bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth of AlN crystals on AlN/SiC seeds by AlN powder sublimation in nitrogen atmosphere
Authors: Noveski V. , Schlesser R. , Mahajan S. , Beaudoin S. , Sitar Z.
Published: August 2016

AlN single crystals were grown on AlN/SiC seeds by sublimation of AlN powder in TaC crucibles in a nitrogen atmosphere. The seeds were produced by metallorganic chemical vapor deposition (MOCVD) of AlN on SiC crystals. The influence of growth temperature, growth time and source-to-seed distance on the crystallinity and the crystal growth rate were investigated. Crystals were grown in an RF heated sublimation reactor at growth temperatures ranging from 1800-2000°C, at a pressure of 600 Torr, nitrogen flow-rate of 100 sccm and source-to-seed distances of 10 and 35 mm. At 1870°C and a source-to-seed distance of 35 mm, isolated crystals were observed with few instances of coalescence. At 1930°C, a source-to-seed distance of 10 mm and longer growth times (~30 hrs), crystal coalescence was achieved. Above 1930°C, the decomposition of SiC was evidently affecting the growth morphology and resulted in growth of polycrystalline AlN. After an initial nucleation period, the observed growth rates (10-30 µm/hr) were in close agreement with predictions of a growth model that assumed gas-phase diffusion controlled growth. Optical and electron microscope observations revealed step-flow growth, while X-ray diffraction results showed the single crystal nature of the grown material. Single crystalline AlN was grown over surface areas of 200-300 mm2 and was transparent and essentially colorless.
A GaN/4H-SiC heterojunction bipolar transistor with operation up to 300°C
Authors: Torvik John T , Leksono M. , Pankove J. I. , Van Zeghbroeck B.
Published: August 2016

We report on the fabrication and characterization of GaN/4H-SiC n-p-n heterojunction bipolar transistors (HBTs). The device structure consists of an n-SiC collector, p-SiC base, and selectively grown n-GaN emitter. The HBTs were grown using metalorganic chemical vapor deposition on SiC substrates. Selective GaN growth through a SiO2 mask was used to avoid damage that would be caused by reactive ion etching. In this report, we demonstrate common base transistor operation with a modest dc current gain of 15 at room temperature and 3 at 300°C.
Strain relaxation in GaN layers grown on porous GaN sublayers
Authors: Mynbaeva M. , Titkov A. , Kryzhanovski A. , Kotousova I. , Zubrilov A.S. , Ratnikov V.V. , Davydov V. Yu. , Kuznetsov N.I. , Mynbaev K. , Tsvetkov D.V. , Stepanov S. , Cherenkov A. , Dmitriev V.A.
Published: August 2016

We have studied epitaxial GaN layers grown by hydride vapour phase epitaxy (HVPE) on porous GaN sublayers formed on SiC substrates. It was shown that these layers can be grown with good surface morphology and high crystalline quality. X-ray, Raman and photoluminescent (PL) measurements showed that the stress in the layers grown on porous GaN was reduced to 0.1-0.2 GPa, while the stress in the layers grown directly on 6H-SiC substrates remains at its usual level of about 1 GPa. Thus, we have shown that growth on porous GaN sublayer is a promising method for fabrication of high quality epitaxial layers of GaN with low strain values.
GaInN/GaN Multi-Quantum Well Laser Diodes Grown by Low-Pressure Metalorganic Chemical Vapor Deposition
Authors: Kung P. , Saxler A. , Walker D. , Rybaltowski A. , Zhang Xiaolong , Diaz J. , Razeghi M.
Published: August 2016

We report the growth, fabrication and characterization of GaInN/GaN multi-quantum well lasers grown on (00·1) sapphire substrates by low pressure metalorganic chemical vapor deposition. The threshold current density of a 1800 μm long cavity length laser was 1.4 kA/cm2 with a threshold voltage of 25 V. These lasers exhibited series resistances of 13 and 14 Ω at 300 and 79 K, respectively.
Atomic force microscopy observation of threading dislocation density reduction in lateral epitaxial overgrowth of gallium nitride by MOCVD
Authors: Marchand H. , Ibbetson J.P. , Fini Paul T. , Kozodoy Peter , Keller S. , DenBaars Steven , Speck J. S. , Mishra U. K.
Published: August 2016

Extended defect reduction at the surface of GaN grown by lateral epitaxial overgrowth (LEO) on large-area GaN/Al2O3 wafers by low pressure MOCVD is demonstrated by atomic force microscopy. The overgrown GaN has a rectangular cross section with smooth (0001) and {110} facets. The density of mixed character threading dislocations at the surface of the LEO GaN is reduced by at least 3-4 orders of magnitude from that of bulk GaN. Dislocation-free GaN surfaces exhibit an anisotropic step structure that is attributed to the orientation dependence of the dangling bond density at the step edges.
Selective Area Growth of GaN Directly on (0001) Sapphire by the HVPE Technique
Authors: Singh Raj , Barrett Richard J. , Gomes John J. , Dabkowski Ferdynand P. , Moustakas T.D.
Published: August 2016

In this paper, we report on the selective area growth (SAG) of GaN directly on patterned c-plane sapphire substrates by hydride vapor phase epitaxy (HVPE). A number of researchers have reported that the HVPE growth technique, unlike the MBE and MOCVD methods, is capable of producing device quality GaN films without the need for any low temperature nucleation/buffer layers. The density of edge dislocations in these HVPE films decreases dramatically as the film thickness is increased, and the dislocation density values for thick films (> 10μm) are comparable to those reported for the best GaN films grown by other methods on c-sapphire. These advantages of the HVPE growth technique makes it possible to achieve high quality selective area growth of GaN directly on c-sapphire substrates.

