Rasmi R. Das

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.

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...

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.

Dielectric relaxation in pulsed laser ablated CaCu3Ti4O12 thin film

The dielectric constant and ac conductivity of CaCu3Ti4O12 thin films deposited on platinized silicon substrate using pulsed laser deposition technique have been measured in the metal-insulator- metal configuration over wide temperature (80–500K) and frequency (100Hz–1MHZ) ranges. The crystallographic structure and the phase purity of the deposited films were investigated using x-ray diffraction and micro-Raman spectroscopy. The dielectric dispersion data have been fitted to Debye-type relaxation with a distribution of relaxation times and an asymmetric distribution of relaxation was observed which increases with increase in ωτ.

Comparison of the Electrical Characteristics of PZT and SBT Thin Films

The electrical properties of lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT) thin films are compared for nonvolatile random access memory applications. PZT thin films have high switchable polarization with poor fatigue resistance and higher leakage current densities as compared to SBT thin films deposited on platinized silicon substrates. Characterization of these films in terms of their domain dynamics under application of sub-switchable and switchable electric fields give valuable insight about the observed differences in the electrical behavior. In the present work the dielectric behaviors of SBT and PZT thin films at sub-switchable electric field were analyzed in terms of Rayleigh law. The reversible and irreversible polarization component of switchable polarization was separated by the measurement of capacitance-voltage (C-V) and polarization hysteresis loops. The conduction mechanisms in these films were evaluated by measuring field dependence of leakage current densities at various temperatures. The observed differences in electrical properties are explained in terms of the intrinsic defects and defect-domain interaction of these two materials.

Polarized Raman Spectra of BaTiO3/SrTiO3 Superlattices

BaTiO 3 /SrTiO 3 superlattices were grown on (001)MgO substrates by pulsed-laser-deposition technique. The thickness of each layer varied from 6 to 25 unit cells. Polarization-dependent Raman spectra confirmed tetragonal symmetry of the superlattices with the polar axis being normal to the layers. The observed optic modes in the superlattices showed intermediate frequencies between corresponding bulk BaTiO 3 and SrTiO 3 values, while the E(TO 1 ) soft mode exhibited upward shift due to significant two-dimensional clamping caused by the lattice mismatch of the constituting layers. The observed downward shift of the soft mode revealed relaxation of two-dimensional stresses with increasing the stacking periodicity of the superlattices. The excitation of folded transverse acoustic phonons in the Raman spectra due to the new periodicity of the Brillouin zone was revealed. The observed data are quantitatively explained by a folding of the corresponding acoustic branch. It is shown that micro-Raman spectroscopy provides determination of a spatial variation of the layer thickness in graded superlattices. On leave from the Faculty of Physics, Rostov State University, Rostov-on-Don, Russia

CaBi2Ta2O9 ferroelectric thin films prepared by pulsed laser deposition

Thin films of CaBi2Ta2O9 (CBT) were deposited on Pt/TiO2/SiO2/Si substrates using the pulsed laser deposition technique at temperatures ranging from 500 to 700 °C. The presence of (115) and (0010¯) orientations confirm the phase formation at the lower temperature (500 °C). Microstructure evolution of CBT films with oxygen pressure of 100–200 mTorr at a substrate temperature of 650 °C shows that the films deposited at lower pressure have a relatively smaller grain size and less surface roughness. The films grown at 650 °C exhibited a maximum polarization of (2Pm) 17 μC/cm2, remanent polarization of (2Pr) 8 μC/cm2 and coercive field of (Ec) 128 kV/cm, with fatigue endurance up to 1010 switching cycles. The higher dielectric constant (∼115 at 100 kHz) with a relatively lower dissipation factor (0.02) at higher growth temperature (700 °C) was explained by the increased grain size. The higher leakage current density (∼10−7 A/cm2) at higher deposition temperature is attributed to the interfacial diffusion of th...