In Situ Characterization of Oxide Thin Film Synthesis and Transformation by Surface and Coherent X-Ray Scattering—APS-U Perspectives

The <i>in situ</i> characterization of thin film synthesis and processing is crucial for advancing the development of multifunctional material heterostructures and devices. The greatly increased brightness and coherence of fourth generation X-ray lightsource like the upgraded APS (APS-U) will implement and deliver the world-class experimental platforms for in situ/operando surface X-ray and coherent X-ray scattering characterization, which will enable transformative investigations into thin film material synthesis and transformation under realistic conditions, which are critical for energy, quantum engineering and information technologies. In this talk, we would like to demonstrate two exemplary studies to highlight synchrotron surface X-ray capabilities to investigate the synthesis and phase transformation of oxide thin films.<br/><br/>Firstly, we explore remote epitaxy, a novel synthesis technique that allows for the fabrication of thin, freestanding single crystals and nanomembranes. This process involves a sacrificial layer, such as graphene, between a thin film and a single-crystalline substrate. This technique can create single crystal heterostructures with optimized properties by minimizing material incompatibilities. However, details of nucleation and growth via remote epitaxy remain largely unknown, necessitating in situ studies with atomic-level resolution. In this context, we will demonstrate our in situ synchrotron X-ray investigation of perovskite oxide thin film growth by molecular beam epitaxy onto graphene-coated SrTiO3 (001) substrates. Using X-ray phase retrieval methods, we reconstructed electron density profiles from X-ray crystal truncation rods measured under various growth conditions. Our in situ observations, combined with post-growth spectroscopy, provide critical insights into the behavior of graphene in the synthesis environment and its effects on complex oxide/graphene heterostructures.<br/><br/>The second example focuses on the topotactic reduction process to achieve superconducting infinite-layer nickelate thin films. In spite of significant progress has been made in the synthesis of parent phase nickelate thin film (e.g., RE0.8Sr0.2NiO3, RE = La, Nd, Pr...), the chemical reduction process to achieve the infinite-layer nickelate structure remains challenging and not fully understood. We will present our<i> in situ</i> synchrotron surface X-ray scattering studies combined with element-specific spectroscopies to probe the key steps of the topotactic reduction of epitaxial Nd0.8Sr0.2NiO3 thin films into Nd0.8Sr0.2NiO2 through a low-temperature reaction with CaH2. Our <i>in situ</i> X-ray observations provide essential structural and chemical insights into the formation of the square-planar structure critical for superconductivity in nickelate heterostructures. We discovered that the infinite-layer phase initiates at the heterointerface and propagates toward the film surface, with a dynamic surface boundary layer introducing hydrogen and removing apical oxygen ions. This study offers precise experimental guidance to improve effective reduction for intrinsic superconductivity behaviors.<br/><br/>In the end, we will give brief perspectives on emerging opportunities in X-ray <i>in situ</i> studies of multifunctional thin film and heterostructures enabled by the exciting advancements at the APS-U beamlines, in particular with enhanced high-energy, coherence and spatiotemporal capabilities (e.g. HESXRD, XPCS), which offer guidance for advancing the field of probing complex processes in thin film synthesis and processing.