Thin MgO Layer Research Articles

Atomic-Scale Mechanism of Enhanced Electron-Phonon Coupling at the Interface of MgB2 Thin Films.

In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors, electron-phonon coupling is the fundamental mechanism of superconductivity. For instance, the superconductivity of magnesium diboride (MgB2) comes from the coupling between E2g modes (in-plane boron-boron bond vibrations) and self-doped charge carriers. In thin films and ceramics of BCS superconductors, interfaces with discontinuous chemical bonds may alter the local electron-phonon coupling. However, such effects remain largely unexplored. Here, we investigate the heterointerface of the MgB2 film on the SiC substrate at the atomic scale using electron microscopy and spectroscopy. We detect the presence of a thin MgO layer with a thickness of ∼1 nm between MgB2 and SiC. Atomic-level electron energy loss spectra (EELS) show MgB2-E2g mode splitting and softening near the MgB2/MgO interface, which enhances electron-phonon coupling at the interface. Our findings highlight the potential of interface engineering to enhance superconductivity via modulating local phonon states and/or electron states.

Impact of the gate work function on the experimental I-V characteristics of MOS solar cells simulated with the Sentaurus TCAD software

In this work, the influence of gate work function on the experimental J-VG characteristics of MOS solar cells was investigated with the aid of the Sentaurus TCAD for 2D numer-ical simulations of TiN/SiOxNy/Si Al/SiOxNy/Si and Al/MgO/Mg/SiOxNy/Si structures aiming at solar cells for en-ergy harvesting applications. The increase of the gate work function led to the increase of the reverse current density as pointed out by the Sentaurus TCAD simulations and by the ex-perimental J-VG characteristics. The work functions of Mg, Al, and un-annealed TiN used in the TCAD simulations were 3.7 eV, 4.1 eV, and 4.4 eV, respectively. It was observed that the onset voltage at 0.5 mA/cm2 in the forward-biasing region was at a lower voltage for TiN (~ - 0.06 V) compared to Al (~ - 0.42 V) and the Al/MgO/Mg stack (~ - 0.47 V). On the other hand, the current density increased steeply in the forward biasing for TiN and Al compared to the Al/ MgO/Mg stack gate and the thin MgO layer between Al and Mg worked as a potential barrier in an opposite direction to the potential barrier of the Mg/Si-OxNy/Si structure, which meant an onset voltage lowering for the Al/MgO/Mg/SiOxNy/Si solar cell. For the Al/MgO/Mg stack, the barrier effect of the MgO layer was fitted as a series re-sistance RS = 100 Ω and an equivalent Al/MgO/Mg work func-tion of 4.15 eV considering a substrate doping NA = 1.2x1016 cm-3 and parallel conductance GP = 0. Also, the experimental JxVG characteristic of the Al/SiOxNy/Si cell was fitted for Al work function of 4.10 eV, a series resistance RS = 100 Ω, a parallel resistance RP = 0.02 Ω (GP = 50 S) and a substrate doping NA = 5.5x1015 cm-3. In this case, the high parallel conductance fitted was attributed to the tunneling through the dielectrics as a pre-dominant effect possibly caused by a high concentration of de-fects in the SiOxNy layer. Finally, the MOS solar cell parameters were relatively lower compared to those of commercial outdoor solar cells, but the power generated by the MOS cells reached the mW range, and the conversion efficiency from light energy into electrical energy was higher (12.7%) than the typical values found for energy-harvesting solar cells.

Effect of electropolishing parameters of WE43 magnesium alloy on corrosion resistance of artificial plasma

In this study, an improved technique is proposed to obtain bare metal surfaces for implants, stents, and medical devices. The aim is to enhance the corrosion resistance and biocompatibility of the WE43 magnesium alloy by modifying its surface characteristics through electropolishing. This is achieved using an electrolytic mixture with varying concentrations of HNO3 and C2H5NO3. The morphology, microstructure, chemical composition, and phase structures of the coating are analyzed by SEM, XPS, and XRD. The corrosion resistance of the surface was analyzed by polarization and bio-corrosion behavior tests. During the electropolishing process, unstable Mg(NO3)2 is easily formed by Mg and HNO3. However, the addition of C2H5OH helps release C2H5NO3 and HNO3 ions from the electrolyte, leading to the formation of a thin and dense layer of MgO on the alloy surface. The results revealed that surface roughness significantly affected the corrosion resistance of the alloy. Specifically, the oxide layer thickness gradually decreased with increasing voltage during electropolishing. However, it was observed that higher electropolishing voltages (beyond 0.5 V) caused surface cracking and reduced the corrosion resistance of the alloy. Therefore, it is necessary to investigate the optimal electrolytic polishing parameters to enhance the corrosion resistance of WE43 magnesium alloy. According to the experimental results, the optimal electrolytic polishing parameters for WE43 magnesium alloy were determined as follows: 2.0 wt% HNO3 and C2 H5OH, a voltage of 0.45 V, and a time of 300 s. These parameters also exhibited the best biocompatibility characteristics.

