bluuebunny/crossref_metadata_2025 huggingface dataset:
The importance of on-site evaluation for placing renewable energy in the landscape: A case study of the Búrfell wind farm (Iceland)
Authors: Frantál Bohumil , Bevk Tadej , Van Veelen Bregje , Hărmănescu Mihaela , Benediktsson Karl
Published: January 2018

Using a case study of the Búrfell wind farm project, a large wind farm proposed in the Central Highlands of Iceland, the authors attempt to provide new insights into the factors shaping subjective landscape perceptions and attitudes to renewable energy developments, and into alternative methods that may be used for their assessment. The research was based on an on-site visit and actual experience of the place, investigated using a combination of mental mapping, the technique of the semantic differential and a questionnaire survey. The results show that participants visiting a landscape and using all sensory organs in combination with mental mapping, can reveal more important information than using only ‘laboratory’ methods with static photographs. The results suggest that the perception of landscape is highly subjective. Those perceiving the landscape as more open, homogenous, industrial, unfamiliar and resilient also consider it more compatible with wind turbines. The perception of the landscape’s compatibility with wind turbines proved to be a dominant factor shaping attitudes towards the project. The acceptance of wind turbines is not, however, inconsistent with the perception of landscape as beautiful, wild and unique. Participants from more densely populated countries and countries with a developed wind energy industry were more tolerant of wind turbines in the Icelandic landscape.
Wind farms and rural tourism: A Portuguese case study of residents’ and visitors’ perceptions and attitudes
Authors: Silva Luís , Delicado Ana
Published: January 2018

Residents’ and visitors’ perceptions of and attitudes towards existing wind farms, as well as the perceived impact of wind farms on tourism, are examined in this article with reference to a built heritage site in the Portuguese countryside. Based on a set of semi-structured interviews, the paper sheds light on the positive impact that the community’s or local actors’ involvement in the constitution, management and decision-making processes has on the residents’ perceptions and attitudes regarding wind farms, and also on the trade-off with the perceived effect of wind farms on local tourism. Moreover, it shows that although most visitors criticised the proximity of wind turbines to medieval architecture, a clear majority of them accepted their presence and virtually all of them stated that these facilities had no impact on their choice of destination.
Landscape degradation at different spatial scales caused by aridification
Authors: Meyer Burghard Christian , Mezősi Gábor , Kovács Ferenc
Published: January 2018

Landscape responses to degradation caused by aridification bring the landscape system into a new equilibrium state. The system transformation may entail irreversible changes to its constituting parameters. This paper analyses the impact of aridification on landscape degradation processes in the sand-covered landscapes of the Hungarian Danube-Tisza Interfluve region at the regional, landscape, and local site scales. Changes in groundwater level (well data), lake surface area (Modified Normalized Difference Water Index) and vegetation cover (Enhanced Vegetation Index) were analysed over time periods of 12–60 years. Significant regional variation in decreasing groundwater levels is observed and limits the regional applicability of this indicator. Applying the lake surface area parameter from remote sensing data demonstrated greater utility, identifying several local lakes in the landscapes which have dried out. Analysis of the vegetation response indicated minor changes over the 2000–2014 time period and did not indicate a landscape system change. Landscape degradation as a result of changes in groundwater, vegetation, land cover and land use is clearly identified exclusively in local lake areas, but at the landscape scale, changes in the water balance are found in phases of system stability and transformation. Thresholds are identified to support policy and management towards landscape degradation neutrality.
A comparison study of steady-state vibrations with single fractional-order and distributed-order derivatives
Authors: Duan Jun-Sheng , Cheng Cui-Ping , Chen Lian
Published: January 2018

We conduct a detailed study and comparison for the one-degree-of-freedom steady-state vibrations under harmonic driving with a single fractional-order derivative and a distributed-order derivative. For each of the two vibration systems, we consider the stiffness contribution factor and damping contribution factor of the term of fractional derivatives, the amplitude and the phase difference for the response. The effects of driving frequency on these response quantities are discussed. Also the influences of the order α of the fractional derivative and the parameter γ parameterizing the weight function in the distributed-order derivative are analyzed. Two cases display similar response behaviors, but the stiffness contribution factor and damping contribution factor of the distributed-order derivative are almost monotonic change with the parameter γ, not exactly like the case of single fractional-order derivative for the order α. The case of the distributed-order derivative provides us more options for the weight function and parameters.
Current superimposition variable flux reluctance motor with 8 salient poles
Authors: Takahara Kazuaki , Hirata Katsuhiro , Niguchi Noboru , Kohara Akira
Published: January 2018

We propose a current superimposition variable flux reluctance motor for a traction motor of electric vehicles and hybrid electric vehicles, which consists of 10 salient poles in the rotor and 12 slots in the stator. However, iron losses of this motor in high rotation speed ranges is large because the number of salient poles is large. In this paper, we propose a current superimposition variable flux reluctance motor that consists of 8 salient poles and 12 slots. The characteristics of the 10-pole-12-slot and 8-pole-12-slot current superimposition variable flux reluctance motors are compared using finite element analysis under vector control.
Impact of Intense Pulsed Light Therapy on the Quality of Life of Rosacea Patients
Authors: Kolontaja-Zaube Inese , Ančupāne Ināra , Dērveniece Andra , Žileviča Aija , Ķikuste Ilze
Published: January 2018

