Anisotropy of core-level spectra and the correlation with transport properties of epitaxial lanthanum nickel oxide thin films

Abstract

We studied the electronic structures of (001), (110), and (111) LaNiO₃ (LNO) films using XPS. Core-level photoemission spectra analysis reveals that the energy separation ΔE from the satellite to main peaks of Ni 2p_1/2 clearly varies in accordance with LNO film orientation. The ΔE in (111) LNO films is 9.71 ± 0.1 eV, larger than the values of 9.39 ± 0.1 and 9.21 ± 0.1 eV in corresponding (001) and (110) films, respectively. This indicates that the Ni 3d–O 2p hopping interaction strength t_pd is larger in the (111) LNO films compared with the other two films. The large aforementioned t_pd in (111) LNO films also changes the electron states near the Fermi level, which suggests improved electrical conductive properties in (111) LNO films compared with corresponding (110) and (001) films.

Introduction

LaNiO₃ (LNO) is an outstanding perovskite oxide not only because of its electric, magnetic, and mass enhancement properties, but also because it does not exhibit low temperature embrittlement [[1], [2], [3]]. Advanced physical vapor deposition techniques allow researchers to grow LNO thin films that have a low electrical resistivity [4,5]. The LNO film has attracted great attention for its promising application as a bottom electrode in the integrated ferroelectric microelectro-mechanical systems (MEMS) devices [6]. To date, the prediction of high-T_C superconductivity and magnetic order in LaNiO₃-based heterostructures has triggered a flurry of new activities on LaNiO₃ films [7]. However, LaNiO3 does not show any magnetic order. It remains a paramagnetic metal down to lowest temperatures with an enhanced effective mass [8,9]. Recent studies have revealed that the epitaxial direction of perovskite oxide films corresponds with distinct electronic structures and spin numbers at the interface [[10], [11], [12]]. The spin states or energy level shift is attributable to Ni 3d–O 2p hybridization, which results from distinct O p-level energy sites, and will affect the magnetic and transport properties of the films [13]. Of particular importance is that Ni 3d–O 2p hybridization plays a key role in LNO conductive properties [14,15]. Under these circumstances, one would expect that physical properties such as transport or interfacial coupling can be modulated by crystal orientation [16,17]. In other words, the Ni 3d electron orbital can affect the conduction band in a different axial orbital [18], such that orientation toward the <100> Ni–O–Ni bonds along the p–d hybridized orbital will lead to anisotropy of electrical conductivity [19,20]. There has been intensive research on (001) LNO film transport properties and electronic structure. However, to clearly understand hybridization strength, t_pd and conductivity of (110) and (111) LNO films are pertinent, because the large hopping interaction strength t_pd in LNO is similar to high-T_C cuprate superconductors [21]. In accordance with the Zaanen–Sawatzky–Allen (ZSA) phase diagram, the energy shift should be taken into account because of different p–d hybridizations in different directions with a configuration–interaction (CI) model [22]. The conductivity difference and magnetic interface in (111) LNO films [15] can be understood in terms of the change in t_pd, in which the spins in e_g orbitals and the electrical carriers are related to the t_pd [[23], [24], [25]].

2p XPS is a useful tool for probing electronic structure—for example, in high-T_C superconductivity and RNiO₃ (R = La to Lu) perovskite structures [24]—and can provide direct experimental interpretations for conductive and other distinct properties of (001), (110), and (111) LNO films. In this paper we report a core-level XPS spectroscopy study on epitaxial LNO thin films grown in different directions. Through analyses of the distinct Ni 2p_1/2 relative energy changes from the main to satellite peak (ΔE), we obtained information in the t_pd strength of the samples [23,24]. We experimentally found that the e_g bandwidth of the epitaxial LNO films corresponds with the ΔE, attributable to the small variation in the Ni–O hybridization strength t_pd [24]. Our direct experimental results on the ΔE and e_g states correspond with the electronic structure change and conduction properties of (001), (110), and (111) LNO films. Researchers have described analogous findings for SrRuO₃ (SRO) and La_2/3Sr_1/3MnO₃ systems, where the e_g state difference in (001), (110), and (111) films affects the carrier concentrations and saturated magnetization [26]. The XPS results correspond with the conductive properties of epitaxial LNO films.