We show in this article that it is possible to obtain elemental compositional maps and profiles with atomic-column resolution across an InxGa1-xAs multilayer structure from 5th-order aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images. The compositional profiles obtained from the analysis of HAADF-STEM images describe accurately the distribution of In in the studied multilayer in good agreement with Muraki's segregation model [Muraki, K., Fukatsu, S., Shiraki, Y. & Ito, R. (1992). Surface segregation of In atoms during molecular beam epitaxy and its influence on the energy levels in InGaAs/GaAs quantums wells. Appl Phys Lett 61, 557-559].
"In order to being able to compare these experimental results to the simulated images, it is necessary to average the intensity over a significant number of atomic columns. For this, we have used a method for measuring the integrated intensities around each atomic column  that has been successfully used previously for the calculation of the composition of different semiconductor materials from experimental HAADF-STEM image   . In the case of more than one MoS 2 layers, the crystal can Fig. 2. Electron beam induced damage at (a) 120 and (b) 200 kV in the surface of MoS 2 layers. "
[Show abstract][Hide abstract] ABSTRACT: In this work we examined MoS2 sheets by aberration-corrected scanning transmission electron microscopy (STEM) at three different energies: 80, 120 and 200kV. Structural damage of the MoS2 sheets has been controlled at 80kV according a theoretical calculation based on the inelastic scattering of the electrons involved in the interaction electron-matter. The threshold energy for the MoS2 material has been found and experimentally verified in the microscope. At energies higher than the energy threshold we show surface and edge defects produced by the electron beam irradiation. Quantitative analysis at atomic level in the images obtained at 80kV has been performed using the experimental images and via STEM simulations using SICSTEM software to determine the exact number of MoS2 layers.
"Figure 5g shows a magnified dumbbell where the upper and lower columns are the anionic and cationic components, respectively. In order to estimate column-by-column the Sb and In contents, a method similar to the one used in [9,19] was performed. The R values corresponding to each sub-net column, Ri, are represented in Figure 5 with colored dots, where higher values (red dots) are associated with atomic columns with higher proportion of heavier elements with respect to the corresponding atomic columns in GaAs. "
[Show abstract][Hide abstract] ABSTRACT: The use of GaAsSbN capping layers on InAs/GaAs quantum dots (QDs) has recently been proposed for micro- and optoelectronic applications for their ability to independently tailor electron and hole confinement potentials. However, there is a lack of knowledge about the structural and compositional changes associated with the process of simultaneous Sb and N incorporation. In the present work, we have characterized using transmission electron microscopy techniques the effects of adding N in the GaAsSb/InAs/GaAs QD system. Firstly, strain maps of the regions away from the InAs QDs had revealed a huge reduction of the strain fields with the N incorporation but a higher inhomogeneity, which points to a composition modulation enhancement with the presence of Sb-rich and Sb-poor regions in the range of a few nanometers. On the other hand, the average strain in the QDs and surroundings is also similar in both cases. It could be explained by the accumulation of Sb above the QDs, compensating the tensile strain induced by the N incorporation together with an In-Ga intermixing inhibition. Indeed, compositional maps of column resolution from aberration-corrected Z-contrast images confirmed that the addition of N enhances the preferential deposition of Sb above the InAs QD, giving rise to an undulation of the growth front. As an outcome, the strong redshift in the photoluminescence spectrum of the GaAsSbN sample cannot be attributed only to the N-related reduction of the conduction band offset but also to an enhancement of the effect of Sb on the QD band structure.
Nanoscale Research Letters 11/2012; 7(1):653. DOI:10.1186/1556-276X-7-653 · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The introduction of semiconductor and metallic nanostructures in photovoltaic materials is an active and promising area of development. This is due to the existence of several characteristics of nanostructures that improve the functionality of photovoltaic materials, for example by an enhancement of efficiency in solar cells, an increase of collected sun light and by the use of up- and down-converters to change the energy of incident photons. Nanostructures for solar cells offer, in comparison with single junction solar cells, new photovoltaic mechanisms for transforming sun light energy into electrical energy. A critical issue to understand the behavior of nanostructures based solar cells is to know their structural characteristics (size, shape, strain, distribution and composition) at nanoscale. In addition to this, the knowledge of the processes of nucleation, segregation and intermixing in nanostructures at nanoscale, and in some cases to atomic scale, is another important point to improve their design. This communication reviews our contribution toward the understanding of these issues and present some prospective studies and experiments in this context, by the use and development of methods based in aberration-corrected scanning transmission electron microscopy, high resolution electron microscopy and other complementary techniques. These methods are applied to several semiconductor nanostructures for intermediate band solar cells and some prospective results are presented in relation to their potential use for nanostructured thin film solar cells. We also present a method to prepare special samples to correlate, in individual or some few nanostructures, high resolution structural properties with other functional properties for the better design of photovoltaic materials. The nanostructures studied by aberration-corrected scanning transmission electron microscopy that are reviewed in this paper consist of III-V quantum dots and wires, silicon wires and strain- compensated stacked nanostructures grown by epitaxial techniques.
Erin C. Young, Benjamin P. Yonkee, Feng Wu, Burhan K. Saifaddin, Daniel A. Cohen, Steve P. DenBaars, Shuji Nakamura, James S. Speck
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