Carrier trapping and luminescence polarization in quantum dashes

Abstract

We study experimentally and theoretically polarization-dependent luminescence from an ensemble of quantum-dot-like nanostructures with a very large in-plane shape anisotropy (quantum dashes). We show that the measured degree of linear polarization of the emitted light increases with the excitation power and changes with temperature in a non-trivial way, depending on the excitation conditions. Using an approximate model based on the k.p theory, we are able to relate this degree of polarization to the amount of light hole admixture in the exciton states which, in turn, depends on the symmetry of the envelope wave function. Agreement between the measured properties and theory is reached under assumption that the ground exciton state in a quantum dash is trapped in a confinement fluctuation within the structure and thus localized in a much smaller volume of much lower asymmetry than the entire nanostructure.Comment: 13 pages, 9 figures; considerably extended, additional discussion and new figures include

Phonon-assisted radiative recombination of excitons confined in strongly anisotropic nanostructures

“…The surface density of the dashes is very high, exceeding significantly 10 10 cm −2 , therefore for single dash spectroscopy the sample was patterned with submicron-size mesas obtained by combining electron beam lithography and reactive ion etching. Some fundamental optical properties of the considered InAs/InGaAlAs/InP QDashes have already been investigated for single nanostructures [22][23][24] as well as for whole ensembles [26,41]. Among others, the problem of luminescence polarization was addressed in case of an ensemble of QDashes [27] where a nontrivial behavior of the temperature dependence of the degree of linear polarization was observed and modeled by assuming the existence of confinement potential fluctuations able to trap the excitons in a smaller volume of effectively lower asymmetry.…”

“…Neither experimental nor theoretical reports regarding the analysis of the influence of the exciton-phonon interaction on the emission spectra of anisotropic dashlike nanostructures are available. InAs/InP QDashes can be especially interesting in that respect due to the possible coexistence of two types of states differing intrinsically in the confinement regime, predicted previously based on polarization-dependent studies of the emission from QDash ensembles [26]. Local widenings, zigzag corners, and other shape or composition fluctuations within a quantum dash act as an additional localization center so that the exciton can be confined in a much smaller and less anisotropic space than in the case of a regular dash where the wave function is delocalized over the entire nanostructure volume [25,26].…”

“…InAs/InP QDashes can be especially interesting in that respect due to the possible coexistence of two types of states differing intrinsically in the confinement regime, predicted previously based on polarization-dependent studies of the emission from QDash ensembles [26]. Local widenings, zigzag corners, and other shape or composition fluctuations within a quantum dash act as an additional localization center so that the exciton can be confined in a much smaller and less anisotropic space than in the case of a regular dash where the wave function is delocalized over the entire nanostructure volume [25,26]. These two types of objects: (i) with a localization center resembling symmetric QDs and (ii) without a localization center, i.e., uniform, strongly anisotropic nanostructures, are expected to differ with respect to interaction with phonons, which can be investigated via the visibility of the phonon-related spectral features [27], which are confinement dependent.…”

Exciton spin relaxation in InAs/InGaAlAs/InP(001) quantum dashes emitting near 1.55μm

“…21 The lack of full polarization of emission in the V-V configuration could be partially caused by (i) random deviations from the V axis orientation in the ensemble of QDashes, (ii) presence of more symmetric structures in the ensemble, (iii) local widenings along QDashes acting as more symmetric trapping centeres, 18 (iv) elliptically polarized emission from trions 8 independent of polarization of excitation, acting as a background for excitonic emission.…”

“…This can be explained in terms of the QDash shape, anisotropic confinement for carriers, a non-uniform strain field and piezoelectricity induced by it, atomistic disorder at interfaces, and finally the local asymmetry of the InAs zinc-blende crystal lattice. 10,[14][15][16][17][18] From a theoretical point of view, these asymmetries with respect to V and H axes lead to a light-hole (lh, j# = "i) admixture to a nominally purely heavy-hole (j* = +i) state, producing hole eigenstates, 19 i.e., j* 0 = + 0 i / j* = +i 6iej# = "i. This converts excitons' polarizations from circular to elliptical, with major axes tilted towards the V axis in the case of both states.…”