Growth Mode Transition in Complex Oxide Heteroepitaxy: Atomically

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Growth Mode Transition in Complex Oxide Heteroepitaxy: Atomically Resolved Studies Alexander Tselev,*,† Rama K. Vasudevan,† Anthony G. Gianfrancesco,‡ Liang Qiao,†,# Tricia L. Meyer,† Ho Nyung Lee,† Michael D. Biegalski,† Arthur P. Baddorf,† and Sergei V. Kalinin*,† †

Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States UT/ORNL Bredesen Center, University of Tennessee, Knoxville, Tennessee 37996, United States

‡

ABSTRACT: We performed investigations of the atomic-scale surface structure of epitaxial La5/8Ca3/8MnO3 thin films as a model system dependent on growth conditions in pulsed laser deposition with emphasis on film growth kinetics. Postdeposition in situ scanning tunneling microscopy was combined with in operando reflective high-energy electron diffraction to monitor the film growth and ex situ X-ray diffraction for structural analysis. We find a correlation between the out-of-plane lattice parameter and both adspecies mobility and height of the Ehrlich−Schwoebel barrier, with mobility of adatoms greater over the cationically stoichiometric terminations. The data suggest that the out-of-plane lattice parameter is dependent on the mechanism of epitaxial strain relaxation, which is controlled by the oxidative power of the deposition environment.

1. INRODUCTION Complex metal oxides with perovskite-related crystal structure (with a general formula ABO3, where A and B are metal cations) in the form of thin films and heterostructures exhibit a large variety of physical and chemical properties making them highly attractive as candidates for a multitude of functionalities in electronics, spintronics, and sensing systems, as well as platforms to study fundamental physical phenomena.1−8 These properties include, but are not limited to, ferroelectricity, magnetic-field-controlled electron transport, ionic phenomena, colossal magnetoresistance, emergent conductivity and magnetization at the interface of two band insulators, multiferroicity, and others. Much of the knowledge regarding complex oxide physics was obtained through thin film growth using pulsed laser deposition.8 Both fundamental research and practical implementation of different functionalities offered by complex oxide films and heterostructures demand understanding and precise control over film growth. However, pulsed laser deposition (PLD) poses a considerable challenge for experimental study. PLD involves a series of highly energetic and nonequilibrium processes including evaporation of the material from the target and transport of the vapor (plasma) to the substrate through background oxygen gas. In the idealized picture of the process, the large increase of local temperature in the laser spot on the target leads to congruent evaporation of the target material. In practice, however, stoichiometric transfer of the target composition to the substrate is difficult to achieve, in particular, due to different scattering patterns of the ionic and atomic species in the dense plasma at the target and in the plasma plume away from the target.9 After the flux arrival onto the substrate surface, nucleation and formation of the film structure take place at a range of length and time scales through multiple physical and chemical steps in interaction with the background © 2016 American Chemical Society

gas. Because of this complexity, and notwithstanding the high efficacy of the method, many details of the film growth process are not well understood, and many fundamental issues related to the growth kinetics and film structure formation remain unresolved. Kinetics of the growth can be monitored in operando by electron and X-ray diffraction. Reflection high-energy electron diffraction (RHEED) is widely used to monitor growth in real time. Because of the high scattering cross-section of electrons and a grazing incidence geometry, this technique is highly surface-sensitive. However, multiple scattering processes involved in electron diffraction hinder accurate interpretation of the data,10,11 therefore limiting the accessible information both on details of the processes taking place on the surface of a growing film and on the final surface structure. The most accurate information on the dynamic processes occurring on the surface of a growing film was obtained using X-ray reflectivity techniques. The most studied system to date is homoepitaxy of SrTiO3 on SrTiO3. Insightful details about surface diffusion, details of the intra- and interlayer transport, as well as length, time, and energy parameters of the growth kinetics in the layer-by-layer growth mode were obtained.12−16 While more difficult to interpret, X-ray reflectivity was employed for heteroepitaxy as well. Willmott et al.17 and Dale et al.18 implemented X-ray reflectivity to heteroepitaxial growth of a manganite compounds La1−xSrxMnO3 (LSMO) on SrTiO3 (001). However, the techniques based on surface reflectivity are limited in that they provide only information averaged over large areas of a sample. Moreover, recent studies showed that Received: December 27, 2015 Revised: March 21, 2016 Published: April 4, 2016 2708

DOI: 10.1021/acs.cgd.5b01826 Cryst. Growth Des. 2016, 16, 2708−2716

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on the mounded surface associated with the 3D growth. We observed clear evidence of a larger adspecies mobility on the cationically stoichiometric surfaces. STM images reveal a complicated pattern of extended nontopographic features over the cationically stoichiometric surfaces, which can be ascribed to the presence of subsurface defects.

