Many novel optoelectronic devices can be realized by the technique of molecular beam epitaxy regrowth on etched surfaces. A smooth etched surface free from contaminants is necessary as this surface determines the success of the regrowth and the etched/regrown interface affects the performance and reliability of the devices. We have systematically studied the contaminants at the etched and regrown interface of Al0.4Ga0.6As for various steps of our in situ process. Using secondary ion mass spectroscopy, transmission electron microscopy, and mass spectroscopic analysis we have pinpointed what we believe to be the sources of carbon, oxygen, and silicon contamination in the etch chamber and have taken steps to eliminate them.