W. Pérez

Ferroelectric properties of laser-ablated Sr1-xAxBi2Ta2O9 thin films (where A=Ba, Ca)

Bismuth-layered ferroelectric thin films of Sr1−xAxBi2Ta2O9, with composition x=0 and 0.2, were fabricated by using the pulsed-laser deposition technique. Structural characterization of the films by x-ray diffraction and atomic force microscopy, revealed that the films are polycrystalline in nature with average grain size of 180 nm. The films displayed spherical grains with a surface roughness of 12 nm. The ferroelectric measurements of Sr0.8Ba0.2Bi2Ta2O9, SrBi2Ta2O9, and Sr0.8Ca0.2Bi2Ta2O9 showed saturated hysteretic behavior with remanent polarization (2Pr) of 23.5, 17.9, 14 μC/cm2 and coercive field (Ec) of 31.06, 74.2, 86.3 kV/cm for a maximum applied electric field of 360 kV/cm. Films exhibited minimal (⩽17%) degradation of polarization for up to 1010 switching cycles. It was observed that the coercive field decreased with increase in the ionic size of partially substituted cations. The leakage current density of films were found to be of the order of ∼10−8 A/cm2 for up to a breakdown field of about ...

High remanent polarization in Sr1−xCaxBi2Ta2O9 ferroelectric thin films

SrBi2Ta2O9 (SBT) thin films with different concentrations of Ca at the Sr-site were grown on platinized silicon substrates. A systematic shift of diffraction lines towards higher diffraction angle confirmed the decrease in lattice parameter and attributed to the smaller ionic radii of Ca than for Sr. The grain size of the films was found to be increased upon Ca incorporation in SBT. The dielectric constant of the SBT films was systematically decreased with increasing Ca contents and it was attributed to lower dielectric permittivity of CaBi2Ta2O9 system. The ferroelectric properties were gradually increased up to 25% Ca at the Sr site, above which the ferroelectric properties started degrading. The maximum value of remanent polarization (∼23.8 μC/cm2) of the films was obtained at 20% Ca substituted SBT. The systematic increase in coercive field was attributed to the higher electronegativity of Ca. The films showed minimal fatigue degradation (<23%) after 1010 switching cycles. Substitution of Ca (up to 25...

Leakage current characteristics of laser-ablated SrBi2Nb2O9 thin films

We have studied the leakage current conduction mechanism of pulsed-laser-deposited SrBi2Nb2O9 (SBN) thin films on platinized silicon substrates. The time-dependent dc leakage current densities of SBN thin films do not follow Curie–von Schweidler power law. Instead the contribution of conduction current is predominantly electronic. At lower fields, the leakage current follows the ohmic behavior, and it increases exponentially for higher fields. The leakage current density of the SBN thin films was studied at elevated temperatures, and the data were fitted with the Schottky emission model. The effective Richardson’s constant was calculated to be about 8.7×10−6 A/cm2 K2. The Schottky barrier height of the SBN thin films was estimated to be 1.37 eV.

Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3

Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO3 over a temperature range of 20–300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a Neel transition at TN ∼ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below TN. Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near TN. An additional phonon anomaly was identified at T* ∼ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman freq...

Influence of Ca on structural and ferroelectric properties of laser ablated SrBi2Ta2O9 thin films

Bismuth layered perovskites of composition Sr1-xCaxBi2Ta2O9 (SCBT) with x=0, 0.1, 0.2, and 0.3 were grown using pulsed laser deposition technique. Influence of Ca on the structural and ferroelectric properties of SrBi2Ta2O9 (SBT) thin films have been investigated. XRD analysis revealed that SCBT thin films were polycrystalline after annealing at 750?C. The decrease in lattice parameter with increasing Ca contents in SBT thin films was attributed to smaller ionic radii of Ca at Sr-site. The grain size and the surface roughness were increased with higher Ca concentration as observed by atomic force microscopy. SBT thin films with 20% Ca doping exhibited enhanced ferroelectric properties with a remanent polarization (Pr) of 23.8 ?C/cm2. The fatigue resistance of Pr in SBT thin films degraded upon Ca incorporation at Sr-site. The coercive field increased with increases in Ca contents in SBT. The dielectric constant of SBT thin films was increased with the increase in grain size. The leakage current density of the SCBT thin films increased upon Ca incorporation.

