In Situ Chemical Analysis of Complex Oxide Interfaces via Auger Electron Spectroscopy

Complex Oxides [1-3] have been investigated for their unconventional physical and chemical properties and ensuing broad technological applications, especially in the epitaxial thin film form. The key to controlling the properties of complex oxides is sensitively tied to their structure, composition, and chemical state of the transition metal ions in these materials. For ultra-high vacuum epitaxial growth methods such as molecular beam epitaxy, <i>in situ </i>structural characterization methods such as reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) have revolutionized our ability to control the structure of thin films, heterostructures and superlattices of complex oxides down to a single unit cell. These methods, RHEED in particular, have been adopted to high-pressure epitaxial growth methods such as Pulsed Laser Deposition (PLD). Nevertheless, the studying of the chemical state and composition of the materials <i>in situ</i>, especially in high-pressure environments, remains challenging due to the need to develop facile, accurate and versatile chemical and compositional analysis probes.<br/><br/>Recent developments in probe design for surface sensitive chemical analysis techniques such as Auger Electron Spectroscopy (AES) have expanded their use in harsh oxygen-rich atmosphere required for oxide thin film growth using PLD. This has been achieved by using a differential pumping system that uses a retarding field analyser (RFA), coupled with a collimator lens. This has led to increased sensitivity and much improved signal-to-background and signal-to-noise. This has further enabled us to measure and understand subtle changes in composition of the films <i>vis-à-vis </i>laser fluence and pressure [4]. Quantitative, real-time measurements of the composition during the growth have also led us to monitor and deliberately switch the surface termination of different oxide films [5].<br/><br/>Oxide heterostructures exhibit novel interfacial phenomena such as two-dimensional electron gases (2-DEGs), emergent ferromagnetism, tunable spin-charge interconversion and topological states. These interfacial phenomena are sensitive to surface reconstructions and charge transfer across the interfaces. To probe and manipulate these interfacial phenomena, it is imperative to study the chemical state of transition metals and oxygen at the single unit cell level. Considering that auger electrons are particularly sensitive to chemical state due to their outer-shell origins, real-time, <i>in situ </i>measurements of oxidation state during growth would help us understand the interplay of charge transfer with lattice, spin, and orbital degrees of freedom. In this work we simultaneously measure the oxidation state, surface composition and in essence the surface termination of complex oxides by monitoring the relative intensity ratios and shifts of satellite peaks in Auger fine spectra of both the transition metal cations and oxygen. Structural characterization using methods such as <i>in situ </i>RHEED and High-Resolution X-Ray Diffraction have been used to corroborate these findings.<br/><br/><br/>REFERENCES :<br/><br/>1) Huang, Z.; Renshaw Wang, X.; Rusydi, A.; Chen, J.; Yang, H.; Venkatesan, T.; Ariando, X. Interface engineering and emergent phenomena in oxide heterostructures. Adv. Mater. 2018, 30, No. 1802439.<br/>2) Ramesh, R. Emerging routes to multiferroics. Nature 2009, 461, 1218−1219.<br/>3) Schlom, D. G.; Chen, L. Q.; Pan, X.; Schmehl, A.; Zurbuchen, M. A. A thin film approach to engineering functionality into oxides. J. Am. Ceram. Soc. 2008, 91, 2429−2454.<br/>4) Orvis, T.; Kumarasubramanian, H.; Surendran, M.; Kutagulla, S.; Cunniff, A.; Ravichandran, J.; <i>ACS Appl. Electron. Mater.</i> 2021, 3, 3, 1422–1428<br/>5) Orvis, T.; Cao, T.; Surendran, M.; Kumarasubramanian, H.; Thind, A.S.; Cunniff, A.; Mishra, R.; Ravichandran, J.; <i>Nano Lett.</i> 2021, 21, 10, 4160–4166