Single Crystalline Substrates Research Articles

Peculiar critical current density and irreversibility line in double perovskite Ba2SmNbO6-doped SmBa2Cu3Oy film prepared by Nd:YAG pulsed laser deposition

We prepared SmBa2Cu3Oy (SmBCO) films including Ba2SmNbO6 (BSNO) by Nd:YAG pulsed laser deposition (PLD) on LaAlO3 single-crystalline substrates. The BSNO formed many nanorods with a large diameter of 35 nm, which is the largest nanorod diameter in REBa2Cu3Oy films, and their in-plane distribution was random. The matching field estimated from the number density of 202 µm−2 was 0.42 T. We measured Jc–B curves at various measurement temperatures and these Jc–B curves had a Jc peak at approximately 0.37 T. Interestingly, the irreversibility line showed a “reverse S” shape and an anomalous curve was observed at approximately 0.37 T near Tc. To the best of our knowledge, this is the first time that these distinctive features were found in this sample. We concluded that these peculiar behaviors originated from the large-diameter, straight, and threading BSNO nanorods.

Intrinsic hard magnetic properties of Sm(Fe1 − xCox)12 compound with the ThMn12 structure

We have grown Sm(Fe1−xCox)12 (x=0, 0.1 and 0.2) films epitaxially on V(001) buffered MgO(001) single crystalline substrates in order to investigate the intrinsic hard magnetic properties of the Co substituted SmFe12 phase with the ThMn12 structure. We found Sm(Fe0.8Co0.2)12 has excellent intrinsic hard magnetic properties with spontaneous magnetization of 1.78T, anisotropy field of 12T and Curie temperature of 859K, all of which are superior to those for Nd2Fe14B. Hence, the Sm(Fe0.8Co0.2)12 compound is promising as permanent magnet material, if it can be stabilized in a bulk form.

Epitaxial Ba2NdFeNb4O15-based multiferroic nanocomposite thin films with tetragonal tungsten bronze structure

High-quality Ba2NdFeNb4O15-based nanocomposite films were grown on Pt/MgO(100) single crystalline substrates. The c-oriented epitaxial films have the unit cell rotated in the plane by ±18.5° with respect to that of the substrate. The room temperature ferroelectric properties of the films were demonstrated at both the macroscopic and the microscopic scales, attesting that the macroscopic ferroelectricity was conserved down to the nanoscale. The presence of a small amount of nanocrystalline BaFe12O19 as a magnetic secondary phase was confirmed by measuring ferromagnetic hysteresis loops. The coexistence of ferroelectric and ferromagnetic phases attested the achievement of nanocomposite films, exhibiting room temperature multiferroic properties.

Oxygen Reduction Reaction at Self-Assembled La-Sr-Co-O Composite PLD Films: Dependence on Temperature and pO2

