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.

KEYWORDS:thin film metal-to-insulator transition oxide nickelate pulsed laser deposition Introduction ARTICLE SECTIONSJump To Perovskite rare-earth nickelates, of which the generic formula is RNiO3 (R = La, Nd, Pr, etc.), have attracted great interest due to a large variety of unusual physical phenomena such as metal-to-insulator phase transitions (except for LaNiO3),(1−4) an octahedral breathing order caused by charge/bond disproportionation,(5,6) nonequilibrium polar metallic states,(7) and unconventional magnetic/orbital orderings.(8−12) Despite these fascinating physical phenomena, it is difficult to fabricate high-crystalline rare-earth nickelates in either bulk or film form.(4,13−21) For bulk rare-earth nickelates, an extremely high oxygen ambient pressure of 4 GPa is required at 900 °C for crystallizing bulk single crystals and for stabilizing the Ni3+ oxidation state in R3+Ni3+O2–3.(15) For rare-earth nickelate films, their crystalline structures and electrical transport behaviors are quite different according to various factors controllable in the film deposition.(17−25) Note that disorders (i.e., strain,(18,19) defects,(26−28) grain boundaries,(29) off-stoichiometry,(20,25,30,31) and crystallinity(17,20)) in complex oxide materials are very susceptible to their film-growth conditions (e.g., temperature and oxygen partial pressure).(32,33) Recently, Hauser et al. have demonstrated that chemical stoichiometry and strain of epitaxial NdNiO3 films strongly depend on growth conditions, resulting in a sizable discrepancy in their electrical transport behaviors.(17) Thus, it is valuable to study how the physical properties of rare-earth nickelates are affected by such disorders arising from a difference in synthetic circumstances. A pulsed laser deposition (PLD) technique has been widely used to synthesize epitaxial rare-earth nickelate films for several decades.(7,16,19,20) In PLD, it is possible to create a plasma state of ionized atoms with an extremely high kinetic energy by irradiating an excimer laser with high energy (ultraviolet (UV) wavelength = 248 nm and photon energy ∼ 5 eV for KrF) onto a ceramic target inside a vacuum chamber.(34,35) The energetic ions generated by laser ablation of the cerami