Phonons and phonon-related effects in prospective nanoscale semiconductor devices

Abstract

The research was devoted to the theoretical investigation of lattice vibrations in low-dimensional heterostructures and bulk materials with strong polaronic coupling. The purpose of the research has been to develop the phonon theory for technologically-important materials such as nitrides of Ga and Al as well as to locate new phonon-related effects which can be utilized in artificially-created heterostructures. The electron-phonon interaction has been considered quantum mechanically.The main findings can be summarized briefly as follows: 1. Consideration of carrier-induced renormalization of acoustic phonon spectra in quantum wires revealed the possibility for the Peierls phase transition into a state with periodic lattice distortion and charge-density waves of macroscopic period in artificially-prepared structures. The phase diagram for this transition has been determined. An analytical dispersion relation for the coupled electron-phonon excitation has been derived.2. It is found that the drift of two-dimensional electrons in quantum wells can lead to efficient amplification (generation) of sub-THz coherent confined acoustic vibrations due to the Cerenkov effect when the velocity of the drifted electrons exceeds the sound velocity in the given medium. A theory has been developed to describe the confinement of acoustic modes propagating along the high-symmetry directions in cubic quantum wells.3. A theory of confinement of optical phonon modes in wurtzite quantum wells has been developed. A formalism has been derived for calculation of electron scattering rates in optically anisotropic (uniaxial) crystals and quantum wells. 4. From the comparison of the energy losses to the lattice as function of the carrier velocity obtained in frameworks of perturbative model and path-integral Thornber-Feynman approach it is found that perturbation theory can be applied for materials with intermediate polaronic coupling such a GaN and AlN. Moreover, the theoretical possibility of unique low-field runaway transport in these materials has been demonstrated.