The formation mechanism of rotational twins was systematically studied in beam-induced lateral epitaxy (BILE) on (1 1 1)B GaAs substrates using in situ scanning electron microscope (SEM) and ex situ atomic force microscope (AFM). As a result, it was found that rotational twins were produced from two-dimensional nuclei on the surface with 2 x 2 reconstruction by introducing stacking faults beneath them. As-trimers on 2 x 2 reconstruction probably induced the change. This was confirmed by the result that the formation of rotational twins was suppressed by the use of substrates with offset angles and a high growth temperature. The former is characterized by the high density of surface steps, which inhibits two-dimensional nucleation by the narrow inter-step distance. The latter brings root 19- x root 19 reconstruction, which includes no As-trimer, and leads to the normal stacking of growth with a smooth surface even with two-dimensional nucleation. (c) 2007 Elsevier B.V. All rights reserved.