Rosacea is a chronic inflammatory skin disorder affecting predominantly adult patients. The aim of the current investigation was to evaluate clinical response by using quality of life assessment before and after an IPL (intensive pulsed light) therapy course for patients sufferring from rosacea treated in the outpatient clinic “Health and Aesthetics” in Rīga during a one-year period (in 2016). All patients presented with typical clinical symptoms of rosacea on the face — acneiform papules, pustules, telangiectasia, centrofacial erythema, and complaints about flushing and burning. In the current study, 100 rosacea patients treated with IPL therapy were selected. Each patient filled in the quality of life questionnaire before and after two courses of IPL therapy. The interval between IPL therapy sessions was one month. The results reflected comprehensive benefit from IPL therapy for all rosacea patients. No serious side effects from IPL therapy were detected. Side effects like mild eythema after procedure (75%), mild oedema (10%), vesiculation (3%), and hyperpigmentation (8%) were completely resolved within four weeks after completion of treatment. In all patients, quality of life assessment showed a statistically significant increase after two courses of IPL therapy.
Hydrodynamics, numerical study and application of spouted bed
Authors: Rahimi-Ahar Zohreh , Hatamipour Mohammad Sadegh
Published: January 2018

This article reviews the major research and development on spouted beds (SBs). Due to its unique structural and flow characteristics, the SB is a very successful system in most applications. Two-phase and sometimes three-phase interactions generate a large number of variables to be noted in each process. Up-to-date information on the fundamentals and applications of SBs has been briefly presented, based on the published works. Thousands of interesting studies on hydrodynamic characteristics, numerical simulations, and new applications of SBs are reported. In the first step, the present work presents a review of hydrodynamic characteristics (circulation of solids in SB, measurement techniques for particle tracking and empirical hydrodynamics, pressure drop, maximum spoutable height, minimum spouting velocity, and diameter of the spout). In the second step, main mathematical models and computational fluid dynamics (CFD) simulation of the SB to predict and analyze different processes are described. Some main mathematical modeling and the recent advances of two fluid methods and discrete element method approaches in CFD simulation of SBs are summarized. In the last step, some new applications of the SB are presented. As the result of this review, we can observe the importance of further development of hydrodynamics structure, working on modeling and related correlations and improve the applications of SBs.
Effect of mineral additives on structure and properties of concrete for pavements
Authors: Sobol Khrystyna , Markiv Taras , Hunyak Oleksii
Published: January 2018

Concrete pavements is an attractive alternative to asphalt pavements because of its lower cost and higher durability. Major contribution to sustainable development can be made by partial replacement of cement in concrete pavement with supplementary cementitious materials of different nature and origin. In this paper, the effect of natural zeolite and perlite additives in complex with chemical admixtures on the structure and properties of concrete for pavement was studied. Compressive and flexural strength test was used to study the mechanical behavior of designed concrete under load. Generally, the compressive strength of both control concrete and concrete containing mineral additives levels at the later ages of hardening. The microstructure analysis of concrete with mineral additives of different nature activity showed the formation of additional amount of hydration products such as tobermorite type calcium hydrosilicate which provide self-reinforcement of hardening concrete system.
PV Module Integrated Dual Boost Isolated dc-dc Converter Fed Three-Phase Micro-Inverter
Authors: Geetha P. , Ravi S.
Published: January 2018

This paper presents a photovoltaic (PV) array-integrated dual-boost isolated dc-dc converter-fed three-phase micro-inverter for the grid-connected applications. The coupled inductor and high-frequency DC link-based dc-dc converter offer more advantages over the conventional dc-dc converters used in the grid-tied PV system. A controller with an inner voltage control loop and frequency control loop is proposed to operate the converters of a micro-inverter to transfer the maximum power generated on the PV array to the grid. The inner voltage control loop generates the required control signal for the proposed high step-up dc-dc converter to extract the maximum possible power from the PV array during sunny days. It eliminates the need of cascaded connection of dc-dc converters. High maximum power point tracking (MPPT) accuracy, high voltage gain, and high energy conversion efficiency can be attained with the proposed system. The principles of operation and design considerations of the dual-boost isolated dc-dc converter-fed micro-inverter are presented. The MATLAB/Simulink simulation results are presented to validate the performance of the proposed three-phase micro-inverter.
Optimization on Additives of SBS and CTR for Interlayer Asphalt in Rigid-Flexible Composite Pavement
Authors: Li Sheng , Yang Fan , Liu Zhao-Hui
Published: January 2018

This research was performed to improve the interlayer shear bond strength of rigid-flexible composite pavement. Based on the laboratory testing, three types of additive, which are listed as follows, were recommended and investigated to determine the optimum content of crumb tire rubber (CTR) and thermoplastic styrene-butadiene-styrene (SBS): 8 % CTR + 3.5 % SBS, 10 % CTR + 3 % SBS, and 12 % CTR + 2.5 % SBS. The high-temperature property and viscous nature of co-modified asphalts were studied by using a rotational rheometer. A temperature sweep and a shear fatigue test were performed to characterize the dynamic mechanical performance and fatigue property of asphalt materials. The results showed that the co-modified asphalt with 8 % CTR and 3.5 % SBS performed best in a viscosity test at 60°C. At 135°C and 165°C, it exhibited good viscous nature and had the desired workability of asphalt pumping and mixing. The high-temperature grade, the dynamic mechanical performance, and fatigue property of this co-modified asphalt were of great satisfaction as well. Therefore, 8 % CTR + 3.5 % SBS is recommended as a modifying material for construction and design purpose.
Optical Gain Spectra in InGaN/GaN Quantum Wells with the Compositional Fluctuations
Authors: Uenoyama Takeshi
Published: January 2018

