Structure and reactivity of thin film gold-aluminum alloys

Gold-aluminum alloys provide a good opportunity to study both the electronic structures of the alloys as well as the reactivity of aluminum in the presence of a relatively non-reactive element. Methods for the formation of pure Au ₂Al and Al₂Au alloy surfaces have been established for studies of the Au-Al thin film alloying using X-ray photoelectron spectroscopy (XPS). As part of this, the effects of annealing time, temperature and ion bombardment on the resultant alloy composition and purity were studied. XPS spectral data for Au(4f) and Al(2p) binding energies of the alloys were reanalyzed using a recently-developed Maximum Entropy Method (MEM).; In the study of electronic structures of thin film Au-Al alloys, the Auger parameter measurements in combination with the Thomas and Weightman model have been used to estimate the charge transfer between Au and Al, while M₃-edge X-ray absorption near-edge structure (XANES) and XPS valance band studies have been used to explore the behaviour of d electrons in alloying. In spite of the fact that Au acts as a net electron acceptor, a significant depletion of Au 5d charge upon alloying has largely compensated for the charge flow onto the Au site. The understanding of electronic structures in Au-Al alloys is further improved from other experimental data: the decrease of lattice parameters in terms of the bonding strength between Au and Al; changes in the position and intensity of the plasmons accompanying the Al1s peak, which is associated with the electronic changes that occur in the atomic environment upon alloying; analysis of intrinsic energy loss structure associated with the Al2p line in order to extract information on charge transfer from Au 5d to Al sp bands.; The oxide growth kinetics of thin film Au-Al alloys as well as pure Al has been studied using XPS across a wide range of water vapour and air exposures. Using XPS to measure oxide growth in the outermost 3–4 nm, the alloys and the pure Al were found to undergo a similar set of oxide growth kinetics. In addition to the three stages of early growth kinetics identified for Al in previous studies, a fourth stage is identified at air exposure doses >10 9 L. The oxidation of the aluminum component causes phase transitions at the near-surface to more Au-rich alloys.; The initial stage of the oxidation process from a different point of view has been revealed through Secondary Ion Mass Spectrometry (SIMS) depth profiling and imaging studies. In addition to the outward diffusion of Al cations from the layer directly beneath the oxide to react with water at the oxide/gas interface, the oxygen anions may also diffuse into the metal to cause internal oxidation. SIMS has also been used to study the structure on the oxide as well as on the internal surfaces (interfaces, grain boundaries) for alloys during the initial and the later stages of oxidation. Al L _2,3-edge XANES results have shown that the innermost oxide structure on alloys is different from that on Al metal, while the oxide structures closer to the outmost surface are however similar for both Al metal and alloys.