Structural and chemical investigation of In0.6Ga0.3As Stranski-Krastanow layers buried in GaAs by transmission electron microscopy

We report a detailed structural and chemical study of buried and free-standing In0.6Ga0.4As Stranski-Krastanow islands. The layers were grown by molecular-beam epitaxy on GaAs(001) substrates. We investigated two different types of samples with nominal In0.6Ga0.4As layer thicknesses of 1.5 and 2 nm. The growth was interrupted for 0, 60, or 180 s prior to the deposition of the IO-nm-thick GaAs cap layer. The chemical and structural analyses of the In0.6Ga0.4As layers were carried out with high-resolution transmission electron microscopy. The chemical morphology of the buried layers was evaluated with the composition evaluation by lattice-fringe analysis (CELFA) method. The free-standing islands were investigated by strain state analysis combined with finite element calculations. The density and size distribution of the islands was obtained by conventional plan-view transmission electron microscopy. We found two types of islands: Coherent islands with a lateral size of approximately 13 nm and large islands (40-100 nm) showing plastical strain relaxation. The density of the defect-free small islands decreases with increasing duration of the growth interruption whereas the density and size of the large islands increases. A detailed study of the wetting layer with the CELFA method revealed about a 4-nm-thick InxGa1-xAs layer. The total amount of In contained in the wetting layer decreases with increasing duration of the growth interruption. Composition profiles in growth direction were measured. Their shape is explained by mainly three effects: Segregation of In, incorporation of migrating In into the growing cap layer, and strain-driven migration of In and Ga. An inhomogeneous In concentration increasing from bottom to top is observed in free-standing islands.