Christophe Durand

Self-assembled growth of catalyst-free GaN wires by metal–organic vapour phase epitaxy

A catalyst-free method for growing self-assembled GaN wires on c-plane sapphire substrates by metal-organic vapour phase epitaxy is developed. This approach, based on in situ deposition of a thin SiN(x) layer (approximately 2 nm), enables epitaxial growth of c-oriented wires with 200-1500 nm diameters and a large length/diameter ratio (>100) on c-plane sapphire substrate. Detailed study of the growth mechanisms shows that a combination of key parameters is necessary to obtain vertical growth. In particular, the duration of the SiN(x) deposition prior to the wire growth is critical for controlling the epitaxy with the substrate. The GaN seed nucleation time determines the mean size diameter and structural quality, and a high Si-dopant concentration promotes vertical growth. Such GaN wires exhibit UV-light emission centred at approximately 350 nm and a weak yellow band (approximately 550 nm) at low temperature.

M-Plane Core-Shell InGaN/GaN Multiple-Quantum-Wells on GaN Wires for Electroluminescent Devices

Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {11-00} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1-8 nm) and In-composition in the wells (∼16%). The core-shell MQW light emission characteristics (390-420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.

Homoepitaxial growth of catalyst-free GaN wires on N-polar substrates

The shape of c-oriented GaN nanostructures is found to be directly related to the crystal polarity. As evidenced by convergent beam electron diffraction applied to GaN nanostructures grown by metal-organic vapor phase epitaxy on c-sapphire substrates: wires grown on nitridated sapphire have the N-polarity ([000math]) whereas pyramidal crystals have Ga-polarity ([0001]). In the case of homoepitaxy, the GaN wires can be directly selected using N-polar GaN freestanding substrates and exhibit good optical properties. A schematic representation of the kinetic Wulffs plot points out the effect of surface polarity.

Integrated Photonic Platform Based on InGaN/GaN Nanowire Emitters and Detectors

We report the fabrication of a photonic platform consisting of single wire light-emitting diodes (LED) and photodetectors optically coupled by waveguides. MOVPE-grown (metal-organic vapor-phase epitaxy) InGaN/GaN p-n junction core-shell nanowires have been used for device fabrication. To achieve a good spectral matching between the emission wavelength and the detection range, different active regions containing either five narrow InGaN/GaN quantum wells or one wide InGaN segment were employed for the LED and the detector, respectively. The communication wavelength is ∼400 nm. The devices are realized by means of electron beam lithography on Si/SiO2 templates and connected by ∼100 μm long nonrectilinear SiN waveguides. The photodetector current trace shows signal variation correlated with the LED on/off switching with a fast transition time below 0.5 s.

Flexible Light-Emitting Diodes Based on Vertical Nitride Nanowires

We demonstrate large area fully flexible blue LEDs based on core/shell InGaN/GaN nanowires grown by MOCVD. The fabrication relies on polymer encapsulation, nanowire lift-off and contacting using silver nanowire transparent electrodes. The LEDs exhibit rectifying behavior with a light-up voltage around 3 V. The devices show no electroluminescence degradation neither under multiple bending down to 3 mm curvature radius nor in time for more than one month storage in ambient conditions without any protecting encapsulation. Fully transparent flexible LEDs with high optical transmittance are also fabricated. Finally, a two-color flexible LED emitting in the green and blue spectral ranges is demonstrated combining two layers of InGaN/GaN nanowires with different In contents.

Single-wire photodetectors based on InGaN/GaN radial quantum wells in GaN wires grown by catalyst-free metal-organic vapor phase epitaxy

We present a letter on single-wire photodetectors based on radial n-i-n multiquantum well (QW) junctions. The devices are realized from GaN wires grown by catalyst-free metalorganic vapor phase epitaxy coated at their top by five nonpolar In0.16Ga0.84N/GaN undoped radial QWs, and are sensitive to light with energy E>2.6 eV. Their photoconductive gain is as high as 2×103. The scanning photocurrent microscopy maps evidence that the detector response is localized at the extremity containing the QWs for both below (at λ=488 nm) and above GaN band gap (at λ=244 nm) excitation. This confirms that the device operates as a radial n-i-n junction.

Correlation of microphotoluminescence spectroscopy, scanning transmission electron microscopy, and atom probe tomography on a single nano-object containing an InGaN/GaN multiquantum well system.

A single nanoscale object containing a set of InGaN/GaN nonpolar multiple-quantum wells has been analyzed by microphotoluminescence spectroscopy (μPL), high-resolution scanning transmission electron microscopy (HR-STEM) and atom probe tomography (APT). The correlated measurements constitute a rich and coherent set of data supporting the interpretation that the observed μPL narrow emission lines, polarized perpendicularly to the crystal c-axis and with energies in the interval 2.9-3.3 eV, are related to exciton states localized in potential minima induced by the irregular 3D In distribution within the quantum well (QW) planes. This novel method opens up interesting perspectives, as it will be possible to apply it on a wide class of quantum confining emitters and nano-objects.

Single-Wire Light-Emitting Diodes Based on GaN Wires Containing Both Polar and Nonpolar InGaN/GaN Quantum Wells

Single-wire light-emitting diodes based on radial p–i–n multi quantum well (QW) junctions have been realized from GaN wires grown by catalyst-free metal organic vapor phase epitaxy. The InxGa1-xN/GaN undoped QW system is coated over both the nonpolar lateral sidewalls and on the polar upper surface. Cathodo- and electroluminescence (EL) experiments provide evidence that the polar QWs emit in the visible spectral range at systematically lower energy than the nonpolar QWs. The EL of the polar or nonpolar QWs can be selectively activated by varying the sample temperature and current injection level.

Catalyst-free growth of high-optical quality GaN nanowires by metal-organic vapor phase epitaxy

Catalyst-free GaN wires with 100–200 nm diameters are grown on bare c-sapphire substrates by a metal-organic vapor phase epitaxy approach using both low V/III ratio and V-III precursor flows that favor a reaction-limited growth regime. The polarity control of the initial seeds allows obtaining pencil-shape wires with very sharp pyramids at their top (∼5 nm diameter). These defect-free nanowires evidence excellent structural and optical properties as shown by a sharp photoluminescence linewidth (1–3 meV at 5 K).

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 In0.2Ga0.8N/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. The electroluminescence of a flexible device presents a cool-white color with a spectral distribution covering a broad spectral range from 400 to 700 nm. Mechanical bending stress down to a curvature radius of 5 mm does not yield any degradation of the LED performance. The maximal measured external quantum efficiency of the white LED is 9.3%, and the wall plug efficiency is 2.4%.