The compositional fluctuations of the In content were found in InGaN/GaN quantum wells and it caused the localized states by the potential fluctuation. We have evaluated the optical gain of GaN based quantum well structures with localized states. The localized states are treated as the subband states of the quantum disk-like dots in the well. It was found that the inhomogeneous broadening played an important role in the optical gain and that it should be reduced to use the benefit of the localized states for laser oscillations.
Pendeo-Epitaxy of Gallium Nitride and Aluminum Nitride Films and Heterostructures on Silicon Carbide Substrate
Authors: Gehrke T. , Linthicum K. J. , Thomson D. B. , Rajagopal P. , Batchelor A. D. , Davis R. F.
Published: January 2018

Pendeo-epitaxy of individual GaN and AlxGa12−xN films and single- and multi-layer heterostructures of these materials have been achieved on a columnar GaN seed layer using metallorganic vapor phase epitaxy. These structures have been characterized using scanning electron microscopy and atomic force microscopy. The RMS roughness value of the grown side wall plane (110) of these structures was 0.099 nm.
Temperature Effect on the Quality of A1N thin Films
Authors: Thompson Margarita P. , Drews Andrew R. , Huang Changhe , Auner Gregory W.
Published: January 2018

AlN thin films were deposited at various substrate temperatures via Plasma Source Molecular Beam Epitaxy. The films were grown on 6H-SiC (0001) substrates. Reflection High Energy Electron Diffraction and Atomic Force Microscopy showed a dramatic change in the surface morphology of the film grown at 640°C. This is attributed to a change in the growth mechanism from pseudomorphic at lower temperatures to three-dimensional at higher than 640°C temperatures. Photoreflectance measurements showed an absorption shift toward 200 nm as the deposition temperature increases which is attributed to the change in the growth mechanism at higher temperatures. X-Ray Diffraction was unable to conclusively determine the AlN (0002) peak due to a significant diffuse intensity from the SiC (0002) peak. A MIS structure was created by depositing Pt contacts on the film grown at 500°C. I-V measurements showed that the Pt/AlN contact is Schottky.
Cubic GaN Heteroepitaxy on Thin-SiC-Covered Si(001)
Authors: Hiroyama Yuichi , Tamura Masao
Published: January 2018

We have investigated the growth conditions of cubic GaN (β-GaN) layers on very thin SiC-covered Si(001) by using gas-source molecular beam epitaxy as functions of SiC layer thickness, Ga-cell temperature and substrate temperature. Under the present SiC formation conditions on Si substrates by carbonization using C2H2 gas, the SiC layers with the thickness between 2.5 and 4 nm result in the epitaxial growth of β-GaN on thus SiC-formed Si substrates. At the highest GaN growth rate of 110 nm/h ( a Ga-cell temperature of 950 °C), β-GaN layers grown at a substrate temperature of 700 °C show a nearly flat surface morphology and the fraction of included hexagonal GaN becomes negligible when compared to the results of β-GaN layers grown under other conditions of Ga-cell and substrate temperatures. Thus obtained β-GaN films have good performance in photoluminescence intensity although the FWHM of band-edge recombination peak is still wider (137 meV) than the reported values for the β-GaN on 3C-SiC and GaAs.
Pressure Dependence of Optical Transitions in InGaN/GaN Multiple Quantum Wells
Authors: Shan W. , Ager J.W. , Walukiewicz W. , Haller E.E. , McCluskey M.D. , Johnson N.M. , Bour D.P.
Published: January 2018

The effect of hydrostatic pressure on optical transitions in InGaN/GaN multiple quantum wells (MQWs) has been studied. Photoluminescence (PL) and photomodulated transmission (PT) measurements were performed under applied pressure to examine the pressure dependence of optical transitions associated with confined states in MQWs. The PL emission from the MQWs was found to shift linearly to higher energy with applied pressure but exhibit a significantly weaker pressure dependence compared to epilayer samples with similar bandgap energies. Similar pressure coefficients obtained by PT measurements rule out the possibility of PL resulting from deep localized states. We show that the difference in the compressibility of InGaN and of GaN induces a tensile strain in the compressively strained InGaN well layers that partially compensates the applied hydrostatic pressure. This mechanical effect is the primary factor for the smaller pressure dependence of the optical transitions in the InGaN/GaN MQWs. At pressure above 100 kbar, the PL signal in MQWs samples is quenched, indicating that the carriers involved in the radiative recombination processes in the well layers originate primarily from the adjacent GaN layers.
P- and N-Type Doping of MBE Grown Cubic GaN/GaAs Epilayers
Authors: As D.J. , Simonsmeier T. , Busch J. , Schöttker B. , Lübbers M. , Mimkes J. , Schikora D. , Lischka K. , Kriegseis W. , Burkhardt W. , Meyer B.K.
Published: January 2018

