Study on modulating the indium composition in InGaN quantum wells to improve the luminous efficiency of GaN LED

This study primarily used metal-organic chemical vapor deposition to grow gallium nitride (GaN) light-emitting diode (LED) structures with InGaN quantum wells (QWs). During the InGaN QW growing process, an identical concentration of trimethylindium gas was prepared and introduced at different times (Before(B), Middle(M), and After(A)) into the QW structures for an investigation of the variation in GaN LED luminous efficacy. Because of segregation resulting from the different concentrations of In content of the InGaN QWs during the process and because of the stress resulting from lattice mismatch between atoms, the interaction between segregation and stress forms quantum dots (QDs). Under processes with the appropriate parameters, the QDs can improve the luminous efficacy of GaN LEDs. Postprocess LEDs were measured for their electroluminescence, photoluminescence, cathodoluminescence, thermal stability, light output power, and external quantum efficiency. The QW structures were analyzed and observed using high-resolution transmission electron microscopy. The results revealed that the Before (B) LED had the greatest light output power at 46.6 mW, an increase of approximately 15.6%. Thermal annealing was then used to treat the LED at 850 °C, after which the photoluminescence intensity increased by 1.7 times.

     

Development of a backscattering type ultraviolet apertureless near-field scanning optical microscope

A backscattering type ultraviolet apertureless near-field scanning optical microscope (ANSOM) for the correlated measurement of topographical and optical characteristics of photonic materials with high optical resolution was developed. The near-field Rayleigh scattering image of GaN covered with periodic submicron Cr dots showed that optical resolution around 40 nm was achievable. By measuring the tip scattered photoluminescence of InGaN/GaN multi quantum wells, the applicability of the developed microscope for high resolution fluorescence measurement was also demonstrated.     

High energy Urbach characteristic observed for gallium nitride amorphous surface oxide

We have observed an “above band-gap” Urbach like characteristic for gallium nitride films (at the high energy side of the band-edge). A combination of X-ray diffraction, secondary ion mass spectroscopy and optical transmission measurements were taken for gallium nitride samples of different thickness. From this data we demonstrate that the high energy Urbach like characteristic is related to the presence of an amorphous surface oxide. It is shown to dominate the absorption spectra of thin gallium nitride samples, for which the influence of surface oxidation is strongest.     

Homogeneous dispersion of gallium nitride nanoparticles in a boron nitride matrix by nitridation with urea

A Gallium Nitride (GaN) dispersed boron nitride (BN) nanocomposite powder was synthesized by heating a mixture of gallium nitrate, boric acid, and urea in a hydrogen atmosphere. Before heat treatment, crystalline phases of urea, boric acid, and gallium nitrate were recognized, but an amorphous material was produced by heat treatment at 400 degrees C, and then was transformed into GaN and turbostratic BN (t-BN) by further heat treatment at 800 degrees C. TEM obsevations of this composite powder revealed that single nanosized GaN particles were homogeneously dispersed in a BN matrix. Homogeneous dispersion of GaN nanoparticles was thought to be attained by simultaneously nitriding gallium nitrate and boric acid to GaN and BN with urea.     

Metalorganic molecular beam epitaxy of GaN and Al(Ga)N on GaAs(001) studied using laser reflectometry and reflectance anisotropy spectroscopy

We report the growth of GaN and AlGaN films on GaAs (0 0 1) substrates in the temperature range 400–800 °C by metalorganic molecular beam epitaxy. An r.f. plasma nitrogen source was used in conjunction with triethylgallium and ethyl-dimethylamine-alane group III sources. Growth was initiated using either a low temperature AlN buffer layer or a graded arsenide-nitride buffer layer. The growth was monitored in real time using in-situ laser reflectometry. The temperature dependence of growth rates for the nitride layers are compared with their arsenide analogs. The relative growth rate of gallium nitride/gallium arsenide from triethylgallium was found to be in the range 54–60%, the Ga incorporation rates are closely comparable when the higher density of GaN is taken into account. The range of growth temperatures for gallium nitride extends to higher temperatures compared with gallium arsenide probably due to lower evaporation rates of Ga bound to the nitride surface. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs (0 0 1)-c (4 × 4) As-stabilized surface at temperatures as low as 400 °C but without the gross faceting that has been observed at higher temperatures.     

Homoepitaxial n-core: p-shell gallium nitride nanowires: HVPE overgrowth on MBE nanowires

We present the homoepitaxial growth of p-type, magnesium doped gallium nitride shells by use of halide vapor phase epitaxy (HVPE) on n-type gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy (MBE). Scanning electron microscopy shows clear dopant contrast between the core and shell of the nanowire. The growth of magnesium doped nanowire shells shows little or no effect on the lattice parameters of the underlying nanowires, as measured by x-ray diffraction (XRD). Photoluminescence measurements of the nanowires show the appearance of sub-bandgap features in the blue and the ultraviolet, indicating the presence of acceptors. Finally, electrical measurements confirm the presence of electrically active holes in the nanowires.     

