Phase Control in Thin Films of Layered Cuprates - ACS Publications

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Article Cite This: Chem. Mater. 2018, 30, 1095−1101

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Phase Control in Thin Films of Layered Cuprates Henrik Hovde Sønsteby,*,† Jon Einar Bratvold,† Kristian Weibye,† Helmer Fjellvåg,† and Ola Nilsen† †

Department of Chemistry, University of Oslo, Oslo 0315, Norway S Supporting Information *

ABSTRACT: Perovskite-related layered cuprates are considered the basis of all high-Tc superconductors. Considerable effort has been put into understanding these materials. Deposition of phase-pure and oriented thin films can help shed light on the unusual interplay between the structure and properties, as the copper layers will be oriented in the same direction throughout the sample. Chemical deposition routes have been considered difficult, because of detrimental decomposition of metalorganic precursors, catalyzed by deposited copper species. In the current paper, we present a route for atomic layer deposition (ALD) of layered lanthanum cuprates. The system exhibits remarkable stoichiometric control, enabling deposition of a variety of lanthanum cuprate species. La2CuO4 films is shown to be tetragonal and (001) oriented on LaAlO3 (100)pc, with the copper planes parallel to the film surface. We go on to demonstrate that the oxygen-stoichiometry of LaCuO3−x films can be tuned by post-treatment, resulting in phase control of the complex perovskite lanthanum cuprate system. Epitaxial films have been obtained for oxygen annealed monoclinic and air annealed orthorhombic variants of LaCuO3−x on LaAlO3 (100)pc substrates with a LaCuO3−x(100)|[640]||LaAlO3(100)pc|[310]pc and LaCuO3−x(100)pc| [110]pc||LaAlO3(100)pc|[110]pc epitaxial relationship, respectively.

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INTRODUCTION The realization of superconductivity at temperatures higher than the boiling point of nitrogen is one of the biggest scientific breakthroughs in contemporary history. The achievement was made possible by the discovery of a simple, yet elusive, group of compounds; the layered cuprates. Because the observation of superconductivity above 30 K in La2−xBaxCuO4 in 1986, all subsequent critical temperature (Tc) records have been set by new variants of layered cuprates.1 This includes the first observation of Tc above the boiling point of nitrogen (YBa2Cu3O7−x, 1987) and the current record of 133 K at atmospheric pressure (HgBa2Ca2Cu3O8+x, 1994).2,3 The layered cuprates have been shown to exhibit an unusual interplay between the electronic spin and lattice degrees of freedom.4,5 The high-Tc cuprates all have one thing in common; the fact that they are all structurally based on a perovskite lattice with layers of weakly coupled CuO layers. Not all layered cuprates are superconducting. Over the last decades, a massive effort has been put into understanding what causes some systems to have this property, while others do not. One of the strategies has been to study the mother systems; the M3+CuO3 perovskite compounds and the M3+2CuO4 Ruddlesden−Popper 1 variants. Two of the most promising systems in this respect have been LaCuO3−x and La2CuO4 (Figure 1).6,7 LaCuO3−x is a perovskite structure with a remarkable versatility in terms of oxygen off-stoichiometry. The oxygen content can be tuned from 3 (x = 0) all the way to 2.5 (x = 0.5), meaning that the formal oxidation state of copper can be varied from +3 to +2. Note that the actual electronic configuration of the formal +3 state is still disputed.8 Three distinct phases are known at © 2018 American Chemical Society

Figure 1. LaCuO3 tetragonal perovskite structure (left) and La2CuO4 tetragonal Ruddlesden−Popper 1 structure (right). The unit cell is marked with a hard black line.

atmospheric pressure. For 0 ≤ x < 0.1 a tetragonal phase is observed, for 0.1 < x < 0.5 a monoclinic phase is observed, whereas for x = 0.5 (formally viewed as La2Cu2O5) a distorted orthorhombic lattice is found. The electrical properties also changes dramatically as a function of the oxygen offstoichiometry. For x = 0.5 the oxide is an insulator, while decreasing x increases the conductivity all the way to a metal Received: November 30, 2017 Revised: January 15, 2018 Published: January 17, 2018 1095

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Chemistry of Materials for x = 0. In addition to this, Ruddlesden−Popper variants, and especially RP1 La2CuO4, can be obtained by varying the La to Cu stoichiometry. The latter structure is the parent structure to the superconducting La2−xMxCuO4 (M = Ca, Sr, Ba). None of the ternary lanthanum cuprates are superconductors without doping, but the electronic interplay in these simple systems is believed to shed some light on the mechanisms behind high-Tc superconduction. LaCuO3−x is also an intrinsically interesting compound, as it is a perovskite oxide exhibiting metallic (albeit low) electric conductivity.6 The tetragonal structure is also a weak ferrimagnet. Very few ternary perovskite oxides are metallic conductors, and even fewer combine this with net magnetic moments. These compounds can be used to obtain epitaxial multilayer systems with combinations of electronically conducting, and/or weakly ferromagnetic materials. In addition, the wide range of materials belonging to the perovskite family often facilitates epitaxial growth of thin films. Procedures for deposition of such materials are highly desired, and could be used to extend the possibilities for designing superstructures.9 Understanding the electronic phenomena in layered cuprate systems is sometimes considered more straightforward using oriented thin films as compared to randomly oriented powders.10 When the cuprate layers are oriented in the thin film plane, the atomic and electronic structure of the compounds can be probed more easily. It is also fundamentally interesting to expand the range of suitable methods for deposition of superconducting thin films, especially for incorporation in multilayer systems. Deposition of epitaxial lanthanum cuprate thin films has previously been achieved by techniques such as PLD, MBE, and RF sputtering, although the elevated temperatures that are often used make it difficult to control the structure or stoichiometry of the resulting films.11−15 Chemical deposition techniques are also prone to catalytic decomposition of precursor molecules by CuOx at elevated temperatures, leading to uncontrolled growth. The latter point has been a limiting factor for atomic layer deposition (ALD) of cuprate systems.16 ALD is a layer-by-layer self-limiting deposition technique, utilizing surface saturation of precursor molecules that do not react with themselves.17 This facilitates growth of thin films with thickness control on the submonolayer level, extreme conformality on high aspect ratio surfaces and high stoichiometric control. Deposition of multilayer systems is attainable by expanding the amount of precursors that can be sequentially pulsed into the reaction chamber. ALD also enables wafer-scale batch deposition of conformal films, holding it in high regard for industrial upscaling. The technique has traditionally been used by industry to deposit pinhole free high-κ materials, but over the past decade interest in depositing epitaxial complex oxides has rapidly increased.9,18 ALD of complex oxide thin films containing copper was unexplored for a relatively long time due to the challenges of finding pairs of precursors that could be used in the same temperature region. The limiting factor has been copper precursor decomposition at temperatures below the applicable temperature range for the other metal precursor(s). Future development of copper precursors with higher thermal stability may help to overcome this challenge. At present, the two best precursor candidates for copper are the metalorganic compounds copper 2,2,6,6-tetramethyl-3,5-heptanedionate (Cu(thd)2) and copper actylacetonate (Cu(acac)2). These

have both been used for deposition of copper oxide at temperatures up to 250 °C.19 Currently, only one complex oxide system containing copper by ALD has been reported; CuCrO2 (delafossite structure) deposited using Cu(thd)2 and Cr(acac)3.20 Note the +1 formal oxidation state of Cu in this structure. In this article, we show that thin films of LaCuO3−x and La2CuO4 are indeed possible to achieve by ALD, using Cu(acac)2 as the copper precursor. The temperature profile for deposition is discussed in detail to show how sensitive these systems are in terms of thermal decomposition. We show that the stoichiometry in the whole La2O3−CuO binary oxide range can be controlled, and that phase control in epitaxial films of the interesting ternary compounds LaCuO3−x and La2CuO4 can be obtained by carefully choosing a substrate with a close lattice match. We go on to discuss the electrical conductivity of the films, and show that we can control the oxygen offstoichiometry in LaCuO3−x by postdeposition treatment. This report is a first step toward using ALD to deposit films of layered cuprates that can be used to understand fundamental electrical phenomena. It also enables deposition of the perovskite structure with metallic conductivity by ALD, which is very attractive in multilayered systems where an electrically conducting layer is needed.

