Surface Of MgO Substrate Research Articles

Realization of tunable fabrication from Cu nanoparticles film towards Cu2O thin film by pulse laser deposition method

In this paper, the tunable fabrication from Cu nanoparticles (NPs) film to Cu2O thin film on single-crystal MgO (100) substrates via pulse laser deposition (PLD) has been realized by precisely adjusting the oxygen pressure. In-situ X-ray photoelectron spectroscopy confirms that the chemical state of Cu is altered from Cu0 to mixed state of Cu0 and Cu1+, and then to Cu1+. X-ray diffraction results reveal the main crystal structure of the films is changed from Cu (200) to Cu2O (110) with the increase of oxygen pressure. The morphology of three typical samples (Cu NPs film, Cu@Cu2O core-shell NPs films and Cu2O thin film) was record by transmission electron microscopy. According to the analysis, the mechanism of oxygen is to absorb on the surface of MgO (100) substrate and act as an oxidant in the growth of the films. The increase of oxygen pressure is beneficial to the formation of Cu@Cu2O core-shell NPs with larger diameters and the transition from Cu@Cu2O core-shell NPs film to continuous Cu2O (110) thin film. This work provides essential information for the fabrication of Cu2O thin films by PLD with Cu target at low oxygen pressure.

Chemical Mechanical Polishing of MgO Substrate and Its Effect on Fabrication of Atomic Step-Terrace Structures on MgO Surface by Subsequent High-Temperature Annealing

Single-crystalline MgO is used as a substrate for the deposition of various functional thin films. The present study focused on the development of a complete sequence of fabricating atomic step-terrace structures on the MgO substrate via a method that includes grinding, precise mechanical polishing, chemical mechanical polishing (CMP), and high-temperature annealing. The effect of a damage-free surface pretreatment on the subsequent high-temperature annealing was investigated. An atomically smooth and damage-free MgO substrate surface with an average surface roughness of 0.05 nm was obtained via a CMP process using a colloidal silica slurry. Atomic step-terrace structures were formed on the substrate after the high-temperature annealing process at 1000 °C for 20 h under atmospheric air. The obtained step height was 0.20 nm, which corresponds to one-half the unit cell of an MgO crystal (0.21 nm). By contrast, when a mechanically damaged MgO substrate was subjected to the annealing process, Ca segregation was observed on the annealed surface, without the formation of an atomic step-terrace structure. CMP was found to be necessary prior to high-temperature annealing to attain atomic step-terrace structures and to avoid the out-diffusion of impurities in the MgO bulk crystals.

Spin canting of Ni/CoO/Fe films grown on curved MgO(0 0 1) substrate

Using element-resolved x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) measurements, we determined the spin orientations of Ni, CoO and Fe films in Ni/CoO/Fe films grown on a curved MgO(001) substrate. We find that the vicinal surface of MgO(001) substrate results in a spin canting towards out-of-plane direction in the Ni and CoO films as a result of the interfacial coupling. The Ni spin canting angle increases monotonically with the vicinal angle in the studied range of 0–17° and the CoO spin canting angle increases more rapidly towards saturation only at a few degrees of the vicinal angle. The uniaxial magnetic anisotropy induced in the Ni layer by the Ni/CoO interfacial coupling is quantitatively determined and is shown to increase monotonically with the vicinal angle. Our result provides a new pathway for tailoring the spin orientation by modifying the substrate surface symmetry in combining with the ferromagnetic/antiferromagnetic interfacial interaction in thin-film based spintronic devices.

