MgO Layer Research Articles

Blocking Si‐Induced Visible Photoresponse in n‐MgxZn1–xO/p‐Si Heterojunction UV Photodetectors Using MgO Barrier Layer

Photodetectors based on n‐Mg0.25Zn0.75O/p‐Si heterojunctions are not only suitable for integration with existing semiconductor technology, but also circumvent the difficulty of stable p‐type doping in MgxZn1–xO. However, the use of Si leads to photoresponse in the visible part of the light spectrum, which renders n‐MgxZn1–xO/p‐Si heterojunction devices unsuitable for visible blind UV photodetection. Herein, it is demonstrated that the visible photoresponse in the n‐Mg0.25Zn0.75O/p‐Si photodetectors can be significantly suppressed by inserting a thin interlayer of MgO at the heterojunction. The MgO layer serves as a blocking layer for the drift of photo‐excited electrons from p‐Si to n‐Mg0.25Zn0.75O, thereby limiting the visible photoresponse. It is found that on increasing the thickness of the MgO interlayer from 3 to 15 nm, the UV to visible rejection ratio increases from ≈25 to 200. This enhancement in the UV to visible rejection ratio demonstrates that n‐Mg0.25Zn0.75O/p‐Si heterojunction devices with MgO interlayer are promising for visible‐blind UV photodetection applications.

Perpendicular magnetic anisotropy in as-deposited CoFeB/MgO thin films

Fabrication of perpendicularly magnetized ferromagnetic films on various buffer layers, especially on numerous newly discovered spin–orbit torque (SOT) materials to construct energy-efficient spin-orbitronic devices, is a long-standing challenge. Even for the widely used CoFeB/MgO structures, perpendicular magnetic anisotropy (PMA) can only be established on limited buffer layers through post-annealing above 300 °C. Here, we report that the PMA of CoFeB/MgO films can be established reliably on various buffer layers in the absence of post-annealing. Further results show that precise control of MgO thickness, which determines oxygen diffusion in the underneath CoFeB layer, is the key to obtain the as-deposited PMA. Interestingly, contrary to the previous understanding, post-annealing does not significantly influence the well-established as-deposited PMA but indeed enhances unsaturated PMA with a thick MgO layer by modulating oxygen distributions, rather than crystallinity or Co– and Fe–O bonding. Moreover, our results indicate that oxygen diffusion also plays a critical role in PMA degradation at high temperatures. These results provide a practical approach to build spin-orbitronic devices based on various high-efficient SOT materials.

Anionic assisted incorporation of WO3 nanoparticles for enhanced electrochemical properties of AZ31 Mg alloy coated via plasma electrolytic oxidation

The present work examined the influence of anion type on incorporating WO 3 into the MgO layer produced via plasma electrolytic oxidation of AZ31 Mg alloy. Here, three different anions, such as aluminate (AlO 2 - ), silicate (SiO 3 2- ), and phosphate (PO 4 3- ) were added separately into an alkaline electrolyte containing WO 3 nanoparticles. The microstructural observations revealed that the incorporation of the WO 3 nanoparticles is affected by the diameter of discharge channels associated with the type of anion added into the electrolyte. The sample produced from phosphate electrolytes had higher thickness but was more porous than those obtained in aluminate or silicate electrolytes. Regardless of anion type, the amounts of WO 3 nanoparticles incorporated into the inner layer of PEO coating were more significant than those incorporated into the outer layer, where a WO 3 -rich inner layer was obtained in the case sample coated in electrolyte with silicate anions. The electrochemical measurements in a 3.5 wt% NaCl solution indicated that the corrosion resistance of the sample coated in silicate electrolyte was superior to other samples in which the sample coated in phosphate electrolyte exhibited the lowest corrosion resistance. This behavior is explained by a mechanism in which every anion produces its microstructural defects under the influence of discharge types, such as type-A, B, C, D, and E, thus, affecting the physical incorporation of WO 3 into the MgO layer under plasma conditions. • Incorporation of WO3 nanoparticles is affected by the anion type. • The anion type affects the types of plasma discharges developed during PEO. • WO3 nanoparticles affected by discharge type affiliated with particular anion. • Effect of SiO3 2- anions on the corrosion behavior of PEO coating is significant