C-plane sapphire substrates were coated with PECVD SiO2 and photolithographically patterned with different size and shape openings. Subsequently, these patterned substrates were introduced in a horizontal, hot-wall quartz reactor for the GaN growth. It was observed that single crystal GaN growth was preferentially initiated in the openings in the oxide layer. This selective area growth was followed by epitaxial lateral overgrowth (ELO), leading to the formation of hexagonal GaN prisms terminated in smooth, vertical (100) facets. We have been successful in shearing these pyramid structures from the sapphire substrates as individual devices, which do not require any post-growth etching for feature definition. This procedure allows for the dramatic reduction of the process complexity and the duration and expense for GaN growth for device applications. Stimulated emission results on these self-formed optical cavities are also presented.
The role of piezoelectric fields in GaN-based quantum wells
Authors: Hangleiter Andreas , Im Jin Seo , Kollmer H. , Heppel S. , Off J. , Scholz Ferdinand
Published: August 2016

In this contribution, we focus on the consequences of the piezoelectric field, which is an inherent consequence of the commonly used wurtzite phase of GaN, on the optical properties of strained GaN-based quantum well structures. We demonstrate that both in GaN/AlGaN and in GaInN/GaN single quantum well structures, the piezoelectric field leads to a Stark-shift of the fundamental optical transitions, which can lead to luminescence emission far below the bulk bandgap. Due to the spatial separation of the electron and hole wavefunctions in such structures, the oscillator strength of these transitions may become extremely small, many orders of magnitude lower than in the field-free case. From specially designed structures, we can even determine the sign of the piezoelectric field and relate it to the polarity of the layers. Under high-excitation conditions, as found in a laser diode, the piezoelectric field is almost completely screened by the injected carriers. As a consequence, the stimulated emission is significantly blue-shifted compared to the photoluminescence, which has sometimes been confused with localization effects.
High quality GaN films - growth and properties
Authors: Paku ,a K. , Baranowski Jacek M. , Leszczyn˜ski M. , Suchanek B. , Wojdak M.
Published: August 2016

Growth of GaN/Al2O3 layers by MOVPE has been investigated. Precise optimization of the growth parameters results in films with extremely high electron mobility: 900 cm2/Vs at 300K and 4000 cm2/Vs at 77K. The influence of the growth parameters on film properties like morphology, crystallographic structure, and the concentration of electrically active defects is presented. The mechanism of dislocation density reduction is proposed to explain the obtained results.
Surface Structures, Surfactants and Diffusion at Cubic and Wurtzite GaN
Authors: Zywietz T. , Neugebauer Jörg , Scheffler M. , Northrup J. , Van de Walle Chris G.
Published: August 2016

Clean and As covered zinc-blende and wurtzite GaN surfaces have been investigated employing density-functional theory calculations. For clean GaN surfaces our calculations indicate the stability of several novel surface structures that are very different from those found on traditional III-V semiconductors. Adding impurities commonly present in significant concentrations during growth strongly modifies surface reconstructions and energies. In particular, we find that arsenic has a low solubility and significantly stabilizes the cubic GaN (001) surface making it interesting as a potential surfactant. Finally, we have studied the diffusion of Ga and N adatoms on both the equilibrium and non-equilibrium surfaces. Our calculations reveal a very different diffusivity for Ga and N adatoms: While Ga adatoms are very mobile at typical growth temperatures, the diffusion of N adatoms is slower by several orders of magnitude. These results give insight into the fundamental growth mechanisms and allow conclusions concerning optimum growth conditions.
Structural properties of MOVPE GaN layers grown by a new multi-buffer aproach
Authors: Kozlowski J. , Paszkiewicz R. , Korbutowicz R. , Panek M. , Paszkiewicz B. , Tlaczala M.
Published: August 2016

GaN undoped layers of good morphology, good crystallinity and electrical properties were grown on c-plane sapphire substrates by the atmospheric pressure MOVPE technique using a new multi-buffer growth approach. A suitable buffer layer growth technique was worked out which enabled growth of GaN layers with properties superior to those grown in a conventional process scheme. Additional buffer layers, deposited with increasing temperature and increasing V/III molar ratio, were inserted between the low temperature buffer layer and the high temperature GaN layer grown on it. The c and a lattice constants of the high temperature GaN overgrown layer were evaluated from X-ray data. The layer mosaicity and c-lattice parameter variation were determined. The relationship between c and a lattice parameters and the second buffer layer growth scheme has been studied. The effect of second buffer layer growth conditions, buffer layer annealing time as well as the influence of V/III molar ratio during the high temperature GaN deposition on the crystalline and electrical properties of overgrown GaN epitaxial layers are presented. Characterization includes surface morphology examination by SEM and Nomarski optical microscope, X-ray diffraction and C-V measurements.
On the Bandstructure in GaInN/GaN Heterostructures - Strain, Band Gap and Piezoelectric Effect
Authors: Wetzel Christian , Nitta Shugo , Takeuchi Tetsuya , Yamaguchi Shigeo , Amano H. , Akasaki I.
Published: August 2016