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.

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.

Dynamics of Photo-Generated Carriers across the Interface between CsPbBr3 Nanocrystals and Au-Ag Nanostructured Film, and Its Control via Ultrathin MgO Interface Layer.

The dynamics and control of charge transfer between optoelectronically interesting and size-tunable halide perovskite quantum dots and other juxtaposed functional electronic materials are important issues for the emergent device interest involving such a family of materials in heterostructure configurations. Herein, we have grown bimetallic Au–Ag thin films on glass by pulsed laser deposition at room temperature, which bear nanoparticulate character, and the corresponding optical absorption spectra reveal the expected surface plasmon resonance signature(s). Subsequently, spin-coated CsPbBr3 nanoparticle films onto the bimetallic Au–Ag films exhibit surface-enhanced Raman scattering as well as strong photoluminescence quenching, the latter reflecting highly efficient transfer of photo-generated carriers across the CsPbBr3/Au–Ag interface. Surprisingly, when an ultrathin MgO (insulating) layer of optimum thickness is introduced between the CsPbBr3 and Au–Ag films, the charge transfer is further facilitated with the average lifetime of carriers becoming even shorter. By changing the thickness of the thin MgO layer, the carrier lifetime can in fact be tuned; with the charge transfer getting fully blocked for thick enough MgO layers, as expected. Our study thus throws light on the charge-carrier dynamics in halide perovskites, which is of importance to emergent optoelectronic applications.

Ultraviolet Photoconductive Detector Based on MgO/a-GaN Heterostructure Grown by Aqueous Method.

Thin MgO nanostructure layer was grown on a-plane GaN film in this study using low temperature aqueous method. The thin MgO layer showed uniform sheet-like morphology. Meanwhile, ultraviolet (UV) photoconductive detector based on MgO/a-GaN heterostructure was fabricated by simple way. The obtained detector displayed excellent UV sensing properties. Results showed that the thin MgO nanostructure layer can effectively decrease the dark current and passivate the surface defects. The facile method will provide a new route to adopt nanostructures for applications in enhancing the performance of GaN-based UV detector.

Instability of UV Photoresponse in MgZnO Thin Films and Its Improvement by MgO Capping

Herein, it is shown that the UV photoresponse characteristics of MgZnO thin films deposited by pulsed laser deposition change rapidly with time if the films are exposed to ambient air and that the response can be stabilized significantly by depositing a thin capping layer of MgO on the surface of MgZnO thin film. The decay time of photocurrent in as‐deposited MgZnO thin films increases from 1.8 to 229 s and the value of peak responsivity increases by about three orders of magnitude as the film is exposed to air for about 106 s. On application of MgO capping layer, the decay time of the photocurrent marginally increases from ≈0.2 to ≈0.4 s and the value of peak responsivity increases only by one order of magnitude over the span of 106 s, thereafter showing tendency of saturation, implying much better stability in performance. The MgO capping layer is transparent to UV, but acts as a diffusion barrier for oxygen, stabilizing the UV photoresponse.

Strained lateral structure and its relaxation in a Cu2O thin crystal epitaxially grown on MgO surface

We measured in-plane X-ray diffraction and investigated the strained lateral structure and its relaxation in a Cu2O thin crystal epitaxially grown in a small gap between paired MgO substrates. Although one side MgO was peeled off to irradiate Cu2O with X-ray, a thin MgO layer remained on Cu2O, and diffraction peaks of (400)-planes in both MgO and Cu2O were detected. The peak has four-fold symmetry on sample rotation, and the lattice constant estimated from the diffraction angle is 4.254 Å. The lattice constant is larger than that of MgO and smaller than that of strain-free Cu2O. Furthermore, it was found that the lattice strain in Cu2O relaxes at a depth of ≈0.15 μm from MgO on the basis of the estimated X-ray penetration depth under consideration of low incident angle, absorption and refraction of X-ray.