Direct probing of temperature-independent bulk half-metallicity in Co2MnSi by spin-resolved hard x-ray photoemission

We have performed the bulk-sensitive spin-resolved hard x-ray photoelectron spectroscopy (HAXPES) to directly clarify the spin-dependent valence band electronic states and spin polarization of $L{2}_{1}$-ordered ${\mathrm{Co}}_{2}\mathrm{MnSi}$ as a predicted half-metal. The spin-resolved valence band HAXPES spectra clearly exhibited the difference in the majority and minority spin states in the bulk region of a 30-nm-thick ${\mathrm{Co}}_{2}\mathrm{MnSi}(001)$ thin film buried under a MgO(2 nm) layer at both temperatures of 21 and 300 K. As expected from the half-metallicity in ${\mathrm{Co}}_{2}\mathrm{MnSi}$, we found that the majority (minority) spin spectrum shows a metallic Fermi edge (a band gap), leading the high spin polarization of \ensuremath{\sim}90% at around the Fermi level at a temperature of 21 K. The spin-resolved HAXPES experiments revealed that the half-metallicity of ${\mathrm{Co}}_{2}\mathrm{MnSi}$ in the bulk region is independent on the temperature up to 300 K. The experiments also revealed the slight changes in the majority spin spectral shapes and the shift of the valence band maximum of the minority spin states with temperature. By comparing the experimental results with the first-principles calculations at a finite temperature obtained by the disordered local moment method, it is suggested that the slight changes in the majority and minority spin HAXPES spectra of ${\mathrm{Co}}_{2}\mathrm{MnSi}$ with temperature is mainly caused by the temperature-dependent Co $3d$ electronic states. It is also suggested that the conduction band minimum of the minority spin states is located sufficiently above the Fermi level at temperature up to 300 K.

Synthesis and flame retardant behavior of siloxane functionalized polyethylene

AbstractTo improve the flame retardancy of polyethyelene (PE), this work realized an effective in‐situ synthesis of siloxane functionalized PE via continuous process with two stages: in the first stage, reactive olefin monomers containing dichlorosilane group were designed and utilized in ethylene copolymerization catalyzed by Ziegler‐Natta catalyst; in the second stage, by injecting dimethylsilanediol (DMS) directly into the unterminated polymerization solution, siloxane was grafted onto PE side group via the reaction between the pendant chlorosilane groups on PE and hydroxyl on DMS. Owing to the carbonization effect of silica produced by siloxane combustion, the combustion residue of PE was significantly increased to 20.8 wt% in the TGA test after grafting 0.58 mol% DMS. The siloxane functionalized PE was then added into PE/magnesium hydroxide (MH) composites to investigate the materials burning behavior. By grafting 0.15 wt% siloxane, the limited oxygen index of PE was increased from 18% to 21% and that of PE/MH increased from 28% to 30%, indicating the flame retardancy of PE was improved with grafted siloxane modification. It was also found that the nanoscale silica in‐situ generated by the graft siloxane groups efficiently assisted the formation of a dense residual layer of MgO and amorphous carbon.

Electrically driven whispering-gallery-mode microlasers in an n-MgO@ZnO:Ga microwire/p-GaN heterojunction.

In emerging miniaturized applications, semiconductor micro/nanostructures laser devices have drawn great public attentions of late years. The device performances of micro/nanostructured microlasers are highly restricted to the different reflective conditions at various side surfaces of microresonators and junction interface quality. In this study, an electrically driven whispering-gallery-mode (WGM) microlaser composed of a Ga-doped ZnO microwire covered by a MgO layer (MgO@ZnO:Ga MW) and a p-type GaN substrate is illustrated experimentally. Incorporating a MgO layer on the side surfaces of ZnO:Ga MWs can be used to reduce light leakage along the sharp edges and the ZnO:Ga/GaN interface. This buffer layer incorporation also enables engineering the energy band alignment of n-ZnO:Ga/p-GaN heterojunction and manipulating the current transport properties. The as-constructed n-MgO@ZnO:Ga MW/p-GaN heterojunction device can emit at an ultraviolet wavelength of 375.5 nm and a linewidth of about 25.5 nm, achieving the excitonic-related recombination in the ZnO:Ga MW. The broadband spectrum collapsed into a series of sharp peaks upon continuous-wave (CW) operation of electrical pumping, especially for operating current above 15.2 mA. The dominant emission line was centered at 378.5 nm, and the line width narrowed to approximately 0.95 nm. These sharp peaks emerged from the spontaneous emission spectrum and had an average spacing of approximately 5.5 nm, following the WGM cavity modes. The results highlight the significance of interfacial engineering for optimizing the performance of low-dimensional heterostructured devices and shed light on developing future miniaturized microlasers.

