Oxide Thin Film Heterostructures Research Articles

Enhancement of thermoelectric performance by breaking thermopower – conductivity trade-off relation in artificially designed oxide thin-film heterostructures

Enhancement of thermoelectric performance by breaking thermopower – conductivity trade-off relation in artificially designed oxide thin-film heterostructures

Magnetic-field–induced Zeeman excitation mode in paramagnetic NdGaO3single-crystal probed by magneto-terahertz spectroscopy

The magnetic field control of quantum matter enables to study various exotic phenomena which emerge in complex oxide materials. The thin film heterostructures of these systems, potentially relevant in oxide electronics, require a great deal of care in distinguishing the properties of the film from those of the substrate underneath. Here, we unravel a low-energy excitation mode in NdGaO3 crystals which is an important constituent of oxide thin film heterostructures. Employing terahertz (THz) time-domain spectroscopy with control of the external magnetic field, an excitation mode resonating at a frequency of 0.2THz was observed along two key (001) and (110) crystallographic orientations. The mode develops and strengthens with an application of increasing magnetic field and weakens with increasing temperature; all attributes conforming to the predictions for the energetics of Zeeman splitting and intra-level transition within Kramer's doublet ground state. A dominance of the magnetic field control of this mode along (110) compared to that along (001) orientation also further unveils a structural control. NdGaO3crystals are widely used as substrates in oxide thin film heterostructures; hence, while realizing ultrafast control of magnetic order, specifically in oxide thin films, the Kramer crystal field splitting mode should be carefully separated from ordered spin resonance mode.

Fabrication and characterization of copper (II) oxide/iron (III) oxide thin film heterostructures for trace arsenic (III) removal in water

In this study, nano-heterostructures based on copper (II) oxide/iron (III) oxide (CuO/α-Fe2O3) thin films were fabricated by a dip-coating technique using aqueous solutions. The heterostructures were deposited on fluorine-doped tin oxide glass substrates varying the CuO film thickness. From a detailed characterization using Fourier transform-infrared and X-ray diffraction the formation of CuO (tenorite)/α-Fe2O3 (hematite) was demonstrated. Atomic force microscopy provided valuable information on the growth of α-Fe2O3 crystals in the heterostructure with a conical-shaped surface. Meanwhile, the field emission scanning electron microscopy cross-section images confirm the formation of well-defined CuO layers under the α-Fe2O3 layers. The optical band gap energies for the heterostructures obtained were estimated from the diffuse reflectance spectra and ranged from 1.41 to 1.51 eV. Photoluminescence analysis revealed an improved separation and faster transfer of photogenerated electrons and holes for the heterostructures. The removal arsenic from an aqueous solution was achieved through the direct adsorption for As(III) and visible light oxidation to As(V). An enhancement of removal efficiency of As(III) for the heterostructures fabricated compared to pristine oxides was obtained.

Atomic-Scale Control of Magnetism at the Titanite-Manganite Interfaces

Complex oxide thin-film heterostructures often exhibit magnetic properties different from those known for bulk constituents. This is due to the altered local structural and electronic environment at the interfaces, which affects the exchange coupling and magnetic ordering. The emergent magnetism at oxide interfaces can be controlled by ferroelectric polarization and has a strong effect on spin-dependent transport properties of oxide heterostructures, including magnetic and ferroelectric tunnel junctions. Here, using prototype La2/3Sr1/3MnO3/BaTiO3 heterostructures, we demonstrate that ferroelectric polarization of BaTiO3 controls the orbital hybridization and magnetism at heterointerfaces. We observe changes in the enhanced orbital occupancy and significant charge redistribution across the heterointerfaces, affecting the spin and orbital magnetic moments of the interfacial Mn and Ti atoms. Importantly, we find that the exchange coupling between Mn and Ti atoms across the interface is tuned by ferroelectric polarization from ferromagnetic to antiferromagnetic. Our findings provide a viable route to electrically control complex magnetic configurations at artificial multiferroic interfaces, taking a step toward low-power spintronics.

Evidence for exchange bias coupling at the perovskite/brownmillerite interface in spontaneously stabilized SrCoO3−δ/SrCoO2.5 bilayers

Interface effect in complex oxide thin film heterostructures lies at the vanguard of current research to design technologically relevant functionality and explore emergent physical phenomena. While most of the previous works focus on the perovskite/perovskite heterostructures, the study on perovskite/brownmillerite interfaces remain at its infancy. Here, we investigate spontaneously stabilized perovskite-ferromagnet (SrCoO3-{\delta})/brownmillertite-antiferromagnet (SrCoO2.5) bi-layer with TN > TC and discover an unconventional interfacial magnetic exchange bias effect. From magnetometry investigations, it is rationalized that the observed effect stems from the interfacial ferromagnet/antiferromagnet coupling. The possibility for coupled ferromagnet/spinglass interface engendering such effect is ruled out. Strikingly, a finite coercive field persists in the paramagnetic state of SrCoO3-{\delta} whereas the exchange bias field vanishes at TC. We conjecture the observed effect to be due to the effective external quenched staggered field provided by the antiferromagtic layer for the ferromagnetic spins at the interface. Our results not only unveil a new paradigm to tailor the interfacial magnetic properties in oxide heterostructures without altering the cations at the interface, but also provide a purview to delve into the fundamental aspects of exchange bias in such unusual systems paving a big step forward in thin film magnetism.

