Effect of SrO Doping on LaGaO3 Synthesis via Magnetron Sputtering

28 Oct 2016 - The high temperature growth behavior of epitaxial LaGaO3 thin films with and without SrO is determined with real-time X-ray scattering. ...
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Effect of SrO Doping on LaGaO3 Synthesis via Magnetron Sputtering Matthew J. Highland,*,† Edith Perret,†,# Chad M. Folkman,†,§ Dillon D. Fong,† Carol Thompson,‡ Paul H. Fuoss,† and Jeffrey A. Eastman† †

Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, United States

‡

ABSTRACT: The high temperature growth behavior of epitaxial LaGaO3 thin films with and without SrO is determined with realtime X-ray scattering. We find SrO alters the thin film growth mode of LaGaO3, both when predeposited on a surface as well as when SrO and LaGaO3 are codeposited. We also find that depositing a small amount of SrO on a LaGaO3 surface induces significant structural rearrangement in the film. We describe mechanisms under which these transformations can occur. The strong effect of SrO on the microstructure of La1−xSrxGaO3 likely has wider implications for other ionically conducting oxide materials.

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when using in situ X-ray scattering to study synthesis via sputter deposition due to the weakly interacting nature of X-rays. In addition to an expanded pressure range, X-rays are not influenced by electric or magnetic fields, and their penetrating power allows one to probe the entire thickness of a film during the deposition process. The interaction between X-rays and a material is primarily elastic and can be largely described by kinematic diffraction theory, simplifying the interpretation of in situ X-ray scattering data. By monitoring variations in intensity at anti-Bragg positions during synthesis, one can determine the growth mode under which the film is being formed. Typical thin film deposition can be described using step-flow, layer-by-layer, or three-dimensional (3D) growth models. When homoepitaxial deposition (depositing a material onto a substrate of the same material) occurs under step-flow conditions, the intensity at an anti-Bragg position remains constant. With incoherent X-ray scattering the surface appears unchanging with time, giving rise to constant intensity at the anti-Bragg position. However, during heteroepitaxial deposition (deposition of a material onto a substrate of a different material) step-flow leads to intensity oscillations as the thickness of the deposited film increases. These oscillations originate from interference between X-rays scattering from the top surface and buried interface of the film, in the same way that thickness fringes form around a film Bragg peak. These oscillations do not occur during homoepitaxial deposition and are termed either Kiessig or thickness oscillations.7 The period of these oscillations is inversely proportional to the out-of-plane reciprocal lattice unit L,

n the continuing effort to develop solid oxide fuel cells (SOFCs) for use in energy technology, a key challenge is lowering the operational temperature of the cell while maintaining a reasonably high ionic conductivity through the solid electrolyte.1 Significant advances have been made over conventional YSZ electrolytes by using doped complex oxides such as La1−xSrxGa1−yMgyO3−δ (LSGM).2 It has also been observed that the ionic conductivity can be enhanced in thin films with well-controlled microstructures. In all of these cases, the ionic conductivity depends strongly on doping and microstructure.3 To better understand aliovalent doping and the interaction between dopants and surfaces/interfaces, we need to be able to observe the doping process in wellcontrolled systems. We therefore must be able to synthesize high-quality films of relevant materials with atomic-level control. Synthesis via 90° off-axis rf-magnetron sputtering is an excellent technique of the growth of thin film oxides since it can yield highly stoichiometric, epitaxial crystals and limits the flux of energetic species to the surface while maintaining a high oxygen activity in the synthesis chamber.4 In addition to growing high-quality oxide thin films, we need to observe the effect of dopants on the structure and growth behavior in real-time. To do so we utilize in situ X-ray scattering and reflectivity, which are ideally suited to penetrate the sputtering environment and provide real-time atomic-level information about the synthesis process, such as distinguishing between thin film growth modes.5 Reflection high energy electron diffraction (RHEED) has been used for in situ monitoring of sputter deposition, but this often requires the use of differentially pumped RHEED sources, as well as a precise balancing of stray magnetic fields, precluding the use of dense sputtering plasmas or applied substrate bias.6 These requirements can, in some cases, limit the range of sputtering conditions that can be studied. Such limitations are not present © 2016 American Chemical Society