A study of the optoelectronic properties of strained 40 nm Ga1−xInxN layers on GaN films is presented. The fact of pseudomorphic strain leads to a new interpretation of the film composition when derived from x-ray scattering. In addition we directly confirm that strain induces huge piezoelectric fields in this uniaxial system by the observation of Franz-Keldysh oscillations in photoreflection. As a function of composition (0 < x < 0.2) and strain we derive the electronic band gap energy and the piezoelectric field strength. We interpret both in terms of effective bowing parameters and piezoelectric coefficients, respectively. From a spatially resolved micro photoluminescence at room temperature we find no evidence for spatial band gap or composition variations of more than 60 meV over the length scale from 1 to 50 μm (x=0.187) in our material. At the same time, an observed discrepancy between photoluminescence peak energy and photoreflection band gap energy increases with x to some 160 meV. We attribute this redshift to photon assisted tunneling in the huge piezoelectric fields (Franz-Keldysh effect).
Luminescence and ESR Spectra of GaN:Si below and above Mott Transition
Authors: Paku ,a K. , Wojdak M. , Palczewska M. , Suchanek B. , Baranowski Jacek M.
Published: August 2016

Investigations of luminescence and ESR of silicon doped GaN layers are presented. The room temperature electron concentration in the investigated layers ranged from 1.7×1017 cm−3to 7×1018 cm−3. The layer with the highest electron concentration has metallic conductivity. The ESR investigation revealed the presence of a characteristic asymmetric resonance whose intensity grows with increasing silicon impurity concentration. This resonance, corresponding to perpendicular g=1.985 and parallel g=1.983 has been observed in Si doped layers with electron concentration below the Mott transition. It seems that the ESR resonance is due to isolated Si donors. It has been found that the total PL emission increases with the silicon concentration, and is strongest in the layer with metallic conductivity. This indicates that silicon impurities eliminate non-radiative recombination centers or they create a new path of radiative recombination. The AFM and low temperature PL measurements indicate that strain relief via creation of pinholes may be responsible for the increase of radiative emission in GaN:Si epilayers.
300°C GaN/AlGaN Heterojunction Bipolar Transistor
Authors: Ren Fan , Abernathy Cammy R. , Van Hove J. M. , Chow P. P. , Hickman R. , Klaasen JJ , Kopf R. F. , Cho Hyun , Jung K. B. , La Roche J. R. , Wilson R. G. , Han J. , Shul R. J. , Baca A. G. , Pearton S.J.
Published: August 2016

A GaN/AlGaN heterojunction bipolar transistor has been fabricated using Cl2/Ar dry etching for mesa formation. As the hole concentration increases due to more efficient ionization of the Mg acceptors at elevated temperatures (> 250°C), the device shows improved gain. Future efforts should focus on methods for reducing base resistance, which are briefly summarized.
Paramagnetic defects in GaN
Authors: Palczewska M. , Suchanek B. , Dwili˜ski R. , Paku ,a K. , Wagner A. , Kami˜ska M.
Published: August 2016

In this work, paramagnetic defects in wurtzite GaN crystals were systematically studied using the Electron Spin Resonance (ESR) technique and using electrical measurements. Three different resonance signals were found. The first had g|| = 1.9514 ± 0.0005 and g⊥ = 1.9486 ± 0.0005, a commonly observed defect in n-type crystals ascribed to the shallow donor of GaN [1]. The second ESR signal, an anisotropic line of g|| = 2.0728 ± 0.0015 and g⊥ = 1.9886 ± 0.0015, was observed only in Mg-doped p-type GaN layers, and was assigned to the Mg acceptor. The last ESR resonance signal, an isotropic line with g = 2.0026 ± 0.0005 was observed only in AMMONO GaN crystals after thermal annealing, as well as in Mg-doped GaN epitaxial layers. It was tentatively identified as due to a deep acceptor.
Raman study of resonance effects in Ga1−xAlxN solid solutions
Authors: Demangeot F. , Frandon J. , Renucci M. A. , Sands H. , Batchelder D. , Ruffenach-Clur S. , Briot Olivier , Gil Bernard
Published: August 2016

The photoluminescence and Raman spectra of several Ga1−xAlxN layers (0 ≤ x ≤ 0.86) grown on sapphire substrates by metal-organic vapor phase epitaxy have been recorded at room temperature, under an excitation at 244 nm. Using the photoluminescence spectra, the variation of the band gap of these alloys can be followed only up to x = 0.5. From resonant Raman scattering, it can be deduced that the band gap energy of the solid solution for x very close to 0.7 corresponds to the incident photon energy (5.08 eV). This result is confirmed by a detailed comparison of the present work with previous experimental data on the A1(LO) phonon peak position, obtained under visible excitation.
Gain Spectroscopy of HVPE-Grown GaN
Authors: Eckey L. , Holst J.-Chr. , Hoffmann A. , Broser I. , Detchprohm T. , Hiramatsu K.
Published: August 2016

We report on photoluminescence and optical gain measurements of highly excited GaN crystals grown by hydride vapor physe epitaxy (HVPE). Inelastic scattering processes of excitons dominate the spontaneous emission spectrum under high excitation up to temperatures of 180 K. Towards room temperature phonon-assisted recombination of excitons and free carriers begins to dominate the spectrum. Similar characteristics are observed in temperature-dependent gain measurements.
Studies of Mg-GaN grown by MBE on GaAs(111)B substrates
Authors: Cheng T. S. , Foxon C. T. , Jeffs N. J. , Dewsnip D. J. , Flannery L. , Orton J. W. , Novikov S. V. , Ber B. Ya , Kudriavtsev Yu A.
Published: August 2016

