STO Substrates Research Articles

Substrate Control of Anisotropic Resistivity in Heteroepitaxial Nanostructured Arrays of Cryptomelane Manganese Oxide on Strontium Titanate

Resistivity and resistance measurements have been carried out for thin films of cryptomelane-type manganese oxide (OMS-2) grown onto (001), (110), and (111)STO single crystals substrates via pulsed laser deposition. While the symmetries of the (001) and (111)STO substrate surfaces give deposits consisting of multiple nanofiber arrays with isotropic in-plane resistivities, only a single nanofiber array is formed on (110)STO giving highly anisotropic electrical properties with very low resistivity values measured parallel to the fibers and similar to the lowest value ever reported.

Comparative study of magnetic and magnetotransport properties of Sm0.55Sr0.45MnO3 thin films grown on different substrates

Highly oriented polycrystalline Sm0.55Sr0.45MnO3 thin films (thickness ∼100 nm) deposited on LaAlO3 (LAO, (001)), SrTiO3 (STO, (001)) and (La0.18Sr0.82) (Al0.59Ta0.41)O3 (LSAT, (001)) single crystal substrates by ultrasonic nebulized spray pyrolysis have been studied. The out of plane lattice parameter (OPLP) of the film on LAO is slightly larger than that of the corresponding bulk. In contrast, the OPLP of the films on STO and LSAT are slightly smaller than the corresponding bulk value. This suggests that the film on LAO is under compressive strain while LSAT and STO are under tensile strain. The films on LAO and LSAT show simultaneous paramagnetic-ferromagnetic (PM–FM) and insulator-metal transition (IMT) temperature at TC/TIM ∼ 165 K and 130 K, respectively. The PM–FM and IM transition occur at TC ∼ 120 K and TIM ∼ 105 K, respectively in the film on STO substrate. At T < TC, the zero field cooled–field cooled (ZFC–FC) magnetization of all the films shows strong bifurcation. This suggests the presence of a metamagnetic state akin to cluster glass formed due to coexisting FM and antiferromagnetic–charge order (AFM–CO) clusters. All the films show colossal magnetoresistance but its temperature and magnetic field dependence are drastically different. The films on LAO and STO show peak CMR around TC/TIM, while the film on LSAT shows MR > 99 % over a very wide temperature range of ∼40 K centred on TC/TIM. In the lower temperature region the magnetic field dependent isothermal resistivity also shows signature of metamagnetic transitions. The observed results have been explained in terms of the variation of the relative fractions of the coexisting FM and AFM–CO phases as a function of the substrate induced strain and oxygen vacancy induced quenched disorder.

Carrier density modulation by structural distortions at modified LaAlO3/SrTiO3 interfaces

In order to study the fundamental conduction mechanism of LaAlO3/SrTiO3 (LAO/STO) interfaces, heterostructures were modified with a single unit cell interface layer of either an isovalent titanate ATiO3 (A = Ca, Sr, Sn, Ba) or a rare earth modified Sr0.5RE0.5TiO3 (RE = La, Nd, Sm, Dy) between the LAO and the STO. A strong coupling between the lattice strain induced in the LAO layer by the interfacial layers and the sheet carrier density in the STO substrate is observed. The observed crystal distortion of the LAO is large and it is suggested that it couples into the sub-surface STO, causing oxygen octahedral rotation and deformation. We propose that the ‘structural reconstruction’ which occurs in the STO surface as a result of the stress in the LAO is the enabling trigger for two-dimensional conduction at the LAO/STO interface by locally changing the band structure and releasing trapped carriers.

Stoichiometry dependence and thermal stability of conducting NdGaO3/SrTiO3 heterointerfaces

The structural and electrical properties of conducting NdGaO3/SrTiO3 (NGO/STO) heterostructures grown at various deposition temperatures were investigated. X-ray diffraction and X-ray photoelectron spectroscopy reveal a strong impact of the growth temperature on both crystallinity and cation stoichiometry of the NGO thin films. This stoichiometry variation significantly affects the electrical properties of the NGO/STO interface. High temperature conductance measurements under oxygen equilibrium conditions show a distinct conductance contribution of the NGO/STO interface up to 1000 K and exclude a conduction effect caused by a mere reduction of the STO substrate. Above 1000 K, the interface conduction is degrading due to a thermal instability. Both stoichiometry variation in as-grown films and thermal instability are attributed to the preferential evaporation of gallium from the NGO thin films at elevated temperatures.