Fabrication and luminescent properties of(In,Fe)Sb/GaAs/InGaAs diodes

The electrical and luminescent characteristics of InGaAs/GaAs spin light-emitting diodes with an injector based on (In,Fe)Sb were investigated and the circular polarization of the electroluminescence was obtained for such structure. It has been established that the deposition of (In,Fe)Sb layer does not introduce any additional defects into the region of near-surface quantum wells, but directly affects the (In,Fe)Sb/GaAs interface. It was found that the application of a thin protective layer of MgO between the ferromagnetic (In,Fe)Sb injector and the light-emitting structure minimizes this effect.

Tunnel spin polarization of Fe/MgO/Si contacts reaching 90% with increasing MgO thickness

Using nonlocal spin-transport devices, the tunnel spin polarization (TSP) of Fe/MgO tunnel contacts on Si is determined as a function of the thickness of the MgO. The TSP, extracted from the magnitude of the nonlocal spin-valve and Hanle signals, increases with MgO thickness and is shown to reach values of around 90% at low temperatures. Such a near-perfect spin polarization of the tunnel current indicates that symmetry-based spin filtering due to coherent tunneling occurs in Fe/MgO tunnel contacts on Si, despite the significant lattice mismatch. For MgO thicknesses below 1 nm, the TSP drops unexpectedly rapidly (to values below 25%), which is attributed to the lower crystalline quality of thin MgO layers. In fact, the data suggest that the first 0.7 nm of MgO on the Si contributes very little to the spin filtering.

Corrosion Behavior of AZ31B Magnesium Alloy in Potassium Nitrate and Sulfate Solutions

In light of low density, high specific strength, and good heat dissipation properties, magnesium alloys are promising for lightweight structure applications; specifically aerospace, automotive, and 3C devices [1]. However, most magnesium alloys have poor corrosion resistance due to the high chemical reactivity and the porous corrosion product, i.e., magnesium hydroxide [2]. Better understanding of the corrosion behavior of magnesium alloys is thus an urgent necessity. Different kinds of anions existing in the service environment generally affect the corrosion behavior of metals. For example, chlorine ions accelerate the corrosion rate of magnesium. Moreover, sulfate ions or the nitrate ions in chloride-containing solutions can inhibit hydrogen evolution. They are so-called the magnesium corrosion inhibitors. Sulfate ions serve as a physical barrier to hinder chlorine ions from attacking the magnesium substrate [3]; however, the hindrance effect of nitrate ions on hydrogen evolution on magnesium is still not well understood [3]. In this research, AZ31B magnesium alloys were immersed in potassium nitrate and potassium sulfate solutions, respectively, to investigate how NO3 -­ and SO4 2- anions influence the corrosion behavior and the microstructure of the resulting corrosion product. Cross-sectional transmission electron microscopy (TEM) characterization results show that the corrosion product layer formed in the potassium sulfate solution is porous and has a non-uniform thickness ranging from 80 nm to 240 nm. In contrast, the corrosion product layer formed in the potassium nitrate solution is uniform in thickness of about 60 nm. Moreover, a thin compact layer was observed at the interface between the AZ31B magnesium substrate and the magnesium hydroxide layer. Scanning TEM - high angle annular dark field line-scan and mapping as well as X-ray photoelectron spectroscopy analysis results further identify that the thin compact layer is magnesium oxide. It is likely that the oxidation of the magnesium substrate by nitrate ions results in the formation of magnesium oxide. Compared with magnesium hydroxide, this thin magnesium oxide layer is more compact; therefore, it can protect the magnesium substrate from corrosion, which, in turn, inhibits the hydrogen evolution. The resulting corrosion product layer is thus more uniform. Finally, EIS was employed to characterize the properties of corrosion products tested in 0.05 M NaCl and 0.1 M Na2SO4 solution. The EIS data were fitted using an equivalent circuit, Rs(Qc(Rc(Qdl Rct L))), which is slightly modified from the equivalent circuit of M. Curioni et al. [4] who studied the corrosion behavior of the pure magnesium in chloride-containing aqueous and a charge transfer resistance (Rct) was adapted to account for the process of electron transfer. This modification is based on the fact that the corrosion product on AZ31B magnesium formed in potassium sulfate or potassium nitrate solution provides to some extent corrosion protection in the test solution. It was found that the AZ31B after potassium nitrate treatment has a higher R­ct value than that after potassium sulfate treatment. This can be attributed to the thin, compact magnesium oxide layer formed at the interface. As a result, the presence of nitrate anions facilitates the formation of a thin compact MgO layer and the resulting corrosion product displays a larger charge transfer in 0.05 M NaCl and 0.1 M Na2SO4 solution compared to the counterpart formed in potassium sulfate solution.

