Wannier–Mott Excitons in Semiconductors

This chapter discusses the concept of Wannier–Mott excitons in bulk and low-dimensional semiconductors, including the nonlinear effects in quantum wells. The low-temperature optical properties of pure semiconductor crystals and of their heterostructures are dominated, just below the band edge, by exciton absorption lines, which are typically arranged in a hydrogenic series. An envelope-function approach is well suited to describe Wannier–Mott excitons and quantum-confined structures. In the most relevant case of a quantum-well thickness L of the order of the Bohr radius, the motion of the electron and of the hole along the growth direction is separately quantized and the binding energy and oscillator strength of the quantum-well exciton are significantly increased compared to the bulk. At high excitation density, nonlinear effects come into play and the picture of excitons as ideal bosons is abandoned. The chapter discusses the correlation effects in semiconductor optics. For appropriate conditions, biexcitons—that is, the molecular states of two excitons bound together—may form and affect the optical properties. The possibility of the Bose–Einstein condensation of excitons is also discussed in the chapter.