Energy losses of 2D electron gas due to near-surface acoustic phonon scattering

We have shown that for quantum wells placed close to the stress-free surface of the semiconductor heterostructure, the energy relaxation rate of two-dimensional electrons interacting with acoustic phonons at low temperatures (Bloch–Grüneisen regime) is changed considerably in comparison with that of a two-dimensional electron gas placed in a bulk of semiconductor. The relaxation rate is enhanced in the case of a semiconductor–vacuum system and is suppressed in the case of the surface covered by a thin metal film. The enhanced energy loss is caused by additional scattering at localized and reflected acoustic waves, and the decrease appears due to suppression of piezoelectric scattering in the vicinity of the metal.     

An electrohydrodynamics model for non-equilibrium electron and phonon transport in metal films after ultra-short pulse laser heating

The electrons and phonons in metal films after ultra-short pulse laser heating are in highly non-equilibrium states not only between the electrons and the phonons but also within the electrons. An electrohydrodynamics model consisting of the balance equations of electron density, energy density of electrons, and energy density of phonons is derived from the coupled non-equilibrium electron and phonon Boltzmann transport equations to study the nonlinear thermal transport by considering the electron density fluctuation and the transient electric current in metal films, after ultra-short pulse laser heating. The temperature evolution is calculated by the coupled electron and phonon Boltzmann transport equations, the electrohydrodynamics model derived in this work, and the two-temperature model. Different laser pulse durations, film thicknesses, and laser fluences are considered. We find that the two-temperature model overestimates the electron temperature at the front surface of the film and underestimates the damage threshold when the nonlinear thermal transport of electrons is important. The electrohydrodynamics model proposed in this work could be a more accurate prediction tool to study the non-equilibrium electron and phonon transport process than the two-temperature model and it is much easier to be solved than the Boltzmann transport equations.     

Optical and intervalley scattering in quantized inversion layers in semiconductors

The momentum relaxation time for scattering of electrons in quantized levels of an inversion layer on a semiconductor surface is calculated for interactions via optical and intervalley phonons. A selection rule is found which prohibits transitions between subbands belonging to the same valley or set of valleys, at least in the zero order to which these scattering processes may occur. Relaxation times for the zero-order interaction and the first-order interaction are obtained for intervalley phonons. The results are applied to the case of a (100)-silicon surface, with electrons in the three lowest subbands (with energy levels E₀, E₁, E'₀) of the two sets of valleys. Agreement with the experimental data of Fang and Fowler is good when the combined effects of intervalley and acoustic scattering are considered.     

Metal‐nanoparticle‐coating‐induced enhancement and weakening of resonant Raman scattering in ZnO: effect of surface electric field

Effects of Ag and Ti nanoparticle coatings on resonant Raman scattering in various ZnO thin films are presented. The longitudinal optical (LO) phonons, irrespective of the ZnO quality, exhibit an enhancement and a weakening by the Ag and Ti nanoparticle coatings, respectively. The enhancement (weakening) is always accompanied by a reduced (an increased) intensity ratio of the second to first‐order LO phonons, which can be associated with changes in the electron‐phonon coupling strength in the probed area of ZnO. Angle‐resolved X‐ray photoelectron spectroscopy provides evidence for the bending of the surface energy bands and their changes induced by the metal coatings. The effect of metal nanoparticle coatings on the Raman scattering of ZnO is thus attributed to the changes in the surface electric field. Copyright © 2011 John Wiley & Sons, Ltd.     

极性半导体的表面电子

有不少的极性半导休,电子与表面声学声子和体纵光学声子的耦合弱,但与表面光学声手的耦台强.本文同时考虑体纵光学声子、表面光学声子以及表面声学声予的影响,研究这类半导体的表面电予的性质,采用线性组合算符和拉格朗日乘子法,导出其有效哈密顿量和重正化质量。     

Long-range order in quasi-one-dimensional conductors

We study the formation of the charge-density wave long-range order in a system of repulsive 1D electrons coupled to 3D phonons. We show that the charge-density wave can be stabilized by interaction with phonons in quasi-1D crystals and semiconducting nanowires. In the case of metallic atomic chains, interaction with phonons of a 3D substrate is not enough, and violation of the translational invariance by commensurable perturbation or disorder is needed. The possibility of stabilization of superconductivity in 1D electrons with attraction by means of tunnel coupling to a 3D metal is considered.     