Growth and characterization of topological insulator Bi2Se3 thin films on SrTiO3 using pulsed laser deposition

Bismuth selenide (Bi2Se3) thin films were grown on SrTiO3(111) (STO) substrates using pulsed laser deposition (PLD). The structural, morphological, electrical, and transport properties were studied at various substrate temperatures (TS) from 120 to 350°C. Amorphous films grown at TS<180°C exhibited semiconducting behavior, and highly c-axis-oriented textured films deposited at TS≥180°C exhibited metallic behavior. Bi2Se3 thin-films were epitaxially grown on STO substrates at 300 and 350°C. Thickness-dependent characteristics were also investigated for optimized Bi2Se3 films deposited at TS=230°C. The semiconducting or metallic characteristics of Bi2Se3 films prepared using PLD were observed through electrical and transport results.

Temperature controlled c axis elongated low symmetry phase BiFeO3 thin film on STO substrate

BiFeO3 thin films with a mixture of tunable R-like and c axis elongated low symmetry phase (T-like phase) are fabricated on STO (001) substrate through controlling of the substrate temperature. Almost pure T-like phase can be grown on STO substrate at 600°C. Comparing with the situations on LAO (001), it is found that, strains from the LAO substrate may be the only reason that induces the T-like phase at higher temperatures. At lower temperatures, the island growth induced strains alone can also generate T-like phase on STO substrate.

XBaTiO3–(1−x)BiFeO3 strained epitaxial thin film with enhanced magnetization

Strained epitaxial xBaTiO3–(1−x)BiFeO3 (x=0–0.2) thin films were grown on STO (001) substrate by radio frequency magnetron sputtering. The crystal structure of the as-deposited thin films were investigated using the combination of powder X-ray diffraction (XRD) and high resolution X-ray diffraction reciprocal space mapping (HRXRD–RSM). The relationship of the crystal structure and magnetic properties of the as-deposited thin films was also investigated. The larger c/a values provide the iron ion with a non-vanished orbital quenching effect, which produces the strong spin-orbital coupling that suppresses the spiral spin structure and initiates a canted antiferromagnetic phase with larger magnetizations at room temperature.

Inhomogeneous Microstructure and Electrical Transport Properties at the LaAlO3/SrTiO3Interface

Medium-energy ion spectroscopy (MEIS), scanning transmission electron microscopy (STEM) and X-ray photoemission spectroscopy (XPS) were used to investigate the composition and microstructure of LaAlO3/SrTiO3 (LAO/STO) interfaces grown by pulsed laser deposition of LAO on TiO2-terminated STO substrates under different oxidizing conditions. MEIS and XPS indicated Sr/La and Al/Ti intermixing within several atomic layers at all studied interfaces. XPS and STEM revealed that La diffuses deeper than Al. Analysis of the MEIS data suggests inhomogeneous lateral distribution of the diffused elements. This is further supported by the observation of a large positive magneto-resistance at low temperatures. We discuss the role of lateral inhomogeneities on the formation of the electron gas at the LAO/STO interface.

Inhomogeneous Microstructure and Electrical Transport Properties at the LaAlO3/SrTiO3 Interface

Medium-energy ion spectroscopy (MEIS), scanning transmission electron microscopy (STEM) and X-ray photoemission spectroscopy (XPS) were used to investigate the composition and microstructure of LaAlO3/SrTiO3 (LAO/STO) interfaces grown by pulsed laser deposition of LAO on TiO2-terminated STO substrates under different oxidizing conditions. MEIS and XPS indicated Sr/La and Al/Ti intermixing within several atomic layers at all studied interfaces. XPS and STEM revealed that La diffuses deeper than Al. Analysis of the MEIS data suggests inhomogeneous lateral distribution of the diffused elements. This is further supported by the observation of a large positive magneto-resistance at low temperatures. We discuss the role of lateral inhomogeneities on the formation of the electron gas at the LAO/STO interface.

ChemInform Abstract: Recent Progress in Nanostructured Multiferroic Bi2FeCrO6 Thin Films.

AbstractReview: 47 refs.

