Top-view SEM images of (a) sample A, (b) sample B, and (c) sample C grown on Si(111) for 4 h. Sample B was grown with the use of a low nitrogen flow rate ∼0.3 sccm during the first 20 min, compared to the 1 sccm used for sample A. The growth conditions of sample C are identical to those of sample B except that the growth temperature was reduced from 740 to 700 °C. It is seen that a lower growth temperature and reduced N/Ga flux ratio can significantly enhance the spontaneous coalescence of nanowire arrays.
Lateral view SEM images of GaN nanowires grown on Si(111) after (a) 40 min, (b) 2 h, and (c) 4 h growth duration.
(a) TEM image for nanowires grown after 40 min, showing the absence of any coalescence. (b)–(d) Representative TEM images for GaN nanowires grown after 4 h, showing coalescence due to variations in the growth direction or enhanced lateral growth. Circles indicate the coalescence interface, and arrows indicate the growth direction of nanowires.
High resolution TEM images for two coalesced nanowires with tilting angles of (a) ∼5° and (b) ∼25°. Arrows indicate the growth direction of the two coalesced branches. (a) Threading dislocations are localized above the coalescence interface. (b) The propagation of threading dislocations along the coalescence interface and the twin boundary can be identified. The lattice spacing along growth direction is 0.256 nm, indicating that nanowire growth is along the c-axis and strain free [for GaN, d(002) = 0.256 nm].
SEM images of GaN nanowires grown (a) on SiOx for 3 h and (b) on Si(111) substrate for 4 h. The SiOx layer is indicated by black arrow in (a).The tilting of nanowires is more pronounced for GaN nanowires grown on SiOx, due to the surface roughness.
(a) Top view and (b) lateral view SEM images of GaN microcrystals grown on SiOx with a growth temperature of 575 °C during the coalescence stage. (c) Top view and (d) lateral view of GaN epilayers grown on SiOx with a growth temperature of ∼545 °C during the coalescence stage. The SiOx layer is indicated by an arrow. The presence of GaN nanowires in the nucleation stage and the subsequent coalescence into microcrystals (b) and smooth epilayers (d) are indicated by parallel lines.
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