This paper discusses the growth of Mg-doped GaN samples using a modified Molecular Beam Epitaxy (MBE) method. Our results suggest that the dopant is incorporated from a surface population maintained by the incident Mg flux by a rapid diffusion mechanism. It follows that the chemical concentration will increase with time of growth and that the p-doping level will also increase progressively with film thickness for a given Mg flux. Increasing the Mg flux to the surface results at first in a higher doping density, but this saturates when the Mg surface concentration reaches a finite value.
Efficient optical activation of ion-implanted Zn acceptors in GaN by annealing under 10 kbar N2 overpressure
Authors: Strite S. , Pelzmann A. , Suski T. , Leszczynski M. , Jun J. , Rockett A. , Kamp Markus , Ebeling K. J.
Published: August 2016

We continue our investigations into the optical activation of Zn-implanted GaN annealed under ever higher N2 overpressure. The samples studied were epitaxial GaN/sapphire layers of good optical quality which were implanted with a 1013 cm−2 dose of Zn+ ions at 200 keV, diced into equivalent pieces and annealed under 10 kbar of N2. The N2 overpressure permitted annealing at temperatures up to 1250°C for 1 hr without GaN decomposition. The blue Zn-related photoluminescence (PL) signal rises sharply with increasing anneal temperature. The Zn-related PL intensity in the implanted sample annealed at 1250°C exceeded that of the epitaxially doped GaN:Zn standard proving that high temperature annealing of GaN under kbar N2 overpressure can effectively remove implantation damage and efficiently activate implanted dopants in GaN. We propose a lateral LED device which could be fabricated using ion implanted dopants activated by high temperature annealing at high pressure.
MOVPE growth optimization of high quality InGaN films.
Authors: der Stricht W. Van , Moerman I. , Demeester P. , Considine L. , Thrush E. J. , Crawley J. A.
Published: August 2016

In this paper growth of high quality InGaN films on (0001) sapphire substrates by atmospheric pressure organometallic vapour phase epitaxy in a vertical rotating disk reactor is investigated. The InGaN layers grown above 800 °C are transparent and show no In-droplets on the surface. The In-content varies between 56 and 9 % for growth temperatures between 700 and 850 °C. The DC X-ray rocking curve of InGaN typically shows a FWHM between 8 and 15 arcmin. Room temperature PL shows an intense band edge emission with a FWHM between 100 and 200 meV for an In-content of 9 and 56 %. The initial efforts on QW growth are discussed.
Theoretical study of point defects in GaN and AlN; lattice relaxations and pressure effects
Authors: Gorczyca I. , Svane A. , Christensen N. E.
Published: August 2016

Native defects and some common dopants (Mg, Zn, and C) in cubic GaN and AlN are examined by means of ab initio theoretical calculations using two methods: i) the Green's function technique based on the linear muffin-tin orbital method in the atomic-spheres approximation; ii) a supercell approach in connection with the full-potential linear muffin-tin-orbital method. We apply the first method to look mainly at the energetic positions of the defect and impurity states in different charge states and their dependence on hydrostatic pressure. The second method allows us to study lattice relaxations. Whereas small relaxations are found near vacancies and substitutional Mg and Zn, the calculations predict large atomic displacements around antisite defects and the substitutional carbon impurity on the cation site.
Optical nonlinearities of Gallium Nitride
Authors: Haag H. , Gilliot P. , Ohlmann D. , Levy R. , Briot Olivier , Aulombard Roger-Louis
Published: August 2016

Luminescence, induced absorption and degenerate four-wave mixing experiments are perform on GaN epilayers grown on a sapphire substrate by MOCVD.

We measure the nonlinear behavior of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I2 line due to the finite donor density in the sample. Higher intensities of excitation induce collision processes between photo-created quasi-particles.

Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, transmission spectra show a red-shift of the absorption edge and of the excitonic resonance.

Pulsed degenerate four-wave mixing experiments were performed using the third harmonic of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time constant of 16 ps has been measured, which is independent of the temperature, spacing of the interference fringe and of the intensity of the pump beams.
Transient four wave mixing experiments on GaN
Authors: Zimmermann R. , Hofmann M. , Weber D. , Möbius J. , Euteneuer A. , Rühle W. W. , Göbel E. O. , Meyer B.K. , Amano H. , Akasaki I.
Published: August 2016

We present four wave mixing experiments on GaN. We find an intrinsic homogeneous broadening of the A-exciton of 1.67 meV. A pronounced beating with a period of 0.52 ps is observed at excitation energies between the A- and the B-exciton and corresponds to an energy splitting of 7.98 meV of A- and B-exciton.
Properties of the Biexciton and the Electron-Hole-Plasma in Highly Excited GaN
Authors: Holst J.-Chr. , Eckey L. , Hoffmann A. , Broser I. , Amano H. , Akasaki I.
Published: August 2016

