Distribution by Scientific Domains

Kinds of GaN

  • hexagonal gan
  • hvpe gan
  • n-type gan
  • p-type gan
  • plane gan
  • semipolar gan

  • Terms modified by GaN

  • gan buffer
  • gan buffer layer
  • gan crystal
  • gan epilayer
  • gan epitaxial layer
  • gan film
  • gan layer
  • gan nanorod
  • gan nanowire
  • gan quantum dot
  • gan substrate
  • gan template
  • gan templates
  • gan thin film

  • Selected Abstracts

    Giant Axonal Neuropathy Locus Refinement To A < 590 KB Critical Interval

    L Cavalier
    Giant axonal neuropathy (GAN) is a rare autosomal recessive neurodegenerative disorder, characterised clinically by the development of chronic distal polyneuropathy during childhood, mental retardation, kinky or curly hair, skeletal abnormalities and, ultrastructurally, by axons in the central and peripheral nervous systems distended by masses of tightly woven neurofilaments. We recently localised the CAN locus in 16q24.1 to a 5-cM interval between the D16S507 and D16S511 markers by homozygosity mapping in three consanguineous Tunisian families. We have now established a contig-based physical map of the region comprising YACs and BACs where we have placed four genes, ten ESTs, three STSs and two additional microsatellite markers, and where we have identified six new SSCP polymorphisms and six new microsatellite markers. Using these markers, we have refined the position of our previous flanking recombinants. We also identified a shared haplotype between two Tunisian families and a small region of homozygosity in a Turkish family with distant consanguinity, both suggesting the occurrence of historic recombinations and supporting the conclusions based on the phase-known recombinations. Taken together, these results allow us to establish a transcription map of the region, and to narrow down the GAN position to a < 590 kb critical interval, an important step toward the identification of the defective gene. [source]

    Long-term great auricular nerve morbidity after sacrifice during parotidectomy,

    THE LARYNGOSCOPE, Issue 6 2009
    William R. Ryan MD
    Abstract Objectives/Hypothesis: To clarify the extent and patient perspectives of great auricular nerve (GAN) morbidity and recovery after nerve sacrifice during parotidectomy 4 to 5 years after surgery. Study Design: Prospective series. Methods: Twenty-two patients who underwent parotidectomy with GAN sacrifice and were previously studied for GAN sensory outcome during the first postoperative year. We performed light touch sensation tests on each patient to develop an ink map representing anesthesia and paresthesia in the GAN sensory territory; patients also completed an outcomes questionnaire. Results: Nineteen (86%) of 22 patients completed follow-up. One patient completed the questionnaire over the phone. The prevalence and average areas of anesthesia and paresthesia decreased since the first postoperative year according to sensory testing and patient scoring. At 4 to 5 years, 47% (9 of 19) of the patients had anesthesia, 58% (11 of 19) had paresthesia, and 26% (5 of 19) had neither anesthesia nor paresthesia during sensory testing. Patients reported that the GAN dysfunction brought them no to mild inference with their daily activities. At a mean point of 2 years, 70% (14 of 20) patients felt that their sensory symptoms had either completely abated or stabilized. Conclusions: The posterior branch of the GAN should be preserved if it does not compromise tumor resection. If this is not possible, the patient and surgeon should be comforted in that only minor, if any, long-term disability will ensue. Laryngoscope, 2009 [source]


    ANZ JOURNAL OF SURGERY, Issue 5 2007
    Dacita T. K. Suen
    Background: The great auricular nerve (GAN) is frequently sacrificed during parotidectomy and causes sensory disturbance of the auricle. Our study is to investigate whether GAN preservation can improve the sensory recovery. Methods: Patients undergoing superficial or total conservative parotidectomy for benign tumours were recruited consecutively from November 1998 to September 2001. Different sensory methods (light touch, two-point discrimination and sharp pain) of the auricle were evaluated by a designated physiotherapist preoperatively as well as at 1, 3, 6 and 12 months postoperatively. The patients and the physiotherapist were blinded to the integrity of the GAN. Long-term subjective assessment was also carried out beyond 2 years postoperatively. Results: A total of 21 patients were recruited for the study. GAN were preserved in 10 patients. The mean follow up was 16 months (12,42 months). There was no difference in sex distribution, type of operation and pathology of parotid tumour between the two groups. No postoperative mortality occurred and postoperative morbidity did not differ between the two groups. Patients with GAN preserved had significantly better light touch and sharp pain recovery at 1 year postoperatively. Subjective assessment of sensory dysfunction also favoured GAN preservation. Conclusion: Great auricular nerve preservation minimizes the postoperative sensory disturbance and should be considered whenever tumour clearance is not compromised. [source]

    Eleven new microsatellites for hop (Humulus lupulus L.)

