MgO Thin Films Research Articles

Morphology, surface topography and optical studies on electron beam evaporated MgO thin films

Electron beam evaporated thin films of MgO powder synthesized by burning of magnesium ribbon in air and sol-gel technique are studied for their microstructure (SEM), surface topography (AFM), and optical transmission behaviour (UV-visible spectroscopy). MgO thin films are shown to be either continuous or have mesh like morphology. The bar regions are believed to be of magnesium hydroxide formed due to absorption of moisture. Their AFM images exhibit columnar/pyramidal/truncated cone structure, providing support to the 3D Stranski-Krastanov model for film growth. Further, they are shown to have high transmittance (∼90%) in the wavelength range 400–600 nm, but absorb radiation below 350 nm substantially giving signature of a band transition.

Solid-phase synthesis of a paramagnetic phase and martensite transformations in Ni/Fe(001)/MgO(001) thin films

Solid-phase synthesis of a paramagnetic phase and martensite transformations in Ni/Fe(001)/MgO(001) thin films

Structural and magnetic features of the solid-state synthesis and martensitic transformations in Ni/Fe(0 0 1)/MgO(0 0 1) thin films

The results of the experimental study of phase transformations occurring in Ni/Fe(0 0 1)/MgO(0 0 1) thin films with 3Fe:1Ni atomic composition during annealing at temperatures up to 650 °C and cooling to liquid nitrogen temperature are presented. Comparative analysis of previous and current results suggests that the paramagnetic phase γ par present in the invar of Ni 1− x Fe x alloys is the γ-Fe 3Ni phase with the lattice parameter a=0.3600 nm. Measurements of magnetization of the samples in dependence of composition allowed one to determine a single-phase region of γ par phase existence in thin films limited by 75 at% Fe. It is shown that the temperature of γ par phase initiation during the solid-state synthesis coincides with the temperature A S of the reverse martensitic α′→γ par transition ( A S= T 0∼550 °C).

A potential for simulating the atomic assembly of cubic AB compounds

Stillinger–Weber interatomic potentials can be used to study the crystal growth of AB compounds with the zinc-blende (B3) structure, but have been unable to be used for other cubic structured compounds. Here we extend a recently modified Stillinger–Weber potential for cubic elements so that it is suited for studying the growth of cubic compounds with the B1 and B2 structures. We also parameterize the potential for the Mg–O system. The potential is shown to accurately model the lattice constants and the cohesive energies of the fcc Mg, fcc O, and the B1 MgO structures. It also correctly ensures that the equilibrium phase of each of these materials possesses the lowest cohesive energy with respect to all their other phases. The potential correctly predicts crystalline growth during molecular dynamics simulations of the vapor deposition of fcc Mg and B1 MgO thin films. The results also reveal the formation of various defects in the films, including islands and twins in Mg, interstitials and disrupted regions on the MgO growth surface, and local un-oxidized regions at the MgO/Mg interface during the oxidation of Mg. The simulation approach enables the study of atomic assembly processes controlling the formation of these defects.

Morphology and optical properties of MgO thin films on Mo(0 0 1)

Thin MgO films with a nominal thickness ranging between 1 and 60 ML have been grown on a Mo(0 0 1) surface. The film morphology was studied by LEED and STM, revealing the presence of a coincidence pattern with the Mo support in the low coverage regime, a dislocation network at medium thickness and a rather flat and defect-poor MgO surface for thicker layers. The MgO optical properties were investigated as a function of film thickness by analyzing electro-luminescence spectra obtained via electron injection from the STM tip into well-defined surface areas. The spectra are characterized by two distinct emission bands at 3.1 and 4.4 eV. Their origin is discussed in the light of earlier photo-luminescence measurements on MgO nanocubes and smokes.

Bulk and surface oxygen vacancy formation and diffusion in single crystals, ultrathin films, and metal grown oxide structures

The neutral oxygen vacancy (OV) energy formation for bulk, subsurface sites at different depths from the surface and various surface sites has been estimated for single crystals, unsupported ultrathin films of MgO, CaO, and BaO, and MgO ultrathin films supported on Ag(001). From the calculated energy barriers for diffusion through the surface and from the surface to the bulk it is found that diffusion is a hindered event, especially for MgO. Nevertheless, diffusion from the terrace to step edges is largely favored while diffusion through terrace sites is less likely and surface to bulk has a very low probability. It is argued that this explains recent scanning tunneling microscopy images for MgO thin films supported on Ag(001) showing OV populating preferentially the step edge sites.

Bonding Trends and Dimensionality Crossover of Gold Nanoclusters on Metal-Supported MgO Thin Films

Bonding of gold clusters, , 16, and 20, on MgO(100) and on thin MgO films supported on Mo(100) is investigated using first-principles density-functional theory. Enhanced adhesive bonding is found for clusters deposited on metal-supported MgO films of thickness up to about 1 nm, or 4 to 5 MgO layers, originating from electrostatic interaction between the underlying metal and metal-induced excess electronic charge accumulated at the cluster interface with the oxide film. The increased wetting propensity is accompanied by a dimensionality crossover from three-dimensional optimal cluster geometries on MgO(100) to energetically favored two-dimensional structures on the metal-supported films.

