Cation-vacancy defects and interfacial reconstruction in hexagonal ferrite heterostructures

Polar narrow bandgap oxides, such as the ferroelectric h-LuFeO₃, are driving interest due to their enhanced optical responsivity in the visible range. The presence of polarization charges combined with the existence of bulk photovoltaic effect lead to a complex interplay between photoresponse and polar domain structure, which itself is critically modulated by the microstructure of samples. Here we present a comprehensive study of h-LuFeO₃ thin films’ microstructure grown on different substrates and buffers: α-Al2O3, α-Al2O3/Pt, YSZ/Pt and YSZ/ITO. Combining the atomic-resolution imaging and spectroscopic capability of scanning transmission electron microscopy with density functional theory calculations, first, we unveil the presence of cation-vacancy defects within the ferroelectric matrix. We show the nature of point defects and how the substrate determines the concentration of cation vacancies in the ferroelectric layer. Second, we identify a structural, chemical and electronic reconstruction at the interface between the h-LuFeO₃ films and the substrate/buffer. This interface reconstruction consists in double Fe-O atomic layers appearing independently from the conductivity of the adjacent film, suggesting that the Fe-O layers can modulate their valence state to compensate the polarity of the interface and contribute to the screening. Finally, we scrutinize the role of point defects as source of uncompensated moments, that may mask or overrule any intrinsic multiferroicity.