MgO Layer Research Articles

Photoelectron spectroscopy of tetraphenylporphyrin layers on MgO films on Ag(100)

The molecular orbitals of magnesium tetraphenyl porphyrin (MgTPP) deposited on epitaxial MgO films on Ag(100) are studied by ultraviolet photoelectron spectroscopy and two-photon photoemission. The energies of the electronic states are pinned to the vacuum level and the band gap is 3.88±0.07 eV largely independent of the MgO layer thickness.

Ultra-high thermal stability of perpendicular magnetic anisotropy in the W buffered CoFeB/MgO stacks with Zr dusting layers

Practical device applications of magnetic multilayers with perpendicular magnetic anisotropy (PMA) usually need to match the mature complementary metal-oxide-semiconductor (CMOS) integrated techniques, which require high temperature annealing during the back-end-of-line process. Here, we report the realization of PMA in the W buffered CoFeB/MgO stack by inserting a thin Zr dusting layer between CoFeB and MgO layers. An ultra-high thermal stability of PMA in the W/CoFeB/Zr/MgO stack is observed, which is robust upon annealing at 600 °C. The establishment of PMA in W/CoFeB/Zr/MgO is due to the formation of an interface layer between CoFeB and MgO doped with oxidized Zr. After annealing at 540 °C, the magnetic interfacial anisotropy density reaches 3.08 erg/cm2, which is much higher than those in previous reports. The results suggest that the W/CoFeB/Zr/MgO stack with extra high annealing stability is a potential candidate to achieving the practical application of spin-logic device that is compatible with the mature CMOS integrated techniques.

Different oxygen migration behaviors at CoFe/MgO and CoFe/HfO2 interfaces and their effects on the magnetic anisotropy

We have studied annealing-induced oxygen migration at CoFe/MgO and CoFe/HfO2 interfaces and its effect on the magnetic anisotropy of Ta/CoFe/MgO (HfO2) films. Through x-ray photoelectron spectroscopy, we found that the Fe–O bonds exist at both CoFe/MgO and CoFe/HfO2 interfaces at the as-deposited state due to the oxidation of interfacial Fe atoms during the deposition of the MgO and HfO2 layers. After annealing, the amount of the Fe–O bonds at the CoFe/MgO interface decreases, whereas at the CoFe/HfO2 interface, it increases, indicating that the oxygen atoms migrate from Fe–O bonds to MgO layers at the CoFe/MgO interface, whereas they migrate from the HfO2 layer to Fe–O bonds at the CoFe/HfO2 interface. Correspondingly, the magnetic anisotropy energy decreases in the Ta/CoFe/MgO film but increases in the Ta/CoFe/HfO2 film after annealing. We attributed these results to interfacial Fe 3d–O 2p orbital hybridization modulated by different oxygen migration behaviors. Our results may improve the understanding of the oxygen migration effect on magnetic anisotropy and anomalous Hall sensitivity in ferromagnet/oxide heterostructures.

Effect of micron-Ti particles on microstructure and mechanical properties of Mg–3Al–1Zn based composites

In the field of metal matrix composites, it is a great challenge to simultaneously improve the strength, ductility and elastic modulus of magnesium matrix composites (MMCs). In this work, Mg–3Al–1Zn composites reinforced with micron-Ti particles were prepared by powder metallurgy. The results show that the yield strength, elongation and elastic modulus simultaneously were increased with increasing content of Ti particles. Through diffusion, Ti and Al formed TiAl phase at the edge of Ti particle. A nanoscale layer of MgO formed between TiAl and magnesium (Mg) matrix. The 9Ti/Mg–3Al–1Zn composite obtained the best comprehensive mechanical properties with yield strength, ultimate tensile strength and elongation of 264 MPa, 294 MPa and 8%, respectively. The increased strength is mainly due to the grain refinement and strong interfacial bonding between Ti particles and Mg matrix. The improved ductility is the result of refined grains, weakened texture and collaborative deformation of Ti particles.

