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Pulsed laser deposition of SmFeAsO1−δ on MgO(100) substrates
Accepted Manuscript Title: Pulsed laser deposition of SmFeAsO1−δ on MgO(100) substrates Author: Silvia Haindl Hiroyuki Kinjo Kota Hanzawa Hidenori Hiramatsu Hideo Hosono PII: DOI: Reference:
S0169-4332(17)32393-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.08.061 APSUSC 36898
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-2-2017 28-6-2017 7-8-2017
Please cite this article as: Silvia Haindl, Hiroyuki Kinjo, Kota Hanzawa, Hidenori Hiramatsu, Hideo Hosono, Pulsed laser deposition of SmFeAsO1minusrmdelta on MgO(100) substrates, (2017), http://dx.doi.org/10.1016/j.apsusc.2017.08.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Pulsed laser deposition of SmFeAsO1−δ on MgO(100) substrates Silvia Haindla,b 1 , Hiroyuki Kinjoa , Kota Hanzawaa , Hidenori Hiramatsua,c , Hideo Hosonoa,c a Laboratory
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for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology, mailbox R3-1, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan b Physikalisches Institut, Universit¨ at T¨ ubingen, 72076 T¨ ubingen, Germany c Materials Research Center for Element Strategy, Tokyo Institute of Technology, Mailbox SE-6, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan
Abstract
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Layered iron oxyarsenides are novel interesting semimetallic compounds that are itinerant antiferromagnets in their ground state with a transition to hightemperature superconductivity upon charge carrier doping. The rare earth containing mother compounds offer rich physics due to different antiferromagnetic orderings: the alignment of Fe magnetic moments within the FeAs sublattice, which is believed to play a role for the superconducting pairing mechanism, and the ordering of the rare-earth magnetic moments at low temperatures. Here, we present thin film preparation and a film growth study of SmFeAsO on MgO(100) substrates using pulsed laser deposition (PLD). In general, the PLD method is capable to produce iron oxyarsenide thin films, however, competition with impurity phase formation narrows the parameter window. We assume that the film growth in an ultra-high vacuum (UHV) environment results in an oxygendeficient phase, SmFeAsO1−δ . Despite the large lattice misfit, we find epitaxial oxyarsenide thin film growth on MgO(100) with evolving film thickness. Bragg reflections are absent in very thin films although they locally show indications for pseudomorphic growth of the first unit cells. We propose the possibility for a Stranski-Krastanov growth mode as a result of the large in-plane lattice misfit between the iron oxypnictide and the MgO unit cells. A columnar 3-dimensional film growth mode dominates and the surface roughness is determined by growth mounds, a non-negligible parameter for device fabrication as well as in the application of surface sensitive probes. Furthermore, we found evidence for a stratified growth in steps of half a unit cell, i.e. alternating growth of (FeAs)− and (SmO1−δ )+ layers, the basic structural components of the unit cell. We propose a simple model for the growth kinetics of this compound. Keywords: thin films, pulsed laser deposition, iron oxypnictides ∗ Corresponding
author
1 [email protected]
Preprint submitted to Journal of LATEX Templates
June 28, 2017
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2010 MSC: 00-01, 99-00
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The quaternary iron oxyarsenide SmFeAsO is a layered semimetal compound with ZrCuSiAs-type structure (P 4/nmm). It was first synthesized in 2000 by Quebe, Terb¨ uchte and Jeitschko [1] who already pointed out that oxygen stabilizes the structure. They reported an a-axis lattice parameter of 3.940(1) ˚ A and a c-axis lattice parameter of 8.496(3) ˚ A, but superconductivity remained a ‘sleeping beauty’ until 2008, when Kamihara et al. [2] reported a Tc of 26 K in the fluorine-doped iron oxyarsenide LaFeAsO. Promptly one month later superconductivity was confirmed in rare earth analogs. Their announcement on the arXiv preprint server triggered the hype for the new iron pnictide superconductors.[3] Until today the highest Tc in the rare-earth oxyarsenide superconductors (‘1111’ - so-called ‘eleven-eleven’) is achieved in F-doped SmFeAsO: approximately 55 K is commonly reported after [4]; a record of 58.1 K was found in F-doped and FeAs-free SmFeAsO.[5] Apart from superconductivity the undoped mother compound offers itself many interesting physical aspects of either different antiferromagnetic orderings in the two sublattices or the tetragonal-to-orthorhombic phase transition in vicinity to the antiferromagnetic ordering of the FeAs sublattice. Especially the latter mentioned phases are believed to have a strong influence on the unconventional superconducting pairing mechanism. Until today a clear separation between purely electronic and purely structural effects on the ordered quantum phases is difficult and is a matter of debate.[6] The majority of studies on SmFeAsO have been carried out on powders or polycrystalline samples, because single crystal growth seems to be nontrivial at very high temperatures of around 1300◦ C or needs a high-pressure cubic anvil technique.2 Single crystal growth of the 1111 phase is still rare and alternative sample preparation techniques such as epitaxial thin film growth methods are highly demanded. Unfortunately, we only have limited knowledge about iron oxyarsenide thin film growth, although a number of reports have been published during the last years with primary interest in superconductivity. We know that epitaxial thin films can be grown by pulsed laser deposition (LaFeAsO, [9]; Fdoped SmFeAsO, [20]) and by molecular beam epitaxy (NdFeAsO, [10]; F-doped SmFeAsO, [11, 12]). In the last decade the problem of F-doping was treated as the main issue in most thin film studies.3 So far, basic film growth aspects have not been an issue. Especially in PLD the growth of iron oxyarsenides suffers
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1. Introduction
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2 The first iron oxyarsenide single crystals were reported by Zhigadlo et al. in Ref. [7] (micrometer size) and later from Ames Laboratory in Ref. [8] (millimeter size). 3 For example, in the case of molecular beam epitaxy (MBE) F-doping was achieved by a F-containing cap layer or by a F-containing substrate. Most recently, progress in the in-situ PLD of F-doped SmFeAsO thin films was made, where CaF2 substrates were used for F-doping but, as we believe, also for stabilizing the ZrCuSiAs structure [20, 21]. A two-step process based on an ex-situ heat treatment was firstly employed successfully for F-doped LaFeAsO in
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from a general off-stoichiometry and oxygen deficiency,4 that is probably one of the main reasons why iron oxypnictide thin film growth has been so difficult to achieve with this method in the past years. In a new approach on PLD of iron oxyarsenides we separate the doping issue from the general film growth analysis. In the following we, therefore, focus on the undoped and oxygen-deficient SmFeAsO compound where we investigate thin film growth on MgO(100) substrates using pulsed laser deposition (PLD). MgO single crystalline substrates are common in thin film growth because of their high melting point, their low cost and their usefulness in different applications. Although MgO is cubic (rocksalt structure) the large lattice misfit strain of (as − af )/af ≈ 7% (at room-temperature) is rated as unfavorable for heteroepitaxial growth of SmFeAsO. as ,af denote the relaxed in-plane lattice parameters of substrate and film, respectively. The thermal expansion of SmFeAsO is unknown for temperatures of several hundred ◦ C but the lattice misfit is not expected to decrease at such high deposition temperatures. Film growth of SmFeAsO on MgO(100) starts, therefore, at the limit of a pseudomorphic growth condition and the critical thickness for an eventually occurring epitaxially strained layer would be only a few unit cells.[23] In this model film growth quickly becomes unstable with increasing thickness and strain release is achieved by the introduction of misfit dislocations with a transition to StranskiKrastanov (SK) growth mode and the preference for 3D columnar growth[24]. The course of this paper is as follows: We will first report on the phase formation and film properties for varying deposition parameters in UHV PLD. Subsequently, based on the evolution of film parameters and surface morphology with increasing thickness, we will discuss the film growth mode in detail. Finally, we propose a simple model that qualitatively captures the essential features of the growth kinetics of this compound.
2. Experimental Procedures
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2.1. Thin Film Preparation For the PLD process in an UHV environment (base pressure ≈ 10−8 mbar) a Spectra Physics Quanta-Ray INDI-40 pulsed Nd:YAG laser (2ω) with a laser wave-length of 532 nm and a laser repetition rate of 10 Hz was used. The peak
Ref. [13] and improved in Refs. [14, 15]. In this case, F-doping was achieved within an ex-situ heat treatment. Finally, film fabrication using metal-organic deposition was attempted for Co-doped NdFeAsO [16] after F-doping turned out to be too complicated. A summary of the progress in thin film growth including iron oxyarsenide superconductors can be found in the review articles [17, 18, 19]. 4 This fact is supported by systematic chemical investigations of bulk synthesis in different atmospheres.[22] Recently, it was shown that oxygen deficiency in SmFeAsO1−δ does not lead to superconductivity as prematurely announced in 2008. In oxygen-deficient iron oxyarsenides the a-axis and c-axis lattice parameters stay nearly unchanged even for large deficiencies (δ ≈ 0.4) compared to a lattice shrinking after element substitution at the oxygen-site. In addition, the fraction of impurity phases like SmAs and Fe notably increase with increasing δ.[22]
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laser energy was measured in front of the laser entrance window of the UHV chamber before and after each film fabrication. The resulting energy density of 2.15 ± 0.63 Jcm−2 on the target is approximated by measuring the imprinted area (ellipse) of the laser spot on the target surface and by taking a 20% loss through the window of the UHV chamber into account. We used a SmFeAsO target prepared by a two-step solid state reaction as described earlier in Ref.[20]. The vertical target-substrate distance during deposition is 2.4±0.2 cm. It was adjusted according to preliminary film growth results. Comparison of the visible extension of the plasma plume upon different laser energies (Fig.1) confirms the limits for a good configuration of the substrate position. A mean target ablation rate of around 3.17 nm s−1 (Table 1) was determined by the weighting the target mass. This method gives a rough estimation of the ablation rates for an uniformly assumed (i.e. idealized) thickness reduction. However, the actual depth of the crater on the target surface generated by the laser pulses must be assumed to be larger (µm size). Repolishing of the target resulted in a thickness reduction of around 200µm. MgO(100) substrates from Furuuchi Chemical Co. with dimensions of 10×10×0.5 mm3 were used. A pre-heat-treatment for MgO(100) substrates at 800◦ C for 1 hour was undertaken ex-situ in a tube furnace. We also compared film growth after a pre-heat-treatment at 1000◦ , however the conditions of the substrate surface were less controllable after annealing (see Appendix for a detailed discussion and Figs.10 and 11). Substrates were then inserted into a high-vacuum loadlock. Before film deposition the substrate was heated in about 1 hour to the deposition temperature Tdep where it was kept for 5 minutes. Substrate heating was performed using a high power laser diode (LUMICS) operating with a wavelength of 975 nm. Temperature control was carried out using a pyrometer (InGaAs detector, spectral range 1.95 – 2.50 µm) and a thermocouple. The thermocouple is, however, not in direct contact with the thin film and monitors the temperature in a distance several cm away from the substrate. The temperatures given by the pyrometer were calibrated in a previous experiment. The measurement error at 800◦ C is ±4◦ C. In the following all given temperatures refer to the calibrated ones.