Received: June 15, 2016 Revised: October 27, 2016 Published: October 28, 2016 6812

DOI: 10.1021/acs.cgd.6b00914 Cryst. Growth Des. 2016, 16, 6812−6816

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seen from the growth oscillations shown in Figure 1a), which were observed at the SrTiO (001/3) position in reciprocal

corresponding to where they are being monitored. For cubeon-cube growth, thickness oscillations appearing at the 201/2 anti-Bragg position will have a period that corresponds to the deposition of two units cells. When ad-atoms on a surface have insufficient mobility to migrate to step-edges, they can cluster to form islands on a terrace (layer-by-layer growth). In this growth mode the intensity is sensitive to the changing morphology of the surface, reaching a maximum when a completed full layer covers the surface and a minimum when the surface is 50% covered with islands. The period of these oscillations corresponds to a deposition thickness equal to the height of the islands, typically one unit cell. Since these oscillations do not depend on the overall film thickness, their period is independent of L. These oscillations can be referred to as roughness oscillations. In systems where heteroepitaxial deposition occurs in a layer-bylayer mode and the incident X-rays can fully penetrate the film, both thickness and roughness growth oscillations modulate the intensity at an anti-Bragg position. Often these oscillations can be distinguished from one another by their relative periods, as we will show below. If ad-atoms incident on the surface cluster to form multiheight surface islands, leading to progressive surface roughening, the growth is termed 3D and will be accompanied by a decay of intensity at the anti-Bragg position due to the increase in surface roughness. Under these conditions any thickness oscillations that may occur would be damped away by the overall decay of intensity. For growth at a fixed temperature, often step-flow growth is associated with high surface mobility, and 3D growth associated with low surface mobility. Intermediate surface mobility is associated with layerby-layer growth. We will use the signatures of each of these growth modes to interpret the effect that SrO has on the growth of LaGaO3 under various conditions. In this study, we have found that the growth behavior and surface roughness of LaGaO3 can be altered significantly by introducing the dopant SrO to the growth process. We find that depositing SrO onto a LaGaO3 surface changes the growth mode of subsequently deposited LaGaO 3 layers, that codeposition of SrO and LaGaO3 has a similar effect on the LaGaO3 growth mode, and that simply adding SrO to a LaGaO 3 surface induces a transformation of the film morphology. We will describe these observations in detail and discuss possible mechanisms by which SrO doping significantly alters the synthesis of LaGaO3.

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Figure 1. Growth mode oscillations observed at the (001/3) position in reciprocal space. Panel a shows LaGaO3 deposited on SrTiO3. Two distinct periods of oscillation are observed, which differ by a factor of 3. Panel b shows the deposition of three monolayers of SrO onto the same LaGaO3 film. Panel c shows deposition of LaGaO3 grown on top of the layer of SrO grown in panel b. Only thickness oscillations are observed (marked by the red dashed-dotted lines) during the deposition.

space. Growth oscillations with two different periods can be seen in Figure 1a. The shorter ∼113 ± 5s period roughness oscillations marked by the blue dashed lines correspond to the completion of a single unit cell. These oscillations modulate longer period ∼336 ± 5 s thickness oscillations, marked by the red dashed and dotted lines corresponding to the deposition three unit cells of LaGaO3. The presence of both thickness and roughness oscillations during growth indicates the film grew in a layer-by-layer mode under these conditions. The fact that the roughness oscillations have a period, which is 1/3 of the period of the thickness oscillation period at 001/3, confirms that the islands formed during the layer-by-layer growth are one unit cell in height. Immediately following the deposition of LaGaO3 onto SrTiO3 shown in Figure 1a), we deposited approximately three monolayers of SrO onto the LaGaO3 surface from a second magnetron source using the same deposition conditions as the LaGaO3 growth. The variation in intensity at the (001/3) position during the SrO deposition is shown in Figure 1c) and corresponds to ∼1 thickness oscillation. The intensity at the (001/3) position recovers to its initial value during the course of this deposition, indicating that the film remains relatively smooth. Immediately following the SrO deposition, LaGaO3 was again deposited, and the growth oscillations are shown in Figure 1c, where only thickness oscillations (with a period of ∼390 ± 5 s, marked by the red dashed-dotted lines) are observed. The lack of roughness oscillations, and the observation that there was no significant loss of intensity on

EXPERIMENTAL SECTION

The heterostructures studied here were synthesized via 90° off-axis rfmagnetron sputtering in a chamber allowing for simultaneous growth and in situ X-ray scattering and reflectivity that is described in detail elsewhere.8 Prior to deposition, single crystal SrTiO3 (001) substrates were treated with a standard HF etch to create a TiO2 terminated surface.9 Substrates were heated to temperatures of 550−800 °C in a 30 mTorr 70/30 Ar/O2 gas environment. Layers were deposited from two off-axis sputtering guns containing LaGaO3 and SrO oxide targets, while measuring scattering at different points on specular or nonspecular crystal truncation rods (CTRs). Postdeposition measurement of the (2 0 L) and (3 0 L) CTRs indicated that the LaGaO3 layers described here remained coherently lattice matched to the SrTiO3 substrate.

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RESULTS AND DISCUSSION When LaGaO3 was grown on TiO2 terminated SrTiO3 at 550 °C, the film formed in a layer-by-layer growth mode as can be 6813

DOI: 10.1021/acs.cgd.6b00914 Cryst. Growth Des. 2016, 16, 6812−6816

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that are slightly modulated by roughness oscillations during the codeposition of LaGaO3 and SrO. The increasing intensity during deposition again indicates that SrO helps to smooth the film surface as compared to the growth of pure LaGaO3 under similar conditions. The diminishing roughness oscillations in the blue curve cannot be due to a loss of X-ray penetration with added thickness since the starting LaGaO3 layer is much thinner for the blue curve. Rather, this is evidence for a transition to step-flow growth during codeposition of SrO and LaGaO3. The change in growth behavior due to the presence of SrO can also be seen when a sub-monolayer coverage of SrO is deposited on a LaGaO3 film at elevated temperatures. Figure 3 a shows the change in intensity at an anti-Bragg position during the deposition of LaGaO3 onto a SrTiO3 substrate and 800 °C. The overall loss of intensity during the deposition indicates significant surface roughening. Figure 3b shows the deposition of SrO onto the same LaGaO3 surface under similar conditions. The green vertical dashed lines mark the opening and closing of the SrO source shutter. While the SrO shutter is open, the intensity at the (201/2) position decreases as a growth oscillation begins. Once the shutter is closed, the intensity starts to steadily increase indicating a smoothing of the surface. The recovery of the surface is enhanced by the addition of SrO and eventually saturates. Figure 4 shows the recovery of intensity at the (201/2) position of the same film shown in Figure 3 as additional SrO is added 1/4 monolayer at a time. As the 1 m.l. and 1.25 m.l. curves in Figure 4 show, once one monolayer of SrO has been deposited, additional doses of SrO do not induce further surface rearrangement. In addition to the reduction in surface roughness, we also observe that SrO deposition results in a small shift of the Bragg peak, as is shown in Figure 5. The 20L CTR from the LaGaO3/SrO film shows a higher intensity near the anti-Bragg position than the CTR from the LaGaO3 film. This is indicative of the LaGaO3/SrO film having a smoother surface structure. This shift in Bragg peak position indicates that the SrO is incorporated into the LaGaO3 film, slightly reducing its lattice parameter. This reduction is consistent with previous results in bulk samples of LaGaO3.10 The 20L CTR from the bare substrate prior to growth is also shown for comparison. The studies described above indicate that the introduction of SrO has a significant effect on the growth behavior and stable

