Low Temperature Epitaxial Oxide Ultrathin Films and Nanostructures by Atomic Layer Deposition

Highly epitaxial and pure (001) CeO₂ ultrathin films have been prepared by atomic layer deposition (ALD) at 275 °C on Y-stabilized ZrO₂ cubic fluorite single crystal substrate using cerium β-diketonate (Ce­(thd)₄) and ozone (O₃) as precursors. Substrate temperature and precursor pulses have been optimized to set the ALD window obtaining a growth per cycle of ≈0.2 Å/cycle. This extremely low growth rate has been identified as a key parameter to ensure epitaxial growth at these low temperatures. Post-thermal treatments at 900 °C in oxygen further improve ALD-CeO₂ film texture while maintaining film stoichiometry and ultrasmooth surface, rms < 0.4 nm. ALD-CeO₂ thin film growth has also been tested on perovskite single crystal substrates, SrTiO₃ and LaAlO₃, exhibiting CeO₂ epitaxial growth and thus validating ALD as an outstanding method for low temperature epitaxial growth. Furthermore, we demonstrate that by combining e-beam lithography and ALD it is feasible to obtain size-controlled CeO₂ nanostructures

Ultrafast Crystallization of Ce_0.9Zr_0.1O_2–<i>y</i> Epitaxial Films on Flexible Technical Substrates by Pulsed Laser Irradiation of Chemical Solution Derived Precursor Layers

The epitaxial growth of Ce_0.9Zr_0.1O_2–<i>y</i> (CZO) thin-films on yttria-stabilized zirconia (YSZ) (001) single crystal and YSZ (001)/stainless steel (YSZ/SS) technological substrates is investigated by pulsed laser irradiation of solution-derived cerium–zirconium precursor layers using a UV Nd:YAG laser source at atmospheric conditions. The influence of laser processing parameters on the morphological and structural properties of the obtained films is studied by atomic force and transmission electron microscopies, as well as X-ray diffractometry. The analyses performed demonstrate that laser treatments enable the epitaxial growth of tens of nanometers thick CZO films with a crystallization kinetic process several orders of magnitude faster than that of conventional thermal annealing. Fully epitaxial films are attained using stainless steel (SS) flexible tapes as a substrate. Even though photochemical mechanisms are not fully discarded, it is concluded that photothermal processes are the main contribution responsible for the fast epitaxial crystallization

Ultrafast Epitaxial Growth Kinetics in Functional Oxide Thin Films Grown by Pulsed Laser Annealing of Chemical Solutions

The crystallization process and physical properties of different functional oxide thin films (Ce_0.9Zr_0.1O_2‑y, LaNiO₃, Ba_0.8Sr_0.2TiO₃, and La_0.7Sr_0.3MnO₃) on single crystal substrates (Y₂O₃:ZrO₂, LaAlO₃, and SrTiO₃) are studied by pulsed laser annealing (PLA). A Nd:YAG laser source (λ = 266 nm, 10 Hz and τ ∼ 3 ns) is employed to crystallize chemical solution deposited (CSD) amorphous/nanocrystalline films under atmospheric conditions. We provide new insight on the influence of photochemical and photothermal interactions on the epitaxial crystallization kinetics of oxide thin films during the transformation from amorphous/polycrystalline material (i.e., atomic diffusion, epitaxial growth rates, and activation energies of nucleation and crystallization). The epitaxial growth is investigated by varying the laser fluence and the applied number of pulses. The morphology, structure, and epitaxial evolution of films are evaluated by means of atomic force and transmission electron microscopies and X-ray diffraction. Highly epitaxial oriented films of 20–40 nm in thickness are obtained by PLA. The crystallization kinetics of laser treatments is determined to be orders of magnitude faster than thermal treatments with similar activation energies (1.5–4.1 eV), mainly due to the large temperature gradients inducing modified atomic diffusion mechanisms derived mainly from photothermal interactions, as well as a minor contribution of photochemical effects. The fast heating rates achieved by PLA also contribute to the fast epitaxial growth due to reduced coarsening of polycrystalline material. The measurement of the physical properties (electrical resistivity and magnetism) of laser processed CSD films has revealed significantly good functionalities, close to those of thermally grown films, but with much shorter processing times

Nanocrystalline Ferroelectric BiFeO₃ Thin Films by Low-Temperature Atomic Layer Deposition

In this work, ferroelectricity is identified in nanocrystalline BiFeO₃ (BFO) thin films prepared by low-temperature atomic layer deposition. A combination of X-ray diffraction, reflection high energy electron diffraction, and scanning transmission electron microscopy analysis indicates that the as-deposited films (250 °C) consist of BFO nanocrystals embedded in an amorphous matrix. Postannealing at 650 °C for 60 min converts the sample to a crystalline film on a SrTiO₃ substrate. Piezoelectric force microscopy demonstrates the existence of ferroelectricity in both as-deposited and postannealed films. The ferroelectric behavior in the as-deposited stage is attributed to the presence of nanocrystals. Finally, a band gap of 2.7 eV was measured by spectroscopic ellipsometry. This study opens broad possibilities toward ferroelectric oxides on 3D substrates and also for the development of new ferroelectric perovskites prepared at low temperature

Direct Monolithic Integration of Vertical Single Crystalline Octahedral Molecular Sieve Nanowires on Silicon

We developed an original strategy to produce vertical epitaxial single crystalline manganese oxide octahedral molecular sieve (OMS) nanowires with tunable pore sizes and compositions on silicon substrates by using a chemical solution deposition approach. The nanowire growth mechanism involves the use of track-etched nanoporous polymer templates combined with the controlled growth of quartz thin films at the silicon surface, which allowed OMS nanowires to stabilize and crystallize. α-quartz thin films were obtained after thermal activated crystallization of the native amorphous silica surface layer assisted by Sr²⁺- or Ba²⁺-mediated heterogeneous catalysis in the air at 800 °C. These α-quartz thin films work as a selective template for the epitaxial growth of randomly oriented vertical OMS nanowires. Therefore, the combination of soft chemistry and epitaxial growth opens new opportunities for the effective integration of novel technological functional tunneled complex oxides nanomaterials on Si substrates

Low AC loss inkjet-printed multifilamentary YBCO coated conductors

Impressive performance has been achieved in (RE)Ba2Cu3O7-delta (REBCO) coated conductors, but for many applications, the high cost and ac losses remain prohibitive. Inkjet printing methods combine scalability and low equipment cost with high-resolution patterning, potentially addressing both issues by enabling the production of multifilamentary coated conductors without subtractive processing. The successful production of multifilamentary superconducting YBa2Cu3O7-delta (YBCO) structures by inkjet printing of a low-fluorine YBCO precursor solution on SS/ABAD-YSZ/CZO substrates is reported. Two approaches have been developed. In the first method, YBCO filaments were directly deposited on the buffered substrate by piezoelectric printing; and in the second approach, CeO2 tracks were first printed to pattern a subsequently overprinted YBCO film, creating a multifilamentary structure by an inverse technique. Scanning Hall probe measurements have been used to compare the filamentary structures and critical currents achieved by both methods, and a J(c) of up to 3 MA cm(-2) has been obtained at 77 K. For the inverse printing approach, the ac hysteresis losses have been also measured and compared with theoretical models