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Figure 1: Extension of the plasma plume of an irradiated SmFeAsO target upon variation of the Nd-YAG (2ω) laser energy between 20 and 27 mJ.
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∆m / time (µg×s−1 )
∆t/ time (nm×s−1 )
25±1 25±3 25±1
5647 11400 17820
0.992 0.939 0.993
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Table 1: Ablation rate measurements for a SmFeAsO sintered target. The idealized thickness reduction ∆t per time is calculated from the measured mass reduction ∆m per time. The data is valid for the ablation using a Nd:YAG(2ω) laser with repetition rate of 10 Hz. For a measured laser energy of 25 mJ the resulting energy density on the target surface is 2.15 ± 0.63 Jcm−2 .
2.2. Thin Film Crystallography, Chemical Analysis and Surface Morphology
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Films were characterized using a Rigaku Smart Lab with CuKα radiation. Standard X-ray diffraction (XRD) θ/2θ-scans were carried out in Bragg Brentano geometry. High resolution scans and X-ray reflectivity (XRR) measurements were carried out in parallel beam geometry (using a cross-beam optics unit and a Ge(220) monochromator in the incident beam path). We have evaluated the c-axis lattice parameter from SmFeAsO(00`) reflections with ` = 1,2,3,5 and 6 by means of a linear extrapolation versus the Nelson Riley function cos(θ)cot(θ)+cos2 (θ)/θ.[25] X-ray reflectivity (XRR) measurements were performed using a Rigaku Smart Lab (CuKα) with a cross-beam optics unit and a Ge(220) monochromator in the incident beam path. We used the GlobalFit program for analyzing the XRR results. Film surface morphologies, grain size as well as film surface roughness were analyzed after preparation using a Bruker AXS MultiMode8 Atomic Force Microscope (AFM) in non-contact mode. The used silicon tips on nitride cantilevers have a resonant frequency of 130 ± 30 kHz and a spring constant of 0.4 Nm−1 (ScanAsyst-Air-HR probes). Scans were performed on 512×512 pixel arrays. The resulting AFM images were flattened by a 3rd order polynomial. Film surface roughness analysis was performed on AFM images after flattening. The root-mean-square (rms) roughness was calculated for 1×1 µm2 and 500×500 nm2 scans. Image and surface roughness analysis was performed after flattening using the WSxM software developed in Ref.[26]. Wavelength-dispersive X-ray fluorescence (XRF) measurements were performed with a Bruker S8-3F Tiger spectrometer in order to estimate the Ca impurity content in MgO substrates.
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3.1. Temperature-Energy-Diagram The effect of deposition temperature and energy density was studied in the range of 700◦ C – 900◦ C and for three different laser energy regimes 22 mJ ± 5
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1 mJ, 25 mJ ± 1 mJ and 28 mJ ± 1 mJ. We confirmed that the small parameter window for single phase and epitaxial iron oxypnictide film growth is extremely small under UHV conditions and when fluorine or oxygen are not supplied. We successively varied the deposition temperature around the optimal value of 860◦ C (Fig. 2). A qualitative difference is observed for deposition temperatures below 850◦ , where SmFeAsO crystallites with (201) and (101) orientation appear. Crystallites with c-axis orientation (00`) appear beyond 800◦ C. A SmAs(002) impurity reflection is present in all samples and its intensity grows with increasing temperature. Additionally, above 875◦ C the Sm2 O3 (222) impurity peak increases. At temperatures between 845◦ C and 860◦ C, films with a large fraction of c-axis oriented SmFeAsO grains could be grown. As mentioned above, all films contain also a small amount of Fe impurity, however Fe(110) and possibly also the Fe(002) reflections are covered by MgO(002) and SmFeAsO(006) peaks and, therefore, not detectable in X-ray diffractograms.
Figure 2: Smoothed XRD θ/2θ-scans (Cu Kα) of SmFeAsO films deposited at different temperatures (E = 25 mJ ± 1 mJ). At the bottom a θ/2θ-scan for MgO is shown for comparison of the background. Left: Full 2θ-range from 5◦ to 75◦ with indexed SmFeAsO(00`) peaks. Right: Zoomed detail (shaded area) from 2θ = 24◦ to 38◦ with the evolution of SmFeAsO(101), SmFeAsO(102), SmFeAsO(003), F = SmAs(002) and = Sm2 O3 (222) reflections with varying deposition temperature.
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A similar trend was found for laser energies of 22 mJ ± 1 mJ and 28 mJ ± 1 mJ. The results are summarized in a qualitative deposition parameter diagram spanned by substrate temperature and laser energy (Fig.3). With increas6
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ing laser energy the c-axis lattice parameter of the films grows. We attribute this effect to the increasing film thickness as it is clear from Fig.1 that with increasing laser energy more material is deposited in the same time interval. The variation of the c-axis lattice parameter with increasing film thickness is discussed below. The temperature dependence of the c-axis parameter is more subtle and displays a maximum near the optimal deposition temperature. Since the amount of impurity phases and misaligned grains varies with substrate temperature as well (compare Fig.2) a final interpretation is difficult. (b)
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Figure 3: (a) Qualitative diagram of SmFeAsO film growth in dependence of substrate temperature and laser energy. Note that Fe and SmAs impurities appear always. Below the optimal growth temperature SmFeAsO(h0`) reflections indicate the presence of misoriented grains, above the optimal temperature the Sm2 O3 impurity content increases. (b) c-axis lattice parameter as function of laser energy. All films were deposited for 10 min in an temperature interval of 560±30◦ C. (c) c-axis lattice parameter as function of substrate temperature for films deposited for 10 min and at a laser energy of 25±1 mJ (shown in Fig.2).
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3.2. Epitaxial Relationship For SmFeAsO films that contain only c-axis oriented grains we confirmed epitaxial growth by in-plane φ-scans. The epitaxial relationship is SmFeAsO(100)[100]kMgO(100)[100] (see Fig. 9 in the Appendix).
3.3. Impurity phases and misoriented grains at the film surface Misoriented grains of the SmFeAsO phase that deviate from c-axis orientation as well as impurity phases (most likely Sm2 O3 grains due to their orientation along the space diagonal) are visible in AFM scans and are responsible for a characteristic surface morphology. In comparison with a film grown near optimally conditions (Fig.4(a)) where the surface is covered uniformly with columnar grains and trenches between them, the film grown at lower temperatures 7
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already displays elongated grains along the MgO[100] and MgO[010] direction (Fig.4(b)). Impurity grains are much larger and of pyramidal shape (Fig.4(c) and (d)). 175
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Figure 4: XRD θ/2θ-scan (top), AFM scan (middle) and schematic annotation of the coverage of misoriented grains (bottom) for films grown at (a) E= 25.7 mJ, T = 875◦ C (b) 25.2 mJ, T = 845◦ C (c) E= 29.3 mJ, T = 880◦ C (d) E = 22.2 mJ, T = 880◦ C. Optimal growth conditions result in a uniformly dense covering with SmFeAsO mounds as given in Fig.(a).