Revival of Smooth MgO(100) Surface: Ar Ion‐Beam Milling and Postvacuum Annealing in Oxygen Atmosphere

A recreation procedure for achieving the smooth and clean surface morphology of the degraded MgO(100) substrates is presented. The surfaces of the degraded MgO(100) substrates are treated with Ar ion‐beam milling (IBM) and annealing in O2 atmosphere. After each treatment, the changes in the MgO(100) substrate surface morphology are characterized using X‐ray diffraction (XRD), a 3D optical profilometer, atomic force microscopy, and Fourier‐transform infrared (FTIR) spectroscopy and compared with a fresh MgO substrate. It is shown that before any treatment, XRD and FTIR spectra show peaks corresponding to hydroxyl and carbonate groups in the degraded samples, whereas after treatment, XRD and FTIR spectra are comparable with the fresh sample. Further, after treatment, the surface roughness Rav value for two used/degraded samples reduces from 129 and 186 nm to 3.96 and 7.91 nm, respectively, after combined Ar IBM and annealing treatment. It is shown that the surface quality of the treated MgO(100) substrate improves in comparison with as it was before any treatment and can be reused for the good quality thin‐film deposition.

Surface preparation and the evolution of atomically flat step terrace morphology of MgO single crystals

We present a preparation method of vicinal stepped MgO (100) substrate surfaces, a prerequisite for high quality thin film growth. Through atomic force microscopy measurements, the surface morphology evolution of (100)-oriented MgO as a function of the annealing conditions is investigated. In particular, we discuss the effect of temperature, time and ambient atmosphere on the evolution of the MgO surface arrangement. At annealing temperature of 1000°C for 40 min, MgO surface morphology displays an atomically flat, single terminated surface with (100)-oriented, sharp terraces and equidistant steps equal to half of the MgO lattice parameter. Longer annealing leads to the formation of bunched terraces with large step heights of 1-5nm. The optimized recipe developed here efficiently removes surface adsorbates, namely hydroxyl and carbonate groups and restores the terraces on the surface.

Layer-by-layer growth of TiN by pulsed laser deposition onin-situannealed (100) MgO substrates

We have grown atomically smooth heteroepitaxial TiN thin films on (100) MgO single crystals by pulsed laser deposition (PLD). To generate optimal conditions for two-dimensional growth of TiN on MgO, the substrates were annealed in-situ using a CO2 laser heater. The MgO surface is smooth and uniformly stepped with a terrace height of half a unit cell; a value of 0.21 nm was measured by atomic force microscopy (AFM). In-situ RHEED oscillations during deposition of TiN indicate two-dimensional growth mode up to a film thickness of about 43 nm. This is confirmed by AFM. The TiN films show smooth, stepped surfaces with a uniform terrace height of 0.211 nm. X-ray diffraction measurements indicate a high film quality, as well. High resolution scans feature intensity fringes around the (200) TiN peak, indicating an abrupt interface between MgO and TiN. On asymmetric reciprocal space maps the peak broadening only in due to finite film thickness proves a high crystalline quality. Maximum electrical conductivity of the films is 2 × 106 S/m at 300 K. AFM topography images (5 × 5 μm2) of the surface of a (100) MgO substrate with a miscut angle of γ = 0.10°, annealed at 950 °C (height scale is 0 to 3.2 nm, left) and a 43 nm thick TiN/MgO thin film grown by PLD (height scale is 0 to 2.4 nm, right). Step height is half a unit cell in both cases.

NO<SUB>2</SUB> Interaction with Au Atom Adsorbed on Perfect and Defective MgO(100) Surfaces: Density Functional Theory Calculations

The interactions of nitrogen dioxide molecule (NO2) on Au atom adsorbed on the surfaces of metal oxide MgO (100) on both anionic (O2-) and defect (F(s) and F(s)(+)-centers) sites have been studied using the Density Functional Theory (DFT) in combination with embedded cluster model. The adsorption energies of NO2 molecule (N-down as well as O-down) on O(-2), F(s) and F(s)(+)-sites were considered. Full optimization for the additive materials and partial optimization for MgO substrate surfaces have been done. The formation energies were evaluated for F(s) and F(s)(+) of MgO substrate surfaces. Some parameters, the Ionization Potential (IP) and electron Affinity (eA), for defect free and defect containing surfaces have been calculated. The interaction properties of NO2 have been analyzed in terms of the adsorption energy, the electron donation (basicity), the elongation of N-O bond length and the charge distribution by using Natural Bond Orbital (NBO) analysis. The adsorption properties were examined by calculation of the Density of State (DOS). The presence of the Au atom increases the surface chemistry of the anionic O(2-)-site of MgO substrate surfaces. On the other hand, the presence of the Au atom decreases the surface chemistry of the F(s) and F(s)(+)-sites of MgO substrate surfaces. Generally, the NO2 molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F(s) and F(s)(+)-centers.