Interface effects in ultra-scaled MRAM cells

The development of advanced magnetic tunnel junctions with a single-digit nanometer footprint can be achieved using an elongated multilayered ferromagnetic free layer structure. We demonstrate the switching of a composite free layer consisting of two ferromagnets separated by an MgO layer and an additional capping MgO layer to boost perpendicular anisotropy. A proper design of the free layer and its interface-induced perpendicular anisotropy helps to achieve reliable switching.

Opposite effects of typical solid inertants on flame propagation in Mg dust clouds versus dust layers

Adding solid inertants to combustible dust is an effective means of reducing dust explosion risk through both preventive and mitigative measures. However, since dust layers and dust clouds differ in their flame spread mechanisms, they may also differ in their response to solid inertants. In this paper, inerting efficacy of four commonly used solid inertants in Mg dust layers and dust clouds was investigated. As concentration increased, all four inertants effectively suppressed burning in Mg dust clouds. However, CaCO3, NaHCO3, and SiO2 greatly increased the fire hazard in Mg dust layers. CO2 produced by thermal decomposition of CaCO3 destroyed the MgO layer formed on the surface of Mg dust layers and caused more burning of Mg vapor in the gas phase. The water vapor produced by NaHCO3 reacted with Mg to generate hydrogen which intensified combustion. SiO2 emitted heat through a substitution reaction with Mg powder to thrust the mixed dust layer upwards, causing violent combustion similar to that in dust clouds. As inertant proportion increased, the fire hazard in dust layers was initially enhanced but was later decreased, although the fire hazard at 70% inertant remained much higher than that with pure Mg dust. Only NaCl, which is chemically stable and does not react with metal dust, can effectively inert both Mg dust layers and dust clouds. Research on the use of solid inertant technology to prevent or mitigate dust explosions should therefore consider the impact on both dust clouds and dust layers.

Strong dual-metal-support interactions induced by low-temperature plasma phenomenon

Supported bimetallic nanoparticles have important industrial applications. However, the direct synthesis of intermetallic alloy nanocatalysts against sintering and aggregation on porous support materials via strong metal-support interactions (SMSIs) remains challenging. Herein, we report a novel synthesis strategy used to achieve low-temperature-plasma-induced SMSIs on a MgO-supported MAg (M = Cu, Fe, or Ni) system by focusing an electric field at a localized interface. The electric-field-controlled discharge plasma intrinsically promoted the formation of a porous MgO structure. This unique structure comprised partially encapsulated MAg nanoparticles (NPs) with a porous MgO layer and co-localized M and Ag NPs in close proximity. Owing to the uniform size distribution of the intermetallic NPs and the unique surface properties of the porous support, the resulting MAg@MgO catalysts exhibited high activity and excellent reusability toward the reduction of nitro and carbonyl groups. In particular, the conversion efficiency of CuAg@MgO (100% after 20 s) was significantly higher than that of the most commonly used catalysts; moreover, its activity did not decrease after 20 cycles of cinnamaldehyde hydrogenation. We believe that this synthesis route provides a universal method for designing and preparing high-activity supported metal catalysts.

