Intraband absorption in Ge/Si self-assembled quantum dots

We have observed intraband absorption in Ge/Si self-assembled quantum dots. The self-assembled quantum dots are grown at 550 degrees C by chemical vapor deposition. Atomic force microscopy shows that the quantum dots have a square-based pyramidal shape (approximate to 100 nm base length) and a density approximate to 2 x 10(9) cm(-2). Intraband absorption in the valence band is observed around 300 meV (4.2 mu m wavelength) using a photoinduced spectroscopy technique. The intraband absorption is in-plane polarized. It is attributed to bound-to-continuum transitions since the intraband energy corresponds to the energy difference between the Si band gap and the photoluminescence energy of the quantum dots. The magnitude of the intraband absorption saturates when the ground level of the quantum dots is filled. This feature allows the measurement of the in-plane absorption cross section of the intraband transition which is found as large as 2 x 10(-13) cm(2). (C) 1999 American Institute of Physics. [S0003-6951(99)01403- 5].