AVS 46th International Symposium
    Organic Electronic Materials Topical Conference Tuesday Sessions
       Session OE+EM+AS-TuM

Paper OE+EM+AS-TuM5
Investigation of the Electronic Structure of Organic Schottky Contacts with Photoemission Spectroscopy: Discrimination Between Interface Dipole, HOMO Offset, Band Bending and Charging Related Spectral Shifts

Tuesday, October 26, 1999, 9:40 am, Room 616/617

Session: Interfaces and Characterization of Organic Thin Films
Presenter: R. Schlaf, US Naval Research Laboratory
Authors: R. Schlaf, US Naval Research Laboratory
G.P. Kushto, US Naval Research Laboratory
L.A. Crisafully, US Naval Research Laboratory
C.D. Merritt, US Naval Research Laboratory
Z.H. Kafafi, US Naval Research Laboratory
Correspondent: Click to Email
We have determined the electronic structure of several organic Schottky contacts formed between the organic luminescent semiconductor tris (8-hydroxyquinolinato) gallium (Gaq@sub 3@) and the metals Al, Mg, Ag, Au, and Pt. The investigated interfaces were prepared by vapor deposition of Gaq@sub 3@ on in-situ deposited metal films or thin sputtered high purity metal foils in a multi step growth procedure. Before growth and after each Gaq @sub 3@ deposition step the samples were characterized in situ using a combination of x-ray and ultraviolet photoemission spectroscopies (XPS, UPS). UPS was used to measure the highest occupied molecular orbital (HOMO) positions and interface dipoles, while XPS was used to determine the band bending (bulk charge redistribution) at the interface. In addition, simultaneous measurements of the high binding energy cutoff of the XP- and UP-spectra allowed the determination of the onset of charging phenomena observed at around 100Å Gaq@sub 3@ film thickness. Our results demonstrate that both, charging and band bending effects, may result in strong shifts of the PES spectra which need to be carefully evaluated in order to accurately determine the orbital line-up and the dipole at the interface. Our experiments suggest that the investigated organic Schottky contacts have large interface dipoles due to the chemisorption of the first organic layer in contact with the metal surface. All investigated interfaces also exhibit strong band bending in the organic layer, which strongly depends on the work function difference between Gaq@sub 3@ and the particular metal in contact.