P. Bhattacharya

Structural and multiferroic properties of La-modified BiFeO3 ceramics

The coexistence of the magnetic and the electrical properties in lanthanum (La)-modified bismuth ferrite (Bi1−xLaxFeO3, x=0.05, 0.1, 0.15, and 0.2) ceramics was studied and compared with those of bismuth ferrite (BiFeO3). The presence of a small secondary phase of BiFeO3 (arises due to excess Bi2O3) was removed on La substitution at the Bi site, as observed in x-ray diffraction (XRD). The effect of La substitution on dielectric constant, loss tangent, and remnant polarization of the samples was studied in a wide range of temperature (77–400K) and frequency (1kHz–1MHz). The variation of magnetization, coercive field, and exchange bias with temperature (2–300K) and La concentration were investigated. These changes in the magnetic parameters with La doping along with those of the electron magnetic resonance parameters measured at 300K and 9.28GHz are understood in terms of increase in the magnetic anisotropy and magnetization. These results also show that stabilization of crystal structure and nonuniformity ...

Fabrication of stable wide-band-gap ZnO/MgO multilayer thin films

We report the fabrication of stable wide-band-gap (∼6 eV) ZnO/MgO multilayer thin films using pulsed-laser deposition on c-plane Al2O3 substrates. The thickness of ZnO layers was varied in the range of 0.75–2.5 nm inside the MgO host with a constant MgO thickness of 1 nm. With a decrease in the thickness of ZnO sublayers, abrupt structural transition from hexagonal to cubic phase was observed. The band gap of the films was found to be influenced by the crystalline structure of multilayer stacks. Thin films with hexagonal and cubic structure exhibited band-gap values of 3.5 and 6 eV, respectively. The x-ray photoelectron spectroscopy analysis confirmed that the Mg content of the films was about 40% and 60% in the hexagonal and cubic phases, respectively. Annealing at 750 °C did not influence the structural and optical properties of the ZnO/MgO multilayers.

Multiferroic properties of Pb(Zr,Ti)O3∕CoFe2O4 composite thin films

In the present work we report multiferroic behavior in lead zirconate titanate (PZT)–cobalt iron oxide (CFO) composite thin films. It is found that upon annealing, the multilayered structures are intermixed at least partially, and CFO is phase separated into PZT matrix to form a composite film. The phase separation behavior has been characterized by x-ray photoelectron spectroscopy depth profiling of the constituent elements in conjunction with dielectric spectroscopy measurements. The composite films exhibited ferroelectric as well as ferromagnetic characteristics at room temperature. The coupling between the ferroelectric and the ferromagnetic order parameters has been demonstrated through the reduction of ferroelectric polarization when measured under an applied magnetic field.

Effect of La substitution on structural and electrical properties of BiFeO3 thin film

The effect of La substitution on the structural and electrical properties of multiferroic BiFeO3 thin films grown on Pt∕TiO2∕SiO2∕Si substrates by pulsed laser deposition has been reported. X-ray diffraction data confirmed the substitutions of La into the Bi site with the elimination of all of the secondary phases. The dielectric constant of the films was systematically increased from 165 to ∼350 and the films showed excellent dielectric loss behavior. We observed a gradual increase in the remnant polarization (2Pr) with lanthanum substitution obtaining a maximum value of ∼42μC∕cm2 at 20mol% La incorporation. The leakage current behavior at room temperature of the films was studied and it was found that the leakage current decreased from 10−4to10−7A∕cm2 for La-substituted films at a field strength of 50kV∕cm. The reduction of dc leakage current of La-substituted films is explained on the basis of relative phase stability and improved microstructure of the material.

Optical properties of Zn1−xCoxO thin films grown on Al2O3 (0001) substrates

Thin films of Zn1−xCoxO (x=1–15%) were grown on an Al2O3 (0001) substrate by pulsed laser deposition and characterized by investigating their photoluminescence (PL) and other optical properties. The films were highly (0001) oriented without any impurity phases as observed in x-ray diffraction. The optical transmittance spectra showed that the band edge of Zn1−xCoxO was decreased with increase of Co concentration along with a subbandgap absorption in the range of 1.8to2.2eV. The near band-edge PL spectrum of ZnO thin films at 77K showed a strong peak of donor bound exciton at 3.307eV along with the free excitons. However, the PL spectra for free excitons of the Zn1−xCoxO films at 77K did not show any significant redshift due to Co incorporation.

Enhanced ferroelectric properties in laser-ablated SrBi2Nb2O9 thin films on platinized silicon substrate

Non-c-axis-oriented SrBi2Nb2O9 (SBN) thin films were grown on Pt/TiO2/SiO2/Si substrates using the pulsed-laser-deposition technique. X-ray diffraction results confirmed the textured growth of SBN thin films along (115) and (200) orientations. The increase in the value of the dielectric permittivity and decrease in the tangential loss of the films with an increase in annealing temperature were attributed to grain size dependence. SBN thin films annealed at 750 °C exhibited a maximum value of the dielectric constant of ∼346 with a dissipation factor of 0.02. Thin films with certain deposition parameters exhibited the highest remanent polarization (Pr) and coercive field, 25.7 μC/cm2 and 198 kV/cm, respectively. There was minimal (<20%) degradation in the switchable polarization (P*−P∧) after 1010 switching cycles. At lower field, the leakage current follows ohmic behavior, and at higher field, up to 100 kV/cm, the leakage current density was about 5×10−7 A/cm2.

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 ...

Leakage current behavior of SrBi2Ta2O9 ferroelectric thin films on different bottom electrodes

SrBi2Ta2O9 thin films on various bottom electrodes/substrates such as Pt/TiO2/SiO2/Si (Pt) and LaNiO3/Pt/TiO2/SiO2/Si (LNO) were grown using the pulsed laser deposition technique. X-ray diffraction studies revealed that as-grown (at 500 °C) films were crystallized in the layered structure after annealing at 800 °C. Films grown on platinized silicon exhibited maximum value of remanent polarization (2Pr∼21.5 μC/cm2) with coercive field (Ec) of ∼67 kV/cm. The dielectric constant and dissipation factor also decreased with the introduction of a 50 nm conducting LaNiO3 oxide electrode at the interface of Pt and SBT thin films, which might be contributed by the high resistive oxide electrode layers. The dc leakage characteristics of the films were studied at elevated temperatures and the data were fitted with the Schottky emission model. The barrier heights of the films grown on Pt and LNO substrates were estimated to be 1.27 and 1.12 eV with optical dielectric constants of 6.3 and 17, respectively. The reductio...

Growth and characterization of c-axis oriented LiNbO3 film on a transparent conducting Al:ZnO inter-layer on Si

C-axis oriented lithium niobate (LiNbO 3 ) films were deposited by pulsed-laser deposition on silicon using a transparent conducting interlayer of aluminum (Al)-doped zinc oxide (ZnO). Only two x-ray diffraction reflections corresponding to (006) and (0012) planes of LiNbO 3 were observed in the films deposited at a 100-mTorr oxygen pressure and a 450–500 °C substrate temperature. Presence of sharp modes corresponding to E(TO) and A(LO), and absence of any superfluous peaks mainly around 900–905 cm −1 in the Raman spectra confirmed the formation of textured LiNbO 3 film. Measured value of direct current and alternate current (AC) conductivities and dielectric constant are 8.6 × 10 -12 Ω −1 cm -1 , 1.16 × 10 -6 Ω −1 cm -1 , and 29.3, respectively, at room temperature. Behavior of dielectric constant and AC conductivity with frequency and temperature are close to the reported single-crystal data.

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...