Epitaxial NiO Research Articles

Resistive Switching Characteristics of NiO Thin Films Influenced by Changes in the Diameter of Nanometer-Scale Top Electrodes.

We investigated the forming, set, and reset voltages affected by the area of the top electrodes of a resistive random access memory (RRAM) capacitor fabricated by epitaxial NiO thin films. NiO RRAM capacitors with Au top electrode with a diameter of 100 μm showed typical unipolar switching characteristics. Au top electrodes with diameters of 10, 20, and 30 nm were formed on the surface of epitaxial NiO thin films by e-beam lithography. The forming, set, and reset voltages tended to decrease as the diameter of NiO RRAM capacitors with nanosized Au top electrodes decreased. As the area of the Au top electrodes increases, the volume of space in which the conductive filaments formed within the NiO RRAM capacitors can be formed becomes larger. This causes the conductive filament to be formed at relatively low energy, giving low forming voltages, while increasing the diversity of paths, causing a wide forming voltage distribution.

Unusual tilted growth and epitaxial relationship of NaCl B1-structured NiO and CrN on r-plane Al2O3

Epitaxial NiO and CrN thin films were deposited on a single-crystal Al2O3(11¯02) (r-plane sapphire) using magnetron sputtering. The two materials were intentionally deposited into two different deposition chamber designs and under different conditions (temperature, pressure, gases, and energy of sputtered particles). Despite the differences in the deposition condition and material system, both materials had the same feature with uncommon tilted epitaxial growth. Through an in-depth x-ray diffraction analysis of the NaCl (B1)-structured materials on r-plane sapphire, the full twin domain epitaxial relations were determined and can be described as (110)NaCl(B1)∥(44¯03)Al2O3 and [11¯2]NaCl(B1)∥[1¯1¯20]Al2O3. This relationship differs from the previously observed orientation of (100)NaCl(B1)∥(11¯02)Al2O3 and [100]NaCl(B1)∥[101¯0]Al2O3. These results are of general relevance for the growth of the extended NaCl (B1)-structured cubic material family onto a r-plane sapphire substrate where similar epitaxial growth can be expected.

Microwave Field-Induced Changes in Raman Modes and Magnetic Force Images of Antiferromagnetic NiO Films

Effective control of domain walls or magnetic textures in antiferromagnets promises to enable robust, fast, and nonvolatile memories. The lack of net magnetic moment in antiferromagnets implies the need for creative ways to achieve such a manipulation. We conducted a study to investigate changes in magnetic force microscopy (MFM) imaging and in the magnon-related mode in Raman spectroscopy of virgin NiO films under a microwave pump. After MFM and Raman studies were conducted, a combined action of broadband microwave (0.01–20 GHz, power scanned from −20 to 5 dBm) and magnetic field (up to 3 kOe) were applied to virgin epitaxial (111) NiO and (100) NiO films grown on (0001) Al2O3 and (100) MgO substrates, following which the MFM and Raman studies were repeated. We observed a suppression of the magnon-related Raman mode subsequent to the microwave exposure. Based on MFM imaging, this effect appeared to be caused by the suppression of large antiferromagnetic domain walls due to the possible excitation of antiferromagnetic spin oscillations localized within the antiferromagnetic domain walls.

Interfacial electronic state between hexagonal ZnO and cubic NiO.

The interface of two dissimilar materials gives rise to a myriad of interesting structural, magnetic, and electronic properties that may be utilized to produce novel materials with unique characteristics and functions. In particular, growing a cubic oxide film on top of a hexagonal oxide substrate results in such unique properties due to the conflict of their respective stabilization mechanisms within the interface layer. This study aims to elucidate the electronic properties of the interface between hexagonal ZnO and cubic NiO by analyzing the interface electronic states within epitaxial NiO films grown on ZnO substrates, expressed in the form of ultraviolet photoemission spectroscopy (UPS) for valence band structure and X-ray absorption spectroscopy (XAS) spectra for conduction band structure. This is accomplished through a modeling approach in which the film, substrate, and interface signals are assumed to be related to each other by a set of mathematical equations, and then rearranging and modulating the equations to obtain unique UPS and XAS spectra that depict the interface electronic states.

