Large Crystallographic Orientation Tilting Induced by Postoxidation

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Large Crystallographic Orientation Tilting Induced by Postoxidation Annealing in Layered Cobaltite CaxCoO2 Thin Films Kouhei Takahashi,* Tsutomu Kanno, Akihiro Sakai, Hideaki Adachi, and Yuka Yamada Advanced Technology Research Laboratories, Panasonic Corporation, 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan ABSTRACT: A drastic change in crystallographic orientation has been observed in incline-oriented thin films of layered cobaltite CaxCoO2 by postoxidation annealing. The as-grown CaxCoO2 films fabricated on nplane sapphire single crystal substrates exhibited an inclined CoO2 plane alignment with respect to the substrate surface by an angle of 58°. Upon postoxidation annealing, we found that the tilt angle of the aligned CoO2 planes changes dramatically up to 80°. The large crystallographic orientation change was observed uniquely in the incline-oriented films and could not be observed in films with low-index plane orientations. We discuss that the large crystallographic orientation tilting results from a strain relief process so as to compensate the internal lattice misfit due to CaxCoO2 lattice modification, which was also induced by the oxidation process.

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ayered cobaltites with a composition of AxCoO2 (A: alkali metals and alkaline earth metals) exhibit a wide variety of physical properties which are intriguing from both fundamental and application aspects. For example, NaxCoO2·yH2O shows unique superconductivity,1 NaxCoO2, SrxCoO2, and CaxCoO2 show fine thermoelectricity,2−4 and LixCoO2 works as a highperformance cathode material for lithium-ion rechargeable batteries.5 These materials consist of alternate stacks of negatively charged CoO2 planes and positively charged A atoms. Charge carriers are basically confined within the CoO2 planes, and thus, the CoO2/Ax stack structure results in highly anisotropic transport properties between parallel and perpendicular to the CoO2 planes. In anisotropic materials, thin film epitaxy is of great interest because it allows us to fabricate crystals in desired orientations. In fact, thin film fabrication of the layered cobaltites has been studied actively in recent years. 4,6−10 We have been investigating epitaxial growth of CaxCoO2 thin films for thermoelectric applications.4,11 Figure 1 shows the crystal structure of CaxCoO2 (x = 0.5). The CoO2 layers form a two-

dimensional triangular lattice of Co ions within the (001) plane. The triangular lattice of CaxCoO2(001) exhibits relatively good matching with the hexagonal lattice of sapphire(0001). Thus, at optimum growth conditions, CaxCoO2 can grow epitaxially on a sapphire (Al2O3) single crystal substrate with an epitaxial relationship of CaxCoO2(001)//Al2O3(0001). The epitaxial thin film growth technique further enabled us to fabricate incline-oriented CaxCoO2 films on Al2O3 substrates with tilted (0001) planes, i.e., CaxCoO2 films with CoO2 planes [(001) plane] ordered tilted with respect to the substrate surface.11 The incline-oriented CaxCoO2 films showed unusually large laser-induced voltages via the unique off-diagonal thermoelectric (ODTE) effect,11−14 which made them potential candidates for thermal/optical sensing device applications. The inclined orientation is essential for the ODTE effect, and its performance highly depends on the CaxCoO2(001) tilt angle. Meanwhile, the crystal lattice of the layered cobaltites shows minor changes depending on the Co oxidation state.15−17 This includes changes in the distance between the adjacent CoO2 planes, the Co−Co distance, A site ordering, etc. However, despite the large amount of research attention directed at the issue of a crystal lattice, little attention has been directed at the issue of thin film orientation, although the crystal lattice is an important factor for texture formation in thin films. Here, we have investigated the impact of postoxidation annealing on the thin film texture of incline-oriented CaxCoO2 thin films by using high-resolution transmission electron microscopy (TEM) and four-circle X-ray diffraction (XRD) analysis. We found that, upon postannealing in an activated oxygen atmosphere, the

Figure 1. Crystal structure of layered cobaltite CaxCoO2. The material is composed of alternate stacks of conductive CoO2 layers and insulative Cax layers. © 2012 American Chemical Society

Received: January 27, 2012 Revised: March 9, 2012 Published: March 20, 2012 1708

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Figure 2. Pole-figure XRD patterns probing the (001) orientation of CaxCoO2 films grown on an n-plane Al2O3 single crystal substrate (a) before and (b) after oxidation annealing for 30 min. Cross-sectional TEM image near the CaxCoO2/Al2O3 n-plane interface and the magnified view of the CaxCoO2 film (upper right panel) (c) before and (d) after oxidation annealing for 30 min. Arrows indicate slight bending of the CoO2 planes. (e) Schematic of the crystallographic orientation change induced by the oxidation process.

