Bismuth surfactant is shown to have dramatic effects on the microstructure of GaAsSb alloys grown on nominally [001] InP substrates by organometallic vapor phase epitaxy. The addition of Bi at a ratio of 1% Bi/Ga in the gas flow results in the formation of CuAu {100} and chalcopyrite {210} ordered and disordered structures. Domains are approximately 10-20 nm in diameter for both undoped and heavily carbon-doped GaAsSb layers, and they account for similar to 40% (CuAu) and similar to 3% (chalcopyrite) of the total surface area. The addition of this small amount of Bi does not affect growth rate, Sb concentration, or surface morphology. Ordered domains with c-axes along the [001] growth direction were not observed. Photoluminescence and optical absorption measurements did not detect band gap changes in GaAsSb samples with and without {100} and {210} ordering, respectively. (c) 2005 Elsevier B.V. All rights reserved.