Growth of Highly Strained CeO2 Ultrathin Films
Yezhou Shi,†,§,∥,⊥ Sang Chul Lee,†,⊥ Matteo Monti,† Colvin Wang,† Zhuoluo A. Feng,‡ William D. Nix,† Michael F. Toney,§ Robert Sinclair,† and William C. Chueh*,†,§,∥ †
Department of Materials Science and Engineering and ‡Department of Applied Physics, Stanford University, Stanford, California 94305, United States § Stanford Synchrotron Radiation Lightsource and ∥Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States S Supporting Information *
ABSTRACT: Large biaxial strain is a promising route to tune the functionalities of oxide thin films. However, large strain is often not fully realized due to the formation of misfit dislocations at the film/substrate interface. In this work, we examine the growth of strained ceria (CeO2) thin films on (001)-oriented single crystal yttria-stabilized zirconia (YSZ) via pulsed-laser deposition. By varying the film thickness systematically between 1 and 430 nm, we demonstrate that ultrathin ceria films are coherently strained to the YSZ substrate for thicknesses up to 2.7 nm, despite the large lattice mismatch (∼5%). The coherency is confirmed by both X-ray diffraction and high-resolution transmission electron microscopy. This thickness is several times greater than the predicted equilibrium critical thickness. Partial strain relaxation is achieved by forming semirelaxed surface islands rather than by directly nucleating dislocations. In situ reflective high-energy electron diffraction during growth confirms the transition from 2-D (layer-by-layer) to 3-D (island) at a film thickness of ∼1 nm, which is further supported by atomic force microscopy. We propose that dislocations likely nucleate near the surface islands and glide to the film/substrate interface, as evidenced by the presence of 60° dislocations. An improved understanding of growing oxide thin films with a large misfit lays the foundation to systematically explore the impact of strain and dislocations on properties such as ionic transport and redox chemistry. KEYWORDS: ceria, yttria-stabilized zirconia, strain, dislocation temperatures.9,10 Both materials crystallize in the cubic fluorite structure under technologically relevant conditions. Whereas ceria is a mixed oxygen ion and electron conductor (typically used as a component in an electrode), YSZ is a pure oxygen ion conductor (typically used as a solid electrolyte). The roomtemperature bulk lattice parameters of 9.5 mol % Y2O3-doped ZrO2 and of undoped CeO2 are 5.142 Å (powder diffraction file (PDF) #04-001-9306) and 5.411 Å (PDF #00-034-0394), respectively. Because of the large misfit (5.0%), it is believed that dislocations form at the ceria/YSZ interface11 at all practical film thicknesses to relieve the misfit strain. Based on the equilibrium theory,12 the critical thickness of ceria, at which misfit dislocations becomes thermodynamically preferable, is estimated as no >0.8 nm (details of the calculation are included in Support Information). Indeed, misfit dislocations are usually
M
isfit strain and dislocations have a profound impact on functional properties of heteroepitaxial thin films, such as charge transport, magnetism, and catalytic activity.1−3 In perovskite oxides, for example, strain has been shown to modulate point defect equilibria, ion transport, and surface reactivity.4−8 A critical step toward realizing strain- and dislocation-tuning of solids is precisely controlling the distribution of interatomic spacing. A biaxially strained thin film is typically prepared by depositing it on the vicinal surface of a single crystal substrate sharing similar in-plane crystal structure, but having different lattice parameters. On the one hand, increasing the film/substrate lattice mismatch increases the strain and potentially amplifies strain-dependent properties. On the other hand, doing so also decreases the film critical thickness, above which misfit dislocations form and commensurate heteroepitaxy is lost. One such strained system exhibiting a significant lattice mismatch is CeO2−x (ceria) thin film on cubic (Zr,Y)O2 (yttriastabilized zirconia, YSZ) single crystals, an important model heterostructure for studying oxygen ion transport and electrochemical hydrogen oxidation and water splitting at elevated © 2016 American Chemical Society
Received: June 20, 2016 Accepted: October 8, 2016 Published: November 7, 2016 9938
DOI: 10.1021/acsnano.6b04081 ACS Nano 2016, 10, 9938−9947
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Figure 1. Reciprocal space mapping for films of different thicknesses near the YSZ (113) peak. The axes are in reciprocal lattice units (h and l r.l.u.) of the substrate. The scattering vector is given as q⃗ = 2π(ha⃗* + kb⃗* + lc⃗*), where a⃗*,b⃗*, and, c⃗* are the reciprocal lattice vectors in the three primary directions of the cubic lattice. The color bar of the diffracted intensity is in log scale. The thickness of the films was determined by X-ray reflectivity, with exception of the relaxed, bulk-like film (∼430 nm), which was estimated from the number of PLD pulses. The brightest spot present in all figures at h = 1, l = 3 is the Bragg peak of the YSZ substrate. As the film thickness increases, the CeO2 films evolve from being coherently strained to being partially strained with a relaxed part due to the formation of dislocations and eventually completely relaxes to its bulk lattice. (a) The substrate without ceria. The thin vertical line across the Bragg point is the crystal truncation rod arising from the smooth surface of the substrate and decreases rapidly and becomes undetectable below l = 2.90. The horizontal dashed line is a guide for the eye. (b) The 1.80 nm ceria is strained to the substrate and adds another vertical component along h = 1.0, indicated by the black arrow. The alignment of the new peak and the YSZ (113) peak along the same h positon shows that in-plane lattice parameter of the ceria film is identical to that of YSZ. (c) The coherently strained film grows as thick as 2.7 nm, leading to a stronger intensity of the vertical component below the dashed line. (d) A weak, broad peak, indicated by the dashed ellipse, emerges at 4.6 nm due to the partial relaxation of the ceria film. The width in l and h directions is due to a small film thickness (l) and small lateral grain size (h). The vertical line is still visible because part of the film remains coherently strained. (e) The broad scattering peak becomes more pronounced as more film is relaxed through the formation of misfit dislocations. (f) Finally, the 430 nm film is completely relaxed with the center of the ceria feature located at h = 0.95 and l = 2.85, consistent with the 5% misfit between YSZ and bulk CeO2.
