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Featured Application: GaN/Ga2O3 core/shell nanowires appear as promising solution to fabricate gas sensors working in a large range of temperature, from ambient temperature to high temperatures. Thanks to their core-shell heterostructures, these systems are based on the NW resistance modulation process, then offering the possibility to improve the sensor performances. The development of sensors working in a large range of temperature is of crucial importance in areas such as monitoring of industrial processes or personal tracking using smart objects. Devices integrating GaN/Ga2O3 core/shell nanowires (NWs) are a promising solution for monitoring carbon monoxide (CO). Because the performances of sensors primarily depend on the material properties composing the active layer of the device, it is essential to control them and achieve material synthesis in the first time. In this work, we investigate the synthesis of GaN/Ga2O3 core-shell NWs with a special focus on the formation of the shell. The GaN NWs grown by plasma-assisted molecular beam epitaxy, are post-treated following thermal oxidation to form a Ga2O3-shell surrounding the GaN-core. We establish that the shell thickness can be modulated from 1 to 14 nm by changing the oxidation conditions and follows classical oxidation process: A first rapid oxide-shell growth, followed by a reduced but continuous oxide growth. We also discuss the impact of the atmosphere on the oxidation growth rate. By combining XRD-STEM and EDX analyses, we demonstrate that the oxide-shell is crystalline, presents the β-Ga2O3 phase, and is synthesized in an epitaxial relationship with the GaN-core. [ABSTRACT FROM AUTHOR]

We investigate the growth mechanism of axially heterostructured InGaN/GaN nanowires (NWs) as a function of the flux conditions. The InGaN heterostructure morphology critically depends on the In/Ga flux ratio affecting the local V/III ratio at the NW growth front. Locally N-rich conditions are associated with tapered island-like morphologies, while metal-rich conditions, leading to the formation of a stable Indium adsorbed layer at the NW growth front, promote the growth of heterostructures with a disk-like shape. Based on experimental results and theoretical predictions, we demonstrate that this indium ad-layer acts as a surfactant inducing a modification of the InGaN heterostructure growth mode. The impact of flux conditions and strain relaxation on the Indium incorporation are also addressed. The resulting insertions present abrupt interfaces and a homogeneous In distribution for In contents up to 40%. [ABSTRACT FROM AUTHOR]

Epitaxial growth of GaN nanowires on graphene is demonstrated using molecular beam epitaxy without any catalyst or intermediate layer. Growth is highly selective with respect to silica on which the graphene flakes, grown by chemical vapor deposition, are transferred. The nanowires grow vertically along their c-axis and we observe a unique epitaxial relationship with the ⟨21̅1̅0⟩ directions of the wurtzite GaN lattice parallel to the directions of the carbon zigzag chains. Remarkably, the nanowire density and height decrease with increasing number of graphene layers underneath. We attribute this effect to strain and we propose a model for the nanowire density variation. The GaN nanowires are defect-free and they present good optical properties. This demonstrates that graphene layers transferred on amorphous carrier substrates is a promising alternative to bulk crystalline substrates for the epitaxial growth of high quality GaN nanostructures. [ABSTRACT FROM AUTHOR]

The polarity of GaN nanowires grown on Si(111) by molecularbeamepitaxy is determined by convergent beam electron diffraction. Thickspecimens lead to unambiguous results showing their N-polarity. Complementarysample etching indicates that all the nanowires have the same N-polarityand are surrounded by a Ga-polar two-dimensional layer. A correlationwith the initial growth stage which uses consecutive exposures ofthe Si substrate to Al and N is established. Al–Si liquid dropletsare formed and produce AlN columnar pedestals playing the role ofseeds for the GaN nanowires. On the surrounding Si surface, the GaNlayer nucleates with the opposite Ga-polarity. [ABSTRACT FROM AUTHOR]