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.