GaN Rods Research Articles

Tuning electronic energy band in a piezoelectric semiconductor rod via mechanical loading

Based on the k·p model with 8 × 8 band Hamiltonian, we demonstrate that the electronic energy band in a wurtzite piezoelectric semiconductor (PSC) rod can be tuned by a pair of uniaxial mechanical tractions. This tuning is achieved by solving two sub-problems: the electro-mechanical fields induced by a pair of axial tractions applied at the two ends of the rod, and the band energy based on the calculated strains and electric potential via the k·p model. Numerical results indicate that, when the magnitude of the applied load is relatively small, the induced PSC fields predicted by the linear model in the rod generally agree with those based on the present rigorous nonlinear model. Otherwise, the linear model will be invalid and the present nonlinear model has to be used to characterize correctly the intrinsic nonlinear property in the PSC rod. It is the first time that the heavy hole (HH), light hole (LH), spin-orbit split-off (SO) and conduction band (CB) in ZnO and GaN rods are all analyzed in details at different locations of the rod under different magnitudes of the applied traction. It is shown that the mechanical load could significantly affect the energy band edges, particularly near the ends of the rod where the loads are applied. Furthermore, the coupling effect of the valence band (VB) and CB is discussed in an InN rod where the material has a narrow bandgap. This work provides a unique opportunity in nanodevice design where nano energy is a key parameter.

Simultaneous Hard X-ray Ptychographic Tomography and X-ray Fluorescence To-mography of Isolated Hollow Core-Shell GaN Rods

Simultaneous Hard X-ray Ptychographic Tomography and X-ray Fluorescence To-mography of Isolated Hollow Core-Shell GaN Rods

Axial Inhomogeneity of Mg-Doped GaN Rods: A Strong Correlation among Componential, Electrical, and Optical Analyses

We systematically characterized the inhomogeneous doping properties along the c-axis of Mg-doped p-GaN microrods. Axial variation of doping concentration and electrical resistance on the p-GaN rod were measured by time-of-flight secondary-ion-mass-spectrometry and four-point probe measurements, respectively. Defects-related optical information was obtained from photoluminescence spectra together with Raman experiments revealing the change of crystal quality and strain along the rod. On the basis of a correlation of these analyses, we confirmed that Mg concentration decreased along the axial direction of the rod, leading to increasing electrical resistance. This axial Mg concentration change was revealed by green luminescence because the intensity of green luminescence sensitively varied with the doping density in both high-doping and low-doping rods. Interestingly, all the resistances at the highly doped rods were higher than the lowly doped rods due to overall mobility degradation at the high-doping rods...

Observation of Anisotropic Growth and Compositional Discontinuity in AlGaN Electron-Blocking Layers on GaN Microrods

In GaN microrods, phase separation of the AlGaN electron-blocking layer is an enormous obstacle for achieving high-efficiency light-emitting diodes, as this phenomenon negatively affects the device efficiency by inducing unwanted band-energy modulations. Here, we found that the AlGaN electron-blocking layer on each m-plane of the GaN microrod appears to be phase separated, and each electron-blocking layer has a different thickness and length. Our careful analysis based on atom probe tomography reveals that the Al distribution in AlGaN is not uniform and that Al-rich and Al-deficient regions are clearly present. In addition, the longer surface diffusion length of Ga adatoms, as compared to Al adatoms, and the different initial strain state of each m-plane in the GaN rods are deeply associated with the different growth rates and inhomogeneous Al composition of AlGaN, resulting in phase separation of the AlGaN electron-blocking layer. These atomic-scale observations in the structural and chemical composition of AlGaN grown on GaN microrods could provide expanded opportunities for building a wide range of high-quality AlGaN electron-blocking layers.

Understanding GaN/InGaN core–shell growth towards high quality factor whispering gallery modes from non-polar InGaN quantum wells on GaN rods

GaN microrods are used as a basis for subsequent InGaN quantum well (QW) and quantum dot deposition by metal-organic vapor phase epitaxy. The coverage of the shell along the sidewall of rods is dependent on the rod growth time and a complete coverage is obtained for shorter rod growth times. Transmission electron microscopy measurements are performed to reveal the structural properties of the InGaN layer on the sidewall facet and on the top facet. The presence of layers in the microrod and on the microrod surface will be discussed with respect to GaN and InGaN growth. A detailed model will be presented explaining the formation of multiple SiN layers and the partial and full coverage of the shell around the core. Cathodoluminescence measurements are performed to analyze the InGaN emission properties along the microrod and to study the microresonator properties of such hexagonal core–shell structures. High quality factor whispering gallery modes with are reported for the first time in a GaN microrod/InGaN non-polar QW core–shell geometry. The GaN/InGaN core–shell microrods are expected to be promising building blocks for low-threshold laser diodes and ultra-sensitive optical sensors.