P-type doping with Mg and n-type doping with Si of cubic GaN (c-GaN) epilayers is reported. Cubic GaN films are grown by rf-plasma assisted MBE on semi-insulating GaAs (001) substrates at a substrate temperature of 720°C. Elemental Mg and Si are evaporated from thermal effusions cells. Secondary ion mass spectroscopy (SIMS), low temperature photoluminescence (PL) and temperature dependent Hall-effect measurements are used to study the incorporation, optical and electrical properties. A Mg related shallow donor-acceptor transiton at 3.04 eV with an acceptor activation energy of EA= 0.230 eV is observed by low temperature PL. At Mg concentrations above 1018 cm−3 the dominance of a broad blue band indicates that also in c-GaN Mg is incorporated at different lattice sites or forms complexes. Si-doped c-GaN epilayers are ntype with electron concentrations up to 5*1019 cm−3. The incorporation of Si follows exactly the vapor pressure curve of Si, indicating a sticking coefficient of 1 for Si in cGaN. With increasing Si-concentration the intensity of the near-band luminescence continuously increases and broadens.
Optical Investigations of AlGaN on GaN Epitaxial Films
Authors: Steude G. , Christmann T. , Meyer B.K. , Goeldner A. , Hoffmann A. , Bertram F. , Christen J. , Amano H. , Akasaki I.
Published: January 2018

We have investigated AlxGa1−xN /GaN heterostructures (0<x<0.22) grown by metal organic vapor phase epitaxy on sapphire with photoluminescence (PL), reflexion and cathodo-luminescence experiments. The energetic positions of the free A-exciton in GaN and AlGaN as a function of the alloy compositions are deduced from temperature dependent PL and from reflexion measurements. We obtain a small bowing parameter and no evidence for a Stokes shift between absorption and emission. The AlxGa1−xN films induce additional compressive strain on the underlying GaN film. Compositional inhomogeneities are present, but the fluctuations are too small to be important for carrier localisation. The broadening of the luminescence line width in the alloy can be described by statistical disorder of a random alloy.
Electron Beam Induced Impurity Electro-Migration in Unintentionally Doped GaN
Authors: Toth M. , Fleischer K. , Phillips M. R.
Published: January 2018

Electron beam induced electromigration of ON+and H+impurities in unintentionally n-doped GaN was investigated using cathodoluminescence (CL) kinetics profiling, CL imaging of regions pre-irradiated with a stationary electron beam, and wavelength dispersive x-ray spectrometry (WDS). The presented results (i) illustrate induced impurity diffusion in wide bandgap semiconductors, (ii) provide experimental evidence for the (VGa−ON)2−model of yellow luminescence in GaN with low Si content1, (iii) confirm the roles of O in frequently reported bound exciton and donor-acceptor pair emissions and (iv) suggest the involvement of ON+and hydrogenated gallium vacancies in a blue emission in autodoped GaN.
Statistical Analysis of Local Composition and Luminescence in InGaN Grown by Molecular Beam Epitaxy
Authors: Einfeldt S. , Böttcher T. , Hommel D. , Selke H. , Ryder P. L. , Bertram F. , Riemann T. , Rudloff D. , Christen J.
Published: January 2018

InGaN layers grown by molecular beam epitaxy are investigated in terms of their compositional homogeneity using transmission electron microscopy and cathodoluminescence spectroscopy performed with high spatial resolution. Strong fluctuations of the indium content were found in bulklike layers, which could be partially reduced by modulating the indium flux during growth, i. e. by nominally growing a short period GaN/InGaN superlattice. For indium compositions above x ≅ 0.1 this approach fails. Strained InGaN in quantum wells exhibits lateral fluctuations on an atomic scale and on a scale of several hundred nanometers. The results are discussed in view of the origin of inhomogeneous indium incorporation.
Ranges of Deposition Temperatures Applicable for Metalorganic Vapor Phase Epitaxy of Gan Films Via the Technique of Pendeo-Epitaxy
Authors: Thomson D. B. , Gehrke T. , Linthicum K. J. , Rajagopal P. , Davis R. F.
Published: January 2018

Pendeo-epitaxy is a type of selective growth of thin films from the sidewalls of etched forms. The resulting films are suspended from the sidewalls and do not interface with the substrate. This process route has advantages over conventional lateral epitaxial overgrowth (LEO) techniques. In this research, pendeo-epitaxial growth of GaN films has been achieved on elongated GaN seed columns. The seed columns were etched from GaN grown on 6H-SiC (0001) substrates via metalorganic vapor phase epitaxy (MOVPE). Silicon nitride mask layers atop the GaN seed columns forced growth from the sidewalls. Pendeo-epitaxial growth of GaN was investigated using several growth temperatures. Higher growth temperatures resulted in improved coalescence due to greater lateral to vertical growth ratios.
Effects of Susceptor Geometry on Gan Growth on Si(111) with a New MOCVD Reactor
Authors: Gao Yungeng , Gulino Daniel A. , Higgins Ryan
Published: January 2018

