Group IV Strained Layer Systems

Abstract

Strain energy in lattice mismatched material systems causes the creation of a misfit dislocation network above a critical thickness and favours growth via three-dimensional nucleation. All group IV semiconductors (diamond, SiC, Si, Ge, Sn) may crystallize in the cubic diamond (zincblende) lattice, but the lattice spacings cover a wide range. Only the completely miscible Si/Ge system exhibits moderate lattice mismatch adjustable from zero to roughly 4% by the choise of the right alloy composition. This alloy is often considered as a model system for strained layer configurations because of the rather low influence of chemistry. At equilibrium only a very limited range of thickness and strains can be realized without severe restrictions by dislocation generation and surface undulations. A frequent strategy for improvement of surface morphology and dislocation suppression utilizes kinetic effects at low growth temperatures. Segregation behaviour and crystal quality are also influenced by decreasing growth temperatures. Recent alternative approaches for improvement of surface morphology under equilibrium conditions and with the aid of surfactants are treated, and advantages/disadvantages of different strategies are compared. We consider the influence of growth parameters in more detail for strained and relaxed SiGe configurations. The impact of SiGe-heterostructure research on device performance is shown on the examples of the heterobipolartransistor, the quantum well field effect transistor, and optical properties of the ultrathin superlattices