Structure, properties and applications of GexSi1-x strained layers and superlattices
The first successful pseudomorphic GexSi1-x strained layers on Si were grown in Germany in 1975. The extensive work that has been done on the production, properties and application of GexSi1-x strained layers and superlattices over the last fifteen years is described in this review. Values of critical layer thickness of the strained layers predicted by equilibrium theories are discussed and compared with experimental results. For x<0.6 the observed values are much larger than the theoretical values. The possible causes of this discrepancy are investigated and it is concluded that the layers must be metastable due to an energy barrier for the production of dislocations. Critical layer thicknesses of superlattices are also discussed and symmetrically strained superlattices are described. Results of band structure calculations of the strained layers and superlattices are given. Strain causes a large reduction in the fundamental indirect band gap of the Si/Ge alloy. Calculated band line-ups at the strained layer-substrate interface are compared with experiment. Modifications in the band structure due to strain have considerable effect on the optical and transport properties of the strained layers. Observed luminescence, photoconductivity and mobility are compared with predictions, taking strain into account. In the case of superlattices, band structure is further modified by zone folding. Superlattice structures which can produce a direct band gap are discussed. Raman scattering has proved a valuable tool for characterizing strained layers and superlattices. In the case of GexSi1-x strained layers, the Raman spectrum shows phonon lines shifted by strain. In the case of superlattices, additional lines due to zone folded LA phonons are observed. Raman measurements may be used to determine strain, thickness of a period and quality of interfaces. This review places emphasis on the use of Si/Ge strained in layers in Si-technology-based integrated circuits. The devices which have been fabricated to exploit properties of these layers include double heterostructure bipolar transistors, optical detectors, modulation doped field-effect transistors and mixed tunnelling avalanche transit time diodes. Progress made in the last 18 months has been rapid; a transistor with a cut-off frequency of 75 GHz was recently fabricated and this device has considerable promise.