Superstructure domain boundaries (linear surface defects) act as island nucleation sites in steady state Si(111) homoepitaxial growth below 650°C. The dependence of the average island size on the deposition rate R allows the estimation of the size of the critical nucleus i∗ to be only one or two atoms. This heterogeneous nucleation mechanism dominates the multilayer steady state growth regime. This is in contrast to the homogeneous nucleation of Si in the submonolayer regime on the perfect Si(111)-(7 × 7) surface, where a critical nucleus size of i∗ ⩾ 5 is reported. Scaling of the temperature of growth and the deposition rate is observed. We do not observe a built up of the lateral and vertical surface roughness during growth: there is no Schwoebel barrier.