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Your search keyword '"J M Ulloa"' showing total 2 results
Start Over Author "J M Ulloa" Topic transmission electron microscopy
2 results on '"J M Ulloa"'

1. Optical Properties and Defect Structure of MOVPE InGaN Films

Author

Robert F. Davis, Horst P. Strunk, D. Hanser, Ana Cremades, Martin Albrecht, Javier Piqueras, and J. M. Ulloa

Subjects
Materials science, Photoluminescence, business.industry, chemistry.chemical_element, Cathodoluminescence, Band bending, chemistry, Transmission electron microscopy, Residual stress, Optoelectronics, Metalorganic vapour phase epitaxy, Luminescence, business, Indium
Abstract

A series of 100 nm thick InGaN films with In content up to 14% has been grown by MOVPE on SiC substrates. Optical characterization was carried out by means of reflectance spectrometry, photoluminescence and cathodoluminescence. Optical properties of the samples have been correlated with the microstructural properties measured by atomic force microscopy, transmission electron microscopy and X-ray diffraction data. Results indicate a dependence of the optical properties on the In content in the sample, as well as on the residual stress in the films induced by Indium incorporation. Part of the strain is reduced elastically by formation of pinholes which reach the InGaN/GaN interface, where first misfit dislocations are observed to form. Our results show that luminescence is directly correlated with the strain distribution in the layers. Pinholes are observed to act as nonradiative recombination sites for carriers, while strain relaxation around pinholes may enhance luminescence emission. We discuss the influence of strain with respect to In incorporation, the appearance of piezoelectric fields and effects due to strain induced band bending.

Published
1999
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2. General route for the decomposition of InAs quantum dots during the capping process.

Author

D González, D F Reyes, A D Utrilla, T Ben, V Braza, A Guzman, A Hierro, and J M Ulloa

Subjects
QUANTUM dots, TRANSMISSION electron microscopy, SOLAR cells, PIEZOELECTRIC materials, TOPOGRAPHY, MORPHOLOGY
Abstract

The effect of the capping process on the morphology of InAs/GaAs quantum dots (QDs) by using different GaAs-based capping layers (CLs), ranging from strain reduction layers to strain compensating layers, has been studied by transmission microscopic techniques. For this, we have measured simultaneously the height and diameter in buried and uncapped QDs covering populations of hundreds of QDs that are statistically reliable. First, the uncapped QD population evolves in all cases from a pyramidal shape into a more homogenous distribution of buried QDs with a spherical-dome shape, despite the different mechanisms implicated in the QD capping. Second, the shape of the buried QDs depends only on the final QD size, where the radius of curvature is function of the base diameter independently of the CL composition and growth conditions. An asymmetric evolution of the QDs’ morphology takes place, in which the QD height and base diameter are modified in the amount required to adopt a similar stable shape characterized by a averaged aspect ratio of 0.21. Our results contradict the traditional model of QD material redistribution from the apex to the base and point to a different universal behavior of the overgrowth processes in self-organized InAs QDs. [ABSTRACT FROM AUTHOR]

Published
2016
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