Uniform sputter deposition of high-quality epitaxial complex oxide thin films

Uniform deposition of high-quality epitaxial complex oxide thin films over a large area is desirable not only for the large scale fabrication of oxide electronics but also for preparing multiple samples with the same growth conditions for various characterization techniques. However, it is particularly challenging to grow uniform thin films of multicomponent oxide systems containing volatile species such as Pb and Bi. By combining a misaligned parallel dual planar magnetron sputtering technique with substrate rotation, the authors have grown uniform epitaxial thin films piezoelectric 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 and multiferroic monodomain BiFeO3 with high-deposition rates over a 2 in. diameter area. These films have excellent uniformity of thickness, stoichiometric compositions, and electrical properties. This technique can be scaled to larger deposition areas by using larger sputtering targets and widely applied to various multicomponent complex oxide thin film heterostructures.

Interfacial control of oxygen vacancy doping and electrical conduction in thin film oxide heterostructures.

Oxygen vacancies in proximity to surfaces and heterointerfaces in oxide thin film heterostructures have major effects on properties, resulting, for example, in emergent conduction behaviour, large changes in metal-insulator transition temperatures or enhanced catalytic activity. Here we report the discovery of a means of reversibly controlling the oxygen vacancy concentration and distribution in oxide heterostructures consisting of electronically conducting In2O3 films grown on ionically conducting Y2O3-stabilized ZrO2 substrates. Oxygen ion redistribution across the heterointerface is induced using an applied electric field oriented in the plane of the interface, resulting in controlled oxygen vacancy (and hence electron) doping of the film and possible orders-of-magnitude enhancement of the film's electrical conduction. The reversible modified behaviour is dependent on interface properties and is attained without cation doping or changes in the gas environment.

Enhanced Coercivity in BiFeO3/SrRuO3 heterostructures

ABSTRACTTransition metal oxide thin film heterostructures have garnered increasing research interest in the last decade due to their multifunctional properties, such as ferromagnetism and ferroelectricity, which may be utilized in next generation device applications. Many previous works reported on the deposition of such structures on oxide substrates such as SrTiO3, which are not compatible with CMOS applications where Si(100) is the mainstay substrate material. BiFeO3(BFO) is a room temperature insulating ferroelectric and antiferromagnet, a well-known multiferroic material. SrRuO3(SRO) is a ferromagnetic metal with the Curie temperature (TC) of 165K. Unexpected properties emerge when these two dissimilar materials are conjoined. However, there has been no report on exploring the magnetic properties of BFO when it is in contact with SRO, and particularly when they are integrated with Si(100) substrates, which is the subject of present study. BFO/SRO thin films have been epitaxially grown on Si (100) substrates by introducing MgO/TiN epitaxial buffer layers using pulsed laser deposition. BFO thin films show room temperature ferroelectricity as observed from piezo force microscopy (PFM) measurements. The magnetic data collected from BFO thin films show typical antiferromagnetic features as expected. The TCof SRO in all the samples studied was found be ∼ 170K, close to the reported value of 165K. Interestingly, we have noticed that the coercive field of SRO layer increased from 4 kOe to 15 kOe (nearly fourfold) by reducing its thickness from 180 to 23nm, while keeping the thickness of BFO layer constant at 100nm. Pinning of Ru ions by ferroelectric domain walls in BFO, strong interfacial exchange coupling and SRO layer thickness could cause the observed enhancement in coercivity. Our near future work will address the precise underlying mechanisms in greater detail.

Epitaxial integration of perovskite-based multifunctional oxides on silicon

We review recent developments in the epitaxial integration of multifunctional oxide thin film heterostructures on silicon (Si). Perovskite oxides have been extensively studied for use in multifunctional devices due to a wide range of functional properties. To realize multifunctional oxide devices, these multifunctional films should be integrated directly on Si, maintaining high crystalline quality. Molecular beam epitaxy growth of epitaxial SrTiO3 (STO) on Si provides a template for incorporating the epitaxial oxide films on Si. However, the dissimilar physical nature of Si from most oxide materials influences the properties of oxide films on Si, especially with regard to structural defects and thermal strains. Therefore, in this review, we present a comprehensive overview of epitaxial integration of various model oxide systems on Si, addressing how STO/Si can be used to explore the novel phenomenon of oxide heterostructures as well as to realize multifunctional devices.

