Mar 30, 2011
Ge(001) surfaces readied for atomic-scale devices
There is a growing interest in Ge for nano- to atomic-scale electronic devices driven by the material's enhanced electrical properties compared with Si and owing to the compatibility of Ge with the Si CMOS platform. The first key step in fabricating any future atomic-scale device on germanium is the routine preparation of a high-quality starting surface. While the preparation of clean and defect-free Si(001) surfaces is well established, obtaining the same result for Ge has been tricky so far due to the lack of a stable Ge native oxide to protect the sample from contaminants.
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.
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.
About the author
The research team includes: Wolfgang Matthias Klesse, PhD student in physics at UNSW working on atomic-scale device fabrication in germanium; Dr Giordano Scappucci, senior research fellow at the University of New South Wales and chief investigator of the research project "Three-dimensional atomically precise circuits in germanium"; Prof. Giovanni Capellini, visiting professor at UNSW from University "Roma TRE", Rome, Italy; Prof. Michelle Simmons, director of the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology.