Free-Standing Self-Assemblies of Gallium Nitride Nanoparticles: A Review
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paper_title: Large-band-gap SiC, III-V nitride, and II-VI ZnSe-based semiconductor device technologies

paper_content:

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.
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paper_title: Stimulated Emission Near Ultraviolet at Room Temperature from a GaN Film Grown on Sapphire by MOVPE Using an AlN Buffer Layer

paper_content:

We report the first observation of the room temperature stimulated emission near UV from a GaN film which was grown by metalorganic vapor phase epitaxy on a (0001) sapphire substrate using an AlN buffer layer. This indicates that the GaN film is promising for the realization of an UV laser diode.
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paper_title: Structure and Heat Capacity of Wurtzite GaN from 113 to 1073 K

paper_content:

High pure wurtzite structure GaN has been synthesized by gas reaction method. Its structure was determined by powder x-ray diffraction using the Rietveld technique. The heat capacity C-p was measured from 113 to 1073 K, which can be represented by C-p = 0.362 + 3.010 x 10(-4)T - 3.411 x 10(3)T(-2) - 7.791 x 10(-8)T(2). NO measurable phase transition was observed in this temperature range.
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paper_title: High‐power InGaN/GaN double‐heterostructure violet light emitting diodes

paper_content:

InGaN/GaN double‐heterostructure light‐emitting diodes were fabricated. The output power was 90 μW and the external quantum efficiency was as high as 0.15% at a forward current of 20 mA at room temperature. The peak wavelengths of the electroluminescence(EL) varied between 411 and 420 nm with changes in the growth temperatures of an InGaN active layer between 820 and 800 °C. The full widths at half maximum of EL were between 22 and 25 nm.
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paper_title: Lattice thermal conductivity of group-IV and III–V semiconductor alloys

paper_content:

The room-temperature thermal conductivity of semiconductor alloys is analyzed using a simplified model of the alloy-disorder scattering. Good agreement is achieved between the present model and published experimental data on various group-IV and III−V semiconductor alloys. A complete set of alloy-disorder parameters are estimated, which makes it possible to calculate the lattice thermal conductivity for optional composition of III−V semiconductor alloys, including III−N alloys. An ordering effect is also examined for the explanation of some intermetallic and semiconductor compounds like CuAu and SiC.
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paper_title: Progress and prospects of group-III nitride semiconductors

paper_content:

Abstract We review recent progress in the group-III nitride and related materials, and electronic and optical devices based on them. Blue and UV (e.g. ultra violet) emitters and detectors, and high temperature/high power electronics which has long been coveted are beginning to be realized either in the laboratory or in the commercial arena, due in part to the breathtaking progress made in the last few years in the art and science of GaN, InN, AlN and their salloys. With brief references to the historical aspect of the relevant developments, this review concerns itself primarily with the current status of wide bandgap gallium nitride and related semiconductors from both the materials and devices points of view. Following a discussion of the structural properties of these materials, their electrical and optical properties are described in detail. The available data on metal contacts, the properties of which are indeed very conducive for the devices mentioned, from the points of view of ohmic contacts and Schottky barriers, are elaborated on. Recent progress on processing issues such as etching are reviewed. The review then embarks on an indash;depth discussion and analysis of field effect transistors, bipolar transistors, light emitting diodes, laser and photo detectors.
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paper_title: Gallium Nitride Schottky betavoltaic nuclear batteries

paper_content:

Abstract Gallium Nitride (GaN) Schottky betavoltaic nuclear batteries (GNBB) are demonstrated in our work for the first time. GaN films are grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD), and then GaN Schottky diodes are fabricated by normal micro-fabrication process. Nickel with mass number of 63 ( 63 Ni), which emits β particles, is loaded on the GaN Schottky diodes to achieve GNBB. X-ray diffraction (XRD) and photoluminescence (PL) are carried out to investigate the crystal quality for the GaN films as grown. Current–voltage ( I – V ) characteristics shows that the GaN Schottky diodes are not jet broken down at −200 V due to consummate fabrication processes, and the open circuit voltage of the GNBB is 0.1 V and the short circuit current density is 1.2 nA cm −2 . The limited performance of the GNBB is due to thin effective energy deposition layer, which is only 206 nm to absorb very small partial energy of the β particles because of the relatively high dislocation density and carrier concentration. However, the conversion efficiency of 0.32% and charge collection efficiency (CCE) of 29% for the GNBB have been obtained. Therefore, the output power of the GNBB are expected to greatly increase with growing high quality thick GaN films.
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paper_title: Metal semiconductor field effect transistor based on single crystal GaN

paper_content:

In this letter we report the fabrication and characterization of a metal semiconductor field effect transistor (MESFET) based on single crystal GaN. The GaN layer was deposited over sapphire substrate using low pressure metalorganic chemical vapor deposition. MESFET devices were fabricated on isolated mesas using TiAu for the source and drain ohmic contacts and silver for the gate Schottky. For devices with a gate length of 4 μm (channel opening, i.e., source to drain separation of 10 μm), a transconductance of 23 mS/mm was obtained at −1 V gate bias. Complete pinch‐off was observed for a gate potential of −12 V.
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paper_title: Stimulated Emission by Current Injection from an AlGaN/GaN/GaInN Quantum Well Device

paper_content:

Quantum well structures composed of GaInN well and GaN barrier were fabricated. Room-temperature stimulated emission by pulsed current injection is observed from group III nitride using the very thin active layer, for the first time.
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paper_title: Thermopower Study of GaN-Based Materials for Next-Generation Thermoelectric Devices and Applications

paper_content:

III-nitride InGaN-based solar cells have gained importance because their band gap can potentially cover most of the solar spectrum, spanning 0.7 eV to 3.4 eV. However, to use these materials to harvest additional energy, other properties such as their thermoelectric properties should be exploited. In this work, the Seebeck coefficient and the electrical conductivity of three InGaN alloys with various indium concentrations and Gd-doped GaN (GaN:Gd) were measured, and the power factor was calculated. We report a Seebeck value of ∼209 μV/K for Gd-doped GaN.
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paper_title: Crystal Growth and Conductivity Control of Group III Nitride Semiconductors and Their Application to Short Wavelength Light Emitters

paper_content:

Recent development of technology and understanding of the growth mechanism in heteroepitaxial growth of nitrides on highly-mismatched substrates have enabled us to grow high-quality GaN, AlGaN, GaInN and their quantum well structures. Conductivity control of both n-type and p-type nitrides has also been achieved. These achievements have led to the commercialization of high-brightness blue, green and white light-emitting diodes and to the realization of short wavelength laser diodes and high-speed transistors based on nitrides. The performance of these devices is still progressing, but still requires advances in many areas of materials science and device fabrication.
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paper_title: Design and simulation of GaN based Schottky betavoltaic nuclear micro-battery

paper_content:

National Natural Science Foundation of China [51075344, 61274120]; Science and Technology Project of Education Department of Fujian Province [JA12258]; Science and Technology Program of Xiamen [3502Z20123008]; Fujian Provincial Major Projects on University-Industry Cooperation in Science and Technology [2013H6023]
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paper_title: Thermoelectric Nanocomposites for Thermal Energy Conversion

paper_content:

Thermoelectric (TE) solid-state devices can convert heat into electricity. TE nanocomposites usually possess better TE performances than traditional TE bulks, such as higher figure-of-merit (ZT) and power output, playing a key role for industrial applications of TE devices, and thereby offering a great promise for improving energy efficiency over the traditional renewable energy systems. This chapter reviews manufacturing, TE properties and devices of TE nanocomposites. These nanocomposites are bottom-up compacted from nanoparticles that are produced by ball-milling . The produced nanocomposites possess unique microstructures to scatter phonons to decrease thermal conductivity and hold electric conductivity while maintaining or increasing the Seebeck coefficient. ZT of these nanocomposites are usually enhanced 10–100 %. TE devices are fabricated from these nanocomposites and their efficiencies are described. Future developments of TE nanocomposites are outlooked at the end of the chapter. The chapter will benefit material scientists, device engineers, and graduate students working in TE fields.
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paper_title: Exciton region reflectance of homoepitaxial GaN layers

paper_content:

Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; EA=3.4776 eV, EB=3.4827 eV, and EC=3.502 eV. The spin‐orbit parameter Δso=19.7±1.5 meV and the crystal field parameter Δcr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).
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paper_title: A high open-circuit voltage gallium nitride betavoltaic microbattery

paper_content:

A high open-circuit voltage betavoltaic microbattery based on a gallium nitride (GaN) p?i?n homojunction is demonstrated. As a beta-absorbing layer, the low electron concentration of the n-type GaN layer is achieved by the process of Fe compensation doping. Under the irradiation of a planar solid 63Ni source with activity of 0.5?mCi, the open-circuit voltage of the fabricated microbattery with 2???2?mm2?area reaches as much as 1.64?V, which is the record value reported for betavoltaic batteries with 63Ni source, the short-circuit current was measured as 568 pA and the conversion effective of 0.98% was obtained. The experimental results suggest that GaN is a high-potential candidate for developing the betavoltaic microbattery.
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paper_title: Nitride-based semiconductors for blue and green light-emitting devices

paper_content:

Recent advances in fabrication technologies for the semiconducting nitrides of the group III elements have led to commercially available, high-efficiency solid-state devices that emit green and blue light. Light-emitting diodes based on these materials should find applications in flat-panel displays, and blue and ultraviolet laser diodes promise high-density optical data storage and high-resolution printing.
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paper_title: Influence of precursor route on the photoluminescence of bulk nanocrystalline gallium nitride

paper_content:

GaN derived from gallium imide, {Ga(NH)3/2}n, typically exhibits yellow defect photoluminescence, with the pyrolysis temperature influencing the intensity of the emission. Pyrolysis of this same precursor in relatively high boiling N,N,N‘,N‘-tetramethyl-1,6-hexanediamine yields blue photoluminescence with an emission maximum near 420 nm. In contrast, GaN derived from pyrolysis of gallazane precursors such as [H2GaNH2]3 yields blue light emission whose quantum yield can be improved by a brief HF etch.
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paper_title: Observation of a quantum-confinement effect with GaN nanoparticles synthesized through a new gas reaction route

paper_content:

