Towards organic film passivation of germanium wafers using diazonium salts: Mechanism and ambient stability

Germanium is well-known for its good electronic properties, but also for the poor passivation quality of its natural or thermally-grown oxide layer. The robust passivation of Ge surfaces is thus a crucial step on the way to its integration in electronics at nanoscale. Before passivation, the natural oxide layer must be removed from the surface. Different methods were investigated and compared. The surfaces were analysed by X-ray photoelectron spectroscopy and by atomic force microscopy to determine the impact of the etching on the roughness. Oxide-free Cl-terminated surfaces were chosen and functionalized with various arenediazonium salts. The grafting is a fast reaction (less than 30 min) and proceeds in soft conditions (room temperature in acetonitrile solutions). The functionalization was found to proceed even at low temperature (18 degrees C). The modified surfaces were mainly studied by XPS and AFM. The morphology of the organic layer was influenced by the temperature and duration of the grafting as well as by the presence of substituents on the aromatic ring. We demonstrated that smooth oxide-free Ge surfaces could be prepared by a cyclic oxidation-rinsing-etching procedure and could be functionalized with various arenediazonium salts. The stability of the organic layer was compared to the stability of a self-assembled monolayer of alkanethiolates. The absolute coverage was also determined. This constitutes a first step towards Ge crystal passivation by organic thin films.