Interface formation between Al and Ca with tris-(8-hydroxyquinoline) aluminum

Using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS), we have investigated the early stages of the interface formation between metals, namely Al and Ca, and tris-(8-hydroxyquinoline) aluminum (Alq(3)). Both interfaces show signs of reaction between the metal and Alq(3). However, the detailed behaviors of the two interfaces are very different. In the case of Al/Alq(3) interface, the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq(3). No evidence of reaction with the carbon was found. Unlike with Ca, little interaction between Al and nitrogen of the pyridyl was observed. UPS spectra show a quick disappearance of the Alq(3) features as early as 0.7 Angstrom of Al deposition, and also suggest the formation of a gap state induced by Al. in the case of Ca/Alq(3), the interface is characterized by a staged interface reaction: for low Ca coverages (Theta(ca) less than or equal to 4 Angstrom), negatively charged Alq(3) radical anions are formed by electron transfer from the Ca. The emergence of new states in the energy gap is observed in the UPS spectra. At higher coverages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq3 molecule.