DRIFTS characterization of a nanostructured gallium nitride powder and its interactions with organic molecules

Nanoparticles of gallium nitride, which are of great interest in optical technology, were synthesized and characterized. The surface chemical composition of these nanoparticles, which can affect the overall properties of the material, was analyzed by diffuse reflectance Fourier transform infrared spectrometry. The interaction of the GaN first atomic layer with acetic acid was investigated as a preliminary step for the deagglomeration study of the nanoparticles in polymer matrixes.     

Steady-state and transient electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide: an updated and critical review

The wide energy gap compound semiconductors, gallium nitride and zinc oxide, are widely recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these wide energy gap compound semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyzes of the electron transport within the wide energy gap compound semiconductors, gallium nitride and zinc oxide. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, zinc-blende gallium nitride, and wurtzite zinc oxide. The electron transport that occurs within bulk zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap compound semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials. The wurtzite gallium nitride and zinc-blende gallium arsenide results, being presented in a previous review article of ours (O’Leary et al. in J Mater Sci Mater Electron 17:87, 2006), are also presented herein for the sake of completeness.     

Steady-State and Transient Electron Transport Within the III–V Nitride Semiconductors, GaN, AlN, and InN: A Review

The III–V nitride semiconductors, gallium nitride, aluminum nitride, and indium nitride, have, for some time now, been recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these III–V nitride semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyses of the electron transport within the III–V nitride semiconductors, gallium nitride, aluminum nitride, and indium nitride. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, aluminum nitride, and indium nitride, for this analysis. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art III–V nitride semiconductor orthodoxy. A brief tutorial on the Monte Carlo approach will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials.     

Gallium nitride formed by vapour deposition and by conversion from gallium arsenide

Attempts to prepare single crystal gallium nitride in thin films and bulk form are reported. The thin films were prepared by reacting GaCl₃ and NH₃ and depositing on to single crystal silicon carbide substrates. The bulk gallium nitride was prepared by the conversion of single crystals of gallium arsenide using an intermediate oxide phase. The structural perfection of the gallium nitride material thus formed has been assessed using X-ray diffraction and electron diffraction techniques. Both methods of preparation produced single phase gallium nitride exhibiting a high degree of structural disorder.     

Nanoporous gallium nitride square microtubes

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.     

Epitaxy of semipolar GaN on a Si(001) substrate with a SiC buffer layer

A new method of synthesis of semipolar gallium nitride on a silicon substrate using the technology of solid-phase epitaxy of 3C-SiC nanocrystals has been suggested. It has been demonstrated that application of buffer layers of 3C-SiC and AlN enables one to form epitaxial layers of semipolar gallium nitride with layer deviation from the polar position of the c axis of a wurtzite crystal by an angle of 48°–51° at the minimal half-width of the X-ray diffraction rocking curve (ω_θ) ～ 24′. The observed bend of a cylindrical character in the structure of GaN/AlN/3C-SiC(001) is explained by the anisotropic deformation of semipolar GaN on silicon.     

Effect of Gallium Nitride Film Growth Conditions on Surface Segregation

The micro-and nanomorphology and local composition of gallium nitride (GaN) films produced by four different procedures have been studied with the aim of detecting autosegregation phenomena. As a result, a complex autosegregation picture has been demonstrated and discussed. Independent of the growth procedure, all of the gallium nitride films have nonstoichiometric chemical compositions (gallium deficiency), with a degree of nonstoichiometry ranging from ~0.30 to <0.10. We discuss the segregation mechanism, which presumably involves predominant selective diffusion of nitrogen atoms to the surface.     

Vapor phase epitaxy of aluminum nitride from trimethylaluminum and molecular nitrogen

We have studied the pyrolysis of trimethylaluminum (TMA) in a nitrogen-containing atmosphere of a vapor phase epitaxy reactor. It is established that, in the presence of gallium nitride coatings in the reactor, the main product of TMA pyrolysis in a nitrogen-hydrogen atmosphere is aluminum nitride. Using this process (without introducing ammonia), we obtained perfect epitaxial aluminum nitride layers.     