High crystalline epitaxial thin films of NiO by plasma-enhanced ALD and their properties

NiO is an interesting transition metal oxide due to its fascinating properties. High crystalline thin films of NiO are preferred for use in a variety of device applications but are challenging to deposit at low temperatures. We have prepared epitaxial thin films of NiO with [111] as the preferred growth direction on a c-plane sapphire substrate at relatively low temperatures using plasma-enhanced atomic layer deposition (PEALD) exploiting a simple nickel precursor with oxygen plasma. The evolution of crystallinity and surface morphology of the films were studied as a function of substrate temperature. Ultra-smooth NiO films with excellent crystallinity were prepared at 250 °C without the necessity for post-annealing. Different microscopic and spectroscopic methods revealed film characteristics. The magnetic properties of (111) oriented epitaxial NiO films prepared using PEALD are explored for the first time, and they are antiferromagnetic in nature.

Influence of epitaxial growth on spin current transmission of NiO films

Abstract This letter reports the effect of the epitaxial growth of NiO films on its spin current transmission. Two sample series of Pt/NiO/CoFeB, including epitaxial and polycrystalline NiO, were prepared using different deposition conditions. The spin current transmission of NiO is evaluated using the thermo-spin effects in the trilayers. Epitaxial NiO exhibits a non-monotonic behavior in the NiO thickness dependence of spin current transmission, while this behavior is smeared out in polycrystalline NiO. The non-monotonic behavior implies the presence of multiple spin transport mechanisms, which becomes apparent with epitaxial growth.

Comparative Study of Spin Pumping in Epitaxial- and Polycrystalline-NiO/Ni80Fe20

We perform a comparative study of spin pumping in NiO/Ni[Formula: see text]Fe[Formula: see text] (Py) by altering the crystallinity of NiO. We simultaneously grow high-quality NiO/Py heterostructures on Si/SiO2 and Al2O3(0001) substrates. Although the growth on Si/SiO2 substrate produced polycrystalline NiO (Poly-NiO), the growth on Al2O3(0001) substrate produced epitaxial NiO (Epi-NiO). Our ferromagnetic resonance measurements reveal the presence of two-magnon scattering (TMS) in the measured linewidth for the Epi-NiO/Py system. By carefully considering the contribution of TMS, we found the effective spin mixing conductance, g[Formula: see text] to be comparable in both Poly-NiO/Py and Epi-NiO/Py, despite the different crystallinity of NiO. Our findings show that g[Formula: see text] of an optimized polycrystalline-NiO/Py system can reach the value for an epitaxial-NiO/Py system.

High Stability Flexible Deep-UV Detector Based on All-Oxide Heteroepitaxial Junction

Transparent flexible electronics constitute a significant research field. Flexible deep-ultraviolet (UV) detectors have received much attention due to their potential in the applications of healthcare, communications, astronomy, and environment monitoring. Recent studies have investigated a variety of flexible photodetectors but show that the transparent, flexible, chemical, and thermal stability performances of these detectors cannot meet the requirements for practical applications. In this study, we demonstrate transparent flexible deep-UV detectors based on the combination of high-quality epitaxial n-type β-Ga2O3 and p-type NiO films as a photodiode on a flexible muscovite substrate. The electrical current of this heterojunction is increased over a 1000 times for on/off ratio under 265 nm wavelength illumination with a reasonable response (<1 s). Moreover, these photodetectors also exhibit good thermal stability as well as excellent mechanical flexibility. Our results exhibit the superior performance of the oxide-based solar-blind deep-UV detectors for advanced flexible sensing and smart applications.

Resistive switching characteristics of epitaxial NiO thin films affected by lattice strains and external forces

We investigated the resistive switching characteristics of NiO films grown epitaxially on two single-crystal Rh and Ni substrates. Two epitaxial NiO films were relatively small compressive strained on the Rh substrate, and confirmed by the x-ray diffraction experiments that the strain was relatively large compressive on the Ni substrate. Comparing the forming, SET, and RESET voltages of NiO resistive switching random access memory capacitors, NiO films grown on the Ni substrates showed relatively higher voltages than Rh substrates. We propose that this compressive strain has an effect on raising the forming, SET, and RESET voltages. Furthermore, we observed that the forming voltage decreases while reducing the compressive strain by applying pressure to the NiO thin film grown on the Ni substrate with an atomic force microscope tip. In addition, we showed that relaxation of compressive strain by external force by first-principles calculations can reduce the oxygen deficiency energy and lower the forming voltage and working voltages, which supports the experimental results well.

Tuning the Electrical Properties of NiO Thin Films by Stoichiometry and Microstructure

Here, the electrical properties of NiO thin films grown on glass and Al2O3 (0001) substrates have been investigated. It was found that the resistivity of NiO thin films strongly depends on oxygen stoichiometry. Nearly perfect stoichiometry yields extremely high resistivity. In contrast, off-stoichiometric thin films possess much lower resistivity, especially for oxygen-rich composition. A side-by-side comparison of energy loss near the edge structure spectra of Ni L3 edges between our NiO thin films and other theoretical spectra rules out the existence of Ni3+ in NiO thin films, which contradicts the traditional hypothesis. In addition, epitaxial NiO thin films grown on Al2O3 (0001) single crystal substrates exhibit much higher resistivity than those on glass substrates, even if they are deposited simultaneously. This feature indicates the microstructure dependence of electrical properties.

Growth of Thin Epitaxial NiO Films on LiNbO3 Substrates

The growth of epitaxial (111) NiO films on (0001) LiNbO3 substrates by reactive magnetron sputtering has been reported for the first time. X-ray diffraction analysis of the obtained films was carried out in combination with layer-by-layer plasma etching. It was shown that a low lattice mismatch allows one to obtain structurally perfect thin layers of NiO on LiNbO3 substrates. With increasing film thickness, the structural perfection of the film deteriorates due to the accumulation of dislocations due to ion bombardment during growth.

Magnetic characteristics of epitaxial NiO films studied by Raman spectroscopy

Raman spectroscopy is utilized to study the magnetic characteristics of heteroepitaxial NiO thin films grown by plasma-assisted molecular beam epitaxy on MgO(100) substrates. For the determination of the Néel temperature, we demonstrate a reliable approach by analyzing the temperature dependence of the Raman peak originating from second-order scattering by magnons. The antiferromagnetic coupling strength is found to be strongly influenced by the growth conditions. The low-temperature magnon frequency and the Néel temperature are demonstrated to depend on the biaxial lattice strain and the degree of structural disorder, which is dominated by point defects.

Epitaxial Growth of Bendable Cubic NiO and In2O3 Thin Films on Synthetic Mica for p- and n-type Wide-Bandgap Semiconductor Oxides

AbstractBendable p-type NiO and n-type In2O3 thin films were epitaxially grown on synthetic mica using mist chemical vapor deposition. It was found that at a growth temperature of 400 °C, epitaxially grown cubic (111) NiO thin films developed twin rotational domains, and the epitaxial relationship between each domain and the substrate was (111) NiO [1-10] or [10-1] || (001) synthetic mica [100]. In the visible light region, the epitaxial NiO thin films showed high transparencies, and their cut-offs appeared in the UV region. Additionally, at a growth temperature of 500 °C, cubic (111) In2O3 thin films with and without Sn doping were epitaxially grown on synthetic mica. As a result of the plasma oscillation of free carriers, Sn-doped In2O3 thin films exhibited reflection characteristics in the infrared region, while maintaining their visible light transmission characteristics. Furthermore, compared with non-doped In2O3, Sn doping decreased the sheet resistance by two digits.

Coherent Transfer of Spin Angular Momentum by Evanescent Spin Waves within Antiferromagnetic NiO.

Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.

Electric field control of magnetization reversal in conducting filament nanostructures in NiO resistive random access memory

Abstract Magnetic nanostructures are attracting attention for use in magnetic storage applications because of the magnetization reversal that occurs when information is read and written. Here, we reveal that conducting filament (CF) nanostructures undergo magnetization switching on NiO thin film. We fabricated epitaxial NiO thin films deposited on a single-crystal Cu substrate through the pulsed laser deposition method. The resistive random access memory characteristics indicated that the current-voltage properties of the Au/NiO/Cu capacitor showed unipolar resistive switching behavior. Using conducting atomic force microscopy (CAFM), various CF nanostructures, i.e., nanodots, nanorods, and nanorings, were formed on the NiO thin film. We studied the reversal in the direction of the magnetization induced by the electric field in the CF nanorods and nanorings on the NiO thin film by using the tip of the CAFM. The CF nanostructures on the NiO thin film have ferromagnetic properties. The coercive field of the nanorings is larger than that of the nanorods, based on Ampere’s law. Our results showed that the magnetic field induced by applying voltage to the tip of the CAFM enables magnetization switching in the nanorods and nanorings.

Distinct domain reversal mechanisms in epitaxial and polycrystalline antiferromagnetic NiO films from high-field spin Hall magnetoresistance

Magnetic-field-induced domain reversal mechanisms of antiferromagnetic (AF) NiO thin films are investigated by spin Hall magnetoresistance (SMR) measurements. The field strength dependence of SMR amplitudes is measured in epitaxial and polycrystalline NiO films. From the field-dependent behavior of SMR amplitude, two distinct domain reversal mechanisms are found for those NiO films. In the epitaxial films, the conventional monodomain formation against the destressing field due to the magneto-elastic coupling is observed. On the other hand, the thermally assisted domain reversal is dominant in the polycrystalline films. Based on our thermal assisted model, the effective values of domain pinning potential and the number of spins contributing to domain reversal in polycrystalline films are determined. These values are quite important to design AF spin memory devices. This study contributes to building a method to determine the key parameters in AF spintronics with polycrystalline thin films, which are free from the lattice mismatching problem.

Plasma-assisted molecular beam epitaxy of NiO on GaN(00.1)

The growth of NiO on GaN(00.1) substrates by plasma-assisted molecular beam epitaxy under oxygen-rich conditions was investigated at growth temperatures between 100°C and 850°C. Epitaxial growth of NiO(111) with two rotational domains, with epitaxial relation NiO(11¯0)‖GaN(11.0) and NiO(101¯)‖GaN(11.0), was observed by X-ray diffraction and confirmed by in situ reflection high-energy electron diffraction as well as transmission electron microscopy (TEM) and electron backscatter diffraction. With respect to the high lattice mismatch of 8.1% and a measured low residual tensile layer strain, growth by lattice matching epitaxy or domain-matching epitaxy is discussed. The morphology measured by atomic force microscopy showed a grainy surface, probably arising from the growth by columnar rotational domains visible in TEM micrographs. The domain sizes measured by AFM and TEM increase with the growth temperature, indicating an increasing surface diffusion length. Growth at 850°C, however, involved local decomposition of the GaN substrate that leads to an interfacial β-Ga2O3(2¯01) layer and a high NiO surface roughness. Raman measurements of the quasiforbidden one-phonon peak indicate increasing layer quality (decreasing defect density) with increasing growth temperature.

