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We report on the growth of InAs Quantum dot stacks of various periods on silicon grown by molecular beam epitaxy. Quantum dot layers of InAs, separated by a very thin GaAs spacer layer, are grown directly on hydrogen terminated (100) Si surface. The dependence of dimensional distribution on the growth parameters like temperature and monolayer coverage is studied by atomic force microscopy. The effects of rapid thermal annealing on the stability of stacked structures are investigated by Raman scattering experiments. The morphological changes are characterized in terms of shifts in the longitudinal optic (LO) and transverse optic (TO) phonon modes of InAs and GaAs forming the structure. Post growth annealing has been found to lead to significant alloying of InAs and GaAs in the successive layers leading to the transformation of three-dimensional quantum dot structure to a two dimensional InGa1As like compositional alloy layer.

The development of Si-based photodetectors is very important due to their compatibility with the state-of-the-art Si planar technology. Photodetectors based on Ge quantum dots were studied. Three p-i-n structures containing Ge dots were grown by molecular beam epitaxy in Stranski-Krastanov mode. The dots were grown embedded in Si spacing layers on Si (100) substrates. The nominal Ge growth thickness in each layer was 1.2, 1.5 and 1.8 nm for the three samples, respectively. Photoluminescence measurement showed that the Ge dot related peak shift to lower energy with increasing the dot layer thickness. The materials were processed into p-i-n photodiodes with conventional processing methods. I-V measurement showed a low dark current density of 3x10 -5 A/cm 2 at -1 V. A strong photoresponse at 1.3-1.55 µm originating from Ge dots was observed. The response peak shifts with the Ge growth thickness. At normal incidence, an external quantum efficiency of 8% was achieved at 2.5 V. The dot layers were considered to trap the light in the intrinsic region, and thus increase the absorption.

Using photoreflectance spectroscopy, fifteen electronic transitions have been measured from a 60 period Si8Ge32 superlattice grown on a Si02Ge0•8 buffer layer on < 100 > Si. The superlattice transitions fit well to a third derivative functional form and most of their energies were determined using a one band envelope-function model, including strain effects. The temperature dependences of the E0 transition in bulk Ge and in the Si8Ge32 superlattice were also fit to a nonlinear functional form.

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