Silicon Photonics: Fundamentals and Devices

2.7 Si-Based Heterostructures

2.7.1 Lattice-Mismatched Heteroepitaxy

Most of the discussions in Section 6 were based on the direct-gap material GaAs and its alloy GaAlAs in which almost perfect lattice matching exists between the two materials.

Heterojunctions made by using lattice-mismatched pairs have been investigated over the last two or three decades, and their novel properties have been exploited to fabricate a large number of electronic and optoelectronic devices. Usually lattice mismatch between the two constituent semiconductors gives rise to strain. The strained layer epitaxy and strained heterojunctions merit an extensive discussion.

The heterostructures involving silicon and other group IV elements and their alloys invariably lead to strained heterostructures. We consider at present heterojunctions made of Si and its alloy Si_1−xGe_x.

The lattice constant of Si is 5.4311 Å and of Ge is 5.6579 Å. This large lattice mismatch, 4%, between the two allows the growth of Ge or the alloy SiGe on Si under restricted conditions. The layers are usually strained, and strain alters the band gap, heterojunctiion band alignment, and band structure in a very complicated way.

2.7.2 Pseudomorphic Growth and Critical Thickness

When an epitaxial layer having a larger lattice constant is deposited on a substrate having a lower lattice constant, usually misfit dislocations develop in the interface ...