MgO Thin Films Research Articles

Experimental and Simulation Study of Magnetron Sputtering MgO Film Thickness Distribution

A physical model of magnetron sputtering process was built, the distribution of MgO film thickness on the substrate was deduced, and the data were analyzed by using the Matlab. nanosized MgO thin film was prepared on Si substrate by magnetron sputtering. SGC-10 was used to measure the thickness of MgO thin film. The results of experiment correspond fairly well with the theory. Both experiment and the theory show that the distribution of the film thickness on the substrate is uneven and it is also influenced by the radius as well as the distance between the target and substrate. The physical model provides a theoretical basis for evaluation and estimation of the film thickness.

A comparative study of room temperature ferromagnetism in MgO films deposited by rf/dc sputtering using high purity Mg and MgO targets

Thin films of nanocrystalline MgO were deposited on glass/Si substrates by rf/dc sputtering from metallic Mg, and ceramic MgO targets. The purpose of this study is to identify the differences in th ...

Synthesis of Doped and non-Doped Nano MgO Ceramic Membranes

Doped and non-doped MgO coated thin films on alumina substrates were prepared using a chelating sol-gel method under controlled conditions to prepare nanomaterials with unprecedented properties. The effect of doping of ZnO on thermal, surface and structural properties was investigated using DTA-TG, BET and XRD respectively. Also microstructural studies and coating thickness measurements of MgO thin film were conducted using SEM. An increase in the thermal stability of MgO with increasing ZnO doping percent was observed. The increase of ZnO doping percent showed a marked decrease in the average particle size of MgO powder as a result of the replacement of some Mg2+ by Zn2+ which has similar ionic radius as Mg2+. This decrease in particle size of MgO was also related to the decrease of the degree of MgO crystalinity. The increase of ZnO doping also showed a marked decrease in coating thickness values of the prepared membranes. This decrease was related to the mechanism of ZnO doping into a MgO crystal lattice.

Ageing and vapor chopping effect on the properties of MgO thin films

Nanoscale magnesium oxide thin films have been prepared on glass substrates by thermal oxidation (in air) of vacuum evaporated magnesium films. The ageing (air exposure) effect for 1, 10, 20 and 30days on the various properties of MgO thin films was studied. The ageing effect on the optoelectronic devices i.e. optical waveguide and plasma display panel’s performance was examined by measuring their optical transmission loss and secondary electron emission yield, respectively. Vapor chopping technique was employed for thin film quality improvement followed by ageing inhibition. The firing voltage for fresh vapor chopped (VC) MgO thin films was in 125–150V range, which shifted to 140–178V after 30days, whereas in the non-chopped (NC) films it increased up to 165–210V from 140–160V. After 30days ageing, NC thin film surface morphology changed from polished to dull due to wrapped moisture like layer, whereas VC thin film morphology was unpretentious. The XPS analysis also confirmed the VC MgO thin films superior sustainability against the ageing than NC films.

Atomic layer deposition of MgO films on yttria-stabilized zirconia microtubes

Deposition of MgO thin film on nanocrystalline yttria-stabilized zirconia microtubes was investigated. The microtubes were prepared by self-formation from threads drawn directly from zirconium butoxide [Zr(OBu)4] precursor and heat treated at 800°C. The tubes possessed 100% tetragonal phase, their typical outer diameter was 50μm, inner diameter 30μm and length 1cm. MgO films were deposited from β-diketonate-type precursor 2,2,6,6-tetramethyl-heptanedionato-3,5-magnesium(II) at 220°C by atomic layer deposition. Thickness of MgO film on microtubes was 15.8nm and growth rate 0.105Å/cycle.

Efficient light incoupling into silicon thin-film solar cells by anti-reflecting MgO/high-compact-AZO with air-side textured glass

Light incoupling effects have been enhanced at front interfaces of silicon (Si) thin-film solar cells. Firstly, a MgO thin film was introduced at glass substrate/Al-doped ZnO (AZO) interface for anti-reflection effect. We additionally found that the surface morphology of AZO films grown on MgO film after texture-etching is dependent on the compactness of AZO. For high-compact AZO films, the texture-etched MgO/AZO double layer exhibited significantly enhanced light-scattering capability. Secondly, we made textured surfaces at air/glass interface through simple plasma-etching without sacrificial layers or masks by optimizing the etching pressure. The additional air-side texture contributed to further improvement of total light scattering from the MgO/AZO-coated glass substrate. Fabricated microcrystalline Si thin-film solar cells employing the MgO coated glass with air-side surface texture showed decreased cell reflectance and increased quantum efficiency. The Jsc increased from 21.7 to 26.5 mA cm−2 and final efficiency of 9.49% was achieved. Based on our experimental results, the suggested structure, the MgO coating on glass substrate of which air-side surface is texture-etched, can offer a promising approach to improve the light incoupling and efficiency of Si thin-film solar cells.