Transmission Electron Microscopy Study of Selective Growth of GaN Rods on the Apex of GaN Stripes

Transmission Electron Microscopy Study of Selective Growth of GaN Rods on the Apex of GaN Stripes

(Invited) Nitride Semiconductor Quantum Structures for Novel Nanophotonic Devices

Group III-nitride semiconductor based low-dimensional nanostructures have attracted a lot of attention owing to their unique optical characteristics and their versatile classical and quantum photonic applications. Here, we present various nitride-based quantum nanostructures grown on pyramidal, annular, columnar, and tapered structures as a template. First, we demonstrated multi-color and broadband visible light emitting diodes (LEDs) based on GaN hexagonal nano-pyramid and hexagonal annular structures. The pyramid LEDs emit a broad-band spectrum originated from quantum dots (QDs), quantum wires, and quantum wells (QWs) with different emission wavelengths, which are formed at the tops, edges, and sidewalls of the pyramids, respectively. The annular and double concentric truncated pyramidal LEDs provide broad-band, white light generation from the QWs formed on various planes. Red-color emission using InGaN/GaN double heterostructures on GaN nano-pyramid structures was also observed [1]. Second, GaN-based rod structures were directly grown on Si substrates without using any catalysts or mask processing and then InGaN/GaN multiple QWs were deposited on the surface of GaN rods. By using tapered GaN/InGaN core−shell QW semiconductor rods having a large gradient in their bandgap energy along their growth direction, highly asymmetric photonic diode behavior was observed with low scattering loss [2]. Third, we developed a dislocation-eliminating top-down chemical vapor etching method for fabricating high-quality GaN nanostructures. InGaN-based single QD arrays were formed on obelisk-shaped GaN nanostructures by growing a ultrathin QW layer. Ultrafast and high efficiency single photon generation was demonstrated by virtue of spontaneous formation of single QD on the apex of tapered GaN nanostructures. Moreover, a broad spectrum of the entire visible range was achieved by growing multiple QW structure with various QW thicknesses on the obelisk-shaped GaN nanostructures. [3]. Finally, we demonstrate an novel approach of the self-aligned deterministic coupling of single QDs to nanofocused plasmonic modes. Site-controlled InGaN QD array was grown by selective growth method using metal-organic chemical vapor deposition, and then a silver film was deposited on the single QD array. Using this approach, we achieved strong spontaneous emission enhancement as high as ~ 22 of QDs over a wide spectral range. Meanwhile, the hybrid nature of exciton polaritons opens up possibilities for developing a new concept nonlinear photonic device. We developed a novel polariton system resulting from strong coupling between a two-dimensional exciton and whispering gallery mode photon using a core−shell hexagonal wire with GaN/InGaN multiple QW [4]. This approach overcomes the major hurdles in the implementation of practical solid-state quantum devices and shows great promise for various applications including quantum cryptography, quantum logic gates, integrated on-chip nano-emitters, and energy-harvesting devices.

Exceptionally long GaN sub-micrometer rods grown by HVPE on a MOCVD-GaN rod template

The growth mechanism of exceptionally long N-polar GaN sub-micrometer (sub-μm) rods has not been revealed to date. In this work, we investigated the effects of the V/III molar ratio and HCl (diluted in carrier gas) on the lengthening of a MOCVD-GaN rod template by HVPE. It is found that a low V/III molar ratio and HCl (diluted in carrier gas) help form vertical sidewalls and suppress the lateral growth on the top part of exceptionally long GaN sub-μm rods. Simulation results revealed that the low V/III molar ratio leads to reactive species distributing almost exclusively on the top part of GaN rods, which can effectively prevent crystal growth on the bottom of GaN rods. Chlorine ions support the growth by etching the sidewalls. After growth, the diameter and length of each GaN rod are more than 1.5 μm and 70–80 μm, respectively. Finally, an empirical growth model was developed to account for the exceptionally long GaN sub-μm rods under HVPE growth.

Selective Area Growth and Characterization of GaN Nanorods Fabricated by Adjusting the Hydrogen Flow Rate and Growth Temperature with Metal Organic Chemical Vapor Deposition**Supported by the Key Program of the National Natural Science Foundation of China under Grant No 61334009, and the National High Technology Research and Development Program of China under Grant No 2014AA032604.