High-excitation processes like biexciton decay and recombination of an electron-hole-plasma are discussed as efficient mechanisms for lasing in blue laser diodes [1]. Therefore, the investigation of these processes is of fundamental importance to the understanding of the properties of GaN as a basic material for optoelectronical applications. We report on comprehensive photoluminescence and gain measurements of highly excited GaN epilayers grown by metal-organic chemical vapor deposition (MOCVD) over a wide range of excitation densities and temperatures. For low temperatures the decay of biexcitons and the electron-hole-plasma dominate the spontaneous-emission and gain spectra. A spectral analysis of the lineshape of these emissions is performed and the properties of the biexciton and the electron-hole-plasma in GaN will be disscused in comparison to other wide-gap materials. At increased temperatures up to 300 K exciton-exciton-scattering and band-to-band recombination are the most efficient processes in the gain spectra beside the electron-hole-plasma.
MOVPE Growth and Structural Characterization of AlxGa1−xN
Authors: Ruffenach-Clur S. , Briot Olivier , Gil Bernard , Aulombard Roger-Louis , Rouviere J. L.
Published: August 2016

The ternary alloy GaAlN has been grown by low pressure MOVPE (76 Torr) using triethylgallium, trimetylaluminum and ammonia as precursors. The alloy layers were grown on (0001) sapphire substrates using a low temperature AlN buffer. All layers were deposited at a growth temperature of 980°C. Only the aluminum/gallium ratio in the gas phase was changed, keeping the total group III molar flow rate and V/III molar ratio constant.

The aluminum incorporation versus gas phase composition was determined experimentally, using energy dispersive analysis of X-rays (EDAX), and X-ray diffraction. We propose a model, taking into account kinetically limited mass transport of group III species in the gas phase, which describes well the data.

The structural quality of the layers was investigated using X-ray diffraction and TEM experiments.

A degradation of the materials quality is observed with increasing Al content. In this case, growth originate on the buffer grains facets resulting in a “ two directional » growth. This phenomenon, being markedly enhanced when increasing the Al content will be detailed in this paper.
Epitaxial Growth and Orientation of GaN on (1 0 0) γ-LiAlO2
Authors: Hellman E. S. , Liliental-Weber Z. , Buchanan D. N. E.
Published: August 2016

The (1 0 0) face of γ-LiAlO2 has attracted attention as a possible substrate for GaN epitaxial growth. This is partly because this face has an excellent lattice and structural match to (1 0 0) GaN. This orientation would have a misfit of only −1.4% along the c-direction and −0.1% along the b-direction of LiAlO2. We find that in practice this orientation relationship does not occur; instead, (0 0 0 1) oriented GaN grows with a small tilt (0.6° towards the c-direction) between the film and substrate. Although the misfit along the substrate b direction is large (−6.3%) for this orientation, the tilt perfectly accommodates the −1.4% misfit in the c direction. We present characterization of these films by RHEED, X-ray diffraction, and TEM. We propose that the tilt is driven by a reduction of interface energy which occurs in polar, incoherent interfaces.
Exciton dynamics in thick GaN MOVPE epilayers deposited on sapphire.
Authors: Allègre J. , Lefebvre P. , Camassel J. , Beaumont B. , Gibart Pierre
Published: August 2016

Time-resolved photoluminescence spectra have been recorded on three GaN epitaxial layers of thickness 2.5 μm, 7 μm and 16 μm, at various temperatures ranging from 8K to 300K. The layers were deposited by MOVPE on (0001) sapphire substrates with standard AlN buffer layers. To achieve good homogeneities, the growth was in-situ monitored by laser reflectometry. All GaN layers showed sharp excitonic peaks in cw PL and three excitonic contributions were seen by reflectivity. The recombination dynamics of excitons depends strongly upon the layer thickness. For the thinnest layer, exponential decays with τ ~ 35 ps have been measured for both XA and XB free excitons. For the thickest layer, the decay becomes biexponential with τ1 ~ 80 ps and τ2 ~ 250 ps. These values are preserved up to room temperature. By solving coupled rate equations in a four-level model, this evolution is interpreted in terms of the reduction of density of both shallow impurities and deep traps, versus layer thickness, roughly following a L−1 law.
Study of high quality AlN layers grown on Si(111) substrates by plasma-assisted molecular beam epitaxy
Authors: Sánchez-García M. A. , Calleja E. , Monroy E. , Sánchez F. J. , Calle F. , Muñoz E. , Sanz. Hervas A. , Villar C. , Aguilar M.
Published: August 2016

High quality AlN layers with full widths at half maximum values of 10 arcmin and average surface roughness (rms) of 48Å were grown by molecular beam epitaxy on Si(111) substrates. A systematic study and optimization of the growth conditions was performed in order to use these AlN layers as buffers in the growth of GaN films. Atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques were employed to determine the surface and structural quality of the layers. Best AlN films were obtained at high substrate temperatures (Tsubs>900°C) and III/V ratios close to stoichiometry. Growth conditions with III/V ratios beyond stoichiometry (Al-rich) did not further improve the crystal quality. In these cases a higher substrate temperature is needed to prevent condensation of Al on the surface. GaN films with full width at half maximum of 10 arcmin and improved optical properties were grown on top of optimized AlN buffer layers.
Time-resolved photoluminescence studies of InGaN/GaN multiple quantum wells
Authors: Allègre J. , Lefebvre P. , Juillaguet S. , Knap W. , Camassel J. , Chen Q. , Khan M. A.
Published: August 2016

