xTiO3 Thin Films Research Articles

Optimized Tunability of Barium Strontium Titanate Thin Films onto Different Plates with Proper Buffer Layers

Misfit strain, polarization, permittivity, and tunability of Ba x Sr1−x TiO3 (BST) thin films on various substrates with varied buffer layers are calculated by a modified thermodynamic model. The lattice parameters of the LaAlO3 (LAO), CaTiO3 (CT), BaZr0.25Ti0.75O3 (BZT), and MgO buffer layers grow consecutively as does the thermal expansion coefficient (TEC) of the Si, Ti, Ni, and stainless steel (SS) substrates. When the TEC of substrates is bigger (such as Ni, SS) than that of the films, introducing buffer layers with larger lattice parameters (such as BZT, MgO) can improve the permittivity and tunability. Conversely, inserting buffer layers with smaller lattice parameters (such as LAO, CT) can enhance the dielectric response when the TEC of substrates is smaller (such as Si, Ti) than that of the films. The relationship between the TEC of substrate and the lattice parameter of buffer layer is primarily linear for achieving high tunability. According to several references, introducing LaNiO3 buffer layers can significantly increase the tunability of BST solid solutions prepared on Si‐based substrates, which is in line with the estimated rule. This law offers a guide for choosing the best substrate and buffer layer when producing BST films with strong dielectric response.

Epitaxial growth by atomic layer deposition and properties of high-k barium strontium titanate on Zintl-templated Ge (001) substrates

This work reports the atomic layer deposition (ALD) and epitaxial growth of 10–12 nm BaxSr1−xTiO3 (x ∼ 0.5) thin films on Zintl-templated Ge (001) substrates, formed with 0.5 monolayers of Ba. The best thin films were grown using two steps. An amorphous 2–3 nm layer was deposited at 225 °C and then was converted to a crystalline film by annealing for 15 min at 650 °C under vacuum to generate the seed layer that had an abrupt interface with the Ge (001) substrate. A 7–10 nm layer was subsequently grown and crystallized at the same conditions as the seed layer. Metal-insulator-semiconductor structures that employed Au electrodes were used to assess leakage currents and dielectric properties. Interfacial effects inherent to the ∼10 nm BaxSr1−xTiO3 films affect the capacitance measurements leading to k of 87 and 140 for 10.9 and 14.6 nm films, respectively. The epitaxial films have high k in the bulk. Using capacitance measurements for BaxSr1-xTiO3 films that are 13–18.4 nm thick, a bulk k of ∼2000 and low interfacial capacitance density (C/A) of ∼100 fF/μm2 were extracted from thickness dependent relationships. The 10 nm films display a high leakage current (∼102 A/cm2 at −1.0 V) that can be reduced (to ∼10−1 A/cm2 at −1.0 V) by capping the film with ∼1 nm Al2O3 using ALD before depositing the Au electrode or by annealing in O2 at 300 °C for 15 min. The thin low-k Al2O3 layer or the GeOx formed during the O2 annealing step, while improving leakage current, introduces a capacitor in series that further reduces the film dielectric constant such that a 9.9 nm Al-capped film and a 9.8 nm O2-annealed film yield dielectric constants of 80 and 41, respectively.

Mechanical control of electrocaloric response in epitaxial ferroelectric thin films

The electrocaloric (EC) effect in dielectrics has shown great potential in the next-generation solid-state refrigeration; however, most dielectrics cannot satisfy the requirement for refrigeration at various temperatures. How to control large EC response to various temperatures is a critical problem for practical cooling applications. In this work, based on entropy analysis, a nonlinear thermodynamic approach considering mechanical loading has been generalized to demonstrate mechanical control of EC response in epitaxial ferroelectric BaxSr1−xTiO3 (BST) thin films. The effects of chemical composition x, in-plane misfit strain um, and out-of-plane external uniaxial stress σ3 on the phase diagrams, phase transition temperatures, and EC response of BST thin films have been studied. The results reveal that a large EC temperature change ΔT appears in the vicinity of c-PE and r-aa phase boundaries, because the polarization component P3 becomes zero across both phase boundaries, leading to a large change of order in dipoles. It is found that the phase structures and the transition temperatures are sensitive to misfit strain and stress. Large EC response in BST thin films can be controlled and shifted to various temperatures over a wide temperature range for practical applications. It also indicates that the peak of ΔT can be shifted to higher temperature under in-plane compressive strain um, while it is shifted to lower temperature under tensile strain um or compressive stress σ3. Furthermore, the optimal combination of misfit strain and stress for the EC temperature change are identified, which further enhances EC response. These insights offer an alternative pathway to control and implement EC refrigeration over a wide range of temperature.

