Iñigo Bretos

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.

Defect-mediated ferroelectric domain depinning of polycrystalline BiFeO3 multiferroic thin films

The ferroelectric domain depinning in a polycrystalline BiFeO3 film is studied by a defect-mediated diffusion mechanism driven by a secondary re-oxidation anneal. The presence of defect complexes (oxygen-vacancy-associated dipoles) responsible for pinning is discussed from the current-voltage (I-V) characteristics of the film. Dissociation of these complexes by re-oxidation anneal produces the effective depinning of domains in the material. The released oxygen vacancies would be compensated at the re-oxidized state due to the valence change of Fe2+ to Fe3+. Improvement on domain mobility results in a larger contribution to ferroelectric switching, showing a room-temperature remanent polarization of 67 μC cm−2.

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.

Metal complexes with N-methyldiethanolamine as new photosensitive precursors for the low-temperature preparation of ferroelectric thin films

New photosensitive precursors for the low-temperature preparation of ferroelectric PbTiO3 thin films are obtained by using a sol–gel synthetic route. The reaction of N-methyldiethanolamine (MDEA) with lead acetate and titanium bis(acetylacetonate) diisopropoxide reagents leads to the formation of metal-MDEA complexes with a significant ultraviolet (UV) absorption due to the chromophore groups arisen from the orbital splitting between the respective metal atom and the organic ligand. Processes of transference of charge between the nitrogen and the metallic centre are responsible for the UV absorption in the resulting sols: ligand to metal charge transfer (LMCT) in the case of the Ti(IV) complex, d0 element, and metal to ligand charge transfer (MLCT) in the case of the Pb(II) one, d10 element. The enhanced photosensitivity of the precursor solutions provided by these complexes is exploited for the fabrication of ferroelectric PbTiO3 films at temperatures compatible with silicon microelectronics (400 °C).

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.

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.

Dielectric properties of Pb0.5Ca0.5TiO3 thin films

A dielectric study is carried out on chemical solution-deposited calcium lead titanate (Pb,Ca)TiO3 thin films, with calcium contents of 50 at. %, by measuring capacitance versus voltage as a function of temperature. A moderate ferroelectric activity is observed that remains in a broad temperature range, and that depends on the applied voltage and thickness of the films. This behavior is associated with the built-in fields generated as result of the space charge inside the films. From the diffusivity of dielectric constant versus temperature (K′−T) curves, the relaxorlike nature of the films is also demonstrated. The feasibility of these films for fabrication of voltage tuneable capacitors and charge store devices (dynamic random access memories) is confirmed.

Processing of Chemical Solution Deposited Ca-Modified PbTiO3 Films for High Frequency Components and Dynamic Random Access Memories

Diol-based sol-gel processes have been used for the synthesis of calcium modified lead titanate, CaxPb1 - xTiO3 (PCT), solutions with x = 0.5. Single perovskite films have been obtained on Pt/TiO2/SiO2/(100)Si substrates. Dielectric properties of the films have been measured to evaluate their applicability in dynamic random access memories (DRAMs) and high frequency (HF) components.

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.

Effects of the Sol-Gel Processing and Sol Aging on the Properties of the High-Ca Modified PbTiO3 Derived Films

Solutions of calcium modified lead titanate with a nominal composition of (Pb 0.5 Ca 0.5 )TiO 3 have been synthesised by two synthetic sol-gel routes. The aging of the synthesised sols was followed by the variation with time of their particle size and of their viscosity. Films were prepared from the sols aged at different times. The thickness, crystalline structure and dielectric properties of these films were measured and the results were related with the synthesis procedure and with their aging behaviour.