We have studied the geometrical anisotropy in the molecular beam epitaxial formation of GaSb/GaAs quantum dot structures grown by a modified Stranski-Krastanov growth mode on (0 0 1) GaAs. Microstructural analysis from both atomic force microscopy and transmission electron microscopy has showed quantum dots elongate along the < 1 1 0 > direction. In particular, the maximum lateral aspect ratio greater than three in the elongated dots is found to exist under high Sb/Ga flux ratio and a suitable As-4 background overpressure. Detailed morphologic analysis by atomic force microscopy reveals that the GaSb dots possess trapezoidal shape with (2 1 1) and (3 1 1) sidewall facets. Probably because of existence of low surface energy for (3 1 1)/(2 1 1) sidewalls, GaSb islands more likely grow up along < 1 1 0 > azimuth other than < 1 1 0 > azimuth and forms the elongated dot structures with a high aspect ratio. In addition, the large anisotropy in polarized photoluminescence is found to exist in these elongated GaSb quantum dots. (c) 2006 Elsevier B.V. All rights reserved.