due to structural tolerance to cation nonstoichiometry and intermixing, structural analysis alone provides inadequate characterization of complex oxide thin films,19 which can, in turn, lead to a misleading interpretation of the observed physical phenomena. These limitations demand atomic-scale studies to determine the interplay between deposition parameters and growth kinetics in oxides, which has been not addressed thus far, in contrast to metal and semiconductor materials.20−22 While the film growth may have many common features for these material classes, complex oxides have significantly different behavior in multiple aspects. One example is mechanisms of accommodation of epitaxial strain as discussed, e.g., in ref 23. Another aspect is directly related to the complex chemical composition of the oxides. It was previously observed that the adatom mobility is a function of film stoichiometry.24,25 Furthermore, since film deposition takes place in a chemically active environment, in addition to kinetic, thermodynamic effects associated with the environment may significantly impact film composition and properties.25,26 In this work, we implemented postdeposition in situ atomicresolution scanning tunneling microscopy (STM) to study evolution of the film surface structure in heteroepitaxy of a hole-doped manganite La5/8Ca3/8MnO3 on single-crystal SrTiO3 (001) substrates. STM was combined with in operando RHEED to monitor the film growth and ex situ X-ray diffraction for structural analysis. We note that observation of atomically resolved surface structures and associated defects is highly desirable not only for understanding of the growth process, but also for in-depth interpretation of images obtained with lower resolution techniques as well as prediction of the film properties and functionality. Previously, high-temperature STM was used by Lippmaa et al.27,28 to study transformation of islands in the case of SrTiO3 on SrTiO3 (001) growth. While being very insightful, the hightemperature STM images of films were lacking atomic resolution. Furthermore, heteroepitaxial growth may have peculiarities, which are not present in homoepitaxy. Atomically resolved STM was applied to visualize formation of the first film layers for homoepitaxy of SrTiO3 on SrTiO3(001) by Iwaya et al.29 and Ohsawa et al.30,31 Later, Shimizu et al.32 employed atomically resolved STM images to elucidate chemical effects of the film−substrate interaction in the heteroepitaxy of La0.7Ca0.3MnO3 on SrTiO3 (001). For our studies, we have chosen to grow a similar Ca-doped lanthanum manganite compound, La 5 / 8 Ca 3 / 8 MnO 3 . La1−xCaxMnO3 is a prototypical colossal magnetoresistance oxide system and properties of PLD-grown La1−xCaxMnO3 thin films have been widely investigated. La5/8Ca3/8MnO3 is semiconducting at room temperature, which simplifies the task of obtaining atomic resolution, in contrast to its higher Curie temperature counterpart LSMO, where atomic resolution is difficult to achieve.33 For depositions, we have chosen a regime, where layer-by-layer growth is at the borderline with mound instability and 3D growth.34 However, the wide parameter window to grow best quality films of La1−xCaxMnO3 cited in the literature covers the conditions used in this work. We explored effects of varying laser fluence, oxygen pressure, and substrate-induced strain. A correlation between epitaxial strain, potential barrier for downhill mass transport, growth mode, and adspecies mobility was observed. The films of this study are slightly Mn-deficient. Despite the deposition regime with Mn-deficiency, nearly cationically stoichiometric terraces with a good structural quality were revealed by STM imaging