Atomic-Scale Insights on Large-Misfit Heterointerfaces in LSMO/MgO/c-Al2O3

Understanding the interfaces in heterostructures at an atomic scale is crucial in enabling the possibility to manipulate underlying functional properties in correlated materials. This work presents a detailed study on the atomic structures of heterogeneous interfaces in La0.7Sr0.3MnO3 (LSMO) film grown epitaxially on c-Al2O3 (0001) with a buffer layer of MgO. Using aberration-corrected scanning transmission electron microscopy, we detected nucleation of periodic misfit dislocations at the interfaces of the large misfit systems of LSMO/MgO and MgO/c-Al2O3 following the domain matching epitaxy paradigm. It was experimentally observed that the dislocations terminate with 4/5 lattice planes at the LSMO/MgO interface and with 12/13 lattice planes at the MgO/c-Al2O3 interface. This is consistent with theoretical predictions. Using the atomic-resolution image data analysis approach to generate atomic bond length maps, we investigated the atomic displacement in the LSMO/MgO and MgO/c-Al2O3 systems. Minimal presence of residual strain was shown at the respective interface due to strain relaxation following misfit dislocation formation. Further, based on electron energy-loss spectroscopy analysis, we confirmed an interfacial interdiffusion within two monolayers at both LSMO/MgO and MgO/c-Al2O3 interfaces. In essence, misfit dislocation configurations of the LSMO/MgO/c-Al2O3 system have been thoroughly investigated to understand atomic-scale insights on atomic structure and interfacial chemistry in these large misfit systems.

Electron-phonon coupling of Fe-adatom electron states on MgO/Ag(100)

We study the strength of the electron-phonon interaction on Fe single adatoms on MgO/Ag(100) based on many-body \textit{ab-initio} spin collinear calculations. In particular, we analyze the relative importance of the substrate and, among other results, we conclude that the interface electron state of Ag(100) plays a prominent role in determining the electron-phonon coupling of localized Fe electron states. The analysis of the hybridization of the adatom with the substrate reveals qualitative differences for even or odd coverages of MgO, affecting significantly the spectral structure and strength of the electron-phonon coupling. Our calculations indicate that the electron-phonon interaction is very strong for $\le$~1 layers of MgO, while it is sharply suppressed for larger coverages, a trend that is consistent with recent experimental findings.

A dual-modification strategy for P2-type layered oxide via bulk Mg/Ti co-substitution and MgO surface coating for sodium ion batteries

P2-type materials are regarded as competitive cathodes for next generation sodium ion batteries. However, the unfavorable P2 → O2 phase transition usually leads to severe capacity decay. Moreover, the cathode material always suffers from destruction of surface crystal structure caused by trace amount of HF. In this study, a dual-modification method containing Mg/Ti co-doping and MgO surface coating is designed to solve the defects of P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode. Results turn out that the P2 structure can be stabilized via Mg/Ti co-substitution and MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na+. Moreover, the as-prepared MgO-coated Na0.67Ni0.17Co0.17Mn0.66Mg0.1O2 exhibits improved electrochemical performance than the raw material. It delivers 111.6 mAh g−1 initial discharge capacity and maintains 90.6% at high current density of 100 mA g−1 within 2–4.5 V, which has been obviously enhanced than that of Na0.67Ni0.17Co0.17Mn0.66O2. The significant improvement can be attributed to the synergistic effect of Mg/Ti co-substitution and MgO surface coating. This dual-modification strategy based on the synergetic effect of Mg/Ti co-doping and MgO surface coating might be a resultful step forward to develop cathode materials for sodium ion batteries.

Negative electron affinity opens quantum well in MgO layers on Ag(100)

Epitaxial MgO films on Ag(100) were studied by photoelectron spectroscopy. From the low-energy part of the spectra we obtain a negative electron affinity of about −0.9 eV. Even though electrons in the lowest conduction band are not confined by a potential barrier at the surface, quantum-well resonances are observed. The dispersion of the conduction band is determined in good agreement with theoretical calculations. Aspects of observing image-potential states predicted by theory on MgO films are discussed.