3.4. Evolution of film structure and surface morphology of SmFeAsO thin films with thickness
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The evolution of the surface morphology of SmFeAsO thin films was studied with increasing thickness close to optimal growth conditions around T = 860◦ C – 870◦ C and E = 25 mJ (Fig.5). A deposition for 10 seconds results in a total film thickness of 2.5 nm showing a fine granulation that already uniformly covers the substrate surface although steps of the substrate surface can still be traced. Similarly, the thin film surface after a deposition time of 30 seconds shows still traces of steps on the initial substrate surface. The film layer covers
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the substrate, however small pits appear. It is possible that these pits in the film surface match original pits on the substrate surface as found in an AFM scan of the bare substrate.5 Considerably increased granularity is observed after a deposition of 1 min. It might be possible that the initial wetting of the substrate is not complete, however, the AFM analysis suggests that the 3-dimensional (3D) like growth of mounds (’wedding cakes’) is driven by stress release. With increasing deposition time a tendency to the formation of steep mounds can be traced indicating a dominance of a 3D growth mode driven by strain. For deposition times larger than 2 min coalescence of mounds is found at the bottom of the AFM scale, new mound formation can be seen at the top of the AFM scale with quadratic shape and facets along MgO<100>. A schematic model is given in the bottom panel of Fig.6. The observed small terrace width is indicative of an existing strong step edge barrier in accordance with the idea that the time for adatoms or -clusters residing on the top island is very large compared to the diffusion time, i.e. the time an adatom or -cluster needs for travelling across the top layer. Driven by the question if a strained SmFeAsO epilayer can be found we look
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Figure 5: Evolution of surface morphology of SmFeAsO thin films with increasing deposition times: (1) 0.17 min, (2) 0.5 min, (3) 1 min, (4) 1.5 min, (5) 2 min, (6) 3 min, (7) 5 min and (8) 10 min. Coverage of the substrate surface is found already for a film with a thickness of 2 nm, although substrate terraces, steps and pits are still visible in the AFM images (1) - (3). The maximum value of the scale is indicated above each image. All scans are made on an area of 1×1 µm2 . For an annotation see Fig.12 in the Appendix.
at the AFM scan on the thinnest film grown. If pseudomorphic growth appears 5 For
a discussion of the MgO substrate surface the reader is referred to the Appendix.
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and under the assumption of preserved unit cell volume under biaxial tensile strain (constant Poisson ratio) we expect a c-axis lattice parameter of the first unit cell to be around 7.44˚ A. Line scans in the AFM scan of the film deposited for 10 seconds (top panel in Fig.6) indicate individual island heights of around 7.44˚ A. However, higher resolution or in-situ analytical methods are required in order to make a conclusive statement. At present, a strained SmFeAsO layer for film thicknesses below 2.5 nm is possible. Consequently, the critical thickness of such an epitaxially strained layer can be estimated to the size of 1 - 3 unit cells. The evolution of thin film crystallinity begins with a deposition time of 1.5 min (Fig.8(a)): SmFeAsO(00`) Bragg reflections are absent in very thin films with thicknesses below 11 nm and start to occur after a deposition time of 1.5 minutes (corresponding to a total film thickness of 11.3 nm). A limiting case is the film deposited for 1 min and a resulting film thickness of 10.5 nm with a enhanced background close to the Bragg peak positions. Although no clear SmFeAsO (00`) reflections could be observed the film surface consists already of steps of height of a unit cell, c, and of a half unit cell, c/2 (with c ≈ 8.5 ˚ A). This result indicates the stratification in the growth of iron oxypnictide unit cells according to its building blocks, with subsequent growth of (SmO)+ and (FeAs)− layers (middle panel in Fig. 6). The different polarity of the layers may have an additional influence on the 3D like growth and the step edge barrier.
A possible reason for the absence of more distinct Bragg reflections in very thin SmFeAsO films can be a strong perturbation of the ordered structure of subsequent (SmO)+ and (FeAs)− layers caused by the introduction of screw dislocations and leading to a large mosaicity in accordance with the X-ray results. Furthermore, the AFM results show that SmFeAsO epilayers in very thin films are not uniformly strained across the whole substrate which also leads to a loss in crystallographic lattice coherence. Total film thicknesses were extracted from Kiessig fringes of the X-ray reflectivity (XRR) measurements (Fig.8(b)). For the fit procedure a two-layer model was used with a SmFeAsO layer of density ρ1 ≈ 7.493 gcm−3 and an approximately 1 nm thick surface layer of low density (see details in Table 2). The roughness of the MgO substrate was set to a value close to 1.0 nm. Fit results of the XRR scans are shown as thin lines in Fig.8(b). The total film thickness, tXRR , between 20 nm and 100 nm is proportional to the square-root of the deposition time (Fig.8(c)).
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SmFeAsO 001 Bragg reflections were modeled by Laue functions in order to estimate the coherently grown film thickness, usually smaller than the total film thickness evaluated from XRR measurements (Fig.7, last three columns in Tab.2). Analysis of the c-axis parameter in SmFeAsO thin films is less trivial. We find that the maximum c-axis lattice parameter increases with √ total film thickness follows a square-root law: c(˚ A)= 8.59 + 0.80 × 10−2 × tXRR − 14 with tXRR given in nm. The increase in c-axis can be understood as result of the lattice 10
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Figure 6: Top panel: Topography of the film surface after deposition for 10 seconds with steps of unit cells and half unit cells: AFM image (200 nm × 200 nm) with positions of the line scans indicated; three different line scans in random directions. The reduced value of the c-axis lattice parameter for a strained epilayer, cs ≈ 7.44˚ A, is indicated (red). Step-sizes of cs , cs /2 but also of c ≈ 8.5 ˚ A (black) can be measured. Middle panel: Topography of the film surface after deposition for 1 min with steps of unit cells and half unit cells: AFM image (250 nm × 250 nm) with positions of the line scans indicated; line scan along SmFeAsO[100]kMgO[100] and line scan along SmFeAsO[010]kMgO[010]. The film shows an enhanced X-ray diffraction background but lacks distinct Bragg reflections, nevertheless step-sizes of c ≈ 8.5 ˚ A and c/2 can be measured. Bottom Panel: Topography of the film surface after deposition for 10 min with evolution of growth mounds. AFM image (200 nm × 200 nm) and two different contour levels (coloured) separated by one unit cell. A schematic growth model of the film growing on the substrate (S) is presented.
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b)
r1 (nm)
r2 (nm)
ρ2 (gcm−3 )
11.3 16.1 22.0 45.4 72.5 73.3 91.1
10.6 15.1 20.9 44.3 70.6 72.5 90.3
0.7 1.0 1.1 0.9 1.9 0.7 0.8
1.8 1.8 1.9 1.3 1.8 1.8 1.3
0.3 0.3 0.2 0.3 <0.1 0.2 0.2
3.1 3.4 3.0 2.6 2.1 1.6 2.0
F W HM (◦ )
N
0.67 0.44 0.25 0.18
12 23 46 78
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tLaue (nm)
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10.3 19.8 39.7 67.0
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Table 2: Fit parameters of XRR results using a two layer model (a) shown in Fig.4(b); b) shown in Figs.8(b) and 4(a)) for films of different deposition times (time): total film thickness tXRR , thickness of the SmFeAsO layer t1 , thickness of the surface layer t2 , roughness of the SmFeAsO layer r1 , roughness of the surface layer r2 , and density of the surface layer ρ2 . The last three columns contain full-width-half-maximum (FWHM) of SmFeAsO 001 Bragg reflections, number of coherently scattering lattice planes N and thickness of the coherently scattering volume, tLaue for selected films shown in Fig. 7.
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Figure 7: SmFeAsO 001 Bragg reflection of films deposited for (a) 2 min (b) 3 min (c) 5 min and (d) 10 min. (e) Evolution of FWHM with increasing film thickness. Note, that tLaue is the estimated thickness of the coherently scattering part of the film.
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relaxation with increasing thickness. However, yet a large spread in c-axis lattice parameters is found for films grown for 10 min. Oxygen-deficiency should not contribute to a large change in unit-cell volume [22]. Therefore, we assume, that the microstructure plays the dominant role. The comparison of two films deposited for 10 min with similar resulting total thickness shows that the c-axis parameter in one film follows the phenomenologically described square-root behavior, whereas the c-axis parameter in the other film is much smaller and close to the value found in thinner films deposited for 1.5 or 2 min (see Fig.8(d)). AFM scans and XRD of the films clearly reveal a different microstructure that may be responsible for more or less strain acting on the grains as shown in Figs.4(a) and (b). Although both films have a similar total thickness of around 73 nm, the film with the smaller c-axis parameter consists of c-axis oriented iron oxypnictide grains and a SmAs impurity phase, whereas the film with the larger
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c-axis parameter consists of c-axis oriented grains as well as a comparable fraction of (102)-oriented grains. The additional (102)-oriented grains differ from c-axis oriented grains in shape that is elongated in [100] and [010] directions of the MgO substrate. We point out again, that the issue of reproducibility in the oxypnictides is sensitively linked with the competition between 1111 phase formation and the formation of impurity phases.
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Figure 8: (a) 2θ/ω scans (Cu Kα) around the (003) reflection of SmFeAsO thin films grown at different deposition times (indicated in minutes). The SmAs(002) impurity reflection is indicated by ?. Note that a clear (003) Bragg reflection occurs only in films deposited for 1.5 min and longer. (b) X-ray reflectivity (thick lines) for SmFeAsO thin films and curve fits (thin lines). (c) Total thickness resulting from XRR fits. The dependence of the film thickness on deposition time can be phenomenologically described by a square-root behavior (curve). (d) c-axis lattice parameter determined by high-resolution X-ray diffraction. The dependence of the maximal c-axis value on the deposition time can be phenomenologically described by a square-root behavior (curve). (e) Resulting growth rate (˚ As−1 ) as function of the total film thickness.
Table 3: Results of thickness series of SmFeAsO thin films on MgO(100) with increasing deposition time (minutes): Thickness and overall growth rate (GR) as determined by XRR, the c-axis lattice parameter is evaluated by XRD, minimum peak-to-peak (p-p) and rms roughness are extracted from AFM scans of the film surface.
