Transition Metal Oxide Thin Films Research Articles

Strongly correlated and topological states in [111] grown transition metal oxide thin films and heterostructures

We highlight recent advances in the theory, materials fabrication, and experimental characterization of strongly correlated and topological states in [111] oriented transition metal oxide thin films and heterostructures, which are notoriously difficult to realize compared to their [001] oriented counterparts. We focus on two classes of complex oxides, with the chemical formulas ABO3 and A2B2O7, where the B sites are occupied by an open-shell transition metal ion with a local moment and the A sites are typically a rare earth element. The [111] oriented quasi-two-dimensional lattices derived from these parent compound lattices can exhibit peculiar geometries and symmetries, namely, a buckled honeycomb lattice, as well as kagome and triangular lattices. These lattice motifs form the basis for emergent strongly correlated and topological states expressed in exotic magnetism, various forms of orbital ordering, topological insulators, topological semimetals, quantum anomalous Hall insulators, and quantum spin liquids. For transition metal ions with high atomic number, spin–orbit coupling plays a significant role and may give rise to additional topological features in the electronic band structure and in the spectrum of magnetic excitations. We conclude this perspective by articulating open challenges and opportunities in this actively developing field.

New approaches for achieving more perfect transition metal oxide thin films

This perspective considers the enormous promise of epitaxial functional transition metal oxide thin films for future applications in low power electronic and energy applications since they offer wide-ranging and highly tunable functionalities and multifunctionalities, unrivaled among other classes of materials. It also considers the great challenges that must be overcome for transition metal oxide thin films to meet what is needed in the application domain. These challenges arise from the presence of intrinsic defects and strain effects, which lead to extrinsic defects. Current conventional thin film deposition routes often cannot deliver the required perfection and performance. Since there is a strong link between the physical properties, defects and strain, routes to achieving more perfect materials need to be studied. Several emerging methods and modifications of current methods are presented and discussed. The reasons these methods better address the perfection challenge are considered and evaluated.

Increasing the silicon solar cell efficiency with transition metal oxide nano-thin films as anti-reflection coatings

Herein, we report the study on RF-sputtered transition metal oxide thin films of Zinc oxide, Magnesium oxide, and Aluminum oxide as an antireflection coating on silicon-based solar cells and their influence on energy conversion. The transmission spectrum of all sputtered metal oxides was studied using a UV-visible spectrophotometer. The phase formation and microstructure analysis of these sputtered oxides were studied using glass for the destructive test along with the device. The x-ray diffraction and cross-section scanning electron microscopy of sputtered glass confirmed a single-phase structure along with nearly equal desired deposition thickness. The thicknesses of sputtered films were estimated using variable angle ellipsometry and the same was confirmed from cross-section scanning electron micrograph. The chemical composition and oxidation state of thin films deposited on glass were established from x-ray photoemission spectroscopy. The ability of a fabricated device deposited with the antireflection layer in converting photon energy to electrical energy was studied using a solar simulator under 1 sun condition. The ability to collect charge carriers of the anti-reflection coated device as a function of wavelength was also studied using quantum efficiency measurement.

Spray-pyrolysis deposited La1−xSrxCoO3 thin films for potential non-volatile memory applications

In this work thin films of the La1-xSrxCoO3 (0.05 < x < 0.26) compound were grown, employing the so-called spray pyrolysis process. The as-grown thin films exhibit polycrystalline microstructure, with uniform grain size distribution, and observable porosity. Regarding their electrical transport properties, the produced thin films show semiconducting-like behavior, regardless the Sr doping level, which is most likely due to both the oxygen deficiencies and the grainy nature of the films. Furthermore, room temperature current-voltage (I-V) measurements reveal stable resistance switching behavior, which is well explained in terms of space-charge limited conduction mechanism. The presented experimental results provide essential evidence regarding the engagement of low cost, industrial-scale methods of growing perovskite transition metal oxide thin films, for potential applications in random access memory devices.

