Effect of Ceramic-Target Crystallinity on Metal-to-Insulator Transition of Epitaxial Rare-Earth Nickelate Films Grown by Pulsed Laser Deposition

Abstract

We demonstrate the effect of the crystallinity of ceramic targets on the electronic properties of LaNiO3 (001) thin films epitaxially grown by pulsed laser deposition (PLD). We prepared two kinds of LaNiO3 targets with different crystallinity by manipulating calcination temperature (i.e., 300 and 1000 °C) in the solid state reaction for ceramic synthesis. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and X-ray photoelectron spectroscopy (XPS) experiments of the as-sintered LaNiO3 ceramic targets clearly show that the LaNiO3 target sintered after high-temperature (1000 °C, high crystallinity) calcination is more oxidized to Ni3+ with better crystallinity than the LaNiO3 target sintered after low-temperature (300 °C, poor crystallinity) calcination. Using these two LaNiO3 ceramics as PLD targets, we fabricated epitaxial LaNiO3/LaAlO3 (001) thin-film heterostructures to examine how target crystallinity affects the physical properties of LaNiO3 films. Intriguingly, the electrical transport properties of the as-grown LaNiO3 thin films are quite different depending on crystallinity of the LaNiO3 ceramic target used for film deposition. In conjunction with subsequent XPS analyses of our LaNiO3 thin films, it appears that LaNiO3 (001) films deposited from the high-temperature-calcined target with better crystallinity are less disproportionate in Ni charge valency with more Ni3+ oxidation states compared with LaNiO3 (001) films deposited from the low-temperature-calcined target with poor crystallinity. This difference in degree of charge disproportionation can induce a discrepancy in the metal-to-insulator transition temperature of ultrathin LaNiO3 (001) films and in their electrical conductance.