an impurity layer formed between said first semiconductor substrate and epitaxial layer extending to the surface of said composite substrate, said impurity layer having an etching rate different from {HEAD ~ etching rates} of said first semiconductor substrate and {TAIL ~ epitaxial layer} 
of
an impurity layer formed between said first semiconductor substrate and epitaxial layer extending to the surface of said composite substrate , said impurity layer having an etching rate different from { HEAD ~ etching rates } of said first semiconductor substrate and { TAIL ~ epitaxial layer }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL