MgO Thin Films Research Articles

Band gap and defect states of MgO thin films investigated using reflection electron energy loss spectroscopy

The band gap and defect states of MgO thin films were investigated by using reflection electron energy loss spectroscopy (REELS) and high-energy resolution REELS (HR-REELS). HR-REELS with a primary electron energy of 0.3 keV revealed that the surface F center (FS) energy was located at approximately 4.2 eV above the valence band maximum (VBM) and the surface band gap width (EgS) was approximately 6.3 eV. The bulk F center (FB) energy was located approximately 4.9 eV above the VBM and the bulk band gap width was about 7.8 eV, when measured by REELS with 3 keV primary electrons. From a first-principles calculation, we confirmed that the 4.2 eV and 4.9 eV peaks were FS and FB, induced by oxygen vacancies. We also experimentally demonstrated that the HR-REELS peak height increases with increasing number of oxygen vacancies. Finally, we calculated the secondary electron emission yields (γ) for various noble gases. He and Ne were not influenced by the defect states owing to their higher ionization energies, but Ar, Kr, and Xe exhibited a stronger dependence on the defect states owing to their small ionization energies.

MgO on Mo(001): Local Work Function Measurements above Pristine Terrace and Line Defect Sites

We present a local spectroscopy study of thin MgO films on Mo(001), investigated with a combined noncontact atomic force microscope (nc-AFM) and scanning tunneling microscope (STM). The work function of Mo(001) and MgO/Mo(001) was measured with field emission resonance (FER) spectroscopy. The work function of the clean Mo(001) surface ΦMo was determined to be 4.5 eV. After the deposition of an 8 atomic layer thick MgO film, it decreased to ΦMgO/Mo = 3.2 eV (ΔΦFER = −1.3 eV). This work function shift ΔΦFER, introduced by the oxide film on the metal work function, was also investigated with contact potential difference (CPD) measurements. Here, a lowering of the work function by ΔΦCPD = −1.1 eV was measured. Furthermore, the influence of line defects, present in the MgO film, on the local work function was investigated and compared to pristine terrace sites of the oxide film. Here, we measured a slightly higher local work function of +200 meV above line defects as compared to pristine oxide terrace sites.

Resputtering effect during MgO buffer layer deposition by magnetron sputtering for superconducting coated conductors

In this study, MgO thin films were deposited by radio-frequency magnetron sputtering. The film thickness in the deposition area directly facing the target center obviously decreased compared with that in other areas. This reduction in thickness could be attributed to the resputtering effect resulting from bombardment by energetic particles mainly comprising oxygen atoms and negative oxygen ions. The influences of deposition position and sputtering pressure on the deposition rate were investigated. Resputtering altered the orientation of the MgO film from (111) to (001) when the film was deposited on a single crystal yttria-stabilized zirconia substrate. The density distribution of energetic particles was calculated on the basis of the measured thicknesses of the MgO films deposited at different positions. The divergence angle of the energetic particle flux was estimated to be approximately 15°. The energetic particle flux might be similar to the assisting ion flux in the ion beam assisted deposition process and could affect the orientation of the MgO film growth.

Cu adatom charging on Mo supported ScN, MgO and NaF

Molybdenum supported thin films of ScN, MgO and NaF with a Cu adatom have been studied in the framework of density functional theory.

Process model-based analysis of highly crystalline and chemically pure molecular beam epitaxy of MgO (111) nano-thin films on 6H-SiC (0001) substrates

A highly two-dimensional and chemically pure MgO (111) thin film grown epitaxially under ultra-high vacuum on a (√3 × √3) R30° reconstructed wideband gap 6H-SiC (0001) substrate surface can serve as a promising minimal mismatch interfacial layer between the substrate surface and a crystalline structure of functional oxide film. However, the reliability and repeatability of the growth process is challenging. In this paper the dynamic termination of the MgO (111) polar oxide during layer-by-layer growth is explained by OH group chemical bonding state of MgO structure. The analysis of causal relationships is conducted at three different thickness levels to account for the sensitivity of film chemistry to the mismatching strain related inherent twist of crystalline structure along the film thickness. The contribution of sources of the undesired bonding states is quantified by examining the process sensitivity trends to the most critical process variable (percentage starting oxygen) at different thickness levels.