2. EXPERIMENTAL SECTION La5/8Ca3/8MnO3 (LCMO) films were epitaxially grown by PLD (KrF laser, λ = 248 nm) from a stoichiometric La5/8Ca3/8MnO3 target at a substrate temperature of 750 °C with in operando monitoring of the growth using RHEED with the incident beam along the [100] azimuth of the substrate. The substrates were attached to stainless steel plates with a very small amount of silver paint and were heated during deposition by an infrared laser from the plate side. The amount of the silver paint was sufficiently small to avoid contamination of the film surface with silver.35 Two types of single-crystal substrates were used for depositions: SrTiO3 (001) (STO) and NdGaO3 (110) (NGO). To obtain single termination, STO substrates were etched in buffered oxide etch solution (pH = 4.5) for 30 s and annealed in air for 4 h at 900 °C. Prior to film deposition, each substrate was inspected with atomic force microscopy (AFM) in tapping mode using the AFM tip oscillation phase signal for detection of the second, SrO, termination on the surface. AFM images revealed that the substrates are nearly single-terminated with small islands of the second termination. Only substrates with a low surface roughness and a small fraction of the minor, SrO, termination were selected for film depositions with following STM experiments. The selection of the substrates was dictated by the necessity of obtaining highly smooth substrate surfaces to ensure a large mobility of adatoms and to minimize the presence of small immobile clusters on the surface, which are highly detrimental for stable STM imaging. The width of the vicinal steps on the STO substrates was between ca. 200 and 400 nm. The NGO substrates were annealed in air for 4 h at 900 °C without prior etch, and showed largely mixed NdO/GaO2 termination with one of the terminations (unidentified) being predominant. The STO and NGO substrates have significantly different lattice mismatches with stoichiometric bulk LCMO. Lattice constants of LCMO, STO, and NGO are 0.3858, 0.3905, and 0.3864 nm, respectively (in pseudocubic notation as adapted everywhere in this paper), which results in a 1.22% lattice mismatch with a tensile stress for a bulk LCMO/STO system and an 8-times smaller lattice mismatch of 0.16% (the same sign) for LCMO/NGO. These two substrates allow us to explore the effects of the epitaxial strain on the growth mode. The depositions were carried out at background oxygen pressures of 50 and 20 mTorr. The target-substrate distance was 70 mm. At 50 mTorr, the deposition on STO substrates was carried out with several laser fluences from 0.8 to 2.6 J/cm2 (with a ∼1 × 3.3 mm2 spot), while at 20 mTorr, the laser fluence was kept at 1.2 J/cm2. The pulse repetition frequency was 10 Hz. Immediately after growth stop, the films were cooled down at an initial rate of 150 °C/min at the deposition pressure. The deposition chamber was evacuated from oxygen at a temperature of 400 °C, and samples were quickly transferred into the STM chamber maintained at a pressure 50 nm and only three top unit cell layers simultaneously exposed to the surface. Remarkably, this indicates that with increasing thickness, the suboptimal layerby-layer growth involving multiple exposed layers evolves into a nearly ideal layer-by-layer growth with the prototypical three exposed layers. However, the large-scale STM images in Figure 2 do not provide any extra information that could not be obtained with employment of the widely used ex situ AFM. To shed light on atomic-scale details of the growth mode change, we performed high-resolution imaging. Figure 3a shows an atomically resolved image of the terraces of one of the mounds on the surface of the 16 u.c. film. The height of the steps between adjacent terraces is half-unit-cell, and the neighboring terraces show clearly different atomic-scale structures. Apparently, the terraces correspond to alternating Aand B-site surface terminations. Both terminations are (√2 × √2)R45°-reconstructed (as defined with respect to the pseudocubic lattice of LCMO), with one of the terminations being distinctly more ordered than the other at the atomic scale. The surface of the 25 u.c. sample also shows two terminations as is evident from the atomically resolved image in Figure 3b. However, the highly ordered termination is clearly a minor termination on this sample, and the other termination is noticeably more disordered than in the case of the 16 u.c. film. The ordered termination can be seen within vacancy islands of the disordered termination and as adatom islands on top of the disordered termination. Upon further increase of the film thickness, the disordered termination fully covers the film surface. A corresponding atomically resolved image of the 250 u.c.-thick film shows only single disordered termination as illustrated in Figure 3c. For interpretation of the STM images and understanding of the growth mode evolution, it is critical to identify the two

Table 1. Major Film Parameters and Corresponding Deposition Conditions Used in This Worka oxygen pressure (mTorr) substrate laser fluence (J/cm2) thickness (u.c.) growth mode

20

50

50

50

STO 1.2

STO 0.8, 1.0, 1.2, 1.6, 2.0 50 3D changing into LBL

STO 2.6

NGO 2.6

16, 25, 50, 250 3D changing into LBL

25 LBL

50 3D

a STO is TiO2-terminated SrTiO3 (001), NGO is mixed-terminated NdGaO3 (110); u.c. stands for unit cell, 1 u.c. ≈ 0.4 nm; LBL stands for layer-by-layer.