Comment on “Observation of anomalous acoustic phonon dispersion in SrTiO3 by broadband stimulated Brillouin scattering” [Appl. Phys. Lett. 98, 211907 (2011)

the probe that is reflected from the interfaces of the sample and the light partially reflected from the strain pulse. At normal incidence, the period can be written as T ¼ k=2nv ,w herek is the probe wavelength, n is the optical index, and v is the sound velocity. By measuring the oscillation period over a wavelength range, the authors of Ref. 1 deduced the longitudinal sound velocity at various acoustic frequencies. Surprisingly, they found that the sound velocity is not constant but varies by 5% in the investigated phonon wave vector range. The intent of this Comment is to warn readers that the conclusion is probably wrong due to the setup itself. We obtained new data which are very close to what is expected from a substrate, and no deviation of the sound velocity is observed. We performed experiments on a 10 � 10 � 0.5 mm 3 bulk commercial (100) STO substrate 3 As in Ref. 1, the substrate was covered by a thin Al film in order to convert light pulses into acoustic pulses. We used a conventional pump and probe setup associated with a femtosecond laser tunable between 690 and 1040nm at a repetition rate of 80MHz with 120 fs optical pulse width. The probe is frequency doubled to reach the blue-UV range. Our data are obtained by tuning the laser wavelength by wavelength. The signal to noise ratio is better than using a spectroscopic setup which permits to study the response of the substrate over a wider wavelength range. In Fig. 1, we plot our experimental data, the data from Ref. 1 and the theoretical data obtained from the Brillouin formula, sound velocity, and optical index are extracted from lit

Structural reorientation of PLD grown La2NiO4 thin films

Single crystal La2NiO4 films were grown on STO (001) substrates by use of pulsed laser deposition (PLD). It was found that these films grow with the c-axis in the out of plane direction up to a certain critical thickness. Films thicker than that resulted in a structural reorientation from c-axis to a-axis out of plane orientation. The evolution of the c and a axis were measured by using a four circle X-ray diffractometer.

Recent progress in nanostructured multiferroic Bi2FeCrO6 thin films

We report the latest progress on the growth and characterization of Bi2FeCrO6 (BFCO), a recently discovered multiferroic system. BFCO thin films and nanostructures exhibit exceptional multiferroic properties at room temperature. The growth of pure BFCO thin films on STO substrates is possible only in a narrow window of deposition parameters (i.e., Oxygen pressure pO2=1.2×10−2mbar and around a substrate temperature TS=680°C). The epitaxial growth stabilizes the metastable single phase of this material and promotes the Fe/Cr cation ordering in both thin films and nanostructures. This cationic ordering which is responsible for good magnetic properties of BFCO is also at the origin of pronounced photovoltaic (PV) properties observed in the epitaxial films grown on STO substrates. The results indicate that the ferroelectric polarization plays a dominant role in the observed PV effect.

Formation of oxygen vacancies and charge carriers induced in then-type interface of a LaAlO3overlayer on SrTiO3(001)

We investigate the formation mechanism of oxygen vacancies and the properties of charge carriers induced in the $n$-type interface of a LaAlO${}_{3}$ (LAO) overlayer on SrTiO${}_{3}$(001) (STO) by carrying out density-functional-theory (DFT) calculations. We find that the formation of a low concentration of oxygen vacancies is dominated by the coupling between oxygen vacancies and the polar electric field in LAO. Oxygen vacancies on the LAO surface, which maximally reduce the electrostatic energy in LAO, are energetically the most stable, and most of the charge carriers induced by the surface vacancies are confined to the interface, forming a two-dimensional electron gas. We also verify that a high concentration of oxygen vacancies in the STO substrate could give rise to a three-dimensional electron gas. In addition, we demonstrate that the band profile at the interface is mainly determined by the concentration of the surface oxygen vacancies.

Unintentional F doping of SrTiO3(001) etched in HF acid-structure and electronic properties

We show that the HF acid etch commonly used to prepare SrTiO3(001) for heteroepitaxial growth of complex oxides results in a non-negligible level of F doping within the terminal surface layer of TiO2. Using a combination of x-ray photoelectron spectroscopy and scanned angle x-ray photoelectron diffraction, we determine that on average ~13% of the O anions in the surface layer are replaced by F, but that F does not occupy O sites in deeper layers. Despite this perturbation to the surface, the Fermi level remains unpinned, and the surface-state density, which determines the amount of band bending, is driven by factors other than F doping. The presence of F at the STO surface is expected to result in lower electron mobilities at complex oxide heterojunctions involving STO substrates because of impurity scattering. Unintentional F doping can be substantially reduced by replacing the HF-etch step with a boil in deionized water, which in conjunction with an oxygen tube furnace anneal, leaves the surface flat and TiO2 terminated.

