Self-induced growth of vertical GaN nanowires on silica

Manipulation of Photoelectrochemical Water Splitting by Controlling Direction of Carrier Movement Using InGaN/GaN Hetero-Structure Nanowires

We report the improvement in photoelectrochemical water splitting (PEC-WS) by controlling migration kinetics of photo-generated carriers using InGaN/GaN hetero-structure nanowires (HSNWs) as a photocathode (PC) material. The InGaN/GaN HSNWs were formed by first growing GaN nanowires (NWs) on an Si substrate and then forming InGaN NWs thereon. The InGaN/GaN HSNWs can cause the accumulation of photo-generated carriers in InGaN due to the potential barrier formed at the hetero-interface between InGaN and GaN, to increase directional migration towards electrolyte rather than the Si substrate, and consequently to contribute more to the PEC-WS reaction with electrolyte. The PEC-WS using the InGaN/GaN-HSNW PC shows the current density of 12.6 mA/cm² at -1 V versus reversible hydrogen electrode (RHE) and applied-bias photon-to-current conversion efficiency of 3.3% at -0.9 V versus RHE. The high-performance PEC-WS using the InGaN/GaN HSNWs can be explained by the increase in the reaction probability of carriers at the interface between InGaN NWs and electrolyte, which was analyzed by electrical resistance and capacitance values defined therein.

Selective Area Epitaxy of GaN Nanowires on Si Substrates Using Microsphere Lithography: Experiment and Theory

GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiO_x/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N₂ flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N₂ flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements.

Investigation of the effect of the doping order in GaN nanowire p-n junctions grown by molecular-beam epitaxy

We analyse the electrical and optical properties of single GaN nanowire p-n junctions grown by plasma-assisted molecular-beam epitaxy using magnesium and silicon as doping sources. Different junction architectures having either a n-base or a p-base structure are compared using optical and electrical analyses. Electron-beam induced current (EBIC) microscopy of the nanowires shows that in the case of a n-base p-n junction the parasitic radial growth enhanced by the magnesium (Mg) doping leads to a mixed axial-radial behaviour with strong wire-to-wire fluctuations of the junction position and shape. By reverting the doping order p-base p-n junctions with a purely axial well-defined structure and a low wire-to-wire dispersion are achieved. The good optical quality of the top n nanowire segment grown on a p-doped stem is preserved. A hole concentration in the p-doped segment exceeding 10¹⁸ cm^-3 was extracted from EBIC mapping and photoluminescence analyses. This high concentration is reached without degrading the nanowire morphology.

Titanium Carbide MXene Nucleation Layer for Epitaxial Growth of High-Quality GaN Nanowires on Amorphous Substrates

Growing III-nitride nanowires on 2D materials is advantageous, as it effectively decouples the underlying growth substrate from the properties of the nanowires. As a relatively new family of 2D materials, MXenes are promising candidates as III-nitride nanowire nucleation layers capable of providing simultaneous transparency and conductivity. In this work, we demonstrate the direct epitaxial growth of GaN nanowires on Ti₃C₂ MXene films. The MXene films consist of nanoflakes spray coated onto an amorphous silica substrate. We observed an epitaxial relationship between the GaN nanowires and the MXene nanoflakes due to the compatibility between the triangular lattice of Ti₃C₂ MXene and the hexagonal structure of wurtzite GaN. The GaN nanowires on MXene show good material quality and partial transparency at visible wavelengths. Nanoscale electrical characterization using conductive atomic force microscopy reveals a Schottky barrier height of ∼330 meV between the GaN nanowire and the Ti₃C₂ MXene film. Our work highlights the potential of using MXene as a transparent and conductive preorienting nucleation layer for high-quality GaN growth on amorphous substrates.

A systematic study of Ga- and N-polar GaN nanowire–shell growth by metal organic vapor phase epitaxy

N-polar and Ga-polar (0001) GaN core–shell wires detached from an AlN/Si(111) growth template. Different facets have been identified, limiting the vertical shell growth extension, modelled by varying surface terminations and different H-passivation.

Spatially controlled VLS epitaxy of gallium arsenide nanowires on gallium nitride layers

MOVPE of Au catalyzed p-GaAs nanowires on n-GaN layers. Left: VLS growth optimization (density and morphology). Middle and right: site-controlled pn-junctions by lateral and vertical anisotropic NWs in structured SiOx openings (scalebar 1 μm).

Direct Growth of Single Crystalline GaN Nanowires on Indium Tin Oxide-Coated Silica

In this work, we demonstrated the direct growth of GaN nanowires on indium tin oxide (ITO)-coated fused silica substrate. The nanowires were grown catalyst-free using plasma-assisted molecular beam epitaxy (PA-MBE). The effect of growth condition on the morphology and quality of the nanowires is systematically investigated. Structural characterization indicates that the nanowires grow in the (0001) direction directly on top of the ITO layer perpendicular to the substrate plane. Optical characterization of the nanowires shows that yellow luminescence is absent from the nanowire's photoluminescence response, attributed to the low number of defects. Conductive atomic force microscopy (C-AFM) measurement on n-doped GaN nanowires shows good conductivity for individual nanowires, which confirms the potential of using this platform for novel device applications. By using a relatively low-temperature growth process, we were able to successfully grow high-quality single-crystal GaN material without the degradation of the underlying ITO layer.

