Oriented BaTiO3 Thin Films Research Articles

Synthesis and characterization of (111)-oriented BaTiO3 thin films

The synthesis of (111)-oriented BaTiO3 thin films on Nb-doped SrTiO3 and bilayers of BaTiO3 and La0.7Sr0.3MnO3 on SrTiO3 are investigated. With increasing thickness the films are found to exhibit a decreasing out-of-plane lattice parameter and increased surface roughness. The BaTiO3 films on doped SrTiO3 are found to be relaxed with indications of increasing defect density with increasing thickness. Through piezoresponse force microscopy, pyroelectric measurements, and tunneling electroresistance measurements, indications of ferroelectric behavior are found in (111)-oriented BaTiO3 down to a thickness of 5 nm.

Current–voltage characterization of epitaxial grown barium titanate thin films on Si substrate

The current–voltage (I–V) characterization was studied on the (111)-oriented BaTiO3 thin films on Pt-buffered Si substrates. The ferroelectric domains polarized downward had a smaller resistance and a better corresponding electrical property. An abnormal asymmetric switching was observed, which is similar to the memristive behavior at the pre-switching state. With increasing bias, the I–V curve under the positive bias tended to be saturated. The film showed a better endurance of the negative bias than the positive one. Such findings are promising for the potential utilization of barium titanate thin films as new type memristive memory application apart from the classic ferroelectric memory.

Ridge waveguide using highly oriented BaTiO3 thin films for electro-optic application

In this work, 750 nm-thick BaTiO3 thin films with highly (0 0 1) preferred orientation were grown on single crystal MgO substrates by RF-sputtering. Hydrogen silsesquioxane (HSQ) resist material based ridge waveguides, which were fabricated on BaTiO3 thin films, were formed by e-beam lithography. Au electrodes were deposited on top of the BaTiO3 films beside the waveguide. Propagation losses of the BaTiO3 ridge waveguide were 3–5 dB/cm in transverse electric polarization. The measured electrooptic coefficient value (r51) was 110 pm/V, which is three times larger than the electrooptic coefficient (r33 = 30.8 pm/V) of single crystal LiNbO3. SiO2 strip waveguide formed by HSQ exhibited light propagation with loss lower than 5 dB/cm. This result demonstrates potential possibility of creating highly oriented and/or epitaxially grown BaTiO3 waveguides and optical components on oxide substrates.

Hydrothermal–galvanic couple synthesis of directionally oriented BaTiO3 thin films on TiN-coated substrates

BaTiO3 films were synthesized on TiN-coated Si substrate below 100°C by a hydrothermal–galvanic couple technique in barium contained alkaline solutions. X-ray diffraction and electron backscatter diffraction results show that the BaTiO3 thin films were directionally oriented grown on the TiN/Si substrates, i.e., (111) BaTiO3 over (111) TiN. The surface morphologies revealed that BaTiO3 nucleated and grew over the TiN surface with a single layer. From kinetic analyses, the growth rates of BaTiO3 films prepared by the hydrothermal–galvanic couple technique were faster than a hydrothermal method. The galvanic effects were confirmed by investigating the induced currents and energies. The galvanic currents were generated and controlled by both the dissolution of TiN and the formation of BaTiO3. The output electric energies increased rapidly with the reaction time and leveled off at the full coverage of BaTiO3.

Preferentially oriented BaTiO3 thin films deposited on silicon with thin intermediate buffer layers

Barium titanate (BaTiO3) thin films are prepared by conventional 2-methoxy ethanol-based chemical solution deposition. We report highly c-axis-oriented BaTiO3 thin films grown on silicon substrates, coated with a lanthanum oxynitrate buffer layer of 8.9 nm. The influence of the intermediate buffer layer on the crystallization of BaTiO3 film is investigated. The annealing temperature and buffer layer sintering conditions are optimized to obtain good crystal growth. X-ray diffraction measurements show the growth of highly oriented BaTiO3 thin films having a single perovskite phase with tetragonal geometry. The scanning electron microscopy and atomic force microscopy studies indicate the presence of smooth, crack-free, uniform layers, with densely packed crystal grains on the silicon surface. A BaTiO3 film of 150-nm thickness, deposited on a buffer layer of 7.2 nm, shows a dielectric constant of 270, remnant polarization (2Pr) of 5 μC/cm2, and coercive field (Ec) of 60 kV/cm.

