This paper presents a new approach to model selection based on hypothesis testing. We ÿrst describe a procedure to generate di erent scores for any candidate model from a single sample of training data and then discuss how to apply multiple comparison procedures (MCP) to model selection. MCP statistical tests allow us to compare three or more groups of data while controlling the probability of making at least one Type I error. The complete procedure is illustrated on several model selection tasks, including the determination of the number of hidden units for feed-forward neural networks and the number of kernels for RBF networks.
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.
In this article a method for determining errors of the strain values when applying strain mapping techniques has been devised. This methodology starts with the generation of a thickness/defocus series of simulated high-resolution transmission electron microscopy images of InAsxP1-x/InP heterostructures and the application of geometric phase. To obtain optimal defocusing conditions, a comparison of different defocus values is carried out by the calculation of the strain profile standard deviations among different specimen thicknesses. Finally, based on the analogy of real state strain to a step response, a characterization of strain mapping error near an interface is proposed.
We show in this work that GaAs capping of 2 ML of GaSb grown by molecular beam epitaxy results in the formation of very small ͑with heights of about 1 nm͒ GaAs x Sb 1−x nanostructures surrounded by a GaAs rich layer. This conclusion is obtained by analyzing the morphology of the GaAs x Sb 1−x nanostructures by high resolution scanning transmission electron microscopy in Z-contrast mode. This result shows that a significant fraction of the Sb atoms must segregate along the growth direction during the GaAs capping process. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3077009͔Research on GaSb/GaAs quantum dots is a topic of increasing interest because this material has very attractive properties, such as its type-II band alignment, 1 opening the possibility to fabricate new devices such as quantum information devices and memories based on quantum dots. 2-4Jiang et al.5 studied by atomic force microscopy ͑AFM͒ uncapped GaSb quantum dots grown by molecular beam epitaxy ͑MBE͒ and they found quantum dots sizes of about 10 nm. Nevertheless, during the process of capping of the GaSb with GaAs, an important fraction of the formed quantum dots seems to disappear, probably due to the exchange reaction of Sb adatoms with As atoms on the GaAs surface layer. 6,7 This idea is supported by the fact that very small ͑approximately 1-2 nm in size͒ GaAs capped GaSb/GaAs nanostructures are observed by cross-sectional scanning tunneling microscopy on samples grown by metalorganic chemical vapor deposition at 470°C. In this work, we investigate by high resolution Z-contrast the nanomorphology of GaAs capped GaSb nanostructures grown at 480°C by MBE. The analysis of the photoluminescence spectra of these nanostructures in a previous work shows evidence of a type-II band alignment, with holes strongly confined ͑501 meV͒ and electrons weakly confined ͑46 meV͒ in the electrostatic potential created by the accumulation of multiple holes inside each quantum dot under optical pumping. The quantum dot photoluminescence at 1057 meV blueshifts as the excitation power is increased.8 Here we present a structural characterization study showing that GaAs capping of GaSb quantum dots results in the formation of very small ͑with heights of about 1 nm͒ nanostructures at the GaAs/ GaSb interface.The analyzed nanostructures were grown by solid source MBE at a growth rate of 0.1 ML/s at 480°C on an n-type GaAs͑001͒ substrate after deposition of an n-type GaAs buffer layer ͑Si: 1 ϫ 10 18 cm −2 ͒. The formation of the GaSb quantum dots was detected by the change in the reflection high energy electron diffraction ͑RHEED͒ pattern after deposition of 1.8 ML of GaSb. The GaSb layer, with a nominal thickness of 2 ML, was then exposed to Sb flux for 20 s and then annealed for an additional 20 s. The GaAs capping was done at 0.4 ML/s in two steps. In the first step, a 10 nm thick GaAs layer was grown at the temperature of quantum dot nucleation. In the second one, a 40 nm thick GaAs layer was deposited at 570°C. During the growth, the As and Sb beam equiv...
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