Atomic arrangements in a triple-period (TP)-A type ordered AlInAs layer were investigated by a cross-sectional. scanning tunneling microscope (XSTM) for the first time. The distributions of cation atoms in the ordered layer were distinguished using the tunneling spectroscopy method. The XSTM image on the cleaved surface of the ordered AlInAs layer revealed the presence of ragged short-range ordering domains comprising periodic structures aligned along the [1 1 2] direction on the whole surface, separated by three-fold periodicity along the [1 1 1] direction. This three-fold periodic structure comprises units of In-In-Al and/or In-Al-Al. That is, the TP-A type ordered AlInAs layer comprises two kinds of three-fold periodic planes; one is composed of two InAs-like planes and one AlAs-like plane, and the other is composed of one InAs-like plane and two AlAs-like planes. In addition, a local two-fold ordered structure having alternating InAs-like and AlAs-like planes can be observed in the STM image. (C) 2004 Elsevier B.V. All rights reserved.