    J. Jak
    Abstract We present a new set of 11 polymorphic microsatellite primer sequences for use with Humulus lupulus. Microsatellite-enriched libraries for GAn and GTn types of repeats were produced. Sequencing of 72 clones revealed 42 unique inserts containing microsatellites, out of which 19 primer pairs were designed and microsatellite amplification was tested on 39 wild hops and cultivars. Eleven primer pairs showed single locus amplification with 2,13 alleles, average 7.2, of which 17 unique alleles were discovered. One primer pair amplified too strong stutter bands, one locus was monomorphic and multilocus amplification was obtained with the remaining six primer pairs. [source]

    Spatially resolved X-ray diffraction as a tool for strain analysis in laterally modulated epitaxial structures

    A. Wierzbicka
    Abstract Spatially resolved X-ray diffraction (SRXRD) is applied for micro-imaging of strain in laterally modulated epitaxial structures. In GaAs layers grown by liquid phase epitaxial lateral overgrowth (ELO) on SiO2 -masked GaAs substrates a downward tilt of ELO wings caused by their interaction with the mask is observed. The distribution of the tilt magnitude across the wings width is determined with ,m-scale spatial resolution. This allows measuring of the shape of the lattice planes in individual ELO stripes. If a large area of the sample is studied the X-ray imaging provides precise information on the tilt of an individual wing and its distribution. In heteroepitaxial GaSb/GaAs ELO layers local mosaicity in the wing area is found. By the SRXRD the size of microblocks and their relative misorientation were analyzed. Finally, the SRXRD technique was applied to study distribution of localized strain in AlGaN epilayers grown by MOVPE on bulk GaN substrates with AlN mask. X-ray mapping proves that by mask patterning strain in AlGaN layer can be easily engineered, which opens a way to produce thicker, crack-free AlGaN layers with a higher Al content needed in GaN-based laser diodes. All these examples show that high spatial and angular resolutions offered by SRXRD makes the technique a powerful tool to study local lattice distortions in semiconductor microstructures. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Bulk growth of gallium nitride: challenges and difficulties

    M. Bockowski
    Abstract The present status of the GaN bulk growth by High Pressure Solution (HPS) method and combination of HPS and Hydride Vapor Phase Epitaxy (HVPE) methods is reviewed. Up to now the spontaneous high pressure solution growth of GaN results in crystals having habit of hexagonal platelets of surface area of 3 cm2 or needles with length up to 1 cm. Recently, the platelets and needles have been used as seeds for the HVPE growth. On the other hand, the LPE technique under pressure with pressure-grown GaN (hp-GaN), GaN/sapphire template, patterned GaN/sapphire template and free standing HVPE GaN as seeds has been examined and developed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Preparation of GaN crystals by heating a Li3N-added Ga melt in Na vapor and their photoluminescence

    Takahiro Yamada
    Abstract GaN crystals were prepared by heating a Ga melt with 1 at% Li3N against Ga at 750 °C in Na vapor under N2 pressures of 0.4,1.0 MPa. The GaN crystals grown at 1.0 MPa of N2 were colorless and transparent prismatic, having a size of approximately 0.7 mm in length. A secondary ion mass spectrometry (SIMS) showed the contaminant of lithium in the obtained crystals. A large broad yellow band emission peak of 2.28 eV was observed at room temperature in the photoluminescence spectrum in addition to the near band emission peak of GaN at 3.39 eV and a small broad satellite emission at 3.24 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    The impact of SiC substrate treatment on the heteroepitaxial growth of GaN by plasma assisted MBE

    A. S. Brown
    Abstract We report on the impact of the preparation of the Si-face 4H-SiC(0001)Si substrate using a Ga flash-off process on the epitaxial growth of GaN by plasma-assisted molecular beam epitaxy. The nucleation, as well as the resultant structural and morphological properties of GaN grown directly on 4H-SiC(0001)Si are strongly influenced by the chemical and morphological modifications of the SiC surface induced by the Ga flash-off process. Herein we describe the impact of the specific concentration of Ga incident on the surface (quantified in terms of monolayer (ML) coverage): of 0.5 ML, 1ML and 2ML. The residual oxygen at the SiC surface, unintentional SiC nitridation and the formation of cubic GaN grains during the initial nucleation stage, are all reduced when a 2 ML Ga flash is used. All of the above factors result in structural improvement of the GaN epitaxial layers. The correlation between the SiC surface modification, the initial nucleation stage, and the GaN epitaxial layer structural quality has been articulated using x-ray photoelectron spectroscopy, x-ray diffraction, atomic force microscopy and spectroscopic ellipsometry data. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Heat-Transport Mechanisms in Superlattices

