Electromodulation of the interband and intraband absorption of Ge/Si self-assembled islands

We have investigated the interband and the intraband absorption properties of Ge/Si self-assembled islands. The investigated structure consists of a p–i–n junction containing Ge/Si self-assembled islands embedded in a Si 0.98 Ge 0.02 waveguiding layer. The variation of transmission associated with carrier injection under forward bias is monitored both in the near-infrared and in the mid-infrared spectral ranges. We show that the carrier injection leads to an absorption resonant at 185 meV which is polarized along the growth axis of the islands. This transition corresponds to an intraband optical transition from the island ground states to the two-dimensional wetting layer states. This assignment is supported by a two-dimensional band structure calculation performed in a 14 band k · p formalism. Meanwhile, the carrier injection leads to a bleaching of the interband absorption. We show that this electroabsorption spectroscopy is a useful tool for the study of self-assembled islands that is complementary of standard photoluminescence, electroluminescence or absorption spectroscopies.