CONTAMINATION IN MOLECULAR-BEAM EPITAXY - THE ROLE OF ARSENIC DRAG EFFECT

We have measured levels of Al, In and Si in nominally pure GaAs layers grown by molecular beam epitaxy as a function of arsenic Aux for a nu mber of different combinations of idling temperatures of Al. In and Si cells, using either As-2 or As-4 arsenic molecules, We find that a co nsiderable number of atoms find their way to the growing layer in spit e of closed shutters and no direct line of sight from their effusion c ell to the wafer surface. The number of these atoms is proportional to the arsenic flux used and to the equilibrium vapor pressure over the considered element. We present arguments for the existence of the ''ar senic drag'' effect which deflects a fraction of the atoms that by-pas sed their effusion cell shutter towards the wafer.