Typically, clean and defect-free Ge(001) surfaces are obtained by repeated sputtering of the sample in ultra-high vacuum until any contaminant is removed. Now researchers from Australia have demonstrated, with a scanning tunneling microscope study, that a simple wet chemical procedure in the clean-room combined with thermal treatment and epitaxial growth in ultra-high vacuum yields Ge(001) surfaces comparable in quality with the ideal Si(001) surface.

Reporting their results in the journal Nanotechnology, the scientists have developed a four-step cleaning process for Ge(001). They are now able to routinely produce clean surfaces with minimal roughness, low defect densities and wide mono-atomic terraces.

Protective layer

First a repeated ex situ wet chemical etching/oxidation step strips and reforms a protective oxide passivation layer. This protective layer is then thermally desorbed in UHV leaving small nanometre-sized Ge "flakes" on the surface. The Ge "flakes" are buried under a thin Ge epitaxial layer grown by molecular beam epitaxy (MBE), without leaving any fingerprint on the surface. A final thermal anneal induces coalescence of atomic terraces increasing their size and reducing the defect density of the final Ge(001) surface.

Such high-quality Ge(001) surfaces are ideal for near-perfect hydrogen passivation and allow the researchers to pattern atomically scaled structures on single terraces as well as micro-sized features, with a clear contrast in STM images.

Having previously demonstrated phosphorus delta doping of the Ge(001) surface, the team is now in a position to fabricate atomically precise, planar electrical devices in germanium using a combination of STM lithography, gas phase doping and MBE. The fabrication and study of atomic scale devices in germanium such as nanowires, tunnelling gaps or quantum dots is now one big step closer.