Fig. (no.)
time (min)
tXRR (nm)
GR (˚ As−1 )
c-axis (˚ A)
p-p (nm)
rms (nm)
5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8
0.17 0.5 1.0 1.5 2.0 3.0 5.0 10.0
2.5 6.7 10.5 11.3 16.1 22.0 45.3 73.3
2.50 2.23 1.74 1.26 1.34 1.22 1.51 1.22
n.a. n.a. n.a. 8.599 8.591 8.613 8.633 8.596
1.9 4.6 9.2 5.7 7.1 6.3 7.2 9.5
0.2 0.3 0.7 0.8 0.9 0.9 0.8 1.1
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Conclusions
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Excluding the doping issue from our actual study we were able to study the intrinsic difficulties of SmFeAsO film growth. UHV PLD faces several challenges, particularly due to off-stoichiometries and an oxygen deficiency. Above, we presented the first detailed growth report and surface morphology analysis of SmFeAsO films grown by UHV PLD. We have confirmed a narrow parameter window for the reduction of impurity phase formation in the absence of oxygen or fluorine supply. We find, in general, SmAs and Fe impurities in competition with SmFeAsO phase formation. The impurity fraction is smallest at the optimal deposition temperature near 860◦ C. A Sm2 O3 impurity phase appears preferentially at higher deposition temperatures and can only be removed by decreasing the substrate temperature. However, at substrate temperatures below the optimal value a small fraction of misoriented SmFeAsO grains appear. Relevant studies on bulk specimens and previous film studies suggest that the as-grown films are oxygen deficient, SmFeAsO1−δ , and we note that the oxygen content in iron oxypnictide plays a crucial role in fabrication as well as in physical properties. Hiramatsu et al. [9] have not found a correlation of in-plane lattice parameters of iron oxypnictide thin films with the substrate lattice parameters. As a consequence, control of epitaxial strain or a coherent film/substrate interface might be difficult to achieve. We find, that the initial SmFeAsO epilayers are strongly perturbed and probably contain already a high density of misfit dislocations at the film/substrate interface, followed by a dominant 3D growth mode of the crystal grains. As a consequence, 7% lattice mismatch is already too high for a 2D growth of iron oxyarsenides, however, SmFeAsO films thicker than 10 nm show an epitaxial relationship to MgO(100) substrates. The increase of the c-axis lattice parameter with film thickness, that can be phenomenologically described by a square-root law, can be understood as result of lattice relaxation. Deviations from this behavior are caused by a different film morphology. In very thin SmFeAsO films below a threshold thickness of roughly 10 nm no Bragg reflections occur. However, we found indications for pseudomorphic growth of a strained epilayer with compressed c-axis lattice parameter. Locally, a Stranski-Krastanov growth mode with a very small critical thickness of 1-3 unit cells is thus possible. Importantly, the constraints in deposition temperature and laser fluence result in a limitation of the variation of kinetic growth conditions by usual means. Therefore, we observe typically growth mounds on the film surface and a tendency for columnar 3D growth that finally governs the roughness of the film surfaces. This result has strong implications for any device fabrication or analytical surface sensitive probes that rely on an atomic smooth surface or a multilayer fabrication. Another important result is that the mounds have a step size of a unit-cell (c ≈ 8.5˚ A) and also half a unit-cell. The growth of SmFeAsO is thus stratified according to its layered structure with (SmO)+ and (FeAs)− layers. This implies that charged antiphase grain boundaries may appear in films, and, that there is a strong step edge barrier that further hinders surface smoothing. We believe that future studies of the initial
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stages of SmFeAsO growth with higher resolution and/or by in-situ methods will improve our understanding of kinetic film growth effects and finally help to find methods for the intelligent design of iron oxyarsenide multilayers.
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Acknowledgments
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The work on iron oxypnictide thin films was carried out at Tokyo Institute of Technology and was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through Element Strategy Initiative to Form Core Research Center. S.H. and H. Hi. acknowledge financial support by German Research Foundation (DFG HA5934/5-1). H. Hi. was also supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research on Innovative Areas Nano Informatics (Grant Number 25106007), and Support for Tokyotech Advanced Research (STAR). All authors thank Dr. Takayoshi Katase for synthesizing the polycrystalline PLD target disk. S. H. wants to thank Rudolf H¨ ubener for reading the manuscript and fruitful discussions. Appendix
Phi-scan of SmFeAsO film on MgO(100)
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Figure 9: XRD φ-scan (Cu Kα) of the SmFeAsO(110) reflection versus the φ-scan of the MgO(200) reflection that confirms cube-on-cube orientation.
Remark on SmFeAsO film growth on CaF2 substrates
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In a previous work [20] we have shown that SmFeAsO film growth on CaF2 (100) substrates results in F-doped iron oxypnictide with superconducting transitions. We also reported characterstic surface defects and cracking of the film along SmFeAsO[100] and [010]. We note here that neither such surface defects nor film cracking was observed in the undoped SmFeAsO films deposited on MgO(100).
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MgO(100) surface morphology The importance of MgO substrates in thin film research has already lead to many studies on the MgO(100) surface under different conditions of treatment and cleaning from adsorbates and contamination.[27] MgO single crystalline substrates contain a small fraction of Ca2+ impurity ions. A thermal treatment of MgO substrates above a temperature of 800◦ C can cause their segregation to the substrate surface as reported in Refs.[28, 29] and even CaOx particles were imaged by non-contact AFM in Ref.[30]. We conclude that due to the high deposition temperatures of around 850◦ C in UHV, the MgO(100) surface is increasingly affected by the segregation of Ca2+ impurity ions and the formation of CaOx . The surface chemical composition thus becomes nonuniform. We confirmed an impurity content of around 400 ppm Ca in the used MgO substrates by comparing the Kα1-line intensities of Ca and Mg using energy-dispersive X-ray fluorescence spectroscopy. In addition, the surface morphology of the MgO substrates was studied by AFM after heat treatment in air and in UHV. The AFM images deliver local information on the surface roughness of the MgO(100) substrates. Untreated substrates have locally smooth surfaces (image (1) in Fig.10) and show facetting along MgO<100> similar to substrates baked in UHV at 850◦ C for 5 min (image (7) in Fig.10). However, still small pits exist, as exemplary shown in image (2) in Fig.10, that cause an increase in surface roughness. Nanometer-sized particles start to cover the surface after a heat treatment in air at 1000◦ C (images (3) & (4) in Fig. 10) but also after a short heat treatment at 850◦ C in UHV atmosphere (image (7) in Fig. 10). According to previous studies on Ca segregation, we believe that these particles are CaOx . Furthermore, as it can be seen in images (4)-(6) in Fig. 10, the surface morphology varies from a rather uniform surface to pronounced steps of around 4˚ A in height and terraces that finally determine the roughness of the substrate surface. The orientation of the terraces changes from parallel to MgO<100> compared to images (1) (3) to parallel to MgO<110> in images (5) & (6) in Fig. 10. Control of the surface condition of the substrates seem to become more difficult the higher the temperature in the pre-heat-treatment and may depend on the amount of Ca impurity. The peak-to-peak roughness of the MgO substrates without pre-treatment and heated only in UHV to the deposition temperature is 1.45 nm (image (7) in Fig. 10). We note that the results for MgO(100) surfaces shown here are only qualitative and surface roughness can increase locally due to the presence of small pits (as seen in image (2) of Fig. 10). For the study above, we decided to perform SmFeAsO film growth on substrates without specific heat treatment or with a pre-heat-treatment at 800◦ C for 1 hour in order to avoid Ca segregation.
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Annotation of AFM images in Figs.5 and 10 References References 380
[1] P. Quebe, L. J. Terb¨ uchte, W. Jeitschko, J. Alloys Comp. 302, 70 (2000) 16
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Figure 10: Surface morphology of MgO(100) substrates after heat treatment as measured by nc-AFM: (1) untreated (2) 1 hour at 800◦ C in air (3) 1 hour at 1000◦ C in air (4) 3 hours at 1000◦ C in air (5) 3 hours at 1000◦ C in air (6) 3 hours at 1000◦ C in air ; and (7) 5 min at around 850◦ at 2.5 10−8 mbar . Scan size is 1×1 µm2 . The scan direction in all images is along MgO[110]. Contour plots and annotations are given in Fig.11.
bake out procedure
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Table 4: Bake out and heat treatment procedures of MgO(100) substrates
roughness
T (◦ C)
time h
atm. –
p-p (nm)
rms (nm)
10-1 10-2 10-3 10-4 10-5 10-6
– 800 1000 1000 1000 1000
– 1 1 3 3 3
– air air air air air
1.22 5.57 1.91 2.70 1.45 1.99
0.14 0.19 0.14 0.25 0.15 0.25
10-7
850
1
UHV
1.28
0.12
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[2] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008) [3] X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, D. F. Fang, Nature 453, 761 (2008)
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Figure 11: Annotation of AFM images displayed in Fig.10.
Figure 12: Annotation of AFM images displayed in Fig.5.