A Physically Transient Self-Rectifying and Analogue Switching Memristor Synapse

A physical transient memristor synapse with self-rectifying and analogue switching behaviors based on Mo/MgO/AZO (Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> 2 wt %, ZnO 98 wt %)/W is presented. By modulating thickness of MgO layer, the device with 5 nm MgO insulator layer shows stable resistive switching memory with a rectification ratio up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and precise tuning synaptic functions. Meanwhile, the formation of Schottky barrier and oxygen vacancy conductive filament is proposed to further explain self-rectifying and analogue switching behaviors. Additionally, the electrical characteristics of the Mo/MgO/AZO/W devices degraded after being immersed in deionized water (DI) for 3 min, which demonstrated the physically transient features successfully. The physical transient self-rectifying memristors have great potential for applications in secure neuromorphic computing systems, and bio-integrated electronics.

A Systematic Assessment of W-Doped CoFeB Single Free Layers for Low Power STT-MRAM Applications

Spin-transfer torque magnetoresistive random access memory (STT-MRAM) technology is considered to be the most promising nonvolatile memory (NVM) solution for high-speed and low power applications. Dual MgO-based composite free layers (FL) have driven the development of STT-MRAMs over the past decade, achieving data retention of 10 years at the cost of higher write power consumption. In addition, the need for tunnel magnetoresistance (TMR)-based read schemes limits the flexibility in materials beyond the typical CoFeB/MgO interfaces. In this study, we propose a novel spacerless FL stack comprised of CoFeB alloyed with heavy metals such as tungsten (W) which allows effective modulation of the magnet properties (Ms, Hk) while retaining compatibility with MgO layers. The addition of W results favours a delayed crystallization process, in turn enabling higher thermal budgets up to 180 min at 400 °C. The presence of tungsten reduces the total FL magnetization (Ms) but simultaneously increasing its temperature dependence, thus, enabling a dynamic write current reduction of ~15% at 2 ns pulse widths. Reliable operation is demonstrated with a WER of 1 ppm and endurance &gt;1010 cycles. These results pave the way for alternative designs of STT-MRAMs for low power electronics.

Resistivity size effect in epitaxial iridium layers

The resistivity size effect in Ir is quantified with in situ and ex situ transport measurements at 295 and 77 K using epitaxial layers with thickness d = 5–140 nm deposited on MgO(001) and Al2O3(0001) substrates. Data fitting with the Fuchs–Sondheimer model of the measured resistivity ρ vs d for single-crystal Ir(001)/MgO(001) layers deposited at Ts = 1000 °C yield an effective electron mean free path λeff = 7.4 ± 1.2 nm at 295 K, a room-temperature bulk resistivity ρo = 5.2 μΩ cm, and a temperature-independent product ρoλeff = (3.8 ± 0.6)×10−16 Ω m2, which is in good agreement with first-principles predictions. Layers deposited at Ts = 700 °C and stepwise annealed to 1000 °C exhibit a unique polycrystalline multi-domain microstructure with smooth renucleated 111-oriented grains that are &amp;gt;10 μm wide for d = 10 nm, resulting in a 26% lower ρoλeff. Ir(111)/Al2O3(0001) layers exhibit two 60°-rotated epitaxial domains with an average lateral grain size of 88 nm. The grain boundaries cause a thickness-independent resistivity contribution Δρgb = 0.86 ± 0.19 and 0.84 ± 0.12 μΩ cm at 295 and 77 K, indicating an electron reflection coefficient R = 0.52 ± 0.02 for this boundary characterized by a 60° rotation about the ⟨111⟩ axis. The overall results indicate that microstructural features including strain fields from misfit dislocations and/or atomic-level roughness strongly affect the resistivity size effect in Ir. The measured ρoλeff for Ir is smaller than for any other elemental metal and 69%, 43%, and 25% below reported ρoλ products for Co, Cu, and Ru, respectively, indicating that Ir is a promising alternate metal for narrow high-conductivity interconnects.

