GaN-based Nanowires Research Articles

Flexible bidirectional self-powered photodetector with significantly reduced volume and accelerated response speed based on hydrogel and lift-off GaN-based nanowires

Due to the wide range of potential applications for next-generation multi-functional devices, the flexible self-powered photodetector (PD) with polarity-switchable behavior is essential but very challenging to be realized. Herein, a wearable bidirectional self-powered PD based on detached (Al,Ga)N and (In,Ga)N nanowires has been proposed and demonstrated successfully. Arising from the photovoltage-competing dynamics across (Al,Ga)N and (In,Ga)N nanowire photoelectrodes, such PD can generate the positive (33.3 mA W−1) and negative (-0.019 mA W−1) photo-responsivity under ultraviolet (UV) and visible illumination, respectively, leading to the bidirectional photocurrent behavior. Thanks to the introduction of quasi solid-state hydrogel, the PD can work without the liquid-electrolyte, thus remarkably reducing the volume from about 482 cm3 to only 0.18 cm3. Furthermore, the use of hydrogel is found to enhance response speed in the UV range by reducing the response time for more than 95%, which is mainly attributed to the increased open circuit potential and reduced ion transport distance. As the GaN connecting segment is pretty thin, the piezoelectric charges generated by stress are proposed to have only a limited effect on the photocurrent density. Therefore, both the stable on-off switching characteristics and photocurrent densities can still be achieved after being bent 400 times. With an excellent flexibility, this work creates opportunities for technological applications of bidirectional photocurrent PDs in flexible optoelectronic devices, e.g., wearable intelligent sensors.

High Piezoelectric Conversion Properties of Axial InGaN/GaN Nanowires.

We demonstrate for the first time the efficient mechanical-electrical conversion properties of InGaN/GaN nanowires (NWs). Using an atomic force microscope equipped with a modified Resiscope module, we analyse the piezoelectric energy generation of GaN NWs and demonstrate an important enhancement when integrating in their volume a thick In-rich InGaN insertion. The piezoelectric response of InGaN/GaN NWs can be tuned as a function of the InGaN insertion thickness and position in the NW volume. The energy harvesting is favoured by the presence of a PtSi/GaN Schottky diode which allows to efficiently collect the piezo-charges generated by InGaN/GaN NWs. Average output voltages up to 330 ± 70 mV and a maximum value of 470 mV per NW has been measured for nanostructures integrating 70 nm-thick InGaN insertion capped with a thin GaN top layer. This latter value establishes an increase of about 35% of the piezo-conversion capacity in comparison with binary p-doped GaN NWs. Based on the measured output signals, we estimate that one layer of dense InGaN/GaN-based NW can generate a maximum output power density of about 3.3 W/cm2. These results settle the new state-of-the-art for piezo-generation from GaN-based NWs and offer a promising perspective for extending the performances of the piezoelectric sources.

Nanoscopic insights into the effect of silicon on core-shell InGaN/GaN nanorods: Luminescence, composition, and structure

GaN-based nanorods and nanowires have recently shown great potential as a platform for future energy-efficient photonic and optoelectronic applications, such as light emitting diodes and nanolasers. Currently, the most industrially scalable method of growing III-nitride nanorods remains metal-organic vapour phase epitaxy: whilst this growth method is often used in conjunction with extrinsic metallic catalyst particles, these particles can introduce unwanted artifacts in the nanorods such as stacking faults. In this paper, we examine the catalyst-free growth of GaN/InGaN core-shell nanorods by metal-organic vapor phase epitaxy for optoelectronic applications using silane to enhance the vertical growth of the nanorods. We find that both the silane concentration and exposure time can greatly affect the nanorod properties, and that larger concentrations and longer exposure times can severely degrade the nanorod structure and thus result in reduced emission from the InGaN QW shell. Finally, we report that the mechanism behind the effect of silane on the nanorod structure is the unintentional formation of an SiNx interlayer following completion of the growth of the nanorod core. This interlayer induces the growth of GaN islands on the nanorod sidewalls, the spatial distribution of which can affect their subsequent coalescence during the lateral growth stages and result in non-uniformity in the nanorod structure. This suggests that careful control of the silane flow must be exerted during growth to achieve both high aspect ratio nanorods and uniform emission along the length of the nanorod.

Shape Evolution of Highly Lattice-Mismatched InN/InGaN Nanowire Heterostructures

We have investigated the structure and shape of GaN-based nanowires grown on (001) Si substrates for optoelectronic device applications. The nanowire heterostructures contained InN disks and In0.4Ga0.6N barrier layers in the active region. The resulting nanowire array comprised two differently shaped nanowires: shorter pencil-like nanowires and longer bead-like nanowires. The two different nanowire shapes evolve due to a variation in the In incorporation rate, which was faster for the bead-like nanowires. Both types of nanowires exhibited evidence of significant migration of both Ga and In during growth. Ga tended to diffuse away and down along the sidewalls, resulting in a Ga-rich shell for all nanowires. Despite the complex structure and great variability in the In composition, the optical properties of the nanowire arrays were very good, with strong luminescence peaking at ∼ 1.63 μm.

