S. Sanguinetti

Thermally activated carrier transfer and luminescence line shape in self-organized InAs quantum dots

We investigated the temperature dependence (10–180 K) of the photoluminescence (PL) emission spectrum of self‐organized InAs/GaAs quantum dots grown under different conditions. The temperature dependence of the PL intensity is determined by two thermally activated processes: (i) quenching due to the escape of carriers from the quantum dots and (ii) carrier transfer between dots via wetting layer states. The existence of different dot families is confirmed by the deconvolution of the spectra in gaussian components with full width half maxima of 20–30 meV. The transfer of excitation is responsible for the sigmoidal temperature dependence of the peak energies of undeconvoluted PL bands.

Optical transitions in quantum ring complexes

High Magnetic Field Center, National Institute for Materials Science, Sakura 3-13, Tsukuba 305-0003, Japan(Dated: February 2, 2008)Making use of a droplet-epitaxial technique, we realize nanometer-sized quantum ring complexes, consistingof a well-defined inner ring and an outer ring. Electronic structure inherent in the unique quantum system isanalyzed using a micro-photoluminescence technique. One advantage of our growth method is that it presentsthe possibility of varying the ring geometry. Two samples are prepared and studied: a single-wall ring and aconcentric double-ring. For both samples, highly efficient photoluminescence emitted from a single quantumstructure is detected. The spectra show discrete resonance lines, which reflect the quantized nature of the ring-type electronic states. In the concentric double–ring, the carrier confinement in the inner ring and that in theouter ring are identified distinctly as split lines. The obse rved spectra are interpreted on the basis of singleelectron effective mass calculations.

Phonon strain shift coefficients in Si1−xGex alloys

A comprehensive study of the biaxial strain-induced shift of the Si1−xGex Raman active phonon modes is presented. High-resolution Raman measurements of Si1−xGex/Si heterostructures have been compared to x-ray diffraction data. Our approach, unlike previous works, is effective to decouple and quantify separately the effect of strain and composition on the phonon frequencies, yielding an accurate determination of the phonon strain shift coefficients in the entire composition range. Our results show that the strain shift coefficients are independent of the composition, a result which is in good agreement with theoretical calculations, performed within the framework of valence force-field theory.

Temperature dependence of the photoluminescence of InGaAs/GaAs quantum dot structures without wetting layer

We analyze the photoluminescence temperature behavior of InGaAs/GaAs quantum dots grown by heterogeneous droplet epitaxy. Morphologically, these dots are nanocrystal InGaAs inclusions in the GaAs matrix, with a concave disk shape and, more important, no wetting layer is connecting the dots. The photoluminescence of the dots does not show any of the typical of the Stranski–Krastanov dots temperature properties, such as sigmoidal peak energy position and linewidth narrowing. We demonstrate that such behavior stems from the lacking of the thermally activated dot–dot coupling channel provided by the wetting layer thus preventing the establishment of a common quasiequilibrium in the whole dot ensemble.

Coupled quantum dot―ring structures by droplet epitaxy

The fabrication, by pure self-assembly, of GaAs/AlGaAs dot-ring quantum nanostructures is presented. The growth is performed via droplet epitaxy, which allows for the fine control, through As flux and substrate temperature, of the crystallization kinetics of nanometer scale metallic Ga reservoirs deposited on the surface. Such a procedure permits the combination of quantum dots and quantum rings into a single, multi-functional, complex quantum nanostructure.

Low density GaAs∕AlGaAs quantum dots grown by modified droplet epitaxy

Low temperature photoluminescence spectroscopy is used to analyze the effects of the Ga coverage and of the postgrowth thermal annealing on the electronic properties of low density (≈1×109cm−2) self-assembled GaAs∕AlGaAs quantum dots (QDs) grown by modified droplet epitaxy (MDE). We demonstrate that with the MDE method it is possible to obtain low density and high efficiency QD samples with high photoluminescence efficiency. Large modifications of the photoluminescence band, which depend on Ga coverage and thermal annealing, are found and quantitatively interpreted by means of a simple model based on the Al-Ga interdiffusion.

Ultra-narrow emission from single GaAs self-assembled quantum dots grown by droplet epitaxy

We realized ultra-narrow excitonic emission from single GaAs/AlGaAs quantum dots (QDs) grown by a refined droplet epitaxy technique. We found that uncapped quantum dots can be annealed at 400 degrees C without major changes in their morphology, thus enabling an AlGaAs capping layer to be grown at that temperature. Consequently, we demonstrate a fourfold reduction of the linewidth of the emission together with an increased recombination lifetime, compared to the conventional droplet epitaxial QDs. The averaged linewidth of neutral excitons measured by micro-photoluminescence on single quantum dots was around 35 microeV.

Modified droplet epitaxy GaAs/AlGaAs quantum dots grown on a variable thickness wetting layer

We show that the use of modified droplet epitaxy allows to tune the wetting layer thickness in GaAs quantum dot structures. Morphological observations demonstrate that the wetting layer at the base of the dots can be controlled or even removed by changing the surface stoichiometry of substrates before droplet formations. Spectroscopical measurements show that the variation of the wetting layer thickness strongly influences the optical properties of the dots. The experimental transition energies of the dots well agree with a theoretical model based on effective mass approximation.

Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and (311) samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics.

Effects of post-growth annealing on the optical properties of self-assembled GaAs/AlGaAs quantum dots

Photoluminescence spectroscopy is used to analyze the effects of post-growth thermal annealing on the electronic properties and capture processes of self-assembled GaAs/AlGaAs quantum dots grown by modified droplet epitaxy. Post-growth annealing induces deep changes in the electronic structure of the quantum dots material, modifying both capture processes and photoluminescence quenching channels. The optical data, together with theoretical models, are used to quantify such structural and electronic modifications in the annealed materials.