Spherical wurtzite GaN nanoparticles with diameters of 5–8 nm were synthesized through a new gas reaction route. The concave-surface-induced nucleation process based on the surface roughness of the substrate played an important role in the formation of quantum dots. Photoluminescence spectrum revealed the quantum-confined excited states in the nanoparticles with features at 3.73 eV (332 nm) and 3.85 eV (322 nm), which show significant blueshift compared with the band-gap emission from bulk GaN crystals. Four blue emission bands with low intensity, centered at 2.85 eV (435 nm), 2.75 eV (451 nm), 2.67 eV (464 nm), 2.62 eV (473 nm) respectively, were also observed. These blue bands may originate from surface states.
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paper_title: Structure and Heat Capacity of Wurtzite GaN from 113 to 1073 K

paper_content:

High pure wurtzite structure GaN has been synthesized by gas reaction method. Its structure was determined by powder x-ray diffraction using the Rietveld technique. The heat capacity C-p was measured from 113 to 1073 K, which can be represented by C-p = 0.362 + 3.010 x 10(-4)T - 3.411 x 10(3)T(-2) - 7.791 x 10(-8)T(2). NO measurable phase transition was observed in this temperature range.
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paper_title: Morphology of GaN in ammonia

paper_content:

chinese acad sci, inst phys, beijing 100080, peoples r china. chinese acad sci, ctr condensed matter phys, beijing 100080, peoples r china. acad sinica, int ctr mat phys, shenyang 110015, peoples r china.;lan, yc (reprint author), chinese acad sci, inst phys, pob 603, beijing 100080, peoples r china
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paper_title: Synthesis, structure, and optical properties of colloidal GaN quantum dots

paper_content:

Colloidal chemistry was used to synthesize GaN quantum dots. A GaN precursor, polymeric gallium imide, {Ga(NH)3/2}n, which was prepared by the reaction of dimeric amidogallium with ammonia at room temperature, was heated in trioctylamine at 360 °C for one day to produce GaN nanocrystals. The GaN particles were separated, purified, and partially dispersed in a nonpolar solvent to yield transparent colloidal solutions that consisted of individual GaN particles. The GaN nanocrystals have a spherical shape and mean diameter of about 30±12 A. The spectroscopic behavior of colloidal transparent dispersion has been investigated and shows that the band gap of the GaN nanocrystals shifts to slightly higher energy due to quantum confinement. The photoluminescence spectrum at 10 K (excited at 310 nm) shows band edge emission with several emission peaks in the range between 3.2 and 3.8 eV, while the photoluminescence excitation spectrum shows two excited-state transitions at higher energies.
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paper_title: Large-band-gap SiC, III-V nitride, and II-VI ZnSe-based semiconductor device technologies

paper_content:

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high‐temperature electronics and short‐wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high‐power operation. The present advanced stage of development of SiC substrates and metal‐oxide‐semiconductor technology makes SiC the leading contender for high‐temperature and high‐power applications if ohmic contacts and interface‐state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high‐temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN‐based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short‐wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN‐based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe‐based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide‐band‐gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.
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paper_title: A soluble salt-assisted facile synthetic route to semiconducting GaN nanoparticles

paper_content:

By using the soluble salt as the dispersant, a facile synthetic route has been developed to prepare GaN nanoparticle powders on a large scale and in a high yield of 95.8% through the direct nitridation of Ga–Na2SO4 mixture at 700 °C and followed by washing with water. This preparation method is also applicable to some other low-melting-point-metal oxides. The simple preparation procedure and the range of potential applications of the products made this work both scientifically and technologically interesting.
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paper_title: Fabrication and photoluminescence of highly crystalline GaN and GaN:Mg nanoparticles

paper_content:

Abstract Well-dispersed nano-sized gallium nitride (GaN) and magnesium-doped GaN (GaN:Mg) particles were successfully prepared from nano-sized gallium oxide (Ga 2 O 3 ) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared from mixtures of gallium nitrate aqueous solution and ammonia solution that were heated to between 600 and 800 °C. Addition of a proportional amount of ammonia into the solution is a key point for production of well-dispersed nano-sized gallium oxide particles. Gallium oxide nanoparticles less than 20 nm in size were obtained when using an NH 3 to Ga(NO 3 ) 3 ratio of 1:1 (mol/mol) with a heating temperature of 800 °C. Nitridation of gallium oxide nanoparticles under a flow of ammonia at a heating temperature of 800 °C resulted in nano-sized GaN particles less than 40 nm in size. We investigated the effects of preparation conditions such as solution composition and operating temperature on the characteristics of gallium nitride particles. This method allows simple doping of GaN with alkali earth metals. It was shown that the doping with magnesium resulted in intense blue luminescence from particles when UV excitation at 254 nm was used.
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paper_title: Scalable preparation and characterization of GaN nanopowders with high crystallinity by soluble salts-assisted route

paper_content:

By using Na3PO4 as a dispersant, soluble salt-assisted route has been further developed to prepare high-crystalline GaN nanoparticles powder on a large scale through the direct nitridation of Ga-Na3PO4 mixture at 750–950 °C and followed by washing with water. The systematical characterizations including XRD, Raman, IR, TEM, XPS, and PL spectrum showed that the as-prepared nanopowders were composed of nanoparticles in diameters of 8–18 nm, hexagonal phase, pure GaN, and had a broad UV centered at 388 nm and blue emissions band centered at around 547 nm. Because of the utilization of the simple reaction between metallic Ga and NH3, the preparation of pure GaN nanopowders becomes very easy, economical, and scalable, suggesting broad application in optoelectronic device material. The interesting results indicate the wide range of soluble salt-assisted route for promising industrial production of GaN nanopowders. In this work, the soluble salt-assisted route has been further developed to prepare high-crystalline wurtzite GaN nanopowders (NPs) on a large scale through the direct nitridation of Ga-Na3PO4 mixture at 750–950 °C and followed by washing with water.
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paper_title: Preparation and optical properties of GaN nanocrystalline powders

paper_content:

Abstract GaN nanocrystalline powders were synthesised by decomposition of gallium nitrate, followed by nitrogenising with ammonia under different temperature. X-ray diffraction (XRD) and the transmission electron microscopy (TEM) indicated that the crystallinity of the powder is improved and the average size of the GaN nanocrystallites increases from 4·8 to 23·9 nm as the temperature increases from 850 to 1050°C. The Raman spectra displayed four broadened peaks corresponding to A1 (LO), A1 (TO), E1 (TO) and E2 (high) modes of wurtzite GaN respectively. Two additional modes at 252 and 421 cm–1 attributed to boundary phonons activated by the finite size effects and octahedral Ga–N6 bonds were observed respectively. A strong blue photoluminescence (∼353 nm) was detected for room temperature measurement, indicating that the GaN nanocrystalline powders have few defects and high quality.
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paper_title: Ammonolytical conversion of microcrystalline gallium antimonide GaSb to nanocrystalline gallium nitride GaN: thermodynamics vs. topochemistry

paper_content:

Reaction of microcrystalline powders of readily available gallium antimonide GaSb with ammonia gas at elevated temperatures afforded in one step high yields of nanocrystalline powders of the semiconductor, gallium nitride GaN. In particular, temperatures of 900–1000 °C and suitable reaction times of 36–170 hours resulted in complete nitridation in the system. GaN was prepared as a mixture of the major stable hexagonal and the minor metastable cubic polytypes. Formation of the cubic GaN was consistent with topochemistry playing a meaningful role in the ammonolysis of the cubic GaSb substrate. Specific experimental conditions, including variations in the reaction temperature/time and manual grinding or high energy ball milling of the substrate, had a significant impact on the final GaN polytype make-up and average crystallite size, the latter ranging from a few to a few tens of nanometers. Under the applied conditions, all by-products were conveniently removed from the reaction mixture as volatile species, thus affording chemically pure GaN nanopowders of very good quality.
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paper_title: The effect of nitridation temperature on the structural, optical and electrical properties of GaN nanoparticles

paper_content:

Synthesis of GaN nanoparticles (NPs) by a novel chemical co-precipitation method and the effect of nitridation temperature on the structural, optical and electrical properties have been reported. X-ray diffraction and high resolution transmission electron microscopy show the hexagonal wurtzite structure and highly crystalline nature of the GaN NPs. A strong blue luminescence was observed for all the GaN NPs at room temperature photoluminescence studies. Nevertheless, red and yellow luminescence were absent at the nitridation temperature of 1000 °C. The phonon frequency mode at the k-point of the Brillouin zone symmetry was observed at 271–273 cm−1 for GaN NPs by micro-Raman spectroscopy, which is normally absent for bulk GaN. The carrier concentration and mobility of GaN NPs synthesized at higher temperature were calculated to be 1.36 × 1017 cm−3 and 433 cm2 V−1 s−1, respectively, by the Raman line shape analysis of the longitudinal-optical-phonon–plasmon coupled mode.
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paper_title: Ammonolysis of polycrystalline and amorphized gallium arsenide GaAs to polytype-specific nanopowders of gallium nitride GaN

paper_content:

Convenient single-step N-for-As metathesis reactions of gallium arsenide GaAs with ammonia NH3 at temperatures in the range 650–950 °C for 6–90 hours afforded in this oxygen-free system high yields of pure nanocrystalline powders of the wide bandgap semiconductor gallium nitride GaN. High energy ball milling via noticeable amorphization of the monocrystalline cubic GaAs substrate enabled complete ammonolysis and nitride preparation at lower temperatures and shorter times relative to manual grinding. Under the applied conditions, all by-products were removed as volatiles affording pure GaN nanopowders. Reaction-controlled average crystallite sizes ranged from a few to a few tens of nanometers. When compared to the related ammonolysis reactions of cubic GaP and cubic GaSb which yielded, respectively, either the hexagonal polytype only or mixtures of mostly hexagonal with some cubic GaN polytypes, here, the nitride could be made both as solely hexagonal and as a mixture of two polytypes in a wide composition range. All this supports diverse reaction pathways which were found to be closely correlated with substrate grain size characteristics. The ball milled fine GaAs particles afforded only hexagonal or hexagonal GaN-enriched mixtures pointing to predominantly thermodynamic reaction control. Under similar conditions, the manually ground coarser GaAs particles yielded cubic GaN-enriched mixtures, instead, consistent with prevailing topochemical control.
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paper_title: Ammonolysis of gallium phosphide GaP to the nanocrystalline wide bandgap semiconductor gallium nitride GaN

paper_content:

