LSAT Substrates Research Articles

Tunable semimetallic state in compressive-strainedSrIrO3films revealed by transport behavior

Orthorhombic SrIrO3 is a typical spin-orbit-coupling correlated metal that shows diversified physical properties under the external stimuli. Here nonlinear Hall effect and weakly temperature-dependent resistance are observed in a SrIrO3 film epitaxially grown on SrTiO3 substrate. It infers that orthorhombic SrIrO3 is a semimetal oxide. However, linear Hall effect and insensitive-temperature-dependent resistance are observed in SrIrO3 films grown on (La,Sr)(Al,Ta)O3 (LSAT) substrates, suggesting a tunable semimetallic state due to band structure change in SrIrO3 films under different compressive strain. The mechanism of this evolution is explored in detail through strain-state analysis by reciprocal space mapping and electron diffraction, carrier density and mobility calculations, as well as electronic band structure evolution under compressive strain (predicted by tight-binding approximation). It might suggest that the strain-induced band shift leads to the semimetallic tuning in the SrIrO3 film grown on from SrTiO3 to LSAT substrates. Our findings illustrate the tunability of SrIrO3 properties and pave the way to induce novel physical states in SrIrO3 such as the proposed topological insulator state in heterostructures.

Surface precipitates formed on annealed LSAT (001) single crystal.

LSAT (La0.3Sr0.7)(Al0.65Ta0.35)O3, which has a complex perovskite structure of (A'A'')(B'B'')O3, is expected as an attracting substrates for GaN and high temperature superconductivity oxides solid thin films from a viewpoint of the suitable lattice matching. To grow high quality thin film, it is very important to prepare step-terrace structure on substrates used for thin film growth. For this purpose, a technique of annealing substrates with mirror surface is often used. However, surface precipitates, called surface mounts, are reported to appear after annealing LSAT substrates [1]. In this study, we investigated the surface precipitates formed on annealed LSAT surfaces by TEM/STEM. Further, we directly confirmed the terminated atomic layers at the annealed LSAT surfaces in the area without surface precipitates.Commercially available LSAT single crystal substrates with (001) surfaces (SHINKOSHA CO.,LTD) were used for TEM/STEM observation. After annealing at 1300°C for 30 min in air, the (001) surface structures were observed from [110] direction using cross sectional thin foils. The thin foils were prepared by joining two annealed LSAT (001) surfaces with glue, grinding, polishing and finally Ar ion milling. TEM/STEM observation was conducted by JEOL ARM-200F (a double Cs-corrector type for TEM/STEM) operated at 200kV.Surface mounds were confirmed to appear on LSAT crystal surface after annealing at the annealing condition used in this study. A typical example is shown in Fig.1. shows TEM bright field image taken from the surface area of LSAT (001) after annealing. The observation direction of the image is [110], which is parallel to the annealed surface. Cross sectional images of surface mounts with 300nm was clearly seen as indicated by the arrows in the image. The height of the mounts is around 20nm, and it is noted that the interfaces between the mounts and LSAT surfaces are hollowed into LSAT crystal with the depth about 10nm. Nano diffractometric and EDS analysis have revealed that the mounts are amorphous structure with mainly Al and Sr. So far, the surface mounts has been considered to be SrO precipitation because the mounts easily dissolve into water. After cleaning in water, smooth surfaces for thin film growth can be considered to be obtained. However, the mounts are revealed to be formed on the hollowed surfaces of LAST as shown in Fig.1. To use LSAT annealed surfaces as suitable substrates for thin film growth, the hollowed surface structure should be considered.jmicro;63/suppl_1/i20/DFU049F1F1DFU049F1Fig. 1.TEM bright field image of surface mounds on LSAT (001) annealed surface. Meanwhile, we determined the terminated atomic layers of the surface by carrying out high resolved HAADF-STEM observation at the areas without the mounts. After annealing, the surfaces without any mounts have atomically flat (001) surface structure at an atomic level. To clarify the terminated atomic layers, it must be necessary to assign a position of a unit cell for atomically resolved HAADF-STEM image. To carry out the assignment, we used an ordered domain structure exiting in LSAT crystal [2]. In the ordered domains, the cations of Al and Ta occupy at B-site periodically. Observing the B-site ordered domains by HAADF-STEM, the respective B-site atomic columns can be distinguished separately with contrast difference due to the atomic number of Al and Ta ions. To use the contrast variation, we can precisely assign the unit cell position for the obtained HAADF-STEM image. As a result, it was found that the annealed surfaces were terminated at B-site at the area without any mounts. Further, EDS analysis has revealed that Al/Ta ratio at terminated B-site layer is different from that of crystals, which is Al/Ta ∼ 1. There is a report informing that La ions tend to vapor during annealing LSAT [1]. The formation of the surface mounts is closely related to the vaporization of La ions near surfaces. The mounts was formed from residual ions of Al and Sr. As a result, the surface areas without any mounts can be considered to include Ta more.

