The enormous progress of communication technologies in the last years has increased the demand for efficient and low-cost optoelectronic functions. For several present and future applications, photonic materials—in which light can be generated, guided, modulated, amplified, and detected—need to be integrated with standard electronic circuits in order to combine the information-processing capabilities of electronics data transfer and the speed of light. Long-distance communications, local-area-networks data transfer, and chip-to-chip or even intrachip optical communications all require the development of efficient optical functions and their integration with state-of-the-art electronic functions. Silicon is the material of choice for reliable and low-cost optoelectronic integrated circuits because it is the leading semiconductor in the electronic arena and since a wellestablished processing technology exists for this material. However Si is characterized by an indirect bandgap and by a weak electro-optic effect. It is therefore not suitable for the implementation of fundamental optical functions such as light emission and modulation. At the moment, hybrid integration of compound-semiconductor optical functions with Si electronic functions is providing the gateway from electronic to photonic technology. However several strategies are being considered to engineer the optical functions of Si and to realize fully Si-based or at least Si-compatible optoelectronics.

Studies by Raman spectroscopy of two kinds of Si/SiGe nanostructures-quantum dot multilayers and planar superlattices-reveal a wide variety of spectral features including first-and second-order Raman scattering, polarized Raman scattering, and low-frequency inelastic light scattering associated with folded acoustic phonons. Here we overview how such features can be employed to semi-quantitatively analyze the strain, chemical composition, and thermal conductivity in these industrially important materials that are widely used for producing electronic and optoelectronic devices.

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Dense nanowire films can be considered as quasi-porous structures with a high surface-to-volume ratio. From this perspective, structural, optical and electrical properties of axial Si-Ge nanowire heterojunctions produced by the vapor-liquid-solid growth method using Au nanoclusters as catalysts are analyzed. The lattice mismatch induced strain is partially relieved due to spontaneous SiGe intermixing at the heterointerface and lateral expansion of the Ge segment of the nanowire. The mismatch in Ge and Si coefficients of thermal expansion and low thermal conductivity of Si-Ge nanowire heterojunctions are found to be responsible for the thermally induced stress detected under intense laser radiation in photoluminescence and Raman scattering measurements. In electrical measurements, the observed non-linear current-voltage characteristics, strong flicker noise, and damped current oscillations with frequencies of 20-30 MHz are explained using a proposed Si-Ge nanowire heterojunction energy band diagram that includes energy states associated with structural imperfections at the nanowire surface.

The nature of the interfaces between SiGe nanostructures (NSs) and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one, two, and three dimensions. The interface sharpness is influenced by many factors including growth conditions, strain, and thermal processing, which can make it difficult to attain the desired structures. This is certainly the case for NS confinement in one dimension. However, axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50–120 nm produce a strong PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, experiments show that two quite different SiGe NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic non-exponential decay time that is remarkably shorter (as much as 1000 times) than that found in conventional Si/SiGe NSs.