the CTR, indicates that LaGaO3 is growing in a step-flow rather than 3D mode. This change in growth mode, from layer-bylayer prior to SrO deposition to step-flow after SrO deposition, is consistent with an enhanced mobility of LaGaO3 species due to the addition of SrO, achieving step-flow behavior at the relatively low temperature of 550 °C. The significant role SrO plays in determining the growth behavior of LaGaO3 can also be seen when SrO and LaGaO3 are codeposited from two sputtering sources. Figure 2 shows

Figure 2. Oscillations observed at the (201/2) position during the growth on LaGaO3 at 800 °C. The starting LaGaO3 layer is ∼9 unit cells thicker for the data shown in blue compared with the data in red, which shows roughness oscillations from the layer-by-layer growth of LaGaO3. The blue curve shows thickness oscillations from the codeposition of LaGaO3 and SrO.

intensity oscillations at the (201/2) position in reciprocal space for LaGaO3 deposited with and without the codeposition of SrO. The red curve in Figure 2 shows roughness oscillations measured at the (201/2) position in reciprocal space during the deposition of LaGaO3 at 800 °C. These data were measured at a fixed incident angle above the critical angle of LaGaO3. For the red and blue curves in Figure 2, the sample surface was already covered in LaGaO3 layers 3 unit cells thick and 12 unit cells thick, respectively. For pure LaGaO3, the intensity at the (201/2) position decays quickly during the deposition, such that only a few rough oscillations can be seen. The blue curve in Figure 2 shows an overall increase of intensity as the film is being grown, which allows us to observe thickness oscillations

Figure 3. Intensity variation at the anti-Bragg position of LaGaO3 film due the introduction of SrO. Panel a shows intensity oscillations observed at the (201/2) position during the deposition of LaGaO3 on SrTiO3 at 800 °C. The red dashed vertical lines mark the opening and closing of the LaGaO3 source shutter. Panel b shows the deposition of a 1/4 monolayer of SrO onto the same LaGaO3 surface. The blue dashed vertical lines mark the opening and closing of the SrO source shutter. 6814

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plane of the crystal carries a charge, with formal charges (LaO)+1 and (GaO2)−1 per unit cell. Therefore, increasing the thickness of a LaGaO3 film leads to a diverging surface energy12 and a so-called polarization catastrophe.13 That surface polarity must be compensated for the film to be stable. The film may be compensated through a number of mechanisms, including adsorbates, surface rumpling, or electronic reconstruction.14 Paisley et al.15 have previously shown that supplying a chargecompensating surfactant can lead to highly smooth surfaces, even when the surfaces are polar. Since Sr doping increases the oxygen vacancy concentration and greatly increases the ionic conductivity of film, both positive and negative charges may be expected to screen polar discontinuities at the film surface and the film interface with SrTiO3. As such, the results presented here are relevant to studies aimed at inducing and utilizing twodimensional (2D) electron gas behavior at complex oxides interfaces.13,15−17 These results yield insight into the effects of doping on thin film oxides and are particularly significant for the study of electrolyte materials for SOFCs in a number of ways. One of the fundamental requirements for fuel cell electrolyte materials is having high ionic conductivity, and previous studies have shown that doping and microstructure both influence the ability to move oxygen vacancies through a material.18 Another important property governing the performance of SOFCs is the ability for the cathode surface to initiate an oxygen reduction reaction (ORR). To move oxygen through a solid electrolyte, molecular oxygen has to be reduced at the surface of the electrolyte through a complex catalytic process. Previous studies have shown that the vacancy concentration and composition can affect the efficiency of the ORR.19 Studies of oxygen reduction activity at SrTi1−xFexO3 surfaces also found that the segregation of Sr to the cathodes surfaces significantly reduces ORR efficiency.20 This work demonstrates that SrO can act as a surfactant during growth and alter the microstucture of LaGaO3 thin films.

