We report results of a surface modification process for (100) GaSb using a gas cluster ion beam (GCIB) technique that removes chemical mechanical polish (CMP)-induced surface damage and replaces the native oxide with an engineered surface oxide, the composition of which depends on the reactive gas employed. X-ray photoelectron spectroscopy of O-2-, CF4/O-2-, and HBr-GCIB surface oxides is presented indicating the presence of mixed Ga- and Sb-oxides, with mostly Ga-oxides at the interface, that desorb at temperatures ranging 530-560 degrees C. Cross-sectional transmission electron microscopy of molecular-beam epitaxy grown GaSb/AlGaSb layers showed that the HBr-GCIB surface produced a smooth dislocation-free substrate-to-epitransition with no discernable interface. Topography of epi surfaces, using atomic force microscopy, showed that GCIB surfaces resulted in characteristic step-terrace formations comprising monatomic steps and wide terraces. The HBr-GCIB process can be easily adapted to a large-scale manufacturing process to produce epi-ready GaSb substrates. (C) 2008 Elsevier B.V. All rights reserved.