Several recent investigations indicated that hetero grain boundaries between perovskite La1-xSrxCoO3- d (P) and Ruddlesden-Popper phase (La1-xSrx)2CoO4+ d (R) may exhibit an accelerated oxygen exchange surface reaction ("TPB effect"), see e.g. [1,2,3]. In the present work, compact La-Sr-Co-O composite films with different fractions of P and R are formed by self assembly on single crystalline YSZ substrates during pulsed laser deposition from a single PLD target [4,5]. The lateral grain size is about 50 nm, and characteristic morphologies can be assigned to the different crystalline phases. In addition to P and R two metastable phases were found: a Co-deficient perovskite phase La0.7Sr0.3Co0.9O3- d, and a higher order Ruddlesden-Popper phase. During the self-organized phase separation, different Sr contents are established in P and R, leading for most sample compositions to a Sr accumulation in R and corresponding depletion in P [4,5]. However, during the PLD process the thermodynamic Sr distribution (as determined in [6] for powder samples annealed at 1200 °C) is not fully reached. Since the oxygen exchange activity of P strongly depends on the Sr content, the knowledge of the actual Sr distribution is required to calculate the correct reference activity for the composite films, i.e. the hypothetical activity in absence of a "TPB effect". At 600 °C, the effective surface rate constant k q measured by impedance spectroscopy of the self-assembled composite films with an overall La:Sr ratio of 7:3 exceed this reference activity by half to one order of magnitude [7]. This corresponds to a strength of the TPB effect normalized to the triple phase density of 2-9×10-13 cm2/s. Furthermore, the Co-deficient perovskite phase exhibits an increase of k q by about two orders of magnitude relative to La0.7Sr0.3CoO3- d(cf. related findings in [8]). The present results are compared to literature, and possible reasons for the increased oxygen exchange activity at hetero grain boundaries are discussed. [1] T. Kawada, M. Sase, M. Kudo, K. Yashiro, K. Sato, J. Mizusaki, N. Sakai, T. Horita, K. Yamaji, H. Yokokawa, Solid State Ionics 177 (2006) 3081 [2] E. J. Crumlin, E. Mutoro, S.-J. Ahn, G. J. la O’, D. N. Leonard, A. Borisevich, M. D. Biegalski, H. M. Christen, Y. Shao-Horn, J. Phys. Chem. Lett. 1 (2010) 3149 [3] W. Ma, J. J. Kim, N. Tsvetkov, T. Daio, Y. Kuru, Z. Cai, Y. Chen, K. Sasaki, H. L. Tuller, B. Yildiz, J. Mater. Chem. A 3 (2015) 207 [4] S. Stämmler, R. Merkle, B. Stuhlhofer, G. Logvenov, J. Maier, poster at SSI-19 in Kyoto, June 2013 [5] S. Stämmler, R. Merkle, B. Stuhlhofer, G. Logvenov, K. Hahn, P. A. van Aken, J. Maier, submitted (2016) [6] S. Stämmler, R. Merkle, B. Stuhlhofer, G. Logvenov, J. Maier, ECS Transact. 68(1) (2015) 579 [7] S. Stämmler, R. Merkle, J. Maier, submitted (2016) [8] A. Takeshita, S. Miyoshi, S. Yamaguchi, T. Kudo, Y. Sato, Solid State Ionics 262 (2014) 378

Crystalline phase and electrical properties of lead‐free piezoelectric KNN‐based films with different orientations

AbstractLead‐free piezoelectric 0.915K0.5Na0.5NbO3‐0.075BaZrO3‐0.01Bi0.5Na0.5TiO3 (KNN‐BZ‐BNT) films were grown on Nb‐doped SrTiO3 substrates with different orientations. The thin films show highly preferential orientations in accordance with the orientations of single crystalline substrates. The films all exhibit a weak contrast and remarkable homogeneity in the local static out‐of‐plane piezoresponse phase images, suggesting a strong self‐polarization. Combining with the analysis of XRD‐RSM, TEM and PFM, the crystalline phase of our samples was determined to be rhombohedral, which has the spontaneous polarization along [111] direction. Thus, the (100)‐oriented film demonstrates the most superior piezoelectric properties. Our detailed studies on structural and electrical properties of KNN‐BZ‐BNT films further clarify the structure‐property relationship and make a step closer to use this lead‐free material for many piezoelectric applications.

Oxygen transport in epitaxial SrTiO<sub>3</sub>/SrTi<sub>1 − <i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3</sub> multilayer stacks

Abstract. Nano-ionic materials made of strontium titanate (SrTiO3, STO) and solid solutions of strontium ferrite in STO (SrTi1 − xFexO3, STF) are grown on single crystalline STO substrates and characterized. Since STF exhibits an oxygen deficiency and, simultaneously, enables oxygen interstitial defects, a space charge area close to the STO | STF interface is present. Oxygen tracer diffusion experiments and impedance spectroscopy at temperatures from 500 to 700 °C and at oxygen partial pressure ranging from 10−3 to 10−23 bar confirm fast oxygen transport caused by enhanced ionic conductivity at the interface. There, an oxygen diffusion coefficient of 3. 4 × 10−10 m2 s−1 at 600 °C and a p type conductivity of about 360 S m−1 at 700 °C are calculated. Such structures open new options in design of nano-ionic materials for oxygen sensors and energy conversion at temperatures lower than those of conventional materials such as yttrium-doped zirconia.