Local hardening of Raman phonons in BaxSr1−xTiO3 thin films deposited by r.f. sputtering

Thin films of nanometric order of a solid solution of barium-strontium titanate BaxSr1−xTiO3 (BSTO), in the entire 0 ≤ x ≤ 1 range, were deposited at 495 °C on quartz by RF reactive co-sputtering. The composition was controlled through the RF power (P) applied to the targets. X ray diffractograms (XRD) reveal that all the samples show the cubic perovskite crystalline structure. The percentage of elements (x) in the films were evaluated by usingof electron dispersion spectroscopy, also through the change of the (110) lattice interplanar spacing determination of BSTO by means of XRD patterns. Experimental data points of concentration of Ba (x) and Sr (1−x) as a function of P describe a symmetric sigmoidal curve. The average crystal size for the whole composition is around 20 nm as estimated from XRD data. Raman measurements in the 100–900 cm−1 range show that optic phonons experience local hardening in the x interval 0.6–0.8. Around this region of x it is, in general, expected in BSTO a crystalline cubic-tetragonal phase transition when is prepared at low substrates temperatures. Distortion of the unit cell is observed in that interval, which probably originates the local hardening of phonons.

The Influence of the Substrate Material on the Structure and Electrophysical Properties of Ba x Sr1 – xTiO3 Thin Films

The high-frequency spraying method has been used to produce Ba x Sr1–xTiO3 thin films on (111)Pt/(100)Si and SiOx/(100)Si substrates. The synthesized films have proven to be single-phase with a polycrystalline structure. In this paper, we show the influence of the platinum sublayer on such parameters as roughness, mean size of grains, and the local polarization of Ba x Sr1–xTiO3 thin films. Possible mechanisms of the obtained results related to the intragrain conductivity and the nature of interaction of a ferroelectric film with its substrate are discussed.

Stoichiometry Calculation in BaxSr1−xTiO3 Solid Solution Thin Films, Prepared by RF Cosputtering, Using X-Ray Diffraction Peak Positions and Boltzmann Sigmoidal Modelling

A novel procedure based on the use of the Boltzmann equation to model the x parameter, the film deposition rate, and the optical band gap of BaxSr1−xTiO3 thin films is proposed. The BaxSr1−xTiO3 films were prepared by RF cosputtering from BaTiO3 and SrTiO3 targets changing the power applied to each magnetron to obtain different Ba/Sr contents. The method to calculate x consisted of fitting the angular shift of (110), (111), and (211) diffraction peaks observed as the density of substitutional Ba2+ increases in the solid solution when the applied RF power increases, followed by a scale transformation from applied power to x parameter using the Boltzmann equation. The Ba/Sr ratio was obtained from X-ray energy dispersive spectroscopy; the comparison with the X-ray diffraction derived composition shows a remarkable coincidence while the discrepancies offer a valuable diagnosis on the sputtering flux and phase composition. The proposed method allows a quick setup of the RF cosputtering system to control film composition providing a versatile tool to optimization of the process.

Thin film ferroelectric structures on diamond for high power microwave applications

A ferroelectric BaxSr1−xTiO3 (BSTO) thin film structure on a diamond substrate was developed for the first time to reduce an overheating in high power microwave applications. The BSTO/SiC/Diamond structure was produced by ion-plasma rf magnetron deposition technique with a silicon carbide (SiC) layer as a buffer to compensate the thermal expansion difference between the BSTO film and the diamond substrate. It was shown by an X-ray diffraction and Rutherford back-scattering analysis that the thin SiC crystalline buffer layer provides the growth of the (100) oriented BSTO perovskite films with good adhesion. The BSTO planar microwave capacitive elements were investigated at a frequency of 3GHz and the non-linear dielectric response of the BSTO capacitor on SiC/diamond substrate was demonstrated based on the temperature and voltage dependencies of the capacitance. The overheating of the BSTO/SiC/diamond capacitor under exposure of high level microwave power was observed to be an order of magnitude lower than that for the capacitors on Al2O3 substrate.

