The determination of the rigid-body translation across anti-phase boundaries in GaAs

Abstract

The growth of a polar material on a non-polar substrate leads to the possibility of anti-phase boundary (APB) formation. Across the boundary there will be bonding between like atoms. The stoichiometry of the material may, or may not, be affected, depending on the relative proportion of A-A and B-B bonds in the boundary. This will depend on the crystallographic orientation of the boundary plane. Because of the different nature of the bonding at an APB, it has been assumed that the physical properties, particularly the electrical characteristics, of the polar layer will be adversely affected by the presence of APBs. Much effort has therefore gone into the development of growth conditions which minimize the likelihood of APB formation, particularly in the GaAs on Si system.Little work, however, has been done on the fine-structure of the APB itself. In this paper the results of a quantitative comparison of the α fringes produced in 2-beam images of APBs, with calculated images will be presented. The theoretical images were calculated using the COMIS program, which uses a manybeam formalism based on the Howie-Whelan equation. The samples were prepared from GaAs epilayers deposited on a (100) Ge substrate. The Ge substrate was removed by mechanical polishing prior to ion milling of the epilayer in a liquid nitrogen cooled specimen stage. The epilayer was examined at a number of low index zone axes, using a number of different 2-beam conditions. A representative selection is shown in figure 1. Images A, B, and C, were obtained close to the [001] zone axis, image D was obtained near the [101] zone axis and shows the horizontal APB nearly flat-on, but out of contrast. Near the [101] zone axis this APB appears as a line defect, and therefore must lie on the (101) plane.