We report both cw and time resolved optical investigations performed on an InGaN/GaN multiple quantum well grown by MOVPE on <0001>-oriented sapphire substrate. At low temperature we find a strong “blue” luminescence band, of which energy position corresponds well with the wavelength of stimulated emission when excited with a nitrogen laser. We show that this PL band appears systematically red-shifted with respect to the QWs features, which supports a standard picture of fluctuations of the indium composition. Coming to the time-resolved data, we find at low temperature at least two “blue” band components which are both associated with long decay times (up to 4-5 ns at 8K). The decay time is temperature dependent and, when rising the temperature, the recombination rate increases. At room temperature, we reach typical values in the range ~100 to 500 ps.
Theoretical Studies Of Hydrogen Passivated Substitutional Magnesium Acceptor In Wurzite GaN
Authors: Torres V. J. B. , Öberg S. , Jones R.
Published: August 2016

Infrared measurements on wurzite GaN codoped with Mg and H reveal strong absorption at 3125 cm−1. Theorectical work provides strong evidence for the H being antibonding to N. We have performed an ab-initio study of Mg-H complexes in wurzite GaN, using the Local Density Approximation on a large H-terminated cluster MgHGa25N26H42. We have investigated the physical properties of three neutral configurations along the c direction. In all configurations Mg sits in a gallium substitutional site. H is then located in the Mg-N bond centre (BC), in the antibonding site on nitrogen side (ABN) or in the antibonding position on magnesium side (ABMg). We found the lowest total energy configuration is hydrogen in the antibonding on the nitrogen site. The stretch mode in this configuration is calculated at 3277 cm−1 which agrees with experiment and previous LDA calculations and we predict an unreported infra red active wag mode at 1311 cm−1. The experimental isotopic shift with D is well reproduced. The BC and ABMg configurations are 0.5 and 3.7 eV higher, producing local modes at 3645 and 2144 cm−1, respectively. No wag modes appear for the BC and ABMg configurations.
Raman study of Ga1−xAlxN solid solutions
Authors: Demangeot F. , Groenen J. , Frandon J. , Renucci M. A. , Briot Olivier , Ruffenach-Clur S. , Aulombard Roger-Louis
Published: August 2016

Long wavelength optical phonons of AlxGa1−xN solid solutions have been identified in the whole compositional range by Raman spectroscopy. The frequencies of A1 and E1 polar phonons increase continuously with x from one-member crystal to the other. A generalization of the dielectric model of Hon and Faust is used to treat the coupling of the longitudinal optic (LO) mode. This approach accounts for the observed frequencies and confirms the so-called one-mode behaviour of polar LO phonons. Moreover,a signature of the coupling of a discrete mode (tentatively associated to silent q=0 B1 mode) with an unidentified continuum has been obtained.
Luminescence Spectra Of Superbright Blue and Green InGaN/AlGaN/GaN Light-Emitting Diodes
Authors: Zolina K. G. , Kudryashov V. E. , Turkin A. N. , Yunovich A. E. , Nakamura Shuji
Published: August 2016

Electroluminescence spectra of superbright blue and green LEDs based on epitaxial InxGa1−xN/AlyGa1−yN/GaN heterostructures with thin quantum well active layers [1] were studied at currents J = 0.01-20 mA. Spectral maxima of blue and green LEDs are ωmax = 2.58-2.75 eV and ωmax = 2.38-2.45 eV, dependent on the active layer In content. The low energy tails of the spectra are exponential with the parameter E0 = 42-50 meV almost independent of the temperature. The high energy tails of the spectra are exponential with a temperature dependent parameter E1= 20-40 meV. Both parameters (E0, E1) are current independent at J > 0.5 mA. The spectral band can be described by taking into account quantum size effects, impurities and electron-phonon interactions in active layers. A structure in the spectra was detected which can be described by the influence of light interference in the GaN layer on the sapphire substrate. Light intensity was a linear function of the drive current over the interval J = 1-20 mA, and was slightly temperature dependent. In the blue LEDs, the efficiency fall off at low currents (J < 0.7 mA) had a I ~ J4-5 dependence at room temperature. The green LEDs showed no such dependence. The influence of tunnel effects on the efficiency at low currents is discussed. Tunnel radiation spectra with maxima moving with the voltage were detected at low currents in III-N structures.
Bandgap Variation at the Isostructural Phase Transformation of Wurtzite InN
Authors: Bellaiche L. , Kunc K. , Besson M.
Published: August 2016

The pressure variation of the bandgap, at the isostructural phase transition of wurtzite InN, is determined theoretically, using the first-principles total-energy pseudopotential method. It is found that the bandgap (as well as the structural parameters) exhibit three different types of behavior, in three regions of pressure. Optical experiments at low temperatures could then be employed to directly identify the two different wurtzite phases of InN.
Alternative N precursors and Mg doped GaN grown by MOVPE
Authors: Beaumont B. , Vaille M. , Lorenzini P. , Gibart Pierre , Boufaden T. , Jani B. el
Published: August 2016

In this paper, we address two different aspects relevant to the growth of GaN. The first part concerns alternative nitrogen source whereas in the second part, we report experimental results on Mg doping. Several nitrogen precursors have been used for the growth of GaN in MOVPE. To produce active species from N2 or NH3, a remote Plasma Enhanced Chemical Vapour Deposition (RPECVD) process has been implemented. In addition, nitrogen organic precursors, triethylamine and t-butylamine were also used. To accurately control the critical parameters of the MOVPE of GaN, we have implemented a laser reflectometry, which allows a real time in situ monitoring of the different steps of the growth. MeCp2Mg was used as Mg precursor for the p doping study. The dependence on the partial pressure of Mg precursor of dopant incorporation, electrical activity and growth rate are reported.
Evidence for Shallow Acceptor Levels in MBE Grown GaN
Authors: Ren B. G. , Orton J. W. , Cheng T. S. , Dewsnip D. J. , Lacklison D. E. , Foxon C. T. , Malloy C. H. , Chen X.
Published: August 2016

We report the results of photoluminescence measurements on a number of GaN thin films grown by MBE on GaAs (111)B substrates. In particular, we draw attention to a new observation of a line at approximately 3.40eV which is accompanied by complex fine structure and interpret it as due to a donor-acceptor (DA) transition. Assuming a donor energy of 30meV, we derive an acceptor binding energy of approximately 80meV which is very much smaller than the accepted value of 250meV for the well established Mg acceptor. However, our result is in agreement with a recent estimate of the hydrogenic acceptor energy as being 85meV.
Raman Determination of the Phonon Deformation Potentials in α-GaN
Authors: Demangeot F. , Frandon J. , Renucci M. A. , Briot Olivier , Gil Bernard , Aulombard Roger-Louis
Published: August 2016

Raman spectroscopy is used to study the effect of the built-in biaxial stress on the E2 and A1 (LO) q = 0 phonon modes of wurtzite GaN layers deposited by metal organic vapor phase epitaxy on (0001) sapphire substrate. By means of phonon frequency shifts, the biaxial pressure coefficients of the mode frequencies are determined and used to calculate the corresponding deformation potentials. Stress calibration has been performed using reflectance data.
High-Power High-Temperature Heterobipolar TransistorWith Gallium Nitride Emitter
Authors: Pankove J. I. , Leksono M. , Chang S. S. , Walker C. , Zeghbroeck B. Van
Published: August 2016

A new heterobipolar transistor was made with the wide bandgap semicon-ductors gallium nitride (GaN) and silicon carbide (SiC). The heterojunction allows high injection efficiency, even at elevated temperatures. A record current gain of ten million was obtained at room temperature, decreasing to 100 at 535°C. An Arrhenius plot of current gain vs 1/T yields an activation energy of 0.43 eV that corresponds to the valence band barrier blocking the escape of holes from the base to the emitter. This activation energy is approximately equal to the difference of energy gaps between emitter and base. This Transistor can operate at high power without cooling. A power density of 30 KW/cm2 was sustained.
Identification of a Cubic Phase in Epitaxial Layers of Predominantly Hexagonal GaN
Authors: Strauss U. , Tews H. , Riechert H. , Averbeck R. , Schienle M. , Jobst B. , Volm D. , Streibl T. , Meyer BK , Rühle W. W.
Published: August 2016

Epitaxial layers of GaN on c-plane sapphire are analyzed by continuous-wave and time-resolved photoluminescence at 4K and by X-ray diffraction. Besides the well-known emissions from hexagonal GaN we observe luminescence bands at 3.279 and 3.15 to 3.21 eV which are identified as the transition of the donor bound exciton and the donor-acceptor pair recombination in cubic GaN, respectively. Measurements of the luminescence decay times are essential for the clarification of the emission processes. Due to the probing depth of about 200 nm in PL we find that the fraction of cubic phase typically decreases with layer thickness. In our best samples, however, we do not detect the cubic phase at all.
Assessing Benthic Barriers vs. Aggressive Cutting as Effective Yellow Flag Iris (Iris pseudacorus) Control Mechanisms
Authors: Tarasoff Catherine S. , Streichert Kailee , Gardner Wendy , Heise Brian , Church John , Pypker Thomas G.
Published: August 2016

An experiment was initiated to study the effects of rubber benthic barriers vs. aggressive cutting on the invasive aquatic emergent plant, yellow flag iris. Treatments were compared against a control at two locations within British Columbia, Canada (Vaseux Lake and Dutch Lake). Yellow flag iris response was significantly different between the two sites, but biologically the results were identical: the benthic barrier killed yellow flag iris rhizomes within 70 d of treatment. Over the extent of the research, at Vaseux Lake the effect of aggressive cutting was no different from the control, while aggressive cutting was statistically no different than the benthic barrier at Dutch Lake. Vegetation regrowth approximately 200 d after the benthic barriers were removed was not detected at either location. These results indicate that rubber benthic barriers may be an effective treatment for yellow flag iris and maybe suitable for other, similar species.
Cow Green Reservoir – update for the 50th anniversary issue
Authors: Blacklock Norman
Published: August 2016

Cow Green was authorised by the Tees Valley and Cleveland Water Act 1967, which received the Royal Assent on 22 March 1967 after lengthy proceedings in both Houses of Parliament. The reservoir is on the river Tees on the boundary of Durham and Westmorland (now Cumbria) and the dam is situated at about 280 m upstream of the Cauldron Snout waterfall – the largest water cascade drop in England.
The Risk-Adjusted Monetary Policy Rule
Authors: Nakata Taisuke , Schmidt Sebastian
Published: August 2016

Macroeconomists are increasingly using nonlinear models to account for the effects of risk in the analysis of business cycles. In the monetary business cycle models widely used at central banks, an explicit recognition of risk generates a wedge between the inflation-target parameter in the monetary policy rule and the risky steady state (RSS) of inflation---the rate to which inflation will eventually converge---which can be undesirable in some practical applications. We propose a simple modification to the standard monetary policy rule to eliminate the wedge. In the proposed risk-adjusted policy rule, the intercept of the rule is modified so that the RSS of inflation equals the inflation-target parameter in the policy rule.
The Essence of the Fact Phenomenon and it’s Functioning in Media
Authors: Komova Maria
Published: August 2016

Application of the factual principle of information presentation is particularly relevant in present conditions of global usage of information space as a confrontation field for different models of development. Main objective of the study is to analyze the basic substantiating status of fact and its interpretation in the media system. Methodology of research is based on systematization and classification. The method of systematization is applied to integrate the nature of the fact phenomenon and its representation in the media system, and to determine the functional characteristics of the factual principle of information presentation in mass media. The method of classification is applied to establish a classification system qualifying different classes of facts.

