E g ¼ 5.5 eV, silicon with a ¼ 5.43 Å and E g ¼ 1.1 eV, germanium with a ¼ 5.66 Å and E g ¼ 0.7 eV, and Sn with a ¼ 6.49 Å and zero band gap. For the formation of a solid solution, the Hume-Rothery rules [1] state that all elements in the alloy should have the same crystal structure and the same valency, which all four have (Sn only up to a temperature of 286 K), crystallizing in the diamond structure with four valence electrons. They also need to have similar electronegativities, which is true with values of 1.90 for Si, 2.01 for Ge, and 1.96 for Sn, while C's electronegativity with 2.55 is much larger. Finally, the atomic radii should not differ by more than 15%. While C is already 34% smaller than the second-smallest element Si and in combination with its large electronegativity is not expected to form stable solid solutions, we find for the other combinations that Ge is 4.2% larger than Si, Sn is 14.7% larger than Ge, and Sn is 19.5% larger than Si. While the size difference for Sn-Ge is just at the borderline and indicates possible, but not effort-free alloying within at least some solubility range, the situation for Sn-Si looks more difficult. Indeed, looking at the equilibrium phase diagrams, the maximum solubility of Sn in Si is 0.1% [2] more than an order of magnitude smaller than that of Sn in Ge, [3] while Si and Ge have an isomorphous phase diagram with solubility over the entire composition range. [4] The first system realized due to the similar properties of its constituents was the well-behaved Si 1Àx Ge x with full miscibility over the entire composition range. In SiGe alloys, the dependence of the bandgap on the Ge concentration is well known for more than 60 years, where the conduction band minimum changes from the Δ-point on the Si-rich side to the L-point above 85% Ge with a sharp kink at the transition composition. [5] Also, the lattice constant follows closely Vegard's Law with only a small bowing deviation. [6] SiGe was introduced into the semiconductor device market in the 1990s by IBM and has by now become a staple for heterojunction bipolar transistors or as a straininducing layer for CMOS transistors.SiGeC (and to a lesser degree SiC) alloys were closely following SiGe and heavily researched in the 1990s to realize stress-compensated alloys with bandgaps larger than Si. It started with Soref 's conjecture that the bandgap of these alloys might follow a linear/exponential interpolation of the bandgaps of the elemental endpoints. [7] Thus, the bandgap of diamond, with 5.5 eV much larger than those of Ge (0.7 eV) and Si (1.1 eV) was