Microstructure and magnetic properties of FeSiCr soft magnetic powder cores with a MgO insulating layer prepared by the sol-gel method

In this study, the electromagnetic properties of FeSiCr soft magnetic powder cores (SMPCs) were optimized by providing a novel insulation process. Magnesium acetate tetrahydrate and oxalic acid dihydrate were used as raw materials for the sol-gel method. The powder was then heat-treated at 700 °C so that it was evenly coated by an insulating layer. Thermogravimetric analysis-differential scanning calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy, and energy dispersive spectroscopy were used to demonstrate that the layer heat-treated at 700 °C was the ideal MgO layer and was uniformly coated on the surface of the FeSiCr powder. Additionally, the potential reactions and growth mechanism of the MgO layer were analyzed. The effects of the magnesium acetate tetrahydrate content on the soft magnetic properties of the FeSiCr@MgO SMPCs were investigated. The optimal soft magnetic performance (Pcv = 248.48 mW/cm3, measured at 2500 kHz and 10 mT) was achieved at a magnesium acetate tetrahydrate content of 0.50 wt%, yielding a direct current-bias performance of up to 92.3% at 100 Oe. In addition, the FeSiCr@MgO SMPCs effectively improved the heat dissipation performance, which is necessary to avoid high-temperature damage, thereby enhancing their applicability in high-frequency and high-current environments.

Impact of MgO spacer layer on microwave performance of MgZnO/ZnO HEMT

This article analyzes the direct current and small-signal parameters of MgZnO/ZnO (MZO) HEMT for microwave application. Further, the impact of the MgO spacer layer on the microwave performance parameters such as transconductance (g m), cut-off frequency (f T), maximum oscillation frequency (f max) and Johnson’s figures of merit (J-FOM) of MZO HEMT has been analyzed. MZO HEMT with MgO spacer results in the enhanced values of two-dimensional electron gas (2DEG) density of 7.2 × 1013 cm−2 and g m of 91 mS mm−1. The values of f T and f max exhibit 3-fold enhancement to 5.57 GHz and to 7.8 GHz, respectively, and J-FOM is increased by 2.93 times with the introduction of MgO spacer layer in HEMT structure. Moreover, the impact of MgO spacer is studied on the off-state breakdown mechanism of MZO HEMT. The off-state breakdown voltage (V br) of MZO HEMT is ∼25 V higher than that for MZO HEMT with an MgO layer. Therefore, there is a trade-off between the microwave performance and the device off-state breakdown voltage. This work is significant for the development of large-area and cost-effective ZnO-based HEMTs for microwave applications.

Role of an in-plane ferromagnet in a T-type structure for field-free magnetization switching

Deterministic magnetization switching driven by current-induced spin–orbit torque (SOT) without an external magnetic field has potential applications in magnetic random access memory. Here, we realized the field-free magnetization switching in a T-type structure (CoFeB/W/CoFeB), where the two CoFeB layers have perpendicular magnetic anisotropy and in-plane magnetic anisotropy (IMA), respectively. We discovered that the direction of symmetry-breaking field is parallel to the magnetization of the bottom CoFeB (IMA), which cannot be explained by a stray field of this layer. In addition, by placing a 2.5-nm thick insulating layer of MgO between the bottom CoFeB and W layer (CoFeB/MgO/W/CoFeB) to block the interlayer exchange coupling and the spin current from the bottom CoFeB, the field-free SOT switching was still achieved, primarily due to the Néel orange-peel effect in our devices. By using micromagnetic simulations, the roughness of angstrom magnitude was introduced into the model to calculate the symmetry-breaking field, finding a qualitative agreement with experiments. Moreover, we obtained the spin Hall angle of CoFeB (θSH = −0.024) by the current-induced hysteresis loop shift method, and the contribution of the effective efficiency χ from the bottom CoFeB was accounted for about 26% of the total in the current-induced SOT switching process. These results indicated that an in-plane ferromagnet layer in the T-type structure provides not only the symmetry-breaking field but also spin current for the field-free SOT magnetization switching.

Standing wave hard X-ray photoemission study of the structure of the interfaces in Ta/Co2FeAl/MgO multilayer

Interfaces of the magnetic layer in the heterostructure Ta/Co2FeAl/MgO have been studied using hard X-ray photoelectron spectroscopy (HAXPES). Depth-selective information with sub-nanometer resolution is obtained using X-ray standing waves. It is found that the two magnetic species namely, Co and Fe behave very differently at the interfaces; 54% of Fe in the magnetic layer gets either oxidised or alloyed with Ta at the interfaces, as against 48% of Co. Oxidation state of both Co and Fe decreases, as one moves away from the MgO interface. On the average about 0.36 nm of Co2FeAl gets oxidised at MgO interface and 0.66 nm gets alloyed with Ta at the bottom interface. Thermal annealing at 200℃ results in further oxidation of Fe, accompanied by a partial reduction of Co. It is suggested that this opposite behaviour of Fe and Co at the MgO interface may be the cause of the thermal annealing induced shift of the centroid of Fe distribution towards MgO layer, as observed in some STEM studies in such heterostructures.

