Determination of interface dipole and band bending at the Ag/tris (8-hydroxyquinolinato) gallium organic Schottky contact by ultraviolet photoemission spectroscopy

A tris ( 8-hydroxyquinolinato) gallium (Gaq(3)) thin film was grown in seve ral steps on a previously in situ evaporated Ag thin film. Ultraviolet phot oemission spectroscopy (UPS) measurements carried out prior to growth and a fter each growth step allowed the determination of the alignment of the hig hest occupied molecular orbital (HOMO) relative to the Fermi level of the A g substrate. The deposition of ultra-thin (submonolayer) initial Gaq(3) fil ms allowed us to distinguish between band bending and interface dipole rela ted high binding energy cutoff (secondary cutoff) shifts, which is necessar y to determine the interface dipole with high precision. In order to determ ine the band bending with high accuracy it was necessary to identify the HO MO position of the submonolayer Gaq(3) films, This was accomplished by remo ving the Ag related emission background in the low coverage spectra using t he Fermi edge intensity as a measure for the Ag related emission. Our resul ts demonstrate that the interface dipole builds up during the growth of the first one or two monolayers during which the HOMO position remains constan t. The offset between the HOMO cutoff(low binding energy cutoff of the UP s pectra) and the Ag Fermi edge was determined to be 1.67 eV, while the inter face dipole amounted to 0.69 eV. Ln order to find an estimate for the align ment of the lowest unoccupied molecular orbital (LUMO) relative to the Ag F ermi edge, the HOEO/LUMO gap (2.70 eV) of Gaq(3) was determined by optical absorption measurements, The LUMO offset; was estimated to be - 1.04 eV, (C ) 2000 Elsevier Science B.V. All rights reserved.