Low-temperature growth of GaAs was investigated using in situ reflectance anisotropy spectroscopy (RAS) for the first time. RAS indicates that the surface during growth is mostly unreconstructed with many dangling bonds. Above a critical thickness anisotropic roughening occurs. When doped with Mn, Si, and Sri the RAS signal showed only contributions due to surface band bending from doping. At very high concentrations the spectra change, indicating a transition from doping to alloying. Alloying occurred above 2-3% Mn or for more than 1% Sn incorporation. (C) 2004 Elsevier B.V. All rights reserved.