Monolithic Integration of Multi-Spectral Optical Interference Filter Array on Thin Film Amorphous Silicon Photodiodes

Hydrogenated amorphous silicon thin film photodetectors combined with optical filters are routinely used for biological detection applications based on fluorescence. Monolithic integration of these filters to the photodetectors further enhances the portability and simplicity of the system. However, due to the multitude of fluorophores used in such applications, it is often necessary to combine multiple filters in a single chip for increased ease of application and portability of the system. In this work, we report a novel facile technique to integrate multiple filters in a chip with four photodiodes, in an innovative methodology that leverages the beneficial characteristics of different types of deposition techniques to fabricate precise multilayer interference filters. Ion beam deposited MgO layers with different thicknesses are successively patterned by lift-off and sandwiched between two standard dielectric mirrors deposited via plasma-enhanced chemical vapor deposition to produce the filter array. The successful integration of the devices is demonstrated through fluorescence measurements of grape fluorescence at two different wavelengths which are utilized in grape maturation monitoring in the viticulture sector.

Room‐Temperature Non‐Local Spin Transport in Few‐Layer Black Phosphorus Passivated with MgO

AbstractBlack phosphorus (BP), a new member of 2D materials, is an ideal selection to construct spin‐based devices due to its tunable direct bandgap and high carrier mobility. Assembling van der Waals heterostructures is the most popular method to create spintronic devices for 2D materials, especially for the easily oxidized BP. However, it is too complicated to be realized for fabricating large‐scale integrated circuits in practical applications. To overcome this flaw, an oxide layer on BP simultaneously serving as the protection layer and barrier to fabricate a Co/MgO/BP‐based non‐local spin valve is employed. The non‐local spin signals demonstrate the diffusion of pure spin current in the BP channel, which is the direct evidence of the spin injection from Co into BP. Combining the Hanle precession measurements with the Bloch equation fitting, the spin transport parameters of the few‐layer BP can be extracted. The spin diffusion length λs and spin relaxation time τs are 6.15 µm and 241.7 ps, respectively. Therefore, the MgO layer in the non‐local spin valve can simplify the fabrication of 2D material‐based spintronic devices and accelerate their applications.

Structural analysis of the ZnO/MgO superlattices on a-polar ZnO substrates grown by MBE

The analysis of heteroepitaxially grown ZnO/MgO superlattices on a-oriented ZnO substrates was described. The influence of unalike natural crystalline structures of ZnO and MgO on the interface’s abruptness and retaining wurtzite crystallinity was studied. Wurtzite structure was found to be retained in the MgO thin barriers up to 1.5 nm, but creation of nonuniform biaxial strains and some interdiffusion of Mg from barriers to ZnO QW’s seemed to be inevitable. The complex strain/composition heterogeneities can be explained by the difference of vapor pressure between Mg (p = 1.93 Pa) and Zn (p = 0.22 Pa) elements at growth temperatures and high mobility of Mg atoms, which are gladly incorporated into ZnO matrix. As a result, a subtle three monolayers in size ZnMgO film was created at the interfaces, which was partly responsible for retaining the wurtzite structure of coherently grown MgO layers.

Demonstration of reconfigurable magnetic tunnel diode and giant tunnel magnetoresistance in magnetic tunnel junctions made with spin gapless semiconductor and half-metallic Heusler alloy

