The mesa-stripe structures of GaAs/AlGaAs multi-quantum-wells (MQWs) were buried by AlGaAs overgrown layers using an in situ fabrication technique, including Cl2 gas etching and molecular-beam epitaxial regrowth without air exposure. In these structures, having stripe widths of less than several microns, the photoluminescence intensities from the MQWs were considerably improved, compared to conventionally fabricated buried structures as well as to as-etched open-sidewall structures. Correspondingly, it was observed by transmission-electron-microscope that the densities of the crystal defects at the in situ regrown interfaces were greatly reduced, indicating great usefulness of in situ techniques in fabricating high-quality microstructures.