Search Results

This chapter discusses the roles of phonon-assisted tunnelling and, to a lesser extent, phonon scattering, in two related types of low-dimensional semiconducting structures: resonant tunnelling devices and superlattices (phonon-assisted tunnelling effects in quantum dots are discussed in Chapter 4). Phonon-assisted tunnelling describes the process in which electron or hole tunnelling is accompanied by the emission or absorption of a phonon. Phonon-assisted tunnelling by longitudinal optic phonons gives rise to satellite lines in the I(V) characteristic of the device. However, phonon-assisted tunnelling by acoustic phonons can only be seen as a change in I(V) produced by a change in the phonon occupation number, such as that resulting from an incident heat pulse. The chapter also includes brief descriptions of work on coherent phonon generation from superlattices, on the effect of surface acoustic waves on single electron transport in quantum wires, and on the role of phonon-assisted tunnelling in quantum cascade lasers.

This chapter examines the interaction between an electron, conceived to be in a particular state described by a wave function, and a lattice vibration, described by a phonon eigenstate. Topics discussed include the adiabatic principle, interaction terms, matrix element for electron-phonon scattering, rigid approximation, electron-lattice interaction in metals, deformation potentials in semiconductors, interaction of carriers with optical modes, and interaction with polar modes.

This chapter describes the main experimental techniques used to measure the drift velocity in superfluid ⁴He at low temperature. The experimental results are then presented by showing the contributions to the ion drag due to the different elementary excitations of the superfluid. The theoretical description of the processes of ion scattering off phonons, rotons, and ³He atomic impurities is also presented, and the theoretical predictions are compared with experimental results. The use of the formalism of the Boltzmann transport equation to predict how the drag force on an ion in the superfluid is determined by the different scattering mechanisms is discussed.

This chapter discusses major scattering processes found in semiconductors, including phonon scattering (deformation scattering, piezoelectric scattering, polar scattering and non-polar scattering), scattering arising from impurities (charged, so a Coulomb scattering, and charge neutral) and scattering arising in compositional randomness, from carrier-carrier events and due to coupled-particle interactions. The discussion starts by making connections between the classical scattering cross-section and its quantum-mechanical origins through the matrix elements for scattering. The ability to write the matrix element is employed for describing scattering by phonons in its various forms, for impurities and their various levels of accuracy of the description. Umklapp processes are described. When multiple scattering processes are present, the resulting transport manifests the processes’ independence and dependence. With an understanding of the scattering, observed behavior in semiconductors of interest is summarized to show their relative importance. The chapter concludes by discussing frequency and high field behavior manifested by electron ensembles.

This chapter analyzes intrasubband scattering processes, namely spontaneous emission, phonon scattering, and elastic scattering. It then discusses experimental measurements of intersubband lifetime using the following techniques: interband pump and probe, intersubband pump and probe, intersubband saturation experiments, and intersubband electroluminescence.

This chapter discusses remote processes that influence electron transport and manifest themselves in a variety of properties of interest. Coulomb and phonon-based interactions have appeared in many discussions in the text. Coulomb interactions can be short range or long range, but phonons have been treated as a local effect. At the nanoscale, the remote aspects of these interactions can become significant. An off-equilibrium distribution of phonons, in the limit of low scattering, will lead to the breakdown of the local description of phonon-electron coupling. Phonons can drag electrons, and electrons can drag phonons. Soft phonons—high permittivity—can cause stronger electron-electron interactions. So, plasmon scattering can become significant. Remote phonon scattering too becomes important. These and other such changes are discussed, together with phonon drag’s consequences for the Seebeck effect, as illustrated through the coupled Boltzmann transport equation. The importance of the zT coefficient for characterizing thermoelectric capabilities is stressed.

This chapter describes the most commonly used approaches for computing the band structure of active materials with intersubband optical transitions. The physics of quantum cascade lasers (QCLs) is discussed in detail, including the mechanisms that limit the threshold current density, threshold voltage, wall-plug efficiency, and temperature sensitivity of state-of-the-art devices. The important roles of phonon and interface roughness scattering in determining threshold are emphasized. The chapter also compares the performance of QCLs to other mid-IR lasers in considerable detail and makes some conclusions as to which sources are preferred depending on the emission wavelength and application. Finally, the physical principles of laser-based frequency combs, including self-starting frequency-modulated QCL combs, are discussed.