Real-time X-ray Scattering Studies of Oxide Epitaxial Growth

University Of Vermont & State Agricultural College, Burlington VT

Investigators

Abstract

This project is jointly funded by the Electronic and Photonic Materials (EPM) and Ceramic (CER) Programs in the Division of Materials Research. NON-TECHNICAL DESCRIPTION: X-rays are an enormously powerful tool for the study of materials: they can measure structures down to atomic dimensions using diffraction because of their short wavelength, and they can reach inside materials because they are not strongly absorbed. Synchrotrons are the brightest sources of X-rays available to scientists. In this project, real-time X-ray scattering is used to study the growth of crystalline metal oxide thin films in a special film deposition chamber customized for installation at a synchrotron source. A movie of the X-ray scattering pattern from the growing surface as a function of time is used to learn about the atomic structure of the crystalline layers as they assemble from the vapor phase, which yields important insights in the quest to create new materials with improved or novel properties. Examples of useful properties include the ability to store an electrical charge via a relative shift of positive and negative ions in the crystal lattice (ferroelectric effect), or to produce an electrical voltage when deformed (piezoelectric effect). These effects, and several other more exotic effects can potentially be modified through growth of artificially-produced thin films (e.g., with alternating layers of two different materials, each only a few atoms thick). Due to their novel properties, metal oxides are significant in a number of applications, including electronic memories, detectors, actuators and energy harvesters. This project provides graduate student training in a highly interdisciplinary area, and it introduces undergraduate students to X-ray science and thin film deposition technology. TECHNICAL DETAILS: This project utilizes a new world-class synchrotron facility at Brookhaven National Laboratory (NSLS II) to carry out in situ time-resolved X-ray scattering studies of the evolution of surfaces, interfaces, and multilayers composed of oxide materials during thin film growth by pulsed laser deposition. Perovskite oxides exhibit an extraordinary variety of complex structural distortions and associated functional properties, which may also be continuously tunable through growth of non-equilibrium structures. However, realizing synthetic structures through the growth of epitaxial thin films and multilayers currently involves a time-consuming trial and error approach, which greatly limits the pace of progress towards new materials and devices. In situ synchrotron X-ray scattering provides the capability to generate reciprocal space maps in a matter of seconds. These maps provides real time information on the evolution of both in and out of plane lattice parameters, polar domain structure, surface roughness and surface termination, in a single snapshot, thus providing critical feedback on the relationship between growth process variables and the resulting atomic structure. This project also focuses on fundamental issues related to the role of impact-induced energetic processes in promoting the formation of planar interfaces, and how such processes can be controlled to produce improved materials. Specific atomic-scale mechanisms are being tested using in situ X-ray scattering to follow the dynamics of surfaces of oxide thin films during deposition. This project also provides an exciting possibility for graduate students to gain expertise in both thin film growth and characterization of materials by advanced methods, and for undergraduate students to participate in an interdisciplinary project involving university-national laboratory collaborations.