Crystallization of functional oxide nanostructures and thin films has attracted great attention in the past decades since it is essential for the development of advanced materials for multiple applications in fields such as energy production and storage, sensing, electronics or photocatalysis. Solution-based methods, as the chemical solution deposition (CSD), have proven cost-effectiveness, great adaptability and scalability in the synthesis of highly crystalline complex oxide heterostructures as compared to more established physical methods. The main challenge is still, however, to unravel the thermodynamic and kinetic mechanisms driving epitaxial growth of CSD-based thin films.
In these works, the epitaxial crystallization process of model functional oxide thin films such as Ce0.9Zr0.1O2-y, LaNiO3, Ba0.8Sr0.2TiO3 and La0.7Sr0.3MnO3 has been studied on single crystal substrates (Y2O3:ZrO2, LaAlO3 and SrTiO3). Different thermal sources have been used to discern the key parameters controlling nucleation and epitaxial growth: pulsed laser annealing (PLA), rapid thermal annealing (RTA) and conventional thermal annealing (CTA). The structural-compositional investigations revealed a conversion from polycrystalline to epitaxial phase in all the investigated oxides. The isothermal evolution toward epitaxial film growth, typical in CTA and RTA methods, followed a self-limited process driven by atomic diffusion, and surface and interface energy minimization. This study clearly evidenced the competition between heterogeneous and homogeneous nucleation barriers, and a fast coarsening of polycrystalline grains in thick films which limits the epitaxial growth rate. Interestingly, the crystallization kinetics of laser treatments was determined to be 4-6 orders of magnitude faster (103-105 nm s-1) than in thermal (RTA, CTA) methods, mainly due to the developed large temperature gradients which induce modified atomic diffusion mechanisms driven 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 physical properties of the obtained films revealed good functionalities, being improved with longer annealing times due to grain boundary healing processes. The general validity of our studies will allow generalizing the implementation of solution derived films in functional devices.
1 Institut de Ciència de Materials de Barcelona ICMAB-CSIC, Spain
2 Institut Català de Nanociència i Nanotecnologia (ICN2), Spain