Effect of Extended Defects on AlGaN Quantum Dots for Electron-Pumped Ultraviolet Emitters.

We study the origin of bimodal emission in AlGaN/AlN QD superlattices displaying a high internal quantum efficiency (around 50%) in the 230-300 nm spectral range. The secondary emission at longer wavelengths is linked to the presence of cone-like domains with deformed QD layers, which originate at the first AlN buffer/superlattice interface and propagate vertically. The cones originate at a 30°-faceted shallow pit in the AlN, which appears to be associated with a threading dislocation that produces strong shear strain.

Long indium-rich InGaAs nanowires by SAG-HVPE.

We demonstrate the selective area growth of InGaAs nanowires (NWs) on GaAs (111)B substrates using hydride vapor phase epitaxy (HVPE). A high growth rate of more than 50m hand high aspect ratio NWs were obtained. Composition along the NWs was investigated by energy dispersive x-ray spectroscopy giving an average indium composition of 84%.

Optical net gain measurement on AlGaN/GaN multi-quantum wells.

We report net gain measurements at room temperature in AlGaN/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm, obtained at the maximum pumping power density of the experimental setup (743 kW/cm).

AlGaN/GaN asymmetric graded-index separate confinement heterostructures designed for electron-beam pumped UV lasers.

We present a study of undoped AlGaN/GaN separate confinement heterostructures designed to operate as electron beam pumped ultraviolet lasers. We discuss the effect of spontaneous and piezoelectric polarization on carrier diffusion, comparing the results of cathodoluminescence with electronic simulations of the band structure and Monte Carlo calculations of the electron trajectories. Carrier collection is significantly improved using an asymmetric graded-index separate confinement heterostructure (GRINSCH).

Solubility Limit of Ge Dopants in AlGaN: A Chemical and Microstructural Investigation Down to the Nanoscale.

Attaining low-resistivity AlGaN layers is one keystone to improve the efficiency of light-emitting devices in the ultraviolet spectral range. Here, we present a microstructural analysis of AlGaN/Ge samples with 0 ≤ ≤ 1, and a nominal doping level in the range of 10 cm, together with the measurement of Ge concentration and its spatial distribution down to the nanometer scale. AlGaN/Ge samples with ≤ 0.

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.

UV Emission from GaN Wires with -Plane Core-Shell GaN/AlGaN Multiple Quantum Wells.

The present work reports high-quality nonpolar GaN/AlGaN multiple quantum wells (MQWs) grown in core-shell geometry by metal-organic vapor-phase epitaxy on the -plane sidewalls of ̅-oriented hexagonal GaN wires. Optical and structural studies reveal ultraviolet (UV) emission originating from the core-shell GaN/AlGaN MQWs. Tuning the -plane GaN QW thickness from 4.

Internal quantum efficiency of AlGaN/AlN quantum dot superlattices for electron-pumped ultraviolet sources.

In this paper, we describe the growth and characterization of ≈530 nm thick superlattices (100 periods) of AlGaN/AlN (0 ⩽ x ⩽ 0.1) Stranski-Krastanov quantum dots (QDs) for application as the active region of electron-beam pumped ultraviolet lamps. Highly dense (>10 cm) QD layers are deposited by molecular beam epitaxy, and we explore the effect of the III/V ratio during the growth process on their optical performance.

Controlling the shape of a tapered nanowire: lessons from the Burton-Cabrera-Frank model.

The propagation of sidewall steps during the growth of nanowires is calculated in the frame of the Burton-Cabrera-Frank model. The stable shape of the nanowire comprises a cylinder section on top of a cone section: their characteristics are obtained as a function of the radius of the catalyst-nanowire area, the desorption-limited diffusion length of adatoms on the terraces, and the sticking of adatoms at step edges. The comparison with experimental data allows us to evaluate these last two parameters for InP and ZnTe nanowires; it reveals a different behavior for the two materials, related to a difference by an order of magnitude of the desorption-limited diffusion length.

Role of Underlayer for Efficient Core-Shell InGaN QWs Grown on -plane GaN Wire Sidewalls.

Different types of buffer layers such as InGaN underlayer (UL) and InGaN/GaN superlattices are now well-known to significantly improve the efficiency of -plane InGaN/GaN-based light-emitting diodes (LEDs). The present work investigates the role of two different kinds of pregrowth layers (low In-content InGaN UL and GaN UL namely "GaN spacer") on the emission of the core-shell -plane InGaN/GaN single quantum well (QW) grown around Si-doped -GaN microwires obtained by silane-assisted metal organic vapor phase epitaxy. According to photo- and cathodoluminescence measurements performed at room temperature, an improved efficiency of light emission at 435 nm with internal quantum efficiency >15% has been achieved by adding a GaN spacer prior to the growth of QW.

Assessment of AlGaN/AlN superlattices on GaN nanowires as active region of electron-pumped ultraviolet sources.

