Flashing lights deal with germanium problem

Researchers have found a way to deal with a problem that currently blocks the use of germanium instead of silicon for advanced transistors. By moving back to germanium as the main channel material, chipmakers could build faster devices.

The big problem with germanium up to now is that the dopants needed to make n-channel transistors can move too easily through the crystal lattice, removing the ability to create a changing concentration of dopants through the transistor channel which is needed for reliable operation.

The phosphorus atoms are strongly redistributed within the material during the annealing process used to repair the lattice after the dopants have been implanted. Silicon has a similar problem with boron, but this has been addressed by adding carbon to the dopant mix.

Scientists at the research centre Forschungszentrum Dresden-Rossendorf (FZD) working as part of an international team, have come up with two methods to prevent the wholesale movement of phosphorus atoms. The germanium samples were heated by short laser 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 phosphorus atoms to move.

An alternative method to suppress phosphorus diffusion in germanium has been investigated by an international team consisting of researchers from Germany, Denmark and the USA, among them physicists from FZD. After ion implantation of phosphorus into germanium the sample was heated to a given temperature and then irradiated by protons. They demonstrated that this treatment leads to the reduction of phosphorus diffusion as well.

Germanium was the basic material of first-generation transistors before it was replaced by silicon at the end of the 1960s, helped by the use of silicon dioxide as a gate insulator. Now that high-k, metal gates have arrived, the traditional manufacturing advantage of using silicon for the transistor has reduced significantly.

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