Soft x-ray appearance potential spectroscopy study of MgO (100) and α-Al2O3 (100) single crystals

Soft x-ray appearance potential spectroscopy (SXAPS) measurements was used to measure on MgO (100) and α-Al2O3 (100) single crystals. Mg 1s, Al 1s, and O 1s SXAPS self-deconvoluted (SD) spectra were obtained. The features of the Mg 1s and O1s SD spectra are in fair agreement with those of the near-edge x-ray absorption fine structure spectra for an MgO thin film (3 ML) on Ag (100). This suggests that the SXAPS spectra reflect electronic states of the relaxed MgO (100) surface. The features of the Al 1s and O 1s SD spectra are in qualitative agreement with those of the electron energy-loss spectroscopy. The SXAPS SD spectra are discussed in terms of antibonding states and partial density of empty states obtained by theoretical calculations for MgO and α-Al2O3, respectively. The present result suggests that the “approximate dipole selection rule” is applicable to the SXAPS spectra of MgO and α-Al2O3, as well as 3d transition metal oxides.

Etch characteristics of MgO thin films in Cl2/Ar, CH3OH/Ar and CH4/Ar plasmas

High density plasma etching of MgO thin films patterned with Ti (or TiN) hard masks was conducted using Cl2/Ar, CH3OH/Ar and CH4/Ar gases. As the concentration of each gas increased, the etch rate of MgO thin films decreased and there was no enhancement of etch profiles except for those obtained using CH4/Ar gas. The etch profiles of MgO thin films under CH4/Ar gas, which is non-corrosive and non-toxic, were improved without redeposition with increasing CH4 concentration. The enhancement of etch profiles in CH4/Ar gas was attributed to both the formation of a protective layer containing hydrogen and hydroxyl species on the sidewall of the patterns and the formation of magnesium compounds during the etching. Optical emission spectroscopy and X-ray photoelectron spectroscopy analyses showed that magnesium hydroxide was formed due to chemical reactions on the film surface under CH4/Ar gas. Additionally, the MgO thin films were found to be etched by a physical sputtering etching mechanism that was influenced by a chemical reaction.

How Contacting Electrodes Affect Single π-Conjugated Molecular Electronic States: Local Density of States of Phthalocyanine Nanomolecules on MgO(001), Cu(111), Ag(001), Fe(001), and Mn(001)

Single molecules have attracted much interest as new materials for future spin electronic devices; however, many open questions still remain. One of them is how the electronic local density of states (LDOS) of single molecules is affected when they are in contact with electrodes. We show a systematic study of the LDOS of π-conjugated phthalocyanine (H2Pc) nanomolecules adsorbed on various electrodes, namely, (1) MgO(001) thin films grown on Ag(001), (2) noble metals of Cu(111) and Ag(001), and (3) 3d magnetic metals of Fe(001) and Mn(001), adupting scanning tunneling spectroscopy techniques with an ultrahigh-vacuum scanning tunneling microscopy setup at room temperature. Since MgO thin films cut the electronic coupling from the substrate Ag(001), we could observe H2Pc molecular states at -1.5 and +1.0 eV. H2Pc molecules on the noble metal substrates form a pattern with a square unit cell of about 1.5×1.5 nm2 and have similar LDOS peaks near the Fermi energy. Strong hybridizations between the substrate 3d spin-polarized states and the molecular π orbitals produce new molecular states of H2Pc molecules adsorbed on Fe(001) and Mn(001) near the energy positions of the Fe(001) minority spin state and the Mn(001) majority spin state, respectively.

Effect of stabilizer on optical and structural properties of MgO thin films prepared by sol–gel method

The effects of monoethanolamine (MEA) and acetylacetone (ACAC) addition as stabilizer on the crystallization behaviour, morphology and optical properties of magnesium oxide were investigated using thermogravimetry (TG/DTG), X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Visible, photoluminescence (PL) and Fourier transform infrared (FTIR) spectroscopy. Stabilizer addition reduces transparency of the films. MgO films prepared at 500 °C showed weak orientation of (200). However, the films prepared by addition of stabilizer are amorphous. MgO powders were prepared for exhibiting the structural properties. The patterns of MgO powders showed a preferred orientation of (200). The addition of stabilizer causes a reduction in grain size. SEM micrographs show that a homogenous and crack-free film can be prepared at 500 °C and addition of stabilizer causes an increase in packing density.