Atomically-smooth single-crystalline VO2 (101) thin films with sharp metal-insulator transition

Atomically-abrupt interfaces in transition metal oxide (TMO) heterostructures could host a variety of exotic condensed matter phases that may not be found in the bulk materials at equilibrium. A critical step in the development of such atomically-sharp interfaces is the deposition of atomically-smooth TMO thin films. Optimized deposition conditions exist for the growth of perovskite oxides. However, the deposition of rutile oxides, such as VO2, with atomic-layer precision has been challenging. In this work, we used pulsed laser deposition to grow atomically-smooth VO2 thin films on rutile TiO2 (101) substrates. We show that an optimal substrate preparation procedure followed by the deposition of VO2 films at a temperature conducive for step-flow growth mode is essential for achieving atomically-smooth VO2 films. The films deposited at optimal substrate temperatures show a step and terrace structure of the underlying TiO2 substrate. At lower deposition temperatures, there is a transition to a mixed growth mode comprised of island growth and layer-by-layer growth modes. VO2 films deposited at optimal substrate temperatures undergo a sharp metal to insulator transition, similar to that observed in bulk VO2, but at a transition temperature of ∼325K with ∼103 times increase in resistance.

Surface-orientation-dependent growth of SrRuO3 epitaxial thin films

The growth of epitaxial transition metal oxide thin films depends on various parameters including the substrate temperature, oxygen partial pressure, kinetics of incoming adatoms, Gibbs free energy, and surface energy. Naturally, the change in the crystallographic surface orientation with a distinctive surface energy also influences the growth rate and growth mode of the epitaxial thin films substantially. Using perovskite SrRuO3 as a model system, we studied the growth characteristics by changing the surface orientation of the SrTiO3 substrate. Employing X-ray diffraction and surface atomic and Kelvin probe force microscopy (KPFM), we observed a systematic decrease in the growth rate and a modification in the growth mode from a two-dimensional growth to a three-dimensional island growth with the change in the surface orientation from (100) to (110) to (111). A spin-polarized density functional theory calculation demonstrated the corresponding difference in the surface energy, which was also confirmed experimentally by the KPFM measurements. The difference in the surface energy could explain the observed change in the growth kinetics, based on the modified classical nucleation theory. A combinatorial method using a polycrystalline epitaxial thin film was employed to generalize our understanding of the crystallographic surface-orientation-dependent thin film growth.

Probing the Interfacial Symmetry Using Rotational Second-Harmonic Generation in Oxide Heterostructures

Transition metal oxide thin films and heterostructures have attracted remarkable and continuous attention due to the incredible variety of electronic and magnetic properties. A significant challeng...

Metal-insulator transition in (111) SrRuO3 ultrathin films

(111)-oriented transition metal oxide thin films provide a route to developing oxide-based topological quantum materials, but the epitaxial growth is challenging. Here, we present the thickness-dependent electronic and magnetic phase diagrams of coherently strained, phase pure (111)-oriented SrRuO3 epitaxial films grown on (111) SrTiO3 substrates using pulsed laser deposition. With decreasing film thickness, it is found that both the metal-to-insulator and magnetic phase transitions occur at the same thickness of 4–5 nm for films grown along both the (111) and the (001) directions. The character of the transport near the metal-insulator transition is, however, distinct for the different directions, which is attributed to the increased electron-electron correlation for (111) SrRuO3. The findings presented here highlight both the broad challenges as well as the possibilities in modifying correlated materials using dimensional tuning of electronic and magnetic properties.

Anisotropic spin-orbit torque generation in epitaxial SrIrO3 by symmetry design

Spin-orbit coupling (SOC), the interaction between the electron spin and the orbital angular momentum, can unlock rich phenomena at interfaces, in particular interconverting spin and charge currents. Conventional heavy metals have been extensively explored due to their strong SOC of conduction electrons. However, spin-orbit effects in classes of materials such as epitaxial 5d-electron transition-metal complex oxides, which also host strong SOC, remain largely unreported. In addition to strong SOC, these complex oxides can also provide the additional tuning knob of epitaxy to control the electronic structure and the engineering of spin-to-charge conversion by crystalline symmetry. Here, we demonstrate room-temperature generation of spin-orbit torque on a ferromagnet with extremely high efficiency via the spin-Hall effect in epitaxial metastable perovskite SrIrO3 We first predict a large intrinsic spin-Hall conductivity in orthorhombic bulk SrIrO3 arising from the Berry curvature in the electronic band structure. By manipulating the intricate interplay between SOC and crystalline symmetry, we control the spin-Hall torque ratio by engineering the tilt of the corner-sharing oxygen octahedra in perovskite SrIrO3 through epitaxial strain. This allows the presence of an anisotropic spin-Hall effect due to a characteristic structural anisotropy in SrIrO3 with orthorhombic symmetry. Our experimental findings demonstrate the heteroepitaxial symmetry design approach to engineer spin-orbit effects. We therefore anticipate that these epitaxial 5d transition-metal oxide thin films can be an ideal building block for low-power spintronics.

Investigation of surface reactions in metal oxide on Si for efficient heterojunction Si solar cells

Recently, the transition metal oxide thin film has been actively investigated for doping-free heterojunction Si solar cells. However, most of the research on characterizing the chemical state and work function of the metal oxide thin film has been conducted on its surface, while there has been little work on the characterization on the subsurface of the metal oxide thin film. Here, we systematically investigate the chemical state and work function of the evaporated nickel oxide (NiOx) thin film on a Si substrate as a function of the depth position. We found that the chemical state of the NiOx thin film is highly affected by the surface chemical reaction. For instance, an air-exposed NiOx surface exists more in nickel hydroxide [Ni(OH)2] than in nickel monoxide (NiO). In addition, we discern that NiOx near the Si substrate exists in nickel silicide (NiSix). The changed chemical state of the NiOx thin film creates a high variation in the work function as a function of the depth position in the range of 4.4–5.4 eV. We also investigate the heterojunction Si solar cell with the NiOx thin film. We found that the performance of the heterojunction Si solar cell was determined according to the air exposure on the NiOx thin film inducing an undesirable chemical reaction. The heterojunction Si solar cell with the air-exposed NiOx thin film shows a relatively low efficiency of 11.84% by the reduced work function of the NiOx thin film, while one with the controlled NiOx thin film exhibits an enhanced efficiency of 14.23%.

Quantification of Charge Transfer at the Interfaces of Oxide Thin Films.

The interfacial electronic distribution in transition-metal oxide thin films is crucial to their interfacial physical or chemical behaviors. Core-loss electron energy-loss spectroscopy (EELS) may potentially give valuable information of local electronic density of state at high spatial resolution. Here, we studied the electronic properties at the interface of Pb(Zr0.2Ti0.8)O3 (PZT)/4.8 nm La0.8Sr0.2MnO3 (LSMO)/SrTiO3 (STO) using valance-EELS with a scanning transmission electron microscope. Modeled with dielectric function theory, the charge transfer in the vicinity of the interfaces of PZT/LSMO and LSMO/STO was determined from the shifts of plasma peaks of valence EELS (VEELS), agreeing with theoretical prediction. Our work demonstrates that the VEELS method enables a high-efficient quantification of the charge transfer at interfaces, shedding light on the charge-transfer issues at heterogenous interfaces in physical and chemical devices.

Analysis of plasma enhanced pulsed laser deposition of transition metal oxide thin films using medium energy ion scattering

In this study, plasma-enhanced pulsed laser deposition (PE-PLD), which is a novel variant of pulsed laser deposition that combines laser ablation of metal targets with an electrically-produced oxygen plasma background, has been used for the fabrication of ZnO and Cu2O thin films. Samples prepared using the PE-PLD process, with the aim of generating desirable properties for a range of electrical and optical applications, have been analysed using medium energy ion scattering. Using a 100 keV He+ ion beam, high resolution depth profiling of the films was performed with an analysis of the stoichiometry and interface abruptness of these novel materials. It was found that the PE-PLD process can create stoichiometric thin films, the uniformity of which can be controlled by varying the power of the inductively coupled plasma. This technique showed a high deposition rate of ∼0.1 nm s−1.

Manipulating the Structural and Electronic Properties of Epitaxial SrCoO2.5 Thin Films by Tuning the Epitaxial Strain.

Structure determines material's functionality, and strain tunes the structure. Tuning the coherent epitaxial strain by varying the thickness of the films is a precise route to manipulate the functional properties in the low-dimensional oxide materials. Here, to explore the effects of the coherent epitaxial strain on the properties of SrCoO2.5 thin films, thickness-dependent evolutions of the structural properties and electronic structures were investigated by X-ray diffraction, Raman spectra, optical absorption spectra, scanning transmission electron microscopy (STEM), and first-principles calculations. By increasing the thickness of the SrCoO2.5 films, the c-axis lattice constant decreases, indicating the relaxation of the coherent epitaxial strain. The energy band gap increases and the Raman spectra undergo a substantial softening with the relaxation of the coherent epitaxial strain. From the STEM results, it can be concluded that the strain causes the variation of the oxygen content in the BM-SCO2.5 films, which results in the variation of band gaps with varying the strain. First-principles calculations show that strain-induced changes in bond lengths and angles of the octahedral CoO6 and tetrahedral CoO4 cannot explain the variation band gap. Our findings offer an alternative strategy to manipulate structural and electronic properties by tuning the coherent epitaxial strain in transition-metal oxide thin films.

Microwave plasma annealing of sol-gel deposited tantalum oxide and zinc oxide films

The sol-gel process allows the high throughput formation of transition metal oxide thin films. Microwave plasma annealing (MwPA) treatments have been performed on thin films of two different transition metal oxides, Ta2O5 and ZnO, selected as representatives of covalently and strongly ionic bonded oxides, respectively. Ta2O5 has been explored as a dielectric barrier for porous silicon structures. The main limitation of the sol-gel spin coating in this case is the surface roughness of the coating, which is highly improved upon Ar MwPA. The treatment leads additionally to a microstructural activation and interface development comparable to a 500 °C thermal annealing. The MwPA of ZnO is a quasi-equivalent process to a 200 °C thermal annealing, preventing grain growth and promoting nanocrystalline phases. This is suggested to have a direct impact on the optical and electronic properties of the ZnO films. The MwPA films show wider optical band gap than thermally annealed ones. An impedance analysis further shows that the MwPA ZnO films present lower equivalent resistance and higher equivalent capacitance than the thermal films. These results are promising for the development of new processing routes for widely demanded transition metal oxide thin films.

Enhanced magnetic and thermoelectric properties in epitaxial polycrystalline SrRuO3 thin films.

Transition metal oxide thin films show versatile electric, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO3 thin films using pulsed laser epitaxy. Using the epitaxial stabilization technique with an atomically flat polycrystalline SrTiO3 substrate, an epitaxial polycrystalline SrRuO3 thin film with the crystalline quality of each grain comparable to that of its single-crystalline counterpart is realized. In particular, alleviated compressive strain near the grain boundaries due to coalescence is evidenced structurally, which induced the enhancement of ferromagnetic ordering of the polycrystalline epitaxial thin film. The structural variations associated with the grain boundaries further reduce the thermal conductivity without deteriorating the electronic transport, and lead to an enhanced thermoelectric efficiency in the epitaxial polycrystalline thin films, compared with their single-crystalline counterpart.

Core-Shell Au@Metal-Oxide Nanoparticle Electrocatalysts for Enhanced Oxygen Evolution.

Enhanced catalysis for electrochemical oxygen evolution is essential for the efficacy of many renewable energy technologies, including water electrolyzers and metal-air batteries. Recently, Au supports have been shown to enhance the activity of many 3d transition metal-oxide thin films for the oxygen evolution reaction (OER) in alkaline media. Herein, we translate the beneficial impact of Au supports to high surface area, device-ready core-shell nanoparticles consisting of a Au-core and a metal-oxide shell (Au@MxOy where M = Ni, Co, Fe, and CoFe). Through a systematic evaluation, we establish trends in performance and illustrate the universal activity enhancement when employing the Au-core in the 3d transition metal-oxide nanoparticles. The highest activity particles, Au@CoFeOx, demonstrate an overpotential of 328 ± 3 mV over a 2 h stability test at 10 mA cm-2, illustrating that strategically coupling Au support and mixed metal-oxide effects in a core-shell nanoparticle morphology is a promising avenue to achieve device-ready, high-performance OER catalysts.

Fabrication and characterization of WO3 thin films on silicon surface by thermal evaporation

The transition metal-oxide thin films of WO3 have been prepared by thermal evaporation at a low deposition rate. The material characterization shows that the thin films have stoichiometric composition and the tungsten ions were fully oxidised into W6+. The surface of the WO3 thin films became smoother when the thickness increased. The results show that the deposition of WO3 thin film follows island growth. The flatband voltage of WO3/Si was extracted. The values on n-Si and p-Si were about 0.75eV and 0eV, respectively. It was also found that large amounts of negative charges were accumulated at the interface of WO3/n-Si because of electron transfer.

