Silicon Photonics: Fundamentals and Devices

2.5 Alloys of Silicon and Other Group IV Elements

2.5.1 Different Alloy Systems

Electronic and optical properties of binary alloy of Si and Ge: Si_1−xGe_x were studied as early as 1958 by Braunstein et al. 7. The widespread use of heterojunctions made of III–V compounds and their binary, ternary, and quaternary alloys in electronic and photonic devices prompted workers to undertake studies of the properties of alloys of group IV elements and to make devices using these alloys grown on Si substrate. Electronic devices like heterojunction bipolar transistors and heterostructure field effect transistors using SiGe–Si combinations have already found commercial applications. Recently it has been found that the addition of C in SiGe alloy introduces a number of beneficial effects. A lot of work has recently been undertaken for tailoring the band gap or even for obtaining direct-gap Group IV material on Si substrate. The materials studied so far are binary alloys of Si, Ge, C, and Sn as well as ternary alloys of Si, Ge, C, and Sn.

In Sections 2.5.2 and 2.5.3, the basic properties of these alloys will be discussed.

2.5.2 Lattice Constants

Solid solutions of Si and Ge can be formed over the complete range of compositions, and the alloy Si_1−xGe_x is thermodynamically stable. Ordered compound SiGe (analogous to SiC) has not been reported. The lattice constant of the alloy SiGe obeys the relationship

(2.19)

where c is the bowing parameter = 25.6 × 10⁻³ Ǻ. In most of the calculations of the parameters ...