MgO Deposition Research Articles

Wear reduction mechanism in modulated turning of nodular graphite iron with coated carbide tool

Modulation-assisted machining (MAM) is used for turning nodular graphite iron (NGI) with coated carbide tools and compared with conventional machining (CM) at the cutting speed of 350 m/min in dry and wet conditions. Significant reductions in flank wear are achieved using MAM as compared to CM turning, in both dry and wet conditions. Dry MAM results in even lower wear than wet MAM condition. The substantial wear reduction in dry MAM is due to the formation of two material layers (i.e., the MgO deposition layer and the stagnated iron layer) and their unique arrangement on the flank face of the tool. The protective effect of these layers and the MAM conditions conducive to the formation of these layers are discussed.

Rapid Laser Processing of Thin Sr‐Doped LaCrO3–δ Interconnects for Solid Oxide Fuel Cells

Rapid laser reactive sintering (RLRS) is an additive manufacturing process which enables the quick and efficient fabrication of a wide range of ceramic‐based cells with various geometries and microstructures. Herein, the preparation of La0.8Sr0.2CrO3–δ interconnects by RLRS for solid‐oxide fuel/electrolysis cells (SOFC/ECs) is demonstrated. Uniform perovskite structure without residual intermediate phases can be achieved by CO2 laser irradiation at 103 W with a scanning speed of 0.07–0.10 mm s−1. Narrowing the width of the underlying MgO substrate and deposition of a terpineol‐based slurry with a solid loading level of less than 4 g m−2 are found to be critical parameters to avoid cracking and delamination. The optimum laser conditions balance Cr loss and densification of La0.8Sr0.2CrO3–δ resulting in an 11‐μm‐thick RLRS‐derived La0.8Sr0.2CrO3–δ film having a high relative density of 80–95% with a low area specific resistance (ASR) of 0.003 Ω cm2 at 600 °C. This ASR is more than 30 times lower than that of furnace‐sintered La0.8Sr0.2CrO3–δ in the same thickness range. The RLRS technique is promising for quick and efficient preparation of dense and thin La0.8Sr0.2CrO3–δ which are key components for highly compact SOFC/ECs.

Overview of catalytic upgrading of biomass pyrolysis vapors toward the production of fuels and high‐value chemicals

The application of heterogeneous catalysis in biomass pyrolysis is considered as one of the most promising methods to improve bio‐oil quality by minimizing its undesirable properties (high viscosity, corrosivity, instability, etc.) and producing renewable fuels and high‐value chemicals. Catalytic fast pyrolysis (CFP) of biomass refers both to the “in situ” and “ex situ or two‐stage” upgrading. A plethora of catalytic materials have been investigated in the literature for both approaches, including conventional microporous zeolites, ordered mesoporous aluminosilicates, promoted or not with several transition metals, as well as various metal (nano)oxide catalysts with Lewis acidity. Recently, hybrid micro/mesoporous and basic materials have been also suggested exhibiting most promising results due to combined micro/mesoporosity and adequate balance of acid and basic sites. Optimum catalysts are required to retain deoxygenation, enhance yields of aromatics, and other valuable compounds (such as phenolics), while limiting coke formation. Coke formation is one of the reasons of catalyst deactivation; a very challenging issue during biomass CFP, especially using zeolites. The deactivation process is affected by many factors, including the composition of the feedstock (inorganic minerals present in the feedstock may also deposit on the catalyst), reaction conditions, and the nature/properties of the catalysts used in CFP. Precoking on the surface of zeolites or MgO deposition are among the proposed effective ways to suppress coke formation and thus, catalyst deactivation.This article is categorized under: Bioenergy > Science and Materials Energy Research & Innovation > Science and Materials Energy and Transport > Science and Materials

Dependence of magnetic anisotropy on MgO sputtering pressure in Co20Fe60B20/MgO stacks

We investigated the dependence of magnetic anisotropy of Ta/Co20Fe60B20/MgO stacks on the Ar partial pressure during MgO deposition, in the range between 0.5 and 15 mTorr. The stacks are studied before and after annealing at 300°C and it is shown that magnetic anisotropy significantly depends on Ar partial pressure. High pressure results in stacks with very low perpendicular magnetic anisotropy even after annealing, while low pressure results in stacks with perpendicular anisotropy even at the as-deposited state. A monotonic increase of magnetic anisotropy energy is observed as Ar partial pressure is decreased.

