. Using an ultra-high-vacuum cross-sectional scanning tunneling microscope (XSTM), we observed the atomic arrangement of a double period (CuPt)-B-type ordered GaAsSb layer grown on (0 0 1) InP by molecular beam epitaxy (MBE). XSTM images of the ordered GaAsSb region revealed that the arrangement of the CuPt-B-type ordered GaAsSb layer was partially and non-uniformly formed in the epitaxial layer. The image also revealed that the ordered region was composed of one GaAs-like planes and one GaSb-like planes alternatively aligned in the [(1) over bar 1 1] and [(1) over bar 1 1] directions. (c) 2006 Elsevier B.V. All rights reserved.