Epitaxial Growth and Ferroelectricity of BaTiO3 on SrRuO3/TiO2 Buffered GaN

Epitaxial growth of (111)-oriented BaTiO3 thin films on GaN/Al2O3 (0001) substrates has been realized by intervening epitaxial (111) SrRuO3/(100)TiO2 buffer layers. Rutile TiO2 was deposited by Laser molecular beam epitaxial firstly and the deposition process of TiO2 layer was in-situ monitored by reflective high energy electron diffractions. After that SrRuO3 conductive films and BaTiO3 ferroelectric films were fabricated via pulsed laser deposition. The perovskite BaTiO3 films were epitaxially grown on wurtzite GaN as revealed by X-ray diffraction. Electrical measurements demonstrate that the BaTiO3 films possess a saturation ferroelectric hysteresis loop, butterfly-shape C-V curve and low leakage current density. These results show that BaTiO3 films with favorable electrical performance could be epitaxially grown on GaN using SrRuO3/TiO2 bufferlayer.

Single-crystal-like, c-axis oriented BaTiO3 thin films with high-performance on flexible metal templates for ferroelectric applications

Epitaxial, c-axis oriented BaTiO3 thin films were deposited using pulsed laser ablation on flexible, polycrystalline Ni alloy tape with biaxially textured oxide buffer multilayers. The high quality of epitaxial BaTiO3 thin films with P4mm group symmetry was confirmed by x-ray diffraction. The microscopic ferroelectric domain structure and the piezoelectric domain switching in these films were confirmed via spatially resolved piezoresponse mapping and local hysteresis loops. Macroscopic measurements demonstrate that the films have well-saturated hysteresis loops with a high remanent polarization of ∼11.5 μC/cm2. Such high-quality, single-crystal-like BaTiO3 films on low-cost, polycrystalline, flexible Ni alloy substrates are attractive for applications in flexible lead-free ferroelectric devices.

Preparation of BaTiO3 Films on Si Substrate with MgO Buffer Layer by RF Magnetron Sputtering

Highly (002)- or (200)-oriented BaTiO3 thin films were successfully grown on a Si substrate with a MgO buffer layer by RF magnetron sputtering. The deposition parameters need to be stringently controlled in order to grow BaTiO3 films with good crystallinity. The sputtering parameters such as substrate temperature, RF power, gas flow ratio, and deposition pressure were varied to obtain the optimum deposition conditions for the BaTiO3 films. The as-deposited films were characterized by X-ray diffraction analysis and atomic force microscopy to analyze their crystalline structure and surface morphology. The full width at half maximum intensity of the BaTiO3 (002) or (200) peak of the sample fabricated under the optimum deposition parameters was only 0.28°. The surface roughness of the BaTiO3 films was about 3.2 nm. The results could be useful in the integration of ferroelectric and semiconductor devices on the same Si substrate.

Thickness Dependence of Electrical Properties of Highly (100)-Oriented BaTiO3 Thin Films Prepared by One-Step Chemical Solution Deposition

Highly (100)-oriented BaTiO3 thin films having different thicknesses of 70–280 nm were deposited on Pt/TiOx/SiO2/Si substrates with LaNiO3 as a buffer layer by one-step chemical solution deposition. X-ray diffraction analyses showed that the LaNiO3 and BaTiO3 layers were all under tensile stress. The tensile stress of the LaNiO3 layer increased after depositing BaTiO3 films. The tensile stress in both the LaNiO3 and BaTiO3 layers decreased with increasing BaTiO3 layer thickness. The thickness dependences of the dielectric and piezoelectric behaviors of the highly (100)-oriented BaTiO3 films were investigated. It was found that the effect of tensile stress on the dielectric and piezoelectric properties is dominant. A reduction in in-plane tensile stress (primarily arising from a thermal expansion mismatch) was considered effective for increasing the dielectric and piezoelectric constants. The measured dielectric constant increased from about 313 for 70 nm films up to 831 for 280 nm films. The local effective piezoelectric coefficient increased from 15 pm/V for 70 nm films up to 45 pm/V for 280 nm films.