Cs-Corrected Scanning Transmission Electron Microscopy Investigation of Dislocation Core Configurations at a SrTiO3/MgO Heterogeneous Interface

Heterostructures and interfacial defects in a 40-nm-thick SrTiO(3) (STO) film grown epitaxially on a single-crystal MgO (001) were investigated using aberration-corrected scanning transmission electron microscopy and geometric phase analysis. The interface of STO/MgO was found to be of the typical domain-matching epitaxy with a misfit dislocation network having a Burgers vector of ½ a(STO) <100>. Our studies also revealed that the misfit dislocation cores at the heterogeneous interface display various local cation arrangements in terms of the combination of the extra-half inserting plane and the initial film plane. The type of the inserting plane, either the SrO or the TiO(2) plane, alters with actual interfacial conditions. Contrary to previous theoretical calculations, the starting film planes were found to be dominated by the SrO layer, i.e., a SrO/MgO interface. In certain regions, the starting film planes change to the TiO(2)/MgO interface because of atomic steps at the MgO substrate surface. In particular, four basic misfit dislocation core configurations of the STO/MgO system have been identified and discussed in relation to the substrate surface terraces and possible interdiffusion. The interface structure of the system in reverse--MgO/STO--is also studied and presented for comparison.

A density functional study of NO2 adsorption on perfect and defective MgO (1 0 0) and Li/MgO (1 0 0) surfaces

The density functional theory (DFT) in combination with embedded cluster model have been used to study the adsorption of nitrogen dioxide molecule (NO2) on Li atom deposited on the surfaces of metal oxide MgO (100) on both anionic (O2−) and defect (Fs and Fs+-centers) sites. The adsorption energy (Eads) of NO2 molecule (N-down as well as O-down) in different positions on O−2, Fs and Fs+-sites is considered. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces. The formation energies have been evaluated for Fs and Fs+ of MgO substrate surfaces. The ionization potential (IP) and electron affinity (eA) for defect free and defect containing surfaces have been calculated. The adsorption properties of NO2 are analyzed in terms of the adsorption energy, the electron donation (basicity), the elongation of NO bond length and the atomic charges on adsorbed materials. The densities of states (DOS) have been calculated and used for examining the adsorption properties. The NO2 molecule is dissociated due to the interaction with the defective substrate surface (Fs-site) producing an oxygen atom strongly chemisorbed to the vacancy of the substrate and gaseous NO far away from the surface. The presence of the Li atom increases the surface chemistry of the anionic O2−-site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the surface chemistry of the Fs and Fs+-sites of MgO substrate surfaces. Generally, the NO2 molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing Fs and Fs+-centers.

Adsorption of SO 2 on Li atoms deposited on MgO (1 0 0) surface: DFT calculations

The adsorption of sulfur dioxide molecule (SO 2) on Li atom deposited on the surfaces of metal oxide MgO (1 0 0) on both anionic and defect ( F s-center) sites located on various geometrical defects (terrace, edge and corner) has been studied using density functional theory (DFT) in combination with embedded cluster model. The adsorption energy ( E ads) of SO 2 molecule (S-atom down as well as O-atom down) in different positions on both of O −2 and F s sites is considered. The spin density (SD) distribution due to the presence of Li atom is discussed. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces (terrace, edge and corner). The oxygen vacancy formation energies have been evaluated for MgO substrate surfaces. The ionization potential (IP) for defect free and defect containing of the MgO surfaces has been calculated. The adsorption properties of SO 2 are analyzed in terms of the E ads, the electron donation (basicity), the elongation of S–O bond length and the atomic charges on adsorbed materials. The presence of the Li atom increases the catalytic effect of the anionic O −2 site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the catalytic effect of the F s-site of MgO substrate surfaces. Generally, the SO 2 molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F s-center.

