Optical Properties of Semiconductors

In previous Chapters, we introduced the reader to the fundamental concepts of quantum mechanics, band structure and semiconductor physics. In this Chapter we have the opportunity to apply this acquired knowledge of the electronic structure of solids to understand the optical properties. We do this by modeling the optical response properties, in particular the permittivity of the solid. We present the formalism which allows one to calculate the permittivity, and then study how this permittivity affects the light penetrating the solid. We shall demonstrate how band structure and free electrons determine the permittivity, and therefore the way light propagates in a solid, and how much of this light gets absorbed. We shall investigate under what circumstances the lattice can couple to photons, and how this coupling can affect the velocity of light in a medium. But we shall see in the next Chapters that band structure depends on the dimensionality of the system, and we have already seen in Chapters 7 and 8 that carriers can be added or neutralized in semiconductors. So it turns out that just in the same way that the energy bands can be engineered, so can the optical properties. Atom by atom growth and miniaturization are modern key engineering tools, but so is the application of external electric and magnetic fields. In the last sections of this Chapter we therefore investigate how an electric or a magnetic field modifies the band structure, and how this reflects on the optical properties. The fundamental concepts developed in this Chapter are a necessary prerequisite to understand the way optical methods can be used to characterize the electronic structure of semiconductors as is described in Chapter 12.