Petr Klenovský

Electronic structure of InAs quantum dots with GaAsSb strain reducing layer: Localization of holes and its effect on the optical properties

The electronic structure of InAs quantum dots covered with the GaAs1−ySby strain reducing layer has been studied using the k⋅p theory. We explain previous experimental observations of the red shift of the photoluminescence emission with increasing y and its blue shift with increasing excitation power. For y>0.19, type-II dots are formed with holes localized in the GaAsSb close to the dot base; two segments at the opposite sides of the dot, forming molecular-like states, result from the piezoelectric field. We also propose an experiment that could be used to identify the hole localization using a vertical electric field.

Excitonic structure and pumping power dependent emission blue-shift of type-II quantum dots

In this work we study theoretically and experimentally the multi-particle structure of the so-called type-II quantum dots with spatially separated electrons and holes. Our calculations based on customarily developed full configuration interaction ap- proach reveal that exciton complexes containing holes interacting with two or more electrons exhibit fairly large antibinding energies. This effect is found to be the hallmark of the type-II confinement. In addition, an approximate self-consistent solution of the multi-exciton problem allows us to explain two pronounced phenomena: the blue-shift of the emission with pumping and the large inhomogeneous spectral broadening, both of those eluding explanation so far. The results are confirmed by detailed intensity and polarization resolved photoluminescence measurements on a number of type-II samples.

Polarization anisotropy of the emission from type-II quantum dots

We study the polarization response of the emission from type-II GaAsSb capped InAs quantum dots. The theoretical prediction based on the calculations of the overlap integrals of the single-particle states obtained in the (k) over right arrow . (p) over right arrow framework is given. This is verified experimentally by polarization resolved photoluminescence measurements on samples with the type-II confinement. We show that the polarization anisotropy might be utilized to find the vertical position of the hole wave function and its orientation with respect to crystallographic axes of the sample. A proposition for usage in the information technology as a room temperature photonic gate operating at the communication wavelengths as well as a simple model to estimate the energy of fine-structure splitting for type-II GaAsSb capped InAs QDs are given.

Excitation intensity dependence of photoluminescence spectra of SiGe quantum dots grown on prepatterned Si substrates: evidence for biexcitonic transition

The pumping intensity (I) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear I dependence of the PL intensity. At approximately I larger than 3 W cm-2, additional bands with a nearly quadratic I dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite no-phonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the no-phonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.

Excitonic fine structure splitting in type-II quantum dots

Excitonic fine structure splitting in quantum dots is closely related to the lateral shape of the wave functions. We have studied theoretically the fine structure splitting in InAs quantum dots with a type-II confinement imposed by a GaAsSb capping layer. We show that very small values of the fine structure splitting comparable with the natural linewidth of the excitonic transitions are achievable for realistic quantum dots despite the structural elongation and the piezoelectric field. For example, varying the capping layer thickness allows for a fine tuning of the splitting energy. The effect is explained by a strong sensitivity of the hole wave function to the quantum dot structure and a mutual compensation of the electron and hole anisotropies. The oscillator strength of the excitonic transitions in the studied quantum dots is comparable to those with a type-I confinement which makes the dots attractive for quantum communication technology as emitters of polarization-entangled photon pairs.

Modelling of electronic states in InAs/GaAs quantum dots with GaAsSb strain reducing overlayer

We present results of our 8-band k.p calculations of the emission energy of InAs/GaAs quantum dots (QDs) covered with GaAs1-xSbx strain reducing overlayer (SRO). In agreement with previous experimental observations we find a strong red shift of the emission with increasing Sb content. We explain this effect by: (1) The lowering of the valence band offset between the QD and the SRO with increasing Sb content resulting in the type-II QDs with holes confined in the SRO for Sb concentration above 14%. (2) The reduction of compressive strain inside the QDs. The contributions of these mechanisms to the total red shift are estimated and compared. For realistic shape and size of the QD and a realistic value of the SRO thickness the previously measured photoluminescence data are reproduced with fairly good accuracy.

Quantum entanglement in lateral GaAs/AlGaAs quantum dot molecules

We calculate the excitonic structure of pairs of GaAs/AlGaAs quantum dots forming lateral molecules and obtain the entanglement of exciton states. The following advantages of the lateral geometry over the vertical one are found: (1) The energy structures of the dots forming a molecule can be in principle identical. (2) Comparable tunneling of electrons and holes ensures a high entanglement of antisymmetric excitons. A drawback of existing structures are very low tunneling energies, which make the entanglement vulnerable against differences in the sizes and shapes of both dots.

Inversion of the exciton built-in dipole moment in In(Ga)As quantum dots via nonlinear piezoelectric effect

We show that anisotropic biaxial stress can be used to tune the built-in dipole moment of excitons confined in In(Ga)As quantum dots up to complete erasure of its magnitude and inversion of its sign. We demonstrate that this phenomenon is due to piezoelectricity. We present a model to calculate the applied stress, taking advantage of the so-called piezotronic effect, which produces significant changes in the current-voltage characteristics of the strained diode-membranes containing the quantum dots. Finally, self-consistent k.p calculations reveal that the experimental findings can be only accounted for by the nonlinear piezoelectric effect, whose importance in quantum dot physics has been theoretically recognized although it has proven difficult to single out experimentally.