Results / findings. Factual principle is implemented within the factual field, which is a set of information about changes in social environment integrated with lexical, grammatical, logical and stylistic ties, with appropriate focus and pragmatic attitude. The factual field is shaped by the interrelated system consisting of basic fact, factual environment and journalistic skills to convince by arguments based on facts. Factual principle of providing information to the media is based on the following functional characteristics: universality, consistency, analyticity, reliability, professionalism of fact presentation. The aim of facts classifications is to differentiate the objects reflected in the media by certain defining characteristics. Classification schemes are created on the basic of various logical processes: species classification is based on differentiation, while typological classification is based on integration features.

Conclusions. The essence of the fact phenomenon and its representation in the media is implemented by the universal application of the factual principle of information presentation. The factual field is an organic component of social interaction, and its consistency is shaped by the interrelation of objective and subjective factors. The classification and typology of facts provide a basis for establishing the specificity of each species and types of facts and of factual information created on their basis. It also helps to navigate in the information environment, to use effectively the factual data to rationalize messages in mass media. Significance of the study: the theory of fact is substantiated, its functional characteristics are determined, and the model of fact classification is developed.
Forms of Teaching Pedagogical Disciplines in Orthodox Religious Educational Institutions of Ukraine (The End of 19th - The Beginning of 20th Century)
Authors: Tverdokhlib Tetiana
Published: August 2016

The main forms of teaching pedagogical disciplines in orthodox religious educational institutions of Ukraine (the end of 19th – the beginning of the 20thcentury) have been characterised in the article on the basis of analyzed pedagogical literature, archival records, and documents in periodicals. The features of conducting lessons, teaching practice, examinations and organisation of making written home compositions in various types of religious educational institutions of Ukraine in the stated time period have been revealed. The essence of "rehearsals" as a form of controlling students’ learning activity in the Kyiv Theological Academy has been revealed, the basic kinds of academists’ works at practical hours on pedagogy have been given; the content and quality of teachers’ work with graduates on their researches have been characterised.
Estimation of Distribution Algorithm for Energy-Efficient Scheduling in Turning Processes
Authors: Wang Fang , Rao Yunqing , Zhang Chaoyong , Tang Qiuhua , Zhang Liping
Published: August 2016

With the increasing concern of environment, the energy-efficiency scheduling of manufacturing industry is becoming urgent and popular. In turning processes, both spindle speed and processing time can affect the final energy consumption and thus the spindle speed and scheduling scheme need to be optimized simultaneously. Since the turning workshop can be regarded as the flexible flow shop, this paper formulates a mixed integer linear programming model for energy-efficient scheduling of flexible flow shop. Accordingly, a new decoding method is developed by considering of the optimization of spindle speed and scheduling scheme simultaneously, and an estimation of distribution algorithm adopting the new decoding method is proposed to solve large-size problems. The parameters of this algorithm are determined by statistical technique with a simplified practical case. In order to validate the proposed method, a case from practical factories is studied, in which the makespan can be shortened by 25.22%, and the consumed energy can be saved by 5.48%. These results demonstrate the effectiveness of the proposed mathematical model and algorithm.
Multiple Disparities in Adult Mortality in Relation to Social and Health Care: Results from Different Data Sources
Authors: Ranabhat Chhabi , Kim Chun-Bae , Park Myung-Bae , Kim Chang-Soo , Jeong Hyoung Sun , Koh Sang Baek , Chang Sei-Jin
Published: August 2016

Background: Disparity in adult mortality (AM) with reference to social dynamics and health care has not been sufficiently examined. This study aimed to identify the gap in the understanding of AM in relation to religion, political stability, economic level, and universal health coverage (UHC). Methods: A cross-national study was performed with different sources of data, using the administrative record linkage theory. We created a new data set using data from the 2013 World Bank data catalogue by region, The Economist (Political instability index 2013), Stuckler David et al. (Universal health coverage, 2010), and religious categories of all UN country members. Descriptive statistics, a t-test, an ANOVA followed by a post hoc test, and a linear regression were used where applicable. Result: The average AM rate for males and females was 0.20 ± 0.10 and 0.14 ± 0.10, respectively. AM was significantly higher in economically weak countries, countries with political instability, countries with traditional religion, without achievement of UHC, and Sub-Saharan Africa (p<0.01). There was high disparity of AM between countries with and without UHC (F = male: 61.89, female: 51.85, p<0.001) and between groups with low and high income (F = male: 36.33, female: 42.39, p<0.001). UHC and political stability would significantly reduce AMR by > 0.41 in both sexes and high economic status would reduce male AMR by 0.44, and female AMR by 0.70, in relation to countries without UHC, with political instability, and low economic status. Conclusions: Disparities in AM can be reduced after the achievement of UHC and economically productive activities for those adults affected by conflict and political unrest.