rotation angle around the surface normal, respectively. A single peak structure can be identified in both Figure 2a and b at φ ≠ 0° or 90°. This indicates uniform and, also, inclined ordering of the (001) planes for both films. In the as-grown film, the maximum intensity is observed at φ of 58°. This angle fairly agrees with the (0001) plane tilt angle β of n-plane Al2O3 substrates (β = 61°), which suggests nearly an epitaxial relationship of CaxCoO2(001)//Al2O3(0001). The slight discrepancy between β and the CaxCoO2(001) tilt angle α in the as-grown film presumably results from a strain relief process which will be mentioned below.19,20 On the other hand, in the oxidized film, the position of the CaxCoO2(001) diffraction peak changed significantly from φ of 58° to 70°. The pole-figure XRD patterns indicate a significant change in the film orientation by the oxidation process. To provide further insight, we show in Figure 2c and d the cross-sectional TEM image of the as-grown film and the film oxidized for 30 min, respectively. In both TEM images, a uniform-tilted stripe structure can be identified in the CaxCoO2 film. The direction and the tilt angle of the stripes are consistent with the CaxCoO2(001) orientation suggested by the pole-figure XRD measurements. Furthermore, the width between each stripe agrees with the distance between the adjacent CoO2 layers of CaxCoO2 (∼5 Å). These features reveal that the stripes in the TEM images corresponds to the aligned CoO2 planes and that α has indeed changed from 58° to 70° by this 30-min oxidation process. A schematic of such a crystallographic orientation change by the oxidation process is depicted in Figure 2e. It is noteworthy that similar oxidation-induced CoO2 plane tilting was observed in other incline-oriented CaxCoO2 films, which were grown on S-plane [(101̅1) plane] Al2O3 substrates with β of 72°. In this case, α changed from 66° to 82° by the same 30min oxidation process (see the TEM images in Figure 3). The present phenomenon is thus not specific to CaxCoO2 films grown on n-plane Al2O3 substrates, and it may appear universally in other incline-oriented CaxCoO2 films with various different tilt angles. The closed circles in Figure 4 show α as a function of postoxidation time. With increasing the postoxidation time, α increased continuously from the initial value of 58° to 80° after a 90-min oxidation. In each sample, we confirmed that β does not change from 61° by the postoxidation process. This means that, although the initial α is determined by β, it has no

inclined CoO2 planes change their tilt angle drastically more than 20°. XRD and electron energy-loss spectroscopy (EELS) indicates modification of the CaxCoO2 lattice by the postoxidation process. We discuss that the large crystallographic orientation tilting results from a strain relief process which compensates the internal lattice misfit induced by the CaxCoO2 lattice modification. 150-nm-thick CaxCoO2 thin films were grown on n-plane [(112̅3) plane] Al2O3 single crystal substrates by magnetron sputtering. Substrate temperature and background pressure were fixed at 450 °C and 5 Pa, respectively, during the sputtering. A mixed gas of Ar and O2 (Ar: 96% and O2: 4%) was used as the sputtering gas. Details of thin film fabrication are described in ref 11. The electrical resistivity ρ of the asgrown films (along the CoO2 planes) was nearly 2 orders of magnitude higher than that of the single crystal.18 Since oxygen vacancy can act as a trap center for electrons, we assume that the as-grown films were in an oxygen deficient state with a composition of CaxCoO2−δ. UV-asher was used to prepare CaxCoO2 films with different oxidation states. CaxCoO2 films were first heated up to 300 °C inside the UV-asher chamber in an air atmosphere. Oxygen gas was then introduced into the chamber under UV lamp exposure with a flow rate of 0.5 L/min. This process creates highly active oxygen gas inside the chamber, which efficiently oxidizes the CaxCoO2 films. The oxidation time was varied from 15 to 90 min for different CaxCoO2 films. After oxidation at 300 °C, CaxCoO2 films were cooled down to room temperature for 2 h without the oxygen gas flow. We confirmed that ρ decreases and approaches the reported value by the annealing process.18 Although the actual oxygen content of each film is unknown, this indicates that the present annealing has gradually filled the oxygen vacancies in the as-grown film. Using energy dispersive X-ray spectroscopy, we confirmed that the Ca composition x of the films was ∼0.5 and does not change by the oxidation process. The structural properties of the thin films were examined by four-circle XRD analysis with Cu Kα radiation and TEM. Figure 2a and b shows pole-figure XRD patterns probing the CaxCoO2 (001) diffraction peak of the as-grown film and the film oxidized for 30 min, respectively. Note that φ and ϕ depicted in Figure 2a represents the tilt angle of the substrate surface with respect to the incident X-ray beam and the sample 1709

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film before and after oxidation for 30 min. The EELS spectrum reflects the chemical bonding state and, thus, gives valuable information on the crystal lattice. One can see that the two spectra in Figure 5a are similar in shape. However, a slight difference can be identified when the two spectra are magnified near the Co L3-edge (see Figure 5b). The EELS spectrum of the as-grown film shows a symmetric peak structure centered around 781.8 eV. On the other hand, in the EELS spectrum of the oxidized film, a shoulder appears around 780.3 eV just below the above-mentioned peak-center (see the arrows in Figure 5b). This implies a change in the chemical bonding state of Co and the surrounding O ions, perhaps related to some changes in the Co−O bond length or distortion of the oxygen octahedron which may lead to changes in the lattice parameter. We emphasize that annealing in vacuum or in air atmosphere did not induce changes in the crystallographic orientation. There was also no sign of lattice modification in such cases. This implies that the CaxCoO2 lattice modification plays an essential role in the CoO2 plane tilting. It has been reported that the strain relief process can result in small crystallographic orientation tilt (misorientation) in epitaxial thin films.19−22 This was observed in various materials in the thin film growth stage. The origin of the small crystallographic orientation tilt was explained by dislocationmediated relaxation of the interface strain, which was formed by the film/substrate lattice mismatch. We assume that the oxidation-induced CoO2 plane tilting is also driven by a similar strain relief mechanism. As indicated above, oxidation induces modification of the CaxCoO2 lattice. Oxidation starts from the CaxCoO2 surface and evolves gradually inside the film with increasing oxidation time. Therefore, lattice inhomogeneity should be formed inside the CaxCoO2 film, reflecting the oxygen concentration distribution. This will introduce considerable strain inside the CaxCoO2 film due to different lattice parameters between the high-oxidized region and the underneath low-oxidized region.23 Lattice modification will advance inside the film and continuously form additional strain until the film is uniformly oxidized. The CoO2 plane tilting presumably occurs so as to compensate the lattice misfit between such differently oxidized regions. It is noteworthy that the aforementioned crystallographic orientation tilt in lattice-mismatched heterostructures emerges only when miscut or vicinal substrate is used.19−22 In epitaxial films grown on miscut substrates, close packed planes are usually misoriented (tilted) from the substrate surface. Edge dislocations can slip on these close packed planes and form misfit dislocations at the film/substrate interface. The Burgers vector of such misfit dislocation will have a component titled against the substrate surface (parallel to the slip plane), which can further be decomposed into two components, i.e., a component (i) parallel and (ii) perpendicular to the substrate surface. The former component relieves lattice misfit, and the latter component introduces the crystallographic orientation tilt.19,20 The previous reports thus explain that crystallographic orientation tilt can develop only by introduction of dislocation that has a Burgers vector component perpendicular to the surface. The situation seems to be similar for the CoO2 plane tilting by postoxidation annealing. The close packed slip plane of CaxCoO2 is the (001) plane, which corresponds to the CoO2 planes. Thus, when the CaxCoO2 film is subjected to strain, defect and dislocation motion is expected to take place within the (001) planes to relieve strain. In the incline-oriented

Figure 3. Cross-sectional TEM image near the CaxCoO2/Al2O3 Splane interface (a) before and (b) after oxidation annealing for 30 min. The arrows indicate slight bending of the CoO2 planes.