Oxide Thin Film Heterostructures on Large Area, with Flexible Doping, Low Dislocation Density, and Abrupt Interfaces: Grown by Pulsed Laser Deposition

Advanced Pulsed Laser Deposition (PLD) processes allow the growth of oxide thin film heterostructures on large area substrates up to 4-inch diameter, with flexible and controlled doping, low dislocation density, and abrupt interfaces. These PLD processes are discussed and their capabilities demonstrated using selected results of structural, electrical, and optical characterization of superconducting (YBa2Cu3O7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO3-based) and dielectric (wide-gap oxide) thin films and multilayers. Regarding the homogeneity on large area of structure and electrical properties, flexibility of doping, and state-of-the-art electronic and optical performance, the comparably simple PLD processes are now advantageous or at least fully competitive to Metal Organic Chemical Vapor Deposition or Molecular Beam Epitaxy. In particular, the high flexibility connected with high film quality makes PLD a more and more widespread growth technique in oxide research.

Modulation doping of a Mott quantum well by a proximate polar discontinuity

We present evidence for hole injection into ${\text{LaAlO}}_{3}/{\text{LaVO}}_{3}/{\text{LaAlO}}_{3}$ quantum wells near a polar surface of ${\text{LaAlO}}_{3}$ (001). As the surface is brought in proximity to the ${\text{LaVO}}_{3}$ layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient are observed below a characteristic coupling length of 10--15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the ${\text{AlO}}_{2}$-terminated ${\text{LaAlO}}_{3}$ surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole doping in oxide thin-film heterostructures.

YBa2Cu3O7/La2/3Ca1/3MnO3 superlattices showing simultaneously ferromagnetic and superconducting order

All oxide thin film heterostructures and superlattices composed of materials of different functionalities such as ferromagnetic [FM], superconducting [SC], piezoelectric or ferroelectric open a new wide field of device oriented fundamental research by exploring the interaction of different long range ordered ground states. To demonstrate these principles we have prepared YBa2Cu3O7 based superlattices [SL's] with La2/3Ca1/3MnO3 as ferromagnetic [FM] part by pulsed laser deposition. The films are characterized with respect to their structual, magnetic and transport properties. Whereas simple heterostructures [single layer La2/3Ca1/3MnO3 and single layer YBa2Cu3O7 50 nm thickness each] reproduce the intrinsic properties of the constituent material rather well [Curie temperature 250 K superconducting transition at T = 75 K] there are some novel effects emerging due to the coupling between the layers observed in the superlattices. The experimental findings are discussed within the frame of a model based on FM interlayer coupling and superconducting proximity effect. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Defects and Interfaces in III-Nitride/Sapphire Thin Film Heterostructures

AbstractHigh quality III-nitrides and zinc oxide thin film heterostructures have been fabricated using pulsed laser deposition (PLD) on highly lattice mismatched sapphire substrates. The structural quality of PLD films grown at 700-800°C is comparable with those grown by MOCVD in the temperature range 950-1050°C. The PLD films were found to be single crystalline with columnar structure having the dislocation density of ~1010 cm2 at a film thickness 150-200 nm. The typical structure of defects and interfaces in GaN and AIN films is analyzed and compared with those obtained from specimens grown by MOCVD at much higher temperatures. Low temperature PLD (< 780°C) does not require additional atomic nitrogen source to compensate for the loss of nitrogen due to thermal decomposition of the nitride layer. At slightly higher temperatures the GaN films exhibit rough morphology and formation of the cubic zinc-blende phase which is more stable against nitrogen deficiency than wurtzite phase. A mechanism of formation of zinc-blende phase has been identified. Growth of GaN on lattice-matched ZnO/sapphire buffer layers showed the formation of cubic spinel ZnGa2O4 intermediate layer.

Microstructure of epitaxial oxide thin film heterostructures on silicon by pulsed laser deposition

The microstructure of epitaxial La0.5Sr0.5CoO3(LSCO)/ferroelectric lanthanum modified lead zirconate titanate (PLZT)/ La0.5Sr0.5CoO3(LSCO)/bismuth titanate(BT)/yttria-stabilized zirconia (YSZ) heterostructures on [001] silicon has been investigated. X-ray diffraction and pole figure analysis reveal epitaxial growth of the PLZT, LSCO, BT, and YSZ layers. High resolution transmission electron microscopy was done to study the crystal defects and interfacial structure.

Oriented growth in oxide thin film heterostructures

In this paper, the authors report the structural analysis of La[sub 0.5]Sr[sub 0.5]CoO[sub 3] (LSCo) / Pb[sub 0.9]La[sub 0.1]Zr[sub 0.2]Ti[sub 0.8]O[sub 3] (PLZT) multilayers grown with and without a bismuth titanate (Bi-Ti) template onto yttria-stabilized zirconia (YSZ) coated (001) Si substrates grown by laser ablation. The tetragonal PLZT was expected to have lattice parameters of approximately a [equals] 0.406 nm and c [equals] 0.411 nm, and the cubic LSCo to have a lattice parameter of 0.381 nm. The Bi-Ti layer was used because, with strongly two-dimensional growth, it was expected to form with the (001) basal plane parallel to the growth surface. Such a layered structure could then provide a template for the oriented growth of any subsequent layers.