Mechanism of strong resonant 1LO Raman scattering

The interaction of electrons with impurities and the quasi-elastic scattering of electrons by acoustic phonons highly enhances the efficiency of resonant 1 LO Raman scattering. As a result, for a wide range of parameter values the efficiency of resonant scattering becomes rather strong and does not depend on the strength of interaction of electrons with the impurities and acoustic phonons, and on the impurity concentration.     

Electron-phonon interaction in impure metals

Making use of the theory for the phonon damping in an impure metal developed recently by Eisenriegler, we study the interaction between electrons and long wave-length longitudinal phonons, and its effects on electronic properties at low temperatures. On account of the inelastic scattering of electrons by impurity ions, a minimum in the electrical resistivity is found when plottedvs temperature, although its value is extremely small.     

The evaluation of the effect of piezo-electric scattering on the cyclotron resonance half-width in weakly polar semiconductors

Following the method given by Srinivas et al., the scattering of electrons on acoustic phonons and its effect on the cyclotron resonance width in CdS and CdTe is calculated. Two mechanisms: scattering by means of a deformation potential and through piezo-electric coupling are considered; the importance of the latter is stressed.     

Convoy electrons produced at glancing angle scattering of MeV HeH+ ions from a crystal surface

Abstract

Convoy electrons produced at glancing angle scattering of MeV HeH⁺ ions from an atomically clean (001) surface of SnTe crystal are observed. Energy spectrum of the convoy electrons shows a peak broader than that at scattering of atomic projectiles and the most probable energy of convoy electrons at HeH⁺ scattering is larger than those at scattering of isotachic He ions. This acceleration of convoy electrons is qualitatively explained by the force due to surface wake induced by Coulomb exploding fragment He²⁺ and H⁺.     

Intensity of intersubband electron scattering by polar optical phonons in semiconductor quantum wires with account of energy level broadening

An approach describing the effect of energy level broadening in semiconductor quantum wires on the intensity of intersubband electron scattering by polar optical phonons is suggested. As a broadening mechanism, scattering by atomic thermal vibrations and by the roughness of the confining surfaces of a quantum system is considered. It is shown that in this case the dependence of the intensity of intersubband scattering of electrons on their kinetic energy has no singularities.     

Surface-enhanced Raman spectroscopy of semiconductor nanostructures

We review our recent results concerning surface-enhanced Raman scattering (SERS) by confined optical and surface optical phonons in semiconductor nanostructures including CdS, CuS, GaN, and ZnO nanocrystals, GaN and ZnO nanorods, and AlN nanowires. Enhancement of Raman scattering by confined optical phonons as well as appearance of new Raman modes with the frequencies different from those in ZnO bulk attributed to surface optical modes is observed in a series of nanostructures having different morphology located in the vicinity of metal nanoclusters (Ag, Au, and Pt). Assignment of surface optical modes is based on calculations performed in the frame of the dielectric continuum model. It is established that SERS by phonons has a resonant character. A maximal enhancement by optical phonons as high as 730 is achieved for CdS nanocrystals in double resonant conditions at the coincidence of laser energy with that of electronic transitions in semiconductor nanocrystals and localized surface plasmon resonance in metal nanoclusters. Even a higher enhancement is observed for SERS by surface optical modes in ZnO nanocrystals (above 10⁴). Surface enhanced Raman scattering is used for studying phonon spectrum in nanocrystal ensembles with an ultra-low areal density on metal plasmonic nanostructures.     

Peripheral phonons and phonon conductivity of a metal at low temperatures

The role of the peripheral and non-peripheral phonons in the estimation of the lattice thermal conductivity of a metal has been studied at low temperatures by calculating their separate contributions towards the total lattice thermal conductivity. The study is made in the temperature range 0.4–2.5 K with the help of the Ziman expression for the scattering of phonons by the charge carriers and the Callaway expression of the phonon conductivity, and Sb is taken as an example. The separate percentage contributions due to peripheral and non-peripheral phonons have also been studied and it is found that the percentage contribution due to peripheral phonons increases with increasing temperature while the percentage contribution due to non-peripheral phonons decreases with increasing temperature. The percentage contributions of the lattice thermal resistivities due to electrons and holes towards the total lattice thermal resistivity of Sb have also been reported in the present note.     

Galvanomagnetic phenomena in semiconductor with a superlattice in the case of inelastic electron scattering

Galvanomagnetic phenomena in semiconductor with a superlattice in static electrical and magnetic fields in the case of strongly inelastic scattering of electrons by Optical phonons are considered.The existance of additional threshold perculiarities arising in magnetic field—current characterstics due to the finite width of conduction band is predicted.     