3. RESULTS We start with results for films deposited at a background oxygen pressure of 50 mTorr and a laser fluence of 2.6 J/cm2. Figure 1a shows the intensity oscillations of the RHEED specular spot versus time for a typical deposition run on an STO substrate. For all deposition runs and regardless of the laser fluence used, we observed intensity oscillations characteristic for the layer-by-layer growth. As seen, both the intensity and amplitude of the intensity oscillations first drop after the deposition start, pass a minimum, then grow, pass a maximum, and after that drop slowly. The exact number of oscillations before the amplitude minimum and the following maximum varied in depositions but generally occurred between 8th and 12th oscillations for the minimum and between 16th and 20th oscillations for the amplitude maximum. The period of RHEED oscillations always remained nearly constant during growth, starting from the very first oscillation. Such RHEED behavior can be interpreted as first roughening of the film surface, then a decrease of the roughness with increasing film thickness and eventual conversion to a stable layer-by-layer growth. This behavior may indicate conversion of the film termination as suggested in ref 36, where very similar trends were observed during growth of LaxSr1−xMnO3 films. To clarify the observed change in the RHEED oscillations, Figure 2a,b shows a large-scale STM image of film surfaces when deposition was stopped at 16th and 25th peaks of the RHEED intensity oscillations, respectively, which approximately correspond to points (I) and (II) along the RHEED intensity curve shown in Figure 1a. Depositions were stopped at the maxima of a RHEED oscillation. The larger-scale morphology of the 16 unit cells (u.c.)-thick film surface (Figure 2a) can be roughly described as consisting of terraced mounds

Figure 1. Typical RHEED specular spot intensity oscillations during depositions on (a) STO and (b) NGO substrates at PO2 = 50 mTorr and a laser fluence 2.6 J/cm2. (c) Same as (a) and (b) for depositions on STO substrates at PO2 = 20 mTorr and a laser fluence 1.2 J/cm2. In (a), I and II approximately indicate peak positions, where depositions were stopped (in different deposition runs) for STM experiments with images shown in Figure 2, panels a and b, respectively. Arrows in (b) and (c) indicate the end of deposition. 2710

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Figure 2. STM images of LCMO films with different thicknesses on STO substrates. (a) 16 u.c. or 6.4 nm, (b) 25 u.c. or 10 nm, and (c) 250 u.c. or 100 nm. All films were deposited at PO2 = 50 mTorr and a laser fluence 2.6 J/cm2. Insets show RHEED patterns from corresponding film surfaces taken in a vacuum immediately after cooling and before transfer into the STM chamber. The step heights seen in images (a) and (b) are 1/2 u.c. and 1 u.c. in image (c).

Figure 3. Atomically resolved images of the surfaces displayed in Figure 2. Film thicknesses: (a) 16 u.c., (b) 25 u.c., and (c) 250 u.c. Labels “A” and “B” mark the (La,Ca)O and MnO2-terminated surface, respectively. The terrace step heights seen in images (a) and (b) are 1/2 u.c. and 1 u.c. in image (c). The green dots in (a) and (b) show the (√2 × √2)R45° reconstruction pattern on the (La,Ca)O-termination. The geometry of the pattern in the images is distorted due to drift.

terminations seen in the STM images. We made detailed analysis of the surface chemical composition of the films employing in situ angular resolved X-ray photoelectron spectroscopy (AR-XPS). The details and results of the ARXPS measurements are published elsewhere.34 The terminations were unambiguously identified as shown in Figure 3. The A-site termination(La,Ca)Ois highly ordered and (√2 × √2)R45°-reconstructed in all STM images, while the atomic structure of the B-siteMnO2termination is poorly ordered and dependent on deposition conditions and film thickness. These differences can be linked to the growth mode as discussed below. Notably, by varying the laser fluence on the target for depositions at 50 mTorr of PO2, we found that the surface morphology is largely independent of the laser fluence in the range 0.8−2.6 J/cm2. To illustrate this, Figure 4 displays large scale and atomically resolved images of a 50 u.c.-thick film deposited at a fluence of 1.2 J/cm2, which shows the same features as the 25 u.c.-thick film deposited at 2.6 J/cm2 in Figure 3b. In contrast to the growth on the STO substrates, LCMO deposition on NGO substrates at PO2 = 50 mTorr and a laser fluence of 2.6 J/cm2 exhibits only RHEED oscillations characteristic of layer-by-layer growth, and no intermittent roughening transition was observed (Figure 1b). STM images of 25 u.c.-thick film on NGO deposited at PO2 = 50 mTorr (Figure 5) reveal a surface morphology with mixed termination and the same atomic-scale structure of the terminations, closely resembling that of the 25 u.c.-thick film deposited on STO.

Figure 4. (a) Larger scale and (b) atomically resolved images of a 50 u.c.-thick film deposited on an STO substrate at PO2 = 50 mTorr and a laser fluence 1.2 J/cm2. The green dots in (b) show the (√2 × √2)R45° reconstruction pattern on the (La,Ca)O-termination. 2711

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Figure 6. (a) Larger scale and (b) atomically resolved images of a 50 u.c.-thick film deposited on an STO substrate at PO2 = 20 mTorr and a laser fluence 1.2 J/cm2. The inset in (a) shows a RHEED pattern from the film surface acquired in a vacuum immediately after cooling. The green dots in (b) show the (√2 × √2)R45° reconstruction pattern on the (La,Ca)O-termination (“A”). (c) A high-resolution image of a portion of the smooth surface of the MnO2-termination (“B”) in panel (b). The image was taken at conditions different from other images shown in the paper: tip bias Vt = −0.7 V and tunneling current It = 130 pA. The green dots in (c) show the (1 × 1) reconstruction pattern. The inset in (c) highlights the electron inhomogeneity over a portion of the MnO2-terminated layer. This image was acquired under the same conditions as the image of the main panel but is shown in a grayscale color map with an enhanced contrast.

