Laser molecular beam epitaxy of vertically self-assembled GaN nanorods on Ta metal foil: role of growth temperature and laser repetition rate

Self-aligned GaN nanorod assembly directly grown on metal foil substrates is very attractive for developing flexible devices. We have investigated the effect of growth temperature (500-700 degrees C) and laser repetition rate (10-30 Hz) on the formation, structure, and morphological and optical properties of GaN nanorods, grown on flexible Ta metal foil by laser molecular beam epitaxy. It is found that the growth temperature plays a critical role in the formation of vertically self-aligned growth of GaN nanorods. High-temperature growth at 700 degrees C yielded a highly dense and vertically aligned nanorod ensemble on Ta foil. The nanorod alignment is highly uniform across the sample and no tilt was observed irrespective of the polycrystalline nature of the Ta foil. The GaN nanorods are characterized by their hexagonal wurtzite structure and c-axis oriented growth. The effect of laser repetition rate on the growth of GaN at 700 degrees C was to alter only the dimension and areal density of GaN nanorods but did not affect the nanorod alignment. The grown GaN nanorod ensemble exhibits high optical quality with an intense and sharp near-band-edge emission in the UV region and negligible yellow band emission. The growth of GaN nanorods on Ta foil is observed to be self-driven without being influenced by the nature of the substrate.