Interface formation of epitaxial MgO/Co2MnSi(001) structures: Elemental segregation and oxygen migration

The interface formation in epitaxial MgO/Co2MnSi(001) films was studied using in-situ X-ray photoelectron spectroscopy (XPS). MgO was deposited on single crystal Co2MnSi(001) layers using e-beam evaporation: a technique which is expected to oxidize the Co2MnSi layer somewhat due to the rise in oxygen partial pressure during MgO deposition while leaving the deposited MgO oxygen deficient. Not unexpectedly, we find that e-beam evaporation of MgO raises the oxygen background in the deposition chamber to a level that readily oxidizes the Co2MnSi surface, with oxygen bonding preferentially to Mn and Si over Co. Interestingly, this oxidation causes an elemental segregation, with Mn-Si effectively moving toward the surface, resulting in an MgO/Co2MnSi interface with a composition significantly differing from the original surface of the unoxidized Co2MnSi film. As MgO is deposited on the oxidized Co2MnSi, the Mn-oxides are reduced, while the Si oxide remains, and is only somewhat reduced after additional annealing in ultrahigh vacuum. Annealing after the MgO is grown on Co2MnSi causes oxygen to move away from the oxidized Co2MnSi interface toward the surface and into the MgO. This observation is consistent with an increase in the tunneling magnetoresistance ratio with post-growth annealing measured in fabricated magnetic tunnel junctions (MTJs). The findings are discussed in light of fabrication of MgO/Heusler based MTJs, where the exponential decay of tunneling probability with contact separation exemplifies the importance of the ferromagnet/tunnel barrier interface.

Naphthalene hydrogenation over Mg-doped Pt/Al2O3

Mg-doped alumina carriers at various alkaline earth contents (0.5–8wt%) were prepared by incipient wetness impregnation of corresponding nitrate on sol-gel Al2O3 support. Pt (1wt%) supported catalysts were obtained by pore-filling deposition from H2PtCl6·6H2O on magnesium-modified carriers. Prepared materials were characterized by N2 physisorption, infrared (IR) and UV–vis spectroscopies and temperature-programmed reduction. Textural properties of alumina substrate were not significantly affected by MgO deposition at any content suggesting well-dispersed magnesia patches. According to CO2 adsorption analyzed by IR, surface basic properties of mixed supports were clearly enhanced with alkaline earth concentration. Reduction temperature of Pt4+ species slightly shifted to lower values by augmenting Mg content in binary supports pointing out to weakening of noble metal-mixed carriers interaction. Reduced (at 623K, under hydrogen flow) noble metal catalysts were tested in liquid-phase naphthalene hydrogenation in tri-phasic slurry batch reactor (T=523K, P=3.92MPa). The reduced Pt catalysts supported over the binary carrier at 2wt% Mg content had the best properties in both naphthalene conversion and consecutive tetralin hydrogenation. Nevertheless, addition of the basic additive at higher loading in the support resulted detrimental on saturating properties of corresponding noble metal-based catalyst. Selectivity to preferred cis-decalin isomer decreased over Pt materials by increasing alkaline-earth concentration in the carrier.

Discharge and structural characteristics of MgO thin films under various O2 and H2 gas flow rates during MgO deposition when using ion plating method in microdischarge cells

ABSTRACTThis paper has investigated the characteristics of structure and discharge for the MgO thin films grown by the ion plating process by varying the oxygen (O2) and hydrogen (H2) flow rates. The structural characteristics of the MgO film are measured by scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). The discharge characteristics are examined based on the firing voltage, secondary electron emission, sustain delay, and address delay. By increasing the O2 flow rates during ion plating process, the grain sizes of the MgO thin films are decreased and the firing voltages are decreased, which would be due to the improvement of secondary electron emission characteristics. In addition, as the O2 gas flow rates are increased in the range from 200 to 260 sccm, the sustain delay times are reduced, on the contrary, the address delay times are increased. Whereas, by increasing the H2 flow rates, the grain sizes of the MgO thin films are increased. In particular, the lower firing voltages are measured at a H2 flow rate of 40 sccm, which also would be due to an enhancement of secondary electron emission characteristics. When H2 flow rates are increased from 0 to 40 sccm, the address delay times are greatly reduced. Whereas, when H2 flow rates are beyond 40 sccm, the address delay times are slightly increased. However, the sustain delay times are slightly increased as the H2 flow rates are increased from 0 to 60 sccm. Accordingly, the optimal control of both O2 and H2 flow rates during the MgO film deposition using ion plating method can contribute to enhancing the discharge characteristics in ac plasma display panels (ac-PDPs).