High quality GaN films on AlN buffer layers were grown on Si(111) with a new, commercial, two-injector vertical rotating disk MOCVD reactor (CVD, Inc.). It was found that the geometry of the susceptor greatly affected the structural quality of the epilayers on Si. For the original susceptor geometry, though single crystal GaN films could be obtained, the films were dark gray in appearance with a rough morphology, and the best x-ray rocking curve FWHM was 2.33°. After modifying the susceptor geometry, transparent, mirror-like single crystal GaN films were obtained with the best x-ray rocking curve FWHM being 0.24°. Photoluminescence (PL) and infrared reflectance (IR) spectra of the grown films were compared. The film growth rate was found to increase with decrease of the growth pressure. A 2-D simulation of the flow, heat transfer, and chemical species transport in the reactor showed a more symmetric flow, larger velocity gradient, and lower upward velocity with the modified susceptor, which may be the main reason for the improvement of the structural quality of the films.
Properties of Epitaxial Zno Thin Films for Gan and Related Applications
Authors: Shen H. , Wraback M. , Pamulapati J. , Liang S. , Gorla C. , Lu Y.
Published: January 2018

In this paper, we present a detailed study of high quality (110) ZnO films, epitaxially grown on R-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). The epitaxial relationships are (11
0 ) ZnO//( 01 2 ) Al2O3 and [0001] ZnO//[011] Al2O3 as confirmed by X-ray diffraction (θ-2θ, and ϕ-scan) and high-resolution cross-sectional transmission electron microscopy (HR-TEM). Low temperature photoluminescence (PL) indicates the ZnO thin films are almost strain free. Optical absorption and reflection measurements with linearly polarized light indicate a strong optical anisotropy. The polarization rotation towards the C-axis associated with the optical anisotropy is utilized to demonstrate an optically addressed ultra-fast, ultraviolet light modulator.
GaN CVD Reactions: Hydrogen and Ammonia Decomposition and the Desorption of Gallium
Authors: Bartram Michael. E. , Creighton J. Randall
Published: January 2018

Isotopic labeling experiments have revealed correlations between hydrogen reactions, Ga desorption, and ammonia decomposition in GaN CVD. Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) were used to demonstrate that hydrogen atoms are available on the surface for reaction after exposing GaN(0001) to deuterium at elevated temperatures. Hydrogen reactions also lowered the temperature for Ga desorption significantly. Ammonia did not decompose on the surface before hydrogen exposure. However, after hydrogen reactions altered the surface, N15H3 did undergo both reversible and irreversible decomposition. This also resulted in the desorption of N2 of mixed isotopes below the onset of GaN sublimation. This suggests that the driving force of the high nitrogen-nitrogen bond strength (226 kcal/mol) can lead to the removal of nitrogen from the substrate when the surface is nitrogen rich. Overall, these findings indicate that hydrogen can influence GaN CVD significantly, being a common factor in the reactivity of the surface, the desorption of Ga, and the decomposition of ammonia.
Effect of Buffer Layer and III/V Ratio on the Surface Morphology of Gan Grown by MBE
Authors: Piquette E. C. , Bridger P. M. , Beach R. A. , McGill T. C.
Published: January 2018

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS.
Relaxation Phenomena in GaN/ AlN/ 6H-SiC Heterostructures
Authors: Edwards N.V. , Batchelor A.D. , Buyanova I.A. , Madsen L.D. , Bremser M.D. , Davis R.F. , Aspnes D.E. , Monemar B.
Published: January 2018

We have developed a method to modulate the strain state (normally > 4 kbar, tensile) of moderately thick (∼2 μm) GaN based structures grown on 6H-SiC to a range 0 to -2 kbar of compressive stresses by introducing a strain-mediating layer (SML) above the standard high-temperature AlN buffer layer. The strain characteristics of subsequently deposited nitride layers can be modulated by changing the growth parameters of the SML layer. This is achieved by in-situ techiniques during crystal growth without degrading the optical and structural properties of the deposited layers.
TEM Study of Defects in Laterally Overgrown GaN Layers
Authors: Liliental-Weber Z. , Benamara M. , Swider W. , Washburn J. , Park J. , Grudowski P. A. , Eiting C. J. , Dupuis R. D.
Published: January 2018

Transmission electron microscopy was applied to study defects in laterally overgrown GaN layers, with initial growth on Al2O3 substrates followed by further growth over SiO2 masks. Dislocations found in the overgrown areas show changes in line direction. Most dislocations propagate along c-planes. In the overgrown material planar defects (faulted loops) are present on c-planes and their presence is most probably related to segregation of excess point defects and impurities present in this material. They appear to be initiated by the fast lateral growth. Some dislocations with screw orientation become helical resulting from climb motion.

