Recent experimental measurements, without any theoretical guidance, showed that isotropic polarization response can be achieved by increasing the number of quantum-dot (QD) layers in a QD stack. Here we analyze the polarization response of multilayer QD stacks containing up to nine QD layers by linearly polarized photoluminescence (PL) measurements and by carrying out a systematic set of multimillion atom simulations. The atomistic modeling and simulations allow us to include correct symmetry properties in the calculations of the optical spectra, a factor critical to explain the experimental evidence. The values of the degree of polarization (DOP) calculated from our model follows the trends of the experimental data. We also present detailed physical insight by examining strain profiles, band edges diagrams, and wave function plots. Multidirectional PL measurements and calculations of the DOP reveal a unique property of InAs QD stacks that the TE response is anisotropic in the plane of the stacks. Therefore, a single value of the DOP is not sufficient to fully characterize the polarization response. We explain this anisotropy of the TE modes by orientation of hole-wave functions along the [$\overline{1}10$] direction. Our results provide a new insight that isotropic polarization response measured in the experimental PL spectra is due to two factors: (i) TM${}_{001}$-mode contributions increase due to enhanced intermixing of HH and LH bands, and (ii) TE${}_{110}$-mode contributions reduce significantly due to hole-wave function alignment along the [$\overline{1}10$] direction. We also present optical spectra for various geometry configurations of QD stacks to provide a guide to experimentalists for the design of multilayer QD stacks for optical devices. Our results predict that the QD stacks with identical layers will exhibit lower values of the DOP than the stacks with nonidentical layers. DOI: http://dx.doi.org/10.1103/PhysRevB.84.115321 ©2011 American Physical Society

We have studied the electronic states of closely stacked InAs/GaAs quantum dots (QDs) with a 4.0-nm spacer layer using linearly polarized photoluminescence (PL) and time-resolved PL measurements. An increase in the stacking-layer number (SLN) leads to an increase in the linear polarization anisotropy in the (001) plane; the [$-$110]-polarization component becomes dominant. These SLN-dependent polarization characteristics result from the valence-band mixing induced by the vertically coupled electronic states. The PL spectrum of the stacked QDs shows clear blueshifts with an increase in the excitation power because of the band filling. In addition, the radiative recombination lifetime has been found to obey the $T^{1/2}$ dependence, which directly confirms the one-dimensional translational motion of excitons in the closely stacked QDs.

We studied the two-step photon absorption (TSPA) process in InAs/GaAs quantum-dot superlattice (QDSL) solar cells. TSPA of subband-gap photons efficiently occurs when electrons are pumped from the valence band to the states above the inhomogeneously distributed fundamental states of QDSLs. The photoluminescence (PL)-excitation spectrum demonstrates an absorption edge attributed to the higher excited states of the QDSLs in between the InAs wetting layer states and the fundamental states of QDSLs. When the absorption edge of the excited state was resonantly excited, the superlinear excitation power dependence of the PL intensity demonstrated that the electron and hole created by the interband transition separately relax into QDSLs. Furthermore, time-resolved PL measurements demonstrated that the electron lifetime is extended by thereby inhibiting recombination with holes, enhancing the second subband-gap absorption.

A comprehensive assessment of the nature of the distribution of sub band-gap energy states in bulk GaAsBi is presented using power and temperature dependent photoluminescence spectroscopy. The observation of a characteristic red-blue-red shift in the peak luminescence energy indicates the presence of short-range alloy disorder in the material. A decrease in the carrier localisation energy demonstrates the strong excitation power dependence of localised state behaviour and is attributed to the filling of energy states furthest from the valence band edge. Analysis of the photoluminescence lineshape at low temperature presents strong evidence for a Gaussian distribution of localised states that extends from the valence band edge. Furthermore, a rate model is employed to understand the non-uniform thermal quenching of the photoluminescence and indicates the presence of two Gaussian-like distributions making up the density of localised states. These components are attributed to the presence of microscopic fluctuations in Bi content, due to short-range alloy disorder across the GaAsBi layer, and the formation of Bi related point defects, resulting from low temperature growth.

We have studied the time-resolved intraband transition from the intermediate state to the continuum state of the conduction band in InAs/GaAs self-assembled quantum dots (QDs) embedded in a one-dimensional photonic cavity structure using two-color photoexcitation spectroscopy. The resonant energy of the photonic cavity was tuned to enhance the intraband transition with an energy smaller than the interband transition energy between the intermediate state and the quantized hole states. The interband photoluminescence intensity was observed to be drastically reduced due to the pumping out of carriers in the intermediate state using near-infrared laser light. We proposed a model describing the carrier relaxation process in the InAs/GaAs QD system, where the two-photon absorption and the Pauli blocking in QDs are considered.