We have created atomic scale modifications on the GaAs(111)B surface by applying voltage pulses between the tip of a scanning tunneling microscope and a GaAs sample under ultrahigh vacuum conditions. A voltage pulse of 5 V (sample negative) for 25 ms results in the creation of a disordered region (approximately, 3 nm×3 nm in area) of As trimers. In addition, surface stacking faults are formed which extend over distances of order 10 nm and terminate on surface defects. A pulse with the same parameters, but opposite polarity, creates a nanometer scale crater. We argue that the smallest features are formed by electric field induced diffusion (for negative bias pulses) or field desorption (positive bias pulses). ©1995 American Institute of Physics.