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EXPERIMENTAL SECTION

Thin films were deposited in an F-120 Sat reactor (ASM Microchemistry). All films were deposited at a reactor temperature of 250 °C. Nitrogen purging gas was supplied from gas cylinders (Praxair, 99.999%), run through a Mykrolis purifier, and maintained at a 300 cm3 min−1 primary flow rate. Reactor operating pressure was maintained at 2.8 mbar throughout the depositions. All depositions were carried out using La(thd)3 (Volatec, 99%) and Cu(acac)2 (Sigma-Aldrich, 97%) as metal precursors. O3 was used as the oxygen source, made from O2 gas (Praxair, 99.996%) using an In USA ozone generator (AC-2505) producing 15 mass % O3 in O2. Pulse durations were 4, 4, and 2 s for La(thd)3, Cu(acac)2 and O3, respectively, unless otherwise stated. All purge durations were set to 3 s. These pulse and purge times are based on experience with achieving self-limiting growth with the same precursors for deposition of similar materials in the same ALD reactors throughout our prior studies. The films were deposited on 1 × 1 cm2 Si (100) substrates for characterization of thickness and conformality, whereas 3 × 3 cm2 Si (100) substrates were used for compositional analysis. Selected compositions were also deposited on LaAlO3 (100) (LAO, Crystal GmbH) substrates for facilitation of epitaxial growth. Thin film thickness was measured using a J. A. Woollam alpha-SE spectroscopic ellipsometer in the range of 390−900 nm. A Cauchy function was successfully used to model the collected data. X-ray diffraction measurements were performed on a Bruker AXS D8 Discover diffractometer equipped with a LynxEye strip detector and a Ge (111) focusing monochromator, providing CuKα1 radiation. Selected samples were also characterized with synchrotron X-ray diffraction at the Swiss-Norwegian Beamlines at the European Synchrotron Radiation Facility (BM01−SNBL @ ESRF) in Grenoble, France. BM01 is a bending magnet beamline station, and at the time of data collection a monochromatic beam with wavelength 0.6745 Å was employed. Slices of reciprocal space were extracted using the CrysAlisPro v171.38.46. Chemical composition was analyzed using a Panalytical Axios Max Minerals X-ray fluorescence (XRF) system equipped with a 4 kW Rhtube. The system is running with Omnian and Stratos options for standardless measurements of thin films. Complementary measurements of composition and evaluation of chemical state was carried out by X-ray photoelectron spectroscopy (XPS), using a Thermo Scientific Theta Probe Angle-Resolved XPS system, also enabling measurements of carbon contaminants and 1096

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Chemistry of Materials possible segregation of the cations in the films. The instrument is equipped with a standard Al Kα source (hν = 1486.6 eV), and the analysis chamber pressure is in the order of 1 × 10−8 mbar. Pass energy values of 200 and 60 eV were used for survey spectra and detailed scans, respectively. The data was corrected for shifts using the peak from adventitious carbon (binding energy = 248.8 eV) as a reference. Resistivity measurements were performed using a 4-point probe and a Keithley model 2400 SourceMeter. The sheet resistivity was recorded by measuring resistance in 10 points from 1 to 10 μA, and averaged over 5 measurements on different spots close to the center of the single crystal samples.

et al.22 It was found that these metal precursors are stable at high temperatures. However, formation of transition metal oxides in the deposition chamber will catalyze further decomposition of the precursor. As the flux of precursor increases with increasing sublimation temperature, this effect becomes more pronounced. The nonuniformity of the CuOx system can be reduced to negligible values by reducing the precursor temperature, and thus the flux, to a minimum. With this in mind, a precursor temperature of 135 °C was chosen as a compromise. All subsequent depositions were carried out with this temperature and a pulse length of 4 s. Furthermore, as observed for other challenging ALD processes, depositing the species in a complex oxide might help stabilize the system.23 It is known that the applicable temperature region for ternary processes, the so-called “ALD window”, is not always a superposition of the binary oxide windows.24 This means that the temperature profile of the system must always be verified for the ternary system as a whole. The La−Cu−O system was investigated by measuring the growth rate of films deposited with 0.75 La/(La+Cu) pulsing ratio, while varying the reaction temperature from 210 to 300 °C (Figure 3). The deposited cation ratio between La and Cu was also measured to understand the nature of the temperature profile.

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RESULTS AND DISCUSSION Deposition of a ternary oxide with ALD is usually carried out by sequentially depositing the binary oxides that constitutes the ternary compound. A requirement is that the binary processes can be deposited at the same temperature. In the La−Cu−O system, the temperature is limited downward to approximately 210 °C, due to a minimum precursor temperature of 185 °C for the La(thd)3 and uncontrolled growth of La2O3 at lower temperatures.21 Alnes et al. reported non-ALD growth of CuOx at depositions temperatures higher than 250 °C.16 As a result of this, a temperature of 235 °C was chosen for initial testing. However, Alnes et al. also reported a small, but persistent, nonuniformity at all deposition temperatures when applying a precursor temperature of 140 °C. No further investigation or explanation of this observation was reported. To examine if the nonuniformity is related to the precursor temperature, we investigated the growth rate of CuOx for varying pulse durations of Cu(acac)2 at different precursor temperatures. To rule out possible effects from other deposition parameters, we maintained extensively long pulses of O3 (5 s) and purges (5 s) (Figure 2).

Figure 3. Growth rate per cycle and cation ratio as a function of reaction temperature. Black dotted line corresponds to growth rate, whereas the blue dashed line corresponds to the deposited cation ratio.

An ALD window was found to exist at temperatures between 210 and 250 °C. Above 250 °C, both the growth rate and the copper content rapidly increased. This points toward decomposition of Cu(acac)2, leading to uncontrolled growth. A large thickness gradient was also observed at higher temperatures, with a thicker film deposited at the precursor inlet. This also suggests decomposition of Cu(acac)2. As a result of these initial tests, all subsequent films were deposited at a reaction temperature of 250 °C. To obtain phase control in the system, a series of films were deposited varying the pulsed ratio (La/La + Cu) between La and Cu from 0 to 1 (Figure 4). Because the growth rates of La2O3 and CuO are similar, a relatively linear compositional relationship was expected. Such a linear relationship was indeed observed, with a small bend around 0.66 (La2CuO4), which might be due to the extraordinary stability of the RP1 phase. This is a feature also seen in other ternary systems.25 The

Figure 2. Growth rate per cycle of CuOx as a function of precursor temperature and pulse duration.

From Figure 2 it is clear that the growth rate keeps increasing as a function of pulse duration for a precursor temperature of 140 °C. At 135 °C, the surface saturates after approximately 3 s, with no increase in growth rate for longer pulse durations. With a precursor temperature of 130 °C the surface does not saturate, even after 4 s. The temperature stability of transition metal β-diketonates has previously been investigated by Nilsen 1097

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In an attempt to controllably obtain the LaCuO3−x and La2CuO4 phases, depositions yielding La/(La + Cu) ratios of 0.5 and 0.67, respectively, were carried out on LaAlO3 (100)pc single-crystal substrates. With a pseudocubic lattice parameter of 3.86, LaAlO3 is thought to facilitate growth of LaCuO3 (100) and La2CuO4 (001), with a very close lattice match. Again, the films were amorphous as deposited, but annealing at 650 °C for 10 min resulted in crystallization and orientation of the films, as observed by XRD. For the sample with a La/(La + Cu) ratio of 0.67, singlephase and (001) oriented tetragonal RP1 La2CuO4 was observed (Figure 6). As can be seen from the structure in Figure 1 this means that the copper layers align parallel to the film surface.

Figure 4. Stoichiometry (blue) and growth rate (black) as a function of the pulsed La/(La + Cu) cation ratio.

growth rate was stable around 0.35 Å/cycle, with controllable stoichiometry in the whole range, facilitating growth of all the discussed phases. All investigated films were deposited with a total of 1000 cation subcycles, resulting in uniform films with 30−40 nm thickness in the explored stoichiometric range. As will be shown later, LaCuO3−x was obtained for a pulsed ratio of La/(La + Cu) = 0.57, whereas the La2CuO4 composition was found at La/(La + Cu) = 0.67. All samples in the compositional series were investigated in terms of electrical conductivity and refractive index (λ = 623.8 nm) (Figure 5). Two distinct regions were observed, separated by a minimum in resistivity at a La/(La + Cu) ratio of 0.19, coinciding exactly with the maximum refractive index of 2.53. The low resistivity could arise from a lanthanum doped CuOtype phase, but it could also be a result of formation of Cumetal during deposition. The higher refractive index might point toward a more metallic-like phase being deposited. The resistivity gradually increases toward CuO, whereas the refractive index drops off toward the literature value of ≈1.6 for CuO.26 All of the samples were also investigated with XRD, but no crystalline phases were observed.

Figure 6. XRD of thin films with La/(La + Cu) ratio of 0.67, showing single-phase and oriented La2CuO4. The intensity is shown on a logarithmic scale. Substrate peaks are marked with a red asterisk (*).