Strain-Induced First-Order Magnetic Phase Transition in Epitaxial Sm0.35Pr0.15Sr0.5MnO3 Thin Film

Epitaxial Sm0.35Pr0.15Sr0.5MnO3 thin films were deposited on LaAlO3 (LAO, (001)), SrTiO3 (STO, (001)), and (La0.18Sr0.82)(Al0.59Ta0.41)O3 (LSAT, (001)) single-crystalline substrates by using pulsed laser deposition technique. In order to examine the strain effect on electronic and magnetic properties, films were studied by X-ray diffraction, electrical resistivity, and dc magnetization measurements. The film grown on LAO substrate is under compressive strain, and it undergoes ferromagnetic → paramagnetic transition at Curie temperature (T C) of ∼ 165 K and metal → insulator transition at ∼ 107 K. The films grown on STO and LSAT substrates are under tensile strain and have T C of ∼ 120 and 130 K, respectively, and show metal → insulator transition at ∼ 145 and 137 K, respectively. At T < T C, the zerofield and fieldcooled magnetization curves of all the films show a huge bifurcation. In the case of films on STO and LSAT substrates, hysteresis is also observed in fieldcooled cooling and warming magnetization vs. temperature measurement protocols at low magnetic field. All the signatures of the firstorder magnetic phase transition are absent in the case of film on LAO substrate. The occurrence and absence of firstorder magnetic phase transition in films on LAO, STO, and LSAT substrates, respectively, have been well explained through the substrateinduced film lattice strain.

Influence of La and Mn vacancies on the electronic and magnetic properties ofLaMnO3thin films grown by pulsed laser deposition

With pulsed laser deposition we have grown c-axis oriented thin films of the nominal composition LaMnO$_{3}$ (LMO) on LSAT(001) substrates. We find that, depending on the oxygen background pressure during growth, the LMO films contain sizeable amounts of La and/or Mn vacancies that strongly influence their electronic and magnetic properties. Specifically, we show that the Mn/La ratio can be systematically varied from 0.92 at 0.11 mbar to 1.09 at 0.30 mbar of oxygen. These cationic vacancies lead to markedly different disorder effects that become most pronounced once the samples are fully oxygenated and thus strongly hole doped. All as-grown and thus slightly oxygen deficient LMO films are ferromagnetic insulators with saturation moments in excess of 2.5 \mu $_{B}$ per Mn ion, their transport and optical properties that can be understood in terms of trapped ferromagnetic polarons. Upon oxygen annealing, the most La-deficient films develop a metallic response with an even larger ferromagnetic saturation moment of 3.8 \mu $_{B}$ per Mn ion. In contrast, in the oxygenated Mn-deficient films the ferromagnetic order is almost completely suppressed to less than 0.5 \mu $_{B}$ per Mn ion and the transport remains insulator-like. We compare our results with the ones that were previously obtained on bulk samples and present an interpretation in terms of the much stronger disorder potential of the Mn vacancies as compared to the La vacancies. We also discuss the implications for the growth of LMO thin films with well-defined physical properties that, for example, are a prerequisite for the study of interface effects in multilayers.

