Vertically aligned InGaN nanowires with engineered axial In composition for highly efficient visible light emission

Comprehensive model toward optimization of SAG In-rich InGaN nanorods by hydride vapor phase epitaxy

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.

Thermodynamics of the Vapor-Liquid-Solid Growth of Ternary III-V Nanowires in the Presence of Silicon

Based on a thermodynamic model, we quantify the impact of adding silicon atoms to a catalyst droplet on the nucleation and growth of ternary III–V nanowires grown via the self-catalyzed vapor–liquid–solid process. Three technologically relevant ternaries are studied: InGaAs, AlGaAs and InGaN. For As-based alloys, it is shown that adding silicon atoms to the droplet increases the nanowire nucleation probability, which can increase by several orders magnitude depending on the initial chemical composition of the catalyst. Conversely, silicon atoms are found to suppress the nucleation rate of InGaN nanowires of different compositions. These results can be useful for understanding and controlling the vapor–liquid–solid growth of ternary III–V nanowires on silicon substrates as well as their intentional doping with Si.

Compositional control of homogeneous InGaN nanowires with the In content up to 90

Homogenous InGaN nanowires with a controlled indium composition up to 90% are grown on GaN/c-Al₂O₃ templates by catalyst-free hydride vapor phase epitaxy using InCl₃ and GaCl as group III element precursors. The influence of the partial pressures on the growth rate and composition of InGaN nanowires is investigated. It is shown how the InN mole fraction in nanowires can be finely tuned by changing the vapor phase composition. Thermodynamic calculations are presented that take into account different interconnected reactions in the vapor phase and show a good agreement with the compositional data. Energy dispersive x-ray spectroscopy profiles performed on single nanowires show a homogenous indium composition along the entire nanowire length. X-ray diffraction measurements performed on nanowires arrays confirm these data. High-resolution transmission electron microscopy analysis shows the wurtzite crystal structure with a reduced defect density for InGaN nanowires with the highest indium content.

Circumventing the miscibility gap in InGaN nanowires emitting from blue to red

Widegap III-nitride alloys have enabled new classes of optoelectronic devices including light emitting diodes, lasers and solar cells, but it is admittedly challenging to extend their operating wavelength to the yellow-red band. This requires an increased In content x in In _x Ga_1-x N, prevented by the indium segregation within the miscibility gap. Beyond the known advantage of dislocation-free growth on dissimilar substrates, nanowires may help to extend the compositional range of InGaN. However, the necessary control over the material homogeneity is still lacking. Here, we present In _x Ga_1-x N nanowires grown by hydride vapor phase epitaxy on silicon substrates, showing rather homogeneous compositions and emitting from blue to red. The InN fraction in nanowires is tuned from x = 0.17 up to x = 0.7 by changing the growth temperature between 630 °C and 680 °C and adjusting some additional parameters. A dedicated model is presented, which attributes the wide compositional range of nanowires to the purely kinetic growth regime of self-catalyzed InGaN nanowires without macroscopic nucleation. These results may pave a new way for the controlled synthesis of indium-rich InGaN structures for optoelectronic applications in the extended spectral range.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

We studied the emission of bare and aluminum quinoline (Alq₃)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq₃ coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq₃ spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq₃ spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Single crystalline ZnO radial homojunction light-emitting diodes fabricated by metalorganic chemical vapour deposition

ZnO radial p-n junction architecture has the potential for forward-leap of light-emitting diode (LED) technology in terms of higher efficacy and economical production. We report on ZnO radial p-n junction-based light emitting diodes prepared by full metalorganic chemical vapour deposition (MOCVD) with hydrogen-assisted p-type doping approach. The p-type ZnO(P) thin films were prepared by MOCVD with the precursors of dimethylzinc, tert-butanol, and tertiarybutylphosphine. Controlling the precursor flow for dopant results in the systematic change of doping concentration, Hall mobility, and electrical conductivity. Moreover, the approach of hydrogen-assisted phosphorous doping in ZnO expands the understanding of doping behaviour in ZnO. Ultraviolet and visible electroluminescence of ZnO radial p-n junction was demonstrated through a combination of position-controlled nano/microwire and crystalline p-type ZnO(P) radial shell growth on the wires. The reported research opens a pathway of realisation of production-compatible ZnO p-n junction LEDs.