Low-dimensional semiconductors

Abstract

Properties of materials depend on size, in addition to the physical properties of their bulk listed in Part C of this volume. Below a value, typically in the nanometer range, effects of quantization become dominant, and periodicities lead to further confinement effects. Such dependence is widely applied to control electronic, magnetic, or other properties just by tailoring the size and shape of a given material; furthermore, materials structured on the nanoscale may have characteristic mechanical properties. The previous chapter (Chap. 28 ) provided a survey of the wide field of nanomaterials and their applications. This chapter focuses on semiconductors, whose electronic and optical properties are commonly classified in terms of dimensionality, according to the number of spatial directions in which the size or structural patterns are smaller than some specific limit. The effects of electronic and optical confinement are discussed, emphasizing the basic principles and selecting only a few widely applied materials. Section 29.1 addresses electronic confinement, covering the changes of the electronic density of states in reduced dimensions and the altered binding energy of confined excitons. Examples are given for the two-dimensional (2-D) structures of quantum wells and superlattices, for one-dimensional (2-D) quantum wires, and for zero-dimensional (0-D) quantum dots. Control of optical modes is considered in Sect. 29.2. Periodic structures with a photonic bandgap in one to three dimensions are considered, and the effect of optical defects used to fabricate waveguides and resonant cavities is pointed out. Finally, this chapter covers metamaterials comprising repeat units, which create magnetic and electric resonances and allow a negative real refractive index in this frequency range.