Highly ordered GaN-based nanowire arrays grown on patterned (100) silicon and their optical properties

Arrays of GaN-based nanowires have been synthesized on patterned silicon without a catalyst. The spatial density, length and average radius of the nanowires can be well-controlled. The GaN core contains two semipolar facets and a controllable polar facet. The nanowire heterostructures exhibit excellent laser behavior.

GaN-based radial heterostructure nanowires grown by MBE and ALD

A combination of molecular beam epitaxy (MBE) and atomic layer deposition (ALD) was adopted to fabricate GaN-based core/shell NW structures. ALD was used to deposit a HfO2 shell of onto the MBE grown GaN NWs. Electron transparent samples were prepared by focussed ion beam methods and characterized using state-of-the-art analytical transmission and scanning transmission electron microscopy. The polycrystalline coating was found to be uniform along the whole length of the NWs. Photoluminescence and Raman spectroscopy analysis confirms that the HfO2 ALD coating does not add any structural defect when deposited on the NWs.

Single GaN-Based Nanowires for Photodetection and Sensing Applications

We present a study of the structural properties of GaN/AlN heterostructured nanowires grown by plasma-assisted molecular-beam epitaxy. We combine transmission electron microscopy measurements with theoretical calculations of the strain distribution and band diagram in order to gain understanding of the potential distribution in these nanostructures. The effects of surface states and of the formation of a core–shell heterostructure are discussed. The structural properties are correlated with the performance of GaN-based nanowire photodetectors and sensors. In particular, we discuss a sensor design where the insertion of an AlN barrier in a GaN nanowire is proposed as a material engineering solution to inhibit the electron transport through the core, confining the current close to the nanowire sidewalls.

Analysis on the Light Extraction Efficiency of GaN-Based Nanowires Light-Emitting Diodes

With the development of single-photon-sources of ultra-efficient light extraction, nanowire structures, with embedded quantum wells, present unprecedented opportunity to enhance the light extraction efficiency (LEE) of light emitting diodes (LEDs). In this paper, the finite-difference time-domain method is carried out in graphics processing units to explore mechanisms by which nanowire arrays can effectively improve LEE. The results show that the LEE strongly depends on the radius of the nanowire and the distance between the nanowires. On analyzing the emitted power distribution of the dipole in the semiconductor nanowire, the LEE of LEDs agrees with guided power portion of the total power with respect to the radius of the nanowire and the distance between the nanowires. The guided mode has an important role in enhancing the LEE of LEDs. The free-space mode, however, becomes the whispering gallery mode which is not important in LEE. The LEE obtains the maximum at the mode cutoff radius. The height is also discussed in this paper. With the optimized parameters, ~11 times increment in LEE is achieved.

Polaron effect on linear and nonlinear intersubband optical absorption of a wurtzite GaN-based nanowire

Based on the density matrix approach and the perturbation treatment, the polaronic effect on the linear and nonlinear intersubband optical absorption coefficients in quasi-one-dimensional wurtzite nanowires (NWs) is investigated. The simple analytical formulas for polaronic states and the linear and nonlinear optical absorption coefficients in the NW systems are also deduced. Numerical calculation on a freestanding wurtzite GaN NW is performed. The polaronic effect and quantum size effect as well as the influence of incident optical intensity on the optical absorption properties are analyzed and discussed. The results show that, the polaronic effect results in significant increase of the absorption coefficients, and obvious decrease of saturation absorption intensity. In order to observe strong optical absorption, a nitride NW with relative small radius should be chosen. Moreover, a comparison with the situation of GaN-based quantum well systems is also done, and the profound physics is analyzed reasonably. • Polaronic states including the IO and QC phonon modes in wurtzite nitride NWs are obtained. • Polaronic effect results in significant increase of the linear and nonlinear absorption coefficients. • Taking the polaronic effect into account, saturation absorption intensity decreases obviously. • GaN-based NWs with smaller radius should be chosen for observing stronger optical absorption.

Optical properties of GaN-based nanowires containing a single Al0.14Ga0.86N/GaN quantum disc

The optical properties of wurtzite GaN nanowires containing single Al0.14Ga0.86N/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter.

GaN-based Nano-Pores and Nano-Wires Fabricated Using Electroless Chemical Etching Process

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GaN-based core-shell nanowires sputtered with Pd target and their annealing characteristics

We have prepared GaN-cored heteronanowires by sputtering with the palladium (Pd) target, subsequently investigating the effects of thermal annealing. The surface of heteronanowires became rough by the thermal annealing, being attributed to the agglomeration of the shell layers into the cluster-like structures. X-ray diffraction, high-resolution transmission electron microscopy, and selected area electron diffraction coincidentally indicated that the shell layer comprised the Ga 2Pd 5 phase. From Gaussian deconvolution studies, we observed that photoluminescence (PL) spectra consisted of red, green, and ultraviolet (UV) emission bands, regardless of shell-coating and subsequent thermal annealing. The integrated PL intensities of red, green, and UV emissions have been changed by the shell-coating and thermal annealing, presumably being associated with the surface-coverage by the shell layer.