A INTRODUCTION

Silicon-germanium (SiGe) alloys are very attractive for monolithic integration of Si-based photonic and high-speed electronic devices with state-of-the-art CMOS chips. Preserving almost 100% compatibility with mainstream Si technology, SiGe heterojunction bipolar transistors (HBTs) provide at least a doubling of the transistor speed [1]. Moreover, the SiGe alloy is a favoured candidate for optoelectronic applications as it can extend the detection wavelength range of Si, enabling the integration of waveguides, photodetectors and potentially also emitters. There are three key parameters of SiGe permitting an enhanced flexibility in material properties and device characteristics: alloy concentration, strain, and heterostructure design in order to confine carriers by reducing the layer thickness to form quantum wells. Si/SiGe quantum wells, wires and dots have attracted much attention because reduced dimensions lead to modifications in the band structure providing a post-growth possibility to realise optoelectronic as well as high-speed electronic devices.

Since visible light emission from porous silicon was observed in 1990 by Canham [2], which could be correlated to the creation of quantum wire-like structures with their modified electronic states, many different approaches have been tried to realise nanostructures. One attractive possibility was to overcome the poor light emission of the indirect gap semiconductors Si and Ge, which gave new hope of realising an all-optical Si-based chip system. A recent review on silicon-based optoelectronics has been published in [3].

The high quality epitaxial material grown at...