The energy relaxation due to electron-two short wave length transverse acoustic phonons in semiconductors

The energy relaxation time, due to the interaction of electrons with two short wave lenght acoustic phonons, is calculated in nondegenerate semiconductors and compared with an experiment in n-type InSb at low temperatures.     

Light scattering from surface acoustic phonons in metals and semiconductors

Several examples of Brillouin scattering from thermally excited surface acoustic phonons in metals and opaque semiconductors are presented. The measured surface velocities are consistently lower by 1–5% than the values calculated from the bulk elastic constants. The spectra also show the continuum of Lamb excitations having some surface-type character with velocities ranging between the bulk transverse velocity and the bulk longitudinal velocity.     

Effect of non-equilibrium acoustic phonons on radiative recombination in semiconductors

We report the first direct observation of radiative no-phonon and acoustic phonon-assisted transitions, determination of relative rates of radiative and non-radiative phonon-assisted decay for centres of various types in semiconductors. We also have suggested improvement of experimental approach to get a form of the distribution function for non-equilibrium acoustic phonons in crystals.     

Exciton energy relaxation on acoustic phonons in double-quantum-well structures

The paper analyzes the rate of energy relaxation involving acoustic phonon emission between exciton states in a double quantum well. A theoretical study is made of the part played by two mechanisms, one of which is a one-step transition with emission of an acoustic phonon and the other is a two-step transition, which includes elastic exciton scattering from interface nonuniformities followed by energy relaxation within an exciton subband. The rate of the two-step transition in real double quantum wells is shown to be higher than that of the one-step transition. As follows from calculations, the fast energy relaxation between exciton states is determined by the elastic scattering of phonons from the interface.     

Energy relaxation of an electron-hole plasma in semiconductors

We calculate the energy relaxation of an electron-hole plasma created by a short laser pulse in semiconductors like Si and GaAs in two cases: (i) when the carrier-carrier collision time is much shorter than the carrier-phonon one, so that a carrier temperature T_c exists. We give the variation of T_c with time; (ii) when there is no carrier temperature and the initial energy distribution is a peaked function of width Δ. We give the time evolution of the system when Δ is much larger and much smaller than the phonon energy.     

Theory of nonlinear effects in semiconductors with a nondiffusion approximation for acoustic phonons

Momentum and energy balance equations in impurity semi-metals and degenerate semiconductors are derived and investigated in a nondiffusion approximation for the acoustic phonons with arbitrary heating and entrainment of electrons and phonons taken into account. It was shown that the diffusion approximation is not satisfied even for relatively weak electrical fields. In cases of thermal entrainment and no heating of the phonons, cases are possible when the electron temperature becomes equal to and less than the lattice temperature, which is associated with radiation obtained from the energy field by electrons in the form of acoustic phonons at a point of acoustic instability. If mutual entrainment and heating of electrons and phonons occurs then the crystal boundaries are the main channel of energy and momentum relaxation. Necessary conditions delimiting the strong changes of all the galvano- and thermomagnetic effects at a point of acoustic instability are found in every specific case.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 11–16, November, 1990.     

Scattering by polar-optical phonons in a quasi-two-dimensional semiconductor

The scattering rate and momentum relaxation time for scattering of electrons in the quasi-two-dimensional quantized levels of an inversion or accumulation layer on a semiconductor surface is calculated for interactions via the polar-optical phonon. This interaction represents an important scattering mechanism in compound semiconcudctors. The quantization of the motion perpendicular to the surface enhances the scattering by this process over the bulk scattering.     

First-principle calculation of the recombination cross-section assisted by acoustic phonons at shallow impurities in semiconductors

Summary An original Monte Carlo study of the equilibrium microscopic and macroscopic recombination cross-sections at shallow impurity centres is presented. Both cross-sections are investigated in lightly dopedp-Si as functions of the temperature and ionized acceptor concentration. In order to treat generation-recombination processes we extend the semi-classical Boltzmann equation through a simulation of the carrier motion in the energy-configuration space of an impurity centre. The analysis of the scattering rates as a function of the total carrier energy enables a microscopic interpretation of the capture process to be carried out. The role of excited levels is naturally included and found to be of main importance at increasing lattice temperatures. Numerical results are then compared with available experiments and existing analytical calculations.     

Anharmonic relaxation of phonons

Using the Ziman scheme of considering a drifting Planck’s distribution, as the eigenvector of the linearized phonon collision operator, the anharmonic relaxation of phonons is discussed. The earlier arbitrariness in the phonon-phonon coupling parameters is removed by formulating explicit expressions for different allowed processes in terms of measurable quantities. Low and high temperature approximations of relaxation rates are also discussed: the results differ from earlier calculations. At low temperatures superthermal or high frequency phonons, which have temperature-independent and equal N- and U-relaxation rates, play important roles in thermal conduction in pure insulators. Presently working as University of Wales Fellow in the Applied Physics Department, UWIST Cathays Park, Cardiff CF1 3NU, U.K.     

On the theory of the collective phenomena in quasi-two-dimensional semiconductors

Results of calculation of the electron-hole liquid ground state energy are presented and their connection with other possible states of a two-dimensional system of electrons and holes is discussed.     

Thermoelectric power in quasi-two-dimensional systems with scattering of current carriers by phonons

The thermopower α in electron systems with a quasi-two-dimensional energy spectrum is investigated in the relaxation-time tensor approximation. The longitudinal and transverse components of the thermopower are calculated for scattering of the current carriers by different types of phonons. It is shown that the anisotropy of the thermopower in such systems is substantial. The dependence of a on the ratio of the Fermi level ɛ _F to the half-width ɛ ₀ of the one-dimensional conduction band is considered. For scattering by acoustical and nonpolar optical phonons, the thermopower changes sign: α becomes positive for ɛ _F<ɛ ₀. Comparison of the theory with published experimental data demonstrates good qualitative agreement. Fiz. Tverd. Tela (St. Petersburg) 39, 1857–1858 (October 1997)     

Dephasing in quantum dots: quadratic coupling to acoustic phonons

A microscopic theory of optical transitions in quantum dots with a carrier-phonon interaction is developed. Virtual transitions into higher confined states with acoustic phonon assistance add a quadratic phonon coupling to the standard linear one, thus extending the independent boson model. Summing infinitely many diagrams in the cumulant, a numerically exact solution for the interband polarization is found. Its full time dependence and the absorption line shape of the quantum dot are calculated. It is the quadratic interaction which gives rise to a temperature-dependent broadening of the zero-phonon line, calculated here for the first time in a consistent scheme.     

Ultrafast relaxation processes in semiconductors

A brief review of theoretical studies concerning ultrafast relaxation processes in direct-gap polar semiconductors is presented. Comparison with measurement of time-resolved optical spectra in GaAs is shown.     

Theoretical study of NMR relaxation due to rattling phonons

We calculate the NMR relaxation rate due to quadrupolar coupling of the nucleus to a local, strongly anharmonic phonon mode. As a model potential for a “rattling” motion we consider a square-well potential. We calculate the free phonon Green's function analytically and derive the low and high temperature limits of the NMR relaxation rate. It is shown that the temperature dependence of the NMR relaxation rate possesses a peak in contrast to harmonic phonons but in qualitative agreement with a recent NMR study on KOs₂O₆. We discuss the influence of phonon renormalization due to electron-phonon interaction.