The pnictogen-metathesis reaction of microcrystalline gallium phosphide GaP with ammonia NH3 at temperatures of 900–1150 °C for 6–60 hours afforded in one step nanocrystalline powders of the wide bandgap semiconductor gallium nitride GaN. A suitable choice of conditions including variations of reaction temperature/time and manual grinding or high energy ball milling of the substrate enabled control over distinct GaN particle morphologies, regularly shaped particles or nanowires, and average crystallite sizes up to a few tens of nanometers. Under the applied conditions, all by-products were conveniently removed as volatiles affording pure GaN nanopowders. In contrast to ammonolysis of the related cubic GaAs and cubic GaSb, which yielded mixtures of the hexagonal and cubic GaN polytypes, here, the nitride was made exclusively as the stable hexagonal variety. All this supported specific reaction pathways with thermodynamics solely controlling the ammonolytical conversion of the cubic GaP substrate to the hexagonal GaN product.
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paper_title: Synthesis and structure of nanocrystal-assembled bulk GaN

paper_content:

A new condensed form of GaN, nanocrystal-assembled bulk (NAB) GaN, was obtained directly from reactions of metal Ga and NH4Cl in liquid ammonia at 350-500 degrees C. High-resolution transmission electron microscopy observations reveal that the NAB GaN consists of well-crystallized nanocrystals with wurtzite structure. The synchronous densificated NAB GaN is transparent to visible light while the constituted nanocrystals have an average size of about 12 nm. A possible synthesis mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.66.Jg; 81.20. -n.
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paper_title: On GaN Crystallization by Ammonothermal Method

paper_content:

GaN crystals are grown using ammonothermal method at pressures below 5 kbar and temperatures below 550°C. In this method, GaN is synthesised from high purity metallic gallium. The main role in the low temperature GaN crystallization is played by the chemically active and dense ammonia and dissolved mineralizer. Morphology of the obtained crystals as well as solubility experiments prove that gallium nitride is dissolved and crystallised from solution. Physical properties of GaN crystals obtained using ammonothermal method depend on the growth conditions and the type of mineralizer. All GaN samples reveal very intensive photoluminescence, also at room temperature. The spectra of crystals grown with lithium compound mineralizer are shifted towards higher energies in comparison to crystals grown with potassium based mineralizer. At helium temperatures, phosphorescence is also observed. PACS numbers: 81.10.Dn
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paper_title: Progress and prospects of group-III nitride semiconductors

paper_content:

Abstract We review recent progress in the group-III nitride and related materials, and electronic and optical devices based on them. Blue and UV (e.g. ultra violet) emitters and detectors, and high temperature/high power electronics which has long been coveted are beginning to be realized either in the laboratory or in the commercial arena, due in part to the breathtaking progress made in the last few years in the art and science of GaN, InN, AlN and their salloys. With brief references to the historical aspect of the relevant developments, this review concerns itself primarily with the current status of wide bandgap gallium nitride and related semiconductors from both the materials and devices points of view. Following a discussion of the structural properties of these materials, their electrical and optical properties are described in detail. The available data on metal contacts, the properties of which are indeed very conducive for the devices mentioned, from the points of view of ohmic contacts and Schottky barriers, are elaborated on. Recent progress on processing issues such as etching are reviewed. The review then embarks on an indash;depth discussion and analysis of field effect transistors, bipolar transistors, light emitting diodes, laser and photo detectors.
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paper_title: Syntheses and structure of nanocrystalline gallium nitride obtained from ammonothermal method using lithium metal as mineralizator

paper_content:

A double layer CeO2/Nb2O5 thin film by magnetron sputtering was prepared. The property and microstructure of the thin film were carefully characterized by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and high resolution transmission electron microscopy, XPS measurement clearly demonstrated that the Nb2O5 layer underlying CeO2 as a electrical-conductive layer could greatly improve the reduction ability of CeO2 thin film compared with that of a single layer CeO2 thin film: Fitting to the XPS experimental data and deconvoluting the spectra showed the increase of Ce3+ concentration in the double layer CeO2/Nb2O5 thin film. The microstructure of the double layer thin film was composed of three distinguishable layers, which are a polycrystalline CeO2 layer, an amorphous Nb2O5 layer, and an intermediate amorphous layer with regard to the silicon substrate. The interface between CeO2 and Nb2O5 was rough and indistinct. A detailed analysis was given from the viewpoint of interdiffusion at the CeO2/Nb2O5 interface. (C) 2000 American vacuum Society. [S0734-2101(00)03406-0].
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paper_title: Ammonothermal Synthesis of Cubic Gallium Nitride

paper_content:

Cubic gallium nitride (zinc blende type) was formed by reactions of Ga, gallium iodides, and gallium imide−iodides in supercritical ammonia under acidic conditions above 250 °C. Solvothermal transport and recrystallization of c-GaN occurred above 440 °C.
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paper_title: Synthesis, Raman scattering, and infrared spectra of a new condensed form of GaN nanophase material

paper_content:

A new form of transparent condensed nanophase material of GaN was synthesized directly by ammono-thermal synthetic route. Nano-sized effects and thermal stability of that material were investigated through Raman scattering and infrared spectra. Compared with bulk GaN, we observed the Raman low-energy-shift of the phonon frequency of E-2(high) and the transverse optical mode [E-1(TO)], the infrared high-energy-shift of omega(T), and the variation of relative intensity I-E2/E1(TO). These characteristics can be attributed to the existence Of the interface effects and the vacancy of N in the GaN nanophase material. This material has a high thermal stability even at 900 degrees C as indicated through infrared and Raman spectral investigation of annealed samples of as-synthesized nanophase material.
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paper_title: Continuous supercritical route for quantum-confined GaN nanoparticles

paper_content:

GaN quantum dots (QDs) are prepared in a one-step continuous process using anhydrous solvents at supercritical conditions (and temperatures below 450 °C) in short residence times, typically less than 25 s. The as-prepared QDs exhibit a strong luminescence in the ultraviolet (λmax ~ 350 nm) in agreement with the synthesis of quantum-confined GaN nanoparticles.
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paper_title: Solvothermal azide decomposition route to GaN nanoparticles, nanorods, and faceted crystallites

paper_content:

We report a straightforward, nonaqueous, solvothermal method for the production of nanoscale gallium nitride structures. Nanoparticles with spherical and rodlike morphologies are produced via in situ gallium azide precursor synthesis and decomposition in superheated toluene or THF. The solution reaction between gallium chloride and sodium azide produces an insoluble azide precursor that solvothermally decomposes to GaN at temperatures below 260 °C. The resulting products are poorly crystalline but thermally stable and crystallize to hexagonal GaN upon annealing at 750 °C. Product morphologies include nanoparticles (ca. 50 nm) and nanorods (ca. 300 nm lengths). Upon annealing, the nanoparticles coalesce into larger organized crystalline structures with hexagonal facets.
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paper_title: A Benzene-Thermal Synthetic Route to Nanocrystalline GaN

paper_content:

A thermal reaction of Li3N and GaCl3 in which benzene was used as the solvent under pressure has been carried out for the preparation of 30-nanometer particles of gallium nitride (GaN) at 280°C. This temperature is much lower than that of traditional methods, and the yield of GaN reached 80%. The x-ray powder diffraction pattern indicated that sample was mainly hexagonal-phase GaN with a small fraction of rocksalt-phase GaN, which has a lattice constant a = 4.100 angstroms. This rocksalt structure, which had been observed previously only under high pressure (at least 37 gigapascals) was observed directly with high-resolution electron microscopy.
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paper_title: Coexistence of wurtzite GaN with zinc blende and rocksalt studied by x‐ray power diffraction and high‐resolution transmission electron microscopy

paper_content:

X‐ray powder diffraction and high‐resolution transmission electron microscopy (HRTEM) have found coexistence of three structures of GaN: wurtzite, zinc blende, and rocksalt in the samples prepared through benzene thermal process at 280–300 °C under pressure about 5 MPa. Previously, GaN with rocksalt structure only appeared at high pressure more than 37 GPa. HRTEM showed that there was a transitional region between the two cubic phases of GaN with zinc blende and rocksalt. X‐ray photoelectron spectra analysis of the sample gave the average composition of Ga0.95N.
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paper_title: Intermediates in Ammonothermal GaN Crystal Growth under Ammonoacidic Conditions

paper_content:

Invited for the cover of this issue is the group of Rainer Niewa from the Institute of Inorganic Chemistry of the University of Stuttgart. The cover image illustrates several gallium-containing species as possible intermediates of GaN crystal growth in supercritical ammonia under ammonoacidic conditions as well as crystals of solid compounds recovered from ammonia.
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paper_title: Formation of GaN nanorods by a sublimation method

paper_content:

Abstract GaN nanorods with diameters of 10–45 nm were formed through a simple sublimation method. They were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). SEM images showed that the nanorods were straight. XRD, SAED and HRTEM indicated that the nanorods were wurtzite GaN single crystals.
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paper_title: Thermoelectric Nanocomposites for Thermal Energy Conversion

paper_content:

Thermoelectric (TE) solid-state devices can convert heat into electricity. TE nanocomposites usually possess better TE performances than traditional TE bulks, such as higher figure-of-merit (ZT) and power output, playing a key role for industrial applications of TE devices, and thereby offering a great promise for improving energy efficiency over the traditional renewable energy systems. This chapter reviews manufacturing, TE properties and devices of TE nanocomposites. These nanocomposites are bottom-up compacted from nanoparticles that are produced by ball-milling . The produced nanocomposites possess unique microstructures to scatter phonons to decrease thermal conductivity and hold electric conductivity while maintaining or increasing the Seebeck coefficient. ZT of these nanocomposites are usually enhanced 10–100 %. TE devices are fabricated from these nanocomposites and their efficiencies are described. Future developments of TE nanocomposites are outlooked at the end of the chapter. The chapter will benefit material scientists, device engineers, and graduate students working in TE fields.
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paper_title: Formation of gallium nitride by a novel hot mechanical alloying process

paper_content:

