Nanometer-scale Si crystals were produced by selective epitaxial growth on Si(001) surfaces passivated with 0.3-nm-thick oxide films. Window areas for the growth were provided by void formation during the thermal decomposition of the oxide films. Dynamical processes of the void formation and epitaxial growth were observed at 630–730 °C by scanning tunneling microscopy. The crystal shape was a quadrangular pyramid and the typical size was 20 nm in length and 0.8 nm in height. The thin oxide films were found to act as masks for the selective epitaxial growth of nanoscale structures. ©1997 American Institute of Physics.