We present a high resolution transmission electron microscopy (HRTEM) and reflection high energy electron diffraction (RHEED) study of the molecular beam homoepitaxy of Si(111) with and without Ga as surfactant. Cross-sectional images show that the growth at low temperature was associated with a periodic stacking fault (ABC/BCA fault type), leading to a high surface roughness. If the Si film was grown with 1/3 monolayer of Ga added prior to the growth, cross-sectional images show a perfectly flat surface, even at growth temperatures as low as 300 degrees C. According to a systematic study of the damping of the RHEED oscillations during the growth with and without Ga and the determination of the 2D nucleation/step-flow transition, no obvious kinetic effect could be attributed to the surfactant action.