High-density and similarly-sized Si nanodots were formed by annealing ultra-thin amorphous Si (a-Si) films deposited on SiO(2)/Si substrates in vacuum. Dependences of density and diameter of the Si nanodots on the a-Si film thickness and, annealing temperature and time were investigated by scanning electron microscopy. It is found that drastic increase (decrease) in the density (diameter) occurred at an a-Si thickness of 1 nm. By agglomeration of sub-nanometer thick a-Si films, a density larger than 10(12) cm(-2), an average diameter smaller than 5 nm, and a dispersion of diameter less than 15% were achieved. (c) 2008 Elsevier B.V. All rights reserved.