Typical MBE consists of three main vacuum chambers: a growth chamber, a buffer chamber, and a load lock. The load lock is used to bring samples into and out of the vacuum environment while maintaining the vacuum integrity of the other chambers. The buffer chamber is used for preparation and storage of samples, but other systems have buffer chambers equipped for material characterization as well. The buffer chamber on the MBE system also acts as a transition tube to allow samples to be transferred under vacuum to two different  growth chambers, which are nearly identical except that one is set up to grow phosphorus containing III-V compound semiconductors and the other is for arsenic-based compounds. The As-based growth chamber is the one that is used primarily for the work reported here. The growth chamber of a generic MBE system and several of its subsystems are illustrated in the next figure.