Browsing by Author "Reichel, Friederike"

Now showing 1 - 1 of 1

Open Access
The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals
(2007) Reichel, Friederike; Mittemeijer, Eric J. (Prof. Dr. Ir.)
This thesis addresses the effect of the parent metal-substrate orientation on the thermodynamics and kinetics of ultra-thin oxide-film growth on bare metals upon their exposure to oxygen gas at low temperatures (up to 650 K). For such thin oxide overgrowths on their metals, the resulting oxide-film microstructures often differ from those predicted by bulk thermodynamics, because of the relatively large contributions of interface and surface energies to the total energetics of the various metal-substrate/oxide-film systems. To this end, a model description has been developed to predict the thermodynamically stable microstructure of a thin oxide film grown on its bare metal substrate as function of the oxidation conditions and the substrate orientation. An amorphous state for ultra-thin oxide films grown on e.g. Al, Ti, Zr or Si can be thermodynamically, instead of kinetically, preferred up to a certain critical oxide-film thickness, because of the lower sum of surface and interface energies as compared to the corresponding crystalline modification. Beyond this critical oxide-film thickness, bulk thermodynamics will strive to stabilize the competing crystalline oxide phase. For Mg and Ni, the critical oxide-film thickness is less than 1 oxide monolayer and therefore the initial development of an amorphous oxide phase on these metal substrates is unlikely. Finally, for Cu and densely packed Cr and Fe metal surfaces, oxide overgrowth is predicted to proceed by the direct formation and growth of a crystalline oxide phase. Further, polished Al single-crystals with {111}, {100} and {110} surface orientations were introduced in an ultra-high vacuum system for specimen processing and analysis. After surface cleaning and annealing, the bare Al substrates have been oxidized by exposure to pure oxygen gas. During the oxidation, the oxide-film growth kinetics has been established by real-time in-situ spectroscopic ellipsometry. After the oxidation, the oxide-film microstructures were investigated by angle-resolved X-ray photoelectron spectroscopy and low energy electron diffraction. Finally, high-resolution transmission electron microscopic analysis was applied to study the microstructure and morphology of the grown oxide films on an atomic scale. The stoichiometric (i.e. Al2O3) oxide films grown on Al{111} are amorphous up to 450 K, whereas at higher temperatures epitaxial crystalline oxide films with a coherent metal/oxide interface develop. The oxide films grown on Al{100} are also overall stoichiometric, have uniform thicknesses and atomically flat metal/oxide interfaces. They are amorphous up to 400 K, but are transformed into crystalline gamma-Al2O3 upon annealing beyond a critical thickness. At more elevated temperatures (> 400 K), a crystalline Al2O3 film with a semi-coherent metal/oxide interface develops. For the crystalline gamma-Al2O3 overgrowth on Al{100}, an unexpected high lattice mismatch (> 15%) between the Al{100} substrate and the gamma-Al2O3 overgrowth is found with a semi-coherent metal/oxide interface. The oxide films grown on Al{110} for temperatures smaller than 550 K are also overall stoichiometric and amorphous. At more elevated temperatures (> 550 K), the original bare Al{110} surface becomes reconstructed at the onset of oxidation and {111}-facets develop. The kinetics of the oxide-film growth on the bare Al{100} and Al{110} substrates can be subdivided into a initial, very fast and a subsequent, very slow oxidation stage. For the oxidation of the bare Al{111} substrate up to 450 K, a distinction between an initial, very fast and a subsequent, very slow oxidation stage cannot be made. Instead, the initial oxide-film growth rate on Al{111} decreases only gradually with increasing oxidation time. The experimental growth curves for the thermal oxidation of Al single-crystals in the temperature regime of 350 – 600 K can be accurately described by considering the coupled currents of Al3+ cations and electrons in an uniform surface-charge field. As such, a gradual transformation of the initial amorphous oxide film on Al{100} into gamma-Al2O3, was observed with increasing oxidation temperature in the range of 350 – 600 K for Al{100}, as well as up to 450 K for Al{110}. Whereas, on Al{111}, the corresponding amorphous-to-crystalline transition was found to be more abrupt.