GaN nanorods are successfully fabricated by adjusting the flow rate ratio of hydrogen (H2)/nitrogen (N2) and growth temperature of the selective area growth (SAG) method with metal organic chemical vapor deposition (MOCVD). The SAG template is obtained by nanospherical-lens photolithography. It is found that increasing the flow rate of H2 will change the GaN crystal shape from pyramid to vertical rod, while increasing the growth temperature will reduce the diameters of GaN rods to nanometer scale. Finally the GaN nanorods with smooth lateral surface and relatively good quality are obtained under the condition that the H2:N2 ratio is 1:1 and the growth temperature is 1030°C. The good crystal quality and orientation of GaN nanorods are confirmed by high resolution transmission electron microscopy. The cathodoluminescence spectrum suggests that the crystal and optical quality is also improved with increasing the temperature.

GaN Rods Grown on Si by SAG-HVPE toward GaN HVPE/InGaN MOVPE Core/Shell Structures

Selective area growth hydride vapor phase epitaxy (SAG-HVPE) and metal organic vapor phase epitaxy (MOVPE) processes are coupled for the synthesis of high quality c-axis InGaN/GaN core/shell structures. The core consists of high aspect ratio GaN rods grown by SAG-HVPE on patterned N-polar AlN on Si(100) substrates. The shell is grown by MOVPE which provides abrupt InGaN/GaN multiquantum wells (MQWs). Microphotoluminescence (μ-PL) analysis performed on the HVPE GaN core exhibit a narrow emission line of 3 meV in line width associated with the neutral-donor bound exciton revealing the excellent optical properties of the GaN material. For the core/shell wire geometry, the silane free HVPE process ensured the whole lateral cladding of the core. The hybrid HVPE core/MOVPE shell structures exhibit high optical quality without yellow luminescence.

Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods.

We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density (>80%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips' broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD's confinement dimensions, rather than significantly increasing the In%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates.

Correction: Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods.

Correction for 'Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods' by M. Conroy et al., Nanoscale, 2016, 8, 11019-11026.

The Role of Si during the Growth of GaN Micro- and Nanorods

The role of Si during the metal–organic vapor phase epitaxy of GaN rods is investigated. Already a small amount of Si strongly enhances the vertical growth of GaN. Reactive ion etching experiments show that the inner volume of the rod is much more strongly etched than the m-plane surface layer. Transmission electron microscopy and energy dispersive X-ray spectroscopy measurements reveal that Si is predominiantly incorporated in the surface layer of the m-plane sidewall facets of the rods. The formation of a SiN layer prevents growth on and etching of the m-planes and enhances the mobility of atoms promoting vertical growth. Annealing experiments demonstrate the extraordinary thermal resistivity in comparison to undoped GaN rod structures and GaN layers. The subsequent InGaN quantum well growth on the GaN rods reveals the antisurfactant effect of the SiN layer. A model based on the vapor–liquid–solid growth mode is proposed. The results help to understand the role of Si during growth of GaN rod structures to improve the performance of rod based light emitting and electronic devices.

Band Engineered Epitaxial 3D GaN-InGaN Core–Shell Rod Arrays as an Advanced Photoanode for Visible-Light-Driven Water Splitting

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Growth of InGaN/GaN core–shell structures on selectively etched GaN rods by molecular beam epitaxy

This work reports on the growth and characterization of InGaN/GaN core–shell structures by plasma-assisted molecular beam epitaxy on an ordered array of top-down patterned GaN microrods fabricated on a GaN/sapphire substrate. Growth of InGaN/GaN is conformal, with axial and radial growth components leading to core–shell structures with clear hexagonal facets. The radial InGaN growth (shell) was confirmed by spatially resolved cathodoluminescence performed using scanning electron microscopy as well as scanning transmission electron microscopy. The In content of the InGaN shell is controlled by means of the growth temperature and the III/V ratio.

