Lattice distortions in three-dimensional coherent In0.35Ga0.65As islands grown by molecular beam epitaxy at 510 °C on GaAs have been imaged by high resolution electron microscopy. The strain fields are determined from the corresponding digital images, either by direct measurement of the lattice distortions or by combining real space and Fourier space information, with an uncertainty Δε=2×10−3. The strain fields are also simulated through finite-element calculations, taking into account the strain relaxation due to the low thickness of the electron-transparent specimens. The significant differences found between experimental and calculated strain fields are attributed to In segregation within the islands. Bidimensional compositional maps are then established showing that the In concentration in the central part of the islands (up to ∼50%) is significantly higher than the nominal concentration (35%), whereas it is lower (down to ∼20%) at the edges of the islands.
The evolution of surface roughness and the subsequent plastic relaxation mechanisms have been studied by transmission electron microscopy (TEM) as a function of the thickness of highly strained In0.30Ga0.70As layers on GaAs(001). The following stages have been observed: formation of coherent islands, coalescence of islands, and nucleation of dislocations at the troughs of the surface ripples. Dislocations are thus systematically generated where the highest stress concentrations are expected, according to recent theoretical predictions. It is the first time such a plastic relaxation mechanism has been observed in highly strained semiconductor heterostructures.
It is demonstrated by using reflection high-energy electron diffraction and transmission electron microscopy that the epitaxial growth of highly strained InxGa1−xAs (x≳0.3) layers on GaAs(001) is improved by a preadsorbed Te surfactant layer. The formation of 3D islands is inhibited by the surfactant action and consequently the onset of plastic relaxation (i.e., the critical thickness) is significantly delayed.
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