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