Interface formation between Yb and tris-(8-hydroxyquinoline) aluminum (Alq(3)) was systematically investigated using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). The interface formation depended critically on the way it was prepared. For the case of Yb deposited on Alq(3), the XPS results revealed staged interface reactions. Upon oxygen exposure, the effect of Yb/Alq(3) interaction can be eliminated. By proper oxidation, the interface of Yb/Alq(3) can form an ohmic contact, leading to good electron injection. On the other hand, when Alq(3) was deposited on Yb, it was consider that the large ratio of Yb to Alq(3) resulted in substantial charge transfer to the Alq(3) molecules to form even the trivalent Alq(3)(3-) radical anions. (C) 2003 Elsevier B.V. All rights reserved.