5 - Scattering processes in heterostructures

Summary

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Introduction

So far our study of quantum heterostructure devices in Chapter 4 has assumed that the devices were small compared to the mean free path of the electron. Transport in this type of device is referred to as ballistic transport. In real crystals the electron is always subjected to some type of scattering. In Chapter 6 we will study the impact of scattering on the electron wave-function and develop a simple three-dimensional quantum transport theory. In preparation for this analysis, we must first study the scattering mechanisms to which an electron is subjected.

Various scattering mechanisms exist in semiconductors. We will consider first scattering by the lattice vibrations, and, in particular, discuss polar, acoustic, and intervalley scattering. Next, we will turn our attention to scattering processes specific to heterostructures and discuss interface roughness scattering and alloy scattering. Finally, we will conclude this chapter with a discussion of electron–electron scattering.

Note that quantum heterostructures such as resonant tunneling diodes (RTDs), superlattices, and quantum wells (e.g., modulation doped field-effect transistors (MODFETs)) are usually undoped to minimize impurity scattering. Therefore impurity scattering is usually small compared to polar scattering and interface roughness scattering.

Phonons and phonon scattering

A crystal can be represented as a network of masses connected by springs. The masses are the atoms and the springs the covalent bonds between the atoms (see Figure 5.1).