Effect of Rb-doping on the dielectric and tunable properties of Ba0.6Sr0.4TiO3 thin films prepared by sol-gel

There is growing interest in BaxSr1−xTiO3 (BST) thin films due to their dielectric tunability and low dielectric loss. Doping small concentrations of elements is an effective way to increase the dielectric tunability and decrease the dielectric loss of BaxSr1−xTiO3 thin films. Many doping elements have been studied to date, but this does not include the use of rubidium (Rb). In this study, we reported here the effect of Rb-doping on the surface morphology, dielectric and tunable properties of Ba0.6Sr0.4TiO3 thin films prepared by the sol-gel method. It was observed that the tunability of Ba0.6Sr0.4TiO3 thin films was increased greatly from 46.53% to 63.37% at 377kV/cm when Rb was doped at a concentration of 4mol%. The observed improvements originate from increased grain size. Doping effects included occupation of the A site of the composites by Rb+ ions, and also enhanced grain growth during the annealing process, due to the relatively low melting point of rubidium oxide. (Ba0.6Sr0.4)96%Rb4%TiO3 thin films exhibited the largest grain size and a surface root-mean-square roughness (RMS) of 2.409nm, the smallest among the various doping concentrations of rubidium used. They also exhibited the best dielectric characteristics (figure of merit value 31.53 at 2MHz). Thus, Rb doping can be considered a promising way to optimize the dielectric properties of BST thin films for their potential application in microwave tunable devices.

Ferroelectric domains in epitaxial PbxSr1−xTiO3 thin films investigated using X-ray diffraction and piezoresponse force microscopy

We present a detailed study of compressively strained PbxSr1−xTiO3 thin films grown by off-axis radio frequency magnetron sputtering on (001)-oriented Nb-doped SrTiO3 substrates. Film tetragonality and the ferroelectric critical temperatures are measured for samples of different composition and thickness and compared with a phenomenological Landau-Devonshire model. 180∘ ferroelectric domains are observed using both X-ray diffraction and piezoresponse force microscopy and domain sizes obtained by the two techniques are compared and discussed.

Small polaron-related recombination in BaxSr1−xTiO3 thin films by cathodoluminescence spectroscopy

Polarons are critical to our understanding of many condensed matter systems, but are difficult to observe and characterize. By combining cathodoluminescence spectroscopy measurements with insights from first-principles calculations, we have identified transitions involving two distinct and simultaneous polaronic states in relaxed BaxSr1−xTiO3 films grown by molecular beam epitaxy. These polaronic optical transitions arise from two discrete Ti-O-Ti bonding configurations simultaneously present in the system, one with a 180° bond angle and the other with buckled geometry due to octahedral tilting. By compositional modulation, we show that we can semi-quantitatively probe the population of each bonding configuration.

Thickness dependence of the properties of epitaxial barium strontium titanate thin films

Heteroepitaxial BaxSr1 − xTiO3 thin films of different thicknesses have been studied in the paraelectric phase at a temperature of 600°C. The lattice parameters of the film have nonlinear dependence on the thickness. The misfit strain and unit cell volume increase with decreasing thickness. The behavior of the misfit strain with variations in the thickness is well described in the model of the double electric layer formed during the synthesis of the film at the interface with the substrate.

Phenomenological theory of phase transitions in epitaxial BaxSr1−xTiO3 thin films on (111)-oriented cubic substrates

A phenomenological thermodynamic theory of BaxSr1−xTiO3 (BST-x) thin films epitaxially grown on (111)-oriented cubic substrates is developed using the Landau-Devonshire approach. The group-theoretical analysis of the low-symmetry phases was performed taking into account two order parameters: the polarization related to ionic shifts in polar zone-center F1u mode and the out-of-phase rotation of TiO6 octahedra corresponding to the R25 zone-boundary mode in the parent cubic phase Pm3¯m. The eight-order thermodynamic potential for BST-x solid solutions was developed and analyzed. We constructed the “concentration-misfit strain” phase diagram for BST-x thin films at room temperature and found that polar rhombohedral R3m phase with the polarization normal to the substrate is stable for x > 0.72 and negative misfit strains, while ferroelectric monoclinic C2 and Cm phases with in-plane polarization are stable for much smaller x and positive or slightly negative misfit strains. We constructed the “temperature-misfit strain” phase diagrams for several concentrations (x = 1, 0.8, 0.6, 0.4, and 0.2). Systematic changes of the phase transition lines between the paraelectric and ferroelectric phases are discussed. The phase diagrams are useful for practical applications in thin-film engineering.

Epitaxy of Li3xLa2/3–xTiO3 Films and the Influence of La Ordering on Li-Ion Conduction

This paper reports on the epitaxial growth of Li3xLa2/3–xTiO3 thin films by means of pulsed laser deposition and their transport properties. A series of films obtained in this study have revealed that the thin film quality and Li-ion conductivity are highly sensitive to growth conditions as well as target composition. The epitaxial thin films are mostly c-axis oriented, containing a small amount of a-axis domains and antiphase boundaries. In-plane conductivity measurements investigating the relationship between these defects and Li-ion conductivity in Li3xLa2/3–xTiO3 show that the defects are not predominantly resistive against the ionic conduction.

