{HEAD ~ A polycrystalline GaN layer} is grown on {TAIL ~ the bulk GaN layer} to form a GaN/substrate bi layer 
is grown on
{ HEAD ~ A polycrystalline GaN layer } is grown on { TAIL ~ the bulk GaN layer } to form a GaN / substrate bi layer
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH