F. Pulizzi

Time-resolved photoluminescence of InAs quantum dots in a GaAs quantum well

We study the time-resolved photoluminescence emission of InAs self-assembled quantum dots (QDs) incorporated in a GaAs/(AlGa)As quantum well. We show that the quantum well confinement affects the decay time of the dot photoluminescence. In addition, we use the strong dependence of the decay time on excitation energy and temperature to shed light on carrier relaxation mechanisms in QDs.

Magnetophotoluminescence study of the influence of substrate orientation and growth interruption on the electronic properties of InAs∕GaAs quantum dots

We have used photoluminescence in pulsed (⩽50T) and dc (⩽12T) magnetic fields to investigate the influence of substrate orientation and growth interruption (GI) on the electronic properties of InAs∕GaAs quantum dots, grown by molecular beam epitaxy at 480°C. Dot formation is very efficient on the (100) substrate: electronic confinement is already strong without GI and no significant change in confinement is observed with GI. On the contrary, for the (311)B substrate strong confinement of the charges only occurs after a GI is introduced. When longer GIs are applied the dots become higher.

Carrier kinetics in a high-optically efficient quantum dot structure

We report measurements of the temperature and excitation energy dependence of the photoluminescence (PL) decay time of InAs quantum dots (QDs) embedded in a GaAs quantum well (QW). The investigated structure displays high PL efficiency up to high temperatures (>300 K) due to the effect of the barriers which inhibit carrier escape. It is therefore an ideal structure for device applications, e.g., QD lasers. In the work described here, we have made a detailed study of the recombination dynamics in a QD/QW structure. In particular, we focus on the temperature dependence of the QD PL decay time which in recent years has been a matter of some controversy. In QDs the radiative lifetime is believed to be independent of temperature and so any temperature dependence must be due to other processes, e.g., carrier relaxation and redistribution. We find that, for non-resonant pumping and at low temperatures, the presence of the barriers slows down the PL decay time. However, at high temperatures and/or for resonant pumping of the dots, the barriers have less effect and the PL decay time is governed by carrier redistribution between the dots.

A grazing incidence small angle X-ray scattering study of the effect of growth interrupt on the structure of InAs quantum dots

There is considerable interest in InAs quantum dots (QD) grown in a GaAs matrix by molecular beam epitaxy. Electrons and holes may be confined in bound states within the dots due to the smaller band gap of InAs. Optical transitions in the infrared energy range offer the potential of opto-electronic devices such as lasers for optical communications. One of the crucial factors that determines the electrical properties of quantum dots is their structure, including strain within the dot and the matrix and changes in composition due to alloying. The pyramid shape of the InAs islands prior to burial by the GaAs cap is well known from standard surface science studies. There is, however, evidence that the dot shape and composition changes dramatically upon burial. Further changes occur if the sample is subjected to post-growth annealing. The details of such processes are poorly understood, but vital for greater understanding and hence control of QD properties. We present grazing incidence small angle X-ray scattering (GISAXS) studies of QDs measured at the ESRF synchrotron. A series of InAs dots were grown on GaAs(001) capped with 25 nm of GaAs. The samples were prepared with different growth interruptions during the production process. The X-ray scattering reveals differences in the shape of the dots that can be correlated with their photoluminescence spectra. The GISAXS signal also reveals a distinct asymmetry in the shape of the dots. Detailed structural information on buried QDs will allow more realistic computer modelling.

Control of the coalescence phenomena in InAs/GaAs quantum dots by using high-index planes

Summary form only given. Two different series of samples were examined with atomic force microscopy (AFM). In the first series of structures (S), an InAs layer of average thickness L = 2.3 MLs was deposited on a GaAs buffer layer at a temperature of 500/spl deg/C. Samples with different substrate orientation, i.e. [100]-, [311]B-, [411]B- and [511]A- orientation, were grown in the same run. Samples of the second series (A) were grown in the same way as those in series S, but were thermally annealed at 530/spl deg/C for 90 seconds. We also explore the possibility of using the [311]B QDs for opto-electronic applications by photoluminescence (PL) studies.

InAs/GaAs quantum dot formation studied by magneto-photoluminescence spectroscopy

InAs/GaAs quantum dots (QDs) formation was studied by photoluminescence (PL) in magnetic fields up to 50 tesla. In the set of samples investigated, the amount of InAs deposited on [100] and [311]B oriented GaAs was varied from 1.4 to 1.9 monolayers in steps of 0.1 monolayer. On the [100] plane, the large PL linewidths reveal the presence of QDs for an InAs coverage of as little as 1.4 ML. However, up to 1.5 ML an asymmetrical PL-line is observed together with a large diamagnetic energy shift between 0 T and 50 T. In field, the different contributions to the PL-line shift differently, separating their centers of mass. This reveals that the PL- line consists of two peaks: a broad one, characteristic of QD luminescence and a narrow one that we attribute to the wetting layer (WL). The changeover to a symmetric PL, line and the sudden drop in diamagnetic shift at 1.6 ML indicate that the luminescence is dominated by the QDs from 1.6 ML on. In this regime, the charges are well confined inside the QDs. From this point, the decrease in localisation with further increase in InAs coverage leads to a slight increase in diamagnetic shift.