Fabrication of an organic superlattice film by molecular beam deposition

A superlattice film of 5, 10, 15, 20 tetraphenylporphyrin (H₂TPP; C₄₄H₃₀N₄) and 5, 10, 15, 20 tetraphenylporphyrinato zinc (ZnTPP; C₄₄H₂₈N₄Zn) was grown on a hydrogen terminated surface of Si (100) wafer which was kept at −25 °C. The organic molecular beam deposition technique was used for the fabrication under ultrahigh vacuum between 3×10⁻⁹ and 3×10⁻¹⁰ Torr. The small angle x‐ray diffraction study has revealed that the period of the superlattice was 4.4 nm, which was also confirmed by the oscillation of the same period observed in a secondary ion mass spectrometry depth profile of Zn. The Fourier‐transform infrared spectroscopy measurement indicated that the quasi‐planar molecules of H₂TPP and ZnTPP were inclined at an angle of 70° with respect to the substrate surface. The film surface was very flat and any roughness could not be detected by field emission secondary electron microscope observation. An atomic force microscope (AFM) therefore was used to investigate the surface. The root mean square roughness calculated from the AFM image was 0.37 nm.