Figure 5. (a) Larger-scale and (b) atomically resolved images of a 25 u.c.-thick film deposited on an NGO substrate at PO2 = 50 mTorr and a laser fluence 2.6 J/cm2. The film lattice orientation is indicated in the pseudocubic notation. The inset in (a) shows a RHEED pattern from the film surface acquired in a vacuum immediately after cooling. The green dots in (b) show the (√2 × √2)R45° reconstruction pattern on the (La,Ca)O-termination.

The disappearance of the mounding instability with increasing thickness suggests that it is possible to preserve it by a slight change in the growth conditions. It was found that a relatively small reduction in the oxygen pressure to 20 mTorr resulted in suppression of the transition to the layer-by-layer growth as evidenced by the RHEED intensity oscillations in Figure 1c. Further growth on STO substrates was carried out at a reduced oxygen background pressure of 20 mTorr. Films of the 50 u.c. thickness still exhibit a mounded morphology (Figure 6a) similar to that seen with 16 u.c. samples deposited at 50 mTorr (Figure 2a). (Upon further increase of the thickness at 20 mTorr, the growth becomes unstable exhibiting features of the Stranski−Krastanov growth mode with formation of large islands.) However, a noticeable difference was found at the atomic scale structure of mound terraces in Figure 6b,c. While one of the terminations is still ordered with a clear (√2 × √2)R45° reconstruction, the terraces of the other termination are atomically smooth without signs of the disorder characteristic for the films deposited at PO2 = 50 mTorr. The difference in the growth behavior of the films deposited on STO and NGO substrate indicates that the observed evolution in the growth mode on STO is associated with alterations of the strain in the films. Because of a large lattice mismatch, films on STO are subject to a significant epitaxial strain as compared to films grown on NGO. To elucidate the relationship between the strain state of the films on STO and the growth mode, we have performed ex situ X-ray diffraction (XRD) characterization of the film lattice parameters. The outof-plane lattice parameters were measured using 2θ−ω scans, while reciprocal space mapping was utilized to determine the in-plane lattice parameters.

The reciprocal space mapping shows that all films are fully strained to substrates, without any measurable difference of the in-plane lattice parameters, as illustrated by Figure 7a,b for the 250 u.c.-thick film (PO2 = 50 mTorr) and a 50 u.c. (PO2 = 20 mTorr) film, respectively. However, the films exhibit different out-of-plane lattice parameters. As an example, Figure 7c displays 2θ−ω XRD curves around the STO (002) showing different positions of the LCMO (002) Bragg peaks for these films. Figure 7d summarizes the measured out-of-plane lattice parameters obtained for different deposition conditions. For depositions on STO, an increasing trend with increasing thickness at PO2 = 50 mTorr is evident (red squares). In contrast, for PO2 = 20 mTorr, the out-of-plane lattice parameter is significantly smaller (blue circle). As a reference, the out-ofplane lattice parameter of the film deposited on NGO at PO2 = 50 mTorr is largest among all the films and exceeds the bulk value by about 1% (magenta diamond); the films on NGO are, therefore, under compressive in-plane strain. Apparently, the out-of-plane lattice parameters of the films on STO are below the bulk value due to the in-plane tensile strain imposed by the substrates with the largest in-plane strain achieved in the films deposited at 20 mTorr. To better identify the cationic content, we performed characterization of magnetic and electron transport properties of the 50 u.c. films deposited at 1.2 J/cm2 and a 50 mTorr 2712

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transition temperature. The insulator-to-metal transition takes place at the transition temperature of the matrix. The films deposited at 20 mTorr do not show an insulator-to-metal transition down to a temperature of 11 K, which is attributed to a lower oxygen content. We note that the reduction of the oxygen content by as little as 2% suppresses the insulator-tometal transition completely in LCMO films on STO.37

4. DISCUSSION Evolution of the Step Edge Barrier. The instability accompanied by formation of self-assembled structures in the form of 3D mounds has been routinely observed in epitaxial growth of a wide range of material systems including metals, semiconductor, and complex oxide thin films. Most frequently, the instability is associated with a higher step edge energy barrier (Erlich−Schwoebel barrier) for adatoms to descend steps resulting in reduction of downhill atomic fluxes.38−40 However, other mechanisms have been proposed as well.41 In our films, the atomically resolved images in Figure 6b clearly reveal concentration of adatoms along the terrace edges, which strongly provide evidence in favor of the Ehrlich−Schwoebel barrier as a cause of the mound instability. In another publication,42 the height of the Ehrlich−Schwoebel barrier for the 20 mTorr films of this study was estimated to be 0.18 ± 0.04 eV for the deposition temperature of 750 °C using a computer-assisted STM image analysis and Monte Carlo simulations based of a simple termination surface model. Comparing the STM images of the films deposited at different pressures and taking into account the X-ray data in Figure 7d, it becomes apparent that a higher Ehrlich−Schwoebel barrier is associated with the smaller out-of-plane lattice parameter, and the barrier is highest in the 20 mTorr films. Apparently, a lower transparency of the Ehrlich−Schwoebel barrier is common to all films deposited on STO at 20 mTorr (with RHEED behavior similar to Figure 1c) and for the films with thicknesses below ∼20 u.c. deposited at 50 mTorr (up to peak I in Figure 1a). The growth mode change observed with the 50 mTorr films on STO (Figure 1a) can be interpreted so that the Ehrlich−Schwoebel barrier height becomes progressively smaller with increasing thickness and out-of-plane lattice parameter until a regime of the nearly perfect layer-bylayer growth sets in. It is worth noting that previously, a similar effect was reported for heteroepitaxial growth of Ag on Pt(111) surfaces; it was, however, associated with the in-plain strain of the films.43,44

Figure 7. (a) and (b) X-ray reciprocal space maps around the LCMO (013) diffraction peak for two films on STO substrates: (a) for a 250 u.c. film deposited at PO2 = 50 mTorr and a laser fluence 2.6 J/cm2 and (b) for a 50 u.c. film deposited at PO2 = 20 mTorr and a laser fluence 1.2 J/cm2. (c) X-ray diffraction 2θ−ω curves around the (002) peaks of LCMO films shown in panels (a) and (b). (d) LCMO out-of-plane lattice parameter and cell volume increase versus film thickness for films deposited on STO (red squares and blue circle) and NGO (magenta diamond) substrates. Error bars at the data points for the films deposited at 50 mTorr on STO (red squares, line is a guide to the eye) reflect variations of the film lattice c-parameter from run to run, including those due to variations of the laser fluence. The film on the NGO substrate was deposition at PO2 = 50 mTorr and a laser fluence 2.6 J/cm2. For this data point, the right axis is not applicable.

oxygen pressure. Measurements of film magnetization as a function of temperature using a SQUID magnetometer yield a Curie temperature of about 150 K (Figure 8a,b). At the same time, the insulator-to-metal transition (evidenced by the resistance drop on cooling in Figure 8c) starts at a temperature of approximately 105 K with only a kink at 150 K in the resistance-vs.-temperature curve. Such a difference clearly points to film inhomogeneity. It can be inferred that the films can be described as consisting of a network of nearly stoichiometric and isolated atomic-scale clusters with higher Curie and insulator-to-metal transition temperatures. The clusters are surrounded by a Mn-deficient matrix with a lower

Figure 8. Magnetic and electric transport data for a 50 u.c.-thick film deposited on an STO substrate at PO2 = 50 mTorr and a laser fluence 1.2 J/cm2. (a) Magnetization χ and its derivative over temperature dχ/dT as functions of temperature. The data were collected on heating after cooling in a magnetic field B = 1 T. (b) Magnetic hysteresis loops obtained at room temperature and at T = 10 K. (c) Resistance as a function of temperature. Arrows indicated the direction of the hysteresis. 2713