MgO and ZnO Composite Thin Films Using the Spin Coating Method on Microscope Glasses

In this study, a new route to produce pure and composite ZnO-MgO thin films has been presented. In the process the pure ZnO thin films were the starting point, ending up with MgO by doping various percentages (from 0% to 100%) of Mg with the help of sol-gel spin coating technique. The crystal phases in all doping levels have been obtained when the samples annealed at 600℃ for a duration of 6 hours. The X-ray diffraction (XRD) spectra, the scanning electron microscopy (SEM) micrographs and UV-Vis absorption spectra have been performed to elucidate the composed film structures.

A Combined Effect of Plasmon Energy Transfer and Recombination Barrier in a Novel TiO2/MgO/Ag Working Electrode for Dye-Sensitized Solar Cells

Novel TiO2/MgO/Ag composite electrodes were applied as working electrodes of dye-sensitized solar cells (DSSCs). The TiO2/MgO/Ag composite films were prepared by dip coating method for MgO thin films and photoreduction method for Ag nanoparticles. The MgO film thicknesses and the Ag nanoparticle sizes were in ranges of 0.08–0.46 nm and 4.4–38.6 nm, respectively. The TiO2/MgO/Ag composite films were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The TiO2/MgO/Ag composite electrodes were sensitized by immersing in a 0.3 mM of N719 dye solution and fabricated for conventional DSSCs.J-Vcharacteristics of the TiO2/MgO/Ag DSSCs showed that the MgO film thickness of 0.1 nm and the Ag nanoparticle size of 4.4 nm resulted in maximum short circuit current density and efficiency of 8.6 mA/cm2and 5.2%, respectively. Electrochemical Impedance Spectroscopy showed that such values of short circuit current density and efficiency were optimal values obtained from plasmon energy transfer by 4.4 nm Ag nanoparticles and recombination barrier by the ultrathin MgO film.

New heteroleptic magnesium complexes for MgO thin film application.

Novel heteroleptic magnesium complexes, [Mg(dmamp)(acac)]2 (), [Mg(dmamp)(tmhd)]2 (), [Mg(dmamp)(tfac)]2 (), and [Mg(dmamp)(hfac)]2 (), were successfully synthesized using dmamp and β-diketonate as ligands. The thermogravimetric analyses and volatile studies of the complexes showed that the complexes and display high volatility and thermal stability. The single crystal X-ray study of the complexes reveals that both the complexes and were crystalized in the triclinic space group and as dimers, where magnesium atoms are in the pentacoordinate state with distorted trigonal bipyramidal geometry.

Role of surface and subsurface defects in MgO thin film: XANES and magnetic investigations

Present study reports the role of surface and subsurface defects in amorphous and polycrystalline MgO thin films. The amorphous film exhibits magnetization of the order of 179emu/cc at room temperature, however, this value is almost 10 times smaller than the value for polycrystalline MgO film. SEM-EDX and RBS studies reveal the absence of magnetic impurities in both films. Observed magnetic properties seem to be originated from the surface states as evidenced by X-ray absorption near edge fine structure study and in correlation with the existing theories of do ferromagnetism in similar oxides

Structural, Morphological and Optical Characterization of Spray Deposited MgO Thin Film

Structural, Morphological and Optical Characterization of Spray Deposited MgO Thin Film

Interplay of uniaxial and cubic anisotropy in epitaxial Fe thin films on MgO (001) substrate

Epitaxial Fe thin films were grown on annealed MgO(001) substrates at oblique incidence by DC magnetron sputtering. Due to the oblique growth configuration, uniaxial anisotropy was found to be superimposed on the expected four-fold cubic anisotropy. A detailed study of in-plane magnetic hysteresis for Fe on MgO thin films has been performed by Magneto Optic Kerr Effect (MOKE) magnetometer. Both single step and double step loops have been observed depending on the angle between the applied field and easy axis i.e. along ⟨100⟩ direction. Domain images during magnetization reversal were captured by Kerr microscope. Domain images clearly evidence two successive and separate 90° domain wall (DW) nucleation and motion along cubic easy cum uniaxial easy axis and cubic easy cum uniaxial hard axis, respectively. However, along cubic hard axis two 180° domain wall motion dominate the magnetization reversal process. In spite of having four-fold anisotropy it is essential to explain magnetization reversal mechanism in 0°< ϕ < 90° span as uniaxial anisotropy plays a major role in this system. Also it is shown that substrate rotation can suppress the effect of uniaxial anisotropy superimposed on four-fold anisotropy.

Work function modulation inMgO/Nb:SrTiO3by utilizing highly nonequilibrium thin-film growth

We have controlled the amount of charged defects in MgO thin films grown on Nb-doped ${\mathrm{SrTiO}}_{3}$ $(\text{Nb}:{\mathrm{SrTiO}}_{3})$ substrate by pulsed laser deposition at room temperature. The work function of $\text{MgO}/\text{Nb}:{\mathrm{SrTiO}}_{3}$ varies from \ensuremath{\sim}2 to \ensuremath{\sim}5 eV depending on the partial oxygen pressure during film growth. The increase in the work function under the oxygen-rich deposition condition is consistent with the dipole moment formation at the interface, which is given by negatively charged ${\mathrm{V}}^{\ensuremath{-}}$ centers in MgO thin films and the corresponding image charges in the conductive substrate. That the formation of charged defects in an insulating capping layer can induce a strong modulation of work function as large as \ensuremath{\sim}3 eV implies that controlling charged defects could be an effective method to design the electronic properties of oxide surfaces.

17.1: Distinguished Student Paper: Origin of Short Statistical Delay of an AC PDP with MgO Nano‐Powders

AbstractExo‐electron currents emitted from MgO Nano‐powders sprayed on MgO thin film were measured using a highly‐sensitive and fast‐responding measuring apparatus. The results indicated that the short statistical delay with the MgO nano‐powders is mainly due to increased exo‐electron currents from MgO nano‐powders.

Bipolar Pulse Bias Effects on the Properties of MgO Reactively Deposited by Inductively Coupled Plasma-Assisted Magnetron Sputtering

MgO thin films were deposited by internal ICP-assisted reactive-magnetron sputtering with bipolar pulse bias on a substrate to suppress random arcs. Mg is reactively sputtered by a bipolar pulsed DC power of 100 kHz into ICP generated by a dielectrically shielded internal antenna. At a mass flow ratio of <TEX>$Ar/O_2$</TEX> = 10 : 2 and an ICP/sputter power ratio of 1 : 1, optimal film properties were obtained (a powder-like crystal orientation distribution and a RMS surface roughness of approximately 0.42 nm). A bipolar pulse substrate bias at a proper frequency (~a few kHz) prevented random arc events. The crystalline preferred orientations varied between the (111), (200) and (220) orientations. By optimizing the plasma conditions, films having similar bulk crystallinity characteristics (JCPDS data) were successfully obtained.

Thickness Dependence of Fe3O4/MgO Thin Film Properties: Density Functional Theory Investigation

In this work, the thickness dependence of Fe3O4(111)/MgO thin films, were simulated using density-functional theory. From the results, even though the total DOS in bulk structure presents a half-metallic behavior, the DOS of all Fe3O4 (111) films demonstrates that they are metallic. The specific magnetization as a function of film thickness is found to satisfy the inverse power law. Indeed, the noncompensation of spin moments is also observed to be the main majority effect in the enhancement of giant magnetization. Moreover, the high spin polarization value is discovered in the compensation models and tends to reach −80% with increasing the thickness.

Growth of magnesio-aluminate spinel in thin-film geometry: in situ monitoring using synchrotron X-ray diffraction and thermodynamic model

Polycrystalline spinel layers were grown experimentally at the contacts between single-crystal corundum substrates and initially amorphous, then polycrystalline MgO thin films. The growth behavior of the spinel layers was monitored in situ using synchrotron X-ray diffraction. The change in the integrated intensity of the 111 spinel Bragg peak was correlated with the thickness of the layer as determined from ex situ TEM characterization of the run products. At \(900\,^{\circ }\hbox {C},\) a transition from linear growth, corresponding to interface reaction control, to parabolic growth, corresponding to diffusion control, occurred at a layer thickness of less than 10 nm. At 1,000 \(^{\circ }\hbox {C},\) growth was largely linear up to a layer thickness in excess of 300 nm. A thermodynamic model was applied to extract the kinetic parameters characterizing interface motion and long-range diffusion from this growth behavior.

Influence of assisting ion beam current on the microstructure and surface morphology of IBAD-MgO thin films for YBCO coated conductors

MgO thin films were grown on Y2O3/Al2O3 buffered C276 tapes by ion beam assisted deposition (IBAD) method and the effects of assisting ion beam current on their microstructure and surface morphology were investigated. In order to evaluate the quality of IBAD-MgO thin films, CeO2 cap layer was deposited directly on the IBAD-MgO template and then YBCO superconducting layer was grown by pulsed laser deposition (PLD) method. Film characterization was performed by in-situ reflection high energy electron diffraction, X-ray diffraction, and atomic force microscopy. The critical current Ic of the YBCO film was measured by the conventional four-probe method. It was found that the orientation, texture degree and surface morphology of the IBAD-MgO thin films were very sensitive to assisting ion beam current. By optimizing the assisting ion beam current, IBAD-MgO thin films had pure (002) orientation, excellent biaxial texture and very smooth surface with a root mean square surface roughness of less than 1nm. The PLD-CeO2 layer had the best out-of-plane texture of Δω=1.6° and in-plane texture of Δφ=4.8° at 140mA of assisting ion beam current. Under optimized experimental condition, high quality YBCO coated conductor with the critical current density Jc of 4.5×106A/cm2 was fabricated at 77K and self field.

XPS and some surface characterizations of electrodeposited MgO nanostructure

Growth conditions, structural, and optical properties of MgO nanostructure have been investigated. Surface composition and shift in binding energy of Mg at 50.8 eV due to oxidation were examined by core‐level spectroscopy. The SEM showed that the film is dense, and grain growth and crystallinity are enhanced by post‐deposition annealing. Grain distribution was appraised within the confinement of 24.51 µm2 from the selected scan areas. X‐ray diffraction studies indicated prominent peaks, which are attributed to (111), (200), and (220) reflections from fairly crystallized and randomly oriented MgO thin film. Plane (111) is found to be the preferred orientation of the film. The film transmitted well across the visible spectrum and the estimated energy band gap is 5.41 eV. Absence of catalyst in the electrolyte solution aided the purity of the sample. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Structural comparison between MgO/Fe(001) and MgO/Fe(001)–p(1×1)O interfaces for magnetic tunneling junctions: An Auger electron diffraction study

Abstract Magnetic tunnel junctions based on MgO(0 0 1) barriers and ferromagnetic electrodes, such as Fe/MgO/Fe, represent a very popular and widely investigated subject in the field of spin-electronics because of the large values of magnetoresistance shown by these systems. In this paper, the structural properties of MgO thin films grown onto Fe(0 0 1) and MgO/Fe(0 0 1)–p(1 × 1)O surfaces, with MgO thickness ranging from 2 to 14 equivalent monolayers, have been investigated by means of Auger electron diffraction. The structural order and the crystal quality of the MgO films result practically independent from the template, the latter being either the clean Fe(0 0 1) surface or the oxidized Fe(0 0 1)–p(1 × 1)O one. This is confirmed by numerical simulations, showing that, apart from the first two MgO layers close to the interface, the structure is unaffected by the choice of the starting surface. By a structural point of view, we can conclude that Fe(0 0 1)–p(1 × 1)O is a good candidate for the role of bottom electrode for the realization of MgO-based magnetic tunnelling junctions, also considering its higher chemical stability and reproducibility if compared to the clean Fe(0 0 1) surface.

Ion-induced secondary electron emission, optical and hydration resistant behavior of MgO, Mg–Mo–O and Mg–Ce–O thin films

Optical transmittance, hydration resistance and secondary electron emission characteristics of e-beam evaporated pure and Mo- or Ce-containing MgO thin films have been investigated. While the increased grain size and pyramidal columnar morphology following incorporation of molybdenum and cerium in MgO are responsible for the excellent discharge characteristics, emergence of neutral {100} and {110} MgO surfaces preferentially give rise to high optical transmittance (~92–100%) and stability against hydration. Further, addition of Mo (or Ce) in MgO causes significant increase in defect density which, in turn, enhances the photoluminescence (PL) emission from 5-, 4- and 3-coordination sites. The changes lead to lowering of the breakdown voltage and hence improvement in the secondary electron emission (SEE) efficiency. These facts have been supported by ion-induced SEE yield (γ) deduced from the a.c. breakdown voltage observed, taking neon as a discharge gas, and determined semi-empirically as well with Hagstrum's theory based on Auger neutralization process using (i) band offset parameters and surface band gap data derived from X-ray photoelectron spectroscopy signal and (ii) information of defect energy levels obtained from photoluminescence (PL) measurements. The experimental values of neon ion-induced SEE yield (γ) are found to be 0.35, 0.42 and 0.39 for MgO, Mg–Mo–O (x=0.035) and Mg–Ce–O (x=0.01) thin films, respectively.