Gas Flow Effects on MgO Thin Films Deposited by E-Beam Evaporation Method

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Microscopic structures of MgO barrier layers in single-crystal Fe∕MgO∕Fe magnetic tunnel junctions showing giant tunneling magnetoresistance

The microscopic structures of MgO(001) barrier layers in magnetic tunnel junctions showing giant tunneling magnetoresistance were characterized by in situ scanning tunneling microscopy. The MgO thin films formed exceedingly flat surfaces, and their terraces were made even flatter by annealing after deposition. This flattening of MgO surfaces apparently promotes coherent transport of electrons, which should enhance the tunneling magnetoresistance ratio. Local tunneling spectroscopy revealed that an annealed MgO layer has a critical thickness between 3 and 5 ML (monolayer), and a continuous film without pinholes can be formed over the thickness.

Influence of bias voltage on morphology and structure of MgO thin films prepared by cathodic vacuum arc deposition

MgO thin films with high optical transmittance were prepared by cathodic vacuum arc deposition technique. Rutherford backscattering spectroscopy, atomic force microscopy and X-ray diffraction were used to investigate the influences of the negative pulse bias voltage on the composition, the morphology and the crystal structure of MgO thin films, respectively. AFM images show that the grain growth is influenced by high energy ions under bias voltage and that the grains deposited at the pulse bias voltage with set value of |Vp|=600 V stack densely and look the largest as compared to those prepared at different set Vp. The RBS spectra indicate that the Mg/O ratio is about 0.95–1.00 in MgO thin films which is nearly the stoichiometric composition of bulk MgO. The Mg/O ratio increases with set |Vp| until |Vp| is 450 V, and then keeps almost unchanged with set |Vp| up to 750 V. The MgO thin films have a combined orientation of (100) and (110). Below −150 V, the (100) orientation is predominant and the intensity ratio of I220/I200 increases with set |Vp|.

CO adsorption on the surface of MgO(0 0 1) thin films

We report infrared reflection–absorption and thermal desorption spectra of CO adsorbed at low temperature (30 K) on the surface of MgO(0 0 1) thin films. The high similarity of the observed IR bands as compared to data reported for MgO(1 0 0) single crystals and sintered MgO particles gives evidence for the identical surface morphology of the different substrates. CO adsorption series taken at constant temperature/variable pressure, variable temperature/constant pressure, as well as isotope mixing experiments reveal almost no diffusion of CO on MgO(0 0 1) at low temperature, but high adsorbate mobility at slightly elevated temperature.

Binding of Single Gold Atoms on Thin MgO(001) Films

In the present Letter the first electron paramagnetic resonance spectra of single metal atoms on a single crystalline oxide surface are presented. For Au atoms on a MgO(001) film investigated here an analysis of the angular dependent resonance positions and the hyperfine coupling to (17)O shows that the atoms are bound on top of oxygen ions on the terrace of the film. This result is in perfect agreement with scanning tunneling microscopy measurements at 5 K presented here. The measured hyperfine matrix components allow an experimental verification of the theoretically proposed binding mechanism of Au atoms on MgO. In particular, the large reduction of the isotropic hyperfine coupling constant of supported Au as compared to free atoms is not due to a charge transfer at the interface but a hybridization of orbitals and a resulting polarization of the unpaired electron.

Growth Mechanism of Textured MgO Thin Films via SSCVD

Magnesium oxide thin films have been deposited with use of single source chemical vapor deposition (SSCVD). The resultant films were examined by using transmission electron microscopy, X-ray texture analysis, and pole figure analysis. Due to the nature of the chemical reactions occurring at the surface during SSCVD growth, which result in a high growth rate/low flux environment, films of (111) orientation have been achieved without an amorphous underlayer, an unusual result for films of this orientation. Moreover the films have a strong degree of biaxial texturing in the x-y plane as found with X-ray texture analysis. These findings have important implications for buffer layers in perovskite thin film devices. The mechanism producing these structures has been revealed by using TEM and is discussed here.

Electron Paramagnetic Resonance and Scanning Tunneling Microscopy Investigations on the Formation of F+ and F0 Color Centers on the Surface of Thin MgO(001) Films

The formation of surface color centers (F(S) centers) by electron bombardment of thin MgO(001) films is investigated using electron paramagnetic resonance and low-temperature scanning tunneling microscopy. At low electron doses both techniques indicate the formation of singly occupied color centers (F(S)(+)), whereas at high electron doses the doubly occupied type (F(S)(0)) is dominant. It is suggested that with increasing electron dose F(S)(+) centers are transformed into F(S)(0). Tunneling spectra of individual F(S)(0) centers reveal a large distribution of energetic positions of occupied and unoccupied states, which is caused by local variations of the coordination number of the defects and explains the broad signals usually detected with integrating spectroscopic techniques.