observed at the interface between ceria thin films and YSZ (for example, see ref 13 and the citations therein). There is an ongoing discussion in the literature on how strain and misfit dislocations, which are believed to come hand-inhand, impact transport properties in heteroepitaxial ceria.9,13−17 For instance, a multilayer structure with alternating ceria and zirconia layers is sometimes employed to investigate the transport properties along the interface, which is negligibly13 or negatively18 impacted by the strain. In addition, it has been shown that the activation barrier to ionic conduction is increased in a free-standing ceria film19−21 when the film is subjected to mechanical twisting and under compressive strain.17,22 The importance of strain and dislocations in ceria films necessitates a deeper understanding of how the strained ceria film relaxes into a dislocated state at very small thicknesses. However, detailed accounts of the growth mechanism of ceria on YSZ are scarce.23 In this work, we investigate the growth of ceria thin films on (001)-oriented YSZ from 1 to 430 nm via pulsed-laser deposition (PLD). Commensurate ceria ultrathin films with a 5% compressive misfit strain were obtained at thicknesses up to approximately 2.7 nm (or roughly five unit cells), significantly exceeding the predicted critical thickness of ∼0.8 nm.12
Combining in situ electron diffraction and X-ray diffraction and high-resolution transmission electron microscopy (HRTEM), we provide unambiguous evidence that strain relaxation drives the growth mode transition from 2-D to 3-D. Instead of directly nucleating misfit dislocations at the film/ substrate interface, nanometer-size islands form on the surface to partially relieve the strain at a film thickness of approximately 1.1 nm (or two unit cells). At larger thicknesses, 60° dislocations (the angle describes the orientational relationship between the Burgers vector and dislocation line) form at the edge of the islands and move toward the interface where they could react to form 90° (Lomer) dislocations. The critical thickness at which islands are predicted to form is in qualitative agreement with our experiment results. Overall, we demonstrate that (1) coherently strained, (2) incoherently strained, and (3) relaxed ceria films (with decreasing compressive strain from 5.0% to zero) can be obtained by varying the film thickness. By understanding the growth and dislocation formation mechanism in ceria thin films, we lay the foundation necessary to engineer tailored interfacial and bulk structure, for example, to impose symmetry-breaking of the cubic lattice in strained films and to modify conductivity in relaxed films along line dislocations. 9939
DOI: 10.1021/acsnano.6b04081 ACS Nano 2016, 10, 9938−9947
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ACS Nano
RESULTS AND DISCUSSION We fabricated ceria films of different thicknesses on atomically flat YSZ by varying the number of pulses in PLD under 0.5 mTorr O2 at 500 °C (see Supporting Information). This growth condition ensures that the as-deposited ceria film is fully oxidized.24 The thicknesses of thin films were measured by Xray reflectivity and ranged from 1.8 to 40.8 nm. For films thinner than 1.8 nm, the thickness was estimated using the number of deposition pulses, assuming a linear relationship between the two. To determine the crystallographic relationship between the CeO2 thin film and the YSZ substrate, we performed X-ray reciprocal space mapping (RSM). In Figure 1, the bright spot visible in all panels at (113) is the scattering from the Bragg peak of the YSZ single crystal. The short vertical line extending from the sharp spot is the crystal truncation rod that originated from the atomically flat surface.25 For the bare YSZ, the intensity along the vertical line (at h = 1.0) decreases rapidly from l = 3.0 toward lower l and becomes undetectable below l = 2.90 (Figure 1a). When a 1.8 nm ceria film is deposited, a thin vertical line appears in the RSM below the YSZ (113) spot at precisely the same h value (h = 1.0) and extends from l = 2.90 to l = 2.75 (Figure 1b), indicating a cubeon-cube growth ((001) YSZ [001] YSZ ∥ (001) CeO2 [001] CeO2). Moreover, the fact that the in-plane lattice parameter of film is the same as that of the substrate confirms that the film is coherently strained to the substrate. We find that the coherent growth can be maintained up to at least a film thickness of 2.7 nm, with only the coherently strained scattering peak of ceria (113) visible (Figure 1c). This result is surprising given that the critical thickness for heteroepitaxy is predicted to be
