This chapter is devoted to the role of impurities in semiconductor structures which consist of different types of semiconductor materials. Semiconductor heterostructures, quantum wells and superlattices are structures in which the individual layers have spatial dimensions comparable to the carrier de Broglie wavelength. As a consequence, quantum effects cannot be neglected in such small semiconductor structures. Furthermore, the spatial dimensions of such structures can be comparable to the Bohr radius of impurities. As a consequence, the characteristics of impurities, e.g. ionization energy and wave function, are changed. The area of quantum semiconductor physics has gained much interest since the 1970s. The physical properties of several semiconductor quantum structures are closely related to the doping and to residual impurities in such structures. Among these structures are selectively doped heterostructures, doping superlattices, doped quantum wells, and doped quantum barriers, which will be discussed in this chapter.

Selectively doped heterostructures

Selectively doped heterostructures are structures which consist of a doped widegap semiconductor and an undoped narrow-gap semiconductor. Selectively doped heterostructures were first realized by Stormer et al. (1978) and Dingle et al. (1978) in an attempt to reduce scattering of carriers by ionized impurities. The electron mobilities obtained in AlxGa1−xAs/GaAs heterostructures at low temperatures can exceed 107 cm2/V s (Pfeiffer et al., 1989a).