{HEAD ~ The barrier layers} are made of {TAIL ~ SiO2} so that the electron affinity is made large, and small alternately 
are made of
{ HEAD ~ The barrier layers } are made of { TAIL ~ SiO2 } so that the electron affinity is made large , and small alternately
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH