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