Figure 4. Recovery of intensity at the (201/2) position measured from the LaGaO3 film shown in Figure 3 800 °C as it is dosed with 1/4 monolayer coverages of SrO. The legend indicates the total SrO coverage of each curve.

Figure 5. 20L crystal truncation rod (CTR) for a bare SrTiO3 substrate (shown in blue) on which was deposited LaGaO3 (shown in red) and then later was deposited LaGaO3 with SrO under similar conditions. The increase of intensity on the rod near the anti-Bragg positions indicates a smoother film was grown with the addition of SrO.

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CONCLUSIONS We have shown that adding SrO to the growth of LaGaO3 thin films significantly alters the growth mode with a tendency to make smoother surfaces. We identified several potential mechanisms by which the introduction of SrO could induce these changes. These results provide insight into the processes by which dopants enter the crystal and under what conditions we can expect dopant solubility. In all of these cases, we observe that the microstructure, defects, and doping are all linked and depend specifically on synthesis conditions.

surface roughness of LaGaO3. SrO has an effect on the surface mobility of LaGaO3 during synthesis. A mechanism that describes this effect stems from the possible nonstoichiometric nature of complex oxide films sputtered from a single source. In previous work we showed that sputter deposition from a single mixed cation source can result in nonstoichiometric cation deposition.11 Specifically, when depositing LaGaO3 from a single mixed cation source under conditions similar to those used here, we found the resulting films to contain small amounts of a second phase, Ga2O3. Additionally, when we grew stoichiometric films using two sputtering sources, we found that those films had much smoother surfaces than nonstoichiometric films. This suggests that the smoothing effect driven by SrO results from a reaction between excess Ga2O3 on the film surface and the SrO. When we introduce SrO, we are providing additional A-site cations for the Ga2O3 to react with and form La1−xSrxGaO3−δ. At these temperatures the SrO has enough mobility to incorporate into the LaGaO3 perovskite phase, creating oxygen vacancies and contracting the lattice in addition to smoothing the film. It is also important to note that the addition of SrO changes the electrical boundary conditions of LaGaO3 thin films. In the pseudocubic [001] direction of a LaGaO3 crystal each atomic

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses #

(E.P.) Université de Fribourg, Chemin du Musée 3, 1700 Fribourg, Switzerland. § (C.M.F.) Seagate LLC, 47488 Kato Rd, Fremont, CA 94538. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), Materials Science and Engineering Division. The use of the Advanced 6815

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(17) Mannhart, J.; Blank, D. H. A.; Hwang, H. Y.; Millis, A. J.; Triscone, J.-M. Two-dimensional electron gases at oxide interfaces. MRS Bull. 2008, 33, 1027−34. (18) Li, S.; Bergman, B. Doping effect on secondary phases, microstructure and electrical conductivities of LaGaO3 based perovskites. J. Eur. Ceram. Soc. 2009, 29, 1139−1146. (19) Huijben, M.; Koster, G.; Kruize, M. K. Defect engineering in oxide heterostructures by enhanced oxygen surface exchange. Adv. Funct. Mater. 2013, 23, 5240−5248. (20) Chen, Y.; Jung, W.; Cai, Z.; Kim, J. J.; Tuller, H. L.; Yildiz, B. Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi1−xFexO3 surfaces. Energy Environ. Sci. 2012, 5, 7979−7988.

Photon Source at Argonne National Laboratory was supported by the DOE, Basic Energy Sciences, under Contract No. DEAC02-06CH11357.

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DOI: 10.1021/acs.cgd.6b00914 Cryst. Growth Des. 2016, 16, 6812−6816