 starting {HEAD ~ the simultaneous depositing} of Al and {TAIL ~ Ga} atom to form adjacent Al and Ga monolayers 
of
starting { HEAD ~ the simultaneous depositing } of Al and { TAIL ~ Ga } atom to form adjacent Al and Ga monolayers
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH