Understanding the Role of Defective Phases on the Conductivity Behavior of Strained Epitaxial LaNiO₃ Thin Films

Rare earth nickelates (RNiO3, where R is rare earth) possess detailed phase diagrams and exhibit a wide variety of physical phenomena such as antiferromagnetism, metal-to-insulator transitions, as well as rich strain-dependent physics. Among them, LaNiO3 is unique as it retains its metallic character down to ultralow temperatures, as well as offering the promise of various topological effects and exotic phenomena. Practically speaking, however, the fabrication of LaNiO3 in thin-film form is challenging since its various oxygen-deficient phases are close in formation energy. Here, we study a series of epitaxial LaNiO3 films fabricated on (001) LaAlO3 substrates by pulsed laser deposition under various growth and postannealing conditions. A complex correlation between structure, processing, and conduction properties is found, which is explored using a host of complementary characterization tools including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transport measurements, and scanning transmission electron microscopy. The conclusions of this study are as follows: (i) the XRD patterns of LaNiO3 films cannot be used to predict whether films will be conductive or insulating, (ii) the surface XPS of defective and “nondefective” LaNiO3–x films can appear identical, despite the bulk having a complex defective structure, and finally, (iii) LaNiO3–x films with up to 50% defective phase can exhibit metallic-like transport behavior down to 40 K.