Electroabsorption spectroscopy of Ge/Si self-assembled islands

We have performed an electroabsorption spectroscopy of Ge/Si self-assembled islands simultaneously in the near-infrared and in the midinfrared spectral range. The investigated structure consists of self-assembled Ge/Si islands embedded in a p-i-n junction. This active region is inserted into a 3-mu m-thick Si0.98Ge0.02 waveguiding layer. Under a positive applied bias, the injected carriers give rise to a current-induced absorption resonant at 185 meV along with an enhanced transmission around 800 meV. The 185-meV resonance is polarized along the growth axis of the islands. The assignment of the optical transitions is made on the basis of a 14-band quantum well k.p calculation. We show that the midinfrared electroabsorption of the islands is associated with a bound-to-continuum transition between the ground states and the wetting layer states. The enhancement of the transmission is correlated to the bleaching of the interband absorption which results from hole injection in the islands. The carrier density and the parameters governing the carrier dynamics in the islands are deduced from the midinfrared modulation amplitude. An Auger recombination coefficient in the islands, C=1.6x10(-30) cm(6) s(-1), is deduced at room temperature. (C) 2005 American Institute of Physics.