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