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of-plain lattice parameter with thickness found in our experiments. The change of the strain accommodation mechanism and generation of volumetric defects are expected to influence strain fields associated with island edges as well, thereby affecting the Ehrlich−Schwoebel barrier height and the growth mode. At PO2 = 20 mTorr, films remained fully strained with preservation of the mound surface morphology. This stabilization may be attributed to the role of oxygen in the activity of different channels of the strain relaxation. In oxides, defect structure and distribution follow the change of the oxidative power of the environment.19,49,50 Therefore, it is possible that at lower oxygen pressure, the formation of the strain-accommodating defects is suppressed due to reduced oxygen content in the films. Adatom Mobility. We turn to the discussion of the adspecies mobility in the layer-by-layer growth mode, which also shows a noteworthy increase with the increasing thickness. Indeed, inspection of the images in Figures 2−4 allows tracing the evolution of the adatom surface mobility with the film thickness. Larger mobility on the surface of the 250 u.c. film can be deduced from the notably larger average adatom island size as well as a larger average distance between adatom islands and vacancy islands in Figure 2c. The smallest island size and distance between islands, as well as the largest density of the vacancy island are seen in the 25 u.c. films (Figure 2b). The adatom mobility is, therefore, the smallest for these films. The 50 u.c. films exhibit an intermediate situation (Figure 4). It is evident that there is a correlation between the adspecies mobility and the out-of-plane lattice parameter of the 50 mTorr films on STO. This suggests that the out-of-plane lattice parameter, and hence, the associated strain, is a key factor determining the surface energy landscape and in controlling the adspecies mobility in the nonstoichiometric films. In this respect the oxide surface resembles that of metals and semiconductors, where the effect of strain on the surface diffusivity is well established.51 Taking into account the large size of the adatom islands on the surface of the 250 u.c.-thick film, the atomic-scale structural disorder of the B-site termination, which is universal to all films of the 50 mTorr series, may seem counterintuitive. Apparently, it cannot be associated with a low adatom mobility and should be attributed to other factors. A high kinetic energy of particles in the laser plume cannot be a cause of the disorder either, since the very same atomic-scale disordered morphology was observed in films deposited at laser fluences ranging from 0.8 J/cm2 to 2.6 J/cm2, where the energy is expected to change significantly (based on the mere observation of the visible laser plume size, which was about three times smaller than the substrate-target distance at the lowest fluence). Taking into account the above-noted correlations between mobility and the out-of-plane lattice parameter, this structural disorder shows the disorder of the potential energy seen by the adatoms on the surface. We speculate that the potential energy at a site of an island edge, where adatoms can come to rest in the layer-bylayer growth mode, is determined not only by the local surface and island edge structure, but also the atomic structure in the layer just below the surface. Defects in the subsurface layer lead to random variations of the potential energy along the edges of growing islands on top, which is reflected in the structural disorder of the B-site terminated surface. Such “electronic roughness” is readily seen in STM images through the ordered

In the perovskite oxides, the out-of-plane lattice parameter is a sensitive indicator of the film stoichiometry. The increased lattice parameter in our manganite films is a sign of cationic nonstoichiometry and/or oxygen deficiency in the films.25,26,45 The repeatedly observed disorder and numerous vacancy islands on the MnO2 termination suggest that the films are Mndeficient. This result is consistent with ref 25, where transition from Mn-enrichment to Mn-deficiency was reported while changing the fluence from 0.35 J/cm2 to 0.63 J/cm2 (by altering the laser spot size) in ablation of a similar manganite compound La0.7Sr0.3MnO3. It is also in accord with a theoretical model by Saenger,9 which predicts that with increasing fluence, the directional pattern of species in the plume will change from more forward-peaked distribution of light species to heavier species. Therefore, the Mn-deficiency in the films may be related purely to plume dynamic effects. Albeit, other effects such as preferential sputtering of Mn on the film surface by energetic species in the plume46 and preferential evaporation of the more volatile Mn from the film surface can also increase the Mn-deficiency. From this perspective, the mounded surface morphology is noteworthy. We note that the mounded morphology provides a specific mechanism of nonstoichiometry accommodation, where A-cation excess results in double-terminated films grown on B-site single-terminated substrates, with edges of the A-terminated terraces capturing the excess A-site cations. In such a case, motion of the terrace edges during growth takes place over a cationically nearly stoichiometric terminations resulting in a high atomic-scale structural perfection of the top layers and eventually cationic stoichiometry of the film bulk. In this scenario, the noticeably smaller out-of-plane lattice parameter of the films with the mounded surface structure can be interpreted so that the composition of the film bulk is closer to stoichiometric. As concluded above, the transition from 3D to layer-by-layer growth (as seen in Figure 1a) in films deposited at PO2 = 50 mTorr reflects reduction of the Ehrlich−Schwoebel barrier. Note that the reduction cannot be associated with the kinetic energy of the arriving particles. At the lower pressure of 20 mTorr, particle kinetic energy is higher,24,47,48 and this should facilitate the downhill particle transport through the Ehrlich− Schwoebel barrier, contrary to the observations. Taking into account the behavior of the out-of-plane lattice constant, it can be speculated that the reduction of the Ehrlich−Schwoebel barrier at PO2 = 50 mTorr occurs due to change of the mechanism of accommodation of the epitaxial strain in the films during growth. Namely, at the initial stage of growth, excess A-site cations are accommodated with formation of mounds. The film is fully strained, both in and out of plane, and the film bulk is cationically nearly stoichiometric. The full tensile epitaxial strain enhances the Ehrlich−Schwoebel barrier at the island step edges, which leads to the mound instability. Once a critical thickness and a critical amount of the strain energy are reached, the out-of-plane strain and the mechanism of nonstoichiometry accommodation change. The exact mechanism of the strain relaxation is a subject of further studies. Tentatively, generation of (La,Ca)O planar inclusions (faults) can be proposed, a scenario resembling that found in the SrTiO3 films with Sr-excess on NGO substrates by Breckenfeld et al.23 The volumetric defects serve as sites for progressive nucleation of misfit dislocations relaxing the tensile epitaxial strain. This reasonably explains the increase of the out2714

