L. P. H. Jeurgens

Growth kinetics and mechanisms of aluminum-oxide films formed by thermal oxidation of aluminum

The growth kinetics and mechanisms of thin aluminum-oxide films formed by the dry, thermal oxidation of a bare Al(431) substrate at a partial oxygen pressure of 1.33×10−4 Pa in the temperature range of 373–773 K were studied using x-ray photoelectron spectroscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (T⩽573 K), an amorphous oxide film develops that attains a limiting (uniform) thickness. At high temperatures (T>573 K), growth is not impeded at a limiting thickness. Kinetic analysis established the occurrences of two different oxide-film growth regimes: an initial regime of very fast oxide-film growth and a second, much slower oxidation stage that is observed only at T>573 K. These results could be discussed in terms of electric-field controlled, interstitial, outward transport of Al cations through a close packing of O anions in the amo...

Structure and morphology of aluminium-oxide films formed by thermal oxidation of aluminium

AbstractThe structure and morphology of thin aluminium-oxide films grown by the dry, thermal oxidation of a bare Al (431) substrateat a partial oxygen pressure of 1.33 =10 Pa in the temperature range of 373–773 K were studied using X-ray photoelectron y4 spectroscopy and high resolution electron microscopy.The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species.At lowtemperatures ( T (573 K ), the mobility of the oxygen species is very low, and an amorphous oxide film of relatively uniform,limiting thickness develops.X-ray photoelectron spectroscopic analysis established the occurrence of a surface-oxide species atthe very surface of these films.At higher temperatures( T )573 K ) an initially amorphous oxide film of less uniform thicknessdevelops that gradually transforms into crystalline g-Al O .At these temperatures an amorphous-to- 23 23 g-Al O transition oxidephase occurs. 2002 Elsevier Science B.V. All rights reserved.

Thermodynamics of reactions and phase transformations at interfaces and surfaces

Abstract Recent advances in the thermodynamic description of reactions and phase transformations at interfaces between metals, semiconductors, oxides and the ambient have been reviewed. Unanticipated nanostructures, characterized by the presence of phases at interfaces and surfaces which are unstable as bulk phases, can be thermodynamically stabilized due to the dominance of energy contributions of interfaces and surfaces in the total Gibbs energy of the system. The basic principles and practical guidelines to construct realistic, practically and generally applicable thermodynamic model descriptions of microstructural evolutions at interfaces and surfaces have been outlined. To this end, expressions for the estimation of the involved interface and surface energies have been dealt with extensively as a function of, e. g., the film composition and the growth temperature. Model predictions on transformations at interfaces (surfaces) in nanosized systems have been compared with corresponding experimental observations for, in particular, ultrathin (< 5 nm) oxide overgrowths on metal surfaces, as well as the metal-induced crystallization of semi-conductors in contact with various metals.

Amorphous versus crystalline state for ultrathin Al2O3 overgrowths on Al substrates

The thermodynamic and kinetic background of the stability of ultrathin (<3nm) amorphous Al2O3 overgrowths on Al{111}, Al{100}, and Al{110} substrates was investigated by thermal oxidation of the bare substrates in pure oxygen gas for oxidation times up to 6000s in the temperature range of T=350–650K. The microstructural evolutions of the developing oxide films were analyzed by angle-resolved x-ray photoelectron spectroscopy, low energy electron diffraction, and high-resolution transmission electron microscopy. For sufficiently small thicknesses, stable amorphous Al2O3 films form on all substrates. The critical thickness values beyond which a crystalline state for the Al2O3 film is thermodynamically preferred can be reliably calculated provided that a layer-by-layer mode of oxide-film growth occurs. With increasing temperature, a transition from a layer by layer to an island-by-layer type of oxide growth mode occurs and, consequently (tensile), growth strain in a crystalline Al2O3 overgrowth can be more re...

Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growth

The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO2 (a-SiO2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the a-Ge substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO2 and a-Ge substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive → tensile → ...

