Scanning tunneling microscopy (STM) is used to observe changes in the surface morphology of a Si (100) surface during the co-deposition of Sb and Si at 500 degreesC. The initial deposition of only 0.01 monolayers of Sb is shown to modify the Si growth mechanism. The roughness of the surface is found to increase during the deposition of Si under a constant Sb flux where the Sb surface coverage is also increasing due to surface segregation. By holding the Sb surface coverage constant and varying the Si deposition, we find that the presence of Sb on the surface inhibits Si attachment at step edges producing three-dimensional stacked island formations. We show that heating the sample for 60 s at 800 degreesC to desorb the surface Sb can return a planar morphology to the Si surface.