Ge(001) surface cleaning methods for device integration

In recent years, research on Ge nanodevices has experienced a renaissance, as Ge is being considered a possible high mobility channel material replacement for Si MOSFET devices. However, for reliable high performance devices, an atomically flat and perfectly clean Ge surface is of utmost importance. In this review, the existing methods for cleaning the Ge(001) surface are reviewed and compared for the first time. The review discusses three broad categories of cleaning techniques that have been successfully demonstrated to obtain a clean Ge surface. First, the use of ultraviolet light and/or oxygen plasma is discussed. Both techniques remove carbon contamination from the Ge surface and simultaneously form an oxide passivation layer. Second, in situ ion sputtering in combination with germanium regrowth, which can lead to extremely clean and well-ordered Ge surfaces, is discussed. Finally, various wet-etching recipes are summarized, with focus on hydrofluoric acid (HF), NH4OH, and HCl. Despite the success of HF for Si surface preparation, it is demonstrated that in the case of Ge, HF is outperformed by other chemicals with respect to surface roughness, carbon and oxide removal efficiency. It is shown that several cleaning methods can lead to a perfectly clean Ge surface, but only a few methods can be considered for actual device integration due to their effectiveness, simplicity, and scaling ability.