Efficient blue-LEDs with a MgO layer as interfacial modification layer

Novel blue light-emitting diodes (LEDs) based on nanostructured ZnSe/ZnS multilayer thin films are demonstrated. By introducing a thin MgO film as an interfacial modification layer, the performance of the devices have been significantly improved. Both photoluminescence and electroluminescence intensity are enhanced. The electroluminescence intensity is up to about 3 times higher than that of the LEDs without MgO layer. The optimum thickness of the MgO layer is about 10 nm. On the one hand, the thin MgO layer can improve the crystallization quality and compactness of the nanostructured multilayer films. On the other hand, the MgO layer can not only block holes to balance charges concentration but also provide a preferable inter-face for suppressing excitons quenching. Our results indicate that MgO can serve as an effective interfacial modification layer in the field of LEDs.

Interfacial influence on electrical injection and transport characterization of CoFeB|MgO|GaAs-InGaAs quantum wells hetero-structure

The quality of interfaces is a key factor for efficient electrical spin injection into quantum well light emitting diodes. Here, we investigate the interfacial influence on the electrical transport properties in CoFeB|MgO|GaAs-InGaAs quantum wells hetero-structure, by considering textured MgO tunnel barrier fabricated by two different techniques: sputtering and molecular beam epitaxy (MBE). From forward and reverse current–voltage characteristics, it is found that the threshold voltage decreases as the annealing temperature changes from room temperature to 300 °C for sputtered samples, however there is not much difference in threshold voltage with the annealing temperature for MBE-grown samples. In combination with transmission electron microscope (TEM) studies, it is found the MgO|GaAs interface by MBE is sharp for both grown and annealed states. However, there is a thin (∼0.4 nm) amorphous MgO layer at the MgO|GaAs interface for sputtered samples with grown state, and the amorphous MgO can be improved and crystallized after annealing. The MgO|GaAs interface plays a role in modulating the band structure and has an influence on electrical injection. Our work demonstrated that CoFeB|MgO|GaAs interfaces are important and can be engineered thanks to the use of two types of growth for the textured MgO tunnel barriers, which ensures efficient electrical injection.

Surface Characterization and Secondary Electron Emission Properties of Alumina Containing MgO Film on Ag-Mg-Al Alloy

Ag-Mg alloy is used as a dynode material in electron multiplier tubes due to the high secondary electron yields (δ) of the surface of MgO film. However, MgO film is readily degraded under strong electron or ion bombardment, which results in a decrease in the lifetime of devices. In this study, alumina-containing MgO films of ~50–150 nm were developed on a Ag-2Mg-2Al alloy (silver alloy containing 2 wt % Mg and 2 wt % Al) after a thermal activation process performed at 500–600 °C under low oxygen pressures of 5.0–20.0 Pa. Auger electron spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the film consists of a thin layer of pure MgO and a relatively thicker layer of alumina-containing MgO located beneath the top MgO layer. The alumina-containing MgO film exhibits high δ value of 7.7 at a primary electron energy of 580 eV and a much better stability under energetic electron bombardment than pure MgO film on Ag-Mg alloy. Alumina has higher bond dissociation energy than MgO, and the presence of alumina in the film contributes to mitigating the dissociation of the MgO film under electron bombardment. The Ag-2Mg-2Al alloy with alumina-containing MgO film is a promising candidate as a dynode material for electron multiplier tubes.

Efficient, Hysteresis-Free, and Stable Perovskite Solar Cells with ZnO as Electron-Transport Layer: Effect of Surface Passivation.

The power conversion efficiency of perovskite solar cells (PSCs) has ascended from 3.8% to 22.1% in recent years. ZnO has been well-documented as an excellent electron-transport material. However, the poor chemical compatibility between ZnO and organo-metal halide perovskite makes it highly challenging to obtain highly efficient and stable PSCs using ZnO as the electron-transport layer. It is demonstrated in this work that the surface passivation of ZnO by a thin layer of MgO and protonated ethanolamine (EA) readily makes ZnO as a very promising electron-transporting material for creating hysteresis-free, efficient, and stable PSCs. Systematic studies in this work reveal several important roles of the modification: (i) MgO inhibits the interfacial charge recombination, and thus enhances cell performance and stability; (ii) the protonated EA promotes the effective electron transport from perovskite to ZnO, further fully eliminating PSCs hysteresis; (iii) the modification makes ZnO compatible with perovskite, nicely resolving the instability of ZnO/perovskite interface. With all these findings, PSCs with the best efficiency up to 21.1% and no hysteresis are successfully fabricated. PSCs stable in air for more than 300 h are achieved when graphene is used to further encapsulate the cells.

Acousto-electric transport in MgO/ZnO-covered graphene on SiC

We investigate the acousto-electric transport induced by surface acoustic waves (SAWs) in epitaxial graphene (EG) coated by a MgO/ZnO film. The deposition of a thin MgO layer protects the EG during the sputtering of a piezoelectric ZnO film for the efficient generation of SAWs. We demonstrate by Raman and electric measurements that the coating does not harm the EG structural and electronic properties. We report the generation of two SAW modes with frequencies around 2 GHz. For both modes, we measure acousto-electric currents in EG devices placed in the SAW propagation path. The currents increase linearly with the SAW power, reaching values up to almost two orders of magnitude higher than in previous reports for acousto-electric transport in EG on SiC. Our results agree with the predictions from the classical relaxation model of the interaction between SAWs and a two dimensional electron gas.

Spin accumulation in permalloy-ZnO heterostructures from both electrical injection and spin pumping

We report the results of room temperature spin injection and detection studies in ZnO using both electrical injection and spin pumping. At ferromagnetic resonance, an interfacial voltage with a constant polarity upon magnetization reversal is observed in permalloy-ZnO heterostructures, which is attributed to spin accumulation after ruling out other origins. Simultaneous electrical injection during spin pumping is achieved in samples with large interface resistance or insertion of a thin MgO layer at the interface of permalloy and ZnO. From the pumping frequency dependence of detected voltage, a spin lifetime of 32 ps is extracted for ZnO at room temperature, despite the fact that there was no Hanle effect observed in the same device using the conventional three-terminal DC measurement.

Electrical spin transport in cylindrical silicon nanowires with CoFeB/MgO contacts

We examined electrical spin transport in cylindrical silicon nanowires (Si NWs) using the lateral nonlocal spin-valve (NLSV) geometry with CoFeB/MgO contacts. The use of a thin MgO layer as the tunnel barrier in the NLSV devices provided an optimum resistance-area product for spin transport measurements in the Si NWs. A robust NLSV spin signal of over 3.95 kΩ and clear minor loops were observed at 1.8 K in the Si NWs heavily doped with phosphorous. Furthermore, the NLSV magnetoresistance was strongly influenced by the local magnetizations resulting from the ferromagnetic (FM) electrodes being attached to the cylindrically shaped Si NW, with these magnetizations differing from those of bulk ferromagnets. These local micro-magnetic configurations of the FM electrodes led to intriguing NLSV spin signals associated with the Hanle effect. Our study of spin transport in the heavily doped Si NWs provides a sound basis for developing applications of nanoscale semiconductor spintronic devices.

Exchange-Biased Anisotropic Magnetoresistive Field Sensor

IrMn exchange-biased anisotropic magnetoresistive sensors have been optimized for increased outputs and reduced noise. An ~ 56% increase in peak-to-peak outputs has been achieved in IrMn exchange-biased NiFe thin-film sensors, relative to sensors without IrMn. Critically, these sensors’ transfer functions show a linear and symmetric magnetic field response with small hysteresis. The tradeoff between detection range and sensitivity has also been explored, and an inverse proportional relationship is observed. In addition, sensor noise performance can be improved with an IrMn exchange bias layer and further enhanced by the addition of a thin MgO layer between the NiFe and the Ta capping layer. The decrease in magnetic 1/ $f$ noise can be attributed to magnetization stabilization by IrMn and a reduction in pinning sites by MgO. This improved performance from IrMn exchange-biased anisotropic magnetoresistive sensors renders them promising for many different applications.