Using a novel ball mill with controlled ball movement and operating at elevated temperature, gallium nitride powder has been synthetized by simple milling of gallium metal in a dry ammonia atmosphere. The structural changes resulting from milling were studied by x‐ray diffractometry and transmission electron microscopy. The elemental composition of the as‐milled product was analyzed using Rutherford backscattering spectroscopy. A nanostructural phase was obtained following milling, which transforms into polycrystalline GaN of the wurtzite‐type structure upon further thermal treatment.
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paper_title: Synthesis of GaN nanocrystallites by pulsed laser ablation in pure nitrogen background gases

paper_content:

GaN is a promising material not only for electronic devices but also for photocatalysts. Synthesis of GaN nanocrystal is a key issue to improve performance for these applications. In the present study, GaN nanocrystallites have been synthesized by pulsed laser ablation (PLA), where safe and inactive pure N2 gases were used as reactive background gases. The third harmonics beam of a Q-switched Nd:YAG laser (355 nm, 10 mJ/pulse, 4 J/(cm2 pulse)) was used to ablate a sintered high purity GaN target. The deposition substrates were not heated. It was clarified that the formed GaN nanoparticles contained a hexagonal system with the wurtzite structure. The diameter of the nanocrystallites was about 10 nm, and showed only little dependence on the background gas pressure, while the porosity of the assembly of nanocrystallites and content of GaN nanocrystallites in the assembly increased with background gas pressure. Highly porous nanometer-sized GaN film obtained at higher gas pressure is considered to be candidate structures for the photocatalysts.
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paper_title: Observation of confinement-dependent exciton binding energy of GaN quantum dots

paper_content:

The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size. This effect was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius. From this observation, absolute values of the exciton binding energy as a function of quantum dot size are determined.
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paper_title: RED EMISSION FROM GaN NANOCRYSTALLINE SOLIDS

paper_content:

The photoluminescence of the transparent GaN nanocrystallite solids is studied. Three new intensive isolated narrow red emission bands and one broad blue band are observed. A mechanism to explain the origin of these bands is proposed. The three red bands may originate from the transitions of deep donor levels to shallow acceptors, while the blue band may be from the transitions of shallow donors to deep levels with ground and excited states.
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paper_title: Observation of a quantum-confinement effect with GaN nanoparticles synthesized through a new gas reaction route

paper_content:

Spherical wurtzite GaN nanoparticles with diameters of 5–8 nm were synthesized through a new gas reaction route. The concave-surface-induced nucleation process based on the surface roughness of the substrate played an important role in the formation of quantum dots. Photoluminescence spectrum revealed the quantum-confined excited states in the nanoparticles with features at 3.73 eV (332 nm) and 3.85 eV (322 nm), which show significant blueshift compared with the band-gap emission from bulk GaN crystals. Four blue emission bands with low intensity, centered at 2.85 eV (435 nm), 2.75 eV (451 nm), 2.67 eV (464 nm), 2.62 eV (473 nm) respectively, were also observed. These blue bands may originate from surface states.
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paper_title: Dielectric properties of GaN nanoparticles

paper_content:

Three kinds of GaN samples (hexagonal coarse-grain powders, hexagonal nanoparticles and cubic nanoparticles) were synthesized by different methods. The room-temperature frequency spectra of the relative dielectric constant and that of the dielectric loss were measured in the frequency range of 100 Hz to 10 MHz. Analysis of these spectra indicates that the grain size of samples has great influence on the dielectric behavior of samples whereas the crystallography symmetry almost has no impact on the relative dielectric constant.
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paper_title: Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots

paper_content:

Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots is reported. Two samples having different average size of quantum dots (QDs) have been investigated at the temperature of 3.5 K. The measurement has revealed that larger-QD sample shows longer photoluminescence (PL) decay time and smaller emission energy than smaller one. The dependence of radiative decay time of the samples on emission energy smoothly connects with each other reflecting the size distribution. The radiative decay time strongly increases by almost three orders magnitude, reaching microseconds, upon increasing the size of QDs. The increase of PL decay time with increasing the size of QDs is attributed to the reduction of oscillator-strength due to the strong built-in electric field in the GaN/AlN heterostructures.
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paper_title: Size and shape dependence on melting temperature of gallium nitride nanoparticles

paper_content:

The study of variation of the size and shape effect on the melting property of gallium nitride nanoparticles with their spherical and cylindrical geometrical feature is theoretically explored. A numerical thermodynamical model has been devoted for the study. A comparative investigation is made between the two shapes, at the range of ∼3 nm dia. The cylindrical GaN nanoparticles, whose melting point has been reported to decrease with decreasing particle radius, become larger than spherical-shaped nanoparticles. The melting temperature obtained in the present study is in line with the function of radius of curvature.
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paper_title: Theoretical predictions of wurtzite III-nitride nano-materials properties.

paper_content:

In this paper, top-down and bottom-up approaches are used to predict material properties of group III-nitride nanostructures. The first approach calculates the melting temperature, melting enthalpy, Debye temperature and energy bandgap of InN, GaN and AlN through classical thermodynamics. The second approach calculates the surface energies in the liquid and solid states of the considered nitrides materials through molecular dynamics. Moreover, the liquid and solid surface energies of the zinc-blende and wurtzite III-V materials are compared. Finally, the phase diagram of a ternary III-nitride nanomaterial, AlGaN, is presented and the variation of its energy bandgap with composition is predicted.
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paper_title: Optical properties of GaN quantum dots

paper_content:

We report on an investigation of the optical properties of GaN quantum dots (QDs) grown by means of metalorganic vapor phase epitaxy. The growth regime for GaN on AlxGa1−xN was observed to change from two- to three-dimensional, forming GaN QDs, when Si was deposited on the AlxGa1−xN surface prior to the GaN growth. These QDs showed a redshift of the photo luminescence (PL) energy from the increased Coulomb energy induced by a compression of the exciton Bohr radius. Furthermore, a diminishing temperature-dependent shift of the PL energy with decreasing QD size caused by a reduction of the longitudinal-optical phonon coupling was found. We also show that the size of the QDs is a critical parameter for the optical nonlinearities. For large dots, the dominant nonlinearity in the PL is the bandgap renormalization but when the size of the dots was reduced below the critical size of 10 nm thick and 30 nm diameter, the state-filling effect became dominant.
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paper_title: Synthesis, structure, and optical properties of colloidal GaN quantum dots

paper_content:

Colloidal chemistry was used to synthesize GaN quantum dots. A GaN precursor, polymeric gallium imide, {Ga(NH)3/2}n, which was prepared by the reaction of dimeric amidogallium with ammonia at room temperature, was heated in trioctylamine at 360 °C for one day to produce GaN nanocrystals. The GaN particles were separated, purified, and partially dispersed in a nonpolar solvent to yield transparent colloidal solutions that consisted of individual GaN particles. The GaN nanocrystals have a spherical shape and mean diameter of about 30±12 A. The spectroscopic behavior of colloidal transparent dispersion has been investigated and shows that the band gap of the GaN nanocrystals shifts to slightly higher energy due to quantum confinement. The photoluminescence spectrum at 10 K (excited at 310 nm) shows band edge emission with several emission peaks in the range between 3.2 and 3.8 eV, while the photoluminescence excitation spectrum shows two excited-state transitions at higher energies.
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paper_title: On GaN Crystallization by Ammonothermal Method

paper_content:

GaN crystals are grown using ammonothermal method at pressures below 5 kbar and temperatures below 550°C. In this method, GaN is synthesised from high purity metallic gallium. The main role in the low temperature GaN crystallization is played by the chemically active and dense ammonia and dissolved mineralizer. Morphology of the obtained crystals as well as solubility experiments prove that gallium nitride is dissolved and crystallised from solution. Physical properties of GaN crystals obtained using ammonothermal method depend on the growth conditions and the type of mineralizer. All GaN samples reveal very intensive photoluminescence, also at room temperature. The spectra of crystals grown with lithium compound mineralizer are shifted towards higher energies in comparison to crystals grown with potassium based mineralizer. At helium temperatures, phosphorescence is also observed. PACS numbers: 81.10.Dn
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paper_title: A GaN bulk crystal with improved structural quality grown by the ammonothermal method.

paper_content:

The realization of high-performance optoelectronic devices, based on GaN and other nitride semiconductors, requires the existence of a high-quality substrate. Non-polar or semipolar substrates have recently been proven to provide superior optical devices to those on conventional c-plane substrates1,2,3,4. Bulk GaN growth enables GaN substrates sliced along various favourable crystal orientations. Ammonothermal growth is an attractive method for bulk GaN growth owing to its potential to grow GaN ingots at low cost. Here we report on improvement in the structural quality of GaN grown by the ammonothermal method. The threading dislocation densities estimated by plan-view transmission electron microscopy observations were less than 1×106 cm−2 for the Ga face and 1×107 cm−2 for the N face. No dislocation generation at the interface was observed on the Ga face, although a few defects were generated at the interface on the N face. The improvement in the structural quality, together with the previous report on growth rate5 and scalability6, demonstrates the commercial feasibility of the ammonothermal GaN growth.
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paper_title: Synthesis of N-deficient GaN nanoparticles and its enhanced dielectric response

paper_content:

Abstract In this paper, GaN nanoparticles were firstly synthesized through a facile solid-state reaction using an organic reagent cyanamide (CN 2 H 2 ) and Ga 2 O 3 as precursors. The structural properties were investigated in detail. It is found that these nanoparticles having average size of 40 nm were N-deficient with the N vacancies reaching as high as 12%. The Raman scattering spectrum of these nanoparticles presented some interesting features. The room-temperature frequency spectrum of the relative dielectric constant ɛ r was measured and indicated that these nanoparticles exhibited sharp enhancement at low frequency range comparing with GaN nanomaterials and N-deficient microparticles. It is thought both the rotation direction polarization (RDP) and the space charge polarization (SCP) process should be responsible for the enhancement of ɛ r in these N-deficient GaN nanoparticles.
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paper_title: Structure and Heat Capacity of Wurtzite GaN from 113 to 1073 K

paper_content:

