Experimental data and a supporting model are presented for the formation of voids in InN nanorods grown by selective area hydride vapor phase epitaxy (HVPE) on patterned GaN/c-Al2O3 templates. It is shown that these voids shape, due to a high lattice mismatch between InN and GaN materials, starts from the base and extends up to a half of the total length of the nanorods. When the effect of the mismatch between substrate and nanorods becomes weaker, the hollow nanotubes close up at the top and further growth proceeds in the standard nanowire geometry without voids. This effect is observed within a wide range of growth conditions during the InN synthesis and must be taken into account for controlling the final structure of InN nanorods for different device applications.
Controlled growth of In-rich InGaN nanowires/nanorods (NRs) has long been considered as a very challenging task. Here, we present the first attempt to fabricate InGaN NRs by selective area growth using hydride vapor phase epitaxy. It is shown that InGaN NRs with different indium contents up to 90% can be grown by varying the In/Ga flow ratio. Furthermore, nanowires are observed on the surface of the grown NRs with a density that is proportional to the Ga content. The impact of varying the NH3 partial pressure is investigated to suppress the growth of these nanowires. It is shown that the nanowire density is considerably reduced by increasing the NH3 content in the vapor phase. We attribute the emergence of the nanowires to the final step of growth occurring after stopping the NH3 flow and cooling down the substrate. This is supported by a theoretical model based on the calculation of the supersaturation of the ternary InGaN alloy in interaction with the vapor phase as a function of different parameters assessed at the end of growth. It is shown that the decomposition of the InGaN solid alloy indeed becomes favorable below a critical value of the NH3 partial pressure. The time needed to reach this value increases with increasing the input flow of NH3, and therefore the alloy decomposition leading to the formation of nanowires becomes less effective. These results should be useful for fundamental understanding of the growth of InGaN nanostructures and may help to control their morphology and chemical composition required for device applications.
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