For the sample with a La/(La + Cu) ratio of 0.5, corresponding to the LaCuO3−x perovskite phase, structure determination was less straightforward. The film did indeed crystallize upon annealing in air at 650 °C for 10 min, but due

Figure 5. Refractive index (left) and resistivity (right) as a function of La/(La + Cu) ratio in the deposited films. All films were deposited on Si(100) substrates. 1098

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Chemistry of Materials to the low intensity of the film reflections and very close lattice match with the substrate, the reflection from the film was only seen as an asymmetric extension of the substrate peak (Figure 7). To further promote crystallization, annealing at 900 °C in

variation in oxygen under-stoichiometry in these samples; XPS, XRD, and resistivity measurements. XPS was used to look for changes in the chemical environment for copper. Detailed scans of the Cu 2p peak were collected both for the sample annealed in air and the samples annealed in O2. It proved challenging to differentiate between the samples (Figure S3). This is also known from theory, since the formal Cu3+ state is predicted to rather be a Cu2+ state with additional oxygen holes.8 In other words, the copper electron configuration may be d9 in both cases, making it very difficult to observe changes in the chemical environment. Conversly, it is possible to distinguish the crystallographic variations for the different oxygen off-stoichiometries by diffraction, with three distinct phases for x = 0.5 (orthorhombic), 0.2 < x < 0.5 (monoclinic) and 0 < x < 0.2 (tetragonal). Coupled θ−2θ scans will only reveal specular reflections, in which the diffracted peaks are predicted to be very close for the orthorhombic and monoclinic cells. The different cells have different off-specular reflections, and this enables differentiation between them. Asymmetric reciprocal space maps around the LaAlO3 (310)pc reflection collected at SNBL@ESRF revealed that the air-annealed sample was indeed the orthorhombic variant, whereas the oxygen-annealed sample yields the monoclinic structure (Figure 9). This also enabled

Figure 7. High-resolution XRD of the (200)-reflection in the LaCuO3||LaAlO3-system for as deposited (blue), air-annealed (black), and oxygen-annealed (red) samples. The data is normalized to the substrate peak. Diffractograms for 2θ = 10−120° can be found in Figure S2.

air was carried out. This resulted in loss of phase control, observed by the appearance of additional reflections corresponding to the RP1-phase (Figure S1). In an attempt to obtain LaCuO3‑x phases with higher oxygen content, while still maintaining a single phase, annealing at 900 °C in 1 atm of O2 for 10 h was carried out. This significantly improved the crystallinity and it was possible to observe the pseudocubic LaCuO3−x (100) orientation of the film. This does not, however, enable differentiation between the different LaCuO3−x structures, as the specular reflections will coincide for all applicable structure types. One of the ideas behind annealing in oxygen was initially to study the possibility of tuning the oxygen off-stoichiometry in LaCuO3−x, obtaining the tetragonal, monoclinic and orthorhombic structures (Figure 8). It has been shown for bulk samples that the oxygen level can be tuned from x = 0 to x = 0.5 by applying different partial pressures of oxygen.27 Three methods of characterization were employed to understand the

Figure 9. Reciprocal space maps around the substrate LaAlO3 (310)pc for the oxygen-annealed (left) and air annealed (right) LaCuO3−x films. For the oxygen-annealed sample a monoclinic LaCuO2.67 (640) reflection is visible, whereas the reflection is absent for the orthorhombic variant due to extinction. The substrate peak shape is pixelated due to overexposure.

determination of an epitaxial relationship in the samples, as the films were also shown to be oriented in-plane. The oxygen annealed monoclinic and air annealed orthorhombic variants exhibits LaCuO3−x(100)|[640]||LaAlO3(100)pc|[310]pc and LaCuO3−x(100)|[110]||LaAlO3(100)pc|[110]pc epitaxial relationships, respectively. The phase control was confirmed by resistivity measurements. The orthorhombic cell has previously been reported to have orders of magnitude higher resistivity than the monoclinic and tetragonal variants. Using a 4-point probe, the resistivity of the air-annealed sample was measured to be 440 mΩ cm, whereas the oxygen-annealed sample was measured to 2.2 mΩ cm. Although it is hard to differentiate the monoclinic and tetragonal cells using resistivity measurements, there were no traces of the tetragonal cell in the XRD data, pointing toward formation of the monoclinic cell for the oxygen-annealed sample.

