Investigation of the interface formation between calcium and tris-(8-hydroxy quinoline) aluminum

X-ray and ultraviolet photoemission spectroscopy investigations reveal strong interactions between Ca and tris-(8-hydroxy quinoline) aluminum $({\mathrm{Alq}}_{\mathrm{3}})$ during the ${\mathrm{Ca/Alq}}_{\mathrm{3}}$ interface formation. The details of the interaction depend on the direction of the interface formation. For the case of Ca deposited on ${\mathrm{Alq}}_{\mathrm{3}},$ a staged interface reaction is observed. For low Ca coverages ${\mathrm{(\Theta }}_{\mathrm{Ca}}\text{⩽4\hspace{0.17em}Å),}$ negatively charged ${\mathrm{Alq}}_{\mathrm{3}}$ 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 ${\mathrm{Alq}}_{\mathrm{3}}$ molecule. On the other hand, for the case of ${\mathrm{Alq}}_{\mathrm{3}}$ deposited on Ca, a strong chemical reaction takes place as soon as ${\mathrm{Alq}}_{\mathrm{3}}$ is deposited, and Ca attacks every constituent of ${\mathrm{Alq}}_{\mathrm{3}}.$ Finally, no interaction occurs between ${\mathrm{Alq}}_{\mathrm{3}}$ and the Ca substrate if the substrate has been passivated by oxygen prior to the ${\mathrm{Alq}}_{\mathrm{3}}$ deposition.