Local heteroepitaxy as an adhesion mechanism in aluminium coatings cold gas sprayed on AlN substrates

Cold gas sprayed Al coatings deposited onto wurtzitic AlN substrates show excellent adhesion. As a possible adhesion mechanism, the local heteroepitaxy between Al and AlN was considered and verified experimentally in Al coatings, which were deposited using magnetron sputtering or cold gas spraying on single-crystalline and polycrystalline AlN substrates. Analysis of the local orientation relationships at the Al/AlN interfaces revealed that preferentially such lattice planes of Al align parallel with the upright lattice planes of AlN, which possess similar interplanar distances. The matching lattice planes in the Al coatings grew as continuations of the lattice planes in the AlN substrates. In all samples under study, the parallel alignment of the lattice planes {220}Al and {110}AlN was found. Additional orientation relationships between Al and AlN arose if parallel lattice planes with similar interplanar spacing could be found in both counterparts via rotation of the lattice planes {220}Al around their normal direction. Still, the oriented growth of Al on AlN is only possible if Al atoms in the deposited coatings are mobile enough to rearrange along the AlN surface. Whereas the mobility of Al atoms in a magnetron sputtering process is expected to be sufficiently high, the intrinsic mobility of Al atoms in the cold gas sprayed particles is anticipated to be low. However, the auxiliary microstructure analyses have shown that local recrystallization and partial melting are two phenomena, which can facilitate the rearrangement of Al atoms within the cold gas sprayed coating.

Fabrication of β-Ga2O3/ZnO heterojunction for solar-blind deep ultraviolet photodetection

A thin-film type β-Ga2O3/ZnO heterojunction was constructed for the first time by radio frequency magnetron sputtering of a β-Ga2O3 layer on a (0001) ZnO single crystalline substrate. The heterojunction presents a typical positive rectification in the dark, and shows a solar-blind deep ultraviolet photoelectric characteristic. Under zero bias, the photodetector based on a β-Ga2O3/ZnO heterojunction exhibits an Iphoto/Idark ratio of ∼14.8 under a 254 nm light illumination with a light intensity of 50 μW cm−2, showing a characteristic of working with zero power consumption. The photocurrent linear increases and the response time decreases with the increase of the light intensity. The photodetector shows a Rλ of 0.35 A W−1 and an EQE of 1.7 × 102% under 254 nm illumination of 50 μW cm−2 and a negative bias of 5 V. This study presents a promising candidate for use in solar-blind deep ultraviolet photodetection.

Metastable phase of lead phthalocyanine films on graphite: Correlation between geometrical and electronic structures

The geometrical and electronic structures of a metastable phase of lead phthalocyanine (PbPc) films on graphite have been studied by combined use of low energy electron diffraction (LEED) and two-photon photoemission (2PPE) spectroscopy. In submonolayer (sub-ML) PbPc films on graphite, islands in a metastable phase are formed just after deposition, as we reported previously by use of photoelectron emission microscopy (PEEM) [I. Yamamoto, N. Matsuura, M. Mikamori, R. Yamamoto, T. Yamada, K. Miyakubo, N. Ueno, and T. Munakata, Surf. Sci. 602, 2232 (2008)]. On single crystalline graphite substrates, the metastable islands produce clearly discernible LEED spots. By comparing the unit cell with that of annealed 1 ML films, molecules in the metastable islands are standing upright with a molecular density 1.8 times higher than that in the well-ordered 1 ML films. The LEED spots for the sub-ML films disappear after annealing. The islands in the metastable phase are surrounded by areas of a two-dimensional (2D) gaslike phase composed of flat-lying molecules. The metastable islands melt into the 2D gas phase, consistent with the PEEM results. In 2PPE spectroscopy, the lowest unoccupied molecular orbital (LUMO) derived level of the metastable phase is clearly distinguishable from that of flat-lying molecules. By tracking the thermal annealing process of the films by 2PPE spectroscopy, we clarify the decay of the LUMO derived peak intensity, the work function shift, and the energy shifts of molecular states associated with the transition from the metastable phase to the 2D gas phase. With this, we demonstrate the complementary capabilities of LEED and 2PPE spectroscopy to probe phase transitions of organic films in a nondestructive manner.

