a leak- valve to the ultra-high vacuum (UHV) chamber (base pressure of p

torr. During exposure the sample remained either at room temperatureor at elevated temperatures up to 600 K. The mechanism we suggestfor the removal of oxygen and carbon is the formation of chemicalproducts like H90 and CH4 and their subsequent desorption. Also abuild—up of gaflium- and arsenic-hydrides and their desorption ispossible. After cleaning a clear ixi-LEED pattern indicates a ratherwell-ordered surface.