Pulsed Laser Deposition of Rocksalt Magnetic Binary Oxides

Magnetic binary oxides with the rocksalt structure constitute an important class of materials for potential applications as electronic or electrochemical devices. Moreover, they often become a theoretical playground, due to the simple electronic and crystal structures, in the quest for novel phenomena. For these possibilities to be realized, a necessary prerequisite would be to grow atomically ordered and controllably-strained binary oxides on proper substrates. Here we systematically explore the use of pulsed laser deposition technique (PLD) to grow three basic oxides that have rocksalt structure but different chemical stability in the ambient atmosphere: NiO (stable), MnO (metastable) and EuO (unstable). By tuning laser fluence FL, an epitaxial single-phase NiO thin-film growth can be achieved in a wide range of growth temperatures 10 ≤ TG ≤750 °C. At the lowest TG, the out-of-plane strain raises to 1.5%, which is five times higher than in NiO film grown at 750 °C. MnO thin films that had long-range order were successfully deposited on the MgO substrates after appropriate tuning of deposition parameters. The growth of MnO phase was strongly influenced by FL and the TG. EuO films with satisfactory quality were deposited by PLD after oxygen availability had been minimized. Synthesis of EuO thin films at rather low TG = 350 °C prevented thermally-driven lattice relaxation and allowed growth of strained films. Overall, PLD was a quick and reliable method to grow binary oxides with rocksalt structure in high quality that can satisfy requirements for applications and for basic research.

A conducting atomic force microscopy study of conducting filament nanobits in the epitaxial NiO thin film prepared precisely controlled by the oxidation time of the single crystalline Ni substrates

We report on the formation of conducting filament (CF) nanodots and the resistive random access memory (RRAM) characteristics of epitaxial NiO thin films with good crystallinity obtained by oxidation of the single crystal Ni substrates. The thickness of the epitaxial NiO thin films with good crystallinity was precisely controlled by the oxidation time. The local current mapping using conducting atomic force microscope (CAFM) showed that the NiO thin films had uniform resistance without conducting defects inside. In particular, it was confirmed that CF nanodots with a diameter of several nanometers in the 5-nm-thick NiO thin film and the storage density of about 16.7 Tbit/in2 was achieved by arranging the CF nanodots.

Temperature-dependence of resistive switching behaviors and tunneling mechanism of (111) epitaxial NiO films on metal seed layer

Since 2000, resistive switching random access memory (ReRAM) based on binary metal oxide has become one of potential candidates for next generation of nonvolatile memory due to its simple structure, high writing/read speed (~5 ns), good endurance (~109 cycles), long retention time (~10 years) and low energy consumption (~ dozens of pJ). NiO has been extensively studied as one of promising materials applied for commercial ReRAMs. However, it exhibits diverse resistive switching (RS) behaviors that heavily dependent on fabrication technologies, methods and conditions. Up to now, RS performances and mechanisms of polycrystalline NiO films have been extensively investigated, while there are few discussions on RS performances of epitaxial NiO films, especially on metal seed layer. On the other hand, with the expanding scope of human activities, researches on stabilities of ReRAM devices under extreme environmental conditions are necessary. In this paper, temperature-dependence of RS behaviors and tunneling mechanism of epitaxial NiO(111) films on Pt seed layers have been discussed. A set of highly textured NiO(111) films on Pt seed layers were prepared by means of Radio Frequency magnetron sputtering. Their microstructural characteristics and temperature-dependence of RS performance were investigated. Microstructural investigation demonstrated the “cube-to-cube” lattice relationship of NiO(111)/Pt(001) interfaces. Under the applied electric field, the drift of oxygen ions in NiO film and the redox of Ag ions near upper electrodes result in the formation/rupture of oxygen-vacancy conductive filaments at a set/reset voltage lower than ± 1 V. Moreover, current-voltage ( I - V ) curves of Ag/NiO(111)/Pt memory cells exhibit stable bipolar switching behaviors in the temperature range from RT to 80°C. Moreover, their Arrhenius form (ln I vs. 1000/ T ) and Schottky tunneling (ln( I / T 2) vs. 1000/ T ) plots are the best linear fitting relationship. It reveals that the leakage current of Ag/NiO(111)/Pt memory cell on high resistance state can be explained by the Schottky tunneling mechanism.