Epitaxial growth of ultrathin MgO layers on Fe3O4(0 0 1) films

The initial growth stages of MgO on Fe3O4 films are studied by means of X-ray photoelectron spectroscopy and low energy electron diffraction to clarify stoichiometric and structural properties of these layered structures. This bilayer structure is important to fabricate high quality magnetic tunnel junctions based on Fe3O4 electrodes and MgO tunneling barriers. For this purpose, the deposition temperature of MgO has been varied between 100°C and 250°C. Initially, MgO grows layer-by-layer on Fe3O4/MgO(001) forming a wetting layer. Depending on the growth temperature, after growth of a 2–3nm thick laminar wetting layer, the MgO films finally start to roughen during growth. Thus the growth of MgO on Fe3O4/MgO(001) is described by a Stranski–Krastanov growth mode. Diffraction experiments show that the magnetite (2×2)R45° superstructure is removed already during the initial stages of MgO deposition. Furthermore, these experiments show that MgO films are rougher for growth at low deposition temperatures.

High-performing and durable MgO/Ni catalysts via atomic layer deposition for CO2 reforming of methane (CRM)

MgO-coated Ni catalysts with two different shell thicknesses were prepared using atomic layer deposition (ALD) and their catalytic activities for the CO2 reforming of CH4 (CRM) were compared with that of bare Ni catalyst. Conversion of CO2 and CH4 in the CRM reaction at 800°C was enhanced with increasing thickness of the ALD-deposited MgO shells. In particular, the catalytic activity of MgO/Ni catalyst prepared using 200 ALD MgO deposition cycles was fairly durable, with negligible deactivation during a 72h CRM reaction. We suggest that the MgO shell, which is highly basic and has high affinity to CO2, likely, accelerated the reverse CO disproportionation, thereby reducing the poisoning of Ni active sites during the CRM reaction. Furthermore, the 200-cycled MgO shell destroyed ensemble of Ni catalysts surface, allowing preservation of Ni active sites with preferential growth of filamentous carbon to layered graphitic ones. We suggest that the preparation of core–shell structures using ALD can be a promising strategy for fabricating highly active and durable catalysts that are operable at relatively high temperatures.

Effects of post-growth annealing in a CoFeB/MgO/CoFeB trilayer structure

CoFeB/MgO/CoFeB tri-layer thin-film stacks have been widely used in the design of STT-RAM devices as functional magnetic-tunnel-junction (MTJ) structures. The materials properties of the CoFeB and MgO layers, including composition and lattice quality, have been extensively researched from the stand point of optimizing for the best MTJ performance. On the other hand, post-growth annealing is required for the MTJ structure to acquire its functional property, i.e. its TMR performance. In this work, we have studied the various possible effects resulting from the post-growth annealing process. Specifically, we show that the post-growth annealing causes boron in the top and bottom CoFeB layers to migrate into the adjacent Ta layers as well as deterioration in lattice quality of the MgO layer. Furthermore, we evaluate other effects that could be possibly induced during the annealing process, including Ta diffusion and layer intermixing in the CoFeB/MgO/CoFeB tri-layer structure. The post-growth annealing causes little change in the Ta diffusion and the layer intermixing. These annealing effects were also evaluated with respect to variations in the MgO growth process; more specifically, an additional natural oxidation treatment during the MgO layer deposition and the insertion of a Fe layer before the MgO layer. Our results indicate that the addition of a natural oxidation process during the MgO deposition process and the insertion of a thin-layer of Fe before the MgO layer both lead to a reduction in the layer intermixing between the MgO and the CoFeB layer and to an improvement in MgO lattice quality. We also show that the post-growth annealing does not alter the beneficial effect of these MgO growth process modifications.

