J. Seebeck

Polarons in semiconductor quantum dots and their role in the quantum kinetics of carrier relaxation

While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.

Influence of carrier-carrier and carrier-phonon correlations on optical absorption and gain in quantum-dot systems

A microscopic theory is used to study the optical properties of semiconductor quantum dots. The dephasing of a coherent excitation and line shifts of the interband transitions due to carrier-carrier Coulomb interaction and carrier-phonon interaction are determined from a quantum kinetic treatment of correlation processes. We investigate the density dependence of both mechanisms and clarify the importance of various dephasing channels involving the localized and delocalized states of the system.

Coulomb scattering in nitride-based self-assembled quantum dot systems

We study the carrier capture and relaxation due to Coulomb scattering in group-III nitride quantum dots on the basis of population kinetics. For the states involved in the scattering processes the combined influence of the quantum-confined Stark effect and many-body renormalizations is taken into account. The charge separation induced by the built-in field has important consequences on the capture and relaxation rates. It is shown that its main effect comes through the renormalization of the energies of the states involved in the collisions and leads to an increase in the scattering efficency.

Relaxation properties of the quantum kinetics of carrier-LO-phonon interaction in quantum wells and quantum dots

The time evolution of optically excited carriers in semiconductor quantum wells and quantum dots is analyzed for their interaction with LO phonons. Both the full two-time Greens function formalism and the one-time approximation provided by the generalized Kadanoff-Baym ansatz are considered, in order to compare their description of relaxation processes. It is shown that the two-time quantum kinetics leads to thermalization in all the examined cases, which is not the case for the one-time approach in the intermediate-coupling regime, even though it provides convergence to a steady state. The thermalization criterion used is the Kubo-Martin-Schwinger condition.

Rabi oscillations in semiconductor quantum dots revisited: Influence of LO-phonon collisions

Coherence properties of semiconductor quantum dots are of central importance for a variety of applications. Previous studies of decoherence in these systems mainly focused on the interaction with LA phonons, only giving rise to dephasing. In contrast, the interaction with LO phonons additionally causes carrier scattering. To show the importance of the combined influence of dephasing and carrier scattering, we revisit the case of Rabi oscillations in semiconductor quantum dots using a quantum-kinetic treatment of the carrier-LO-phonon interaction.

Excitation-induced energy shifts in the optical gain spectra of InN quantum dots

A microscopic theory for the optical absorption and gain spectra of InN quantum-dot systems is used to study the combined influence of material properties and interaction-induced effects. Atomistic tight-binding calculations for the single-particle properties of the self-assembled quantum-dot and wetting-layer system are used in conjunction with a many-body description of Coulomb interaction and carrier phonon interaction. We analyze the carrier-density and temperature dependence of strong excitation-induced energy shifts of the dipole-allowed quantum-dot transitions.

Ultrafast quantum kinetics of carrier-LO-phonon interaction in quantum dots and quantum wells

The role of polarons in the quantum kinetics of carrier-phonon interaction is emphasized. Scattering processes involving polarons allow ultrafast relaxation even if the phonon energy is not resonant with the electronic transitions.

Quantum kinetic effects in the optical absorption of semiconductor quantum-dot systems

A microscopic theory is used to study the optical properties of semiconductor quantum dots. The dephasing of the coherent polarization due to carrier-carrier Coulomb interaction and carrier-phonon interaction is determined from quantum kinetic equations. We investigate the density dependence of the dephasing mechanisms, and compare the relevance of various interaction processes. The failure of frequently used approximations based on the GKBA with free single-particle energies is demonstrated for pure dephasing processes involving only the localized quantum-dot states.

Quantum kinetic approach to electron-LO-phonon relaxation: Is there a phonon bottleneck problem in optoelectronic devices?

The time evolution of optically excited carriers in semiconductor quantum wells and quantum dots is analyzed for their interaction with LO-phonons. Both the full two-time Greens function formalism and the one-time approximation provided by the generalized Kadanoff-Baym ansatz are considered, in order to compare their description of relaxation processes. It is shown that the two-time quantum kinetics leads to thermalization in all the examined cases, which is not the case for the one-time approach in the intermediate-coupling regime, even though it provides convergence to a steady state. The thermalization criterion used is the Kubo-Martin-Schwinger condition.

A microscopic theory for optical gain in semiconductor quantum dots

We study the optical properties of semiconductor quantum dots by means of a quantum-kinetic theory. The excitation-induced dephasing and the corresponding line-shifts of the interband transitions due to carrier-carrier Coulomb interaction and carrier-phonon interaction are determined and used in conjunction with the usual ingredients of a gain calculation like Coulomb enhancement and State filling to set up a microscopic calculation of the quantum dot gain. We find that for very high carrier densities in QD systems the maximum of the optical gain can decrease with increasing carrier density due to a delicate balancing between state filling and dephasing.