H. Hara, T. Watanabe, H. Takeya, T. Yamaguchi, H. Kumakura, Y. Takano, Supercond. Sci. Technol. 26, 085023 (2013)
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S0169-4332(17)32393-0 http://dx.doi.org/doi:10.1016/j.apsusc.2017.08.061 APSUSC 36898
To appear in:
APSUSC
Received date: Revised date: Accepted date:
20-2-2017 28-6-2017 7-8-2017
Please cite this article as: Silvia Haindl, Hiroyuki Kinjo, Kota Hanzawa, Hidenori Hiramatsu, Hideo Hosono, Pulsed laser deposition of SmFeAsO1minusrmdelta on MgO(100) substrates, (2017), http://dx.doi.org/10.1016/j.apsusc.2017.08.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Pulsed laser deposition of SmFeAsO1−δ on MgO(100) substrates Silvia Haindla,b 1 , Hiroyuki Kinjoa , Kota Hanzawaa , Hidenori Hiramatsua,c , Hideo Hosonoa,c a Laboratory
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for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology, mailbox R3-1, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan b Physikalisches Institut, Universit¨ at T¨ ubingen, 72076 T¨ ubingen, Germany c Materials Research Center for Element Strategy, Tokyo Institute of Technology, Mailbox SE-6, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan
Abstract
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Layered iron oxyarsenides are novel interesting semimetallic compounds that are itinerant antiferromagnets in their ground state with a transition to hightemperature superconductivity upon charge carrier doping. The rare earth containing mother compounds offer rich physics due to different antiferromagnetic orderings: the alignment of Fe magnetic moments within the FeAs sublattice, which is believed to play a role for the superconducting pairing mechanism, and the ordering of the rare-earth magnetic moments at low temperatures. Here, we present thin film preparation and a film growth study of SmFeAsO on MgO(100) substrates using pulsed laser deposition (PLD). In general, the PLD method is capable to produce iron oxyarsenide thin films, however, competition with impurity phase formation narrows the parameter window. We assume that the film growth in an ultra-high vacuum (UHV) environment results in an oxygendeficient phase, SmFeAsO1−δ . Despite the large lattice misfit, we find epitaxial oxyarsenide thin film growth on MgO(100) with evolving film thickness. Bragg reflections are absent in very thin films although they locally show indications for pseudomorphic growth of the first unit cells. We propose the possibility for a Stranski-Krastanov growth mode as a result of the large in-plane lattice misfit between the iron oxypnictide and the MgO unit cells. A columnar 3-dimensional film growth mode dominates and the surface roughness is determined by growth mounds, a non-negligible parameter for device fabrication as well as in the application of surface sensitive probes. Furthermore, we found evidence for a stratified growth in steps of half a unit cell, i.e. alternating growth of (FeAs)− and (SmO1−δ )+ layers, the basic structural components of the unit cell. We propose a simple model for the growth kinetics of this compound. Keywords: thin films, pulsed laser deposition, iron oxypnictides ∗ Corresponding
author
1 [email protected]
Preprint submitted to Journal of LATEX Templates
June 28, 2017
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2010 MSC: 00-01, 99-00
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The quaternary iron oxyarsenide SmFeAsO is a layered semimetal compound with ZrCuSiAs-type structure (P 4/nmm). It was first synthesized in 2000 by Quebe, Terb¨ uchte and Jeitschko [1] who already pointed out that oxygen stabilizes the structure. They reported an a-axis lattice parameter of 3.940(1) ˚ A and a c-axis lattice parameter of 8.496(3) ˚ A, but superconductivity remained a ‘sleeping beauty’ until 2008, when Kamihara et al. [2] reported a Tc of 26 K in the fluorine-doped iron oxyarsenide LaFeAsO. Promptly one month later superconductivity was confirmed in rare earth analogs. Their announcement on the arXiv preprint server triggered the hype for the new iron pnictide superconductors.[3] Until today the highest Tc in the rare-earth oxyarsenide superconductors (‘1111’ - so-called ‘eleven-eleven’) is achieved in F-doped SmFeAsO: approximately 55 K is commonly reported after [4]; a record of 58.1 K was found in F-doped and FeAs-free SmFeAsO.[5] Apart from superconductivity the undoped mother compound offers itself many interesting physical aspects of either different antiferromagnetic orderings in the two sublattices or the tetragonal-to-orthorhombic phase transition in vicinity to the antiferromagnetic ordering of the FeAs sublattice. Especially the latter mentioned phases are believed to have a strong influence on the unconventional superconducting pairing mechanism. Until today a clear separation between purely electronic and purely structural effects on the ordered quantum phases is difficult and is a matter of debate.[6] The majority of studies on SmFeAsO have been carried out on powders or polycrystalline samples, because single crystal growth seems to be nontrivial at very high temperatures of around 1300◦ C or needs a high-pressure cubic anvil technique.2 Single crystal growth of the 1111 phase is still rare and alternative sample preparation techniques such as epitaxial thin film growth methods are highly demanded. Unfortunately, we only have limited knowledge about iron oxyarsenide thin film growth, although a number of reports have been published during the last years with primary interest in superconductivity. We know that epitaxial thin films can be grown by pulsed laser deposition (LaFeAsO, [9]; Fdoped SmFeAsO, [20]) and by molecular beam epitaxy (NdFeAsO, [10]; F-doped SmFeAsO, [11, 12]). In the last decade the problem of F-doping was treated as the main issue in most thin film studies.3 So far, basic film growth aspects have not been an issue. Especially in PLD the growth of iron oxyarsenides suffers
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2 The first iron oxyarsenide single crystals were reported by Zhigadlo et al. in Ref. [7] (micrometer size) and later from Ames Laboratory in Ref. [8] (millimeter size). 3 For example, in the case of molecular beam epitaxy (MBE) F-doping was achieved by a F-containing cap layer or by a F-containing substrate. Most recently, progress in the in-situ PLD of F-doped SmFeAsO thin films was made, where CaF2 substrates were used for F-doping but, as we believe, also for stabilizing the ZrCuSiAs structure [20, 21]. A two-step process based on an ex-situ heat treatment was firstly employed successfully for F-doped LaFeAsO in
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from a general off-stoichiometry and oxygen deficiency,4 that is probably one of the main reasons why iron oxypnictide thin film growth has been so difficult to achieve with this method in the past years. In a new approach on PLD of iron oxyarsenides we separate the doping issue from the general film growth analysis. In the following we, therefore, focus on the undoped and oxygen-deficient SmFeAsO compound where we investigate thin film growth on MgO(100) substrates using pulsed laser deposition (PLD). MgO single crystalline substrates are common in thin film growth because of their high melting point, their low cost and their usefulness in different applications. Although MgO is cubic (rocksalt structure) the large lattice misfit strain of (as − af )/af ≈ 7% (at room-temperature) is rated as unfavorable for heteroepitaxial growth of SmFeAsO. as ,af denote the relaxed in-plane lattice parameters of substrate and film, respectively. The thermal expansion of SmFeAsO is unknown for temperatures of several hundred ◦ C but the lattice misfit is not expected to decrease at such high deposition temperatures. Film growth of SmFeAsO on MgO(100) starts, therefore, at the limit of a pseudomorphic growth condition and the critical thickness for an eventually occurring epitaxially strained layer would be only a few unit cells.[23] In this model film growth quickly becomes unstable with increasing thickness and strain release is achieved by the introduction of misfit dislocations with a transition to StranskiKrastanov (SK) growth mode and the preference for 3D columnar growth[24]. The course of this paper is as follows: We will first report on the phase formation and film properties for varying deposition parameters in UHV PLD. Subsequently, based on the evolution of film parameters and surface morphology with increasing thickness, we will discuss the film growth mode in detail. Finally, we propose a simple model that qualitatively captures the essential features of the growth kinetics of this compound.
2. Experimental Procedures
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2.1. Thin Film Preparation For the PLD process in an UHV environment (base pressure ≈ 10−8 mbar) a Spectra Physics Quanta-Ray INDI-40 pulsed Nd:YAG laser (2ω) with a laser wave-length of 532 nm and a laser repetition rate of 10 Hz was used. The peak
Ref. [13] and improved in Refs. [14, 15]. In this case, F-doping was achieved within an ex-situ heat treatment. Finally, film fabrication using metal-organic deposition was attempted for Co-doped NdFeAsO [16] after F-doping turned out to be too complicated. A summary of the progress in thin film growth including iron oxyarsenide superconductors can be found in the review articles [17, 18, 19]. 4 This fact is supported by systematic chemical investigations of bulk synthesis in different atmospheres.[22] Recently, it was shown that oxygen deficiency in SmFeAsO1−δ does not lead to superconductivity as prematurely announced in 2008. In oxygen-deficient iron oxyarsenides the a-axis and c-axis lattice parameters stay nearly unchanged even for large deficiencies (δ ≈ 0.4) compared to a lattice shrinking after element substitution at the oxygen-site. In addition, the fraction of impurity phases like SmAs and Fe notably increase with increasing δ.[22]
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laser energy was measured in front of the laser entrance window of the UHV chamber before and after each film fabrication. The resulting energy density of 2.15 ± 0.63 Jcm−2 on the target is approximated by measuring the imprinted area (ellipse) of the laser spot on the target surface and by taking a 20% loss through the window of the UHV chamber into account. We used a SmFeAsO target prepared by a two-step solid state reaction as described earlier in Ref.[20]. The vertical target-substrate distance during deposition is 2.4±0.2 cm. It was adjusted according to preliminary film growth results. Comparison of the visible extension of the plasma plume upon different laser energies (Fig.1) confirms the limits for a good configuration of the substrate position. A mean target ablation rate of around 3.17 nm s−1 (Table 1) was determined by the weighting the target mass. This method gives a rough estimation of the ablation rates for an uniformly assumed (i.e. idealized) thickness reduction. However, the actual depth of the crater on the target surface generated by the laser pulses must be assumed to be larger (µm size). Repolishing of the target resulted in a thickness reduction of around 200µm. MgO(100) substrates from Furuuchi Chemical Co. with dimensions of 10×10×0.5 mm3 were used. A pre-heat-treatment for MgO(100) substrates at 800◦ C for 1 hour was undertaken ex-situ in a tube furnace. We also compared film growth after a pre-heat-treatment at 1000◦ , however the conditions of the substrate surface were less controllable after annealing (see Appendix for a detailed discussion and Figs.10 and 11). Substrates were then inserted into a high-vacuum loadlock. Before film deposition the substrate was heated in about 1 hour to the deposition temperature Tdep where it was kept for 5 minutes. Substrate heating was performed using a high power laser diode (LUMICS) operating with a wavelength of 975 nm. Temperature control was carried out using a pyrometer (InGaAs detector, spectral range 1.95 – 2.50 µm) and a thermocouple. The thermocouple is, however, not in direct contact with the thin film and monitors the temperature in a distance several cm away from the substrate. The temperatures given by the pyrometer were calibrated in a previous experiment. The measurement error at 800◦ C is ±4◦ C. In the following all given temperatures refer to the calibrated ones.