Compositional tuning of atomic layer deposited MgZnO for thin film transistors

Thin film transistors (TFTs) have been fabricated using magnesium zinc oxide (MgZnO) layers deposited by atomic layer deposition at 200 °C. The composition of the MgZnO is systematically modified by varying the ratio of MgO and ZnO deposition cycles. A blue-shift of the near band-edge photoluminescence after post-deposition annealing at 300 °C indicates significant activation of the Mg dopant. A 7:1 ratio of ZnO:MgO deposition cycles was used to fabricate a device with a TFT channel width of 2000 μm and a channel length of 60 μm. This transistor yielded an effective saturation mobility of 4 cm2/V s and a threshold voltage of 7.1 V, respectively. The on/off ratio was 1.6×106 and the maximum interface state density at the ZnO/SiO2 interface is ∼6.5×1012 cm−2.

Electrophoretic deposition of crack-free magnesium oxide-coated tin oxide film and its application in dye-sensitized solar cells

Magnesium oxide-coated tin oxide aggregates were prepared by a facile method. The aggregates consisted of nanoparticles which provided a large surface area of 74.27 m2g−1. Crack-free MgO-coated SnO2 porous films were prepared using electrophoretic deposition method. The causes which prevent the forming of cracks were also discussed. For the application in dye-sensitized solar cells, the electrophoresis conditions, such as the amount of MgO loading and deposition duration, were optimized. The MgO-coated SnO2 photoanodes showed excellent photovoltaic response in the long wavelength region between 550nm and 700nm The optimized MgO-coated SnO2 photoanodes showed an overall power conversion efficiency above 7.0% under AM 1.5G illumination (100 mW cm−2), having a short circuit current density, open circuit voltage, and fill factor of 20.0mAcm−2, 0.618V and 0.57, respectively.

Optimization of tunneling magnetoresistance of MgO based tunnel junctions by tuning the stage impedance for radio frequency sputtering of the barrier

In this article, we describe a method to maximize the tunneling magnetoresistance of magnetic tunnel junctions with MgO tunneling barrier by optimizing the stage impedance for the RF MgO deposition. The impedance can be varied continuously using a matchbox that is in place to apply a RF bias to the stage. It is measured by means of a network analyzer connected to a dummy wafer on the stage. For positive stage reactance, both resistance area product and tunneling magnetoresistance are observed to drop, related to the stage impedance resonating with the plasma sheath capacitance. At high negative stage reactance, resputtering is minimized and tunneling magnetoresistance maximized.

Large perpendicular magnetic anisotropy at Fe/MgO interface

A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

High resolution photoemission study of interface formation between MgO and the selenium passivated InAs (1 0 0) surface

Interface formation between an ultra thin MgO layer and the selenium passivated InAs surface has been investigated by soft X-ray photoemission spectroscopy. Atomic hydrogen cleaning of the native oxide covered InAs at 360°C produced an oxide and carbon free InAs surface. The selenium passivation of the atomically clean InAs showed evidence of arsenic replacement in the near surface region. Subsequent MgO deposition resulted in the appearance of an oxidized indium signal indicating that the bonding interaction between the MgO and the substrate is via indium–oxide bond formation. The conduction and valence band offsets were also estimated for this dielectric–semiconductor structure.

High‐resolution photoemission comparison study of interface formation between MgO and the atomically clean and Se‐passivated Ge(100) surfaces

AbstractThe interface formation between MgO dielectric layers and the atomically clean and selenium‐passivated Ge(100) surfaces has been studied using high‐resolution synchrotron radiation based photoemission spectroscopy. Atomically clean germanium was prepared by argon ion bombardment and sub‐ sequent annealing at 550 °C. MgO deposition on this surface resulted in interface oxide formation whereas no oxidation states were observed for a selenium‐treated germanium surface indicating the effectiveness of this interlayer at producing an abrupt MgO/Ge(100) interface. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evidence for boron diffusion into sub-stoichiometric MgO (001) barriers in CoFeB/MgO-based magnetic tunnel junctions