High pure wurtzite structure GaN has been synthesized by gas reaction method. Its structure was determined by powder x-ray diffraction using the Rietveld technique. The heat capacity C-p was measured from 113 to 1073 K, which can be represented by C-p = 0.362 + 3.010 x 10(-4)T - 3.411 x 10(3)T(-2) - 7.791 x 10(-8)T(2). NO measurable phase transition was observed in this temperature range.
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paper_title: Ga-vacancy-induced room-temperature ferromagnetic and adjusted-band-gap behaviors in GaN nanoparticles

paper_content:

Room-temperature ferromagnetism has been found in Ga-deficient GaN grown using the direct reaction of Ga\(_{2}\)O\(_{3}\) powder with NH\(_{3}\) gas. The observed magnetism in GaN induced by Ga vacancies is investigated both experimentally and theoretically. First-principles calculations reveal that the spontaneous spin polarization is created by the 3.0 \(\mu _\mathrm{B}\) local moment for GaN and magnetism originates from the polarization of the unpaired 2\(p\) electrons of N surrounding the Ga vacancy. At the same time, the band gap can be also adjusted by changing the Ga-vacancy concentration.
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paper_title: Yellow and green luminescence in a freestanding GaN template

paper_content:

We have studied a broad photoluminescence band in high-mobility freestanding 200-μm-thick GaN template prepared by hydride vapor-phase epitaxy. Variable-excitation intensity and energy experiments showed two defect-related bands: a yellow luminescence (YL) band at about 2.15 eV and a green luminescence (GL) band at about 2.43 eV. In contrast to epitaxial GaN samples prepared by both vapor-phase and molecular-beam epitaxy, the YL in the sample studied is weak and can be easily saturated. However, the GL is dominant. We attribute the GL to isolated defects involving gallium vacancies and the YL to the same defect, but bound to dislocations, or possibly to structural surface defects.
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paper_title: Luminescence properties of defects in GaN

paper_content:

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...
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paper_title: On the phenomenon of large photoluminescence red shift in GaN nanoparticles

paper_content:

We report on the observation of broad photoluminescence wavelength tunability from n-type gallium nitride nanoparticles (GaN NPs) fabricated using the ultraviolet metal-assisted electroless etching method. Transmission and scanning electron microscopy measurements performed on the nanoparticles revealed large size dispersion ranging from 10 to 100 nm. Nanoparticles with broad tunable emission wavelength from 362 to 440 nm have been achieved by exciting the samples using the excitation power-dependent method. We attribute this large wavelength tunability to the localized potential fluctuations present within the GaN matrix and to vacancy-related surface states. Our results show that GaN NPs fabricated using this technique are promising for tunable-color-temperature white light-emitting diode applications.
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paper_title: Conversion Efficiency Enhancement of GaN/In$_{0.11}$Ga$_{0.89}$N Solar Cells With Nano Patterned Sapphire and Biomimetic Surface Antireflection Process

paper_content:

In this study, p-i-n double-heterojunction GaN/ In0.11Ga0.89 N solar cells grown by metal-organic chemical vapor deposition on pattern sapphire substrate are presented. The solar cell with standard process has a conversion efficiency of 3.1%, which corresponds to a fill factor of 58%, short circuit current density of 2.86 mA/cm2 , and open circuit voltage of 1.87 V under AM1.5G illumination. To further improve the conversion efficiency of the GaN/ In0.11Ga0.89 N solar cells, two-dimensional polystyrene nanospheres were deposited and self-organized as mask in the anisotropic inductively coupled plasma reactive ion etching process to form a biomimetic surface roughing texture. The surface morphology of the solar cell shows a periodically hexagonal bead pattern and the beads are formed in a diameter of 160 nm with a period of 250 nm. An increase of 15% in short circuit current density is found, thus improving the conversion efficiency to 3.87%. If we optimize the structure for 180 nm of the height and 375 nm of the period, a 10% gain can be expected when compared to the current structure.
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paper_title: Photocatalytic Overall Water Splitting on Gallium Nitride Powder

paper_content:

Gallium nitride (GaN) powder was studied as a photocatalyst for overall water splitting. The photocatalytic activity of GaN for the reaction was found to be strongly dependent on the crystallinity ...
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paper_title: Growth of Semiconducting GaN Hollow Spheres and Nanotubes with Very Thin Shells via a Controllable Liquid Gallium-Gas Interface Chemical Reaction

paper_content:

An in situ liquid gallium–gas interface chemical reaction route has been developed to synthesize semiconducting hollow GaN nanospheres with very small shell size by carefully controlling the synthesis temperature and the ammonia reaction gas partial pressure. In this process the gallium droplet does not act as a catalyst but rather as a reactant and a template for the formation of hollow GaN structures. The diameter of the synthesized hollow GaN spheres is typically 20–25 nm and the shell thickness is 3.5–4.5 nm. The GaN nanotubes obtained at higher synthesis temperatures have a length of several hundreds of nanometers and a wall thickness of 3.5–5.0 nm. Both the hollow GaN spheres and nanotubes are polycrystalline and are composed of very fine GaN nanocrystalline particles with a diameter of 3.0–3.5 nm. The room-temperature photoluminescence (PL) spectra for the synthesized hollow GaN spheres and nanotubes, which have a narrow size distribution, display a sharp, blue-shifted band-edge emission peak at 3.52 eV (352 nm) due to quantum size effects.
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paper_title: Fabrication and photoluminescence of ordered GaN nanowire arrays

paper_content:

Large-scale of crystalline GaN nanowires (diameter∼50 nm) have been fabricated through chemical-vapor deposition in the nanochannels of the anodic alumina template. X-ray diffraction and selected area electron diffraction pattern investigations indicate that the nanowires are single crystal with hexagonal wurtzite structure. A typical scanning electron microscopy image and the energy dispersive x-ray spectroscopy results indicate that indium nanoparticles only act as catalyst in GaN nanowires growth. At room temperature, photoluminescence spectrum of the GaN nanowire arrays shows a visible broadband with three peaks, which are located at about 363, 442, and 544 nm. The light emission may be attributed to GaN band-edge emission, the existence of defects or surface states, and the interaction between the ordered GaN nanowires and anodic alumina membrane. The growth mechanism of crystalline GaN nanowires is discussed. The method makes it possible to synthesize other nitride nanowire arrays.Large-scale of crystalline GaN nanowires (diameter∼50 nm) have been fabricated through chemical-vapor deposition in the nanochannels of the anodic alumina template. X-ray diffraction and selected area electron diffraction pattern investigations indicate that the nanowires are single crystal with hexagonal wurtzite structure. A typical scanning electron microscopy image and the energy dispersive x-ray spectroscopy results indicate that indium nanoparticles only act as catalyst in GaN nanowires growth. At room temperature, photoluminescence spectrum of the GaN nanowire arrays shows a visible broadband with three peaks, which are located at about 363, 442, and 544 nm. The light emission may be attributed to GaN band-edge emission, the existence of defects or surface states, and the interaction between the ordered GaN nanowires and anodic alumina membrane. The growth mechanism of crystalline GaN nanowires is discussed. The method makes it possible to synthesize other nitride nanowire arrays.
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paper_title: Large-area synthesis of diameter-controllable porous single crystal gallium nitride micro/nanotube arrays

paper_content:

Large-area porous single crystal gallium nitride micro/nanotube arrays were prepared by a simple method using zinc oxide arrays as the template. This is the first synthesis of such a type of porous GaN micro or nanotubes. The inner diameters of these tubes could be accurately controlled by varying the diameters of zinc oxide micro/nanowires. The area of these micro/nanotube arrays could be achieved at a desired large value by changing the area of zinc oxide micro/nanowire arrays. The deoxidization of zinc oxide at the beginning of the epitaxial growth is the key step to obtain these porous single crystal micro/nanotubes. With large specific surface area, these tubes would be advantageous in potential photocatalysis, nanoelectronic, optoelectronic and biochemical-sensing applications.
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paper_title: Synthesis of GaN–carbon composite nanotubes and GaN nanorods by arc discharge in nitrogen atmosphere

paper_content:

A method using an arc discharge in a nitrogen atmosphere for synthesizing large quantities of gallium nitride (GaN)–Carbon composite nanotubes and GaN nanorods is reported. The reaction is achieved by a dc arc discharge between a graphite anode filled with a mixture of GaN, graphite, and nickel powders and a graphite cathode in a nitrogen atmosphere. The GaN are presented as rodlike fillings in the composite tubes and the isolated GaN nanorods have diameters in the range of 7–45 nanometers and a length of up to 40 μm. The outer graphitic shells of the composite carbon nanotubes have thicknesses ranging from 1 to 8 nm. It was found that the use of a nitrogen atmosphere plays a crucial role for the growth of the GaN nanorods fillings and the individual GaN nanorods.
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paper_title: One-dimensional gallium nitride micro/nanostructures synthesized by a space-confined growth technique

paper_content:

Hexagonal GaN prismatic sub-micro rods and cone nanowires have been synthesized in a large scale by a novel and controllable space-confined growth method. The as-synthesized GaN products are highly crystalline with a wurtzite structure. The prismatic rods have lengths of 15∼20 μm and diameters of 400∼500 nm enclosed by hexagonal smooth side surfaces and a pyramidal end. And the cone nanowires have average diameters of 150∼200 nm and lengths up to several tens of μm with a cone tip. The photoluminescence (PL) studies reveal prominent band-gap UV emission properties of GaN products and narrow FWHM, indicating the excellent luminescent performance and high crystal quality. For field emission characteristic of GaN nanowires, the turn-on field is about 9.5 V/μm and the current density reaches 1.0 mA/cm2 at an electric field of 18 V/ μm. The contrast experiments indicate a novel growth control can be achieved by using a graphite tube as reaction vessel. The sealed graphite tube combined with metallic initiator is greatly responsible for large-scale and uniform preparation of GaN prismatic rods and cone nanowires. Highly symmetric GaN hexagonal micropyramids are grown on a bare Si substrate. The growth mechanism and the control function of the graphite tube are demonstrated. These low-dimensional structures not only enrich semiconducting GaN family, but also are good building blocks for optoelectronic devices.
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paper_title: Growth of Semiconducting GaN Hollow Spheres and Nanotubes with Very Thin Shells via a Controllable Liquid Gallium-Gas Interface Chemical Reaction

paper_content:

