Corundum Substrates Research Articles

Ab Initio Modelling of Metal Adhesion to Ceramics with Surface Defects

Ab initio slab simulations are performed for Ag adhesion to MgO and a-Al 2O3 (corundum) surfaces.We demonstrate that the point surface defects on MgO(001) surface increase markedly the adhesion energy and cause redistributio n of the electron density across the interface. The results for electron ( Fs° = O vacancy with two trapped electrons, i.e. x V O in Kroger-Vink notation) and hole ( Vs° = Mg vacancy with two holes trapped by nearest O 2- ions, i.e. x V Mg ) centers in the Ag atom adhesion at different surface covera ges are analyzed. For 1/4 Ag monolayer (ML) coverage, we observe small silver adhesion energies over O 2- and Mg 2+ ions on a regular MgO(001) substrate (0.23 and 0.22 eV per Ag atom, respectively), with negligible interfacial charge transfer towards metal atoms. However, for Ag atoms positioned over the point surface defects on the magnesia (001) substrate binding energies increase by more than an order of magnitude (7.6 and 12.7 eV, respectively) and are combined with a marked charge transfer (~1 e towards Ag atom over s F center and ~1.5 e towards nearest O 2- ions from Ag atom over s V center). We have studied also silver adhesion on both Al- and O-terminated corundum (0001) substrate surfaces. The latter case could be naturally considered as Ag adhesion on a defective Al-terminated corundum surface with external aluminium vacancies. The dependence of the adhesion energy on the interfacial distance has been analyzed for the two most favorable Ag adsorption positions and f or two metal coverages (a 1/3 ML of the Ag(111) plane and a full Ag(111) monolayer). An alogously to regular and defective magnesia substrate, two different termi nations (Al- and O-) give rise to qualitatively different results. In the former case the small adhesion energies per Ag atom (0.15- 0.25 eV for 1 ML and 0.40-0.55 eV for 1/3 ML) are accompanied by a minor interfacial charge transfer, indicating physisorption , which may be explained by a weak atomic polarization. In contrast, for O-terminated corundum, substantial adhesion energies per Ag atom (4.5-5 eV at 1 ML and 10.5 -12 eV at 1/3 ML) combined with a noticeable charge transfer from silver atoms towards the substrate (0.5-0.9 e) give a clear indications for a strong interfacial ion bonding.

Ab initio modelling of silver adhesion on the corundum (0001) surface

The Ag/α-Al2O3(0001) interface was simulated using ab initio slab calculations. We have studied silver adhesion on both Al- and O-terminated corundum substrates. The latter case may be considered as silver adhesion on a defective Al-terminated corundum surface with external aluminium vacancies. The dependence of the adhesion energy on the interfacial distance has been analyzed for the two favorable Ag adsorption positions and for two metal coverages (a 1/3 monolayer of the Ag(111) crystallographic plane and a full Ag(111) monolayer, 1 ML). The two different terminations (Al- and O-) give rise to qualitatively different results. In the former case, the small adhesion energies per Ag atom are accompanied by a minor interfacial charge transfer, indicating physisorption, which may be explained by a weak atomic polarization. In contrast, for O-terminated corundum, substantial adhesion energies per Ag atom combined with a noticeable charge transfer towards the substrate up to formation of Ag+ ions give a clear indication for a strong interfacial ion bonding. We compare these results with the data obtained earlier for the perfect and defective Ag/MgO(001) interfaces and found some qualitative similarities in metal adhesion nature on both alumina and magnesia substrates.

The adhesion properties of the Ag/α-Al 2O 3( [formula omitted]) interface: an ab initio study

Ab initio computer simulations of the atomic and electronic structure of the Ag/α-Al 2O 3(0 0 0 1) (corundum) interface have been performed for a periodic two-dimensional slab model using the Hartree–Fock method and a posteriori electron correlation corrections. We have considered both Al- and O-terminated corundum substrate surfaces. The dependence of the adhesion energy on the interfacial distance has been analyzed for the two most favorable Ag adsorption positions over corundum and for two different metal coverages (a 1/3 monolayer (ML) of the Ag(1 1 1) crystallographic plane and a full Ag(1 1 1) monolayer). The two different terminations (Al- and O-) give rise to qualitatively different results. The former case corresponds to the most stable termination of the pure corundum (0 0 0 1) substrate where small adhesion energies per Ag atom (0.15–0.25 eV for 1 ML and 0.40–0.55 eV for 1/3 ML) are accompanied by minor interfacial charge transfer, indicating physisorption, which may be explained by a weak atomic polarization. In contrast, for O-terminated corundum, substantial adhesion energies (3–5 eV per Ag atom at 1 ML coverage and 6–11 eV for 1/3 ML) combined with noticeable charge transfer from silver atoms towards the substrate (0.5 e to 0.9 e) are clear indications of a strong interfacial ion bonding. For both terminations, the observed difference in Ag adhesion energies for 1/3 ML and 1 ML coverages arises from a transition from directed Ag–O bonding towards a more delocalized electron density distribution in the complete monolayer. The results of our calculations are compared with available experimental studies and theoretical simulations for various Me/Al 2O 3 interfaces.

New routes to SnO 2 heteroepitaxy

We report the atomic-layer-deposition growth of SnO 2 thin films from SnCl 4 and H 2O 2 (case A), and SnI 4 and O 2 (case B) on (0 1 2)-oriented α-Al 2O 3 (corundum) substrates. In case A, the growth rate had a maximum at 250–500°C and slowly decreased at higher temperatures. In case B, the growth rate increased monotonically with temperature, increase being relatively slow at 500–750°C. At 500–600°C, the films grew three times faster in case B than in case A. The phase content, crystalline properties, purity, and thickness of the films were characterized by X-ray diffraction, X-ray reflection, and X-ray fluorescence spectroscopy. The films grown at 250°C and higher temperatures had tetragonal (cassiterite) structure. At 700°C in case A and 400–750°C in case B, the overgrowth could be considered as epitaxial. The orientational relationship has been determined to be (1 0 1)[0 1 0] cassiterite||(0 1 2)[1 0 0] corundum.

Preparation of AlVO 4-films for sensor application via a sol-gel/spin-coating technique

The preparation of thin AlVO 4 films on corundum substrates for possible sensor application is described. Thermally stable, crack-free thin films having thicknesses typically below 0.1 μm were obtained by a sol-gel process using alkoxide reagents combined with a spin-coating technique. The films were structurally characterized by XRD, Raman and FTIR spectroscopy and their morphology was analyzed by SEM. XRD showed that the films consisted almost exclusively (> 90%) of AlVO 4 after calcination at 923 K.

FTIR spectroscopic studies on the adsorption of an oxidized starch on some oxide minerals

Fourier-transformed IR spectroscopy was used to study the adsorption of oxidized starch on hematite, corundum, and quartz substrates in relation to their flotation. The difference spectrum of the adsorbed starch on hematite indicates surface reactions involving -CH2OH and -COH groups and shows similarities with that of the ferric-starch system and suggests chemical interaction between the Fe of hematite and starch adsorbate. The IR spectra of starch adsorbed on corundum and quartz show no evidence of chem. interaction, although H bonding is apparent. The extent of adsorption of starch on hematite, corundum, and quartz decreases in the indicated order. The evidence for chem. interaction of starch with hematite is further obtained by XPS of starch adsorbed on hematite and shoewd a noticeable decrease in peak positions of O(1s) and C(1s) of free starch. The Moessbauer spectrum of hematite with starch adsorbed on it complements these findings.

コランダムとルチルの規則的成長に関する実験的研究

Experiments on the crystal growth of materials belonging to the A12O3-TiO2 system and on the high-temperature behaviour of natural star-sapphire were made with special reference to the genesis of regular arrangement of the ruitle needle crystals in and on corundum crystals. The crystals were grown by using direct fusion, flux-fusion and hydrothermal methods under the various conditions of temperature, chemical concentration, cooling mode, temperature gradient and oxygen partial pressure. The following results were obtained: (1) When a thin layer of melt made from TiO2 powder or from mixture of Al2O3 and TiO2 powder was slowly cooled on (0001) face of corundum substrate, the regular arrangement of acicular rutile crystals was easily formed on the substrate. (2) The βAl2TiO5 crystals grown from the melt on the substrate did not show the regular arrangement. (3) When crystals of corundum and rutile were synchronously grown by the monotonous slow-cooling of flux-melt in air, no regular intergrowth appeared in both as-grown and aged corundum crystals. (4) When the intermitent flucutation of temperature was introduced during the slow-cooling in the above method, the conspicuous zonal structure composed of various imperfections appeared in the as-grown corundum crystals. After aging in air, the regular arrangement of rutile appeared on the (0001) faces and along the imperfection zones of corundum crystals. (5) When rutile was crystallized synchronously with corundum or epitaxially on the (0001) face of corundum from flux-melt in oxygen gas flow or from hydrothermal solution, no regular intergrowth was formed. (6) In the presence of flux-melt, the regular arrangement of rutile on the corundum substrate was promoted in air or in reducing atmosphere. (7) Epitaxial growth of corundum on a rutile single crystal did not occur in any of the growth methods mentioned above. (8) When natural star-sapphires were heated above 1200°C, the regular arrangement of rutile needles fell into disorder from surface to inside and finally disappeared with leaving small gas-bubbles. Ti-atoms tended to migrate predominantly towards the surface and into the small cracks where rutile was recrystallized. Based on these results and those reported by other authors, critical examinations were made as to such problems as solubility of Ti into corundum, stability of βAl2TiO3, removal of Ti from corundum technology for synthesis, mechanical behaviour, epitaxis and natural occurrence. As a result of examinations, it seems that the “precipitation from solidsolution” theory, which is generally accepted as to the genesis of regular intergrowth in star-sapphire, is less plausible. Following two theories were developed on the genesis ofstar-sapphire. (1) Secondary and epitaxial crystallization of rutile along open spaces and imperfection-zones such as shear planes, tension cracks, zonal structure and inclusions, most of which are parallel to (0001) face. TiO2-component is supplied not only from the external sources, but also from the internal sources such as the imperfection-zones with higher Ti-concentration. (2) Synchronous and parallel growth of corundum and dendritic rutile crystals, being followed by survival of the twig crystals which preserve proper orientation for epitaxis.

Characterization of metal films on corundum substrates

Gold layers on corundum, α-Al2O3, substrates, with intermediate sputtered copper and chromium layers, are used for microwave integrated circuit applications. The existance of “islands” of monocrystalline Au and Cu in otherwise polycrystalline material has been verified using X-ray and electron diffraction techniques. In these monocrystalline regions, a discrete Cr layer, under the Au and Cu layers, is no longer present. Investigations on specially prepared samples established the monocrystalline Cu to be a twinned epitaxic layer with (111)Cu//(0001)α-Al2O3; [¯1¯12] Cu//[2¯1¯10]α-Al2O3. In a sample consisting of a single Cr layer on α-Al2O3, the Cr was found to be epitaxic with (110) Cr//(0001)α-Al2O3. Epitaxic “islands” are detrimental to device performance, and their formation has been suppressed by reducing the sputtering power density.

