The fabrication, by droplet epitaxy, of a class of quantum nanostructures characterized by a regular, nanometres high, flat disks with a diameter of hundreds of nanometres and a hole at the centre encircled by a ring a few nanometres high, is presented here. A detailed analysis of the growth kinetics performed via in situ and ex situ probes allows us to propose a working model for the formation of these structures.

We present a detailed characterization of the growth dynamics of Ga(Al)As(111)A surfaces. We develop a theoretical growth model that well describes the observed behavior on the growth parameters and underlines the Ehrlich-Schwöbel barrier as leading factor that determines the growth dynamics. On such basis we analyze the factors that lead to the huge observed roughness on such surface orientations and we identify the growth conditions that drive the typical three-dimensional growth of Ga(Al)As (111)A towards atomically flat surface. GaAs/Al 0.30 Ga 0.70 As quantum wells realized on optimized surface (< 0.2 nm roughness) show a record low emission linewidth of 4.5 meV.

Several semiconductor quantum dot technologies have been investigated for the generation of entangled photon pairs. Among the others, droplet epitaxy enables control of the shape, size, density, and emission wavelength of the quantum emitters. However, the fraction of entanglement-ready quantum dots that can be fabricated with this method is still limited to values around 5%, and matching the energy of the entangled photons to atomic transitionsa promising route towards quantum networking -remains an outstanding challenge.Here, we overcome these hurdles by introducing a modified approach to droplet epitaxy on a high symmetry (111)A substrate, where the fundamental crystallization step is performed at a significantly higher temperature as compared to previous reports. Our method improves drastically the yield of entanglement-ready photon sources near the emission wavelength of interest, which can be as high as 95% thanks to the low values of fine structure splitting and radiative lifetime, together with the reduced exciton dephasing offered by the choice of GaAs/AlGaAs materials. The quantum dots are designed to emit in the operating spectral region of Rb-based slow-light media, providing a viable technology for quantum repeater stations.Keywords: Quantum dots, entanglement, droplet epitaxy, fine structure splitting, rubidium, resonant two-photon excitation Under the ongoing effort to develop practical quantum technologies, the search for a suitable entangled photon source is an active research direction, as it plays a role in key quantum communication protocols and some approaches to quantum computation. 1,2 Above all, it is a fundamental requirement for the realization of repeaters capable to transfer quantum entanglement over long distances.Epitaxial quantum dots (QDs) are a promising alternative to parametric down-converters, given their ability to generate photons ondemand with high efficiency and their compatibility with semiconductor foundries. 3,4 In order to use QD entanglement resources in reallife technologies, two main roadblocks have to be overcome. The first is related to the difficulty of consistently finding emitters capable 1 arXiv:1710.03483v1 [cond-mat.mes-hall]

Structural and optical properties of GaAs(001) surfaces thermally annealed in dry N 2 atmosphere Surface ordering of (In,Ga)As quantum dots controlled by GaAs substrate indexes Appl. Phys. Lett. 85, 5031 (2004); 10.1063/1.1823590Planar InAs growth on GaAs(001) and subsequent quantum dot formation by a surface induced morphological instability J.We present accurate measurements of Ga cation surface diffusion on GaAs surfaces. The measurement method relies on atomic force microscopy measurement of the morphology of nano-disks that evolve, under group V supply, from nanoscale group III droplets, earlier deposited on the substrate surface. The dependence of the radius of such nano-droplets on crystallization conditions gives direct access to Ga diffusion length. We found an activation energy for Ga on GaAs(001) diffusion E A ¼ 1:3160:15 eV, a diffusivity prefactor of D 0 ¼ 0.53(Â2:161) cm 2 s À1 that we compare with the values present in literature. The obtained results permit to better understand the fundamental physics governing the motion of group III ad-atoms on III-V crystal surfaces and the fabrication of designable nanostructures. V C 2014 AIP Publishing LLC.

A theoretical model is suggested which describes the yield stress dependence on grain size in fine-grained materials, based upon competition between conventional dislocation slip, grain boundary diffusional creep (Coble creep) and triple junction diffusional creep. In the framework of the model, the contribution of diffusional creep mechanisms to plastic deformation increases with reduction of grain size, causing the abnormal Hall-Petch dependence in the range of small grains. A grain size distribution is incorporated into the consideration to account for a distribution of grain sizes occurring in real specimens. The results of the model are compared with experimental data from Cu and shown to be in good agreement. Ó

We present the fabrication of axial InAs/GaAs nanowire heterostructures on silicon with atomically sharp interfaces by molecular beam epitaxy. Our method exploits the crystallization at low temperature, by As supply, of In droplets deposited on the top of GaAs NWs grown by the self-assisted (self-catalyzed) mode. Extensive characterization based on transmission electron microscopy sets an upper limit for the InAs/GaAs interface thickness within few bilayers (≤1.5 nm). A detailed study of elastic/plastic strain relaxation at the interface is also presented, highlighting the role of nanowire lateral free surfaces.

A temperature activated crossover between two nucleation regimes is observed in the behavior of Ga droplet nucleation on vicinal GaAs(111)A substrates with a miscut of 2° towards $$(\bar{1}\bar{1}2)$$ ( 1 ¯ 1 ¯ 2 ) . At low temperature (<400 °C) the droplet density dependence on temperature and flux is compatible with droplet nucleation by two-dimensional diffusion. Increasing the temperature, a different regime is observed, whose scaling behavior is compatible with a reduction of the dimensionality of the nucleation regime from two to one dimension. We attribute such behavior to a presence of finite width terraces and a sizeable Ehrlich-Schwöbel barrier at the terrace edge, which hinders adatom diffusion in the direction perpendicular to the steps.

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