Chemical reaction between BaFe2(As,P)2 superconducting thin film and LSAT substrate

BaFe2(As0.67P0.33)2 (Ba122:P) thin films were fabricated on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (LSAT) (100) substrates by a pulsed laser deposition method using a second-harmonic Nd:YAG laser. Superconducting Ba122:P thin films with c-axis orientation and in-plane alignment were successfully obtained. Detailed structural properties on the films were investigated by X-ray diffraction. The X-ray pattern suggested the existence of some apatite-type phase. Transmission electron microscope observation revealed the formation of a reacted layer between the film and the substrate. The reacted layer had a Ba(Sr)-rich and P-rich composition. These analyses indicated the formation of an apatite material at the interface of the film and the substrate.

Thickness dependent functional properties of PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 heterostructures

The ultra thin ferroelectric PbZr0.52Ti0.48O3 (PZT) films with various thicknesses ranging from 100 (P100) to 10 (P10) nm were grown on La0.67Sr0.33MnO3/(LaAlO3)0.3(Sr2AlTaO6)0.7 (LSMO/LSAT) (001) substrates deposited by pulsed laser deposition technique. The x-ray diffraction patterns of the heterostructures show only (00l) (l = 1 and/or 2) reflections corresponding to the LSAT substrate, PZT, and LSMO layers. The atomic force microscopy studies show that the root mean square surface roughnesses of P100 and P10 films are 2.39 and 0.99 nm, respectively. An increase of both real (ε′) and imaginary (ε″) permittivities was observed when thickness of PZT increases from 10 nm to 100 nm. Temperature dependent ε′ presents an anomaly, related to ferromagnetic-metal to paramagnetic-insulator transition of the LSMO, in the range of 350–500 K. The dielectric anomalies and magnetic phase transition temperature shifted to the lower temperature values with decrease in the PZT films thicknesses. The values of the remanent polarization (Pr) and coercive field (Ec) of the heterostructures were in the range of 24–42 μC/cm2–170–1300 kV/cm. An appreciable increase of saturated magnetization (Ms) was observed with increase of PZT layer thickness. The average Ms values of PZT/LSMO heterostructure were 170, 150, 100, and 45 emu/cm3 for 100, 50, 25, and 10 nm at 300 K, respectively. Enhancement in magnetization with increase in PZT thickness may be due to the interface effect between PZT/LSMO layers.

Optical spectra of LaMn0.5Ga0.5O3: A contribution to the assignment of the electronic transitions in manganites

We have prepared a LaMn0.5Ga0.5O3 thin film deposited by Pulsed Laser Deposition on a LSAT substrate and we have measured its transmittance in the visible and near UV range, from 300 to 10K. The aim was to shed light on the origin of the electronic bands in the LMO family through a comparison with the corresponding spectra collected previously on the parent compound. Two out of the four bands detected have been assigned to the intersite d–d transitions between Mn+3 ions. The other ones were ascribed to the p–d Mn–O charge-transfer transitions. We are thus led to a “mixed” interpretation where both a Mott–Hubbard and a charge-transfer approach concur to interpret the electronic spectrum of those manganites.

Labyrinth-type domain structure of heteroepitaxial SrMnO2.5 film

SrMnO2.5 films grown on SrTiO3 and LSAT substrates were prepared and oxygen vacancies in the films were directly confirmed by annular bright-field imaging in a scanning transmission electron microscope. The SrMnO2.5 films show a unique maze like pattern of domains, i.e., a labyrinth-type domain structure, that arises from relaxation of strain induced by lattice mismatches. The behavior of the domain widths is explained by the geometrical relationship for estimating of the average distance of misfit dislocations. We present useful principles for controlling the domain structures with a view to potential practical applications.

