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The possibility of an independent tuning of the electron and hole confinement in InAs/GaAs quantum dots (QDs) by using a thin GaAsSbN capping layer (CL) is studied. By controlling the Sb and N contents in the quaternary alloy, the band structure of the QDs can be broadly tuned and converted from type-II in the valence band (high Sb contents) to type-I and to type-II in the conduction band (high N contents). Nevertheless, the simultaneous presence of Sb and N is found to induce strain and composition inhomogeneities in the CL and to degrade the photoluminescence of the structure.

The distribution of Bi atoms in epitaxial GaAs(1−x)Bix is analyzed through aberration-corrected Z-contrast images. The relation between the atomic number and the intensity of the images allows quantifying the distribution of Bi atoms in this material. A bidimensional map of Bi atoms is extracted showing areas where nanoclustering is possible and evidencing the location of Bi at As-substitutional positions in the lattice. The distribution of Bi atoms differs from a random spatial pattern of Bi atoms in the material.

The effect of low N-alloying on the structure of capped InAs/GaAs quantum dots is analyzed by transmission electron microscopy related techniques. A statistical study of interplanar distances in InAsN quantum dots shows an increase in the lattice parameter compared to the InAs case. We suggest that the addition of nitrogen blocks the Ga/In exchange processes during the quantum dot capping process, leading to an In enrichment. The observed strong photoluminescence redshift of InAsN structures must therefore be attributed not only to the effect of nitrogen in the band gap, but also to a minor depletion of indium during the capping process.

High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is a powerful tool to quantify size, shape, position, and composition of nano-objects with the assessment of image simulation. Due to the high computational requirements needed, nowadays it can only be applied to a few unit cells in standard computers. To overpass this limitation, a parallel software (SICSTEM) has been developed. This software can afford HAADF-STEM image simulations of nanostructures composed of several hundred thousand atoms in manageable time. The usefulness of this tool is exemplified by simulating a HAADF-STEM image of an InAs nanowire.

Atomic steps at growth surfaces are important heterogeneous sources for nucleation of epitaxial nano-objects. In the presence of misfit strain, we show that the nucleation process takes place referentially at the upper terrace of the step as a result of the local stress relaxation. Evidence for strain-induced nucleation comes from the direct observation by postgrowth, atomic resolution, Z-contrast imaging of an InAs-rich region in a nanowire located on the upper terrace surface of an interfacial diatomic step., This work was supported by the DOE Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (MV and SJP), the SANDiE European Network of Excellence (Contract No. NMP4-CT-2004-500101), the Spanish MEC (TEC2005-05781-C03-01 y 02, NAN2004-09109-C04-01, and Consolider-Ingenio 2010 CSD2006-00019), the CAM (S 0505ESP 0200), and the Junta de Andalucia (PAI research groups TEP-120 and TIC-145; Project No. PAI05-TEP-00383).