Frequency modulated atomic force microscopy on MgO(001) thin films: interpretation ofatomic image resolution and distance dependence of tip–sample interaction

Atomically resolved images on a MgO(001) thin film deposited on Ag(001) obtained inultrahigh vacuum by frequency modulated atomic force microscopy at low temperature arepresented and analysed. Images obtained in the attractive regime show a differenttype of contrast formation from those acquired in the repulsive regime. For theinterpretation of the image contrast we have investigated the tip–sample interaction.Force and energy were recovered from frequency shift versus distance curves.The derived force curves have been compared to the force laws of long-range,short-range and contact forces. In the attractive regime close to the minimum of theforce–distance curve elastic deformations have been confirmed. The recovered energycurve has been scaled to the universal Rydberg model, yielding a decay length ofl = 0.3 nm and ΔE = 4.2 aJ (26 eV) for the maximum adhesion energy. A universal binding-energy–distance relation isconfirmed for the MgO(001) thin film.

Microstructure and morphology of MgO thin film with different magnetic underlayers

The MgO thin films were deposited on various magnetic layers at room temperature by magnetron sputtering. The microstructure and surface morphology of the MgO films were investigated by high-resolution transmission electron microscopy. The results showed that bottom ferromagnetic layer and the MgO thickness are essential factors affecting the quality of the MgO layer. The surface roughness increases with the thickness of the MgO layer. Highly (0 0 1) oriented MgO crystalline films were grown on CoFeB and CoFeC underlayers. On the contrary, the crystallinity of the MgO layer is relatively poor and it shows a wavy interface as the MgO layer was deposited on the Co underlayer.

Orientation of MgO thin films grown by pulsed laser deposition

Pulsed laser deposition technique has been employed to grow MgO thin films with preferred orientation on Si(1 0 0) and SiO 2/Si(1 0 0) substrates. The orientation of MgO thin films has been investigated in detail by varying deposition parameters. XRD analyses showed that the preferred orientation of MgO thin films changed from (1 1 1) to (1 0 0) as laser fluence decreased and oxygen pressure increased to some extent. Substrate temperature seemed to have little influence on the orientation of MgO thin films deposited at high laser fluences. TEM images of the (1 0 0)-oriented MgO thin films on Si(1 0 0) deposited at 400 °C and the laser fluence of 3 J/cm 2 in the oxygen pressure of 200 mTorr demonstrated that there existed a thin amorphous oxide layer of about 2 nm at MgO/Si interfaces due to the oxidation of the Si surface by the residual oxygen. MgO films with controlled orientations are suitable as a buffer layer for the growth of high-quality ferroelectric and superconducting overlayers.

P-88: The Characteristics of Sustain and Address Discharge on the MgO Thin Films in Alternating-Current Plasma Display Panels

The characteristics of sustain and address discharge for ion beam evaporated MgO thin films are investigated as a function of oxygen flow rates. The experimental results reveal that the secondary electron emission coefficient of MgO thin films enhanced due to improving the film properties with increasing the oxygen flow rates during film deposition. The X-Y firing voltage is decreased because the secondary electron emission enhances due to changing the MgO surface properties with increasing the O2 flow rates. Also, the sustain delay becomes faster because the secondary electron emission from MgO film increases, whereas the address delay grows slower as the oxygen gas flows. Accordingly, the optimum condition of MgO film must be considered simultaneously the sustain and address delay in order to obtain a stable discharge.

P-89: The Effect of Dehydration of MgO Protecting Layer on the XPS Spectra and the Electrical Properties in AC-PDP

As the e-beam evaporated MgO thin films were exposed to the air, the shiny surface was changed to be dull and the firing voltage was also increased by about 25 volts due to the hydration of surface. To figure out the effect of hydration on the electrical property, XPS analysis was conducted. XPS spectra show that the density of state, after etching the films, was changed. Especially the valence band edge was shifted to lower binding state and the intensity of valence band was increased. Similar behavior was observed when the films were heat treated. It means that the electrons can be ejected more easily and the ejected electrons have more energy after dehydration. Consequently the electrical property could have improved after dehydration.

Identification of Color Centers on MgO(001) Thin Films with Scanning Tunneling Microscopy

Localized electronic defects on the surface of a 4 monolayer (ML) thin MgO(001) film deposited on Ag(001) have been investigated by low-temperature scanning tunneling microscopy and spectroscopy. Depending on the location of the defect, we observe for the first time different defect energy levels in the band gap of MgO. The charge state of defects can be manipulated by interactions with the scanning tunneling microscope tip. Comparison with ground state energy levels of color centers on the MgO surface obtained from embedded cluster calculations corroborates the assignment of the defects to singly and doubly charged color centers.