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