Jeffrey A. Bellotti

In-plane microwave dielectric properties of paraelectric barium strontium titanate thin films with anisotropic epitaxy

In-plane dielectric properties of ⟨110⟩ oriented epitaxial (Ba0.60Sr0.40)TiO3 thin films in the thickness range from 25–1200nm have been investigated under the influence of anisotropic epitaxial strains from ⟨100⟩ NdGaO3 substrates. The measured dielectric properties show strong residual strain and in-plane directional dependence. Below 150nm film thickness, there appears to be a phase transition due to the anisotropic nature of the misfit strain relaxation. In-plane relative permittivity is found to vary from as much as 500–150 along [11¯0] and [001] respectively, in 600nm thick films, and from 75 to 500 overall. Tunability was found to vary from as much as 54% to 20% in all films and directions, and in a given film the best tunability is observed along the compressed axis in a mixed strain state, 54% along [11¯0] in the 600nm film for example.

Room-temperature tunable microwave properties of strained SrTiO3 films

Structural distortion of ferroelectric thin films caused by film strain has a strong impact on the microwave dielectric properties. SrTiO3 thin films epitaxially grown on (110)DyScO3 substrates using molecular beam epitaxy are extremely strained (i.e., ∼1% in-plane tensional strain) from 3.905A of bulk SrTiO3. The room-temperature in-plane dielectric constant and its tuning of the films at 10GHz are observed to be 6000 and 75% with an electric field of 1V∕μm, respectively. The control of strain in SrTiO3 provides a basis for room-temperature tunable microwave applications by elevating its phase-transition peak to room temperature.Structural distortion of ferroelectric thin films caused by film strain has a strong impact on the microwave dielectric properties. SrTiO3 thin films epitaxially grown on (110)DyScO3 substrates using molecular beam epitaxy are extremely strained (i.e., ∼1% in-plane tensional strain) from 3.905A of bulk SrTiO3. The room-temperature in-plane dielectric constant and its tuning of the films at 10GHz are observed to be 6000 and 75% with an electric field of 1V∕μm, respectively. The control of strain in SrTiO3 provides a basis for room-temperature tunable microwave applications by elevating its phase-transition peak to room temperature.

Effect of misfit strains on fourth and sixth order permittivity in (Ba0.60,Sr0.40)TiO3 films on orthorhombic substrates

The in-plane dielectric response of [110] oriented Ba0.60Sr0.40TiO3 epitaxial films grown on [100] NdGaO3 is used to determine the field induced polarization at 10GHz. The nonlinear polarization curve is used to determine the linear and nonlinear permittivity terms for the in-plane principal directions, [001] and [1¯10]. Studied films are in the thickness range of 75–1200nm, and clearly show the influences that drive tunability down with increasing residual strain. The variation of the tunability, along the [001] direction, proves to be less sensitive to residual strain then the [1¯10] direction, although [1¯10] is capable of greater tunability at low residual strains.

In-plane anisotropy in the microwave dielectric properties of SrTiO3 films

Microwave tunable dielectric properties of strained (001) SrTiO3 thin films epitaxially deposited on (110) DyScO3 substrates were studied for in-plane film orientations ([100], [010], [110], and [−110]). A significant in-plane anisotropy in dielectric constant and tuning was observed in these SrTiO3 films. The highest dielectric constant and tuning at room temperature are observed along the [010] direction of the SrTiO3 film (1000 A thick) (3500 and 70% at 1V∕μm, respectively), the lowest ones are observed along the [100] direction (i.e., 2000 and 50% at 1V∕μm, respectively). The dielectric constant and tuning along [−110] and [110] are about 2500 and 30% at 1V∕μm, respectively, which are intermediary to those along the [010] and [100] directions. The dielectric Q(=1∕tanδ) does not show any large difference for the four directions (i.e., Q∼10−20). Also, the phase-transition peak for the [−110] and [110] directions of the SrTiO3 film (300 A thick) is observed at 275 K, which is lower than that for the [010...

Directionally dependent ferroelectric phase transition order of anisotropic epitaxial BaxSr1−xTiO3 thin films

Epitaxial Ba0.5Sr0.5TiO3 thin films were grown by pulsed-laser deposition on (100) LaAlO3 substrates in two distinct strain states, c∕a>1 and c∕a 1, and continuous and negative for the film with c∕a<1. These symmetry changes in the c∕a ratio with temperature were correlated with measurements of the in-plane dielectric properties, and showed that the polarization in these two types of structures has characteristically different behavior that is highly directionally-dependent.

Anisotropic thermal expansion of strontium barium niobate

Strontium barium niobate is a tungsten-bronze ferroelectric crystal having a tetragonal unit cell. Low-temperature x-ray diffraction studies were performed on a single crystal of Sr0.75Ba0.25Nb2O6 to determine the thermal expansivity along the a- and c-axes. Negative thermal expansion was observed along the c direction while a positive thermal expansion was measured along the a axis. The anisotropic thermal expansion behavior is explained as arising due to the geometry of the crystal structure.

Phase transition in Sr0.75Ba0.25NbO3 near the Curie temperature

Strontium barium niobate has the tungsten-bronze structure with a tetragonal unit cell, and it exhibits negative thermal expansion along the c axis between −120°C and room temperature while having a positive thermal expansion along the a axis for the same temperature range. At higher temperatures, close to the Curie temperature and above, the negative thermal expansion along the c axis changes to positive thermal expansion. The a axis lattice parameter as a function of temperature shows a change in slope at the Curie temperature. These results indicate the presence of a second-order phase transition near the Curie temperature.

Strain-induced anisotropy in microwave dielectric properties of (Ba,Sr)TiO3 thin films with directly applied uniaxial ⟨100⟩ stress

The microwave dielectric properties and their change with applied uniaxial ⟨100⟩ stress in an epitaxially deposited Ba0.6Sr0.4TiO3 thin film on (001) MgO have been measured as a function of dc bias (0 to ±40V) and frequency (1–20 GHz). A significant increase in the relative dielectric constant (from 480 to 530) is observed when uniaxial stress is applied. Similarly, the dielectric tuning of the film at an applied electric field of 4×104V∕cm increases from 43% to 48% at 10 GHz. These increases correspond to an extremely small increase of the in-plane lattice parameter of the film, a, from 3.965 to 3.967 A, (i.e., ∼0.05%) due to the applied uniaxial tensional stress.

STRAIN TENSOR EFFECTS ON SrTiO 3 INCIPIENT FERROELECTRIC PHASE TRANSITION

ABSTRACT Structural distortion of ferroelectric thin films caused by film strain has a strong impact on the microwave dielectric properties. SrTiO3 thin films epitaxially grown on (110) DyScO3 substrates using molecular beam epitaxy (MBE) are extremely strained (i.e., ∼ 1% in-plnae tensional strain) from 3.905 Å of bulk SrTiO3. The room temperature dielectric constant and its tuning of the films are observed to be 6000 and 75% with an electric field of 1 V/μ m, respectively. The control of strain in SrTiO3 provides a basis for room temperature tunable microwave applications by elevating its phase transition peak to room temperature. Also, a significant in-plane anisotropy in dielectric constant and tuning was observed in these SrTiO3 films. The observed in-plane anisotropic dielectric properties have been interpreted based on the phenomenological thermodynamics of film strain.

Nonlinear Dielectric Properties of Paraelectric (Ba,Sr)TiO3 Thin Films with Isotropic and Anisotropic Epitaxy

The effects of isotropic and anisotropic epitaxy on the nonlinear dielectric response in (Ba0.6,Sr0.4TiO3) thin films were investigated experimentally. Under isotropic epitaxy, it is shown that the misfit is detrimental to the non-linear response of the films with decreasing film thickness. Nonlinear dielectric properties of films with in-plane compression exhibit more pronounced strain dependence, while the same does not set-in in film under tension down to 200 nm in thickness. In the case of anisotropic epitaxy, tunability exhibits directional dependence in the plane of the film and possesses orthorhombic symmetry in accordance to the Curie principle. The nonlinear dielectric response was found to be greater along the compressed directions.