An in situ liquid gallium–gas interface chemical reaction route has been developed to synthesize semiconducting hollow GaN nanospheres with very small shell size by carefully controlling the synthesis temperature and the ammonia reaction gas partial pressure. In this process the gallium droplet does not act as a catalyst but rather as a reactant and a template for the formation of hollow GaN structures. The diameter of the synthesized hollow GaN spheres is typically 20–25 nm and the shell thickness is 3.5–4.5 nm. The GaN nanotubes obtained at higher synthesis temperatures have a length of several hundreds of nanometers and a wall thickness of 3.5–5.0 nm. Both the hollow GaN spheres and nanotubes are polycrystalline and are composed of very fine GaN nanocrystalline particles with a diameter of 3.0–3.5 nm. The room-temperature photoluminescence (PL) spectra for the synthesized hollow GaN spheres and nanotubes, which have a narrow size distribution, display a sharp, blue-shifted band-edge emission peak at 3.52 eV (352 nm) due to quantum size effects.
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paper_title: Gallium nitride porous microtubules self-assembled from wurtzite nanorods

paper_content:

Abstract Wurtzite gallium nitride microscale tubules were synthesized through chemical vapor deposition method. The tubules were self-assembled from nanorods without any templates. The self-assemblies were up to 100 μm long and 10 μm in diameter. The tubules were porous. The unique structures of the tubules caused red-shifts of Raman modes and yellow band in photoluminescence spectra. A possible growth mechanism of the porous microtubules was proposed based on experimental observations.
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paper_title: Faceted and Vertically Aligned GaN Nanorod Arrays Fabricated without Catalysts or Lithography

paper_content:

Monocrystalline, vertically aligned and faceted GaN nanorods with controlled diameter have been synthesized by selective organometallic vapor phase epitaxy (OMVPE) onto GaN exposed at the bottom of pores in silicon dioxide templates patterned by reactive ion etching through self-organized porous anodic alumina films. This process is free of foreign catalysts, and the nanorod diameter control is achieved without the need for low-throughput nanolithographic techniques. The use of conventional OMVPE growth conditions allows for the straightforward adaptation of conventional doping and heterostructure growth as will be necessary for the fabrication of nanorod-based strain-relaxed electrically pumped lasers and light-emitting diodes.
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paper_title: Gallium nitride porous microtubules self-assembled from wurtzite nanorods

paper_content:

Abstract Wurtzite gallium nitride microscale tubules were synthesized through chemical vapor deposition method. The tubules were self-assembled from nanorods without any templates. The self-assemblies were up to 100 μm long and 10 μm in diameter. The tubules were porous. The unique structures of the tubules caused red-shifts of Raman modes and yellow band in photoluminescence spectra. A possible growth mechanism of the porous microtubules was proposed based on experimental observations.
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paper_title: Liesegang Rings Engineered from Charged Nanoparticles

paper_content:

Functionalized nanoparticles (NPs) serve as building blocks of self-organizing chemical patterns comprising periodic zones of nanoparticle precipitation. In contrast to ions, which underlie most pattern-forming chemical systems and whose properties cannot be readily modified, NPs allow for flexible adjustment of particle charges and/or material properties. In particular, changes in the particle charges control the precipitation behavior and ultimately the morphologies of the emerging patterns. The phenomenon of NP-based periodic precipitation is explained by reaction−diffusion modeling and can be used for the fractionation of NPs of different sizes.
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paper_title: Yellow and green luminescence in a freestanding GaN template

paper_content:

We have studied a broad photoluminescence band in high-mobility freestanding 200-μm-thick GaN template prepared by hydride vapor-phase epitaxy. Variable-excitation intensity and energy experiments showed two defect-related bands: a yellow luminescence (YL) band at about 2.15 eV and a green luminescence (GL) band at about 2.43 eV. In contrast to epitaxial GaN samples prepared by both vapor-phase and molecular-beam epitaxy, the YL in the sample studied is weak and can be easily saturated. However, the GL is dominant. We attribute the GL to isolated defects involving gallium vacancies and the YL to the same defect, but bound to dislocations, or possibly to structural surface defects.
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paper_title: Nanoporous gallium nitride square microtubes

paper_content:

Porous gallium nitride microtubes were self-fabricated from gallium nitride submicron irregular structures. The microtubes were of square cross-section. Electron diffractions indicated that the microtubes were composed of zincblende gallium nitride. Electron energy-loss spectrum and photoluminescence spectrum of the microtubes were collected and compared with that of single crystals.
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paper_title: Nanoparticle ring formation in evaporating micron-size droplets

paper_content:

Self-assembly process of CoPt3 particles into a ring pattern (ring diameter ranging from 0.6 to 1.5 μm, particle diameter 6 nm) results from phase separation in the thin film of a binary mixture, giving rise to a bilayer structure and subsequent decomposition of the top layer into droplets. Evaporation of the droplet leads to a shrinking of its contact line, and the particles located at the contact line follow its motion and self-assemble along the line.
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paper_title: Synthesis and structure of nanocrystal-assembled bulk GaN

paper_content:

A new condensed form of GaN, nanocrystal-assembled bulk (NAB) GaN, was obtained directly from reactions of metal Ga and NH4Cl in liquid ammonia at 350-500 degrees C. High-resolution transmission electron microscopy observations reveal that the NAB GaN consists of well-crystallized nanocrystals with wurtzite structure. The synchronous densificated NAB GaN is transparent to visible light while the constituted nanocrystals have an average size of about 12 nm. A possible synthesis mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.66.Jg; 81.20. -n.
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paper_title: Synthesis and structure of nanocrystal-assembled bulk GaN

paper_content:

A new condensed form of GaN, nanocrystal-assembled bulk (NAB) GaN, was obtained directly from reactions of metal Ga and NH4Cl in liquid ammonia at 350-500 degrees C. High-resolution transmission electron microscopy observations reveal that the NAB GaN consists of well-crystallized nanocrystals with wurtzite structure. The synchronous densificated NAB GaN is transparent to visible light while the constituted nanocrystals have an average size of about 12 nm. A possible synthesis mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.66.Jg; 81.20. -n.
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paper_title: Lattice thermal conductivity of group-IV and III–V semiconductor alloys

paper_content:

The room-temperature thermal conductivity of semiconductor alloys is analyzed using a simplified model of the alloy-disorder scattering. Good agreement is achieved between the present model and published experimental data on various group-IV and III−V semiconductor alloys. A complete set of alloy-disorder parameters are estimated, which makes it possible to calculate the lattice thermal conductivity for optional composition of III−V semiconductor alloys, including III−N alloys. An ordering effect is also examined for the explanation of some intermetallic and semiconductor compounds like CuAu and SiC.
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paper_title: Syntheses and structure of nanocrystalline gallium nitride obtained from ammonothermal method using lithium metal as mineralizator

paper_content:

A double layer CeO2/Nb2O5 thin film by magnetron sputtering was prepared. The property and microstructure of the thin film were carefully characterized by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and high resolution transmission electron microscopy, XPS measurement clearly demonstrated that the Nb2O5 layer underlying CeO2 as a electrical-conductive layer could greatly improve the reduction ability of CeO2 thin film compared with that of a single layer CeO2 thin film: Fitting to the XPS experimental data and deconvoluting the spectra showed the increase of Ce3+ concentration in the double layer CeO2/Nb2O5 thin film. The microstructure of the double layer thin film was composed of three distinguishable layers, which are a polycrystalline CeO2 layer, an amorphous Nb2O5 layer, and an intermediate amorphous layer with regard to the silicon substrate. The interface between CeO2 and Nb2O5 was rough and indistinct. A detailed analysis was given from the viewpoint of interdiffusion at the CeO2/Nb2O5 interface. (C) 2000 American vacuum Society. [S0734-2101(00)03406-0].
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paper_title: Gallium nitride porous microtubules self-assembled from wurtzite nanorods

paper_content:

Abstract Wurtzite gallium nitride microscale tubules were synthesized through chemical vapor deposition method. The tubules were self-assembled from nanorods without any templates. The self-assemblies were up to 100 μm long and 10 μm in diameter. The tubules were porous. The unique structures of the tubules caused red-shifts of Raman modes and yellow band in photoluminescence spectra. A possible growth mechanism of the porous microtubules was proposed based on experimental observations.
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paper_title: Synthesis and structure of nanocrystal-assembled bulk GaN

paper_content:

A new condensed form of GaN, nanocrystal-assembled bulk (NAB) GaN, was obtained directly from reactions of metal Ga and NH4Cl in liquid ammonia at 350-500 degrees C. High-resolution transmission electron microscopy observations reveal that the NAB GaN consists of well-crystallized nanocrystals with wurtzite structure. The synchronous densificated NAB GaN is transparent to visible light while the constituted nanocrystals have an average size of about 12 nm. A possible synthesis mechanism is discussed. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 78.66.Jg; 81.20. -n.
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paper_title: Synthesis of GaN–carbon composite nanotubes and GaN nanorods by arc discharge in nitrogen atmosphere

paper_content:

A method using an arc discharge in a nitrogen atmosphere for synthesizing large quantities of gallium nitride (GaN)–Carbon composite nanotubes and GaN nanorods is reported. The reaction is achieved by a dc arc discharge between a graphite anode filled with a mixture of GaN, graphite, and nickel powders and a graphite cathode in a nitrogen atmosphere. The GaN are presented as rodlike fillings in the composite tubes and the isolated GaN nanorods have diameters in the range of 7–45 nanometers and a length of up to 40 μm. The outer graphitic shells of the composite carbon nanotubes have thicknesses ranging from 1 to 8 nm. It was found that the use of a nitrogen atmosphere plays a crucial role for the growth of the GaN nanorods fillings and the individual GaN nanorods.
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paper_title: Formation of GaN nanorods by a sublimation method

paper_content:

