A technique for the selective-area growth of GaAs on a nanometer scale is described. The technique comprises nitrogen (N)-passivation mask formation, scanning-tunneling-microscopy (STM) pattern modification, and metalorganic molecular-beam epitaxy. GaAs (001) surfaces are passivated with N radicals dissociated fromN2molecules and are modified by STM on a nanometer scale. GaAs nanostructures are then grown on the modified areas using trimethylgallium and tertiarybutylarsine. Uniform 6-nm-high and50×50-nm2dots were formed on50×50-nm2STM-modified areas. The advantage of the technique is that size-controlled nanostructures can be fabricated in specific positions and these nanostructures are free from contamination because all processes are performed in a vacuum. ©1997 American Institute of Physics.