In this paper we present the first application of the selective area growth by periodic supply molecular beam epitaxy (PSE/MBE) to the fabrication of highly uniform GaAs submicrometer structures grown on GaAs(0 0 1) substrates patterned with a SiO2 mask. By a combination of selective and lateral growth, the merging of epilayers from adjacent open windows occurs and submicrometer structures of ridge and groove are fabricated above the buried SiO2 mask. We have studied the morphology dependence of the epilayers on the geometry of the pattern, that is, on the window width and on the mask width. It was found that the geometry has a strong influence both on the morphology and on the possibility of merging of epilayers. The geometry has also a strong correlation with the deposition time and the different morphologies can be regarded as a time evolution of the growth of the epilayers. Besides, the morphology was found to be influenced by the crystallographic orientation of the line-shaped open windows. At optimized growth conditions and PSE parameters and under optimized geometry, we were able to grow uniform GaAs structures of excellent smoothness above the buried oxide mask.