Removal of scratch on the surface of MgO single crystal substrate in chemical mechanical polishing process

Etching and chemical mechanical polishing (CMP) experiments of the MgO single crystal substrate with an artificial scratch on its surface are respectively performed with the developed polishing slurry mainly containing 2 vol.% phosphoric acid (H 3PO 4) and 10–20 nm colloidal silica particles, through observing the variations of the scratch topography on the substrate surface in experiments process, the mechanism and effect of removing scratch during etching and polishing are studied, some evaluating indexes for effect of removing scratch are presented. Finally, chemical mechanical polishing experiments of the MgO substrates after lapped are conducted by using different kinds of polishing pads, and influences of the polishing pad hardness on removal of the scratches on the MgO substrate surface are discussed.

Epitaxial B2-NiAl layers formed by nanosecond laser irradiation of thin Al/Ni bilayers

The results of experiments on the synthesis of epitaxial B2-NiAl layers by means of nanosecond laser irradiation of sequentially deposited thin nickel and aluminum layers (Al/Ni bilayers) on a MgO(001) substrate surface are presented. Features of the phase formation under the laser action and during the combustion wave initiation are considered. The rapid formation of an epitaxial B2-NiAl layer is explained in terms of a martensitic-like mechanism of the transfer of reacting atoms via a layer of reaction products. It is suggested that this mechanism can compete with diffusion via grain boundaries and dislocation, thus explaining the ultrafast transfer of reacting atoms via a layer of reaction products for various methods of initiation of the solid-state synthesis.

The manipulation of the carbon nanofibers growth into lateral or vertical direction with respect to the substrate synthesized with iridium catalyst

Carbon nanofibers could be grown on the smooth plane area on MgO substrate surface in the lateral direction to the substrate by the catalytic role of iridium. In the case of the rough surface area of the MgO substrate, the vertical growth of the carbon nanofibers was dominant. Laterally-grown carbon nanofibers formed a self-assembled interconnection network. The surface electrical resistivity of the self-assembled lateral carbon nanofibers network decreased with increasing substrate temperature. A scanning tunneling spectroscopy measurement revealed the band gap existence between −1 and 1 V, which confirmed the semiconductor characteristics of the self-assembled lateral carbon nanofibers network.

Dependence of crystalline orientation on film thickness of sputtered YBa 2Cu 3O 7− x on an MgO substrate

We have systematically investigated dependence of crystalline orientation on film thickness of sputtered YBa 2Cu 3O 7− x (YBCO) on an MgO substrate by the X-ray diffraction (XRD) method, the φ diffraction method and atomic force microscope (AFM). From examination of crystalline orientation by XRD method and in-plane orientation by the φ diffraction method, for thinner film below about 500 nm, the YBCO film is oriented with the c-axis perpendicular to the MgO substrate surface. For thicker film above about 500 nm, the c-axis oriented grains in the YBCO film vanish entirely and the a-axis oriented grains are formed. The rectangular images of a-axis oriented grains are also observed on the surface by AFM. We found that the crystalline orientation change drastically from c-axis oriented films to a-axis oriented films with increasing film thickness.

MgO substrate surface optimization for YBaCuO thin film growth

To obtain high quality Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (YBaCuO) films grown on [001] MgO substrates, we have investigated the influence of high temperature substrate annealing prior to film deposition. In particular, we discuss the effect of annealing conditions (atmosphere, temperature and time) on MgO substrate morphology observed with atomic force microscopy. Thermal treatment under specific conditions resulted in the formation of [001]-oriented terraces bounded by [100]-oriented steps on the substrate surface. An improvement in the crystalline quality, as well as better DC electrical properties and microwave surface resistance values, has been observed for films sputtered onto substrate surfaces exhibiting such regular nanoscale step structures.

