Special features of the photoluminescence of self-assembled Ge(Si)/Si(001) islands grown on a strained Si1?x Gex layer

The effect of the predeposition of strained Si_{1 ? x} Ge_x layers (x ≤ 20%) on photoluminescence (PL) of self-assembled Ge(Si)/Si(001) islands is studied. A shift of the PL peak related to dome-shaped islands (domes) to lower energies, with respect to the PL peak related to pyramidal islands is observed; this shift is related to a much larger height of the domes compared to that of pyramids. It is found that, as the Ge content in the Si_{1 ? x} Ge_x layer (x) becomes higher than 0.1, two separate peaks appear in the broad PL band related to the islands; these peaks are attributed to the zero-phonon and phonon-assisted optical transitions in the islands. The appearance of these transitions is caused by a change of the TO-phonon type involved in radiative recombination: a TO_Ge-Ge phonon is replaced by a TO_Si-Ge phonon with a shorter wavelength.