Homoepitaxial Growth of Metal Halide Crystals Investigated by Reflection High-Energy Electron Diffraction

We report the homoepitaxial growth of a metal halide on single crystals investigated with in situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM). Epitaxial growth of NaCl on NaCl (001) is explored as a function of temperature and growth rate which provides the first detailed report of RHEED oscillations for metal halide growth. Layer-by-layer growth is observed at room temperature accompanied by clear RHEED oscillations while the growth mode transitions to an island (3D) mode at low temperature. At higher temperatures (>100 °C), RHEED oscillations and AFM data indicate a transition to a step-flow growth mode. To show the importance of such metal halide growth, green organic light-emitting diodes (OLEDs) are demonstrated using a doped NaCl film with a phosphorescent emitter as the emissive layer. This study demonstrates the ability to perform in situ and non-destructive RHEED monitoring even on insulating substrates and could enable doped single crystals and crystalline substrates for a range of optoelectronic applications.

Suppressed weak antilocalization in the topological insulator Bi2Se3 proximity coupled to antiferromagnetic NiO.

Time-reversal symmetry (TRS) breaking of the topological insulators (TIs) is a prerequisite to observe the quantum anomalous Hall effect (QAHE) and topological magnetoelectric effect (TME). Although antiferromagnetism as well as ferromagnetism could break the TRS and generate massive Dirac surface states in the TIs, no attention has been paid to the antiferromagnet-TI heterostructures. Herein, we report the magnetotransport measurements of Bi2Se3 proximately coupled to antiferromagnetic NiO. Thin films of Bi2Se3 were successfully grown on the NiO (001) single crystalline substrates by molecular beam epitaxy. Unexpectedly, we observed a strong suppression of the weak antilocalization effect, which is similar to the case of TIs coupled to the ferromagnetic materials. For the 5 nm-thick Bi2Se3 sample on NiO, we even observed a crossover to weak localization at 2 K. These behaviors are attributed to the strong magnetic exchange field from the Ni 3d electrons. Our results show the effectiveness of the antiferromagnetic materials in breaking the TRS of TIs by the proximity effect and their possible applications for QAHE and TME observations.

In situ crystallization study of impurity phases in Bi–Fe–O thin films grown by atomic layer deposition

We report on the phase evolution of pure and Fe-doped bismuth oxide (Bi–O and Bi–Fe–O) thin films grown by atomic layer deposition. In situ X-ray diffraction was used to map the evolution of phase formation and orientational growth of the oxides during the gradual crystallization of the films from the amorphous state on single-crystalline substrates. The formation of (001)-oriented Bi2O2.3 was observed at temperatures as low as 300 °C on (001)SrTiO3, with a gradual transformation into Bi2O3. A similar crystallization of Fe-doped Bi2O3 on (111)ZrO2(Y2O3) leads to the formation of (111)-oriented δ-Bi2O3 above 400 °C instead of the sillenite phase. We thus demonstrate that epitaxial stabilization of the metastable phases can take place during atomic layer deposition of oxides as well as post-deposition annealing. In situ crystallization of the amorphous Bi–Fe–O films revealed the evolution of oxide phases in the film with the sillenite composition. Our results are important for the design of annealing procedures to obtain phase-pure epitaxial BiFeO3 thin films from amorphous stoichiometric Bi–Fe–O grown by atomic layer deposition.

Current-voltage characteristics and electroresistance in LaMnO3-δ/La0.7Ca0.3MnO3/LaAlO3 thin film composites.

In this communication, we report results of the electrical transport properties across the interface of composites consisting of n-type LaMnO3-δ (LMO) and p-type La0.7Ca0.3MnO3 (LCMO) manganites grown on LaAlO3 (LAO) single crystalline substrates using low cost wet chemical solution deposition (CSD) and sophisticated, well-controlled dry chemical vapor deposition (CVD) chemical techniques. The XRD ϕ-scan studies reveal the single crystalline nature of both bilayered composites, with parallel epitaxial growth of LMO and LCMO layers onto the LAO substrate. The valence states of Mn ions in both layers of both composites were identified by performing X-ray photoelectron spectroscopy (XPS). The I-V characteristics of the LMO/LCMO interfaces show strong backward diode-like behavior at higher applied voltages well above the crossover voltage (VNB). Below VNB, the interfaces demonstrate normal diode-like characteristics throughout the studied temperature range. The electric field-induced modulation of the LMO/LCMO junction resistance of the interfaces has been observed. Electric field-dependent electroresistance (ER) modifications at different temperatures have also been studied. The electrical transport properties have been discussed in the context of various mechanisms, such as charge injection, tunneling, depletion region modification and thermal processes across the interface. The effects of structurally and chemically developed sharp interfaces between the LMO and LCMO layers on the transport properties of the presently studied bilayered thin film composites have been discussed on the basis of correlation between the physicochemical characterization and charge transport behavior. A comparison of different aspects of the transport properties has been presented in the context of the structural strain and crystallinity of the composites grown using both wet and dry chemical techniques.

Experimental investigations of atomic ordering effects in the epitaxial GaxIn1-xP, coherently grown on GaAs (100) substrates

A range of structural and spectroscopic techniques were used for the study of the properties of epitaxial GaxIn1-xP alloys with an ordered arrangement of atoms in a crystal lattice grown by MOCVD on single-crystalline substrates of GaAs (100). The appearance of atomic ordering in the coherent growth conditions of the ordered GaxIn1-xP alloy on GaAs (100) resulted in cardinal changes of the structural and optical properties of semiconductor in comparison to disordered alloys, including the change of the crystal lattice parameter and, consequently, reduced crystal symmetry, decreased band gap and formation of two different types of surface nanorelief.This is the first report of the calculation of parameters of the crystal lattice in GaxIn1-xP with ordering taking into account the elastic stresses dependent on long-range ordering. Based on the variance analysis data with regard to the IR-reflection spectra as well as the UV-spectroscopy data obtained in the transmission-reflection mode, the main optical characteristics of the ordered GaxIn1-xP alloys were determined for the first time, namely, refractive index dispersion and high-frequency dielectric constant. All of the experimental results were in good agreement with the previously developed theoretical beliefs.

Microstructure and in-plane component of L10-FePt films deposited on MgO and MgAl2O4 substrates

Microstructures of FePt films deposited on MgO and MgAl2O4 single crystalline substrates were studied using aberration corrected scanning transmission electron microscopy (STEM). The [100] and [010] variants of L10-FePt were observed in-the-plane of the FePt grains deposited on the MgAl2O4 substrate, which causes in-plane component in magnetization curves. The d-spacing of the (002) planes of L10-FePt deposited on MgAl2O4 is larger than that deposited on MgO. The reduction of lattice mismatch between FePt and the substrate results in the formation of out-of-plane tensile strain leading to increase in d002 and the formation of the in-plane variants in the L10-FePt grains.

Metal-insulator transition in epitaxial NdNiO3 thin film: A structural, electrical and optical study

NdNiO3 thin film has been prepared by pulsed laser deposition on LaAlO3 (001) single crystalline substrate. Temperature-dependent resistivity measurement shows a sharp metal-insulator transition in such thin film. The phase transition temperature can be tuned from 90K to 121K by changing the thickness of thin film. The structure evolution during phase transition is studied by Raman spectroscopy. Optical conductivity reveals that the variation carrier density in the process of phase transition. The results of structural, electrical and optical studies provide useful insights to understand the mechanism of metal-insulator transition of NdNiO3 thin film.