Here, we report fabrication of a high quality exchanged biased trilayer magnetic tunnel junction (MTJ) utilizing a magnetron sputtering system. The MTJ is composed of a type-II spin gapless semiconductor (SGS) Ti2CoSi inverse Heusler alloy (used as a lower electrode), a thin MgO layer (used as a tunnel barrier), and a half-metallic ferromagnet (HMF) Co2MnSi Heusler alloy (used as an upper electrode). Spin dependent transport properties reveal that the micro-fabricated MTJ can act as a reconfigurable magnetic tunnel diode, which lets the electric current to flow in either the forward or reverse path relying on the relative alignment of the magnetization direction of the upper and lower magnetic electrodes. A considerably high on/off current ratio (∼103) and a significantly low turn on voltage (VT) of 0.09 V have been achieved at 5 K for both parallel and antiparallel configurations. Another important characteristic shown by our fabricated MTJ is that it exhibits extremely large tunnel magnetoresistance ratios of 892% at 5 K and 197% at room temperature, which brings to light the utmost importance of using the combination of HMF and SGS materials as magnetic electrodes in a tunnel junction for potential applications in modern spintronic devices. All these exceptional features can undoubtedly nominate CMS/MgO/TCS MTJs as a promising candidate to serve as memory or logic elements in next generation ultra-high density magnetoresistive random access memories together with contemporary spin based electronic devices.

Spin Reflection-Induced Field-Free Magnetization Switching in Perpendicularly Magnetized MgO/Pt/Co Heterostructures.

Field-free magnetization switching is critical towards practical, integrated spin-orbit torque (SOT)-driven magnetic random-access memory with perpendicular magnetic anisotropy. Our work proposes a technique to modulate the spin reflection and spin density of states within a heavy-metal Pt through interfacing with a dielectric MgO layer. We demonstrate tunability of the effective out-of-plane spin torque acting on the ferromagnetic Co layer, enabling current-induced SOT magnetization switching without the assistance of an external magnetic field. The influence of the MgO layer thickness on effective SOT efficiency shows saturation at 4 nm, while up to 80% of field-free magnetization switching ratio is achieved with the MgO between 5 and 8 nm. We analyze and attribute the complex interaction to spin reflection at the dielectric/heavy metal interface and spin scattering within the dielectric medium due to interfacial electric fields. Further, through substituting the dielectric with Ti or Pt, we confirm that the MgO layer is indeed responsible for the observed field-free magnetization switching mechanism.

Study of the relationship between magnetic anisotropy and composition ratio of Fe oxide to Fe at CoFeB/MgO film interface

Ta (5)/CoFeB (1)/MgO (2)/Ta (2) (in nm) multilayers were deposited on silicon substrates by magnetron sputtering, and the MgO layer was prepared by two methods at the same argon gas flow rate: the radio frequency (RF) sputtering method of MgO target and the reactive sputtering method of direct current (DC) sputtering Mg target with introducing oxygen at the same time. The sample prepared by the former method was denoted as Sample A, and the latter was denoted as Sample B. The results indicate that as-deposited Sample A shows in-plane magnetic anisotropy (IMA) with Keff = -0.79 × 106erg/cm3. However, the as-deposited Sample B shows perpendicular magnetic anisotropy (PMA) with Keff = 1.69 × 106erg/cm3, and the Keff value increases to 3.17 × 106erg/cm3 after annealing at 250℃. X-ray photoelectron spectroscopy (XPS) results illustrate that the films show PMA when the composition ratio (ε) of Fe oxide to Fe in CoFeB/oxide interface of Sample A and B annealed below 300 ℃, is approximately in the range of 0.65–0.75. When ε is beyond the above range, the film shows IMA. Moderate oxidation state at the interface results in the formation of PMA and it was improved by forming effective orbital hybridization of Fe 3d and O 2p. High resolution transmission electron microscopy (HRTEM) analysis reveals that no crystallization appears in CoFeB and MgO layer of the film before and after annealing, and crystallization has no effect on the interface chemical states in this system.

Enhanced Mg/graphene interface adhesion using intermediate MgO layers: First-principles prediction and analysis

Graphene-reinforced Mg matrix composites suffer seriously from the weak Mg/graphene interfacial bonding. In this study, a first-principles study was performed to evaluate the feasibility of improving the Mg/graphene bonding using an in-situ formed intermediate MgO layer. The calculated interface adhesion strengths suggested a relative ordering (from high to low) of Mg(0001)/MgO(11–1) > MgO(11–1)/graphene > Mg(0001)/graphene. The enhanced Mg/MgO/graphene interface bonding can be attributed to the newly formed strong ionic and covalent interactions at the Mg/MgO and the MgO/graphene interfaces, respectively, which replace the otherwise very weak van der Waals bonding between Mg and graphene.