Exploration of the nonideal behavior observed in engineered, multilayer MgO/Ag/MgO photocathodes

Improving photocathode performance by increasing electron emission while lowering the angular spread of emitted electrons can improve particle accelerator performance, expanding the reach of both fundamental and applied science. Materials science expertise is needed to design new photocathodes with these desired properties. In this work, we have undertaken a study of the electronic structure of the interfaces in a multilayer photocathode structure consisting of MgO/Ag/MgO to explore how the fabrication process can lead to nonideal interfaces compared to those constructed in simulations. To study how the fabrication affects the interfaces, hard x-ray photoemission spectroscopy was used to probe the chemistry of the buried interfaces within the thin film multilayer structure of Ag and MgO. In these multilayer structures, we observed that the silver layers were predominantly metallic. A small high binding energy (ΔE=0.69 eV) peak was also observed in the Ag 3d core level in the samples. This peak is shifted in the opposite direction of the binding energy shift in silver oxides, suggesting that this peak is not due to formation of silver oxides at the interfaces with the MgO. Two possible explanations for the origin of this peak then are charge transfer at the interface from the Ag to the oxide monolayer or the formation of silver nanoparticles during the growth process. Based upon simple depth profiling analysis, we postulate that the former is the more likely explanation but cannot rule out the latter. In addition, the O 1s and Mg 1s core level indicated the presence of Mg(OH)2. The MgO layers react with H2O in the vacuum chamber or ideal gas used as a buffer during sample transfer. Since the theory predicts strong dependence upon the number of MgO layers surrounding the Ag, the formation of Mg(OH)2 likely contributes to the nonideal behavior, even given the similarity in the electronic structure to MgO (large bandgap insulator) and Mg(OH)2. The speed at which this reaction occurs would significantly limit the lifetime and the utility of the MgO/Ag multilayer photocathodes. In order to custom engineer multilayer photocathodes, complete control over the growth process will be needed to ensure that the ideal surfaces are formed. Using nonreactive materials would greatly increase the lifetime of the engineered photocathodes.

Toward two-dimensional hybrid organic-inorganic materials based on a I-PE/UHV-PVD system for exceptional corrosion protection

The combination of inorganic thin films with highly functionalized organic molecules has been desired for state-of-the-art applications in electrochemistry and catalytic science. To address this need, we report a rapid, new strategy to fabricate functionalized two-dimensional hybrid organic-inorganic (2D HOI) materials through the combination of interface-plasma electrolysis (I-PE) with ultrahigh-vacuum physical vapor deposition (UHV-PVD), where benzene-1.3.5-tricarboxylic acid (BTC) was used as the organic compound. We demonstrate growth control of organic thin films assembled by reaction sites in BTC molecules, such as carbonyl groups and hydroxyl groups, on a porous inorganic layer. Beginning with a defective surface and organic molecules, we rely on the evaporation of BTC molecules during UHV-PVD to induce layer-by-layer (LbL) deposition, i.e., consecutive formation of multilayers of the organic constituents on 2D HOI materials. DFT calculations were carried out to understand the organic-inorganic interfaces, the adsorption behavior, bonding, and the energetics between BTC molecules and the inorganic surface. Theoretical calculations reveal a robust reciprocal interaction between parallel adsorption configurations of BTC molecules and neighboring MgO layer, facilitated by the formation of a BTC….MgO bond. More interestingly, upon contact with reactive solution, the molecules in the self-assembled multilayers rearranged to a flower-like structure with holes (diameter > 100 nm), which facilitated the corrosive penetration of the magnesium substrate. Finally, on the basis of impedance results, we conclude that the fabricated 2D HOI materials exhibit exceptional corrosion protection.