Effect of annealing on structural and electrical properties of laser ablated Sr0.8Ba0.2Bi2Ta2O9 thin films

Ferroelectric thin films of Sr0.8Ba0.2Bi2Ta2O9 were grown on Pt/TiO2/SiO2/Si substrates by using the pulsed laser deposition technique and the effect of annealing on structural and electrical properties of the films has been investigated. The films deposited at 500 °C substrate temperature and 200 mTorr oxygen pressure showed amorphous nature. Perfect crystalline films were obtained after annealing at 750 °C. Increase in grain size with increasing annealing temperature was observed by atomic force microscopy. Good hysteretic behavior with remanent polarization (2Pr) of 11 and 25 μC/cm2 and coercive field (Ec) of 32 and 34 kV/cm with fatigue endurance of up to 1010 switching cycles were obtained with the films annealed at 750 and 800 °C. Moreover, the films annealed at 800 °C had a maximum dielectric constant of ∼375. Increase in dielectric constant with increasing annealing temperature is attributed to the increased grain size, whereas the lower dielectric constant and higher dissipation factor of the 850...

Electrical properties of non c-axis oriented SrBi2(Ta0.95V0.05)2O9 thin films

Pulsed laser deposition technique was used to grow off c-axis oriented SrBi2(Ta0.95V0.05)2O9 (SBTV) ferroelectric thin films. X-ray diffraction studies revealed the c-axis suppression in the films grown at lower substrate temperature (∼350°C) followed by annealing at higher temperatures (≥650°C). In-plane lattice parameters of the films were decreased with increase in annealing temperature. SBTV films annealed at 750°C exhibited enhanced ferroelectric properties with remanent polarization (2Pr) of ∼31.5 μC/cm2 and coercive field (Ec) of ∼157 kV/cm. The dielectric permmitivity of the films increased with increase in annealing temperature and it was attributed to the grain size dependence. The films annealed at 750°C showed maximum value of dielectric permittivity ∼172 with a tangential loss of 0.1, at 100 kHz. Higher value of dissipation factor at lower annealing temperature is explained in terms of space charge accumulation at grain boundaries. The leakage current densities of the films follow ohmic behavior at low field regime and space charge limited current dominates at higher fields.

Synthesis and Structural Characterization of (SrCa)Bi 2 Ta 2 O 9 Ceramics

(Sr 1 m x Ca x )Bi 2 Ta 2 O 9 ceramics with compositions x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 have been synthesized by conventional solid state route. Structural analysis of pellets were investigated using x-ray diffraction and micro-Raman spectroscopy. Substitution of Ca ion at Sr site was confirmed by decrease in lattice parameter calculated from x-ray diffraction data. Lowest Raman modes at 28.7 and 58 cm m 1 were found to increase in frequency with increasing Ca concentration and this was attributed to the lower mass and lower ionic radii of Ca. The temperature variation Raman studies showed a softening of the lowest phonon mode with increasing temperature. The increase in transition temperature with concentration of Ca in SBT compounds may be due to the decrease in tolerance factor.

Ferroelectric Properties in Ca Substituted SrBi2Nb2O9 Thin Films

We have studied the influence of partial substitution of Ca (0%, 5%, 10% and 15%) on the structural and electrical properties of SrBi 2 Nb 2 O 9 (SBN) thin films. Pulsed laser deposition technique was used to grow thin films on platinized silicon substrates. X-ray diffraction data confirmed the polycrystalline nature of the films with preferential orientation along the polarization axis. Incorporation of Ca at Sr-site of SBN reduced the lattice parameter and was attributed to the smaller ionic radii of Ca. The soft mode frequency at 29.5 cm -1 of SBN shifted towards higher wavenumber implied increasing the transition temperature upon Ca substitution. The grain sizes of SBN thin films vary from oval to rod like structures and improving the homogeneous distribution of grains with the increase in Ca contents. The ferroelectric properties of the films enhanced with maximum remanent polarization of 35.3 mC/cm 2 . The increase in the coercive field with incorporation of Ca at Sr-site was attributed to the higher electronegativity of Ca. The SBN thin films with the incoporation of Ca exhibited poor leakage current behavior. The reduction of dielectric permittivity with Ca concentration was attributed to the lower dielectric permittivity of CaBi 2 Nb 2 O 9 .

FERROELECTRIC PROPERTIES AND INTERFACIAL CHARACTERISTICS OF Ca SUBSTITUTED STRONTIUM BISMUTH TANTALATE THIN FILMS

In this study, we have investigated the structural, interfacial and ferroelectric properties of Sr1-xCaxBi2Ta2O9 thin films grown on Pt/TiO2/SiO2/Si substrates using pulsed-laser-deposition technique. The decrease in lattice parameters with increasing Ca content was attributed to the smaller ionic radius of Ca. Atomic force microscopy shows that the average grain size and surface roughness of the films increases with the incorporation of Ca. Films with x=0.2 exhibited a maximum remanent polarization of ~23.8 μC/cm2 with a coercive field of 175 kV/cm. The higher remanent polarization was attributed to the increased grain size and to the increase in the lattice mismatch between TaO2 and SrO planes. The presence of metallic bismuth at the interface of the film and the substrates was confirmed using XPS depth profile analysis. The current transport property of the thin film capacitors suggests a bulk-limited dc-current conduction mechanism.