Whispering gallery modes in GaN microdisks, microrods and nanorods grown by MOVPE

AbstractSelf‐assembled GaN microdisks, micro‐ and nanorods were grown by a mask‐free and self‐catalyzed metal‐organic vapor phase epitaxy method. The morphology of the structures is analyzed by scanning electron microscopy. All rods have a hexagonal shape with smooth sidewall facets and sharp edges. Depending on the deposition of a GaN nucleation layer prior to growth of GaN rod structures the diameter of the rods can be adjusted. Nanorods with diameters down to 400 nm as well as microrods and disks with diameters up to 5 µm can be achieved. The optical properties of the rods are investigated utilizing a combined scanning electron microscopy and cathodoluminescence system. Electron beam excitiation of GaN rods leads to a typical GaN spectrum showing GaN near band edge emission at 369 nm and broad yellow defect luminescence at 560 nm. Enhanced emission is located at the top of the rod. By fixing the focussed electron beam at a sidewall position of a single GaN rod, whispering gallery modes are observed in the cathodoluminescence spectrum. Depending on the diameter of the micro‐/nanorods the spectral position of the modes as well as the spectral distance between the modes can be adjusted. Quality factors up to 475 are measured. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Controlling morphology and optical properties of self-catalyzed, mask-free GaN rods and nanorods by metal-organic vapor phase epitaxy

A simple self-catalyzed and mask-free approach will be presented to grow GaN rods and nanorods based on the metal-organic vapor phase epitaxy technique. The growth parameter dependent adjustment of the morphology of the structures will be discussed. Rods and nanorods with diameters reaching from a few μm down to 100 nm, heights up to 48 μm, and densities up to 8⋅107 cm–2 are all vertically aligned with respect to the sample surface and exhibiting a hexagonal shape with smooth sidewall facets. Optical properties of GaN nanorods were determined using cathodoluminescence. It will be shown that the optical properties can be improved just by reducing the Ga precursor flow. Furthermore, for regular hexagonal shaped rods and nanorods, whispering gallery modes with quality factors up to 500 were observed by cathodoluminescence pointing out high morphological quality of the structures. Structural investigations using transmission electron microscopy show that larger GaN nanorods (diameter > 500 nm) contain threading dislocations in the bottom part and vertical inversion domain boundaries, which separate a Ga-polar core from a N-polar shell. In contrast, small GaN nanorods (∼200 nm) are largely free of such extended defects. Finally, evidence for a self-catalyzed, Ga-induced vapor-liquid-solid growth will be discussed.

Self-organized and self-catalyst growth of semiconductor and metal wires by vapour phase epitaxy: GaN rods versus Cu whiskers

Abstract Wires represent a new class of nanostructures that offer unprecedented freedom in materials design and new physical properties. Amongst the very different growth mechanisms reported in literature, the vapour-phase growth of self-catalyzed wires has the advantages of simplicity and rapidity with a low level of contaminants. The elaborations of semiconducting and metallic wires are usually considered as very distinct fields and no significant analogies have been noticed yet. This paper illustrates significant similarities of the mechanisms involved in the GaN and Cu wire growths that highlight firstly the role of the substrate surface preparation (with the deposition of an intermediate layer on the substrate surface impacting the nucleation seeds) and secondly the role of the different diffusion paths contributing to the one-dimensional growth in particular the influence of the surrounding gas phase and respective diffusion lengths on the substrate surface and wire sidewall. Experimental data describing the evolution of the wire diameter and length as a function of the growth time are quantitatively analyzed to evidence different growth regimes.

High quality factor whispering gallery modes from self-assembled hexagonal GaN rods grown by metal-organic vapor phase epitaxy

Self-assembled GaN rods were grown on sapphire by metal-organic vapor phase epitaxy using a simple two-step method that relies first on a nitridation step followed by GaN epitaxy. The mask-free rods formed without any additional catalyst. Most of the vertically aligned rods exhibit a regular hexagonal shape with sharp edges and smooth sidewall facets. Cathodo- and microphotoluminescence investigations were carried out on single GaN rods. Whispering gallery modes with quality factors greater than 4000 were measured demonstrating the high morphological and optical quality of the self-assembled GaN rods.

Selective Growth of GaN Rods on the Apex of GaN Pyramids by Metal Organic Vapor Phase Epitaxy

We report on the growth and characterization of GaN rods selectively grown on the apex of hexagonal GaN pyramids. SiO2 near the apex of the hexagonal GaN pyramids was removed by an optimized photolithography process and subsequently subjected to Au deposition and selective growth of GaN rods by metal organic vapor phase epitaxy (MOVPE). It was observed that there were preferred GaN rods orientations toward <1100> directions. The GaN rods had triangular cross section enclosed with (1122), (1122), and (0001) side facets. A particular feature was that each rod has sharp edge at its very end. We found that the GaN rods could be formed not by vapor–liquid–solid (VLS) process but Ga–Au intermediate state. This work opens up new growth methods for position and density controlled III–nitride nano- and micro-structures which have potential use in high functional devices, such as field emitters and gas sensors.