Synthesis of nanocrystalline GaN from Ga2O3 nanoparticles derived from salt-assisted spray pyrolysis

Gallium nitride (GaN) nanoparticles were successfully produced from nano-sized gallium oxide (Ga₂O₃) particles under a flow of ammonia gas. The gallium oxide nanoparticles were prepared by salt-assisted spray pyrolysis (SASP). Highly crystalline Ga₂O₃ nanoparticles with an average diameter of approximately 10 nm were obtained at various temperatures when a flux salt (LiCl, 5 mol/l) was added to the precursor solution. The effects of the crystallinity of the Ga₂O₃ particles and nitridation time on transformation to GaN were characterized using X-ray diffraction and scanning/transmission electron microscopy. Highly crystalline GaN nanoparticles with a mean size of 23.4 nm and a geometric standard deviation of 1.68 nm were obtained when Ga₂O₃ nanoparticles with relatively low crystallinity were used as the starting material. The resulting GaN nanoparticles showed a photoluminescence peak at 364 nm under UV excitation at 254 nm.     

Substrates for wide bandgap nitrides

Different substrates for gallium nitride growth are discussed. The commercially relevant substrates, silicon carbide and sapphire, and the two most promising alternatives, silicon and gallium nitride, are compared in terms of suitability for epitaxial processes and in their effects on devices. An estimation on future market success is given.     

Biocompatible nano-gallium/hydroxyapatite nanocomposite with antimicrobial activity

Intensive research in the area of medical nanotechnology, especially to cope with the bacterial resistance against conventional antibiotics, has shown strong antimicrobial action of metallic and metal-oxide nanomaterials towards a wide variety of bacteria. However, the important remaining problem is that nanomaterials with highest antibacterial activity generally express also a high level of cytotoxicity for mammalian cells. Here we present gallium nanoparticles as a new solution to this problem. We developed a nanocomposite from bioactive hydroxyapatite nanorods (84?wt %) and antibacterial nanospheres of elemental gallium (16?wt %) with mode diameter of 22?±?11?nm. In direct comparison, such nanocomposite with gallium nanoparticles exhibited better antibacterial properties against Pseudomonas aeruginosa and lower in-vitro cytotoxicity for human lung fibroblasts IMR-90 and mouse fibroblasts L929 (efficient antibacterial action and low toxicity from 0.1 to 1?g/L) than the nanocomposite of hydroxyapatite and silver nanoparticles (efficient antibacterial action and low toxicity from 0.2 to 0.25?g/L). This is the first report of a biomaterial composite with gallium nanoparticles. The observed strong antibacterial properties and low cytotoxicity make the investigated material promising for the prevention of implantation–induced infections that are frequently caused by P. aeruginosa.     

Lattice resolved annular dark-field scanning transmission electron microscopy of (Al, In)GaN/GaN layers for measuring segregation with sub-monolayer precision

We have performed lattice resolved annular dark-field (Z-contrast) scanning transmission electron microscopy and combined this with energy-dispersive X-ray spectroscopy as well as simulations to measure quantitatively segregation across strained interfaces in AlGaN/GaN and GaN/InGaN multiple quantum wells of nominal thicknesses between 8 and 0.25 nm. The compositional profiles obtained were corrected for detector dark current and non-linearity of the Z-contrast imaging process before we fitted exponential functions to the profiles across the interface regions. From these, we could highly accurately determine the layer widths, interface widths, and segregation lengths. Experimental values of the segregation lengths calculated varied from 0.3 nm (for InGaN-on-GaN) to 1.4 nm (for AlGaN-on-GaN), with error bars of only ±0.05 nm. A comparison with simulations based on a simple two-state-exchange model for surface segregation shows that the segregation energy for indium atoms is about an order of magnitude smaller than both the corresponding segregation energy for aluminium/gallium atoms and the activation energies for surface segregation of cations in these nitride systems.     

Growth of axial SiGe heterostructures in nanowires using pulsed laser deposition

Axial heterojunctions between pure silicon and pure germanium in nanowires have been realized combining pulsed laser deposition, chemical vapor deposition and electron beam evaporation in a vapor-liquid-solid nanowire growth experiment using gold nanoparticles as catalyst for the 1D wire growth. Energy dispersive x-ray mappings and line scans show a compositional transition from pure silicon to pure germanium and vice versa with exponential and thus comparably sharp transition slopes. Based on these results not only Si-Ge heterojunctions seem to be possible using the vapor-liquid-solid growth process but also heterojunctions in optoelectronic III-V compounds such as InGaAs/GaAs or group III nitride compounds such as InGaN/GaN as well as axial p-n junctions in Si nanowires.     

Chemical Vapor Deposition of GaN from Gallium and Ammonium Chloride

Thick polycrystalline gallium nitride films were grown by a two-step chemical vapor deposition process using gallium metal and ammonium chloride as starting reagents, at deposition rates of up to 50 m/h. The deposits were examined by scanning electron microscopy and photoluminescence (PL) spectroscopy. The results demonstrate that the proposed process can be used to grow high-quality, stoichiometric gallium nitride layers with a perfect crystal structure, as evidenced by the high intensity of the 380-nm exciton peak in the room-temperature PL spectra of the films and also by electron-microscopic examination.