Formation of voids and also a high dislocation density was observed at the boundaries where two overgrowing fronts meet. Tilt and twist components were observed for these boundaries that were different for different overgrown strips grown in the same crystallographic direction suggesting that the GaN subgrain orientations on the two sides of a SiO2 mask are responsible for the final tilt and twist value.
Epitaxial Lateral Overgrowth of GaN with Chloride-Based Growth Chemistries in Both Hydride and Metalorganic Vapor Phase Epitaxy
Authors: Zhang R. , Zhang L. , Hansen D.M. , Boleslawski Marek P. , Chen K.L. , Lu D.Q. , Shen B. , Zheng Y.D. , Kuech T.F.
Published: January 2018

Epitaxial lateral overgrowth (ELO) of GaN on SiO2-masked (0001) GaN substrates has been investigated by using chloride-based growth chemistries via hydride vapor phase epitaxy (HVPE) and metal organic vapor phase epitaxy (MOVPE). Diethyl gallium chloride, (C2H5)2GaCl, was used in as the MOVPE Ga precursor. The lateral and vertical growth rates as well as the overgrowth morphology of ELO GaN structures are dependent on growth temperature, V/III ratio and the in-plane orientation of the mask opening. A high growth temperature and low V/III ratio increase the lateral growth rate and produce ELO structures with a planar surface to the GaN prisms. High-quality coalesced and planar ELO GaN has been fabricated by both growth chemistries. The use of the diethyl gallium chloride source allows for the benefits of HVPE growth to be realized within the MOVPE growth environment.
Computational Materials Science, an Increasingly Reliable Engineering Tool: Anomalous Nitride Band Structures and Device Consequences
Authors: Sher A. , van Schilfgaarde M. , Berding M. A. , Krishnamurthy S. , Chen A.-B.
Published: January 2018

Computational materials science has evolved in recent years into a reliable theory capable of predicting not only idealized materials and device performance properties, but also those that apply to practical engineering developments. The codes run on workstations and even now are fast enough to be useful design tools. A review will be presented of the current status of this rapidly advancing field. Examples of the accuracy of the codes are displayed by comparing the predicted atomic volumes, and cohesive and excess energies of several materials with experiment. As a further demonstration of the methods, the band structures of AlN, GaN, and InN in wurtzite and zinc blende structures will be presented. There are anomalies in the conduction and valence bands of these materials. Some consequences on light emitting and power devices made from these materials will be examined.
Interface Effects on the Persistent Photoconductivity in Thin GaN and AlGaN Films
Authors: Seifert O. P. , Kirfel O. , Munzel M. , Hirsch M. T. , Parisi J. , Kelly M. , Ambacher O. , Stutzmann M.
Published: January 2018

Thin films of GaN and its alloy AlGaN are investigated with respect to their properties of the persistent photoconductivity (PPC). In this work, we show that the film-substrate interface plays an important role for the metastable electrical effect. Strongly absorbed bandgap light causes an increase of photoconductivity which is about one order of magnitude higher when the sample is illuminated from the substrate side near the interface than from the growth side. To access the interface properties at the substrate, we use temperature-dependent Hall effect measurements. The smallest PPC effect was observed for the GaN film with the best interface properties grown on SiC.
Characterization of Flicker Noise in GaN Based Modfets at low Drain Bias
Authors: Ho W.Y. , Fong W.K. , Surya C. , Tong K.Y. , Kim W. , Botcharev A. , Morkoc H.
Published: January 2018

We report systematic characterizations of flicker noise in GaN based MODFETs. Flicker noise was measured across the channel of the devices from room temperature to 130 K. The voltage noise power spectra, SV(f) were found to be proportional to 1/fγ, where γ depends on the device temperature as well as the gate bias. Study of SV(f) as a function of the biasing condition was conducted in detail and was found to vary as VD2/(VG−VT)β where β changes with temperature from about 2.1 at room temperature to about 0.9 at 130K. Analyses of the data showed that the noise originated from thermal activation of carriers to localized states in the channel area. The data suggested that the trapping and detrapping of carriers did not lead to fluctuations in the carrier concentration as postulated in the McWhorter’s model. However, more work is needed to determine if surface mobility fluctuations played key role in the 1/f noise.
Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth
Authors: Coltrin Michael E. , Willan Christine C. , Bartram Michael E. , Han Jung , Missert Nancy , Crawford Mary H. , Baca Albert G.
Published: January 2018

Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1μm nucleation zones. Although emission is comprised of both UV (∼365nm) and yellow (∼550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.
Electrical Characterization of Sputter Deposition Induced Defects in n-GaN
Authors: Auret F. D. , Goodman S. A. , Koschnick F. K. , Spaeth J.-M. , Beaumont B. , Gibart P.
Published: January 2018

We have used current-voltage (I-V) measurements to assess and compare the electrical characteristics of resistively evaporated and sputter deposited Au Schottky contacts on epitaxially grown GaN. These I-V measurements revealed that resistively deposited Au contacts exhibited excellent rectification properties: high barrier height, low reverse current and good ideality factor (n = 1.04). In contrast, sputter deposited contacts had poor characteristics: low barrier height, high reverse current and non-linear forward I-V characteristics. The cause of this is thought to be defects introduced at and near the surface during sputter deposition. Deep level transient spectroscopy (DLTS) showed that at least four defects, with energy levels at 0.22±0.02 eV, 0.30±0.01 eV, 0.40±0.01 eV and 0.45±0.10 eV below the conduction band, were introduced in the GaN during sputter deposition. The first of these defects has similar electronic properties as a radiation induced defect in GaN, speculated to be the nitrogen vacancy, while the second appears to be the same as a defect in the as-grown material. The latter two defects have not previously been observed in as-grown or processed epitaxial GaN.
Effect of Oxygen Ion Implantation in Gallium Nitride
Authors: Jiang W. , Weber W.J. , Thevuthasan S. , Exarhos G.J. , Bozlee B.J.
Published: January 2018