Infrared reflection spectroscopy and optical constants of LiNbO3films on crystal substrates

We have measured infrared reflectivity spectra of thin lithium niobate films of nanometer thickness, grown by a pulsed laser deposition technique using KrF-excimer laser (λ=248 nm) on the single crystalline substrates (sapphire, MgO, NdGaO3 and SrTiO3). Using the dispersion analysis technique, we have calculated thicknesses and optical constants of the films. The phonon parameters of the substrates and films are obtained.

Planar current anisotropy and field dependence of Jc in coated conductors assessed by scanning Hall probe microscopy

The local distribution of the critical current density, Jc, of coated conductors and YBa2Cu3O (YBCO) films on single crystalline substrate was investigated by scanning Hall probe microscopy. The high spatial resolution of the measurements enabled an assessment of dependence of the local Jc on the local magnetic induction, B, and electric field, E. The derived Jc(B)-dependence agreed well with the global Jc obtained from magnetization loops and provided values of Jc at very low fields, which are inaccessible to magnetization and transport measurements. The anisotropic current flow within the film plane was investigated in YBCO films on miscut SrTiO3 substrates and a GdBa2Cu3O film on an MgO buffer layer prepared by inclined substrate deposition on a Hastelloy substrate. The c-axis currents calculated from the Hall maps were significantly larger than previously reported data obtained from direct transport measurements. The planar current anisotropy at 77 K was found to be highly influenced by the microstructure which can either deteriorate the current flowing across the ab-planes or cause enhanced pinning and increase the critical current flowing parallel to the ab-planes.

Atomic Resolution Interfacial Structure of Lead-free Ferroelectric K0.5Na0.5NbO3 Thin films Deposited on SrTiO3

Oxide interface engineering has attracted considerable attention since the discovery of its exotic properties induced by lattice strain, dislocation and composition change at the interface. In this paper, the atomic resolution structure and composition of the interface between the lead-free piezoelectric (K0.5Na0.5)NbO3 (KNN) thin films and single-crystalline SrTiO3 substrate were investigated by means of scanning transmission electron microscopy (STEM) combining with electron energy loss spectroscopy (EELS). A sharp epitaxial interface was observed to be a monolayer composed of Nb and Ti cations with a ratio of 3/1. The First-Principles Calculations indicated the interface monolayer showed different electronic structure and played the vital role in the asymmetric charge distribution of KNN thin films near the interface. We also observed the gradual relaxation process for the relatively large lattice strains near the KNN/STO interface, which remarks a good structure modulation behavior of KNN thin films via strain engineering.

Applicability of MOS structures in monitoring catalytic properties, as exemplified for monolayer-iron-oxide-coated porous platinum films

Metal Oxide Semiconductor (MOS) capacitor devices comprised of monolayer iron oxide-coated as well as non-coated polycrystalline Pt deposited on oxidized silicon carbide substrates have been fabricated and their usefulness as realistic model systems in catalyst studies/development was evaluated. The CO oxidation characteristics of both iron oxide- and non-coated Pt catalysts were investigated using mass spectrometry, monitoring the carbon dioxide production rate for different combinations of carbon monoxide (CO) and oxygen concentrations at various temperatures. Additionally, the output capacitance of the MOS model catalysts was recorded for each individual CO oxidation activity. A low-temperature shift in CO oxidation characteristics for the monolayer-coated compared to the non-coated Pt catalysts was observed, similar to that previously reported for monolayer iron oxide grown on single-crystalline Pt substrates. A strong correlation between the output capacitance of the MOS structures and the CO oxidation characteristics was found for both monolayer- and non-coated model catalysts. Furthermore, the devices exhibit retained MOS electrical output and CO oxidation characteristics as well as an unaffected catalyst surface composition, as confirmed by photoelectron spectroscopy, even after 200h of continuous model catalyst operation. In addition to the implications on practical applicability of monolayer iron oxide coating on widely used polycrystalline Pt films in real-world catalysts and sensors, the findings also point to new possibilities regarding the use of MOS model systems for in situ characterization, high-throughput screening, and tailoring of e.g. catalyst- and fuel-cell-electrode materials for specific applications.