Insight into the Ga/In flux ratio and crystallographic plane dependence for MBE self-assembled growth of InGaN nanorods on patterned sapphire substrates
A controllable self-assembled growth using molecular beam epitaxy (MBE) for dense, uniform, and high-aspect-ratio InGaN nanorods (NRs) is achieved through regulating Ga/In flux ratio and employing high Miller index planes of patterned sapphire substrate (PSS). It is clearly demonstrated that both the low Ga/In flux ratio and high Miller index plane of PSS patterns facilitate the three-dimensional growth mode for InGaN NRs and simultaneously suppress the NR coalescence. Lower Ga/In flux ratio favors higher density, larger aspect ratio, and smaller coalescence degree of InGaN NRs through enhancing axial growth and inversely suppressing radial growth. The specific surface structures of high Miller index planes, e.g., the well-organized step-terrace and irregular bulge structures, critically affect the morphology, dimension, density, and crystallographic orientation of MBE self-assembled NRs. Especially, the narrow and ordered step-terrace structure in C3-plane—(4 -5 1 38) plane on hexagonal pyramid, favors the highest density, largest aspect ratio, and best uniformity of semipolar InGaN NRs, thus contributing to optimal photoluminescence performance. A thorough understanding about the effect mechanism of the Ga/In flux ratio and crystallographic plane on the MBE self-assembled growth behaviour for InGaN NRs was obtained through experimental and theoretical exploration. This work contributes to deeply understanding the MBE self-assembled growth mechanism, as well as controllably fabricating dense, well-separated, and uniform InGaN NRs, thus contributing to the enhanced performance of NR-based optoelectronic devices.