Mask-less MOVPE of arrayed n-GaN nanowires on site- and polarity-controlled AlN/Si templates

Process diagram for achieving pure Ga-polar and site-controlled growth of n-GaN nanowires on conductive n-Si-AlN templates using MOVPE.

Direct Growth of III-Nitride Nanowire-Based Yellow Light-Emitting Diode on Amorphous Quartz Using Thin Ti Interlayer

Consumer electronics have increasingly relied on ultra-thin glass screen due to its transparency, scalability, and cost. In particular, display technology relies on integrating light-emitting diodes with display panel as a source for backlighting. In this study, we undertook the challenge of integrating light emitters onto amorphous quartz by demonstrating the direct growth and fabrication of a III-nitride nanowire-based light-emitting diode. The proof-of-concept device exhibits a low turn-on voltage of 2.6 V, on an amorphous quartz substrate. We achieved ~ 40% transparency across the visible wavelength while maintaining electrical conductivity by employing a TiN/Ti interlayer on quartz as a translucent conducting layer. The nanowire-on-quartz LED emits a broad linewidth spectrum of light centered at true yellow color (~ 590 nm), an important wavelength bridging the green-gap in solid-state lighting technology, with significantly less strain and dislocations compared to conventional planar quantum well nitride structures. Our endeavor highlighted the feasibility of fabricating III-nitride optoelectronic device on a scalable amorphous substrate through facile growth and fabrication steps. For practical demonstration, we demonstrated tunable correlated color temperature white light, leveraging on the broadly tunable nanowire spectral characteristics across red-amber-yellow color regime.

Droplet epitaxy mediated growth of GaN nanostructures on Si (111) via plasma-assisted molecular beam epitaxy

We demonstrate that the use of a GaN seeding layer prepared prior to the growth of epitaxial GaN on Si (111) can lead to the formation of oriented arrays of Y-shaped nanoislands and nanowires and affects the surface density of the nanostructures.

Thin-Wall GaN/InAlN Multiple Quantum Well Tubes

Thin-wall tubes composed of nitride semiconductors (III-N compounds) based on GaN/InAlN multiple quantum wells (MQWs) are fabricated by metal-organic vapor-phase epitaxy in a simple and full III-N approach. The synthesis of such MQW-tubes is based on the growth of N-polar c-axis vertical GaN wires surrounded by a core-shell MQW heterostructure followed by in situ selective etching using controlled H₂/NH₃ annealing at 1010 °C to remove the inner GaN wire part. After this process, well-defined MQW-based tubes having nonpolar m-plane orientation exhibit UV light near 330 nm up to room temperature, consistent with the emission of GaN/InAlN MQWs. Partially etched tubes reveal a quantum-dotlike signature originating from nanosized GaN residuals present inside the tubes. The possibility to fabricate in a simple way thin-wall III-N tubes composed of an embedded MQW-based active region offering controllable optical emission properties constitutes an important step forward to develop new nitride devices such as emitters, detectors or sensors based on tubelike nanostructures.

Polarity-Induced Selective Area Epitaxy of GaN Nanowires

We present a conceptually novel approach to achieve selective area epitaxy of GaN nanowires. The approach is based on the fact that these nanostructures do not form in plasma-assisted molecular beam epitaxy on structurally and chemically uniform cation-polar substrates. By in situ depositing and nitridating Si on a Ga-polar GaN film, we locally reverse the polarity to induce the selective area epitaxy of N-polar GaN nanowires. We show that the nanowire number density can be controlled over several orders of magnitude by varying the amount of predeposited Si. Using this growth approach, we demonstrate the synthesis of single-crystalline and uncoalesced nanowires with diameters as small as 20 nm. The achievement of nanowire number densities low enough to prevent the shadowing of the nanowire sidewalls from the impinging fluxes paves the way for the realization of homogeneous core-shell heterostructures without the need of using ex situ prepatterned substrates.

Strain-Induced Band Gap Engineering in Selectively Grown GaN-(Al,Ga)N Core-Shell Nanowire Heterostructures

We demonstrate the selective area growth of GaN-(Al,Ga)N core-shell nanowire heterostructures directly on Si(111). Photoluminescence spectroscopy on as-grown nanowires reveals a strong blueshift of the GaN band gap from 3.40 to 3.64 eV at room temperature. Raman measurements relate this shift to compressive strain within the GaN core. On the nanoscale, cathodoluminescence spectroscopy and scanning transmission electron microscopy prove the homogeneity of strain-related luminescence along the nanowire axis and the absence of significant fluctuations within the shell, respectively. A comparison of the experimental findings with numerical simulations indicates the absence of a significant defect-related strain relaxation for all investigated structures, with a maximum compressive strain of -3.4% for a shell thickness of 50 nm. The accurate control of the nanowire dimensions, namely, core diameter, shell thickness, and nanowire period, via selective area growth allows a specific manipulation of the resulting strain within individual nanowires on the same sample. This, in turn, enables a spatially resolved adjustment of the GaN band gap with an energy range of 240 meV in a one-step growth process.

Epitaxy of GaN Nanowires on Graphene

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.