Mid-infrared light absorption by photo-excited charge carriers in Ge/Si quantum dots

Mid-infrared optical absorption spectra of Ge/Si quantum dot structures in polarized light were obtained under conditions of additional interband optical excitation. Photo-excited electron-hole pairs captured to quantum dots cause mid-infrared absorption changes for certain light polarization in spectral range of 0.25 – 0.6 eV. The sign of the effect is found to be different in different spectral ranges. There is an increase of absorption at the long-wavelength edge of the spectrum and a decrease of absorption at the short-wavelength edge. Absorption increase is considered to be related to contribution of optical transitions of non-equilibrium holes from the quantum dot ground states, while absorption decrease is associated with suppression of interband-like processes with generation of hole inside the dot and bound electron outside of the dot under conditions of full occupation of appropriate discrete states.