| Popis: |
X-ray photoemission has been used to investigate the formation of Y-Ba-Cu films on Si(100) and as an in situ probe of their subsequent oxidation to yield the associated oxide ceramic. The layers are prepared by coevaporation of the metallic components under ultrahigh vacuum, and pure alloy phases can be deposited at 300 K; reaction with the underlying substrate resulting in loss of Cu and incorporation by Si in the film takes place, however, at higher temperatures. Room-temperature oxidation stabilizes the film against this interaction and results in the preferential oxidation and surface segregation of barium at the expense of Cu. This segregation process becomes even more apparent during higher temperature (∼600 K) oxidation reactions. Chemical shifts and associated effects in x-ray photoelectron spectra are used to infer information on the chemical changes that occur in the film as oxidation proceeds. The thin-film phases prepared in situ in this work reveal a very similar surface composition to bulk superconducting samples prepared ex situ. This suggests that the surface segregation in bulk samples does not simply result from reaction with species such as water vapor, but instead may represent an equilibrium state of the oxide-oxygen interface. |