In this paper, we describe the design and characterization of 400 nm long (88 periods) Al Ga N/AlN (0 ≤ x ≤ 0.1) quantum dot superlattices deposited on self-assembled GaN nanowires for application in electron-pumped ultraviolet sources. The optical performance of GaN/AlN superlattices on nanowires is compared with the emission of planar GaN/AlN superlattices with the same periodicity and thickness grown on bulk GaN substrates along the N-polar and metal-polar crystallographic axes.

Impact of kinetics on the growth of GaN on graphene by plasma-assisted molecular beam epitaxy.

The growth of GaN on graphene by molecular beam epitaxy was investigated. The most stable epitaxial relationship, i.e.

High Lateral Breakdown Voltage in Thin Channel AlGaN/GaN High Electron Mobility Transistors on AlN/Sapphire Templates.

In this paper, we present the fabrication and Direct Current/high voltage characterizations of AlN-based thin and thick channel AlGaN/GaN heterostructures that are regrown by molecular beam epitaxy on AlN/sapphire. A very high lateral breakdown voltage above 10 kV was observed on the thin channel structure for large contact distances. Also, the buffer assessment revealed a remarkable breakdown field of 5 MV/cm for short contact distances, which is far beyond the theoretical limit of the GaN-based material system.

Correlative investigation of Mg doping in GaN layers grown at different temperatures by atom probe tomography and off-axis electron holography.

Correlation between off-axis electron holography and atom probe tomography (APT) provides morphological, chemical and electrical information about Mg doping (p-type) in gallium nitride (GaN) layers that have been grown at different temperatures at a nanometric scale. APT allows access to the three-dimensional distribution of atoms and their chemical nature. In particular, this technique allows visualisation of the Mg-rich clusters observed in p-doped GaN layers grown by metal-organic chemical vapour deposition.

Si Doping of Vapor-Liquid-Solid GaAs Nanowires: n-Type or p-Type?

The incorporation of Si into vapor-liquid-solid GaAs nanowires often leads to p-type doping, whereas it is routinely used as an n-dopant of planar layers. This property limits the applications of GaAs nanowires in electronic and optoelectronic devices. The strong amphoteric behavior of Si in nanowires is not yet fully understood.

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-AlO templates by catalyst-free hydride vapor phase epitaxy using InCl 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.

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 Ga 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.

Dopant radial inhomogeneity in Mg-doped GaN nanowires.

Using atom probe tomography, it is demonstrated that Mg doping of GaN nanowires grown by Molecular Beam Epitaxy results in a marked radial inhomogeneity, namely a higher Mg content in the periphery of the nanowires. This spatial inhomogeneity is attributed to a preferential incorporation of Mg through the m-plane sidewalls of nanowires and is related to the formation of a Mg-rich surface which is stabilized by hydrogen. This is further supported by Raman spectroscopy experiments which give evidence of Mg-H complexes in the doped nanowires.

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 H/NH 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.

Self-catalyzed GaAs nanowires on silicon by hydride vapor phase epitaxy.

Gold-free GaAs nanowires on silicon substrates can pave the way for monolithic integration of photonic nanodevices with silicon electronic platforms. It is extensively documented that the self-catalyzed approach works well in molecular beam epitaxy but is much more difficult to implement in vapor phase epitaxies. Here, we report the first gallium-catalyzed hydride vapor phase epitaxy growth of long (more than 10 μm) GaAs nanowires on Si(111) substrates with a high integrated growth rate up to 60 μm h and pure zincblende crystal structure.

Composition Analysis of III-Nitrides at the Nanometer Scale: Comparison of Energy Dispersive X-ray Spectroscopy and Atom Probe Tomography.

The enhancement of the performance of advanced nitride-based optoelectronic devices requires the fine tuning of their composition, which has to be determined with a high accuracy and at the nanometer scale. For that purpose, we have evaluated and compared energy dispersive X-ray spectroscopy (EDX) in a scanning transmission electron microscope (STEM) and atom probe tomography (APT) in terms of composition analysis of AlGaN/GaN multilayers. Both techniques give comparable results with a composition accuracy better than 0.

Flexible Photodiodes Based on Nitride Core/Shell p-n Junction Nanowires.

A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane transferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p-n junction nanowires containing InGaN/GaN quantum wells grown by MOVPE.

Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors.

We report the first demonstration of flexible white phosphor-converted light emitting diodes (LEDs) based on p-n junction core/shell nitride nanowires. GaN nanowires containing seven radial InGaN/GaN quantum wells were grown by metal-organic chemical vapor deposition on a sapphire substrate by a catalyst-free approach. To fabricate the flexible LED, the nanowires are embedded into a phosphor-doped polymer matrix, peeled off from the growth substrate, and contacted using a flexible and transparent silver nanowire mesh.