Figure 4. CoO2 plane tilt angle (closed circles) and distance between the adjacent CoO2 planes (closed triangles) as a function of postoxidation time in a CaxCoO2 film grown on an n-plane Al2O3 substrate. The dotted lines are only a guide to the eye.

relevance to the subsequent CoO2 plane tilting induced by postoxidation. To discuss the oxidation-induced tilting of CoO2 planes, it should be noted that the present oxidation process also made a notable influence on the CaxCoO2 lattice. The closed triangles in Figure 4 represent the distance between the CoO2 planes d(001) as a function of postoxidation time. We see that d(001) decreases with increasing the postoxidation time. This suggests considerable modification of the CaxCoO2 lattice by the postoxidation process. We also show in Figure 5a the EELS spectra near the Co L3-edge and the L2-edge of the CaxCoO2

Figure 5. (a) EELS spectra of the CaxCoO2 film near the Co-L2 and L3 edges before and after oxidation annealing for 30 min. (b) Magnified EELS spectra near the Co-L3 edge. The two spectra are vertically offset for clarity. 1710

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In summary, we have examined the influence of postoxidation annealing on the texture of incline-oriented CaxCoO2 thin films. Upon oxidation under 300 °C, we found that the aligned CoO2 planes change their tilt angle dramatically over 20°. XRD and EELS measurements indicated modification of the CaxCoO2 lattice by the oxidation process. We conclude that the large tilting of CoO2 planes originates from a dislocationmediated strain relief process so as to compensate the internal lattice misfit due to the CaxCoO2 lattice modification. The same phenomenon can be expected in other layered cobaltites with the same AxCoO2 composition that grows epitaxially on Al2O3 substrates. The result provides important insights into device fabrication using layered cobaltite thin films.

CaxCoO2 films, the Burgers vector of the dislocations formed within the (001) plane will have an inclined component reflecting the tilted (001)-orientation. This condition should allow both strain relief and crystallographic orientation tilt in the incline-oriented films. In fact, CaxCoO2 films fabricated on a-plane [(112̅0) plane] and c-plane [(0001) plane] Al2O3 substrates, which grew with (100)-orientation and (001)orientation, showed no orientation change under the present oxidation process. The CoO2 plane tilting thus occurred only in the incline-oriented CaxCoO2 films. This is consistent with the emergence of misorientation in lattice-mismatched heterostructures. Lastly, we point out that the crystallographic orientation tilt observed here is extremely larger than that reported previously in lattice-mismatched heterostructures fabricated on miscut substrates; that is, the misorientation angle in the latticemismatched heterostructure is typically less than 1°,19−22 whereas the change in the tilt angle in the present case exceeds 20°. The difference between the two phenomena may be related to the different areas and amount of dislocation formation. In the case of lattice-mismatched heterostructures, misfit dislocations that induce crystallographic orientation tilt exist only at the film/substrate interface. On the other hand, we expect formation of misfit dislocations at various levels inside the CaxCoO2 film because it needs to compensate the lattice misfit between the differently oxidized regions of the CaxCoO2 film, which varies spatially and temporally during the oxidation process. The strain relief process should thus take place in multiple levels in this case, which may add up to a large crystallographic orientation tilting as a whole. To support this aspect, slight bending of the CoO2 planes can be identified in multiple locations in the TEM images after oxidation (see the arrows in Figures 2d and 3b). TEM also clarifies that the oxidation induces a considerable amount of defect sites inside the CaxCoO2 film (see the right area of the magnified image in Figure 2d, for example). The absolute value of the crystallographic orientation tilt angle in this mechanism is governed by the spacing of the dislocations.19,20 However, the defect sites are observed randomly at multiple levels inside the CaxCoO2 film and make it difficult to give a quantitative discussion. Further structural analysis will be essential to elucidate the detailed defect dynamics that governs this process. The final concern left to discuss is about the possible consequences of CoO2 plane tilting for other properties of the CaxCoO2 films. The greatest impact is presumably expected on the ODTE characteristics. As we explained above, inclineoriented films allow generation of large voltage signals via the ODTE effect. The resultant voltage is known to be proportional to sin 2α.11−14 This means that tilting of CoO2 planes (change in α) can significantly affect the output voltage generated by the ODTE effect. Namely, α deviating from 45° by the oxidation process shall decrease the ODTE voltage from the original value, whereas α approaching to 45° shall increase the ODTE voltage. Indeed, the influence of oxidation on other physical quantities such as the Seebeck coefficient needs to be taken into account to evaluate the total influence of oxidation on the ODTE effect. Nevertheless, the present phenomenon may be useful to adjust α at an angle which gives the highest voltage signal in conjunction with changes in other physical quantities that govern the performance of the ODTE effect. Although this is an interesting matter to consider, it is beyond the scope of the present work and should be examined thoroughly elsewhere.

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Corresponding Author

*Phone: +81-774-98-2566. Fax: 1-774-98-2585. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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