O. V. Proshina

Polaron in quantum nanostructures

Abstract The interest to blue–green lasers results in fabrication of nanostructures based on wide band gap ionic materials. The strong electron–phonon interaction with longitudinal optical phonons results in extra confinement of electrons within the nanostructure. This paper deals with theory of large radius polaron in quantum dots, wires and wells. It is shown that the binding polaron energy increase with decreasing of nanostructure dimensionality. The localized electrons are shown to interact with long wavelength optical phonons with wave vector q ⩽1/ L , L being the characteristic size of nanostructure.

Effect of the Spectrum of Elementary Excitations on Spinodal Decomposition of Semiconductor Alloys

A new theoretical approach to the determination of the critical temperature of spinodal decomposition of alloys is suggested. The approach is based on the inclusion of the dependences of elementary excitations existing in the system on the alloy composition. It is shown that the most substantial effect on the critical temperature is produced by equilibrium plasma oscillations. An analytical expression for the critical temperature in terms of the band -structure parameters of the alloy is derived. The role of other excitations existing in the system is analyzed. The calculated critical temperatures are correlated with the currently available theoretical and experimental results.

The role of surface phonons in the formation of the spectrum of polaron states in quantum dots

The theory of large-radius polarons in quantum dots is developed with the difference in dielectric properties between the materials of the quantum dot and surrounding matrix taken into consideration. It is shown that the magnitude of the polaron effect is essentially dependent on the spectrum of surface optical phonons. The polaron shift of the size-quantized energy levels for electrons and holes is determined taking into account their interaction with phonons in the bulk and surface phonons. The conditions in which the interaction with surface phonons prevails are defined. It is shown that, in the II–VI compounds, the energy of the interaction can be higher than 10 meV and, hence, should be taken into account in calculating the energy spectrum. An approximate method for treating the polaron states is developed. The method provides a means for determining the magnitudes of the polaron shift in differently arranged heterostructures. It is found that the results obtained for the effect of surface phonons on the polaron states by the approximate method and by exact calculations are in good agreement.

The role of interface phonons in the formation of polaron states in quantum wells

A theory of a large-radius polaron in a quantum well is developed with consideration of the interaction of charged particles with different branches of the phonon spectrum. It is shown that, in narrow quantum wells, the major contribution to the polaron binding energy is made by interaction with symmetric interface phonons. As a result of such interaction, the polaron binding energy is defined by the effective mass of charge carriers in the quantum well and by the polarization properties of barriers. The possibilities of the formation of a polaron exciton in a quantum well in the case of strong interaction of charged particles with optical phonons are analyzed. The conditions in which the polarization fields produced by the electron and hole do not substantially compensate each other are established.

Interface Phonons and Polaron Effect in Quantum Wires

The theory of large radius polaron in the quantum wire is developed. The interaction of charge particles with interface optical phonons as well as with optical phonons localized in the quantum wire is taken into account. The interface phonon contribution is shown to be dominant for narrow quantum wires. The wave functions and polaron binding energy are found. It is determined that polaron binding energy depends on the electron mass inside the wire and on the polarization properties of the barrier material.

Polar state of a particle with a degenerate band spectrum in a quantum dot

The energies of electron and hole polarons in spherical quantum dots based on materials with a high degree of ionicity are found. It is established that consideration of the valence-band degeneracy causes the hole polaron binding energy to be greater than the electron polaron binding energy. The polaron effects increase with decreasing quantum dot radius. Interband optical transitions are accompanied by partial compensation of the polaron effects, because the emergent electron and hole tend to create polarization potential wells with opposite signs. It is shown that complete compensation of the polaron effects does not occur when the valence-band degeneracy is taken into account. Therefore, interband transitions are accompanied by polarization of the medium. Such polarization is manifested by the appearance of a series of intense phonon replicas of the lines for the electronic transitions.

Polaron mass of charge carriers in semiconductor quantum wells

A theory of the interaction of charge carriers with optical phonons in a quantum well is developed with consideration for interface optical phonons. The dependence of the polaron effective mass on the quantum-well dimensions and dielectric characteristics of barriers is analyzed in detail. It is shown that, in narrow quantum wells, a quasi-two-dimensional polaron can be formed. In this case, however, the interaction parameters are defined by the charge-carrier effective mass in the quantum well and by the frequencies of interface optical phonons. If barriers are made of a nonpolar material, the polaron effective mass depends on the quantum-well width. As the quantum-well width is increased, a new mechanism of enhancement of the electron–phonon interaction develops. The mechanism is implemented, if the optical phonon energy is equal to the energy of one of the electronic transitions. This condition yields an unsteady dependence of the polaron effective mass on the quantum-well width.

Resonance enhancement of electron-phonon interaction in nanostructures

The theory of electron-phonon interaction in quantum well is developed taking into account the influence of interface optical phonons. A detailed analysis of the dependence of polaron effective mass on the quantum well size and dielectric characteristics of barrier material is performed. It is shown that quasi-two-dimensional polaron may arise in narrow quantum wells. However, the interaction parameters are determined by effective mass of carriers in the quantum well and interface optical phonons frequencies. If the barriers are made of non-polar material, the polaron effective mass depends on the quantum well width. By increasing the quantum well width, a new mechanism of amplification of the electron-phonon interaction is realized. It occurs in the case of coincidence of the optical phonon energy with the energy of one of the electronic transitions. This leads to a nonmonotonic dependence of polaron effective mass on the quantum well width.

Interface Phonon Influences on Formation of Polaron States in Quantum Nanostructures

In the present work the theory of polaron states in the quantum wells, quantum wires and quantum dots is developed with due regard for the interaction of charge particles both with bulk and with interface optical phonons. The polaron binding energies are obtained. For the quantum dot case, the contributions of bulk and interface phonons have commensurable quantities. The interface phonon role is much more important in the quantum wells and wires. The essential strengthening as well as the attenuation of polaron interaction is possible with the different interrelations of the structure and the barrier dielectric properties.

The Influence of Surface Phonons on Polaron States in Quantum Dots

The influence of the surface phonons on the polaron effect in a quantum dot is investigated. We consider the polar quantum dot embedded into the polar matrix. The polaron energy shift for the electron and hole ground states is calculated. It is shown that the contribution of the surface phonons may exceed the bulk phonon contribution.