Influence of Substrate on Structure and Magnetic Properties of Bi&lt;sub&gt;0.9&lt;/sub&gt;Nd&lt;sub&gt;0.1&lt;/sub&gt;FeO&lt;sub&gt;3 &lt;/sub&gt;Thin Films by Radio Frequency Magnetron Sputtering Method

Bi0.9Nd0.1FeO3 (BNFO) films were deposited on Si (100) and (La,Sr)(Al,Ta)O3 (100) (LAST) substrate by radio frequency (RF) magnetron sputtering method respectively. The structure，morphology and magnetic properties were studied. X-ray diffraction (XRD) result indicates that the BNFO films on different substrate adopted different orientation. Cross-section scanning electron microscopy shows that the film thickness is 145 nm．Magnetic properties measurement shows that the film on Si(100) substrate has the larger saturation magnetization (Ms) of 3 686 emu/cm3, while the Ms value of the BNFO films on LSAT(100) substrate is only 1 213 emu/cm3.

Laser Induced Thermoelectric Voltage Effect of LaNiO&lt;sub&gt;3&lt;/sub&gt;/LaAlO&lt;sub&gt;3&lt;/sub&gt; Multilayer

LaNiO3 single layer, LaNiO3/LaAlO3 superlattice on vicinal-cut single crystalline LaAlO3 (100) and LSAT (100) substrates were prepared by pulsed laser deposition technique. Single-phased LaNiO3 polycrystal target, which is synthesized by sol-gel method, is employed in the thin film deposition process. The structure and physical properties of both the targets and thin films are analyzed. X-ray diffraction results show LaNiO3 single layer is of c-orientation and textural growth, therefore LaNiO3/LaAlO3 forms not superlattice but multilayer. The ultra-violet LITV (laser induced thermoelectric voltage) effects of the LaNiO3/LaAlO3 mutilayer and LaNiO3 single layer were investigated. A comparison study of the influence of LaNiO3 targets prepared by different sol-gel method on the physical property, and LITV effect of thin films was performed as well.

Effects of oxygen content on the structural and transport properties in epitaxial [formula omitted] thin films

Epitaxial Nd0.7Sr0.3MnO3−x (NSMO) thin films, 120nm thickness, were grown coherently on the (001) (LaAlO3)0.3 (Sr2AlTaO6)0.7 (LSAT) substrates by using pulsed-laser deposition (PLD). Effects of oxygen contents controlled by post-annealing in vacuum on the structural and transport properties of the NSMO films were carefully studied by x-ray reciprocal space maps (RSMs) and resistivity measurements. It is clearly shown that both the out-of-plane lattice parameters of the films and the transition temperatures Tp (peak resistance) can simultaneously change as the oxygen contents, the in-plane lattice coherency is maintained consistently with that of the LSAT substrate at the same time. The larger altered transport properties are chiefly ascribed to the increase of Mn3+ ions and the distortion of MnO6 octahedra in films. The oxygen compositions are deduced from the out-of-plane lattice parameters of coherency epitaxy NSMO films. The Kröger–Vink notation was used for explaining the correlation of oxygen vacancy and transport properties.

Effect of misfit strain on multiferroic and magnetoelectric properties of epitaxial La0.7Sr0.3MnO3/BaTiO3 bilayer

La0.7Sr0.3MnO3/BaTiO3 (LSMO/BTO) bilayer films were grown on (0 0 1) oriented SrTiO3 (STO), LaAlO3 (LAO) and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates by pulsed laser deposition. The bilayer deposited on the LSAT substrate presented the best ferromagnetic properties due to the smallest lattice mismatch between LSAT and LSMO. However, the best ferroelectric properties and strongest magnetoelectric (ME) effect existed in the bilayer on the STO substrate due to the minimal compressive strain in the BTO film. All these showed similar frequency-dependent ME behaviour from 0.1 to 100 kHz, and the largest value of ME voltage coefficient (αE) was obtained on the STO substrate. The values of αE were around 263 mV cm−1 Oe−1, 202 mV cm−1 Oe−1 and 169 mV cm−1 Oe−1 at 1 kHz on STO, LSAT and LAO substrates, respectively, which were at least one order of magnitude higher than previously reported values for similar composites and were ascribed mainly to the rather low dielectric constant of the BTO film.

