{HEAD ~ Double heterojunctions} are employed in {TAIL ~ two different modes}: bulk mode or quantum well mode 
are employed in
{ HEAD ~ Double heterojunctions } are employed in { TAIL ~ two different modes } : bulk mode or quantum well mode
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH