Unoccupied electronic states and energy level alignment at interfaces between Cu-phthalocyanine films and semiconductor surfaces

Abstract

Thin films of Cu-phthalocyanine (CuPc) were thermally deposited in UHV on silicon, on highly ordered pyrolytic graphite (HOPG) and on ZnO substrates. The surface potential and the density of unoccupied electron states (DOUS) located 1–25 eV above vacuum level were measured during the film deposition using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Analysis of the TCS data allowed us to obtain DOUS features of the CuPc and to study charge transfer during the interface formation. A significant electron transfer from the CuPc film to SiO 2/n-Si and n-Si(1 0 0) was observed and the charge transfer layer in the film extended up to 10 nm. Band bending in the substrates due to the electron transfer was also monitored. A less significant charge transfer was observed at the interfaces between the CuPc film and the ZnO(0 0 0 1), SiO 2/p-Si and HOPG substrates. A model for the extended charge-transfer layer (extended interface dipole) is suggested to explain the observed spectroscopic features of the interfaces.