Evidence of boron diffusion into the MgO barrier of a CoFeB/MgO based magnetic tunnel junction has been identified using analytical scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy. Structures were deposited by DC/RF-magnetron sputtering, where defective, sub-stoichiometric MgO barriers degrading device performance have been previously mitigated against by deposition of thin Mg layers prior to MgO deposition. We show that despite the protection offered by the Mg layer, disorder in the MgO barrier is still evident by STEM analysis and is a consequence of the oxidation of the Co40Fe40B20 surface during MgO deposition. Evidence of boron diffusion from CoFeB into the MgO barrier in the as-deposited and annealed structure is also presented, which in the as-deposited case we suggest results from the defective structures at the barrier interfaces. Annealing at 375 °C results in the presence of B in the trigonal coordination of [BO3]3− in the MgO barrier and partial crystallization of the top electrode (we presume there is also some boron diffusion into the Ta capping layer). The bottom electrode, however, fails to crystallize and much of the boron is retained in this thicker electrode. A higher annealing temperature or lower initial boron content is required to crystallize the bottom electrode.

Spin-Based MOSFETs for Logic and Memory Applications and Spin Accumulation Signals in CoFe/Tunnel Barrier/SOI Devices

New innovative ferromagnetic source/drain technologies on Si for next-generation-transistor applications are researched and developed using CoFe/AlOxn+-Si and CoFe/MgO n+-Si junctions. As evidence of the spin accumulation in the n+-Si conduction channels, nonlocal spin signals and four-terminal nonlocal-Hanle signals are presented for CoFe/MgO/SOI devices. The spin diffusion times determined by four-terminal nonlocal-Hanle signals are consistent with those observed in three-terminal Hanle signals. The relatively long spin diffusion time of τs=1.4 nsec and relatively large spin polarization P=0.43 at room temperature for CoFe/MgO/SOI devices were observed, when fitting to the existing diffusion model for spin injection and accumulation. We have observed the marked enhancement of the absolute value of three-terminal voltage changes via Hanle-type spin precessions (|ΔV|) as a function of interface resistance in the temperature range between 20 K and 300 K. We also have observed the asymmetric bias voltage dependence on ΔV. In terms of the reason of marked enhancement of |ΔV| as a function of interface resistance, the spin absorption into ferromagnet would be most effective. For the explanation of the asymmetric bias voltage dependence, we should take into account two additional possible origins. Moreover, we succeed in decreasing the interface resistance for CoFe/MgO/ n+-Si junctions down to 36 Ωμm2 by using evaporation method for MgO deposition.

Improvement of Mobility in ZnO Thin Film Transistor with an Oxygen Enriched MgO Gate Dielectric

Theperformanceofbottom-gateZnOthinfilmtransistors(TFTs)usingMgOgatedielectricsevaporatedwithandwithoutintroducing oxygen have been investigated. The oxygen introduced during MgO deposition improves the field-effect mobility significantly as compared to the device without introducing oxygen during MgO deposition. The oxygen-introduced MgO exhibits a dielectric constant of 10.9 and the field-effect mobility of the TFT device is enhanced to 78.3 cm 2 /V s. The threshold voltages can also be related to whether the oxygen is introduced into MgO or not. The interface between oxygen-introduced MgO and ZnO is examined and its connection with mobility enhancement is discussed.

MgO Layer Thickness Dependence of Structure and Magnetic Properties of L10-FePt/MgO/GaAs Structures

We investigated the MgO layer thickness dependences of the structure and magnetic properties of L10-FePt/MgO/GaAs structures. To examine how the crystallinity and growth morphology of the MgO layer affect the L10-FePt layer, two kinds of preparation method were employed for MgO deposition: electron beam (EB) evaporation and sputter deposition. The MgO layer deposited by EB evaporation included a large strain because of the cube-on-cube epitaxial relationship despite a large lattice mismatch between MgO and GaAs. For the MgO layer prepared by sputtering, on the other hand, an amorphous MgO layer was initially grown on the GaAs substrate. Subsequently, a crystalline MgO layer was grown in the (001) direction. In the case of the EB-deposited MgO, as the MgO layer thickness increased, the degree of chemical order of the L10-FePt layer increased from 55 to 81% owing to the improvement of the crystallinity of the MgO layer. The improvement of chemical order also led to the increase in the remanent magnetization of L10-FePt from 84 to 98%.