Impact of substrate on magnetic phase coexistence in bicritical Sm 0.53Sr 0.47MnO 3 thin films

Sm 0.53Sr 0.47MnO 3 thin films were deposited on single crystal LaAlO 3 (LAO/(001)) and SrTiO 3 (STO/(001)) substrates by DC magnetron sputtering. The θ - 2 θ and ω - 2 θ scans show that these films have very good crystallinity and the films on LAO and STO are under compressive and tensile strain, respectively. The films on LAO and STO substrates show ferromagnetic (insulator–metal) transition at T C ∼ 126 K (at T I M ∼ 128 K ) and 120 K ( T I M ∼ 117 K ), respectively. The magnetic state at T < T C is akin to cluster glass, which is formed by the presence of charge ordered-antiferromagnetic clusters in the ferromagnetic matrix. Huge drop in the resistivity at T I M and the associated hysteresis with respect to cooling and warming cycles reveal the bicritical and the first order nature of phase transition, which is also confirmed by the Banerjee criterion. The differences and similarities in the functional properties of films are explained in terms of substrate modified magnetic phase coexistence.

Study of defects in an electroresistive Au/La2/3Sr1/3MnO3/SrTiO3(001) heterostructure by positron annihilation

Defects in an ultrathin Au/La2/3Sr1/3MnO3/SrTiO3 (Au/LSMO/STO) heterostructure displaying electroresistive behavior were studied using variable energy positron annihilation spectroscopy. Vacancy-like defects were found to be the dominant positron traps in the LSMO and STO thin perovskite oxides with a number density &amp;gt;1017 cm−3 and 2 × 1017 cm−3 in the STO substrate. High defect density was revealed by strong positron trapping at the Au/LSMO interface. Oxygen deficiency in LSMO would be the main source of these traps. Besides, a low density of sub-nano voids of ∼6 Å was found in the substrate and in the thin LSMO/STO films.

Electrochemical Stability of Self-Assembled Alkylphosphate Monolayers on Conducting Metal Oxides

Alkylphosphate self-assembled monolayers (SAMs) were prepared on Nb-doped SrTiO(3) (Nb-STO) conducting metal oxide substrates. Unlike thiols on gold, the alkylphosphate SAMs on Nb-STO exhibited an electrochemical stability over a wide voltage range from -2 to 2 V. Cyclic voltammetry showed that the SAM modification inhibited the electrochemical activity of the underlying conducting substrate with an efficiency dependent on the chain length. Impedance spectroscopy showed that SAM-modified Nb-STO substrates have a significantly higher resistance than bare substrates.

Substrate dependent transport and magnetotransport in manganite multilayer

We report magnetotransport properties of La 0.5Pr 0.2Sr 0.3MnO 3 (LPSMO) {5 layers}/La 0.5Pr 0.2Ba 0.3MnO 3 (LPBMO) {4 layers} manganite multilayers grown on single crystalline STO ( h 0 0) and NGO ( h 0 0) substrates using pulsed laser deposition (PLD) technique. An appreciable magnetoresistance (MR) ∼56% (80 kOe field) at room temperature (RT) is exhibited by the heterostructure deposited on STO substrate having field coefficient of resistance (FCR) ∼35% (100 Oe) while the multilayer deposited on NGO substrate exhibits MR∼61% (80 kOe field) and FCR∼18% (5 kOe) at RT. The observed values of temperature coefficient of resistance (TCR) are ∼3.15% and 2.77% at RT for the multilayers grown on STO and NGO substrates, respectively. A comparison of the field sensitivity of the multilayered structure studied with those reported for LPSMO/Al 2O 3/LPSMO (FCR∼20% at 220 K) and LPSMO/STO film (FCR∼13% at 250 K) shows that the multilayer exhibits higher field sensitivity, which can be attributed to the improved LPSMO–LPBMO interfaces. In this communication, the results of the structural, transport and magnetotransport studies on LPSMO/LPBMO multilayered systems have been discussed.

Molecular beam epitaxy of ferromagnetic γ′-Fe 4N thin films on LaAlO 3(1 0 0), SrTiO 3(1 0 0) and MgO(1 0 0) substrates

We have attempted to grow a-axis-oriented γ′-Fe 4N epitaxial films with smooth surfaces at around 400 °C on LaAlO 3(LAO)(1 0 0), SrTiO 3(STO)(1 0 0) and MgO(1 0 0) substrates by molecular beam epitaxy using solid Fe and a radio-frequency NH 3 plasma as the Fe and N sources, respectively. The lattice mismatch of these substrates to γ′-Fe 4N is 0%, 3% and 11%, respectively. Epitaxial growth of γ′-Fe 4N films was successfully achieved on the STO(1 0 0) and LAO(1 0 0) substrates. It was found from reflection high-energy electron diffraction and X-ray diffraction that the a-axis orientation of γ′-Fe 4N was degraded with increasing lattice mismatch. High-angle annular dark-field scanning transmission electron microscopy and electron energy-loss spectroscopy measurements showed that amorphous Al–O layers were present at the γ′-Fe 4N/LAO interface.