Application of convergent-beam illumination methods to the study of lattice distortion across the interface

Abstract

Convergent beam illumination methods were applied to the study of the lattice parameter change in the GaAs layers grown by molecular beam epitaxy (MBE) at 195°C (called LT-GaAs layers) with 1.5% of As excess. This excess of As leads to the lattice parameter change. Tetragonal distortion with a change of the lattice parameter along the growth direction was observed in four independently measured samples. The strain accumulated in the layer would explain the formation of pyramidal defects in these layers at a specific layer thickness related to the As concentration. An artifact due to the surface relaxation of the TEM thin foil is also considered. Surface relaxation was found to be present in the sample and the model was presented for the explanation of Kikuchi line shifts in the large-angle convergent-beam electron diffraction (LACBED) pattern and convergent-beam imaging methods (CBIM) pattern. The lowering the symmetry of CBED patterns from tetragonal to orthorombic observed in this study is most possible influenced by surface relaxation as well, however, the fact that similar results were obtained on four independent samples leaves this question still open for the future study.