The growth, crystallographic and electrical assessment of epitaxial layers of aluminium nitride on corundum substrates

Epitaxial layers of aluminium nitride have been grown on corundum substrates by vapour phase deposition, with aluminium chloride and ammonia as reactants. The structural perfection of the AIN material thus formed has been assessed using optical, X-ray diffraction and electron diffraction techniques. Although some oriented and unoriented polycrystalline deposits have been found, many highly oriented or single crystal layers have been produced. The two main epitaxial relationships developed are (2 1 1 0) A1N//(01 1 2) A1 2O 3 and (2 1 1 6) A1N//(01 1 2) A1 2O 3. The factors which govern the occurrence of these relationships are discussed. Measurements of the piezoelectric coupling of aluminium nitride on α-A1 2O 3 have been made and simple delay lines have been fabricated.

Growth of epitaxial layers of gallium nitride on silicon carbide and corundum substrates

Single crystal epitaxial layers of gallium nitride have been grown on α-SiC and α-Al 2O 3 substrates in the temperature range 1000– 1150 °C. At lower temperatures polycrystalline deposits are obtained, while at higher temperatures extensive decomposition of the layers becomes apparent. Epitaxial relationships developed are (0001) α−SiC ‖ (0001) GaN, (10 1 0) α−Al 2O 3 , ‖ (10 1 5) GaN, (0001) α− Al 2O 3 ‖ (0001) GaN and (11 2 0) α−Al 2O 3 ‖ (0001) GaN. The GaN adheres badly to the α-SiC and tends to shatter upon cooling. The best GaN surface finishes are obtained with (0001) Czochralski and (11 2 0) flame fusion corundum substrates.

Stress in silicon films deposited heteroepitaxially on insulating substrates with particular reference to corundum

The stresses in a thin silicon film deposited epitaxially on to a corundum substrate, arising from the differences in thermal expansion, are considered theoretically and the results are found to be in agreement with published experimental data. It is shown that the stress problem in such films cannot be overcome in the context of the two-layer system, though some alleviation can be obtained by the application of stringent dimensional conditions. Theoretical considerations show that the use of a third layer of material as a backing for the corundum substrate can reduce the stress at the silicon-corundum interface to zero and that at the silicon surface to a negligibly small value.

On the Properties of Yttria-Stabilized Zirconia Thin Films Prepared by Sol-Gel Method

The synthesis of nanostructured films of 20 mol% Y2O3 stabilized ZrO2 on corundum (Al2O3) substrates was performed from different sols using dip-coating technique. All obtained samples were repeatedly annealed at 800 °C temperature after each dipping procedure and fully characterized by X-ray diffraction (XRD) analysis. XRD data exhibited that at 800 °C temperature nano-sized Y0.2Zr0.8O2 thin films with cubic (Fm-3m) crystal structure have been formed. The morphological features of obtained coatings were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface tension and hydrophility of the synthesized films were determined by contact angle measurements (CAM).http://dx.doi.org/10.5755/j01.ms.17.2.491

Silicon/Corundum Epitaxy

The crystallographic aspects of silicon/corundum epitaxy were investigated. Epitaxial deposition was carried out on corundum substrates of two orientations, and the observed crystallographic relationships were shown to be determined by three-dimensional matching between the silicon structure and the sublattice of possible cation sites in corundum. Single-crystal silicon layers were deposited onto (0001) and (8̄629) corundum substrates. For growth on (0001) corundum, (111) silicon is parallel to the interface with [1̄10] silicon parallel to [2̄110] corundum. For growth on (8̄629) corundum, the silicon plane parallel to the interface is irrational, but the epitaxial relationship closely approximates to the following: (001) silicon parallel to (0001) corundum with [1̄10] silicon parallel to [2̄110] corundum. These two epitaxial relationships are the only ones which permit three noncoplanar symmetry axes of the silicon to coincide with three such axes of the cation site sublattice of the corundum. They insure compatible surface symmetries and similar surface densities on an arbitrary deposit/substrate interface. In general, the silicon atom density on the interface does not exceed the cation site density. When it does so, however, the rational crystallographic relationship is modified slightly in order to lower the density of silicon atoms below that of the possible cation sites.