THz behavior originates from different arrangements of coalescent GaN nanorods grown on Si (111) and Si (100) substrates

•Two GaN nanorods were grown on Si (111) and Si (100) by molecular beam epitaxy.•The two GaN nanorods exhibited identical physical, chemical and optical properties.•However, we observed a clear difference between the two GaN nanorods in THz behavior.•We proved the difference is due to aligned or random orientation of two GaN nanorods.