R. Poyato

Stress-induced suppression of piezoelectric properties in PbTiO3:La thin films via scanning force microscopy

Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity o...

Photo-sensitive sol–gel solutions for the low-temperature UV-assisted processing of PbTiO3 based ferroelectric thin films

Photo-sensitive PbTiO3 solutions were synthesised by a sol–gel process that combines the use of a diol solvent, HO(CH2)3OH, and a titanium alkoxide modified with a β-diketonate, Ti(OC3H7)2(CH3COCHCOCH2)2. O-Nitrobenzyl alcohol (NBA), 1-hydroxy-cyclohexyl-phenyl-ketone (HPK) and acetylacetone (ACACH) were added to the synthesised sols as UV-activators. Solutions with nominal compositions of Pb0.76Ca0.24TiO3 were prepared. These solutions have a maximum of absorption in UV between 200 and 300 nm. Pyrolysis of the solutions was studied by means of thermogravimetry and differential thermal analysis (TGA/DTA). Films were deposited by spin-coating onto TiO2/Pt/TiO2/Ti/SiO2/(100)Si substrates. They were irradiated at 250 °C with light of λ ∼ 225 nm and then treated at 550 or 450 °C for 420 s with Rapid Thermal Processing (RTP) in air, using a heating rate of ∼30 °C s−1. Crystalline phases and structures developed in the films were studied by X-ray diffraction with the Bragg–Brentano geometry and with grazing incidence (XRD and GIXRD). Films showed the formation of a single perovskite phase. These films have spontaneous pyroelectricity with a maximum value of the pyroelectric coefficient of γs ∼ +12 × 10−9 C cm−2 K−1. Non-irradiated films prepared at 550 °C did not show a defined ferroelectric response. The ferroelectric properties measured in the films processed at 550 °C under UV were similar to those reported for films prepared at higher temperatures without irradiation. Values of the remanent polarisation of Pr ∼ 22 µC cm−2 and of the coercive field of Ec ∼ 170 kV cm−1 were obtained in these irradiated films prepared at 550 °C.

Effects of substrate annealing and post-crystallization thermal treatments on the functional properties of preferentially oriented (Pb,Ca)TiO3 thin films

〈111〉 and 〈001〉,〈100〉 preferentially oriented calcium-modified lead titanate thin films have been studied. Effects of the substrate annealing and post-crystallization thermal treatment of the films on the texture and ferro-, piezo-, and pyroelectric properties have been analyzed and discussed. The annealing of the substrate has effect on the texture of 〈001〉,〈100〉-oriented films and, as a consequence, produces an increase in the net polarization in the perpendicular direction to the plane of the film and in the pyroelectric coefficient. The annealing of the substrate does not affect the texture of 〈111〉-oriented films, but the electrical properties at the electrode–substrate interface and, as a consequence, gives place to high internal electric fields leading to the highest spontaneous piezo- (5 pm V−1) and pyroelectric coefficients (25.0×10−9 C cm−2 K−1). The asymmetry that characterizes the functional properties of 〈111〉-oriented films under poling with negative or positive polarity is also consequence ...

Creep-resistant composites of alumina and single-wall carbon nanotubes

Composites of alumina (Al2O3) ceramic and single-wall carbon nanotubes (SWNTs) have been tested in uniaxial compression at 1300 and 1350°C (Ar atmosphere), and they have been found to be about two orders of magnitude more creep-resistant compared to a pure alumina of about the same grain size (0.5μm). This is attributed to partial blocking of grain-boundary sliding by SWNTs in the composites. Since the grain boundaries in the ceramic/SWNTs composites are amenable to being engineered, this constitutes an attractive approach to the design of creep-resistant ceramic composites.

Stress-induced depolarization of (Pb, La)TiO3 ferroelectric thin films by nanoindentation

Electrical depolarization has been observed in lanthanum-modified lead titanate ferroelectric thin films stressed by nanoindentation. A spherical metallic indenter was used as a top electrode to locally pole the films and then to measure the depolarization current intensity. The current intensity had distinctive maxima at given indentation forces. These are related to the stress thresholds for the depolarization mechanism, which is probably 90° domain wall movements. Knowledge of the depolarization stresses is necessary for the design of microelectromechanical systems that include a ferroelectric layer.

