VO2 films on the relaxor ferroelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (PMN-PT) provide a promising candidate for the realization of a “Mottronic” device. VO2 undergoes a first-order structural phase transition at about 340 K and simultaneously switches from insulating to metallic behavior by a five orders of magnitude resistance drop. Importantly, the insulator-to-metal transition can also be driven by out-of-plane compressive lattice strain as being mediated, for example, by a PMN-PT substrate.
Here, we present a hard x-ray photoelectron spectroscopy (HAXPES) study of the electronic structure of VO2/PMN-PT interfaces across the strain- and temperature-induced phase transition. The in operando monitoring of the shapes and positions of characteristic core-level emissions directly reveals strain-dependent changes of the electronic structure and phase transition temperature of the VO2 film as well as bias-dependent changes of the electronic energy-level alignment at the VO2/PMN-PT interface.
Since the latter is a key effect determining electrical interface functionality, we further comprehensively determine the energy-level alignment at simple metal/PMN-PT interfaces (Au/PMN-PT and SrRuO3/PMN-PT). The bias-dependent average shifts of the PMN-PT core levels are found to have two dominant contributions on the 0.1–1-eV energy scale: one depending on the metal electrode and the remanent electric polarization and the other correlated with electric-field-induced strain. Element-specific deviations from the average shifts are smaller than 0.1 eV and appear to be related to predicted dynamical charge variations in PMN-PT. The results in particular suggest electric-field-induced modifications of the polarization distribution as a novel way to control the barrier height at metal-ferroelectric interfaces.
Overall, our results establish HAXPES as a powerful tool for the in operando investigation of functional interfaces.