Microstructure and ionic conductivity of LLTO thin films: Influence of different substrates and excess lithium in the target

The deposition of single phase Li3xLa2/3−xTiO3 (LLTO) thin films remains very challenging. The growth of the perovskite phase is in competition with the insulating La2Ti2O7 phase when prepared at high oxygen pressure by PLD. Nevertheless, we have achieved epitaxial growth of LLTO on different (001) oriented substrates such as LaAlO3, SrTiO3 and MgO despite a large lattice mismatch of up to +8.8%. We also determined the percentage of lithium excess in the target necessary to reach a maximum ionic conductivity. However, the presence of the blocking La2Ti2O7 phase strongly hinders the lithium ion migration and reduces the total conductivity compared to bulk properties.

Fabrication and characterization of compositionally graded PbxSr1−xTiO3 thin films by the Sol–gel method

Different compositionally graded PbxSr1−xTiO3 (x = 0.2, 0.3, 0.4, 0.5 and 0.6; PST) thin films were deposited on LaNiO3(LNO)-buffered stainless steel (SS) substrates. Both LNO and PST thin films were prepared by the sol–gel method. X-ray diffraction and scanning electron microscopy analysis were used to investigate the crystallinity and surface morphology of PST thin films. It showed that the crack-free PST thin films presented pure perovskite phase. The dielectric permittivity was measured in the frequency range of 103–106Hz. The tunable value of compositionally graded PST films can be as high as about 30 % (measured at E = 300 kV/cm, 105Hz). The LNO-buffered layers can effectively reduce the dissipation constant of PST films on SS, which was about 3 % and in the same level with the PST films deposited on traditional Si substrates. The temperature and frequency stability of tunability has been improved. The value of (Δnr/nrTo) % (rate of tunability change) could be less than 5 % in the most range of temperature, which demonstrated that PST could be a promising candidate material for tunable devices applications.

Temperature Stabilization of BST Thin Films: A Critical Review

A review of the temperature stability issues associated with BaxSr1−xTiO3 (BST) thin films and a summary of innovative materials designs, and novel process science solutions which serve to help mitigate this temperature sensitivity is presented. The experimental results from the literature and from our own work are reviewed; the correspondence between the theoretical results and the measured properties of tunable materials is critically analyzed; and reasons for discrepancy between literature results are discussed. Particular emphasis is concentrated on tunable phase shifters and filters to enable phased array antennas, radars and other advanced communications devices. This critical review provides the foundation to spawn new materials research solutions to further enable the development of BST films for microwave device applications.

Amorphous and perovskite Li3xLa(2/3)−xTiO3 (thin) films via chemical solution deposition: solid electrolytes for all-solid-state Li-ion batteries

Thin films of amorphous and crystalline perovskite Li3xLa(2/3)−xTiO3 (LLT) (x = 0.117) are prepared by means of aqueous chemical solution deposition onto rutile TiO2 thin films as an anode, yielding an electrochemical half-cell. The Li-ion conductivity of the pin-hole free, amorphous LLT thin film (90 nm thick) is 3.8 × 10−8 S cm−1 on Pt and 1.3 × 10−8 S cm−1 on rutile TiO2, while measuring perpendicular to the thin film direction with impedance spectroscopy. Grazing angle attenuated total reflectance-Fourier transform infrared spectroscopy shows that all organic precursor molecules have been decomposed at 500 °C. In addition, in situ (heating) X-ray diffraction analysis shows that phase pure crystalline perovskite LLT (x = 0.117) is formed on top of the rutile TiO2 anode at 700 °C. Furthermore, thickness control is possible by varying the precursor solution concentration and the number of deposition cycles. The current study presents a promising synthesis route to develop all-solid-state battery devices based on multi-metal oxide materials using aqueous precursor chemistry.