    Yee Kan Koh
    Abstract The heat transport mechanisms in superlattices are identified from the cross-plane thermal conductivity , of (AlN)x,(GaN)y superlattices measured by time-domain thermoreflectance. For (AlN)4.1 nm,(GaN)55,nm superlattices grown under different conditions, , varies by a factor of two; this is attributed to differences in the roughness of the AlN/GaN interfaces. Under the growth condition that gives the lowest ,, , of (AlN)4 nm,(GaN)y superlattices decreases monotonically as y decreases, ,,=,6.35,W m,1 K,1 at y,=,2.2,nm, 35 times smaller than , of bulk GaN. For long-period superlattices (y,>,40,nm), the mean thermal conductance G of AlN/GaN interfaces is independent of y, G,,,620 MW m,2 K,1. For y,<,40,nm, the apparent value of G increases with decreasing y, reaching G,,,2 GW m,2 K,1 at y,<,3,nm. MeV ion bombardment is used to help determine which phonons are responsible for heat transport in short period superlattices. The thermal conductivity of an (AlN)4.1 nm,(GaN)4.9,nm superlattice irradiated by 2.3 MeV Ar ions to a dose of 2,×,1014 ions cm,2 is reduced by <35%, suggesting that heat transport in these short-period superlattices is dominated by long-wavelength acoustic phonons. Calculations using a Debye-Callaway model and the assumption of a boundary scattering rate that varies with phonon-wavelength successfully capture the temperature, period, and ion-dose dependence of ,. [source]

    Single-Crystalline Gallium Nitride Microspindles: Synthesis, Characterization, and Thermal Stability ,

    F. Xu
    Abstract This paper describes a facile procedure for synthesizing high-quality gallium nitride microspindles on a large scale using a solid-state reaction of GaI3, NaNH2, and NH4Cl in a sealed system at 500,°C for 6,h. The structures, compositions, and morphologies of the as-synthesized products are derived from X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FE-SEM). The selected-area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) images show that the as-synthesized GaN spindles are composed of many single-crystalline platelets. The GaN microspindles show different optical properties depending on their shape (e.g., nanowires or nanoparticles) in photoluminescence (PL) emission spectroscopy and Raman spectroscopy. The possible growth mechanism of GaN microspindles is controlled by linear kinetics with a driving force proportional to the difference between a local supersaturation and an equilibrium chemical potential. Furthermore, the thermal stability of the GaN microspindles is investigated under various annealing conditions and discussed on the basis of additional TEM and XRD analyses. [source]

    Increased Color-Conversion Efficiency in Hybrid Light-Emitting Diodes utilizing Non-Radiative Energy Transfer

    ADVANCED MATERIALS, Issue 5 2010
    Soontorn Chanyawadee
    An efficient hybrid color-conversion light-emitting device consisting of colloidal nanocrystal quantum dots (NQDs) and a surface-patterned GaN-based LED is demonstrated (see figure). Excitation in a surface-patterned LED is efficiently transferred to NQD emitters via non-radiative energy transfer. A twofold enhancement of the NQD emission is achieved. [source]

    Nanometer-Thick Single-Crystal Hexagonal Gd2O3 on GaN for Advanced Complementary Metal-Oxide-Semiconductor Technology

    ADVANCED MATERIALS, Issue 48 2009
    Wen Hsin Chang
    Hexagonal-phase single-crystal Gd2O3 is deposited on GaN in a molecular beam epitaxy system (see image). The dielectric constant is about twice that of its cubic counterpart when deposited on InGaAs or Si. The capacitive effective thickness of 0.5,nm in hexagonal Gd2O3 is perhaps the lowest on GaN-metal-oxide-semiconductor devices. The heterostructure is thermo dynamically stable at high temperatures and exhibits low interfacial densities of states after high-temperature annealing. [source]