Highly conductive and flexible electrodes based on ultrathin aluminum-doped zinc oxide epitaxial films

Aluminum-doped zinc oxide (AZO) thin films are popular transparent electrodes for optoelectronics and photovoltaics. However, next generation flexible devices demand not only high conductivities but also low thicknesses in AZO electrodes to stay robust against deformations. The two requirements contradict each other due to the scattering of carriers at grain boundaries in polycrystalline AZO electrodes. Herein, this work develops ultrathin, highly conductive, single-crystalline, and mechanically robust AZO electrodes on flexible Hastelloy substrates. The epitaxial growth of AZO on polycrystalline Hastelloy is empowered by an advanced buffer architecture topped with a biaxially-textured MgO layer. Thereby, a c-axis epitaxial strain is generated in the AZO electrodes and then regulated by varying the thickness of the electrodes. Abnormally high carrier concentrations are detected in the AZO electrodes likely caused by the piezopotential generated by the epitaxial strain. While the highest resistivity value of the epitaxial AZO electrodes is within the mainstream values of AZO electrodes on various substrates, a resistivity as low as ~22 μΩ•cm is demonstrated in the ultrathin electrode with a thickness of ~28 nm. The mechanical durability of the ultrathin AZO electrode is validated by bending tests of 1000 cycles.

Interfacial microstructure and strengthening mechanisms of Cf/Mg composite with double-layer interface

In order to overcome the interfacial incompatibility of carbon fiber reinforced magnesium matrix (Cf/Mg) composites, a double-layer interface (ZrO2–MgO) is designed in this work. Carbon fiber was modified with ZrO2 coating by sol-gel process. Microstructural examination reveals that MgO layer forms on the surface of ZrO2 coating by ZrO2 reacting with Mg during the composite fabrication. Such double-layer interface could inhibit Al4C3 and hence prevent fiber damage. Meanwhile, the wettability was improved for the reaction between ZrO2 and Mg. Thus the tensile strength of ZrO2-Cf/AZ91D composite was 68.0% higher than that of the unmodified one. Due to the fiber bundle pull-out, debonding and crack deflection, the toughness of Cf/Mg composite with double-layer interface is increased simultaneously.

Evolution of Inclusions in Magnesium–Calcium-Treated Liquid Iron

With the development of clean steel technology, the control of non-metallic inclusions in steel is of increasing importance. Magnesium–calcium treatment can effectively balance the castability of molten steel and the control on inclusion size, which is an inclusion modification approach with application prospect. In view of this, how three addition methods (i.e., adding Mg before Ca, adding Mg after Ca, and adding Mg together with Ca) influenced the modification effect of inclusions in liquid iron was experimentally studied, and how these inclusions evolved with time was discussed in this paper. The results demonstrated that despite the sharp difference in their inclusion evolution, composite inclusions with a magnesium aluminate spinel (MAS) core and an outer CaO–Al2O3–MgO layer were formed by all the three addition methods, with the average inclusion size of 1–2 μm. Furthermore, thermodynamic calculation was adopted to reveal the transformation relationship between MAS and calcium aluminate in each of the three addition methods, and clarify the formation and disappearance mechanisms of the intermediate product CaS in the process of Mg–Ca treatment. The thermodynamic calculation results agreed well with the experiment data.

Influence of Graphite Layer on Electronic Properties of MgO/6H-SiC(0001) Interface.

This paper concerns research on magnesium oxide layers in terms of their potential use as a gate material for SiC MOSFET structures. The two basic systems of MgO/SiC(0001) and MgO/graphite/SiC(0001) were deeply investigated in situ under ultrahigh vacuum (UHV). In both cases, the MgO layers were obtained by a reactive evaporation method. Graphite layers terminating the SiC(0001) surface were formed by thermal annealing in UHV. The physicochemical properties of the deposited MgO layers and the systems formed with their participation were determined using X-ray and UV photoelectron spectroscopy (XPS, UPS). The results confirmed the formation of MgO compounds. Energy level diagrams were constructed for both systems. The valence band maximum of MgO layers was embedded deeper on the graphitized surface than on the SiC(0001).

