Chemical mechanical polished (CMP) (100) GaSb substrates were processed using gas cluster ion beams (GCIB) to improve surface smoothness, reduce subsurface damage, and produce a thermally desorbable oxide layer for molecular-beam epitaxy (MBE) overgrowth. In this article, we report the growth of GaSb/AlGaSb epilayers on GCIB processed GaSb substrates. The substrates were processed using either O-2 or CF4/O-2 as the gas cluster in a dual-energy recipe that included a moderate energy (10 keV) etch step followed by a low-energy (3 keV) smoothing step, with a relatively low total dose of 4 x 10(15) ions/cm(2). Half of each wafer was masked such that the epitaxial layers were grown on both CMP and GCIB polished surfaces. Atomic force microscopy showed the elimination of CMP surface scratches on the GCIB processed surfaces. X-ray photoelectron spectroscopy results indicate that the surface oxide composition and thickness can be engineered through the GCIB process recipes. AlGaSb marker layers were used to chart the evolution of the overgrown layers. Cross-sectional transmission electron microscope images of the substrate/epi-interface show that the CMP finished regions contained defects that propagated into the epilayers as compared to the GCIB finished region that showed no penetrating defects, indicating an improved substrate/epi-interface. This work demonstrated that GCIB processing of semiconductor materials has the potential to produce "epiready" surfaces. (C) 2004 American Vacuum Society.