Epitaxial single crystal GaN films (2.0 μm thick) were implanted 60° off the <0001> surface normal with 600 keV O+ ions at 190 or 210 K over a range of ion fluences from 4.8×1017 to 5.0×1020 ions/m2. The implantation damage, as determined by in-situ Rutherford Backscattering Spectrometry in a <0001> channeling geometry (RBS/C), ranged from dilute defects up to the formation of a disorder saturation state that was not fully amorphous. The relative disorder on the Ga sublattice exhibited a sigmoidal dependence on ion fluence. Results show that GaN crystals are extremely resistant to the ion implantation damage as compared to other ceramic materials like SiC. An asymmetric shape in the angular scan curve around the <0001> axis, which might be associated with the Ga lattice distortion in the crystal structure, was observed for the as-irradiated material to the highest ion fluence (5.0×1020 O+/m2) at 210 K. Comparisons of Ga disorder depth-profiles from the experiment and SRIM97 simulations suggest that the damage peaks shift to greater depths at the low irradiation temperature (210 K). Significant recovery of these defects was not observed in the isochronal annealing steps (20-min) up to 970 K.
Thermal Annealing of InGaN/GaN Strained-Layer Quantum Well
Authors: Chan Michael C.Y. , Tsang Kwok-On , Li E. Herbert , Denbaars Steven P.
Published: January 2018

Quantum well (QW) material engineering has attracted a considerable amount of interest from many people because of its ability to produce a number of optoelectronic devices. QW composition intermixing is a thermal induced interdiffusion of the constituent atoms through the hetero-interface. The intermixing process is an attractive way to achieve the modification of the QW band structure. It is known that the band structure is a fundamental determinant for such electronic and optical properties of materials as the optical gain, the refractive index and the absorption. During the process, the as-grown square-QW compositional profile is modified to a graded profile, thereby altering the confinement profile and the subband structure in the QW. The blue-shifting of the wavelength in the intermixed QW structure is found in this process.

In recent years, III-nitride semiconductors have attracted much attention. This is mainly due to their large bandgap range from 1.89eV (wurtzite InN) to 3.44eV (wurtzite GaN). InGaN/GaN quantum well structures have been used to achieve high lumens blue and green light emitting diodes. Such structures also facilitate the production of full colour LED displays by complementing the colour spectrum of available LEDs.

In this paper, the effects of thermal annealing on the strained-layer InGaN/GaN QW will be presented. The effects of intermixing on the confinement potential of InGaN/GaN QWs have been theoretically analysed, with sublattices interdiffusion as the basis. This process is described by Fick’s law, with constant diffusion coefficients in both the well and the barrier layers. The diffusion coefficients depend on the annealing temperature, time and the activation energy of constituent atoms. The optical properties of intermixed InGaN/GaN QW structure of different interdiffusion rates have been theoretically analyzed for applications of novel optical devices. The photoluminescence studies and the intermixed QW modeling have been used to understand the effects of intermixing.
XPS Study of Oxygen Adsorption on (3×3) Reconstructed MBE Grown Gan Surfaces
Authors: Beach R. A. , Piquette E. C. , McGill T. C. , Watson T. J.
Published: January 2018

The incorporation of oxygen onto the (3×3) reconstructed surface of GaN(0001) has been studied using X-ray Photoelectron Spectroscopy (XPS). It was found that the (3×3) reconstruction corresponds to a fractional Ga adlayer atop a Ga terminated GaN surface. Our measurements indicate a surface coverage of 1.15 ± 0.2 monolayers of relaxed Ga on the surface. The binding energy separation between the relaxed surface Ga3d core level and bulk Ga3d level was measured to be 1.1 ± 0.1 eV. A metallic component extending from the bulk GaN valence band maximum out to 0 eV was also present in the XPS spectrum. The separation between the bulk valence band maximum and the Fermi level of the metallic component was found to be 2.1 ± 0.1 eV. The relaxation of the surface Ga was found to decrease with oxygen exposure indicating Ga-O bonding, with oxygen adsorption terminating at 1.3 ± 0.2 monolayers. The O1s core level was found to have a FWHM of 2.0 ± 0.1 eV.
Electrical Characterization of MOVPE-Grown P-Type GaN:Mg Against Annealing Temperature
Authors: Fujita Shizuo , Funato Mitsuru , Park Doo-Cheol , Ikenaga Yoshifumi , Fujita Shigeo
Published: January 2018

Hall effect measurements have been applied for the electrical characterization of p-type Mg-doped GaN grown by metalorganic vapor-phase epitaxy on sapphire substrates in terms of annealing temperature for dehydrogenation (N2 annealing) and hydrogenation (H2 annealing) of the acceptors. With the N2 annealing temperature from 600 to 900 °C for dehydrogenation, both hole concentration and mobility increases, showing more activation of acceptors and less incorporation of unfavorable scattering centers probably originating from Mg-H bondings. The N2 annealing at higher than the growth temperature results in reduced hole concentration, but the mobility gets higher. Some defects compensating acceptors may be induced at high temperature annealing, but they seem to be no scattering centers and be inactivated by successive hydrogenation and re-dehydrogenation at the optimum dehydrogenation temperature 900 °C. The electrical degradation of GaN due to thermal damage is not very destructive and can be well recovered by annealing treatments.
Behavior of W and WSix Contact Metallization on n- and p- Type GaN
Authors: Cao X. A. , Ren F. , Lothian J. R. , Pearton S. J. , Abernathy C. R. , Zolper J. C. , Cole M. W. , Zeitouny A. , Eizenberg M. , Shul R. J.
Published: January 2018