Annealing effect of thermal spike in MgO thin film prepared by cathodic vacuum arc deposition

MgO films were prepared by using pulsed cathodic vacuum arc deposition technique. The substrate bias voltage was in the range of −150 to −750 V. Film structure was investigated by X-ray diffraction (XRD). The annealing effect of thermal spike produced by the impacting of energetic ions was analyzed. The calculated results showed that the lifetime of a thermal spike generated by an energetic ion with the energy of 150 eV was less than one picosecond and it was sufficient to allow Mg2+ or O2- to move one bond length to satisfy the intrinsic stress relief in the affected volume. The MgO(200) lattice spacings of the films deposited at different bias voltages were all larger than the ideal value of 2.1056 Å. As the bias amplitude increased the lattice spacing decreased, which indicated that the compressive stress in the film was partially relieved with increasing impacting ion energy. The stress relief also could be reflected from the film orientation with bias voltage. The biaxial elastic modulus for MgO(100), MgO(110) and MgO(111) planes were calculated and they were M(100) = 199 GPa, M(110) = 335 GPa and M(111) = 340 GPa, respectively. The M values indicated that the preferred orientation will be MgO(200) due to the minimum energy configuration when the lattice strain was large. It was confirmed by the XRD results in our experiments.

Electron quantization in arbitrarily shaped gold islands on MgO thin films

Low-temperature scanning tunneling microscopy has been employed to analyze the formation of quantum well states (QWS) in two-dimensional gold islands, containing between 50 and 200 atoms, on MgO thin films. The energy position and symmetry of the eigenstates are revealed from conductance spectroscopy and imaging. The majority of the QWS originates from overlapping Au 6p orbitals in the individual atoms and is unoccupied. Their characteristic is already reproduced with simple particle-in-a-box models that account for the symmetry of the islands (rectangular, triangular, or linear). However, better agreement is achieved when considering the true atomic structure of the aggregates via a density functional tight-binding approach. Based on a statistically relevant number of single-island data, we have established a correlation between the island geometry and the gap between the highest-occupied and the lowest-unoccupied molecular orbital in the finite-sized islands. The linear eccentricity is identified as a suitable descriptor for this relationship, as it combines information on both island size and island shape. Finally, the depth of the confinement potential is determined from the spatial extension of QWS beyond the physical boundaries of the Au islands. Our paper demonstrates how electron quantization effects can be analyzed in detail in metal nanostructures. The results may help elucidating the interplay between electronic and chemical properties of oxide-supported clusters as used in heterogeneous catalysis.

Room temperature ferromagnetism of amorphous MgO films prepared by pulsed laser deposition

Amorphous MgO thin films were prepared by pulsed laser deposition (PLD) under various oxygen pressures. The structural, magnetic, and optical properties of the films were investigated. All as-deposited samples exhibit room temperature ferromagnetism, which depend strongly on oxygen pressure. It is found that the saturation magnetization (M s) initially increases with the oxygen pressure, the maximum M s of 8.57 emu/cm3 is obtained for the MgO film deposited under an oxygen pressure of 2 mTorr. However, the M s significantly reduces at higher oxygen pressures. Further X-ray photoelectron spectroscopy and photoluminescence demonstrate that the long-range magnetic order in amorphous MgO films can be attributed to the nonstoichiometry effect and the presence of Mg vacancies.

Magnetism of Amorphous and Nano-Crystallized Dc-Sputter-Deposited MgO Thin Films.

We report a systematic study of room-temperature ferromagnetism (RTFM) in pristine MgO thin films in their amorphous and nano-crystalline states. The as deposited dc-sputtered films of pristine MgO on Si substrates using a metallic Mg target in an O2 containing working gas atmosphere of (N2 + O2) are found to be X-ray amorphous. All these films obtained with oxygen partial pressure (PO2) ~10% to 80% while maintaining the same total pressure of the working gas are found to be ferromagnetic at room temperature. The room temperature saturation magnetization (MS) value of 2.68 emu/cm3 obtained for the MgO film deposited in PO2 of 10% increases to 9.62 emu/cm3 for film deposited at PO2 of 40%. However, the MS values decrease steadily for further increase of oxygen partial pressure during deposition. On thermal annealing at temperatures in the range 600 to 800 °C, the films become nanocrystalline and as the crystallite size grows with longer annealing times and higher temperature, MS decreases. Our study clearly points out that it is possible to tailor the magnetic properties of thin films of MgO. The room temperature ferromagnetism in MgO films is attributed to the presence of Mg cation vacancies.

Improvement of Discharge Properties by Printing ${\rm LaB}_{6}$ Films on MgO Protective Layers in Plasma Display Panel

In this letter, a screen-printed LaB6 discontinuous film is coated on MgO thin film to improve the discharge properties of MgO protective layer in alternating current plasma display panel (AC PDP). High transmittance of is obtained by adjusting the concentration ratio of polycrystal LaB6 powders and organic solvent of screen-printed slurry. The LaB6/MgO double protective layer almost shows lower firing voltage and shorter discharge delay time than conventional MgO protective layer, likely owing to the low work function of LaB6. Therefore, it is highly suitable to be applied for AC PDP with low electrical power consumption.

Nucleation and growth of MgO atomic layer deposition: A real-time spectroscopic ellipsometry study

The atomic layer deposition (ALD) of MgO thin films from bis(cyclopentadienyl) magnesium and H2O was studied using in-situ real-time spectroscopic ellipsometry (SE), ex-situ x-ray photoelectron spectroscopy, and grazing-incidence x-ray diffraction. It is found that the initial growth is not linear during the first ten cycles, and magnesium silicate forms spontaneously on the SiO2/Si substrates at 250 °C. Submonolayer sensitivity of SE is demonstrated by the analysis of each half-cycle and self-limiting adsorption, revealing characteristic features of hetero- and homo-MgO ALD processes.

Characterization of MgO Thin Films Grown on Carbon Materials by Molecular Beam Epitaxy

MgO thin films were grown on highly oriented pyrolytic graphite (HOPG) and (001)-oriented diamond substrates by molecular beam epitaxy. The crystal structure of MgO was characterized by reflection high-energy electron diffraction and X-ray diffraction. We found that the MgO grew as (001)- and (111)-oriented textured structures on the HOPG and diamond substrates, respectively. The MgO(001) showed a smaller misalignment of the crystal orientation on the HOPG than on amorphous CoFeB, which indicates that the MgO(001) on graphene can be useful as a tunnel barrier for efficient spin injection into semiconductors.

Competition between (001) and (111) MgO thin film growth on Al-doped ZnO by oxygen plasma assisted pulsed laser deposition

We report on the study of epitaxial MgO thin films on (0001) Al-doped ZnO (Al: ZnO) underlayers, grown by oxygen plasma assisted pulsed laser deposition technique. A systematic investigation of the MgO thin films was performed by X-ray diffraction and atomic force microscopy, along with the current-voltage characteristics. A distinguished behavior was observed that the preferred MgO orientation changes from (111) to (001) in the films as the growth temperature increases. Two completely different in-plane epitaxial relationships were also determined from X-ray diffraction as: [110]MgO//[112¯0]Al: ZnO and [110]MgO//[11¯00]Al: ZnO for (001) MgO with 60° rotated triplet domains, and [110]MgO//[112¯0]Al: ZnO for (111) MgO with 180° rotated twin. The pronounced temperature dependence indicates a reconciliation of the nucleation driving forces among surface, interfacial, and strain energy for heteroepitaxy of cubic MgO on hexagonal Al: ZnO. The related interfacial atomic registry is considered to be important to the formation of unusual (001) MgO on hexagonal crystals. In addition, the electrical characterization revealed a dramatic reduction of the leakage current in (001) MgO thin films, whereas the small grain size of (111) MgO is identified by atomic force microscopy as a main cause of large leakage current.

Preparation of Pd–MgO model catalysts by deposition of Pd from aqueous precursor solutions onto Ag(0 0 1)-supported MgO(0 0 1) thin films

The preparation of Pd–MgO model catalysts via liquid-phase deposition of Pd from aqueous Pd precursor solutions was studied. Thin, single-crystalline MgO(001) films grown on a Ag(001) substrate were used as support and allowed surface science techniques such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature programmed desorption to be applied for characterization. Thin MgO(001) films were unstable and rapidly dissolved in acidic and neutral environments, but remained stable in thickness in alkaline solutions after an initial dissolution of a few layers of MgO. Pd was deposited by exposure of the thin film MgO substrate to alkaline (pH 12) precursor solutions containing Pd-hydroxide complexes. Scanning tunneling microscopy images taken from ultrathin MgO films revealed the formation of Pd particles 3nm in diameter after thermal decomposition of the precursor at 600K, as well as roughening of the MgO substrate, including the formation of etch pits, which leads to partial exposure of the Ag substrate. For Pd deposited on thick MgO films, the formation of Pd nanoparticles by thermal decomposition of the adsorbed Pd-hydroxide precursor was followed by X-ray photoelectron spectroscopy. Pd–MgO model catalysts with similar Pd coverage prepared either by liquid-phase deposition or physical vapor deposition in UHV exhibited similar properties, as revealed by their comparable behavior in CO adsorption and CO oxidation.

X-ray absorption study of the local structure at the NiO/oxide interfaces

This work reports an X-ray absorption near-edge structure (XANES) spectroscopy study at the Ni K-edge in the early stages of growth of NiO on non-ordered SiO2, Al2O3 and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq-ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq-ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross-linking bonds Ni-O-M (M = Si, Al, Mg) at the interface.