Oct 19, 2011
Time-resolved fluorescence spectroscopy looks deeper into germanium nano-islands in silicon
Researchers at Aarhus University, Denmark, are studying the light-emission of germanium (Ge) nano-islands embedded in crystalline silicon (Si). Such islands may one day enable optical activity inside silicon chips, although a number of obstacles remain in reaching this goal. At present, cryogenic temperatures are required for light emission, and Auger recombination of charges seriously prevent efficient light emission.
It has been demonstrated that the instantaneous recombination rate indeed grows quadratically with the number of electron-hole pairs (excitons) in the nano-islands, when this number is large, thus confirming the expectations from the three-body nature of the Auger-recombination process. By optical excitation, a large number of excitons can be created simultaneously in the islands leading to an initially fast Auger-recombination process (see top image – right) followed by much slower two-body dynamics. This separation of timescales suggests that the amount of detected light reflects directly how many excitons are present in the islands (vertical scale of the graph) since the Auger process is absent for single excitons. This interpretation was confirmed using independent information from the spectral properties of the emitted light.
Understanding geometrical effects
The characteristic time of the Auger-recombination process was found to be 10 ns. This timescale is very sensitive to the exact geometry of the Ge/Si interface, and hence the results may potentially lead to a better understanding of such geometrical effects of the recombination. In addition, the temporal record of the emission process presents conveniently the emission characteristics as a function of the number of excitons in a single experimental run. Detailed theoretical models of the Auger-recombination process specific to Ge islands in Si are not available at present, but would be very desirable.
The Ge islands were prepared by molecular beam epitaxy leading to a crystalline structure of both islands and surrounding silicon. The islands have a diameter of around 20 nm and a height of a few nm (see the dark areas in the top image – left). The optical excitation of electron-hole pairs was exploiting a frequency-doubled Ti:sapphire laser, which was mode-locked to obtain pulses of duration 100 fs. The fluorescence-detection apparatus limited the temporal resolution to 1 ns.
More information can be found in the journal Nanotechnology.
About the author
All authors are employed at the Department of Physics and Astronomy, Aarhus University. Brian Julsgaard, postdoc, is experienced in time-resolved fluorescence techniques in semiconductor nanostructures; Peter Balling, associate professor, contributes with ultra-fast laser techniques; John Lundsgaard Hansen, research technician, undertakes the actual sample fabrication; Axel Svane, associate professor, works in the field of solid-state theory; Arne Nylandsted Larsen, professor, is experienced with group-IV molecular beam epitaxy and nano-island formation.