Composition of self assembled Ge hut clusters

Determining the exact composition of self-assembled nanostructures is crucial for any fundamental investigation as well as for any potential application. Here, we present data on the interdiffusion behavior and segregation during growth of single and multiple layers of self assembled Ge hut clusters. Our results suggest that the critical thickness is reduced in upper island layers due to the strain fields of buried hut clusters in the underlying layers. Applying selective wet-chemical etching, we provide direct experimental evidence that the base of the Ge clusters intermixes with Si if hut clusters are stacked vertically. The inhomogeneous strain from the buried islands can explain the enhanced intermixing. Photoluminescence spectra for Si capped single and stacked hut cluster island layers are presented and we find that intermixing with Si during overgrowth is significant even in single layers of hut cluster islands. By reducing the substrate temperature during growth of the Si cap layer, the photoluminescence peak energy of the hut clusters is redshifted from about 800 to less than 640 meV. It is noticeable that this recombination energy is smaller than the fundamental bandgap of bulk Ge (0.74 eV) and is smaller than any emission energy yet detected from a pseudomorphic SiGe heterostructure grown on Si(001).