ABSTRACT
Moiré-like fringes are observed in transmission electron microscopy images of coherently strained semiconductor islands. They are due to the misfit between the island and the underlying substrate and they can be used to determine the chemical composition of these islands by measuring the fringe spacing of the Moiré-like system. The results of a simple kinematical analysis are shown to be very similar to those of dynamical two-beam calculations. The interest of the kinematical analysis is that, contrary to the dynamical two-beam calculations, it makes it possible to understand how Moiré-like fringes are related to parallel Moiré fringes and how the fringe spacing is related to the strain field at the apex of the island, and then to the mean composition of the island.
ABSTRACT
The continuous displacement field within elastically relaxed GaInAs islands was calculated from digitized HREM images of [110] cross sections of In0.35Ga0.65As layers grown on GaAs by molecular beam epitaxy. Experimental maps of the deformations parallel to the interface (epsilonx) and along the growth direction (epsilonz) were drawn and compared with the ones calculated via the finite element method. It was found that epsilonx exp was systematically higher than epsilonx calc and the significant maximum observed for epsilonz exp within the island could not be found for epsilonz calc. These discrepancies were attributed to a variation of the chemical composition in the island. The maps showing the indium concentration gradient drawn from HREM and FE calculations were compared to quantitative profiles for indium concentration obtained by nanometric X-ray microanalysis in TEM. The measured gradient within the island backs our assumption.
