Sexithiophene films on clean and oxidized Si(111) surfaces: Growth and electronic structure

The growth and the electronic properties of sexithiophene thin films on silicon surfaces have been studied by angle resolved ultraviolet photoelectron spectroscopy while morphology and crystalline order have been assessed by atomic force microscopy and x-ray diffraction. The influence of the surface modification of the substrate as well as of the growth temperature and the growth rate were investigated in ultrahigh vacuum through controlled multistep growth sequences ranging from (sub)monolayer coverage up to thick films. Depending on the preparation, two electronically distinct sexithiophene $\left(6\mathrm{T}\right)$ films could be produced, as revealed by their ionization potentials which differed by $\sim 0.7\mathrm{eV}$ . The evolution of the electronic energy levels with film thickness is considered in terms of the concepts of interface dipole and band bending, with the latter being argued to be inappropriate. Irrespective of substrate or growth conditions the thick $6\mathrm{T}$ films were found to all have the same work function of $\sim 0.4\mathrm{eV}$ , which is related to near vertically oriented molecules on the films surface. Against general expectations $6\mathrm{T}$ grown at a low rate on the reactive $\mathrm{Si}\left(111\right)$ substrate and those grown at high rates on the oxide surface produces films that are more uniform and crystalline than those grown at low rates on the passive oxidized $\mathrm{Si}$ surfaces. The latter promotes the formation of small three-dimensional crystallites and a much poorer morphology for planar devices.