An atomistic model is presented based on scanning tunneling microscopy
results and tight binding calculations which explains the observation
of disorder on the GaAs(001)-(2 X 4) surface grown by molecular beam
epitaxy. Calculations show that occupation by As of vacant Ga sites in
the missing dimer trenches of the (2 X 4) unit cell is responsible fo
r the surface disorder in the form of kinks in the dimer rows. The dis
ordered surface is energetically favorable for a range of additional A
s coverage up to 0.25 monolayer.