Silicon has been the dominant semiconductor for electronic devices for more than three decades. However, there is now a growing need to integrate electronic components with optoelectronics for telecommunications and connections between computers. Electro-optical devices, such as light-emitting diodes, vertical-cavity surface-emitting lasers, and photodetectors based on combinations of III-V semiconductor structures with conjugated silicon substrates have been demonstrated.[1-4] Although III-V semiconductors provide high coupling efficiency and low loss in fiber-optical communications, incorporating them in the well-established Si technology is difficult and expensive. It is therefore desirable to grow the active materials directly on a Si substrate.Owing to their having a narrower bandgap than Si, Si 1-x Ge x /Si heterostructures are used to fabricate optical devices that extend the range of applications of Si technology. Si-Ge/Si superlattices, quantum wells, and Ge quantum dots (QDs) have been used in modulators and photodetectors. [5][6][7][8][9][10][11][12][13][14] Both experimental and theoretical investigations, including pyramid-shaped Si-Ge superlattice QDs, [15] Ge-island resonant-cavity-enhanced detectors, [16] photonic-band engineering in opals by growth of Si/Ge multilayer shells, [17] epitaxial growth of Ge nanowires on Si substrates, [18,19] tensile epitaxial Ge films on Si substrates, and relaxed Si-Ge/Si quantum well structures, [20,21] have been actively pursued.In the present study, taking advantage of the comparably low etching rate and uniform size of self-assembled Ge-QD multilayers, a method was developed to fabricate multilayered Ge/SiO 2 core/shell nanostructures with a lenticular shape (Ge@SiO 2 nanolenses) with excellent uniformity over a large area. The fabrication process is compatible with Si/Si-Gebased integrated-circuit technology. [14,22,23] The multilayered Ge@SiO 2 nanolenses exhibited an emission at between 1.5 and 1.55 lm in their photoluminescence (PL) spectrum, and an increase of about 77 % in reflectivity in their reflectance spectrum around 1.5 lm over conventional self-assembled Ge-QD multilayers. Figure 1a is a cross-sectional transmission electron microscopy (XTEM) image depicting the as-prepared self-assembled Ge-QD multilayer structure on Si (001). The as-grown samples consisted of 10 periods of 13.1 eq-ML Ge layers (eq-ML= equivalent monolayer) followed by a 20 nm thick Si-spacer layer. In this geometry, strain fields of underlying Ge islands penetrated into the Si-spacer layer, creating a strain-energy modulation at the surface, and inducing stacks of vertically aligned, and laterally more homogeneous, islands in the upper layers.[24] As the number of multilayers increased, the dot-size uniformity improved, and the average dot size increased. This gradual increase in size within QD multilayers can be attributed to the reduction of the wetting-layer thickness in the upper layers caused by the local strain fields. It was evident that the Ge islands were well aligned fo...

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