Effect of Quantum Confinement on Electrons and Phonons in Semiconductors

Abstract

In Chap. 5 we studied the Gunn effect as an example of negative differential resistance (NDR). This effect is observed in semiconductors, such as GaAs, whose conduction band structure satisfies a special condition, namely, the existence of higher conduction minima separated from the band edge by about 0.2–0.4 eV. As a way of achieving this condition in any semiconductor, Esaki and Tsu proposed in 1970 [9.1] the fabrication of an artificial periodic structure consisting of alternate layers of two dissimilar semiconductors with layer thicknesses of the order of nanometers. They called this synthetic structure a superlattice. They suggested that the artificial periodicity would fold the Brillouin zone into smaller Brillouin zones or “mini-zones” and therefore create higher conduction band minima with the requisite energies for Gunn oscillations. With the development of sophisticated growth techniques such as molecular beam epitaxy (MBE) and metal–organic chemical vapor deposition (MOCVD) discussed in Sect. 1.2, it is now possible to fabricate the superlattices (to be abbreviated as SLs) envisioned by Esaki and Tsu [9.1]. In fact, many other kinds of nanometer scale semiconductor structures (often abbreviated as nanostructures) have since been grown besides the SLs. A SL is only one example of a planar or two-dimensional nanostructure. Another example is the quantum well (often shortened to QW). These terms were introduced in Sects. 1.2 and 7.1.5 but have not yet been discussed in detail. The purpose of this chapter is to study the electronic and vibrational properties of these two-dimensional nanostructures.