Silicon-germanium alloys and heterostructures: Optical and electronic properties

The study and characterization of Ge-Si structures began shortly after the discovery and initial development of the transistor, motivated in part by fundamental curiosity and in part by the hope of improving the performance of semiconductor devices.1–5 Ge and Si are miscible, and alloy solid solutions can be prepared in bulk or by epitaxial growth.6 However, while Ge and Si share a common crystal structure, the diamond cubic lattice, they do not have the same lattice parameter. The lattice parameter of Ge-Si alloys obeys Vegard's law, varying linearly between 5.43 Å for Si and 5.65 Å for Ge.7 Consequently, Ge-Si alloys cannot be grown epitaxially on Si or Ge substrates without introducing large amounts of strain (see Figure 1).