The Initial, Thermal Oxidation of Zirconium at Room Temperature

Angle-resolved x-ray photoelectron spectroscopy (ARXPS) and in situ spectroscopic ellipsometry have been used to investigate the initial oxidation of polycrystalline zirconium at room temperature in the partial oxygen pressure range of 1.3×10−7–1.3×10−4Pa. Detailed quantitative analysis of the measured Zr3d ARXPS spectra of the oxidized metal allowed separation of the intrinsic and extrinsic metallic and oxidic contributions to the spectra. It was shown that, in addition to the metallic contribution from the substrate and the oxidic contribution from stoichiometric ZrO2, two additional suboxidic components are contained in the measured Zr3d spectra of the oxidized Zr metal. As evidenced by angle-resolved XPS and in situ ellipsometry, both of these components can be attributed to a gradient of Zr enrichment in the region of the oxide film adjacent to the metal∕oxide interface (with the highest Zr enrichment at the metal∕oxide interface). Investigation of the oxide-film growth kinetics at various pO2, as de...

Theoretical Analysis of Melting Point Depression of Pure Metals in Different Initial Configurations

Abstract A general equation is derived for melting point depression (MPD) of pure metals, consisting of three terms: MPD due to high gas pressure, MPD due to high strain energy, and MPD due to small size of the metal. Particular equations are derived for different configurations of the solid metal, including grains embedded within a matrix. The equations obtained in this paper can be used to design nano-joining structures with improved MPD.

Metal-catalyzed growth of semiconductor nanostructures without solubility and diffusivity constraints

and vapor–solid–solid (VSS) [ 11–14 ] mechanisms, requiring substantial solubility and diffusivity of the semiconductor in the catalyst and thus high growth temperatures. The present study reveals, using in situ heating electron microscopy, that metal-catalyzed growth of semiconductor nanostructures can be realized without these constraints, thereby enabling strikingly low growth temperatures. The growth mechanism has been unraveled at the atomic scale for the Al-catalyzed growth of Si nanostructures at 150 ° C. Growth starts with wetting of high-angle grain boundaries (GBs) in the Al catalyst, by Si in its amorphous form, and continues by nucleation and growth of nanostructured crystalline Si (c-Si) in the template as precisely defi ned by the Al grain boundary network. The disclosed mechanism breaks solubility and diffusivity limits that have hitherto dictated selection of metal catalysts and growth temperatures and thereby opens new perspectives for direct fabrication of nanostructure devices on heat-sensitive substrates. The fundamental understanding of the metal-catalyzed growth mechanisms has been advanced by recent developments of in situ heating electron microscopy techniques. [ 12–15 ] Metalcatalyzed VLS growth of semiconductor nanowires proceeds by the precipitation of semiconductor material out of metal-semiconductor eutectic melt droplets, which are supersaturated by ceaseless exposure to gas-phase semiconductor reactants above the eutectic temperature. [ 8–10 ] More recent works have demonstrated that semiconductor nanowires can also be grown via a VSS growth mechanism below the eutectic temperature, at which the

Unexpected room-temperature ferromagnetism in bulk ZnO

It is demonstrated that a transition from paramagnetic behavior to clear room-temperature ferromagnetism (RTFM) exists in pure bulk ZnO. A significant enhancement of RTFM has been observed in argon-annealed ZnO samples. Quantitative chemical analysis unambiguously indicates that oxygen-related vacancies at surface play a crucial role in this observed RTFM. We suppose that the surface magnetic states, paramagnetic in the pure nanoparticles, are converted to ferromagnetic phase after mechanical compaction. Additionally, it is found that weakly adsorbed carbon species could block the exchange coupling between isolated magnetic moments in the surface layers.

Laminates of zinc oxide and poly(amino acid) layers with enhanced mechanical performance

In order to improve the resistance of solution-derived zinc oxide thin films against mechanical stress, nanostructured composite systems of soft organic and brittle ZnO layers were prepared by a bio-inspired process. As the organic component, polyelectrolyte multilayers were prepared by dip-coating using polystyrene sulfonate and poly(amino acids). The organic–inorganic laminates have typical properties in common with nacre: they consist of nanocrystals in a matrix of biomolecules, they exhibit a texture and they proved to be harder than the monolithic mineral.