Structural and electrical properties of alkyl/perfluoroalkyl-thiol self-assembled monolayer on germanium as passivation layer

For several decades, germanium has been considered one of most attractive semiconductors within the channel design of the next generation MOSFET. However, its use is compromised by the poor quality of germanium oxide. This motivates the present study, focused on the structural and electrical characterization of self-assembled monolayers (SAMs) of fluoro-alkylthiols and variable length alkylthiols on Ge, which may be considered alternate passivation and insulating layers to replace germanium oxide. The de-oxidation/grafting technique in ethanol-water mixed solution has been adapted in this work. The results show that it provides better results than the more usual acid treatment. Indeed, smoother functionalized Ge surfaces with well-organized SAMs have been obtained, as assessed by ellipsometry, goniometry, and atomic force microscopy. X-ray photoelectron spectroscopy (XPS) analyses demonstrate the removal of oxide from as-functionalized Ge surface. From current-voltage measurements of the various SAMs using liquid gallium-indium eutectic contacts, it appears that fluoro-alkyl SAMs have allowed to decrease the current by more than two decades compared to bare (oxidized) Ge surfaces. Statistical analyses of the electrical characteristics are correlated with spectroscopic studies of the molecular levels, using inverse photoemission spectroscopy for probing the unoccupied levels (LUMO), XPS for the occupied levels (HOMO), and DFT calculations.