『Introduction』
For decades, advanced microelectronics has delivered, year by year, products with more functions and higher performance at the same costs. This has been achleved by continuous lateral shrinkage of monolithic integrated device dimensions and by relying on slmple material concepts with silicon as semiconductor, silicon oxides as dlelectrics and aluminium as interconnect metal. With the 100 nm length approaching, traditional trade-offs fail and we see a paradigm shift requiring sophisticated materials science from semiconductors to dielectrics and metals.

A few years after the inventlon of the bipolar transistor, the basic electronic semiconductor material changed from germanium to silicon. During that swltch around 1960, considerable interest was focused on bulk, unstrained SiGe alloys. Advanced epitaxy methods like molecular beam epitaxy or chemical vapour deposition have enabled the growih of high quality, thin, strained SiGe layers on Si substrates since around 1985. The availability of strained SiGe/Si structures strongly stimulated the research on silicon-based heterostructure devices resulting within a few years in the fastest silicon-based transistors and other very attractive options. However, it was only in 1998 when the volume production of the SiGe heterobipolar transistor (HBT) circuits for mobile communication started, that a broad public audience became aware ofthis new strained layer heterostructure material which is in the main not available in bulk form. The technology involved in applying this material system will spread to other traditional and novel device areas: carbon bandgap engineering, strain adjustment techniques, quantum confmement and self-assembling.

This book is based partly on a revised version of the EMIS Datareviews Series No. 12, Properties of Strained and Relaxed Silicon Germanium (INSPEC, IEE, London, 1995); but the dramatic increase of industrial relevance and the need to cover completely new subjects, such as carbon containing alloys, quantum size effects and self-assembling, forced a rlgorous revision.

The book is organised to meet three different demands of a reader. In Chapter I some general properties of strained layer sy-stems whlch need caution are summarised. The SiGe:C heterostructure can be considered as a model for the investigation of stress driven phenomena because of the chemical similarity ofthe materials involved, which minimises additional chemical effects.

The specific material data for strained and relaxed alloys of SiGe and SiGe:C are given in Chapters 2 to 6. Basically, the different propertles are given as functions of the parameters Ge content, x, and film strain, 8. To a frrst approximation, some properties, e.g. the elastic stiffness constants, can be considered as linear functions ofthe chemlcal composition (Vegard's law). Some properties, e.g. the lattice constants, vary monotonically, but not linearly with composition. Some other properties, e.g. the thermal conductivity, depend even non-monotonically on the chemical composition. Strain dependence can often be approximated by a linear law at least for a given sign of the strain (compressive or tensile). For a few known cases, but not in general, the temperature dependence of the properties is explicitly glven. In most cases the doping dependence of the alloy properties has not yet been explored. For the parent materials, see either Landolt-Bornstein, New Series, Group 111, Volume 17a (Springer-Verlag, Berlin, 1982) or Properties ofCrystalline Silicon, No. 20 in the EMIS Datareviews Series (INSPEC, IEE, London, 1999).

In Chapter 7 some device relevant structures are selected out from a much larger variety. Band offsets, doping effects, adjustment of strain In multiple layer structures, formation of quantum wells and superlattices should be demonstrated for a certain set of parameters. The heterobipolar transistor has already proved its importance for analog, mixed analog/digital and high frequency circuits. This success has paved the way to the consideration of other high impact device applications of thls silicon-based material system. Superior field effect transistors with symmetrical, high mobility n- and p-channels are a unique opportunity given by this material system. Higher refractive index, smaller bandgap and increased absorption are the Ingredients needed for silicon-based optoelectronics with optical waveguides, near-infrared receivers and integrated fast electronics.