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

Abstract

A tris (8-hydroxyquinolinato) gallium (Gaq 3) thin film was grown in several steps on a previously in situ evaporated Ag thin film. Ultraviolet photoemission spectroscopy (UPS) measurements carried out prior to growth and after each growth step allowed the determination of the alignment of the highest occupied molecular orbital (HOMO) relative to the Fermi level of the Ag substrate. The deposition of ultra-thin (submonolayer) initial Gaq 3 films allowed us to distinguish between band bending and interface dipole related high binding energy cutoff (secondary cutoff) shifts, which is necessary to determine the interface dipole with high precision. In order to determine the band bending with high accuracy it was necessary to identify the HOMO position of the submonolayer Gaq 3 films. This was accomplished by removing the Ag related emission background in the low coverage spectra using the Fermi edge intensity as a measure for the Ag related emission. Our results demonstrate that the interface dipole builds up during the growth of the first one or two monolayers during which the HOMO position remains constant. The offset between the HOMO cutoff (low binding energy cutoff of the UP spectra) and the Ag Fermi edge was determined to be 1.67 eV, while the interface dipole amounted to 0.69 eV. In order to find an estimate for the alignment of the lowest unoccupied molecular orbital (LUMO) relative to the Ag Fermi edge, the HOMO/LUMO gap (2.70 eV) of Gaq 3 was determined by optical absorption measurements. The LUMO offset was estimated to be −1.04 eV.