    Recent Progress in GaN-Based Light-Emitting Diodes

    ADVANCED MATERIALS, Issue 45 2009
    Haiqiang Jia
    Abstract In the last few years the GaN-based white light-emitting diode (LED) has been remarkable as a commercially available solid-state light source. To increase the luminescence power, we studied GaN LED epitaxial materials. First, a special maskless V-grooved c -plane sapphire was fabricated, a GaN lateral epitaxial overgrowth method on this substrate was developed, and consequently GaN films are obtained with low dislocation densities and an increased light-emitting efficiency (because of the enhanced reflection from the V-grooved plane). Furthermore, anomalous tunneling-assisted carrier transfer in an asymmetrically coupled InGaN/GaN quantum well structure was studied. A new quantum well structure using this effect is designed to enhance the luminescent efficiency of the LED to ,72%. Finally, a single-chip phosphor-free white LED is fabricated, a stable white light is emitted for currents from 20 to 60,mA, which makes the LED chip suitable for lighting applications. [source]

    Nanowire-Templated Epitaxial Growth: Nanowire-Templated Lateral Epitaxial Growth of Low-Dislocation Density Nonpolar a -Plane GaN on r -Plane Sapphire (Adv. Mater.

    ADVANCED MATERIALS, Issue 23 2009
    George Wang and co-workers report on p. 2416 that low dislocation density a -plane GaN films can be grown by the coalescence of vertically-aligned, single-crystalline GaN nanowires on lattice-mismatched r -plane sapphire. In this technique, shown by the artists' rendering on the inside cover, the nanowires facilitate dramatic strain relaxation in the suspended GaN film, leading to a large reduction in defects. [source]

    Nanowire-Templated Lateral Epitaxial Growth of Low-Dislocation Density Nonpolar a -Plane GaN on r -Plane Sapphire

    ADVANCED MATERIALS, Issue 23 2009
    Qiming Li
    Coalescence of a vertically aligned GaN nanowire array on r -plane sapphire, a technique called nanowire-templated lateral epitaxial growth, is used to grow low-dislocation density a -plane GaN. The resulting film is connected to the lattice-mismatched substrate by nanowires, which facilitates dramatic strain relaxation and leads to a significant reduction in defects. [source]

    Direct Observation of Inversion Domain Boundaries of GaN on c -Sapphire at Sub-ĺngstrom Resolution,

    ADVANCED MATERIALS, Issue 11 2008
    Fude Liu
    Inversion domain boundaries (IDBs) of GaN are studied by a high-resolution technique. The IDB separates adjacent domains of opposite polarity. The image shows a GaN IDB in the [bar;2110] projection. The theoretical IDB structure fits the experimentally obtained structure well. The inset is an image acquired from a very thin region on the right side of the IDB. It can indicate the polarity of GaN directly. [source]

    Nonquasi-static large-signal model of GaN FETs through an equivalent voltage approach

    A. Santarelli
    Abstract A new empirical nonlinear model of GaN-based electron devices is presented in the article. The model takes into account low-frequency dispersion due to self-heating and charge-trapping phenomena and provides accurate predictions at frequencies where nonquasi-static effects are important. The model is based on the application of a recently proposed equivalent-voltage approach and is identified by using pulsed measurements of drain current characteristics and pulsed S-parameter sets. Full experimental validation on a GaN on SiC PHEMT is provided at both small- and large-signal operating conditions. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2008. [source]

    Piezoelectric field around threading dislocation in GaN determined on the basis of high-resolution transmission electron microscopy image

    Summary A new method of determining the piezoelectric field around dislocations from high-resolution transmission electron microscopy images is presented. In order to determine the electrical potential distribution near a dislocation core, we used the distortion field, obtained using the geometrical phase method and the non-linear finite element method. The electrical field distribution was determined taking into account the inhomogeneous strain distribution, finite geometry of the sample and the full couplings between elastic and electrical fields. The results of the calculation for a transmission electron microscopy thin sample are presented. [source]