Spontaneous pyro- and piezoelectricity of sol-gel La-modified lead titanate thin films

Abstract Polycrystalline La-modified lead titanate thin films have been processed, by a diol-based sol-gel route, on Si-based substrates. Multiple deposition and crystallization steps were carried out in order to promote heterogeneous nucleation and, as a consequence, a preferred crystallographic orientation and self polarization. Crystallization was achieved by rapid thermal processing at 650°C for 50s, at a heating rate of ∼30°C/s. The films developed either a <111> or a mixed <100>/<001> preferred orientation, which were determined by quantitative texture analysis of X-ray pole figures. Hysteresis cycles of the piezoelectric d33 coefficient versus electric field were obtained by a double-beam laser interferometric technique and found to strongly depend on the film texture. <111> oriented films present a thickness-dependent spontaneous pyroelectric coefficient and field endurance of the piezoelectric coefficient. These films are thus, appropriate to be used in non-switching applications, such as pyroelectric infra-red detectors and integrated piezoelectric sensors.

Oriented growth of sol–gel‐modified PbTiO3 thin films on Si‐based substrates

A sol–gel Pb0.76Ca0.24TiO3 solution was deposited by spin-coating onto Pt/Ti/SiO2/Si(100), Ti/Pt/Ti/SiO2/ Si(100) and Pt/TiO2/SiO2/Si(100) substrates. As-prepared substrates and those subjected to a thermal treatment in air at 650 °C for 1800 s were used. The wet films were dried at 350 °C for 60 s and crystallized in air by rapid thermal processing (RTP) at 650 °C for 50 s, with a heating rate of ∼30 °C s−1. This procedure was repeated four times to obtain crystalline films with a thickness of ∼250–300 nm. Perovskite films with different textures were obtained after crystallization. This texture was a consequence of the multilayer structure of the substrate. The formation of Pb0.76Ca0.24TiO3 films with a preferred orientation in the 〈 111〉 direction of the perovskite was observed when Ti was present at the top surface of the Pt. Texture of the films was studied by x-ray diffraction (XRD) and the structure of the substrate surfaces was studied by grazing incidence x-ray diffraction (GIXRD). Spontaneous pyroelectric coefficients were measured in the films. Copyright

Low temperature preparation of piezoelectric thin films by ultraviolet-assisted rapid thermal processing

Abstract Ca- and La-modified lead titanate ferroelectric thin films were prepared by a sol–gel method from photosensitive solutions and with an ultraviolet (UV)-assisted rapid thermal processor, including an arrangement of excimer lamps. The diol-based route was used in the preparation of the precursor solutions, whose UV absorption and thermal decomposition were determined. Multiple deposition and crystallization steps of the deposited solution were used to promote preferential orientation. A comparative ferro and piezoelectric study of films prepared by conventional rapid thermal processing at 650°C and films prepared at 550°C with an intermediate step of UV irradiation at 250°C will be presented to assess the value of these films for their use in integrated piezoelectric sensors and microelectromechanical systems. Piezoelectric d 33 and e 31 coefficients were determined by double-beam laser interferometry and by the direct piezoelectric effect on cantilevers, respectively. The effect of the substrate and processing method on the preferential crystalline orientation of the films and the corresponding piezoelectric properties will be reported. The role of the composition will also be discussed.

Reduced dielectric dispersion in ferroelectric (Pb,La)TiO3/(Pb,Ca)TiO3 thin-film multilayer heterostructures due to a mechanical stress relaxation mechanism

The dielectric dispersion of (Pb,Ca)TiO3/(Pb,La)TiO3/(Pb,Ca)TiO3 and (Pb,La)TiO3/(Pb,Ca)TiO3/(Pb,La)TiO3 ferroelectric thin-film multilayer heterostructures onto Si-based substrates has been studied and compared with that of identically prepared (Pb,La)TiO3 and (Pb,Ca)TiO3 films. Grazing incidence x-ray diffraction analysis reveals that the tetragonal distortion of the heterostructures is higher than the ones for the single-component films, which evidences a mechanical stress relaxation mechanism in the heterostructures. A lower dielectric dispersion has been obtained in the heterostructures with respect to the single-component films. This is also a consequence of the stress relaxation by plastic deformation through vacancy diffusion present in the former. Vacancy diffusion decreases the number of VPb–VO defect-dipoles, which could contribute to the dielectric permittivity at low frequencies in the heterostructures.

Effect of the substrate heterostructure on the texture of lanthanum modified lead titanate thin films

Abstract Modified lead titanate thin films with a nominal composition of Pb 0.88 La 0.08 TiO 3 (PTL) have been prepared by chemical solution deposition (CSD) onto annealed Ti/Pt/Ti/(100)Si and Pt/TiO 2 /(100)Si. Composition and heterostructure of the PTL films on the two substrates have been studied by Rutherford backscattering spectroscopy (RBS). Composition of the PTL films on the two substrates is similar and close to the nominal one. However, the Ti/Pt/Ti/(100)Si and Pt/TiO 2 /(100)Si substrates induces different textures in the PTL films. The films on Ti/Pt/Ti/(100)Si developed a preferred orientation, whereas the films on Pt/TiO 2 /(100)Si have a mixed preferred orientation.