Preparation of Lithium Containing Oxides by the Solid State Reaction of Atomic Layer Deposited Thin Films

Lithium containing multicomponent oxides are important materials for both lithium-ion batteries and optical applications. In most cases thin films of these materials are desired. Atomic layer deposition (ALD) is a thin film deposition method that is known to deposit high quality films by sequential self-limiting surface reactions. However, the reactivity of lithium ions during the deposition process can pose challenges for the control of the film growth and even destroy the self-limiting nature of ALD completely. In this paper, we have studied the combination of atomic layer deposition and solid state reactions for the generation of lithium containing multicomponent oxide films. Atomic layer deposited transition metal oxide thin films were covered with ALD-grown lithium carbonate, and the films were annealed to produce lithium tantalate, titanate, and niobate. Lithium carbonate was chosen as the source of lithium because it is easy to deposit by ALD and can be handled in air. The films were analyzed as-de...

Tuning the Performance of Pt/HfO2/Ti/Pt ReRAM Devices Obtained from Plasma-Enhanced Atomic Layer Deposition for HfO2 Thin Films

The market of portable electronics and Green IT solutions demands for non-volatile memory circuits with high speed data access, high reliability, and low power consumption. Resistive switching random access memories (ReRAM) based on the valence change mechanism (VCM) represent a promising concept. VCM-type memristive devices are generally based on transition metal oxide thin films sandwiched between metal electrodes that differ in respect of work function and oxidation enthalpy. ReRAM cells built from insulating metal oxides with a high band gap, like e.g. TiO2, HfO2, Al2O3, typically require an electroforming step to enable resistive switching. Decisive for a stable device operation are a homogeneous microstructure and a controlled defect density in the oxide films of a few nm in thickness. Atomic layer deposition (ALD) can fulfill these requirements by its unique surface-reaction controlled self-limiting growth enabling a control of the film defect structure even for thicknesses in the nm-regime and for low growth temperatures. During the electroforming process, an electronically conducting filament is formed a significantly increased density of oxygen vacancies. The stable VCM-type resistive switching is related to drift/diffusion processes of oxygen vacancies (or oxygen ions, respectively) which depend on the oxidation/reduction enthalpies of the materials involved in the switching event. In this presentation we are going to discuss possibilities to tune the resistive switching performance of nanostructured ReRAM devices by means of a variation of metal oxide bilayer structures with respect to the layer thickness and the stack sequence. This study was performed on various bilayers based on combinations of TiO2/Al2O3 [1] and TiO2/HfO2 [2] with a total thickness < 10 nm integrated into nano-crossbar structures with a size of 100 x 100 nm2. Addressing the ReRAM device performance we discuss effects on the electroforming voltage, necessary SET- and RESET-voltages, achievable resistance ratios, switching stability and variability. In addition, changes in the resistive switching polarity, i.e. from bipolar to complementary-type, which have been observed for certain stacks, are discussed. This work was supported in part by the Deutsche Forschungsgemeinschaft (SFB917).

A Comparative Characterization Study of Molybdenum Oxide Thin Films Grown Using Femtosecond and Nanosecond Pulsed Laser Deposition

ABSTRACTGroup 6 transition metal oxide thin films are in large demand for photocatalysis, heterogeneous catalysis, fuel cell, battery and electronic applications. Pulsed laser deposition offers an inexpensive method for the preparation of nanostructured thin films that may be suitable for heterogeneous catalysis. We have synthesized molybdenum oxide thin films using two types of pulsed laser deposition (PLD). The first method utilizes femtosecond laser-based PLD (f-PLD) while the second method uses an excimer (nanosecond) laser-based PLD (n-PLD). The PLD films have been deposited using f-PLD and, separately, n-PLD on glass and silicon substrates and subsequently annealed to 450 °C for up to 20 hours in air using a Linkam stage. SEM, XRD and Raman spectroscopic characterization shows that the f-PLD films are substantially more textured and partially crystalline prior to annealing whereas the n-PLD-grown thin films are much smoother and predominantly amorphous. A 3-dimensional nano-crystalline structure is evident in the post-annealed f-PLD synthesized thin films, which is desirable for catalytic applications. XPS elemental analysis shows that the stoichiometry of the f-PLD and n-PLD thin films is consistent with the presence of MoO2 and MoO3. Our results are discussed in terms of thin film growth models suitable for f-PLD vs n-PLD.