Low field anisotropic colossal magnetoresistance in Sm0.53Sr0.47MnO3 thin films

Sm0.53Sr0.47MnO3 (SSMO) thin films (thicknesses ∼200 nm) were deposited by on-axis dc magnetron sputtering on the single crystal LSAT (001) substrates. These films are oriented along the out of plane c-direction. The ferromagnetic and insulator-metal transition occurs at TC ∼ 96 and TIM ∼ 91 K, respectively. The magnetization easy axis is observed to lie in the plane of the film while the magnetic hard axis is found to be along the normal to this. The magnetotransport of the SSMO films, which was measured as a function of angle (θ) between the magnetic field (H) and plane of the film, shows colossal anisotropy. Magnetoresistance (MR) decreases drastically as θ increases from 0° (H//easy axis) to 90° (H//hard axis). The out-of-plane anisotropic MR is as high as 88% at H = 3.6 kOe and 78 K. The colossal anisotropy has been explained in terms of the magnetic anisotropies at play and the magnetic domain motion in applied magnetic field.

Influence of substrates on epitaxial growth of B-site-ordered perovskite La2NiMnO6 thin films

Epitaxial La2NiMnO6 films were grown on (100)-oriented SrTiO3 and (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates under various growth conditions by pulsed laser deposition. Ferromagnetism and transport properties of B-site-ordered perovskite are greatly influenced by growth conditions. Optimum growth conditions of epitaxial La2NiMnO6 films, which exhibit the ferromagnetism similar to bulk properties, are different for SrTiO3 and LSAT substrates probably due to the difference in surface migration. Since the initial stage of epitaxial growth might vary with the type of substrates, the ordering of Ni and Mn ions is different. Therefore, the ferromagnetism of La2NiMnO6 depends on the epitaxial growth conditions and type of substrates.

An electrode-free method of characterizing the microwave dielectric properties of high-permittivity thin films

A thin dielectric resonator consisting of a dielectric substrate and the thin film deposited upon it is shown to suffice for microwave characterization and dielectric parameter measurement of high-permittivity thin films without electrodes. The TE01δ resonance mode was excited and measured in thin (down to 0.1 mm) rectangular- or disk-shaped low-loss dielectric substrates (D∼10 mm) with permittivity ε′≥10 inserted into a cylindrical shielding cavity or rectangular waveguide. The in-plane dielectric permittivity and losses of alumina, DyScO3, SmScO3, and (LaAlO3)0.29(SrAl1/2Ta1/2O3)0.71 (LSAT) substrates were measured from 10 to 18 GHz. The substrate thickness optimal for characterization of the overlying thin film was determined as a function of the substrate permittivity. The high sensitivity and efficiency of the method, i.e., of a thin dielectric resonator to the dielectric parameters of an overlying film, was demonstrated by characterizing ultrathin strained EuTiO3 films. A 22 nm thick EuTiO3 film grown on a (100) LSAT substrate and strained in biaxial compression by 0.9% exhibited an increase in microwave permittivity at low temperatures consistent with it being an incipient ferroelectric; no strain-induced ferroelectric phase transition was seen. In contrast, a 100 nm thick EuTiO3 film grown on a (110) DyScO3 substrate and strained in biaxial tension by 1% showed two peaks as a function of temperature in microwave permittivity and loss. These peaks correspond to a strain-induced ferroelectric phase transition near 250 K and to domain wall motion.