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Figure 8. Structures of the stoichiometric tetragonal LaCuO3 (left), the monoclinic LaCuO3−x (0.1 < x < 0.5, middle) and orthorhombic LaCuO2.5 (right). Note the capping of the copper oxygen octahedrons for the oxygen understoichiometric structures. The monoclinic variant is imaged for x = 0.33 for clarity.

CONCLUSIONS In the current paper, we demonstrate an expansion of the ALD technique into deposition of complex layered cuprates, also 1099

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the LaCuO3‑δ perovskites. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 15269−15275. (7) Hirayama, T.; Nakagawa, M.; Sumiyama, A.; Oda, Y. Superconducting properties in La2CuO4+δ with excess oxygen. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 58, 5856−5861. (8) Mizokawa, T.; Fujimori, A.; Namatame, H.; Takeda, Y.; Takano, M. Electronic structure of tetragonal LaCuO3 studied by photoemission and x-ray-absorption spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 9550−9556. (9) Sønsteby, H. H.; Nilsen, O.; Fjellvåg, H. Functional Perosvskites by Atomic Layer Deposition - an Overview. Adv. Mater. Interfaces 2017, 4, 1600903. (10) Habermeier, H.-U. Science and technology of cuprate-based high temperature superconductor thin films, heterostructures and superlatticesthe first 30 years. Low Temp. Phys. 2016, 42, 840−862. (11) Yamamoto, H.; Matsumoto, O.; Tsukada, A.; Naito, M. MBE growth and properties of T′-La2CuO4 thin films. Phys. C (Amsterdam, Neth.) 2010, 470, 1025−1028. (12) Baiutti, F.; Christiani, G.; Logvenov, G. Towards precise defect control in layered oxide structures by using oxide molecular beam epitaxy. Beilstein J. Nanotechnol. 2014, 5, 596−602. (13) Wimbush, S. C.; Takayama-Muromachi, E. Pulsed Laser Deposition of T, T* and T′ La1.85Y0.15CuO4. AIP Conf. Proc. 2005, 850, 475−476. (14) Yu, H.-C.; Chen, Y.-H.; Liao, C.-L.; Fung, K.-Z. Preparation and characterization of RF-sputtered Sr-doped lanthanum cuprate thin films on yttria-stabilized zirconia substrates. J. Alloys Compd. 2005, 395, 286−290. (15) Gupta, A.; Hussey, B. W.; Guloy, A. M.; Shaw, T. M.; Saraf, R. F.; Bringley, J. F.; Scott, B. A. Growth of Thin Films of the Defect Perovskite LaCuO3‑δ by Pulsed Laser Deposition. J. Solid State Chem. 1994, 108, 202−206. (16) Alnes, M. E.; Monakhov, E.; Fjellvåg, H.; Nilsen, O. Atomic Layer Deposition of Copper Oxide using Copper(II) Acetylacetonate and Ozone. Chem. Vap. Deposition 2012, 18, 173−178. (17) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. (Washington, DC, U. S.) 2010, 110, 111−131. (18) McDaniel, M. D.; Ngo, T. Q.; Hu, S.; Posadas, A.; Demkov, A. A.; Ekerdt, J. G. Atomic layer deposition of perovskite oxides and their epitaxial integration with Si, Ge, and other semiconductors. Appl. Phys. Rev. 2015, 2, 041301. (19) Miikkulainen, V.; Leskelä, M.; Ritala, M.; Puurunen, R. L. Crystallinity of inorganic films grown by atomic layer deposition: Overview and general trends. J. Appl. Phys. (Melville, NY, U. S.) 2013, 113, 021301. (20) Tripathi, T. S.; Niemela, J.-P.; Karppinen, M. Atomic layer deposition of transparent semiconducting oxide CuCrO2 thin films. J. Mater. Chem. C 2015, 3, 8364−8371. (21) Nieminen, M.; Putkonen, M.; Niinistö, L. Formation and stability of lanthanum oxide thin films deposited from β-diketonate precursor. Appl. Surf. Sci. 2001, 174, 155−166. (22) Nilsen, O.; Fjellvåg, H.; Kjekshus, A. Inexpensive set-up for determination of decomposition temperature for volatile compounds. Thermochim. Acta 2003, 404, 187−192. (23) Sønsteby, H. H.; Weibye, K.; Bratvold, J. E.; Nilsen, O. Rubidium containing thin films by atomic layer deposition. Dalton Trans. 2017, 46, 16139−16144. (24) Nilsen, O.; Rauwel, E.; Fjellvåg, H.; Kjekshus, A. Growth of La1‑xCaxMnO3 thin films by atomic layer deposition. J. Mater. Chem. 2007, 17, 1466−1475. (25) Bratvold, J. E.; Fjellvåg, H.; Nilsen, O. Atomic Layer Deposition of oriented nickel titanate (NiTiO3). Appl. Surf. Sci. 2014, 311, 478− 483. (26) Tahir, D.; Tougaard, S. Electronic and optical properties of Cu, CuO and Cu2O studied by electron spectroscopy. J. Phys.: Condens. Matter 2012, 24, 175002. (27) Bringley, J. F.; Scott, B. A.; Poppelmeier, K. R.; Taylor, G. A.; Dabrowskiu, B., Synthesis of the Perovskite Series LaCuO3‑δ. In