Epitaxially induced high temperature (>900K) cubic-tetragonal structural phase transition in BaTiO3 thin films

For (001)c oriented BaTiO3 thin films, it has been found that epitaxial constraint can result in a dramatic increase in the temperature of a tetragonal (T) structural phase transition. For 2000-Å-thick films grown directly on SrTiO3 substrates, a T→cubic (C) phase transition was found on heating at >950K, where the lattice constant changed smoothly with temperature. It was also found for films of the same thickness that the T→C transition is nearly restored to that of bulk crystals by the use of a buffer layer, which relaxes the epitaxial constraint. These results provide evidence of an epitaxially induced high temperature structural phase transition in BaTiO3 thin films, where the ferroelectric (internal) and structural (external) aspects of the phase transition are decoupled.

Structural and optical characterization of epitaxial waveguiding BaTiO3 thin films on MgO

Epitaxial waveguide structures of c-axis oriented BaTiO3 thin films on MgO(001) have been grown by pulsed laser deposition. The structural properties of the samples have been characterized by Rutherford backscattering spectrometry/ion channeling (RBS/C), x-ray diffraction, and atomic force microscopy. We found excellent crystalline quality even up to thicknesses of a few microns. This has been confirmed by RBS/C minimum yield values of 2%–3%, a full width at half maximum of 0.36° of the BaTiO3 (002) rocking curve, and a rms roughness of 1.1 nm for a 950 nm BaTiO3 film. The out-of-plane refractive index was measured to be close to the extraordinary bulk value with the birefringence being about one third of the bulk value. Waveguide losses of 2.9 dB/cm have been demonstrated.

Preparation and Characterization of BaTiO3 Thin Films on MgO-buffered Si(100) Substrates by RF Sputtering

We report the results of a study on the deposition and characterization of partially oriented BaTiO3 thin films on MgO-buffered Si(100) by radio-frequency magnetron sputtering. The structural and morphological characteristics of the MgO buffer layer were investigated as a function of substrate temperature. The x-ray θ-2θ, φ scans, and observation of surface morphology revealed that MgO grew with a tendency of (001) orientation. Partially (00l) or (h00) textured BaTiO3 thin films were obtained on Si(100) with the MgO buffer layer while randomly oriented BaTiO3 thin films with large-scale cracks on the surface were made without the MgO layer. Pt/BaTiO3/Pt multistructures were formed on Si(100), MgO/Si(100), and MgO(100) single crystal substrates to conduct preliminary electrical measurements for metal-insulator-metal type capacitor. Comparison of the crystallographic orientation, morphology, and electrical properties between the BaTiO3 films on Si(100) with and without the MgO buffer layer supported the favorable role of the MgO layer as a buffer for the growth of BaTiO3 films on Si(100).

Preparation of Epitaxial BaTiO3 Thin Films by the Dipping-pyrolysis Process

Epitaxial BaTiO3 thin films were prepared on SrTiO3 (100) substrates by the dipping-pyrolysis process using metal naphthenates as starting materials. Highly oriented BaTiO3 thin films were crystallized by heat treatment at 800 °C and higher, from amorphous precursor films pyrolyzed at 470 °C. XRD pole-figure and reciprocal-space mapping analyses showed that the films were epitaxially grown on SrTiO3substrates and were pseudocubic phase with an a║/aτ ratio of 1.003, smaller than the c0/a0 ratio (=1.011) of bulk tetragonal BaTiO3.

Electrical properties and poling of BaTiO3 thin films

Dielectric and ferroelectric properties were measured for both highly a-axis textured and randomly oriented BaTiO3 thin films deposited on Pt coated MgO substrates via metal-organic chemical-vapor deposition. Dielectric constants on the order of ε∼10 and an absence of ferroelectric hysteresis were generally observed for the as-deposited films. Upon applying an electric field exceeding a threshold value of ∼50–100 kV/cm across the film, both the dielectric constant and the ferroelectric hystersis were enhanced significantly as a result of poling. The dielectric constant increased to ε∼100, and the spontaneous polarization of the a-axis textured film reached Ps≥15 μC/cm2. The measured dielectric and ferroelectric properties of the BaTiO3 thin films depended on the microstructure of the films.