control of the electron flow across interfaces is what makes electronic
devices. The relaxor ferroelectric 0.72PbMg1/3Nb2/3O3-0.28TiO3
(PMN-PT) provides a fertile platform for the study of novel bias-dependent
electrical polarization and lattice strain effects on electronic behavior at
interfaces. For example, at the interface to a metal the fundamental electrical
interface property, the Schottky barrier height, becomes dependent on the
magnitude and direction of the polarization, whereas at the interface to a strongly
correlated material the field-induced strain may drive a metal-to-insulator
transition. A major challenge, however, is to directly probe such modifications
of the interfacial electronic structure in device-like structures.
Here, we demonstrate
that hard x-ray photoelectron spectroscopy (HAXPES) is a powerful tool to image
the electronic energy-level alignment at buried interfaces in operando as a function of electrical bias. For the cases of metal/PMN-PT
and VO2/PMN-PT interfaces, we comprehensively track the bias
dependence of the substrate and electrode core levels as well as of the valence
density of states. These spectroscopic data provide an intriguingly direct view
on how key electrical parameters such as the Schottky barrier height and the
density of states at the Fermi level can be tuned in situ by electrical polarization and field-induced lattice strain.
The findings may in particular open novel ways to control the conductivity
across metal-ferroelectric interfaces and of thin VO2 layers,
illustrate the power of the experimental technique, we will finally present the
upcoming fascinating possibility of performing femtosecond time-resolved HAXPES
measurements at the European XFEL, which may enable us to film the electron
dynamics at buried interfaces on fundamental time scales.