Influence of Si(001) substrate misorientation on morphological and optical properties of Ge quantum dots

We have investigated the correlation between morphological and optical properties of Ge dots deposited by molecular-beam epitaxy on a Si(001) surface and on a high-index Si(118) vicinal surface. Ge islands were confined on the top of an undulated Si0.5Ge0.5 template layer according to the Stranski-Krastanov growth mode. Atomic force microscopy measurements reveal that the main effect of the vicinal substrate is to transform hut islands on a nominal (001) substrate into wire-shaped islands on (118) substrates. We have observed a direct correlation between the elongated shape and polarization anisotropy of optical transitions in island. The island photoluminescence (PL) emission is partially (similar to 25%) polarized for dots deposited on a (118) substrate. PL spectroscopy investigations as a function of temperature and excitation power are reported. The results show that the PL of islands strongly depends on the pump excitation power: it broadens and is blueshifted by 28 and 14 meV/decade for structures grown on (001) and (118), respectively, as the excitation power density increases. The significant blueshift is interpreted in terms of band bending in type II recombination. Moreover, a detailed analysis of (118) island PL band shows the presence of two main peaks that could be attributed to the different island morphologies clearly identified by transmission electron and atomic force microscopies: wire islands and dome islands. (c) 2005 American Institute of Physics.