Mechanism of NdBCO film growth on YBCO-seeded MgO substrate using liquid phase epitaxy

YBa 2Cu 3O 7− x (YBCO) seed films were used to grow NdBa 2Cu 3O 7− x (NdBCO) superconducting films on MgO substrates by liquid phase epitaxy. The film microstructures were characterized using optical and electron microscopies to clarify the growth mechanism. It was found that most of the seed grains decomposed at the high processing temperature and dissolved when they touched the solution. The NdBCO grains were formed first by the quasi-homo-epitaxial growth of NdBCO units on the few surviving YBCO seed grains and then grew pendently to cover the large area of the bare MgO substrate surface region quickly by lateral overgrowth. A micrometer-thick melt layer was entrapped between the film and the substrate. Through the few links provided by the surviving seed grains, a stable film/substrate orientation relationship could still be maintained.

On the role of silver evaporation during YBa 2Cu 3O 7− x film processing

YBa 2Cu 3O 7− x (YBCO) films were simultaneously grown on both silver and MgO substrates by metalorganic chemical vapor deposition. Studies show that the transportation of silver occurs via the evaporation from the silver substrate at high deposition temperatures. The silver vapor can migrate over a distance of a few millimeters on the surface of MgO substrate. We argue that the neighboring silver substrate could act as the source of the silver vapor. The presence of silver vapor may play a role as a catalyst for the microstructure change of YBCO film on the MgO substrate.

Growth of atomically flat homoepitaxial magnesium oxide thin films by metal-organic chemical vapor deposition

Homoepitaxial magnesium oxide thin films have been grown on MgO(1 0 0), (1 1 0) and (1 1 1) substrates by the plasma enhanced metal-organic chemical vapor deposition. Atomic force microscopy measurements indicate that the as-received MgO substrate surface morphology is greatly improved as a result of MgO homoepitaxial growth. Various oxygen partial pressures and plasma powers were selected to investigate the role of atomic oxygen species during the growth of the MgO thin films. Oxygen plasma conditions were analyzed by Langmuir probe measurements. These studies reveal that higher concentrations of atomic oxygen effectively assist the layer-by-layer growth kinetics, which are critical to generating atomically flat surfaces. Smooth surfaces, with roughness as low as 0.036 nm, are achieved on MgO(1 0 0) substrates under the conditions of 2.1 mTorr oxygen partial pressure and 1300 W plasma power at 720°C.

Initial growth mechanism of YBCO films in liquid phase epitaxy process

The initial stage of growth for YBa 2Cu 3O x (Y123) films on (100) MgO substrates by liquid phase epitaxy (LPE) was investigated. In the initial stage, Y123 seed films partially dissolved into BaO–CuO solution, and Y123 islands were epitaxially grown from the undissolved seed grains on an MgO substrate within 1 s. It was found by XRD analysis that the LPE films with grain orientation of fourfold symmetry on the surface of MgO substrate were grown from not only a fourfold symmetrical seed film but also an eightfold symmetrical one. This phenomenon cannot be fully explained by the coarsening mechanism due to the curvature difference. The difference in the interface energies between Y123 and MgO should be considered to understand the preferential dissolution and growth.

Interface Characterization of AlN/TiN/MgO(001) Prepared by Molecular Beam Epitaxy

The AlN/TiN/MgO(001) interfaces, prepared by molecular beam epitaxy, have been characterized by cross-sectional high-resolution electron microscopy (HREM). The thin TiN buffer layer, with the thickness of 40 nm, is epitaxially grown on the MgO(001) substrate. Owing to the same structure-type as well as the small mismatch of their lattice parameters, the growth is governed by the parallel orientation relationship of (001)TiN||(001)MgO, (010)TiN||(010)MgO, and (111)TiN||(111)MgO. Two kinds of processes of the hexagonal AlN epitaxial growth on the as-received TiN(001), differed by the (0001)AlN plane parallel to, and the (1012) plane approximately parallel to the MgO substrate surface, respectively, are identified, and within them, several cases are classified which are based on the consideration of crystallographic symmetry. Theoretical calculations based on the geometrical model that was recently proposed and applied to a number of epitaxial systems have been carried out to rationalize these observations.