    Isotope and disorder effects in the Raman spectra of LiHxD1,x crystals

    V. G. Plekhanov
    Most of the physical properties of a solid depend on its isotopic composition in some way or another. Scientific interest, technological promise and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled crystals. A great number of stable isotopes and well-developed methods for their separation have made it possible to grow crystals of C, LiH, ZnO, ZnSe, CuCl, GaN, GaAs, CdS, Cu2O, Si, Ge and ,-Sn with a controllable isotopic composition. Among these compounds, LiH possesses the largest value of the isotope effect. The great number of theoretical and experimental data suggest that the isotopic composition of a crystal lattice exerts some influence on the vibrational properties of crystals. These effects are fairly large and can be readily measured by modern experimental techniques (ultrasound, Brillouin and Raman scattering and neutron scattering). In addition, crystals of different isotopic compositions possess different Debye temperatures. This difference between an LiH crystal and its deuterated analogue exceeds a 100 K. Very pronounced and general effects of isotopic substitution are observed in phonon spectra. The scattering lines in isotopically mixed crystals are not only shifted (the shift of LO lines exceeds 100 cm,1) but are also broadened. This broadening is related to the isotopic disorder of the crystal lattice. It is shown that the degree of a change in the scattering potential is different for different isotopic mixed crystals. In the case of germanium and diamond crystals, phonon scattering is weak, which allows one to apply successfully the coherent potential approximation (CPA) for describing the shift and broadening of scattering lines. In the case of lithium hydride, the change in the scattering potential is so strong that it results in phonon localization, which is directly observed in experiments. The common nature of the isotopic and disorder effects in a wide range of crystals is emphasized. Copyright © 2001 John Wiley & Sons, Ltd. [source]

    Verification of a distortion in the microstructure of GaN detected by EXAFS using ab initio density functional theory calculations

    Nicholas Dimakis
    X-ray absorption fine structure (XAFS) measurements on a series of epitaxially grown GaN samples have shown a distortion in the microstructure of GaN. More specifically the central N atom is 4-fold coordinated but the four Ga atoms are not equidistant. It has been shown that 2.9 to 3.5 of them (depending on the growth conditions) are found in the expected from XRD distance of 1.94 Ĺ and the remaining are at a distance longer by approximately 15%. Second derivative calculation of the conformation energy using the Density Functional Theory (DFT) is used to investigate if the symmetric GaN cluster as given by XRD is the most energetically favorable configuration and if not which distorted structure corresponds to the most energetically favorable one. A very good agreement between DFT results and experimental XAFS spectra has been found. Generalization this technique to other dislocated clusters is also discussed. [source]

    Preparation and Cathodoluminescence of Mg-Doped and Zn-Doped GaN Powders

    Hui-Li Li
    In this paper, undoped, Mg- and Zn-doped gallium nitride powders were prepared by direct nitridation of Ga2O3 under a flowing NH3 gas. The phase purity, morphology and cathodoluminescence spectra were presented. The Ga2O3 powders can be completely nitridized to GaN at 1000°C. The resultant GaN powders agglomerated together with submicron-sized polyhedral crystals. At room temperature, the Mg- and Zn-doped powders exhibit bright blue-violet emission at around 3.05 and 2.81 eV, respectively. This provides clear evidence that magnesium or zinc is incorporated into the GaN powders as an acceptor and suggests that the luminescent materials are promising candidates for optoelectronic applications. [source]

    Atomic scale defect analysis in the scanning transmission electron microscope,

    Ilke Arslan
    Abstract Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope provide the ability to investigate the structure,composition,property relationship at individual defects on the atomic scale. In this article, the main principles behind the techniques will be described. The application of these methods to the analysis of individual dislocations in GaN will also be discussed. In this case, the atomic scale methods indicate that many of the structural and electronic properties of dislocations are modified by the presence of impurities, such as oxygen. Microsc. Res. Tech 69:330,342, 2006. Published 2006 Wiley-Liss, Inc. [source]

    Use of confocal and multiphoton microscopy for the evaluation of micro-optical components and emitters

    J.M. Girkin
    Abstract We report on the application of confocal and multiphoton microscopic techniques for the evaluation of the latest generation of micro optical components. The optical emitting characteristics of arrays of matrix addressable GaN micrometer-sized light emitting diodes (micro-LEDs) have been measured using a commercial confocal microscope utilising the LEDs' own emission along with reflection confocal microscopy to determine the surface structure. Multiphoton induced luminescence from the 10,20-micron diameter emitters has also been used to examine the structure of the device and we compare this with electrically induced emission. In related work, the optical properties of micro lens arrays (10,100-micron diameter) fabricated in SiC, Sapphire, and Diamond have been determined using transmission confocal microscopy. Such optical microscopy techniques offer a simple, non-destructive method to determine the structure and performance of such novel devices. Microsc. Res. Tech. 64:293,296, 2004. © 2004 Wiley-Liss, Inc. [source]

    Behavioral modeling of GaN-based power amplifiers: Impact of electrothermal feedback on the model accuracy and identification