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Figure 1: Extension of the plasma plume of an irradiated SmFeAsO target upon variation of the Nd-YAG (2ω) laser energy between 20 and 27 mJ.
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∆m / time (µg×s−1 )
∆t/ time (nm×s−1 )
25±1 25±3 25±1
5647 11400 17820
0.992 0.939 0.993
3.25 3.02 3.23
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Table 1: Ablation rate measurements for a SmFeAsO sintered target. The idealized thickness reduction ∆t per time is calculated from the measured mass reduction ∆m per time. The data is valid for the ablation using a Nd:YAG(2ω) laser with repetition rate of 10 Hz. For a measured laser energy of 25 mJ the resulting energy density on the target surface is 2.15 ± 0.63 Jcm−2 .
2.2. Thin Film Crystallography, Chemical Analysis and Surface Morphology
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Films were characterized using a Rigaku Smart Lab with CuKα radiation. Standard X-ray diffraction (XRD) θ/2θ-scans were carried out in Bragg Brentano geometry. High resolution scans and X-ray reflectivity (XRR) measurements were carried out in parallel beam geometry (using a cross-beam optics unit and a Ge(220) monochromator in the incident beam path). We have evaluated the c-axis lattice parameter from SmFeAsO(00`) reflections with ` = 1,2,3,5 and 6 by means of a linear extrapolation versus the Nelson Riley function cos(θ)cot(θ)+cos2 (θ)/θ.[25] X-ray reflectivity (XRR) measurements were performed using a Rigaku Smart Lab (CuKα) with a cross-beam optics unit and a Ge(220) monochromator in the incident beam path. We used the GlobalFit program for analyzing the XRR results. Film surface morphologies, grain size as well as film surface roughness were analyzed after preparation using a Bruker AXS MultiMode8 Atomic Force Microscope (AFM) in non-contact mode. The used silicon tips on nitride cantilevers have a resonant frequency of 130 ± 30 kHz and a spring constant of 0.4 Nm−1 (ScanAsyst-Air-HR probes). Scans were performed on 512×512 pixel arrays. The resulting AFM images were flattened by a 3rd order polynomial. Film surface roughness analysis was performed on AFM images after flattening. The root-mean-square (rms) roughness was calculated for 1×1 µm2 and 500×500 nm2 scans. Image and surface roughness analysis was performed after flattening using the WSxM software developed in Ref.[26]. Wavelength-dispersive X-ray fluorescence (XRF) measurements were performed with a Bruker S8-3F Tiger spectrometer in order to estimate the Ca impurity content in MgO substrates.
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3.1. Temperature-Energy-Diagram The effect of deposition temperature and energy density was studied in the range of 700◦ C – 900◦ C and for three different laser energy regimes 22 mJ ± 5
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1 mJ, 25 mJ ± 1 mJ and 28 mJ ± 1 mJ. We confirmed that the small parameter window for single phase and epitaxial iron oxypnictide film growth is extremely small under UHV conditions and when fluorine or oxygen are not supplied. We successively varied the deposition temperature around the optimal value of 860◦ C (Fig. 2). A qualitative difference is observed for deposition temperatures below 850◦ , where SmFeAsO crystallites with (201) and (101) orientation appear. Crystallites with c-axis orientation (00`) appear beyond 800◦ C. A SmAs(002) impurity reflection is present in all samples and its intensity grows with increasing temperature. Additionally, above 875◦ C the Sm2 O3 (222) impurity peak increases. At temperatures between 845◦ C and 860◦ C, films with a large fraction of c-axis oriented SmFeAsO grains could be grown. As mentioned above, all films contain also a small amount of Fe impurity, however Fe(110) and possibly also the Fe(002) reflections are covered by MgO(002) and SmFeAsO(006) peaks and, therefore, not detectable in X-ray diffractograms.
Figure 2: Smoothed XRD θ/2θ-scans (Cu Kα) of SmFeAsO films deposited at different temperatures (E = 25 mJ ± 1 mJ). At the bottom a θ/2θ-scan for MgO is shown for comparison of the background. Left: Full 2θ-range from 5◦ to 75◦ with indexed SmFeAsO(00`) peaks. Right: Zoomed detail (shaded area) from 2θ = 24◦ to 38◦ with the evolution of SmFeAsO(101), SmFeAsO(102), SmFeAsO(003), F = SmAs(002) and = Sm2 O3 (222) reflections with varying deposition temperature.
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A similar trend was found for laser energies of 22 mJ ± 1 mJ and 28 mJ ± 1 mJ. The results are summarized in a qualitative deposition parameter diagram spanned by substrate temperature and laser energy (Fig.3). With increas6
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ing laser energy the c-axis lattice parameter of the films grows. We attribute this effect to the increasing film thickness as it is clear from Fig.1 that with increasing laser energy more material is deposited in the same time interval. The variation of the c-axis lattice parameter with increasing film thickness is discussed below. The temperature dependence of the c-axis parameter is more subtle and displays a maximum near the optimal deposition temperature. Since the amount of impurity phases and misaligned grains varies with substrate temperature as well (compare Fig.2) a final interpretation is difficult. (b)
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Figure 3: (a) Qualitative diagram of SmFeAsO film growth in dependence of substrate temperature and laser energy. Note that Fe and SmAs impurities appear always. Below the optimal growth temperature SmFeAsO(h0`) reflections indicate the presence of misoriented grains, above the optimal temperature the Sm2 O3 impurity content increases. (b) c-axis lattice parameter as function of laser energy. All films were deposited for 10 min in an temperature interval of 560±30◦ C. (c) c-axis lattice parameter as function of substrate temperature for films deposited for 10 min and at a laser energy of 25±1 mJ (shown in Fig.2).
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3.2. Epitaxial Relationship For SmFeAsO films that contain only c-axis oriented grains we confirmed epitaxial growth by in-plane φ-scans. The epitaxial relationship is SmFeAsO(100)[100]kMgO(100)[100] (see Fig. 9 in the Appendix).
3.3. Impurity phases and misoriented grains at the film surface Misoriented grains of the SmFeAsO phase that deviate from c-axis orientation as well as impurity phases (most likely Sm2 O3 grains due to their orientation along the space diagonal) are visible in AFM scans and are responsible for a characteristic surface morphology. In comparison with a film grown near optimally conditions (Fig.4(a)) where the surface is covered uniformly with columnar grains and trenches between them, the film grown at lower temperatures 7
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Figure 4: XRD θ/2θ-scan (top), AFM scan (middle) and schematic annotation of the coverage of misoriented grains (bottom) for films grown at (a) E= 25.7 mJ, T = 875◦ C (b) 25.2 mJ, T = 845◦ C (c) E= 29.3 mJ, T = 880◦ C (d) E = 22.2 mJ, T = 880◦ C. Optimal growth conditions result in a uniformly dense covering with SmFeAsO mounds as given in Fig.(a).
3.4. Evolution of film structure and surface morphology of SmFeAsO thin films with thickness
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The evolution of the surface morphology of SmFeAsO thin films was studied with increasing thickness close to optimal growth conditions around T = 860◦ C – 870◦ C and E = 25 mJ (Fig.5). A deposition for 10 seconds results in a total film thickness of 2.5 nm showing a fine granulation that already uniformly covers the substrate surface although steps of the substrate surface can still be traced. Similarly, the thin film surface after a deposition time of 30 seconds shows still traces of steps on the initial substrate surface. The film layer covers
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the substrate, however small pits appear. It is possible that these pits in the film surface match original pits on the substrate surface as found in an AFM scan of the bare substrate.5 Considerably increased granularity is observed after a deposition of 1 min. It might be possible that the initial wetting of the substrate is not complete, however, the AFM analysis suggests that the 3-dimensional (3D) like growth of mounds (’wedding cakes’) is driven by stress release. With increasing deposition time a tendency to the formation of steep mounds can be traced indicating a dominance of a 3D growth mode driven by strain. For deposition times larger than 2 min coalescence of mounds is found at the bottom of the AFM scale, new mound formation can be seen at the top of the AFM scale with quadratic shape and facets along MgO<100>. A schematic model is given in the bottom panel of Fig.6. The observed small terrace width is indicative of an existing strong step edge barrier in accordance with the idea that the time for adatoms or -clusters residing on the top island is very large compared to the diffusion time, i.e. the time an adatom or -cluster needs for travelling across the top layer. Driven by the question if a strained SmFeAsO epilayer can be found we look
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Figure 5: Evolution of surface morphology of SmFeAsO thin films with increasing deposition times: (1) 0.17 min, (2) 0.5 min, (3) 1 min, (4) 1.5 min, (5) 2 min, (6) 3 min, (7) 5 min and (8) 10 min. Coverage of the substrate surface is found already for a film with a thickness of 2 nm, although substrate terraces, steps and pits are still visible in the AFM images (1) - (3). The maximum value of the scale is indicated above each image. All scans are made on an area of 1×1 µm2 . For an annotation see Fig.12 in the Appendix.
at the AFM scan on the thinnest film grown. If pseudomorphic growth appears 5 For
a discussion of the MgO substrate surface the reader is referred to the Appendix.