AFM Observations of Domain Structures in Thin Films of BiFeO3

We have observed surfaces of multiferroic thin films of the BiFeO3 using the atomic force microscopy. Heterostructures BiFeO3/YBa2Cu3O7 on LSAT substrates were studied by various AFM techniques to reveal ferroic properties. LFM and tapping mode revealed domains of average widths: 36 nm and 58 nm for samples of 25 nm and 200 nm thick, respectively. Domain structure observations and its transformation have been performed by piezoresponse technique. We have also recorded features indicating presence of the magnetic domain structure by magnetic force method. Domain widths were estimated and compared with a classical model.

Barium Strontium Zirconate Titanate (Ba,Sr)(Zr,Ti)O3 Thin Films for Tunable Microwave Applications

(Ba0.55Sr0.45)(Zr0.1Ti0.9)O3 (BSZT) thin films were deposited by pulsed laser deposition on LSAT(001) substrates. X-ray diffraction characterization reveals a good quality of crystallization and epitaxial nature of the films. The in-plane dielectric properties of the films were characterized over a wide frequency range from 50 MHz to 20 GHz by using ring resonator-structured samples. Based on the S-parameter measurements and electromagnetic simulation it was found that microwave dielectric constant of the BSZT thin film is ∼350. Large dielectric tunability was also observed in this material.

An Epitaxial Transparent Conducting Perovskite Oxide: Double-Doped SrTiO3

Epitaxial thin films of strontium titanate doped with different concentrations of lanthanum and oxygen vacancies were grown on LSAT substrates by pulsed laser deposition technique. Films grown with 5−15% La doping and a critical growth pressure of 1−10 mTorr showed high transparency (>70−95%) in the UV−visible range with a sheet resistance of 300−1000 Ω/◻. With the aid of UV−visible spectroscopy and photoluminescence, we establish the presence of oxygen vacancies and the possible band structure, which is crucial for the transparent conducting nature of these films. This demonstration will enable development of various epitaxial oxide heterostructures for both realizing opto-electronic devices and understanding their intrinsic optical properties.

Epitaxial-strain effect on charge/orbital order in Pr0.5Ca0.5MnO3 films

Effect of growth orientation on charge- and orbital-ordering (CO-OO) phenomena has been studied for Pr0.5Ca0.5MnO3 epitaxial thin films fabricated on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates by means of resistivity, synchrotron x-ray diffraction, and polarized optical microscopy measurements. CO-OO transition is observed around 220 K for a film grown on an LSAT (011) substrate [(011) film], similarly to a bulk sample, while a film grown on a (001) plane of LSAT [(001) film] shows much higher transition temperature around 300 K. The domain size of OO is approximately three times as large in the (011) film as in the (001) film. These results demonstrate that various properties of CO-OO phenomena can be controlled with the growth orientation via the epitaxial strain from the substrate.

Observation of Strain Effects of SrTiO3 Thin Films by Terahertz Time-Domain Spectroscopy with a 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Emitter

We measured strain effects of SrTiO3 (STO) thin films by terahertz time-domain spectroscopy (THz-TDS) with a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal that was expected to be a broadband THz emitter. STO thin films were deposited on MgAl2O4, MgO, and LSAT substrates, and lattice constants of the films changed from the value of unstrained STO bulk because of lattice mismatches between STO and the substrates. The dielectric dispersions of the STO thin films deposited on MgO and MgAl2O4 were measured up to 6.0 THz by the broadband THz-TDS system and all the features of dielectric peaks of soft modes responsible for ferroelectricity were clearly observed. The soft-mode frequencies of the films shifted from the original value of STO bulk owing to the strain effects. In particular, the softening of the soft mode of the film on MgAl2O4, which implied that the in-plane tensile strain enhanced ferroelectric fluctuations, was of interest.