Dec 17, 2009
STM tip performs atomic-scale lithography on germanium
The first transistors were built in germanium due to its appealing electrical properties. The semiconductor industry then switched to silicon due to the benefits of the material's natural dielectric – silicon dioxide – which germanium did not have. Recently, the lack of a suitable dielectric in germanium has been overcome thanks to progress in alternative high-k dielectrics, developed within the complementary metal-oxide-semiconductor (CMOS) platform that are compatible with germanium. As a consequence there is a renewed interest in germanium-based electronics.
Over the last five years the Atomic Electronic Devices Group led by Michelle Simmons at the University of New South Wales, Sydney, has developed a radical technology to make fully functional atomic-scale devices in silicon using a scanning tunneling microscope (STM). The team is now extending its innovative technique to the fabrication and study of germanium-based atomic-scale transistors.
Fabricating atomic-scale features
As reported in the journal Nanotechnology, the group has used the STM tip to pattern nano to atomic scale features on a germanium(001) surface passivated beforehand with a 1 atom thick hydrogen layer. Thinking in terms of conventional lithography, the hydrogen atoms act as a resist on the surface.
Next, the highly confined electron beam from the STM tip is used under certain bias/current conditions to selectively desorb, in ultra-high vacuum, hydrogen atoms from the resist, thereby exposing the germanium surface underneath.
In another recent study, published in Appl. Phys. Lett., the researchers have demonstrated the ability to dope germanium at high concentrations using phosphine gas with a process that is selective against the STM-patterned hydrogen mask.
By combining these two techniques, the team is now in the position to use STM lithography to precisely place well defined numbers of dopants in germanium. Possible applications include the creation of ultra-sharp doping profiles for use as atomically abrupt source/drain regions for high-mobility germanium transistors, and atomic-scale wires for use as interconnects, particularly interesting for ultra-scaled CMOS technology.
About the author
Dr Giordano Scappucci is a senior research fellow working in Prof. Michelle Simmons group at the University of New South Wales, Sydney, Australia. He is chief investigator of the research project "Next generation computing: high mobility germanium transistors", initiated together with collaborator A/Prof. Giovanni Capellini, University "Roma TRE", Rome, Italy.