Chapter 11 - Concluding considerations

Summary

Now there is one outstandingly important fact regarding Spaceship Earth, and that is that no instruction book came with it.

R. Buckminster Fuller, Operating Manual for Spaceship Earth, 1969

Pervasive role of phonons in modern solid-state devices

As illustrated throughout this book, phonon effects are pervasive in modern solidstate devices. As is illustrated by the many examples for Chapters 7–10, the importance of these effects is usually at least as great for dimensionally confined structures as for bulk structures. Indeed, in Chapter 7 the effects of dimensional confinement were seen to be important even for biological structures! In this case, a cylindrical shell immersed in a fluid (Sirenko et al., 1996b) was used to model the vibrational behavior of microtubules (MTs) immersed in water. In addition, the examples of Chapters 7 and 9 illustrate that the elastic continuum model provides an accurate description of acoustic phonons in dimensionally confined structures of many geometries including thin films, nanowires with rectangular and circular cross sections, and a variety of dot-like structures. These structures will inevitably be pervasive as elements of nanoscale structures mimicking the well known and larger microelectromechanical structures. Indeed, Cleland and Roukes (1996) reported a technique for fabricating nanometer-scale mechanical structures from bulk, single-crystal Si substrates. As another example of acoustic phonon effects in dimensionally confined structures, it was recently predicted theoretically that Cerenkov-like effects lead to the generation of high-frequency confined acoustic phonons in quantum wells (Komirenko et al., 2000b); see Section 10.6.

In Chapter 8, values of carrier–optical-phonon scattering rates calculated for a variety of dimensionally confined semiconductor structures were found to exceed 10¹³ s^–1.