Hydrostatic pressure effect on the electron mobility in a ZnSe/Zn1－xdx Se strained heterojunction

With a memory function approach, this paper investigates the electronic mobility parallel to the interface in a ZnSe/Zn1-xCdxSe strained heterojunction under hydrostatic pressure by considering the intersubband and intrasubband scattering from the optical phonon modes. A triangular potential approximation is adopted to simplify the potential of the conduction band bending in the channel side and the electronic penetrating into the barrier is considered by a finite interface potential in the adopted model. The numerical results with and without strain effect are compared and analysed. Meanwhile, the properties of electronic mobility under pressure versus temperature, Cd concentration and electronic density are also given and discussed, respectively. It shows that the strain effect lowers the mobility of electrons while the hydrostatic pressure effect is more obvious to decrease the mobility. The contribution induced by the longitudinal optical phonons in the channel side is dominant to decide the mobility. Compared with the intrasubband scattering it finds that the effect of intersubband scattering is also important for the studied material.     

Electron energy losses by excitation of transition radiation

Inelastic scattering of KeV electrons in thin films with energy losses corresponding to the infrared spectral region is discussed taking retardation into account, that is allowing for radiative modes. For sufficiently thin target films Cerenkov radiation does not play any role. Rather it turned out even for nonrelativistic electrons, that in the spectral region of negligible absorption, where for example excitation of phonons or surface phonons does not occur, the energy loss spectrum is exclusively due to the creation of transition radiation. By means of high resolution spectroscopy of electrons inelastically scattered from TiO₂ films it is shown experimentally that this conclusion is indeed correct.     

Theory of surface polarons

We investigate the possibility that an electron may be trapped at the surface of an ionic crystal by the electron-surface optical phonon interaction. For a model Hamiltonian used earlier in a discussion of the inelastic scattering of low energy electrons by surface optical phonons, we find that for all values of the polaron coupling constant α, this trapping does occur. We refer to these surface states as surface polaron states. By the use of a variational procedure, we calculate the surface polaron binding energy and effective mass as a function of α. The present treatment does not include the coupling of the electron to bulk LO phonons.     

Effective interaction of electrons in quantum wells

Effects of electron-phonon interaction on the interaction between electrons in semiconductor quantum wells are considered. It is found that the direct Coulomb potential between electrons in a quantum well is smaller than that in bulk semicondutors. The antisymmetric modes of the confined bulk phonons and interface phonons have no contribution to the effective interaction of electrons. If a well is narrow enough, the effective interaction between electrons caused by interaction with interface phonons may exceed that by interaction with confined bulk phonons. In narrower wells the effective interaction potential of electrons produced by phonons is stronger, but decreases rapidly with increasing distance between electrons.     

Scattering of electrons in the GaAs/AlAs transistor structure

This paper reports on the results of a self-consistent calculation of the rates of electron scattering from surface roughnesses, acoustic phonons, and polar optical phonons in a transistor structure based on a GaAs quantum wire in an AlAs matrix at temperatures T = 77 and 300 K. The rates of electron scattering are calculated in the electric-quantum limit approximation with due regard for both the collisional broadening of the electron energy spectrum and the Pauli principle. The influence of the gate voltage on these rates is investigated. The wave function of electrons and the energy level of their quantum ground state are determined by the self-consistent solution of the Poisson and Schrödinger equations.     

Transient magnetoresponse of photoexcited electrons: Negative cyclotron absorption and evolution of Faraday angle

The transient magnetooptical response of electrons with partly inverted initial distribution produced by an ultrashort optical pulse near the optical phonon energy is studied theoretically. Transient cyclotron absorption and Faraday rotation of polarization plane are considered for bulk semiconductors (GaAs, InAs, and InSb) as well as for a GaAs-based quantum well. Damping of the response due to electron momentum relaxation associated with elastic scattering from acoustic phonons is taken into account in calculations, as well as the evolution of the electron distribution due to quasi-elastic energy relaxation at acoustic phonons and effective inelastic transitions accompanied by spontaneous emission of optical phonons. Nonstationary negative absorption in the cyclotron resonance conditions and peculiarities of Faraday rotation of the polarization plane associated with partial inversion of the initial distribution are considered. The possibility of transient enhancement of the probe field under cyclotron resonance conditions is indicated.