Abstract GaN nanorods with diameters of 10–45 nm were formed through a simple sublimation method. They were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). SEM images showed that the nanorods were straight. XRD, SAED and HRTEM indicated that the nanorods were wurtzite GaN single crystals.
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paper_title: A GaN bulk crystal with improved structural quality grown by the ammonothermal method.

paper_content:

The realization of high-performance optoelectronic devices, based on GaN and other nitride semiconductors, requires the existence of a high-quality substrate. Non-polar or semipolar substrates have recently been proven to provide superior optical devices to those on conventional c-plane substrates1,2,3,4. Bulk GaN growth enables GaN substrates sliced along various favourable crystal orientations. Ammonothermal growth is an attractive method for bulk GaN growth owing to its potential to grow GaN ingots at low cost. Here we report on improvement in the structural quality of GaN grown by the ammonothermal method. The threading dislocation densities estimated by plan-view transmission electron microscopy observations were less than 1×106 cm−2 for the Ga face and 1×107 cm−2 for the N face. No dislocation generation at the interface was observed on the Ga face, although a few defects were generated at the interface on the N face. The improvement in the structural quality, together with the previous report on growth rate5 and scalability6, demonstrates the commercial feasibility of the ammonothermal GaN growth.
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paper_title: Single gallium nitride nanowire lasers

paper_content:

There is much current interest in the optical properties of semiconductor nanowires, because the cylindrical geometry and strong two-dimensional confinement of electrons, holes and photons make them particularly attractive as potential building blocks for nanoscale electronics and optoelectronic devices1,2, including lasers3,4and nonlinear optical frequency converters5. Gallium nitride (GaN) is a wide-bandgap semiconductor of much practical interest, because it is widely used in electrically pumped ultraviolet–blue light-emitting diodes, lasers and photodetectors6,7. Recent progress in microfabrication techniques has allowed stimulated emission to be observed from a variety of GaN microstructures and films8,9. Here we report the observation of ultraviolet–blue laser action in single monocrystalline GaN nanowires, using both near-field and far-field optical microscopy to characterize the waveguide mode structure and spectral properties of the radiation at room temperature. The optical microscope images reveal radiation patterns that correlate with axial Fabry–Perot modes (Q ≈ 103) observed in the laser spectrum, which result from the cylindrical cavity geometry of the monocrystalline nanowires. A redshift that is strongly dependent on pump power (45 meV μJ cm−2) supports the idea that the electron–hole plasma mechanism is primarily responsible for the gain at room temperature. This study is a considerable advance towards the realization of electron-injected, nanowire-based ultraviolet–blue coherent light sources.
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paper_title: Bulk GaN single crystals: growth conditions by flux method

paper_content:

Hexagonal GaN platelet crystals with a size of 1-4 mm have been grown by a Li-based flux method. The influence of growth conditions such as the molar ratio of starting materials, temperature, pressure, the position of Li3N in the crucible on the growth of GaN single crystals was studied. The quality of GaN single crystal was checked by optical microscope and X-ray rocking curve. (C) 2002 Elsevier Science B.V. All rights reserved.
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paper_title: Thermopower Study of GaN-Based Materials for Next-Generation Thermoelectric Devices and Applications

paper_content:

III-nitride InGaN-based solar cells have gained importance because their band gap can potentially cover most of the solar spectrum, spanning 0.7 eV to 3.4 eV. However, to use these materials to harvest additional energy, other properties such as their thermoelectric properties should be exploited. In this work, the Seebeck coefficient and the electrical conductivity of three InGaN alloys with various indium concentrations and Gd-doped GaN (GaN:Gd) were measured, and the power factor was calculated. We report a Seebeck value of ∼209 μV/K for Gd-doped GaN.
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paper_title: Wurtzite-type faceted single-crystalline GaN nanotubes

paper_content:

We report on the direct fabrication of single-crystalline wurtzite-type hexagonal GaN nanotubes via a newly designed, controllable, and reproducible chemical thermal-evaporation process. The nanotubes are single crystalline, have one end closed, an average outer diameter of ∼300nm, an inner diameter of ∼100nm, and a wall thickness of ∼100nm. The structure and morphology of the tubes are characterized using a scanning electron microscope and a transmission electron microscope. The cathodoluminescence of individual nanotubes is also investigated. The growth mechanism, formation kinetics, and crystallography of GaN nanotubes are finally discussed.
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paper_title: Single crystal growth of gallium nitride in supercritical ammonia

paper_content:

Gallium nitride single crystals with 2 µm diameters and lengths up to several millimeters were grown in supercritical ammonia. The colorless transparent hexagonal nitride crystals were grown at 600 °C for 4 days with a vertical temperature gradient of 150 °C. Several amide catalysts and temperature gradients were studied. The wurtzite structure (space group P63mc) of the crystals was confirmed by X-ray powder diffraction. The chemical mechanisms of the nitride growth and the transport in supercritical ammonia are discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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paper_title: Core/Multishell Nanowire Heterostructures as Multicolor, High-Efficiency Light-Emitting Diodes

paper_content:

We report the growth and characterization of core/multishell nanowire radial heterostructures, and their implementation as efficient and synthetically tunable multicolor nanophotonic sources. Core/multishell nanowires were prepared by metal-organic chemical vapor deposition with an n-GaN core and InxGa1-xN/GaN/p-AlGaN/p-GaN shells, where variation of indium mole fraction is used to tune emission wavelength. Cross-sectional transmission electron microscopy studies reveal that the core/multishell nanowires are dislocation-free single crystals with a triangular morphology. Energy-dispersive X-ray spectroscopy clearly shows shells with distinct chemical compositions, and quantitatively confirms that the thickness and composition of individual shells can be well controlled during synthesis. Electrical measurements show that the p-AlGaN/p-GaN shell structure yields reproducible hole conduction, and electroluminescence measurements demonstrate that in forward bias the core/multishell nanowires function as light-...
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paper_title: UV photovoltaic cells fabricated utilizing GaN nanorod/Si heterostructures

paper_content:

Abstract Well-aligned GaN nanorods were synthesized on Si substrates to form heterojunctions with potential applications in photovoltaic (PV) cells. Transmission electron microscopy images, selected area electron diffraction pattern images, and photoluminescence spectra showed that the GaN crystalline nanorods were preferentially oriented along the [0 0 0 1] direction and were very well-aligned perpendicular to the Si (1 1 1) substrate. An ultraviolet (UV) PV cell based on GaN nanorod/n-Si heterojunctions was fabricated, and current–voltage curves under an illumination with UV light exhibited obvious PV effect with a power conversion efficiency of about 1%.
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paper_title: Vertically Aligned p-Type Single-Crystalline GaN Nanorod Arrays on n-Type Si for Heterojunction Photovoltaic Cells

paper_content:

Vertically aligned Mg-doped GaN nanorods have been epitaxially grown on n-type Si substrate to form a heterostructure for fabricating p−n heterojunction photovoltaic cells. The p-type GaN nanorod/n-Si heterojunction cell shows a well-defined rectifying behavior with a rectification ratio larger than 104 in dark. The cell has a high short-circuit photocurrent density of 7.6 mA/cm2 and energy conversion efficiency of 2.73% under AM 1.5G illumination at 100 mW/cm2. Moreover, the nanorod array may be used as an antireflection coating for solar cell applications to effectively reduce light loss due to reflection. This study provides an experimental demonstration for integrating one-dimensional nanostructure arrays with the substrate to directly fabricate heterojunction photovoltaic cells.
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paper_title: Photocatalysis Using GaN Nanowires

paper_content:

The photocatalytic activity of GaN nanowires was investigated for the use of GaN nanowires as photocatalysts in harsh environments. GaN nanowires with diameters of 20–50 nm and lengths of 4–6 µm were prepared by Ni catalyst-assisted metal−organic chemical vapor deposition. Comparisons of GaN nanowires with GaN submicron dot arrays and thin films showed that GaN nanowires exhibit much better photocatalytic activity, resulting from a high surface-to-volume ratio. In addition, GaN nanowires exhibited good ability to photodegrade organic dye at various pHs, even under strong acidity and alkalinity. The photocatalytic activity of GaN nanowires was also compared with that of ZnO and TiO2 nanowires.
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paper_title: Epitaxial Growth of InGaN Nanowire Arrays for Light Emitting Diodes

paper_content:

Significant synthetic challenges remain for the epitaxial growth of high-quality InGaN across the entire compositional range. One strategy to address these challenges has been to use the nanowire geometry because of its strain relieving properties. Here, we demonstrate the heteroepitaxial growth of InxGa1–xN nanowire arrays (0.06 ≤ x ≤ 0.43) on c-plane sapphire (Al2O3(001)) using a halide chemical vapor deposition (HCVD) technique. Scanning electron microscopy and X-ray diffraction characterization confirmed the long-range order and epitaxy of vertically oriented nanowires. Structural characterization by transmission electron microscopy showed that single crystalline nanowires were grown in the ⟨002⟩ direction. Optical properties of InGaN nanowire arrays were investigated by absorption and photoluminescence measurements. These measurements show the tunable direct band gap properties of InGaN nanowires into the yellow-orange region of the visible spectrum. To demonstrate the utility of our HCVD method for ...
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paper_title: GaN Nanowire Arrays for High-Output Nanogenerators

paper_content:

Three-fold symmetrically distributed GaN nanowire (NW) arrays have been epitaxially grown on GaN/sapphire substrates. The GaN NW possesses a triangular cross section enclosed by (0001), (2112), and (2112) planes, and the angle between the GaN NW and the substrate surface is ∼62°. The GaN NW arrays produce negative output voltage pulses when scanned by a conductive atomic force microscope in contact mode. The average of piezoelectric output voltage was about −20 mV, while 5−10% of the NWs had piezoelectric output voltages exceeding −(0.15−0.35) V. The GaN NW arrays are highly stable and highly tolerate to moisture in the atmosphere. The GaN NW arrays demonstrate an outstanding potential to be utilized for piezoelectric energy generation with a performance probably better than that of ZnO NWs.
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paper_title: High efficiency photoelectrochemical water splitting and hydrogen generation using GaN nanowire photoelectrode

paper_content:

