High-quality ZnO layers are grown on Zn-polar ZnO substrates by surfactant-mediated plasma-assisted molecular-beam epitaxy (P-MBE) using atomic hydrogen as a surfactant. Careful investigation with atomic force microscopy (AFM) and reflection high-energy electron diffraction (RHEED) reveals that two-dimensional growth is preserved down to 400 degrees C by irradiating atomic hydrogen during growth, while the low-temperature limit of two-dimensional growth is 600 degrees C without atomic hydrogen irradiation. The crystal quality of ZnO layers grown at 400 degrees C by surfactant-mediated MBE is evaluated to be the same as those grown at 600 degrees C by conventional MBE in terms of X-ray diffraction and photo luminescence properties. (C) 2007 Elsevier B.V. All rights reserved.