DOI: 10.1021/acs.cgd.5b01826 Cryst. Growth Des. 2016, 16, 2708−2716

Crystal Growth & Design B-termination as evident from the inset in Figure 6c. The bright protrusions seen through the ordered structure in the images remarkably resemble those on the rutile TiO2(110) surface, where they were produced by subsurface impurities. 52 Furthermore, it was evidenced that the chemical activity of the surface at the impurity sites is altered.53 Finally, we also emphasize that the smoother surface of the B-terminated terraces in films grown in the 3D mode, especially at PO2 = 20 mTorr, is indicative of a higher mobility of adatoms. This enhanced mobility is clearly visible in the relaxation of the intensity of the RHEED specular spot toward higher values right after the end of deposition in Figure 1c. Relaxation after stop of depositions in the layer-by-layer mode was significantly weaker or even had the opposite direction as seen in Figure 1b for the NGO substrate. This behavior can be linked to the cationic composition of the mound terraces by analogy with the results of refs 24 and 25, where the highest mobility of the adatoms was found to be corresponding to the stoichiometric compositions. The higher structural perfection of the surface and subsurface atomic layers associated with the nearly perfect cationic stoichiometry of terraces can only intensify this effect.

ACKNOWLEDGMENTS

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REFERENCES

This research was sponsored by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division (A.T., R.K.V., A.G.G., T.L.M., H.N.L., S.V.K.). Research was conducted at CNMS, which also provided support (A.P.B., M.D.B., L.Q.) and which is a DOE Office of Science User Facility. A.G. acknowledges fellowship support from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education.

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5. CONCLUSION In conclusion, employing a combination of in operando RHEED with in situ atomically resolved STM imaging and XRD, we explored effects of varying laser fluence, oxygen pressure, and epitaxial strain on growth mode and atomic-scale surface structure in PLD of LCMO films on STO substrates. We observe a correlation between out-of-pane lattice parameter and growth mode. The tensile epitaxial strain causes the appearance of a non-negligible barrier for the downhill adatom transport resulting in mound instability of the film surface (3D growth mode). However, at 50 mTorr deposition pressure, the 3D growth transforms into a layer-by-layer mode with increasing thickness, signaling disappearance of the barrier. In the layer-by-layer regime, the out-of-plane lattice parameter of the films increase with increasing thickness accompanied by an increase in the adatom surface mobility. The mound instability could be preserved by a relatively small reduction of the background oxygen pressure down to 20 mTorr, which was accompanied by a significant decrease of the out-of-plane lattice parameter. These observations are interpreted as evolution of strain and nonstoichiometry accommodation mechanisms in the films as a function of oxidative power of the environment. In the mounded structure, each terrace is atomically smooth and remains nearly cationically stoichiometric as evidenced by the high degree of atomic order revealed by STM imaging. We observed clear evidence of a larger adspecies mobility on the stoichiometric surfaces. STM images reveal a pattern of extended nontopographic features over the cationically stoichiometric surfaces, which can be ascribed to presence of subsurface defects.

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AUTHOR INFORMATION

Corresponding Authors

*(A.T.) E-mail: [email protected]. *(S.V.K.) E-mail: [email protected]. Present Address

# School of Materials, The University of Manchester, Grosvenor Str., Manchester M13 9PL, UK.

Notes

The authors declare no competing financial interest. 2715

DOI: 10.1021/acs.cgd.5b01826 Cryst. Growth Des. 2016, 16, 2708−2716

Crystal Growth & Design

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NOTE ADDED IN PROOF After the paper was accepted for publication, the authors became aware about another similar study of homoepitaxy of SrTiO3 on SrTiO3(110) in ref 54.

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DOI: 10.1021/acs.cgd.5b01826 Cryst. Growth Des. 2016, 16, 2708−2716