New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field. 68.90. + g, 73.50.Pz, Self-assembled InAs-GaAs quantum dots (QDs) provide nearly ideal examples of zero-dimensional semiconductor systems [1] and are hence of considerable contemporary interest for the study of new physics and potential device applications. However, very little is known experimentally about the nature of the QD carrier wave functions and their response to applied fields. Numerous calculations of the electronic structure of QDs have been performed [2][3][4][5], but in the absence of definitive structural information they assume idealized QD shapes, usually pyramidal [6]. However there is evidence that in many cases the dots more closely approximate to lens shaped [7], and may also contain significant concentrations of Ga [8], rather than being pure InAs. In view of the uncertainties in shape and composition, the applicability of the calculated electronic structure to real dots must at best be regarded as approximate at the present time.Consequently, experimental information on the nature of the wave functions is urgently required, to provide a reliable guide to theory, and a firm basis for the interpretation of experiments. In this paper we demonstrate that photocurrent spectroscopy under electric field F provides important, new information on the carrier wave functions, and by comparison with theory, on the composition, shape and effective height of the dots. We show that the QDs possess a permanent dipole moment, implying a finite spatial separation of the electron and hole for F 0. Contrary to the predictions of all recent calculations, we demonstrate that the holes are localized above the electrons in the QDs. This "inverted" alignment can only be explained by assuming nonuniform Ga incorporation in the nominally InAs QDs. As a result of our work the extensive previous theoretical modeling of the electronic structure of InAs QDs will need to be reexamined.Two types of dots were studied, both grown by molecular-beam epitaxy on ͑001͒ GaAs substrates at 500 ± C. The first type (samples A C) was deposited at 0.01 monolayers per second (ML͞s) to give a density ഠ1.5 3 10 9 cm 22 , base size 18 nm, and height 8.5 nm [ Fig. 1(a)], as determined from transmission electron microscopy (TEM). The second type (sample D) had a higher deposition rate of 0.4 ML͞s, resulting in a density ഠ5 3 10 10 cm 22 and size 15 3 3.5 nm. The asymmetric shaped QDs, sitting on an ...

No abstract

The effects of excess electron occupation on the optical properties of excitons ͑X͒ and biexcitons (2X) in a single self-assembled InGaAs quantum dot are investigated. The behavior of X and 2X differ strongly as the number of excess electrons is varied with the biexciton being much more weakly perturbed as a result of its filled s-shell ground state, a direct manifestation of shell-filling effects. Good correlation is found between charging thresholds observed from s-shell recombination perturbed by p-shell occupation, and direct observation of p-shell recombination.

Articles you may be interested inElectronic structure of InAs quantum dots with GaAsSb strain reducing layer: Localization of holes and its effect on the optical properties

The use of a high-growth-temperature GaAs spacer layer is demonstrated to significantly improve the performance of 1.3μm multilayer self-assembled InAs∕InGaAs dot-in-a-well lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics. Incorporation of these spacer layers allows the fabrication of multilayer quantum-dot devices emitting above 1.3μm, with extremely low room-temperature threshold current densities and with operation up to 105°C.

Charged (X*) and neutral ͑X͒ exciton recombination is reported in the photoluminescence spectra of single In͑Ga͒As quantum dots. Photoluminescence excitation ͑PLE͒ spectra show that the charged excitons are created only for excitation in the barrier or cladding layers of the structure, consistent with their charged character, whereas the neutral excitons in addition show well-defined excitation features for resonant excitation of the dots. The PLE spectra for X and X* exhibit a clear anticorrelation in the region of the wetting layer transition, showing that they compete for photocreated carriers.

Self-assembled semiconductor quantum dots (QDs) exhibit fully quantized electronic states and high radiative efficiencies. This makes them highly suitable both for fundamental physics studies of zero-dimensionality, atomic-like semiconductor systems and applications in a range of novel electro-optical devices. This review discusses recent important advances in the study and application of semiconductor QDs. Using a wide range of optical spectroscopy techniques, it is possible to obtain a detailed understanding of the electronic structure and dynamical carrier processes. Such an understanding is required for the implementation of a wide range of QD-based devices.

No abstract