Interface structure and precrystalline order in complex-oxide solid phase epitaxy: direct insights from in situ x-ray scattering

NON-TECHNICAL DESCRIPTION Developing new ways to synthesize very thin layers of materials raises important scientific questions. There are many applications of these thin films, ranging from quantum electronics to biotechnology. This work focuses on the transformation from the disordered amorphous form of materials to yield crystalline layers. Forming crystalline thin films from the amorphous form of the same compound can use structural information from an existing crystal, through a process termed solid-phase epitaxy (SPE). The key advantage of SPE lies in expanding the range of materials that can be formed into crystalline thin films. Studying SPE has been highly challenging because the process involves the rearrangement of atoms under conditions that have been hard to characterize. The volumes of materials are small, the steps occur very quickly, and the structural and chemical changes have subtle signatures. This project probes the transformation using precise X-ray techniques that capture structural and chemical features of the transition as it happens. The specific results include revealing the atomic-scale steps of crystallization, the mechanisms involved in the flow of amorphous metal oxides, linking preexisting short-range order in the disordered amorphous phase to crystallization, and changes in the chemistry of the metal ions. Metal oxide materials have fascinating and technologically important properties that can be improved by the insights resulting from this project. The impact of this work includes improving metal oxide materials for use in microelectronics, digital memories and optics, and superconducting materials for quantum devices. The scientific concepts involved could be generalized to other classes of materials, including biomaterials and emerging semiconductors. The project will result in instructional materials and demonstrations disseminating the underlying concepts. It will also promote the professional preparation of early career scientists and engineers by introducing important concepts in materials synthesis and advanced techniques in X-ray characterization. TECHNICAL DESCRIPTION The kinetics of crystal growth often limit the range of materials that can be crystallized, potentially limiting the applications of crystal growth. The problem is particularly challenging in complex oxides with in which there are multiple possible metal ion valences and structural phases. Crystallization of epitaxial thin films starting with the solid amorphous form of the complex oxide, termed solid phase epitaxy (SPE), occurs via different kinetic phenomena than the conventional direct epitaxial crystal growth from a vapor and thus is a solution to challenging epitaxial growth problems. This project probes the atomic-scale processes that occur at the moving amorphous/crystalline interface using in situ synchrotron X-ray characterization during crystallization. The key scientific issues probed in this work include the relaxation of the mechanical stress resulting from the density difference between amorphous an crystalline oxides, the influence of precrystalline order within the amorphous complex oxide, and changes in the chemical valence of the metal ions before and during crystallization. This project also probes processes competing with uniform crystal growth, including instabilities that can distort the shape of the amorphous/crystalline interface and the nucleation of competing crystalline phases. The project focuses on a series of complementary perovskite complex oxide compositions that share similar chemical and structural features. The resulting insights are broadly applicable across fields beyond the applications of perovskite oxides, in areas ranging from biomineralization to superconductivity. The development of in situ characterization methods supports the future application of sophisticated X-ray coherent scattering techniques. The project provides educational and professional development opportunities including through in-depth interactions with scientists at national laboratories and in the development, implementation, and use of cutting-edge in situ X-ray scattering instrumentation. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.