are below 1×10−5 at 77 K except for those of NdGaO3 and ZrO2 ceramic. This suggests that the tanδ values of MgO, LaAlO3, YAlO3, LaSrGaO4, and sapphire do not limit the quality factors of microwave passive components fabricated using high-T
c superconducting films. It is also demonstrated that the tanδ of the substrate material is strongly affected by impurities.

Relative permittivity and dielectric loss tangent of substrate materials for high-Tc superconducting film

Superconducting Ba2YCu3O7-y films have been grown epitaxially on (100) and (110) SrTiO3 substrates at a substrate temperature of 600°C using an evaporator combined with an ECR ion source for oxidation. Three molecular beams (BaF2, Y and Cu) were deposited simultaneously under the oxygen ion-beam (50 eV) bombardment. The as-deposited films showed zero resistance at 80–82 K with thicknesses between 200 and 700 nm on both (100) and (110) substrates. The oxygen ion beam was very useful to oxidize the metal sufficiently, to dissociate the BaF2 molecules and to lower the substrate temperature.

As-Deposited Superconducting Ba2YCu3O7-y Films Using ECR Ion Beam Oxidation

Epitaxial growth of 1-2-3 superconductor films is reported on a new substrate material that possesses desirable dielectric properties. Magnetron-sputtered EuBa2Cu3O7-y films on YAlO3(001) show excellent properties with a Tc of 91 K. Films are epitaxially grown with the c-axis oriented normal to the surface, and the a- and b-axes ordered in the plane. The high-frequency (50 GHz) surface resistance of a film is 2 mΩ at 30 K. The dielectric constant of YAlO3 is 16. The dielectric loss tangent at 10 GHz is as low as 7.3×10-5 at 300 K and 2×10-6 at 40 K. YAlO3-based films have considerable potential for high-frequency application.

https://iopscience.iop.org/article/10.1143/JJAP.29.L1452/pdf

Epitaxial Growth of EuBa2Cu3O7-yFilms on YAlO3Single Crystals

Effects of ion irradiation on the critical current density of high-Tc superconducting Ba2YCu3O7-δ thin films have been studied. For the film subjected to a fluence of 5.02×1011 ions/cm2 of 514 keV O+, the critical current density determined from I-V characteristics was enhanced by about 10% from 53 to 75 K. The temperature dependence of the critical current density was not affected by ion irradiation. Irradiation effects on the critical temperature of these films have also been investigated.

Effects of Ion Irradiation on the Critical Current Density of Ba2YCu3O7-δ Thin Films

Effect of 28 MeV electron irradiation was investigated on the critical current density (Jc) of high-Tc superconductor Ba2YCu3O7-δ thin films. For the film irradiated with a fluence of 4.7×1016 electrons/cm2, Jc determined from I-V characteristics at temperatures from 75 to 85 K was enhanced by about 8%. The enhance ratio of Jc due to irradiation increased with fluence over the range investigated, while the critical temperature (Tc) was unchanged. The temperature dependence of Jc was not affected by electron irradiation.

Shugo Kubo

Papers

Relative permittivity and dielectric loss tangent of substrate materials for high-Tc superconducting film

As-Deposited Superconducting Ba2YCu3O7-y Films Using ECR Ion Beam Oxidation

Epitaxial Growth of EuBa2Cu3O7-yFilms on YAlO3Single Crystals

Effects of Ion Irradiation on the Critical Current Density of Ba2YCu3O7-δ Thin Films

Enhancement of Transport Critical Current Density of Ba 2 YCu 3 O 7-δ Thin Film by 28 MeV Electron Irradiation