We have studied the photoelectrochemical properties of both undoped and Si-doped GaN nanowire arrays in 1 mol l 1 solutions of hydrogen bromide and potassium bromide, which were used separately as electrolytes. It is observed that variations of the photocurrent with bias voltage depend strongly on the n-type doping in GaN nanowires in both electrolytes, which are analyzed in the context of GaN surface band bending and its variation with the incorporation of Si-doping. Maximum incident-photon-to-current-conversion efficiencies of 15% and 18% are measured for undoped and Si-doped GaN nanowires under 350 nm light illumination, respectively. Stable hydrogen generation is also observed at a zero bias potential versus the counter-electrode. (Some figures may appear in colour only in the online journal)
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paper_title: Photoelectrochemical Properties of p-Type GaN in Comparison with n-Type GaN

paper_content:

We studied the photoelectrochemical properties of p-type GaN compared with those of n-type GaN. Band-edge potentials of p-type GaN were determined from the Mott–Schottky plot, and we clarified the potentials to be identical to those of n-type GaN. Photocurrent in p-type GaN shows its potential as an electrode for photoelectrolysis of water.
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paper_title: Improved photoelectrochemical water splitting efficiency of nanoporous GaN photoanode

paper_content:

Nanoporous (NP) GaN was used as a photoanode during photoelectrochemical water splitting and its efficiency and stability were compared with those of planar GaN. The NP GaN showed larger photocurrent density under illumination, which indicated more hydrogen generation. The enhancement was higher for a large applied voltage where the water splitting process was limited by charge transfer between semiconductor and electrolyte. Photocurrents decreased with time during the water splitting; however, the photocurrent degradation was less severe for NP GaN. It was attributed to efficient hole transport from photoanode to electrolyte in NP GaN due to its large surface area.
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paper_title: Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation

paper_content:

Hydrogen gas generation from a counterelectrode was clearly observed for the first time using light-illuminated n-type GaN as a working photoelectrode in an electrolyte. The application of extra bias to a working electrode was required to obtain a sufficient volume of generated gas. The reactions at the GaN photoelectrode were both GaN decomposition and water oxidization, simultaneously.
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paper_title: Photocatalysis Using GaN Nanowires

paper_content:

The photocatalytic activity of GaN nanowires was investigated for the use of GaN nanowires as photocatalysts in harsh environments. GaN nanowires with diameters of 20–50 nm and lengths of 4–6 µm were prepared by Ni catalyst-assisted metal−organic chemical vapor deposition. Comparisons of GaN nanowires with GaN submicron dot arrays and thin films showed that GaN nanowires exhibit much better photocatalytic activity, resulting from a high surface-to-volume ratio. In addition, GaN nanowires exhibited good ability to photodegrade organic dye at various pHs, even under strong acidity and alkalinity. The photocatalytic activity of GaN nanowires was also compared with that of ZnO and TiO2 nanowires.
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paper_title: Si/InGaN core/shell hierarchical nanowire arrays and their photoelectrochemical properties.

paper_content:

Three-dimensional hierarchical nanostructures were synthesized by the halide chemical vapor deposition of InGaN nanowires on Si wire arrays. Single phase InGaN nanowires grew vertically on the sidewalls of Si wires and acted as a high surface area photoanode for solar water splitting. Electrochemical measurements showed that the photocurrent density with hierarchical Si/InGaN nanowire arrays increased by 5 times compared to the photocurrent density with InGaN nanowire arrays grown on planar Si (1.23 V vs RHE). High-resolution transmission electron microscopy showed that InGaN nanowires are stable after 15 h of illumination. These measurements show that Si/InGaN hierarchical nanostructures are a viable high surface area electrode geometry for solar water splitting.
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paper_title: Photoelectrochemical reaction and H2 generation at zero bias optimized by carrier concentration of n-type GaN.

paper_content:

The authors studied the photoelectrochemical properties dependent on carrier concentration of n-type GaN. The photocurrent at zero bias became the maximum value at the carrier concentration of 1.7×1017cm−3. Using the sample optimized carrier concentration, the authors achieved H2 gas generation at a Pt counterelectrode without extra bias for the first time. The authors also discussed the mechanism of the dependence of photocurrent on the carrier concentration of GaN.
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paper_title: GaN Nanowire Arrays for High-Output Nanogenerators

paper_content:

Three-fold symmetrically distributed GaN nanowire (NW) arrays have been epitaxially grown on GaN/sapphire substrates. The GaN NW possesses a triangular cross section enclosed by (0001), (2112), and (2112) planes, and the angle between the GaN NW and the substrate surface is ∼62°. The GaN NW arrays produce negative output voltage pulses when scanned by a conductive atomic force microscope in contact mode. The average of piezoelectric output voltage was about −20 mV, while 5−10% of the NWs had piezoelectric output voltages exceeding −(0.15−0.35) V. The GaN NW arrays are highly stable and highly tolerate to moisture in the atmosphere. The GaN NW arrays demonstrate an outstanding potential to be utilized for piezoelectric energy generation with a performance probably better than that of ZnO NWs.
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paper_title: Transport properties, specific heat and thermal conductivity of GaN nanocrystalline ceramic

paper_content:

The structural and transport properties (resistivity, thermopower and Hall effect), specific heat and thermal conductivity have been measured for GaN nanocrystalline ceramic prepared by hot pressing. It was found that the temperature dependence of resistivity in temperature range 10-300 K shows the very low activation energy, which is ascribed to the shallow donor doping originating in amorphous phase of sample. The major charge carriers are electrons, what is indicated by negative sign of Hall constant and Seebeck coefficient. The thermopower attains large values (-58 {mu}V/K at 300 K) and was characterized by linear temperature dependence which suggests the diffusion as a major contribution to Seebeck effect. The high electron concentration of 1.3x10{sup 19} cm{sup -3} and high electronic specific heat coefficient determined to be 2.4 mJ/molK{sup 2} allow to conclude that GaN ceramic demonstrates the semimetallic-like behavior accompanied by very small mobility of electrons ({approx}0.1 cm{sup 2}/V s) which is responsible for its high resistivity. A low heat conductivity of GaN ceramics is associated with partial amorphous phase of GaN grains due to high pressure sintering. - Graphical Abstract: Thermal resistivity and thermopower measurements indicates the high phonon scattering and lack of phonon-drag contribution to thermopower in GaN nanoceramics pressedmore » under 4 GPa at 800 {sup o}C.« less
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paper_title: Shape evolution and self assembly of monodisperse PbTe nanocrystals.

paper_content:

In this communication, we report our recent achievement in synthesis and self-assembly of both spherical and cubic PbTe nanocrystals using a high-temperature solution-phase synthesis approach. The possible mechanism of nanocrystalline evolution from spherical to cubic structure has also been discussed. It is possible to use the highly orientated PbTe nanocrystals as building blocks to achieve thickness-controlled film for further manipulation into thermoelectric devices.
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Title: Free-Standing Self-Assemblies of Gallium Nitride Nanoparticles: A Review

Section 1: Introduction
Description 1: This section introduces the fundamental properties and applications of gallium nitride (GaN) based semiconductors and highlights their significance since the 1990s.

Section 2: Chemical Vapor Deposition
Description 2: This section discusses the synthesis of GaN nanoparticles using chemical vapor deposition (CVD) methods, including reaction mechanisms, particle characteristics, and challenges associated with the method.

Section 3: Nitridation
Description 3: This section covers the production of single-phase and high-purity GaN nanoparticles through nitridation of Ga-based materials, detailing various precursor materials and reaction conditions.

Section 4: Solvothermal Techniques
Description 4: This section describes various solvothermal methods for synthesizing GaN nanoparticles under supercritical conditions, emphasizing the role of different solvents and mineralizators.

Section 5: Ball-Milling Techniques
Description 5: This section explains the industrial production of GaN nanoparticles using ball-milling techniques, including mechanical grinding, mechanical alloying, and the characteristics of the nanoparticles produced.

Section 6: Other Techniques
Description 6: This section summarizes alternative methods for producing GaN nanoparticles, such as pulsed laser ablation, sol-gel methods, and synthesis from organometallic gallium azides.

Section 7: Physical Properties of Crystalline Nanoparticles
Description 7: This section examines the size-dependent physical properties of GaN nanoparticles, including band-gap energies, melting temperatures, dielectric behaviors, and radiative decay times.

Section 8: Defects of GaN Nanoparticles
Description 8: This section discusses various types of defects in GaN nanoparticles, their formation during different synthesis methods, and their effects on material properties.

Section 9: Defect Effects on Physical Properties
Description 9: This section elaborates on how vacancies, surface defects, and grain boundaries affect the optical, magnetic, and dielectric properties of GaN nanoparticles.

Section 10: Applications of Nanoparticles
Description 10: This section highlights the use of GaN nanoparticles in various applications, such as solar cells, photocatalysts for water splitting, and in optoelectronic devices.

Section 11: Free-standing Self-assembly of GaN Nanoparticles
Description 11: This section focuses on the self-assembly of GaN nanoparticles into 2D and 3D free-standing structures, including methods to achieve such assemblies and their structural characteristics.

Section 12: Optical Properties of GaN Self-assemblies
Description 12: This section covers the photoluminescence and Raman scattering properties of GaN self-assemblies, influenced by their nanoparticle constituents.

Section 13: Potential Applications of GaN Self-Assemblies
Description 13: This section explores the envisioned applications of free-standing GaN self-assemblies in renewable energy, photocatalytic water splitting, piezoelectric nanogenerators, and thermoelectric devices.

Section 14: Conclusions
Description 14: This section summarizes the synthesis methods, properties, and potential applications of GaN nanoparticles and their self-assemblies, emphasizing their high surface/volume ratios and tunable band-gaps.