New classes of Sn-containing group IV semiconductors are described. Novel CVD routes lead to growth of a broad range of Ge 1-ySn y alloys and compounds directly on Si substrates. The direct bandgap (E 0) and optical transitions E 0 + A 0, E 1, E 1 + Δ 1, E 0′, and E 2 of Ge 1-ySn y, exhibit strong nonlinearities in the compositional dependence, and their bowing parameters correlate with those in Ge 1-xSi x, suggesting a scaling behavior for the electronic properties. The Ge 1-ySn y films can be used as "virtual substrates" for the subsequent growth of Ge 1-x-ySi xSn y ternaries. These are created for the first time and exhibit unprecedented thermal stability, superior crystallinity and unique optical and strain properties such as adjustable bandgaps, and controllable strain states (compressive, relaxed, and tensile). The synthesis of Ge 1-x-ySi xSn y makes it possible to decouple strain and bandgap and adds new levels of flexibility to the design of group IV devices. The Ge-Si-Sn system also represents a new class of "designer" templates for the monolithic integration of III-V and II-VI semiconductors with Si electronics.