Finite-element calculations are used to study strain fields in vertically aligned InAs islands in GaAs. Such strain fields are found to be quite different from those of uncovered islands and nearly insensitive to the position of the island in the stacking. The driving force for vertically self-organized growth is known to be the interacting strain fields induced by the islands. The calculation of strain fields by the finite-element method makes it possible to model the correlations between adjacent InAs layers. A kinetic approach based on the effect of strain on surface diffusion is first proposed. A thermodynamic model is then analyzed to predict local island nucleation probabilities. Pairing probabilities of correlation between stacked islands, first calculated in the case of the InAs/GaAs system, are extended to the case of III-V semiconductors with a cubic crystalline structure. They are shown to be essentially dependent both on the ratio between the spacer layer thickness and the island height and on the lattice mismatch between islands and spacer layers.

Residual stresses and clamped thermal expansion in LiNbO3 and LiTaO3 thin films Appl. Phys. Lett. 101, 122902 (2012) Tribological properties of nanocrystalline diamond films deposited by hot filament chemical vapor deposition AIP Advances 2, 032164 (2012) The combined effect of surface roughness and internal stresses on nanoindentation tests of polysilicon thin films J. Appl. Phys. 112, 044512 (2012) Mechanism for atmosphere dependence of laser damage morphology in HfO2/SiO2 high reflective films Finite element ͑FE͒ analysis and transmission electron microscopy ͑TEM͒ observations have been used to model stress relaxation in InAs quantum dots deposited on ͑001͒ GaAs. TEM observations show that these islands are coherently strained and the corresponding strain contrast is simulated using the dynamical electron diffraction contrast theory. The dot strain fields used for the TEM contrast simulations are deduced from FE calculations. These calculations show that elastic stress relaxation mainly occurs at the crest of the island and that the underlying substrate is under tension. That experimental TEM images and simulated images should match shows that the FE method of determination of the dot strain fields is valid ͑even in the case of microscopic objects͒, and that the shape of islands can be specified.

We propose a method to determine the indium concentrations x and y in the InyAl1−yAs/InxGa1−xAs metamorphic structures. This approach is based on the combination of two experimental techniques: (i) reciprocal space mapping (RSM) to determine the average In composition in the InAlAs layers and (ii) transmission electron microscopy (TEM) using the intensity measurements of the chemically sensitive (002) reflection from dark-field images to determine the composition in the InGaAs quantum well. We apply this method to a InyAl1−yAs/InxGa1−xAs metamorphic high electron mobility transistor, with x and y approximately equal to 0.35. Furthermore, we present an original and straightforward way to evaluate experimental errors in the determination of composition and strain with the RSM procedure. The influence of these errors on the TEM results is discussed. For In concentrations in the 30%–40% range, the accuracy of this simple method is about 0.5% on the In composition in the InGaAs quantum well.

International audienceThe waveguideproperties are reported for wide bandgap gallium nitride(GaN) structures grown by metal organic vapor phase epitaxy on sapphire using a AlN/GaN short period-superlattice (SPS) buffer layer system. A detailed optical characterization of GaN structures has been performed using the prism coupling technique in order to evaluate its properties and, in particular, the refractive index dispersion and the propagation loss. In order to identify the structural defects in the samples, we performed transmission electron microscopy analysis. The results suggest that AlN/GaN SPS plays a role in acting as a barrier to the propagation of threading dislocations in the active GaN epilayer; above this defective region, the dislocations density is remarkably reduced. The waveguide losses were reduced to a value around 0.65dB/cm at 1.55μm, corresponding to the best value reported so far for a GaN-based waveguide