Sputter-deposited W-based contacts on p-GaN (NA∼1018 cm−3) display non-ohmic behavior independent of annealing temperature when measured at 25°C. The transition to ohmic behavior occurs above ∼250°C as more of the acceptors become ionized. The optimum annealing temperature is ∼700°C under these conditions. These contacts are much more thermally stable than the conventional Ni/Au metallization, which shows a severely degraded morphology even at 700°C. W-based contacts may be ohmic as-deposited on very heavily doped n-GaN, and the specific contact resistance improves with annealing up to ∼900°C.
Contact Resistance of InGaN/GaN Light Emitting Diodes Grown on the Production Model Multi-Wafer MOVPE Reactor
Authors: Chuang R.W. , Zou A.Q. , Lee H.P. , Dong Z.J. , Xiong F.F. , Shih R. , Bremser M. , Juergensen H.
Published: January 2018

We report both the device fabrication and characterization of InGaN/GaN single quantum well LEDs grown on sapphire substrates using multi-wafer MOVPE reactor. To improve current spreading of the LEDs, a self-aligned process is developed to define LED mesa that is coated with a thin, semi-transparent Ni/Au (40 Å/40 Å) layer. A detailed study on the ohmic contact resistance of Ni/Cr/Au on p-GaN versus annealing temperatures is carried out on transmission line test structures. It was found that the annealing temperatures between 300 to 500 °C yield the lowest specific contact resistance rc ( 0.016 Ω-cm2 at a current density of 66.7 mA/cm). Based on the extracted rc from the transmission line measurement, we estimate that the contact resistance of the p-type GaN accounts for ∼ 88% of the total series resistance of the LED.
Ensemble Monte Carlo Study of Electron Transport in Bulk Indium Nitride
Authors: Bellotti E. , Doshi B. , Brennan K. F. , Ruden P. P.
Published: January 2018

Ensemble Monte Carlo calculations of electron transport at high applied electric field strengths in bulk, wurtzite phase InN are presented. The calculations are performed using a full band Monte Carlo simulation that includes a pseudopotential band structure, all of the relevant phonon scattering agents, and numerically derived impact ionization transition rates. The full details of the first five conduction bands, which extend in energy to about 8 eV above the conduction band minimum, are included in the simulation. The electron initiated impact ionization coefficients and quantum yield are calculated using the full band Monte Carlo model. Comparison is made to previous calculations for bulk GaN and ZnS. It is found that owing to the narrower band gap in InN, a lower breakdown field exists than in either GaN or ZnS.
Symmetry of Electrons and Holes in Lightly Photo-Excited InGaN LEDs
Authors: Kennedy T.A. , Glaser E.R. , Carlos W.E. , Ruden P.P. , Nakamura Shuji
Published: January 2018

The symmetry of the recombining electrons and holes in lightly photo-excited InGaN LEDs revealed through ODMR is related to the physical structure, band structure and defects present. Calculations of the electron-g within the k• p formalism give the average shift from the free-electron value for GaN but are not fully reconciled with the anisotropy. This theory is also extended to InGaN alloys for both pseudomorphic and relaxed layers. The average shift is close to the experimental values for the green LED. The strongly reduced hole anisotropies seen experimentally are explained by a recently published theory for acceptors in GaN.
Fabrication of Smooth GaN-Based Laser Facets
Authors: Stocker D. A. , Schubert E. F. , Boutros K. S. , Redwing J. M.
Published: January 2018

A method is presented for fabricating fully wet-etched InGaN/GaN laser cavities using hotoenhanced electrochemical wet etching followed by crystallographic wet etching. Crystallographic wet chemical etching of n- and p-type GaN grown on c-plane sapphire is achieved using H3PO4 and various hydroxides, with etch rates as high as 3.2.μm/min. The crystallographic GaN etch planes are {0001}, {100}, {10}, {10}, and {103}. The vertical {100} planes appear perfectly smooth when viewed with a field-effect scanning electron microscope (FESEM), indicating a surface roughness less than 5 nm, suitable for laser facets. The etch rate and crystallographic nature for the various etching solutions are independent of conductivity, as shown by seamless etching of a p-GaN/undoped, high-resistivity GaN homojunction.
Patterning III-N Semiconductors by Low Energy Electron Enhanced Etching (LE4)
Authors: Gillis H.P. , Christopher M.B. , Martin K.P. , Choutov D.A.
Published: January 2018

Fabricating device structures from the III-N wide bandgap semiconductors requires anisotropoic dry etching processes that leave smooth surfaces with stoichiometric composition after transferring high-resolution patterns with vertical sidewalls. The purpose of this article is to describe results obtained by a new low-damage dry etching technique that provides an alternative to the standard ion-enhanced dry etching methods in meeting these demands for processing the III-N materials.