N. A. Bannov

Temperature dependence of electron mobility in a free — Standing quantum well

Abstract We have studied theoretically the low-field electron transport in a free-standing quantum well where the deformation potential scattering of electrons by acoustic phonons is the major mechanism of the electron relaxation. The quantization of acoustic phonons, their multisubband spectrum, and the exact form of the dilatational acoustic modes are taken into account. We have numerically solved the kinetic equation for electrons in the low electric field limit. At low lattice temperature the obtained electron distribution function has several peaks associated with scattering by different dilatational phonons. The electron mobility has temperature dependence similar to that described by the Bloch-Gruneisen formula, however we have obtained T −3 dependence of mobility in the low temperature region.

Electron momentum relaxation time and mobility in a free‐standing quantum well

Kinetic characteristics of the electron transport in a free‐standing quantum well are studied theoretically. The quantization of acoustic phonons in a free‐standing quantum well is taken into account and electron interactions with confined acoustic phonons through the deformation potential are treated rigorously. The kinetic equation for the electron distribution function is solved numerically for nondegenerate as well as degenerate electron gases and the electron momentum relaxation time and the electron mobility are obtained. At high lattice temperatures the electron momentum relaxation time is very similar to that obtained in the test particle approximation. Its dependence on the electron energy has steps which occur at the threshold energies for the dilatational phonons because an additional electron scattering by the corresponding acoustic phonon becomes important. The first mode makes the main contribution to the electron scattering, the contributions of the zeroth and the second modes are also impo...

Coupled electron and nonequilibrium optical phonon transport in a GaAs quantum well

The self-consistent Monte Carlo technique has been used to solve coupled nonlinear kinetic equations for electrons and optical phonons confined in a GaAs quantum well. We have studied the influence of nonequilibrium phonons on quasi-two-dimensional electron transport for a lattice temperature of 30 K and for a wide range of applied electric fields. A substantial difference in generation and decay times as well as the confinement inside the GaAs/AlAs heterostructure-bounded active region lead to a significant growth of nonequilibrium optical-phonon population generated by a heated electron gas. We have found that when the phonon generation (as well as phonon reabsorption by the quasi-two-dimensional carriers) becomes significant, there are substantial effects on transport in the quantum well. We show that for low electron concentrations, the hot optical-phonon distribution reflects the main features of the carrier distribution; indeed, it preserves an average quasi-momentum in the forward (opposite to elec...

Radiation patterns of acoustic phonons emitted by hot electrons in a quantum well

The acoustic phonon radiation patterns and acoustic phonon spectra due to electron–acoustic‐ phonon interaction in a double barrier quantum well have been investigated by solving both the kinetic equations for electrons and phonons. The acoustic phonon radiation patterns have strongly pronounced maximum in the directions close to the perpendicular to the quantum well direction. The radiation pattern anisotropy is explained in terms of possible electron transitions, electron distribution function, and the Hamiltonian of electron–phonon interaction. It was shown that, the simple assumption that emitted phonons always have a perpendicular wave‐vector component of the order of 2π/a, where a is the width of the quantum well, cannot explain the strong anisotropy of the radiation patterns. More detailed analysis is required and has been carried out. The emitted acoustic phonon spectra have maxima at energies 2πℏu/a, where u is the sound velocity.

Electron mobility and terahertz absorption due to interactions with confined acoustic phonons in a free-standing quantum well

Abstract Electron transport and optical parameters of a free-standing quantum well are studied theoretically. Electron interactions with photons and confined acoustic phonons through the deformation potential are treated rigorously. The quantum kinetic equation for the electron distribution function is solved numerically for nondegenerate as well as degenerate electron gases and the electron momentum relaxation time, the electron mobility and the absorption coefficient in tetrahertz range are obtained. Both the electron transport and optical parameters exhibit peculiarities, related to the confined acoustic phonon spectrum. They are displayed as peaks at the electron momentum relaxation time dependence on energy and at the second derivative of the absorption coefficient over photon energy. The positions of the peaks is associated with the energies of confined acoustic phonons.

Low‐frequency noise in the low dimensional semiconductor structures

We present here the results on electron transport and noise (diffusion) in low‐dimensional semiconductor structures obtained self‐consistently by the Monte Carlo technique. Electron scattering by confined and localized optical phonons in quasi‐one‐dimensional quantum wires and quasi‐two‐dimensional quantum wells is taken into account. In spite of the considerable differences in phonon spectra and their coupling with electrons, and in spite of electron quantization, the diffusivity (noise) and transport parameters in low‐dimensional structures only slightly differ from corresponding bulk material characteristics at room temperature. At low lattice temperatures the existence of several optical phonon modes in low‐dimensional structures causes electron streaming at several frequencies.

Hot Phonon Emission in a Double Barrier Heterostructure

Hot acoustic phonon emission represents one of the major channels for thermal energy removal from heterostructures; in addition, detection of phonons emitted by hot electrons provides a valuable tool for investigation of electron-phonon interactions in heterostructures.1 We have studied the acoustic phonon emission by hot electrons in double barrier heterostructures. We have solved the electron kinetic equation to obtain the electron distribution function, which has been used to solve the kinetic equation for phonons and to determine the radiation and absorption patterns for the energy carried by acoustic phonons. The radiation and absorption patterns have highly pronounced maxima inside the solid angle close to the normal to the quantum well direction (z-direction). These orientational dependencies are related to the quantum confinement of electrons and uncertainty in z-component of their wave vectors.

Multimode Regime of Hot Acoustic Phonon Propagation in Two Dimensional Layers

At present there is a considerable interest toward new type of nanostructures: freestanding quantum wells (FSQWs) and free-standing quantum wires (FSQWIs). An important peculiarity of free-standing structures is the quantization of acoustic phonons. The acoustic phonon quantization has been observed both in optical and electrical experiments.1,2 The acoustic phonon modes in free-standing structures and their interaction with electrons have been studied in a number of papers (see e.g. Refs.3–5 and references therein). Due to complicated spectrum of acoustic phonons, their propagation in FSQWs and FSQWIs differs significantly from propagation of bulk acoustic phonons. This is important for heat pulses propagation in such structures and for heat removal from free-standing structures.

LOCALIZED ACOUSTIC PHONONS IN LOW DIMENSIONAL STRUCTURES

Modern microfabrication techniques allow the creation of new free-standing quantum nanostructures which attract considerable attention and are studied by several research groups. These structures are, in fact, solid plates (slabs) or rods (bars) connected to a solid substrate by a side of the smallest cross-section. The major feature of free-standing structures is that the smallest dimensions of the structures may be as small as a few interatomic distances. This attribute gives rise to new interesting physical phenomena and opens new possibilities for applications. First of all, the electrons (holes) in these structures are quantized. In fact, free-standing structures represent waveguides for electron waves which have features substantially different from more conventional quantum structures. Such waveguides may have very high potential energy barriers for electrons, so new effects related to hot but quantized electrons are possible. The phonon subsystem will also undergo significant modification. In this paper we will present a review of the experimental and theoretical results of other authors as well as results of our own investigation on the quantization of the acoustic phonons in free-standing structures.