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and under the assumption of preserved unit cell volume under biaxial tensile strain (constant Poisson ratio) we expect a c-axis lattice parameter of the first unit cell to be around 7.44˚ A. Line scans in the AFM scan of the film deposited for 10 seconds (top panel in Fig.6) indicate individual island heights of around 7.44˚ A. However, higher resolution or in-situ analytical methods are required in order to make a conclusive statement. At present, a strained SmFeAsO layer for film thicknesses below 2.5 nm is possible. Consequently, the critical thickness of such an epitaxially strained layer can be estimated to the size of 1 - 3 unit cells. The evolution of thin film crystallinity begins with a deposition time of 1.5 min (Fig.8(a)): SmFeAsO(00`) Bragg reflections are absent in very thin films with thicknesses below 11 nm and start to occur after a deposition time of 1.5 minutes (corresponding to a total film thickness of 11.3 nm). A limiting case is the film deposited for 1 min and a resulting film thickness of 10.5 nm with a enhanced background close to the Bragg peak positions. Although no clear SmFeAsO (00`) reflections could be observed the film surface consists already of steps of height of a unit cell, c, and of a half unit cell, c/2 (with c ≈ 8.5 ˚ A). This result indicates the stratification in the growth of iron oxypnictide unit cells according to its building blocks, with subsequent growth of (SmO)+ and (FeAs)− layers (middle panel in Fig. 6). The different polarity of the layers may have an additional influence on the 3D like growth and the step edge barrier.
A possible reason for the absence of more distinct Bragg reflections in very thin SmFeAsO films can be a strong perturbation of the ordered structure of subsequent (SmO)+ and (FeAs)− layers caused by the introduction of screw dislocations and leading to a large mosaicity in accordance with the X-ray results. Furthermore, the AFM results show that SmFeAsO epilayers in very thin films are not uniformly strained across the whole substrate which also leads to a loss in crystallographic lattice coherence. Total film thicknesses were extracted from Kiessig fringes of the X-ray reflectivity (XRR) measurements (Fig.8(b)). For the fit procedure a two-layer model was used with a SmFeAsO layer of density ρ1 ≈ 7.493 gcm−3 and an approximately 1 nm thick surface layer of low density (see details in Table 2). The roughness of the MgO substrate was set to a value close to 1.0 nm. Fit results of the XRR scans are shown as thin lines in Fig.8(b). The total film thickness, tXRR , between 20 nm and 100 nm is proportional to the square-root of the deposition time (Fig.8(c)).
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SmFeAsO 001 Bragg reflections were modeled by Laue functions in order to estimate the coherently grown film thickness, usually smaller than the total film thickness evaluated from XRR measurements (Fig.7, last three columns in Tab.2). Analysis of the c-axis parameter in SmFeAsO thin films is less trivial. We find that the maximum c-axis lattice parameter increases with √ total film thickness follows a square-root law: c(˚ A)= 8.59 + 0.80 × 10−2 × tXRR − 14 with tXRR given in nm. The increase in c-axis can be understood as result of the lattice 10
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Figure 6: Top panel: Topography of the film surface after deposition for 10 seconds with steps of unit cells and half unit cells: AFM image (200 nm × 200 nm) with positions of the line scans indicated; three different line scans in random directions. The reduced value of the c-axis lattice parameter for a strained epilayer, cs ≈ 7.44˚ A, is indicated (red). Step-sizes of cs , cs /2 but also of c ≈ 8.5 ˚ A (black) can be measured. Middle panel: Topography of the film surface after deposition for 1 min with steps of unit cells and half unit cells: AFM image (250 nm × 250 nm) with positions of the line scans indicated; line scan along SmFeAsO[100]kMgO[100] and line scan along SmFeAsO[010]kMgO[010]. The film shows an enhanced X-ray diffraction background but lacks distinct Bragg reflections, nevertheless step-sizes of c ≈ 8.5 ˚ A and c/2 can be measured. Bottom Panel: Topography of the film surface after deposition for 10 min with evolution of growth mounds. AFM image (200 nm × 200 nm) and two different contour levels (coloured) separated by one unit cell. A schematic growth model of the film growing on the substrate (S) is presented.
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b)
r1 (nm)
r2 (nm)
ρ2 (gcm−3 )
11.3 16.1 22.0 45.4 72.5 73.3 91.1
10.6 15.1 20.9 44.3 70.6 72.5 90.3
0.7 1.0 1.1 0.9 1.9 0.7 0.8
1.8 1.8 1.9 1.3 1.8 1.8 1.3
0.3 0.3 0.2 0.3 <0.1 0.2 0.2
3.1 3.4 3.0 2.6 2.1 1.6 2.0
F W HM (◦ )
N
0.67 0.44 0.25 0.18
12 23 46 78
(b)
(c)
tLaue (nm)
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10.3 19.8 39.7 67.0
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Table 2: Fit parameters of XRR results using a two layer model (a) shown in Fig.4(b); b) shown in Figs.8(b) and 4(a)) for films of different deposition times (time): total film thickness tXRR , thickness of the SmFeAsO layer t1 , thickness of the surface layer t2 , roughness of the SmFeAsO layer r1 , roughness of the surface layer r2 , and density of the surface layer ρ2 . The last three columns contain full-width-half-maximum (FWHM) of SmFeAsO 001 Bragg reflections, number of coherently scattering lattice planes N and thickness of the coherently scattering volume, tLaue for selected films shown in Fig. 7.
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relaxation with increasing thickness. However, yet a large spread in c-axis lattice parameters is found for films grown for 10 min. Oxygen-deficiency should not contribute to a large change in unit-cell volume [22]. Therefore, we assume, that the microstructure plays the dominant role. The comparison of two films deposited for 10 min with similar resulting total thickness shows that the c-axis parameter in one film follows the phenomenologically described square-root behavior, whereas the c-axis parameter in the other film is much smaller and close to the value found in thinner films deposited for 1.5 or 2 min (see Fig.8(d)). AFM scans and XRD of the films clearly reveal a different microstructure that may be responsible for more or less strain acting on the grains as shown in Figs.4(a) and (b). Although both films have a similar total thickness of around 73 nm, the film with the smaller c-axis parameter consists of c-axis oriented iron oxypnictide grains and a SmAs impurity phase, whereas the film with the larger
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c-axis parameter consists of c-axis oriented grains as well as a comparable fraction of (102)-oriented grains. The additional (102)-oriented grains differ from c-axis oriented grains in shape that is elongated in [100] and [010] directions of the MgO substrate. We point out again, that the issue of reproducibility in the oxypnictides is sensitively linked with the competition between 1111 phase formation and the formation of impurity phases.
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Figure 8: (a) 2θ/ω scans (Cu Kα) around the (003) reflection of SmFeAsO thin films grown at different deposition times (indicated in minutes). The SmAs(002) impurity reflection is indicated by ?. Note that a clear (003) Bragg reflection occurs only in films deposited for 1.5 min and longer. (b) X-ray reflectivity (thick lines) for SmFeAsO thin films and curve fits (thin lines). (c) Total thickness resulting from XRR fits. The dependence of the film thickness on deposition time can be phenomenologically described by a square-root behavior (curve). (d) c-axis lattice parameter determined by high-resolution X-ray diffraction. The dependence of the maximal c-axis value on the deposition time can be phenomenologically described by a square-root behavior (curve). (e) Resulting growth rate (˚ As−1 ) as function of the total film thickness.
Table 3: Results of thickness series of SmFeAsO thin films on MgO(100) with increasing deposition time (minutes): Thickness and overall growth rate (GR) as determined by XRR, the c-axis lattice parameter is evaluated by XRD, minimum peak-to-peak (p-p) and rms roughness are extracted from AFM scans of the film surface.