Enhancing the Schottky-barrier height by inserting a thin MgO layer between Au and annealed-ZnO

The effects of a thin MgO insertion layer between Au and annealed-ZnO on the Schottky-barrier diodes (SBDs) of Au/annealed-ZnO were studied. The structure of Au/as-deposited ZnO (SBD_A) presents a rather leaky (near ohmic) characteristic owing to the presence of oxygen vacancies on the ZnO surface. After post-annealing of ZnO in air, the Au/annealed-ZnO (SBD_B) exhibits a rectifying behavior as a result of the compensated oxygen vacancies. Upon inserting a MgO film between the Au and annealed-ZnO, the leakage current is significantly reduced by approximately 3-orders of magnitude in the Au/MgO/annealed-ZnO structure (SBD_C), largely enhancing the rectification ratio from 10.3 to 731. This is because the oxygen vacancies are further reduced by the MgO modification of ZnO. The post-ZnO annealing and MgO insertion increased the Schottky-barrier height (SBH) from 0.58 to 0.68 and 1.1 eV in SBD_A, SBD_B, and SBD_C, respectively. Hence, the MgO insertion not only decreases the oxygen vacancies on the ZnO surface, it also increases the SBH. X-ray diffraction, X-ray photoelectron spectroscopy, and band-diagrams were employed to elucidate the mechanisms of the increase in SBH.

Laser induced crystallization of Co\u2013Fe\u2013B films

Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized tunneling magnetoresistance (TMR) devices. In the case of Co–Fe–B TMR devices, used most successfully so far in applications and devices, Co–Fe–B layers are initially deposited in an amorphous state and annealed post-deposition to induce crystallization in Co–Fe, thereby increasing the device performance. In this work, first direct proof of locally triggered crystallization of 10 nm thick Co–Fe–B films by laser irradiation is provided by means of X-ray diffraction (XRD) using synchrotron radiation. A comparison with furnace annealing is performed for benchmarking purposes, covering different annealing parameters, including temperature and duration in the case of furnace annealing, as well as laser intensity and scanning speed for the laser annealing. Films of Co–Fe–B with different stoichiometry sandwiched between a Ru and a Ta or MgO layer were systematically assessed by XRD and SQUID magnetometry in order to elucidate the crystallization mechanisms. The transformation of Co–Fe–B films from amorphous to crystalline is revealed by the presence of pronounced CoFe(110) and/or CoFe(200) reflexes in the XRD θ-2θ scans, depending on the capping layer. For a certain window of parameters, comparable crystallization yields are obtained with furnace and laser annealing. Samples with an MgO capping layer required a slightly lower laser intensity to achieve equivalent Co–Fe crystallization yields, highlighting the potential of laser annealing to locally enhance the TMR ratio.

Perpendicular magnetic anisotropy in ultra-thin Cu2Sb-type (Mn–Cr)AlGe films fabricated onto thermally oxidized silicon substrates

Perpendicularly magnetized films exhibiting small saturation magnetizations (Ms) are essential for spin-transfer-torque magnetoresistive random access memories (MRAMs). In this study, the intermetallic compound (Mn–Cr)AlGe with a Cu2Sb crystal structure was investigated as a material exhibiting low Ms (∼300 kA/m) and high-perpendicular magnetic anisotropy energy (Ku). The layer thickness dependence of Ku and effects of Mg-insertion layers at the top and bottom (Mn–Cr)AlGe|MgO interfaces were studied for film samples fabricated onto thermally oxidized silicon substrates to realize high Ku values in the thickness range of a few nanometers. The values of Ku were approximately 7×105 and 2×105 J/m3 at room temperature for 5 and 3 nm-thick (Mn–Cr)AlGe films, respectively, with an optimum annealing temperature of 400 °C and Mg-insertion thicknesses of 1.4 and 3.0 nm for the top and bottom interfaces, respectively. The Mg insertions were relatively thick compared with results of similar studies of the insertion effect on magnetic tunnel junctions. Cross-sectional transmission electron microscope images revealed that the Mg-insertion layers acted as barriers to the interdiffusion of Al atoms as well as oxidization from the MgO layers. The Ku at a few-nanometer thicknesses was comparable to or higher than those reported for perpendicularly magnetized CoFeB films, which are conventionally used in MRAMs, whereas the Ms value was one third or smaller than those of the CoFeB films. The developed (Mn–Cr)AlGe films are promising materials because of their magnetic properties and their compatibility to the silicon process in film fabrication.