    Roberto Quaglia
    Abstract In this article, we discuss the accuracy of behavioral models in simulating the intermodulation distortion (IMD) of microwave GaN-based high-power amplifiers in the presence of strong electrothermal (ET) feedback. Exploiting an accurate self-consistent ET model derived from measurements and thermal finite-element method simulations, we show that behavioral models are able to yield accurate results, provided that the model identification is carried out with signals with wide bandwidth and large dynamics. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2789,2792, 2009; Published online in Wiley InterScience ( DOI 10.1002/mop.24732 [source]

    Linearity-optimized GaN HEMT Doherty amplifiers using derivative superposition technique for WCDMA applications

    Yong-Sub Lee
    Abstract Gallium nitride (GaN) high-electron mobility transistor (HEMT) Doherty amplifiers with the optimized linearity for wide-band code division multiple access (WCDMA) applications are represented. At a 7-dB back-off output power, the measured single-carrier WCDMA results show two-way and three-way GaN HEMT Doherty amplifiers with an adjacent channel leakage ratio (ACLR) of ,43.2 and ,48.2 dBc at ±2.5 MHz offset frequency with a drain efficiency of 43.1% and 30.9%, respectively. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 701,705, 2008; Published online in Wiley InterScience ( DOI 10.1002/mop.23181 [source]

    Tilt of InGaN layers on miscut GaN substrates

    M. Kry
    Abstract We report on the crystallographic orientation of InGaN layers grown on GaN substrates with a miscut with respect to c -planes up to 2.5°. The samples were examined using high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM). Because of the large (up to about 2% in this study) lattice mismatch between InGaN and GaN, an additional tilt between the c lattice planes of InGaN and GaN was observed and explained by using the Nagai model [J. Appl. Phys. 45, 3789 (1974)]. We observed that for part of the samples, this tilt is about 10% smaller compared to the one predicted by the model. The experimental data are important for understanding the microstructure of InGaN layers grown on substrates of non-perfect morphology. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Polarization of eigenmodes in laser diode waveguides on semipolar and nonpolar GaN

    Jens Rass
    Abstract Recent calculations of the eigenmodes in waveguides grown on semipolar GaN suggest that the optical polarization of the emitted light as well as the optical gain depends on the orientation of the resonator. Our measurements on separate confinement heterostructures on semipolar (112) and (102) GaN show that for laser resonators along the semipolar [11] and [011] directions (i.e. the projection of the c -axis onto the plane of growth) the threshold for amplified spontaneous emission is lower than for the nonpolar direction and that the stimulated emission is linearly polarized as TE mode. For the waveguide structures along the nonpolar [100] or [110] direction on the other hand, birefringence and anisotropy of the optical gain in the plane of growth leads not only to a higher threshold but alsoto a rotation of the optical polarization which is not any more TE- or TM-polarized but influenced by the ordinary and extraordinary refractive index of the material. We observe stimulated emission into a mode which is linearly polarized in extraordinarydirection nearly parallel to the c -axis. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Inversion domain boundaries in GaN studied by X-ray microprobe

    Gema Martínez-Criado
    Abstract In this study, we report on the application of synchrotron spectro-microscopic techniques to the examination of inversion domain boundaries formed intentionally in a GaN-based lateral polarity heterostructure. Using X-ray sub-microbeams, no evidence of field-driven electrodiffusion effects has been observed on spatially separated inversion domain boundaries. In addition, XANES data around the Ga K-edge strongly supported hexagonal Ga site configurations, suggesting high local order reconstruction. Based on inner-shell excited luminescence on the micrometer scale, the uniform spectral distribution of the radiative centers was discussed. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Electronic structure of GaN(0001)-2 × 2 thin films grown by PAMBE

    R. Gutt
    Abstract Gallium nitride thin films were grown on silicon carbide (0001) by plasma-assisted molecular beam epitaxy (PAMBE). The samples were cooled down in nitrogen plasma and characterized in situ by reflection high energy electron diffraction (RHEED), photoelectron spectroscopy (XPS/UPS), and atomic force microscopy (AFM) revealing stoichiometric and smooth GaN films virtually free of contaminations. We present valence band data obtained by UPS with strong emission from surface states inside the fundamental band gap. These states and the observed 2 × 2 surface reconstruction are highly sensitive towards residual molecules. Once these surface states have disappeared the original state could not be recovered by surface preparation methods underlining the necessity of in situ investigations on as-grown surfaces. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Efficiency droop in nitride-based light-emitting diodes

    Joachim Piprek
    Abstract Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several explanations of the efficiency droop have been proposed in recent years, but none is widely accepted. This feature article provides a snapshot of the present state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop. Illustration of LED efficiency droop (details in Fig. 13). [source]