Soft X-ray photoelectron spectroscopy of metal-phthalocyanines on the (001) surface of GaAs and Ge

The interaction between selected metal-phthalocyanine (MPc) molecules and the clean (001) surfaces of gallium arsenide and germanium is probed by soft x-ray photoelectron spectroscopy (SXPS) using synchrotron radiation. As-3d and Ga-3d core spectra show no significant change when approximately one monolayer of any of three MPc molecules is deposited; also, the metal atom spectrum comprises a single component. In some contrast, deposition on the Ge(001) surface leads to changes in both substrate and deposit core spectra. Upon deposition of approximately one monolayer of any of three MPc molecules, approximately 50% of the 'up' dimer-atom intensity is shifted to higher binding energy, indicative of an electron-removing interaction. A corresponding electron-receiving shift to lower binding energy is observed in the metal core spectra of two of the three molecules, SnPc and PbPc; significantly, MgPc shows no such satellite feature. Since MgPc is planar while SnPc and PbPc are non-planar, it seems reasonable to attribute the two component spectrum of the latter to the two possible geometries of metal atom in 'contact' and 'non-contact' with the surface.