A study of the stability of ultra thin films of rhodium deposited on silver(100), gold deposited on copper(100), and gold deposited on silicon(100)

The top layer selectivity of Positron annihilation induced Auger Electron Spectroscopy (PAES) has been utilized to monitor the changes occurring when thin film-substrate systems formed at low temperatures are annealed at elevated temperatures. Positron Annihilation induced Auger Electron Spectroscopy (PAES), developed at UTA, provides information pertaining almost exclusively to the topmost atomic layer. This selectivity stems from the fact that the positron becomes localized just outside the surface before annihilation and, as a result, almost all of the annihilation leading to the excitation of Auger electron takes place in the topmost atomic layer. Three different thin film substrate systems (Rh/Ag(100), Au/Cu(100) and Au/Si(100)) were studied. It was found that Rh film deposited at 173 K remained on the top of the system, and the migration of Ag to the surface to form the Ag-Rh-Ag sandwich structure started at 373 K and was completed at 473 K. These two findings indicate that the null finding for ferromagnetism for a monolayer of Rh deposited on Ag at low temperature was not due to the presence of Ag layer. The Au/Cu(100) 2-D surface alloy formation mechanism was studied as a function of temperature. Our measurements indicated that a Au overlayer deposited at 193 K alloyed with the substrate to form a stable 2-D surface alloy at approximately 273 K. No further changes in this layer were noted up to the highest temperature measured (398 K). The interface structure of an annealed Au/Si(100) systems was investigated by taking advantage of the complimentary probing depths of EAES and PAES. EAES results showed that non-equilibrium phase formed at low temperature (193 K) started to transform into an equilibrium phase around room temperature. PAES spectra indicated that the annealed Au/Si(100) system had a Au layer on the top of the system. The presence of a Au layer on the top of the system was confirmed by ion sputtering depth profile analysis using PAES and EAES. A comparison of profiles obtained using EAES and PAES demonstrated that PAES has significantly better depth resolution in studies of ultra-thin films due to the fact that, in contrast to EAES, PAES samples only the topmost layer.