Germanium Nanoelectronics

In the world of miniaturization we are always thinking of faster and smaller things. The use of germanium instead of silicon as primary material of transistor would provide us smaller transistor chip which are faster than its previous counterparts.

During the production of transistor, foreign atoms such as phosphorus and boron are implanted into the semiconductor material so that it becomes partly conducting. But this production step damages the material; it must be repaired by subsequent annealing. As the phosphorus atoms are strongly redistributed within the material during annealing, it has not been possible to manufacture large scale integrated transistors using germanium.

Scientists of the research center Forschungszentrum Dresden-Rossendorf (FZD) surmount this problem with two novel techniques.

Germanium was the basic material of first-generation transistors before it was replaced by silicon at the end of the 1960s. This was due to the excellent electronic properties of the interface between the semiconductor silicon and its insulating and passivating oxide. However, this advantage cannot be utilized if transistor dimensions are further reduced since the oxide must then be replaced by so-called high-k dielectrics. This again stimulates science and industry to search for the most suitable basic material. Higher switching speeds could also be achieved using germanium and some other semiconductors.

By inserting foreign atoms the conductivity of semiconductors can be varied in a purposeful way. One possibility is ion implantation (ions are charged atoms) with subsequent heat treatment, which is called annealing. Annealing of the germanium crystal is necessary as the material is heavily damaged during implantation, and leads to the requested electronic properties. While these methods allow for the manufacturing of p-channel transistors (PMOS) according to future technology needs, it was not possible to produce corresponding n-channel transistors (NMOS) using germanium. This is due to the strong spatial redistribution (diffusion) of the phosphorus atoms which have to be used in manufacturing the n+ regions.

Physicists from the FZD applied a special annealing method that enables repairing the germanium crystal and yields good electrical properties without the diffusion of phosphorus atoms. The germanium samples were heated by short light pulses of only a few milliseconds. This period is sufficient in order to restore the crystal quality and to achieve electrical activation of phosphorus, but it is too short for the spatial redistribution of the phosphorus atoms. The light pulses were generated by the flash lamp equipment which was developed at the research center FZD. Analysis of the electrical and structural properties of the thin phosphorus-doped layers in germanium was performed in close collaboration with colleagues from the Belgian microelectronics center IMEC in Leuven and from the Fraunhofer-Center for Nanoelectronic Technologies (CNT) in Dresden.