{HEAD ~ The GaN/substrate bi layer} may be cooled from the growth temperature to {TAIL ~ an ambient temperature}, wherein GaN material cracks laterally and separates from the substrate, forming a free standing article 
{ HEAD ~ The GaN / substrate bi layer } may be cooled from the growth temperature to { TAIL ~ an ambient temperature } , wherein GaN material cracks laterally and separates from the substrate , forming a free standing article
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