The utilization of germanium as the basic material for electronic switches would enable the production of even faster chips with a higher degree of integration. However, there is a number of problems yet to be solved. Until recently it has not been possible to manufacture a certain type of transistors (NMOS) based on germanium with a technologically interesting degree of integration. Two innovative procedures, successfully deployed by scientists from the FZD and their international colleagues, take remedial action here. 

Higher switching speeds than with silicon could be achieved by using germanium or other semiconductors. Germanium is particularly attractive, for it could be integrated into technological procedures easily. It has been the basic material of the first generation of transistors until it was superseded by silicon in the 1960s. This was due to the excellent electronic characteristics of the interface between the semiconducting silicon and its passivating and isolating oxide. However, this advantage cannot be used when further minimizing the transistors in the integrated circuitry, for the oxide needs to be replaced by so-called high-k materials. Hence, the basic materials question arises again.

By integrating foreign atoms, the conductivity of semiconductors can be deliberately altered. Ion-Implantation (ions are charged atoms) with subsequent tempering, called healing, is one possibility. The semiconductor-crystal’s healing is essential, for the material is damaged severely during implantation. Only after this process the desired characteristics will emerge. Although by utilizing this methods p-Channel-Transistors (PMOS) can be established based on germanium with dimensions according to a future 22-nanometer technology, this hasn’t been accomplished for n-Channel-Transistors. This is due to the strong spatial reallocation of the foreign phosphorous atoms required for the generation of the n+-areas.

Physicists from the Forschungszentrum Dresden-Rossendorf have succeeded in recreating the quality of the germanium crystal after implanting phosphorous ions into germanium by utilizing a specific healing method, as well as attaining good electric properties without a strong reallocation of phosphorous atoms occurring. In order to do so, a germanium sample has been heated up with a light flash for only a few milliseconds. This timeframe is too short to allow for the phosphorous atoms’ diffusion as it can be observed in the usual healing process. The light flashes are generated in a flashbulb-installation developed at the FZD. The analysis of the electric and structural characteristics of the phosphorous-doped layers in the germanium has taken place in close cooperation with the Belgian Microelectronic Research Center IMEC in Leuven and the Dresden Fraunhofer Center Nanoelectronic Technologies (CNT).

An alternative method to suppress the diffusion of phosphorous in germanium is examined by an international team, including scientists from Germany, Denmark and the USA as well as physicists from the FZD. After implanting ions of phosphorous into germanium, the sample is heated up and irradiated with protons. The proton-irradiation then leads to a reduced phosphorous diffusion. These results can be explained by the influence of certain crystallographic defects which destroy the lattice defects responsible for the phosphorous atoms mobility.These findings show that the generation of n-Channel-Transistors on a germanium basis with dimensions according to the largest scaled degree of intergration is possible.

Publications:
C. Wündisch, M. Posselt, B. Schmidt, V. Heera, T. Schumann, A. Mücklich, R. Grötzschel, W. Skorupa, T. Clarysse, E. Simoen, H. Hortenbach, "Millisecond flash lamp annealing of shallow implanted layers in Ge", in: Applied Physics Letters. 95 (2009), 252107.
DOI: 10.1063/1.3276770.H. Bracht, S. Schneider, J. N. Klug, C. Y. Liao, J. Lundsgaard Hansen, E. E. Haller, A. Nylandsted Larsen, D. Bougeard, M. Posselt, C. Wündisch, "Interstitial-Mediated Diffusion in Germanium under Proton Irradiation", in: Physical Review Letters 103 (2009), 255501,
DOI: 10.1103/PhysRevLett.103.255501.

Further Information:
Dr. Matthias Posselt / Clemens Wündisch
Forschungszentrum Dresden-Rossendorf (FZD)
Institut für Ionenstrahlphysik und Materialforschung
Phone: +49 351 260 - 3279 / - 3032
m.posselt@fzd.de  c.wuendisch@fzd.de

Press Contact:
Dr. Christine Bohnet
Forschungszentrum Dresden-Rossendorf (FZD)
Bautzner Landstr. 400
01328 Dresden
Phone: +49 351 260 - 2450
+49 160 969 288 56
presse@fzd.de
http://www.fzd.de

Further Information Online:
http://www.fzd.de

Source: Forschungszentrum Dresden - Rossendorf e.V.

• Away from Silicon – towards Germanium?