J. Mendiola

Piezoelectric properties of Ca-modified PbTiO3 thin films

Low‐field piezoelectric coefficient and strain were investigated in Ca‐modified PbTiO3 thin films by means of optical interferometry. A remanent piezoelectric coefficient of 70 pm/V was observed that agrees well with the values previously reported for the bulk ceramics of the same composition. The efficient poling of the films can be achieved at room temperature under the field of 300 kV/cm. Electrically induced strains as high as 0.8% were observed using a bipolar driving field of 1 MV/cm. Strain response under a unipolar driving field exhibited a good linearity and small hysteresis. These properties combined with the low dielectric constant and high dielectric strength are shown to be attractive for micromechanical applications.

Growth of KNbO3 thin films on MgO by pulsed laser deposition

Crystalline and stoichiometric KNbO3 thin films have been grown on (100) oriented MgO substrates by pulsed laser deposition technique. Electron microprobe analysis and Rutherford backscattering spectroscopy of the films show a progressive loss of K with increasing substrate‐target distance. To compensate for this K loss the ceramic KNbO3 targets were enriched with K2CO3 powder, pressed at room temperature, and sintered at 650 °C. For a substrate‐target distance of 6 cm, targets with [K]/[Nb] molar ratio=2.85 yield stoichiometric KNbO3 films. A partial oxygen pressure of 2×10−2 mbar was optimum for growing transparent films. Films grown between 650 and 700 °C show the KNbO3 crystalline phase with its (110) axis preferentially oriented perpendicular to the surface of the substrate. At these temperatures KNbO3 diffusion into the MgO substrate is observed. Films grown from KNbO3 single crystal targets only contain a Mg4Nb2O9 crystalline layer.

On the effects of stresses in ferroelectric (Pb,Ca)Tio3 thin films

Abstract Grazing angle X-ray diffraction, profilemetry, permittivity vs. temperature and Raman spectrometry measurements have been carried out on Ca-substituted lead titanate thin films prepared by sol–gel and deposited on Pt/TiO 2 /SiO 2 /(100)Si substrates. Films with thicknesses of 150 and 350 nm were obtained. The crystalline (Pb,Ca)TiO 3 films were randomly oriented with a and c parameters slightly different from those of bulk ceramics. Profilemetry measurements indicated that the films were under tensile stresses, developed during the drying of the deposited wet layer and during the thermal treatment of crystallization of the amorphous film. These stresses were confirmed by the shift obtained in the Raman frequencies of these films. As a consequence of these stresses, a positive shift of the maximum of the permittivity vs. temperature is measured.

Pulsed laser deposition of KNbO3 thin films

The laser ablation of stationary KNbO3 single crystal targets induces a Nb enrichment of the target surface. In rotated targets this effect is observed only in those areas irradiated with low laser fluence. The composition of the plasma formed close to the target surface is congruent with the target composition; however, at further distances K-deficient films are formed due to the preferential backscattering of K in the plasma. This loss may be compensated for by using K-rich ceramic targets. Best results so far have been obtained with [K]/[Nb] = 2.85 target composition, and crystalline KNbO3 films are formed when heating the substrates to 650 °C. Films formed on (100)MgO single crystals are usually single phase and oriented with the (110) film plane parallel to the (100) substrate surface. (100)NbO may coexist with KNbO3 on (100)MgO. At substrate temperatures higher than 650 °C, niobium diffuses into MgO forming Mg4Nb2O9 and NbO, leading to K evaporation from the film. Films formed on (001) alpha-Al2O3 (sapphire) show the coexistence of (111), (110), and (001) orientations of KNbO3, and the presence of NbO2 is also observed. KNbO3 films deposited on (001)LiNbO3 crystallize with the (111) plane of the film parallel to the substrate surface. For the latter two substrates the Nb diffusion into the substrate is lower than in MgO and consequently the K concentration retained in the film is comparatively larger.

Rapid thermal processing of strontium bismuth tantalate ferroelectric thin films prepared by a novel chemical solution deposition method

Abstract Strontium bismuth tantalate (SBT) thin films have been deposited on Pt/TiO 2 /(100)Si substrates by chemical solution deposition (CSD), using air-stable solutions synthesised by a sol–gel method. Solutions with different Bi/Sr ratios have been tested for the deposition of the films. These ratios and the type of thermal treatment used for the crystallisation of the films have effect on the microstructure. Adequate ferroelectric responses have been measured in films with a Bi/Sr ratio of 2.75 and prepared with a direct thermal treatment that consists of a rapid thermal processing (RTP) at 650°C with a heating rate of ∼200°C/s. A coercive field of E c ∼60 kV/cm and a remanent polarisation of P r ∼11 μC/cm 2 have been measured in these films. They retain their P r up to ∼10 5 s and they are fatigue-free up to ∼10 10 cycles.

Combined X-ray Texture-Structure-Microstructure Analysis Applied to Ferroelectric Ultrastructures: A Case Study on Pb 0.76 Ca 0.24 TiO 3

A new methodology for a quantitative description of texture, structure and other microstructural parameters of thin layers using X-ray diffraction is presented and applied to the case of a ferroelectric thin film of Pb 0.76 Ca 0.24 TiO 3 on a Pt/TiO 2 /SiO 2 /Si substrate. The approach allows the quantitative texture analysis of the ferroelectric thin film and the Pt electrode, refining simultaneously their structure, layers thickness, mean crystallite size and microstrain state. The powerfulness of this methodology is discussed and compared with other approaches.

Ultrathin ferroelectric strontium bismuth tantalate films

Continuous strontium bismuth tantalate films with an average thickness of ∼36 nm have been prepared by spin-coating deposition of solutions onto Pt/TiO2/SiO2/(100)Si substrates and by rapid thermal processing. The films microstructure was formed by rod-shaped grains and fine equiaxed grains. This densely packed microstructure makes the growth of a continuous ultrathin film possible without shorts in the capacitor. The films exhibited values of coercive field of Ec∼226±40 kV/cm and of remanent polarization of Pr∼7.4±1.5 μC/cm2. They retain their Pr up to ∼105 s and do not fatigue up to ∼1010 cycles.

c-Axis oriented sol–gel (Pb,Ca)TiO3 ferroelectric thin films on Pt/MgO

Abstract Ca-modified lead titanate thin films were prepared on (111)Pt/(100)MgO substrates by spin-on sol–gel process. Two layers of 0.3 mol/l solution were deposited on the substrate and each of them subjected to drying and rapid thermal processing (RTP) for crystallization, which reduces tensile strains. Single perovskite films with a preferred orientation in the direction perpendicular to the plane of the substrate were obtained as measured by X-ray diffraction analysis. Pyroelectricity of these films was measured before and after a polarization process being the pyroelectric coefficient close to ≈2.5×10 −8 C cm −2  K −1 . Ferroelectric properties were also measured, obtaining values of P R of ≈25 μC cm −2 and E c of ≈110 kV.cm −1 . Compressive strains resulting from the differences between thermal expansion coefficients of the substrate and the film cause the (001) orientation of the (Pb,Ca)TiO 3 perovskite layer, that is very convenient for pyroelectric devices.

X-Ray damage on tgs: A thermodynamic theory

Dielectric measurements on the effects produced in TGS by exposure to the radiation of a Cu conventional X-ray diffraction tube have been made with a non-disturbing method. A decrease of the dielectric constant with irradiation time is observed; it is less and less pronounced as the irradiation temperature approaches Tc. Above that no change is observed after 2 hours of irradiation. If a crystal irradiated above Tc is cooled and irradiated again at room temperature, no change occurs to the ferroelectric properties; it appears as if the previous irradiation protects the crystal against further irradiation effects (“vaccine effect”). The results have been explained fairly well by a modification of the free entalphy expression developed in powers of P and introducing odd terms for the coefficients as a function of the irradiation time.

Conditioning effects on RTP (Pb,Ca)TiO3 thin films

Abstract Calcium modified lead titanate thin films have been spin-coated on Pt/TiO 2 /SiO 2 /(100)Si substrates from sol-gel synthesized solutions containing a 10 mol% excess of PbO. Amorphous layers have been crystallized by a rapid thermal treatment at 650°C for 50 s, using a heating rate of ∼30°C/s. Further thermal conditioning treatments were applied to the films, to study their effects on the ferroelectric response, by means of measurements of hysteresis loops, switching current curves, pyroelectricity, permittivity and leakage current densities. The origin of the leakages in these films is related with the nature of the electrode-ferroelectric interfaces. A mechanism based on the redistribution of charged defects after the thermal conditioning treatments is suggested. These treatments influence the trapping level and lead to the control of pinning of ferroelectric domains. This causes a direct effect on the switching process.