Fig. (no.)
time (min)
tXRR (nm)
GR (˚ As−1 )
c-axis (˚ A)
p-p (nm)
rms (nm)
5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8
0.17 0.5 1.0 1.5 2.0 3.0 5.0 10.0
2.5 6.7 10.5 11.3 16.1 22.0 45.3 73.3
2.50 2.23 1.74 1.26 1.34 1.22 1.51 1.22
n.a. n.a. n.a. 8.599 8.591 8.613 8.633 8.596
1.9 4.6 9.2 5.7 7.1 6.3 7.2 9.5
0.2 0.3 0.7 0.8 0.9 0.9 0.8 1.1
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Conclusions
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Excluding the doping issue from our actual study we were able to study the intrinsic difficulties of SmFeAsO film growth. UHV PLD faces several challenges, particularly due to off-stoichiometries and an oxygen deficiency. Above, we presented the first detailed growth report and surface morphology analysis of SmFeAsO films grown by UHV PLD. We have confirmed a narrow parameter window for the reduction of impurity phase formation in the absence of oxygen or fluorine supply. We find, in general, SmAs and Fe impurities in competition with SmFeAsO phase formation. The impurity fraction is smallest at the optimal deposition temperature near 860◦ C. A Sm2 O3 impurity phase appears preferentially at higher deposition temperatures and can only be removed by decreasing the substrate temperature. However, at substrate temperatures below the optimal value a small fraction of misoriented SmFeAsO grains appear. Relevant studies on bulk specimens and previous film studies suggest that the as-grown films are oxygen deficient, SmFeAsO1−δ , and we note that the oxygen content in iron oxypnictide plays a crucial role in fabrication as well as in physical properties. Hiramatsu et al. [9] have not found a correlation of in-plane lattice parameters of iron oxypnictide thin films with the substrate lattice parameters. As a consequence, control of epitaxial strain or a coherent film/substrate interface might be difficult to achieve. We find, that the initial SmFeAsO epilayers are strongly perturbed and probably contain already a high density of misfit dislocations at the film/substrate interface, followed by a dominant 3D growth mode of the crystal grains. As a consequence, 7% lattice mismatch is already too high for a 2D growth of iron oxyarsenides, however, SmFeAsO films thicker than 10 nm show an epitaxial relationship to MgO(100) substrates. The increase of the c-axis lattice parameter with film thickness, that can be phenomenologically described by a square-root law, can be understood as result of lattice relaxation. Deviations from this behavior are caused by a different film morphology. In very thin SmFeAsO films below a threshold thickness of roughly 10 nm no Bragg reflections occur. However, we found indications for pseudomorphic growth of a strained epilayer with compressed c-axis lattice parameter. Locally, a Stranski-Krastanov growth mode with a very small critical thickness of 1-3 unit cells is thus possible. Importantly, the constraints in deposition temperature and laser fluence result in a limitation of the variation of kinetic growth conditions by usual means. Therefore, we observe typically growth mounds on the film surface and a tendency for columnar 3D growth that finally governs the roughness of the film surfaces. This result has strong implications for any device fabrication or analytical surface sensitive probes that rely on an atomic smooth surface or a multilayer fabrication. Another important result is that the mounds have a step size of a unit-cell (c ≈ 8.5˚ A) and also half a unit-cell. The growth of SmFeAsO is thus stratified according to its layered structure with (SmO)+ and (FeAs)− layers. This implies that charged antiphase grain boundaries may appear in films, and, that there is a strong step edge barrier that further hinders surface smoothing. We believe that future studies of the initial
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stages of SmFeAsO growth with higher resolution and/or by in-situ methods will improve our understanding of kinetic film growth effects and finally help to find methods for the intelligent design of iron oxyarsenide multilayers.
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Acknowledgments
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The work on iron oxypnictide thin films was carried out at Tokyo Institute of Technology and was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through Element Strategy Initiative to Form Core Research Center. S.H. and H. Hi. acknowledge financial support by German Research Foundation (DFG HA5934/5-1). H. Hi. was also supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research on Innovative Areas Nano Informatics (Grant Number 25106007), and Support for Tokyotech Advanced Research (STAR). All authors thank Dr. Takayoshi Katase for synthesizing the polycrystalline PLD target disk. S. H. wants to thank Rudolf H¨ ubener for reading the manuscript and fruitful discussions. Appendix
Phi-scan of SmFeAsO film on MgO(100)
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Figure 9: XRD φ-scan (Cu Kα) of the SmFeAsO(110) reflection versus the φ-scan of the MgO(200) reflection that confirms cube-on-cube orientation.
Remark on SmFeAsO film growth on CaF2 substrates
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In a previous work [20] we have shown that SmFeAsO film growth on CaF2 (100) substrates results in F-doped iron oxypnictide with superconducting transitions. We also reported characterstic surface defects and cracking of the film along SmFeAsO[100] and [010]. We note here that neither such surface defects nor film cracking was observed in the undoped SmFeAsO films deposited on MgO(100).
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MgO(100) surface morphology The importance of MgO substrates in thin film research has already lead to many studies on the MgO(100) surface under different conditions of treatment and cleaning from adsorbates and contamination.[27] MgO single crystalline substrates contain a small fraction of Ca2+ impurity ions. A thermal treatment of MgO substrates above a temperature of 800◦ C can cause their segregation to the substrate surface as reported in Refs.[28, 29] and even CaOx particles were imaged by non-contact AFM in Ref.[30]. We conclude that due to the high deposition temperatures of around 850◦ C in UHV, the MgO(100) surface is increasingly affected by the segregation of Ca2+ impurity ions and the formation of CaOx . The surface chemical composition thus becomes nonuniform. We confirmed an impurity content of around 400 ppm Ca in the used MgO substrates by comparing the Kα1-line intensities of Ca and Mg using energy-dispersive X-ray fluorescence spectroscopy. In addition, the surface morphology of the MgO substrates was studied by AFM after heat treatment in air and in UHV. The AFM images deliver local information on the surface roughness of the MgO(100) substrates. Untreated substrates have locally smooth surfaces (image (1) in Fig.10) and show facetting along MgO<100> similar to substrates baked in UHV at 850◦ C for 5 min (image (7) in Fig.10). However, still small pits exist, as exemplary shown in image (2) in Fig.10, that cause an increase in surface roughness. Nanometer-sized particles start to cover the surface after a heat treatment in air at 1000◦ C (images (3) & (4) in Fig. 10) but also after a short heat treatment at 850◦ C in UHV atmosphere (image (7) in Fig. 10). According to previous studies on Ca segregation, we believe that these particles are CaOx . Furthermore, as it can be seen in images (4)-(6) in Fig. 10, the surface morphology varies from a rather uniform surface to pronounced steps of around 4˚ A in height and terraces that finally determine the roughness of the substrate surface. The orientation of the terraces changes from parallel to MgO<100> compared to images (1) (3) to parallel to MgO<110> in images (5) & (6) in Fig. 10. Control of the surface condition of the substrates seem to become more difficult the higher the temperature in the pre-heat-treatment and may depend on the amount of Ca impurity. The peak-to-peak roughness of the MgO substrates without pre-treatment and heated only in UHV to the deposition temperature is 1.45 nm (image (7) in Fig. 10). We note that the results for MgO(100) surfaces shown here are only qualitative and surface roughness can increase locally due to the presence of small pits (as seen in image (2) of Fig. 10). For the study above, we decided to perform SmFeAsO film growth on substrates without specific heat treatment or with a pre-heat-treatment at 800◦ C for 1 hour in order to avoid Ca segregation.
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Annotation of AFM images in Figs.5 and 10 References References 380
[1] P. Quebe, L. J. Terb¨ uchte, W. Jeitschko, J. Alloys Comp. 302, 70 (2000) 16
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Figure 10: Surface morphology of MgO(100) substrates after heat treatment as measured by nc-AFM: (1) untreated (2) 1 hour at 800◦ C in air (3) 1 hour at 1000◦ C in air (4) 3 hours at 1000◦ C in air (5) 3 hours at 1000◦ C in air (6) 3 hours at 1000◦ C in air ; and (7) 5 min at around 850◦ at 2.5 10−8 mbar . Scan size is 1×1 µm2 . The scan direction in all images is along MgO[110]. Contour plots and annotations are given in Fig.11.
bake out procedure
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Table 4: Bake out and heat treatment procedures of MgO(100) substrates
roughness
T (◦ C)
time h
atm. –
p-p (nm)
rms (nm)
10-1 10-2 10-3 10-4 10-5 10-6
– 800 1000 1000 1000 1000
– 1 1 3 3 3
– air air air air air
1.22 5.57 1.91 2.70 1.45 1.99
0.14 0.19 0.14 0.25 0.15 0.25
10-7
850
1
UHV
1.28
0.12
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[2] Y. Kamihara, T. Watanabe, M. Hirano, H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008) [3] X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, D. F. Fang, Nature 453, 761 (2008)
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[4] Z.-A. Ren, L. Wei, J. Yang, W. Yi, X.-L. Shen, C. Zheng, G.-C. Che, X.-L. Dong, L.-L. Sun, F. Zhou, Z.-X. Zhao, Chin. Phys. Lett. 25, 2215 (2008) [5] M. Fujioka, S. J. Denholme, T. Ozaki, H. Okazaki, K. Deguchi, S. Demura,
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Figure 11: Annotation of AFM images displayed in Fig.10.
Figure 12: Annotation of AFM images displayed in Fig.5.
H. Hara, T. Watanabe, H. Takeya, T. Yamaguchi, H. Kumakura, Y. Takano, Supercond. Sci. Technol. 26, 085023 (2013)
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[6] J. Pelliciari, M. Dantz, Y. Huang, V. N. Strocov, L. Xing, X. Wang, C. Jin, T. Schmitt, Appl. Phys. Lett. 109, 122601 (2016) [7] N. D. Zhigadlo, S. Katrych, Z. Bukowski, S. Weyeneth, R. Puzniak, J. Karpinski, J. Phys. Condensed Matter 34, 342202 (2008) 18
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[8] J.-Q. Yan, S. Nandi, J. L. Zarestky, W. Tian, A. Kreyssig, B. Jensen, A. Kracher, K. W. Dennis, R. J. McQueeney, A. I. Goldman, R. W. McCallum, T. A. Lograsso, Appl. Phys. Lett. 22, 222504 (2009)
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[9] H. Hiramatsu, T. Katase, T. Kamiya, M. Hirano, H. Hosono, Appl. Phys. Lett. 93, 162504 (2008)
[10] T. Kawaguchi, H. Uemura, T. Ohno, R. Watanabe, M. Tabuchi, T. Ujihara, K. Takenaka, Y. Takeda, H. Ikuta, Appl. Phys. Express 2, 093002 (2009)
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