with epitaxial growth on suitable substrates. The perovskite and RP1 lanthanum cuprates were chosen as model systems, as they are the parent structures of most high Tc superconductors. Phase control is achieved by carefully controlling the stoichiometry and annealing parameters, resulting in singlephase epitaxial films of LaCuO3−x and La2CuO4. We go on to show that the oxygen deficiency in LaCuO3−x can be tuned to some extent by applying different annealing schemes. Annealing in oxygen atmosphere leads to the monoclinic phase with low electric resistivity. Air-annealing leads to the orthorhombic LaCuO2.5 (La2Cu2O5) structure exhibiting a higher electric resistivity. This route for atomic layer deposition enables continued investigation of the electronic phenomena of layered cuprate thin films.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05005. Diffractogram for LaCuO3−x samples annealed in air at 900 °C; full diffractogram of LaCuO3−x samples as deposited, annealed at 650 °C in air, and annealed at 900 °C in 1 atm O2, respectively; XPS Cu 2p peak for sample annealed at 650 °C in air and annealed at 900 °C in 1 atm O2, respectively (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Henrik Hovde Sønsteby: 0000-0002-5597-1125 Ola Nilsen: 0000-0002-2824-9153 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dmitry Chernyshov at the SwissNorwegian Beamlines at the European Synchrotron Radiation Facility for his contribution to collecting and understanding synchrotron XRD data. This work was partially performed within the RIDSEM-project, financed in full by the Research Council of Norway (project number 272253).

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REFERENCES

(1) Bednorz, J. G.; Müller, K. A. Possible high-Tc superconductivity in the Ba−La−Cu−O system. Z. Phys. B: Condens. Matter 1986, 64, 189−193. (2) Wu, M. K.; Ashburn, J. R.; Torng, C. J.; Hor, P. H.; Meng, R. L.; Gao, L.; Huang, Z. J.; Wang, Y. Q.; Chu, C. W. Superconductivity at 93 K in a new mixed-phase Y-Ba-Cu-O compound system at ambient pressure. Phys. Rev. Lett. 1987, 58, 908−910. (3) Lee, S.-I.; Lee, S.; Mun, M.-O.; Bae, M.-K. Synthesis and Superconductivity of the HgBa2Ca2Cu3O8+x superconductors. Phys. C (Amsterdam, Neth.) 1994, 235−240, 895−896. (4) Keimer, B.; Kivelson, S. A.; Norman, M. R.; Uchida, S.; Zaanen, J. From quantum matter to high-temperature superconductivity in copper oxides. Nature 2015, 518, 179−186. (5) Kalantar-zadeh, K.; Ou, J. Z.; Daeneke, T.; Mitchell, A.; Sasaki, T.; Fuhrer, M. S. Two dimensional and layered transition metal oxides. Appl. Mater. Today 2016, 5, 73−89. (6) Bringley, J. F.; Scott, B. A.; La Placa, S. J.; McGuire, T. R.; Mehran, F.; McElfresh, M. W.; Cox, D. E. Structure and properties of 1100

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Chemistry of Materials Inorganic Syntheses: Nonmolecular Solids; John Wiley & Sons: New York, 2007; Vol. 30, pp 218−222.

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DOI: 10.1021/acs.chemmater.7b05005 Chem. Mater. 2018, 30, 1095−1101