Ferroelectric Perovskites Investigated by Synchrotron X-ray Scanning Tunneling Microscopy and First-principles Simulations

Ferroelectrics can add an extensive array of functionalities to various electronic devices, such as field effect transistors, non-volatile memories, and micro electromechanical system (MEMSs), where interfacial properties play a critical role. Despite this, understanding the role of polarization on the surface structure and chemistry has remained a longstanding challenge, mainly because this ideally requires a combination of both atomic scale imaging and chemical spectroscopy. Here we investigate the role polarization switching on the morphology, chemical composition, and surface reactivity of well-defined MBE-grown ferroelectric thin films using a combination of synchrotron X-ray scanning tunneling microscopy (SX-STM), ab-initio DFT+U electronic state calculations and simulated X-ray adsorption spectroscopy (XAS). SX-STM is a new tool that combines the chemical fingerprinting of XAS with signal collection via an STM tip, enabling chemical specificity at high lateral resolution and the possibility of in-situ electric biasing and gas dosing on the surface. We complement these measurements, and elucidate the effect of polarization switching on the core-hole excitations, using simulated, many-body XAS, calculated with the Bethe-Salpeter equation (BSE). This approach captures the interplay between polarization and lattice distortion at the atomic scale, including the complex physics at the surface seen in the experimental measurements. We find that polarization switching from downward to upward in (001) single crystalline BaTiO₃ thin films increases the number of excitations to unoccupied states and consequently the intensity of X-ray absorption across Ba M, Ti L and O K edges as well as a stronger binding strength with molecular O₂ as a model reactant. Harnessing the ability of investigating and ultimately control of ferroelectric interfaces down to atomic level can aid strategies in the design of heterostructures with tailored properties.