Atomic-scale structural characterization of grain boundaries in epitaxial Ge/Si microcrystals by HAADF-STEM

The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: Sigma 3{111} coherent twin boundaries, Sigma 3{112} incoherent twin boundaries, and Sigma 9{122} and Sigma 27{552} grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Sigma 3{112} incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Sigma 9{122} and Sigma 27{552} grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.