M. Lourdes Calzada

Activated Solutions Enabling Low‐Temperature Processing of Functional Ferroelectric Oxides for Flexible Electronics

Functional ferroelectric oxides for flexible electronics are achieved from activated solutions enabling low-temperature processing and large-area deposition directly on polymeric substrates. This processing technology reaches the lower limit temperature of crystallization at 300 °C, using a strategy that combines seeded diphasic precursors and photochemical solution deposition. Properties of these materials are comparable to those of high-temperature-processed counterparts and organic ferroelectrics.

Low‐Temperature Liquid Precursors of Crystalline Metal Oxides Assisted by Heterogeneous Photocatalysis

The photocatalytically assisted decomposition of liquid precursors of metal oxides incorporating TiO2 particles enables the preparation of functional layers from the ferroelectric Pb(Zr,Ti)O3 and multiferroic BiFeO3 perovskite systems at temperatures not exceeding 350 ºC. This enables direct deposition on flexible plastic, where the multifunctionality provided by these complex-oxide materials guarantees their potential use in next-generation flexible electronics.

Photochemical solution processing of films of metastable phases for flexible devices: the β-Bi 2 O 3 polymorph

The potential of UV-light for the photochemical synthesis and stabilization of non-equilibrium crystalline phases in thin films is demonstrated for the β-Bi2O3 polymorph. The pure β-Bi2O3 phase is thermodynamically stable at high temperature (450–667 °C), which limits its applications in devices. Here, a tailored UV-absorbing bismuth(III)-N-methyldiethanolamine complex is selected as an ideal precursor for this phase, in order to induce under UV-light the formation of a –Bi–O–Bi– continuous network in the deposited layers and the further conversion into the β-Bi2O3 polymorph at a temperature as low as 250 °C. The stabilization of the β-Bi2O3 films is confirmed by their conductivity behavior and a thorough characterization of their crystal structure. This is also supported by their remarkable photocatalytic activity. Besides, this processing method has allowed us for the first time the preparation of β-Bi2O3 films on flexible plastic substrates, which opens new opportunities for using these materials in potential applications not available until now (e.g., flexible photocatalytic reactors, self-cleaning surfaces or wearable antimicrobial fabrics). Therefore, photochemical solution deposition (PCSD) demonstrates to be not only an efficient approach for the low temperature processing of oxide films, but also an excellent alternative for the stabilization of metastable phases.

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.

Pyrochlore-to-perovskite transformation during rapid heating of sol-gel (Pb,La)TiO 3 thin films

Phases appearing in lanthanum-modified lead titanate thin films prepared by a diol-based sol-gel method and crystallized by rapid heating were studied. The results clearly indicate that a phase transformation from a pyrochlore structure to the perovskite phase occurs in Pb-deficient films during the thermal treatment, which involves a heating rate higher than 500 °C min −1 . The rate of this transformation is a function of the lead content of the films, decreasing as lead volatilizes. Temperatures higher than 650 °C or soak times longer than 2 h make possible the complete pyrochlore-to-perovskite transformation without any lead excess in the films.

A UV-absorber bismuth(III)-N-methyldiethanolamine complex as a low-temperature precursor for bismuth-based oxide thin films

Novel synthetic methods in solution that reduce the formation temperature of bismuth-based electronic oxides are essential for their successful integration with substrates of low thermal stability within micro- and flexible-electronic devices. This has become crucial for these oxides, since they appear as promising low-toxic functional materials alternative to other electronic oxides containing heavy metals. However, this is a challenge, since the crystallization of bismuth oxides occurs at high temperatures. To overcome these problems, we synthesize here a UV-absorber charge transfer metal complex in solution between the Bi(III) ion and an alkanolamine, N-methyldiethanolamine (Bi(III)–mdea). We take advantage of the photoreactivity of this complex to prepare bismuth-based oxide thin films at low temperature, which cannot be achieved by traditional thermal processing methods. Room temperature stable oxide thin films of the high-temperature δ-Bi2O3 phase are prepared from these solutions by UV-irradiation and annealing at 350 °C. The efficiency of this synthetic strategy is additionally proven for the low temperature preparation of thin films of much more complex bismuth based functional oxides: the multiferroic bismuth ferrite, BiFeO3, and the relaxor-ferroelectric perovskite of bismuth, sodium and barium titanate, (Bi0.5Na0.5)0.945Ba0.055TiO3.

(Pb,La)TiO3∕(Pb,Ca)TiO3 ferroelectric heterostructures for nonvolatile memories

The existence of voltage shifts in the ferroelectric loops measured in (Pb,La)TiO3∕(Pb,Ca)TiO3 heterostructures onto Si-based substrates after poling at 150°C has been studied. Results have been compared with the measured ones in (Pb,La)TiO3 and (Pb,Ca)TiO3 single-component films, which have been prepared in identical conditions. Lower voltage shifts have been obtained in the heterostructures. The reduced number of VO–VPb defect-dipoles present in the heterostructures as a consequence of the mechanism of stress relaxation via plastic deformation through vacancies diffusion that takes place in these films results in a reduced electron trapping, and thus, in a lower voltage shift of the loop.

(Pb,Ca)TiO3∕(Pb,La)TiO3∕(Pb,Ca)TiO3 heterostructure characterized as ferroelectric multifunctional material

Ferroelectric thin films are multifunctional materials with applications in a wide range of microelectronic and microelectromechanical devices. The recent investigations on multilayer heterostructures builtup with ferroelectric layers of different compositions have shown an enhancement of the dielectric, ferroelectric, and pyroelectric properties with respect to the single-component films. In this work, the structural, dielectric, ferroelectric, and pyroelectric properties of a (Pb,Ca)TiO3∕(Pb,La)TiO3∕(Pb,Ca)TiO3 multilayer heterostructure have been studied and compared with identically prepared (Pb,Ca)TiO3 and (Pb,La)TiO3 films. The heterostructure shows a higher dielectric permittivity in the whole temperature interval up to the transition temperature, and a higher tetragonal distortion with respect to the single-component films, revealing a lower residual stress in the heterostructure. Optimized ferroelectric and pyroelectric properties have been achieved in the heterostructure, with maximum values of ...

Solution processing and crystallization of ferroelectric samarium modified lead titanate thin films

Samarium-doped lead titanate thin films have been prepared by a sol–gel technique. The processing of precursor solutions through two different synthetic routes and containing differing excesses of PbO is described. The films deposited from these solutions are crystallized by thermal treatments at temperatures higher than 550 °C. The crystalline structure, composition and microstructure of the resulting films are studied by grazing incidence X-ray diffraction analysis, energy dispersive spectroscopy, quantitative scanning electron microscopy and transmission electron microscopy. It is observed that the film microstructure and the development of non-ferroelectric phases in these films are influenced by both the solution chemistry and the lead oxide stoichiometry. Preliminary results concerning the ferroelectric behaviour of these films are also given.

Corrigendum: Active layers of high-performance lead zirconate titanate at temperatures compatible with silicon nano- and microelectronic devices

Applications of ferroelectric materials in modern microelectronics will be greatly encouraged if the thermal incompatibility between inorganic ferroelectrics and semiconductor devices is overcome. Here, solution-processable layers of the most commercial ferroelectric compound ─ morphotrophic phase boundary lead zirconate titanate, namely Pb(Zr0.52Ti0.48)O3 (PZT) ─ are grown on silicon substrates at temperatures well below the standard CMOS process of semiconductor technology. The method, potentially transferable to a broader range of Zr:Ti ratios, is based on the addition of crystalline nanoseeds to photosensitive solutions of PZT resulting in perovskite crystallization from only 350 °C after the enhanced decomposition of metal precursors in the films by UV irradiation. A remanent polarization of 10.0 μC cm−2 is obtained for these films that is in the order of the switching charge densities demanded for FeRAM devices. Also, a dielectric constant of ~90 is measured at zero voltage which exceeds that of current single-oxide candidates for capacitance applications. The multifunctionality of the films is additionally demonstrated by their pyroelectric and piezoelectric performance. The potential integration of PZT layers at such low fabrication temperatures may redefine the concept design of classical microelectronic devices, besides allowing inorganic ferroelectrics to enter the scene of the emerging large-area, flexible electronics.