Surface termination and stoichiometry of LaAlO₃(001) surface studied by HRTEM

Highlights

• Atomic structures of LaAlO₃(001) surfaces are investigated using HRTEM and density functional theory (DFT) calculations.

• A La vacancy-mediated La-O termination is determined under negative-Cs imaging (NCSI) mode.

• The surface stabilization mechanisms are discussed in detail.

Abstract

As an important topic of condensed matter physics, metal oxide surfaces often exhibit exotic properties such as high catalytic activity, enhanced ferroelectricity and electronic phase transition, originating from the different local symmetry with respect to the bulk. As the structure determination of oxide surfaces presents challenges to conventional surface science techniques like scanning tunneling microscopy, aberration-corrected transmission electron microscopy (TEM) has been increasingly used to solve structures of oxide surfaces. In this work, the (001) surface of LaAlO₃, one of the most used components of oxide heterostructures, has been investigated. Our TEM experiments and extensive image simulations show that the La-O terminated LaAlO₃(001) surface undergoes significant reconstructions, forming La vacancies on the surface layer. Energetically, the LaAlO₃(001) surface is stable with the reconstructed La-O termination in a wide range of oxygen chemical potentials. Polarity compensation, reduced density of states at the Fermi level and bond enhancement of subsurface oxygen anions all contribute to the stabilization of the reconstructed surface.

Introduction

Surface atomic and electronic reconstructions have always been a fundamental topic in many fields (Goniakowski et al., 2008; Diebold et al., 2010; Marks and Peng, 2016; Shi et al., 2016), such as electronic devices and catalysis. Intrinsic symmetry broken and polarity compensation process can induce structural relaxations, reconstructions and new electronic states on the surface. For example, different local oxygen stoichiometries lead to John-Teller domains on the La_5/8Ca_3/8MnO₃(001) surface (Gai et al., 2014). Transitions between insulating and metallic states have been observed on KTaO₃ (Setvin et al., 2018), SrRuO₃ (Lee et al., 2019), etc. Coupling between surface polarity and ferroelectric polarization has been reported in BiFeO₃ (Xie et al., 2017; Jin et al., 2017) and Pb(Zr_0.2Ti_0.8)O₃ (Gao et al., 2016). Surface reconstructions of catalysts (Sun et al., 2020a; Capdevila-Cortada and Lopez, 2017; Duan et al., 2019; Yuan et al., 2020; Sun et al., 2020b) and electrodes (Amos et al., 2016; Wang et al., 2017; Hess and Yildiz, 2020; Zhang et al., 2020) can greatly modulate their performance during chemical processes. Besides the intrinsic process, external stimuli can also modify surface structures, such as heating (Xu et al., 2016), water leaching (Lee et al., 2019), electron irradiation (He et al., 2012) and gas environment (Yuan et al., 2016) and chemical reactions (Sun et al., 2020a). When surfaces of different materials are in contact with each other to form a heterostructure, polarity-driven atomic and electronic reconstructions can also lead to exotic emergent properties, such as two-dimensional electron gas and interfacial magnetism in the LaAlO₃/SrTiO₃ system (Ohtomo and Hwang, 2004; Reyren et al., 2007; Bert et al., 2011; Yu and Zunger, 2014). Therefore, deep understanding of the surface structures and the driving forces behind them is crucial to clarify mechanisms of emergent properties and eventually control them.