Control of tensile stress on inducing formation and tunability of (100) oriented Pb x Sr1−x TiO3 thin films

(100) Oriented Pb x Sr1−x TiO3 (PSTO) thin films are prepared on indium tin oxide (ITO)/glass substrates by sol–gel technique while inserting doped PbTiO3 (PTO)-inducing layer in between. The effect of tensile stress in PSTO on tunability and (100) orientation of the thin films was investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscope, and atomic force microscope, respectively. Results show that PSTO thin film deposited on doped PTO has (100) oriented structure while it is randomly oriented when deposited directly on the ITO/glass substrate. Lattice mismatch between PSTO and PTO appears, in which the in-plane lattice constant c is 0.3922–0.3924 nm in the former and 4.02–4.07 nm in the latter, respectively, contributing tensile stress in the PSTO due to different lattice constants between them. The stress in the PSTO thin film is 3.04, 3.15, 3.59 and 4.47 GPa when the doped PTOs are Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively. The orientation degrees of PSTO thin films are from 89.63, 90.31, 91.92 to 93.29 % with increasing stress of PSTO on Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively. Tunabilities of the well-oriented PSTO thin films increase in ascending order of 63 < 65 < 69 < 73 % when induced by oriented PTO layers of Fe–PTO, Tb–PTO, Co–PTO and Zn–PTO, respectively, which is in accordance with the degree of (100) orientation appearing in the thin films. The high tunability appears in the PSTO thin film while high (100) orientation is derived from the tensile stress. It is much higher than that of randomly oriented PSTO thin film.

Enhanced dielectric properties from barium strontium titanate films with strontium titanate buffer layers

In order to enhance the permittivity and tunability of the dielectric component, a thin film dielectric composite consisting of a radio frequency sputtered SrTiO3 (STO) buffer layer and metalorganic solution deposited Mg-doped BaxSr1−xTiO3 (Mg-BST) thin film overgrowth was developed using affordable industry standard processes and materials. The effect of the STO buffer layer thickness on the dielectric response of the heterostructure was investigated. Our results demonstrate that the composite film heterostructure, evaluated in the metal-insulator-metal configuration Pt/STO/Mg-BST/Pt on sapphire substrate, with the thinner (9–17 nm) STO buffer layers possessed enhanced permittivity (εr ∼ 491) with respect to the thicker 41 nm buffer layer (εr ∼ 360) and that of a control Mg-BST film without a STO buffer layer (εr ∼ 380). Additionally, the composite film with the thinner buffer layers were shown to have low losses (tan δ ∼ 0.02), low leakage characteristics (J = 7.0 × 10−9 A/cm2), high breakdown voltage (VBR &amp;gt; 10 V), a large grain microstructure (∼125 nm), and smooth pin-hole free surfaces. The enhanced permittivity of the composite dielectric film resulted from three major factors: (i) the template-effect of the thin STO buffer layer on the thicker Mg-BST over-layer film to achieve a large grain microstructure, (ii) the low viscosity of the metallo-organic solution deposition (MOSD) solution, which ensured heterogeneous nucleation of the Mg-BST overgrowth film on the surface of the STO buffer layer, and (iii) minimization of the low permittivity grain boundary phase (TiO2−x phase). The dielectric response of the BST can be explained using a thermodynamic model taking into account interlayer electrostatic and electromechanical interactions. Additionally, Mg doping of the BST enabled low loss and low leakage characteristics of the heterostructure. The large permittivity, low loss, low leakage characteristics, and defect free surfaces of the composite dielectric heterostructure promote tunable device miniaturization and hold the potential to enable enhanced electromagnetic coupling in ferromagnetic/high permittivity dielectric heterostructures, which in turn would facilitate the realization of integrated charge mediated voltage controlled magnetic radio frequency/microwave communication devices.

Enhanced dielectric and tunable properties of barium strontium titanate thin films through introducing Nd(Zn1/2Ti1/2)O3 and adjusting Ba/Sr

Ba0.6Sr0.4TiO3 (BST) and 0.06Nd(Zn1/2Ti1/2)O3–0.94Ba x Sr1−x TiO3 (NZT–BST) thin films with x = 0.6, 0.7, 0.75, and 0.8 were fabricated on Pt/Ti/SiO2/Si substrates by sol–gel method. The structures, surface morphology, dielectric, and ferroelectric properties, and thermal stability of BST and NZT–BST thin films were investigated as a function of NZT and Ba content. It was found that introducing NZT into BST decreased significantly dielectric loss, however, along with the tunability. On this basis, increasing Ba/Sr in NZT–BST thin films led to the simultaneous increase of dielectric constant and tunability of thin films. As a result, optimized dielectric and tunable properties were obtained for 0.06Nd(Zn1/2Ti1/2)O3–0.94Ba0.7Sr0.3TiO3 thin film with the highest FOM value of 43.22. It awakens us that, for reducing dielectric loss, introducing a certain amount of low permittivity oxides or non-ferroelectrics like NZT into weak ferroelectric perovskite tunable materials, not into paraelectric perovskite tunable materials, may obtain more excellent dielectric and tunable performances.