Buried quantum well structures have been fabricated in GaAs/AIGaAs system using an in situ lithography process. The process utilizes an ultrathin oxide layer formed in situ on a GaAs surface as a mask against Cl2 gas etching. An electron beam (EB)-induced Cl2 gas etching is used to locally remove the oxide mask for positive-type lithography. For negativetype lithography, the oxide mask is selectively formed on a GaAs surface by EB-stimulated oxidation. Subsequent Cl2 gas etching results in the formation of isolated quantum wells. After removing the oxide mask, overgrowth using molecular beam epitaxy is successfully carried out on the patterned surface. The cathodoluminescence image of the buried quantum well demonstrates the high quality of the resulting structure formed by this “in situ EB lithography” process. The photoluminescence intensity from the quantum well of the processed sample is proved to be the same order of magnitude compared with that from a successively grown sample, showing that the use of the oxide mask causes no serious degradation in the processed interface.