Compared to surfaces of metals and semiconductors, to which atomically-resolved scanning tunneling microscopy (STM) (Gai et al., 2014; Setvin et al., 2017) and non-contact atomic force microscopy (NC-AFM) (Lauritsen and Reichling, 2010) have been successfully applied, surface atomic structures of insulators are difficult to be imaged by these techniques directly and unambiguously. Low conductivity limits the resolution of STM (Setvin et al., 2017), and the contrast in NC-AFM images depends heavily on the tip, causing difficulties in the interpretation of image information (Lauritsen and Reichling, 2010). Different from the scanning probe microscopy techniques mentioned above, transmission electron microscopy (TEM) has no requirement on the conductivity of materials and with the realization of aberration-correction (Haider et al., 1998), the point resolution of transmission electron microscopy (TEM) has reached 1 Å scale. Combining a negative value of spherical aberration C_S and an overfocus, high resolution TEM (HRTEM) proves to be a powerful tool for material structure investigations (Jia et al., 2003, 2004; Urban, 2008). Aided by image simulations, the surface profile imaging method has been successfully applied in investigating surface structures in various materials, including insulators (He et al., 2012; Yuan et al., 2016; Lin et al., 2014; Crosby et al., 2018; Yu et al., 2010; Liu et al., 2018; Huang et al., 2017a, b).

LaAlO₃ (LAO) has been widely used as a stable substrate in functional metal oxide film growth and one of the components of heterointerfaces. It has been used to offer strain to mediate lattice structures and consequential band structures of the adjacent layers (Fowlie et al., 2017; Beltrán and Muñoz, 2017; Middey et al., 2016; Kim et al., 2016; Frano et al., 2013), couple with other distortion type or provide structural confinement to create new ferroelectric states (Bea et al., 2009; Schlom et al., 2007; Rondinelli and Fennie, 2012), serve as electron acceptor or donor to generate new electronic phases (Ohtomo and Hwang, 2004; Wu et al., 2017; Stemmer and James Allen, 2014), etc. In all these applications, the atomic and electronic configurations of LAO surfaces play a crucial role. For example, ionization of the spontaneously formed oxygen vacancies on LaAlO₃ surfaces can lead to conductivity of LaAlO₃/SrTiO₃ interface (Yu and Zunger, 2014). Other effects of terminations (Hwang et al., 2012; Mannhart and Schlom, 2010; Schmidt et al., 2006) of and point defects (Yu and Zunger, 2014; Sorokine et al., 2012) on LAO surfaces on the performance of heterojunctions have also been extensively investigated.

In the LaAlO₃ bulk structure, the oxygen ions and the aluminum ions form an array of corner-sharing AlO₆ octahedra and the lanthanum ions are located in the center of the unit cells with aluminum ions in the corners. From another view, the structure can be built up by the LaO-AlO₂ stacking sequences repeating along [001] direction, resulting in two kinds of bulk-truncated surfaces terminated with LaO and AlO₂ layers, respectively. According to Tasker’s classification (Tasker, 1979), both of the LaO- and AlO₂-terminated LAO (001) surfaces belong to the Type III polar surface, with charged atomic planes and net dipole moment normal to the surface. The polarity needs to be compensated in order to reach an electrostatically stable state (Marks and Peng, 2016; Noguera, 2000). Much research has been done on LAO (001) surfaces, mainly focusing on the bulk-truncated surfaces with little consideration about the real surface atomic configuration (Schmidt et al., 2006; Yao et al., 1998; Francis et al., 2001; Kawanowa et al., 2002; Tang et al., 2007; Nomura et al., 2011; Liu et al., 2012). A 5 × 5 reconstruction was proposed by Wang and Shapiro (1995) and a √5×√5 R26.6° reconstruction was obtained through transmission electron diffraction combined with direct methods by Lanier et al. (2007). Recently, a 1 × 4 reconstruction was imaged using non-contact atomic force microscopy (Katsube and Abe, 2018). Some first-principles studies have investigated the point defects on the LAO (001) surfaces (Knizhnik et al., 2005; Jin-Long et al., 2008; Seo and Demkov, 2011; Guan et al., 2018). The precious works demonstrate that the surface atomic structure is closely related with sample preparation processes.

In this study, the atomic structures and stability of the LAO (001) surface have been studied by combining aberration-corrected TEM with density functional theory (DFT) calculations. The surface La cation vacancies have been observed and a reconstruction model based on polarity compensation mechanism is proposed. Our results show that polarity compensation, reduced density of states at the Fermi level and bond strengthening effect all contribute to the LAO (001) surface stabilization.