bluuebunny/crossref_metadata_2025_split huggingface dataset:
Influence of Growth Conditions on Phase Separation of InGaN Bulk Material Grown by MOCVD
Authors: Huang Yong , Jani Omkar , Park Eun Hyun , Ferguson Ian 
Published: April 2011

ABSTRACTDifferent types of phase separation in thick InGaN layers were studied using photoluminescence (PL) and x-ray diffraction (XRD). InGaN films of 100 nm in thickness were grown on 2 μm GaN templates with an In molar fraction ranging from 0% to 20% by metal organic chemical phase deposition (MOCVD). It is shown that suppression of the phase separation in InGaN can be made possible by increasing the TMIn flow rate, decreasing the layer thickness and decreasing the growth rate. Based on the results, two types of phase separation, microscopic quantum dots and macroscopic domains, are proposed accordingly. The influence of the growth conditions on each type is summarized respectively in this paper.
Deep centers in GaN layers grown on epitaxial lateral overgrowth templates by metalorganic chemical vapor deposition
Authors: Chevtchenko Serguei A , Xie J. , Fu Y. , Ni X. , Morkoç H. 
Published: April 2011

ABSTRACTThe dependence of traps and their concentration in GaN on the quality of templates, on which the layers are grown, has been studied by deep-level transient spectroscopy (DLTS). Thin GaN layers studied were grown on GaN templates employing conventional epitaxial lateral overgrowth (ELO) and nano-ELO with SiNx nanonetwork. The concentrations of traps found in these layers were compared with a reference sample grown on a standard GaN template not utilizing ELO. Two traps A (0.55 eV – 0.58 eV) and B (0.20 eV – 0.23 eV) were detected in the temperature range from 80 K to 400 K. A reduction of traps in layers grown on the ELO and nano-ELO templates was noted. We attribute this reduction to the reduction of threading dislocation density and as a result reduced capture of point defects and complexes as part of dislocation core structure and/or reduced formation of defects and complexes in the vicinity of line defects where the formation can be energetically favorable.
Characterization of Non-Polar Surfaces in HVPE Grown Gallium Nitride
Authors: Lai Kun-Yu , Grenko Judith A. , Wheeler V. D. , Johnson Mark , Preble E. A. , Williams N. Mark , Hanser A. D. 
Published: April 2011

ABSTRACTNon-polar surfaces of HVPE grown GaN were characterized by cathodoluminescence (CL), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). Both of a- and m-plane GaN were prepared by growing thick GaN along the c-axis, and cutting in transverse orientations. The exposed non-polar surfaces were prepared by mechanical polishing (MP) and chemically mechanical polishing (CMP). Non-uniform luminescent characteristics on a- and m-plane GaN were observed in CL images, indicating a higher concentration of impurities in the area of more luminescence. CL spectra from the bulk samples revealed two peaks: 364 nm and 510 nm, related to band edge and impurity defects respectively. The detection by SIMS confirmed that oxygen was inhomogeneously incorporated during the growth of thick GaN layers. Surface qualities of a- and m-plane GaN were also investigated. The lower optical intensities from a-plane GaN at low acceleration voltages indicated more surface damages were introduced during polish. The optical intensity difference from the two samples was reduced at higher acceleration voltages. Similar CL intensities at low acceleration voltages from a- and m-plane GaN substrates prepared by CMP indicated improved surface qualities.
Thermodynamic Analysis of Impurities in the Sublimation Growth of AlN Single Crystals
Authors: Du Li , Edgar James H 
Published: April 2011

ABSTRACTThe vapor phase species responsible for the transport of impurities in the sublimation-recondensation growth of bulk AlN crystals was predicted by thermodynamic analysis. AlN powder containing oxygen was investigated in Al-O-N system for an inert reactor. Dialuminum monoxide (Al2O) is strongly favored over all other possible oxygen containing species including NO and NO2. For AlN crystal growth in a graphite furnace, the Al-O-C-N system was studied. CO is the main species containing carbon and oxygen, and has a partial pressure more than one hundred times higher than all other carbon or oxygen containing species. Its partial pressure even exceeds that of Al vapor. Pure AlN growth on SiC seed was represented in the Al-N-Si-C system. SiC is not stable at high temperatures, the presence of nitrogen accelerates the decomposition of the SiC, and the most probable volatile silicon and carbon species originating from the SiC seed are Si, CN and C2N2.
Optical Properties of Nearly Lattice-matched AlInN/GaN Single Quantum Wells with Varying Well-widths
Authors: Tan Lay-Theng , Martin Robert W , Watson Ian M , O'Donnell Kevin P 
Published: April 2011

ABSTRACTSingle GaN quantum wells in the nearly lattice-matched GaN/Al1−xInxN system have been studied using photoluminescence (PL) and PL excitation (PLE) spectroscopy. The structures were grown on free-standing GaN and sapphire substrates. Selectively excited PL is able to distinguish luminescence originating from the wells, barriers and the underlying GaN buffer layers. The PL spectra show that the quantum well transition energy decreases as the well-width increases. This manifestation of the quantum confined Stark effect (QCSE) results from intense spontaneous polarization fields, which are present even in the absence of strain in the QW layer. Power dependent PL measurements provide information on the screening of the internal fields. PLE data provide an estimation of the band gap and enable one to determine the energy shift between emission and absorption in the Al1−xInxN barriers.
Nanoparticle-loaded encapsulation materials for light-emitting diode applications
Authors: Mont Frank Wilhelm , Luo H. , Schubert M. F. , Kim J. K. , Schubert E. F. , Siegel R. W. 
Published: April 2011

ABSTRACTNanoparticle-loaded encapsulants provide unique optical and material properties for the enhancement of light extraction efficiency in light-emitting diodes (LEDs). We report on the uniform dispersion of TiO2 nanoparticles with average diameter of 40 nm in epoxy, and the demonstration of a refractive index (n)of 1.68 at 400 nm wavelength, higher than that of pure epoxy (n = 1.53). It is found that proper chemical surfactants and nanoparticle preparation are critical to eliminate agglomeration of nanoparticles. Theoretical analysis of optical scattering in nanoparticle-loaded encapsulation materials reveals that although the size and loading factor of nanoparticles greatly influence scattering, specular transparency of the encapsulant film occurs if the thicknesses of the films are kept below the optical scattering length. Furthermore, the encapsulants benefit from an optimized scattering coefficient as demonstrated by three-dimensional ray-tracing simulations showing light extraction efficiency enhancements greater than 50%.
Comparison between the Photoluminescence Mappings Under Selective and Non-selective Excitation and the Electroluminescence Mappings of the Epitaxial Wafers with InGaN-LED Structure
Authors: Tadatomo Kazuyuki , Shimoike Osamu , Noda Hiromichi , Hiraoka Masahiro , Yoshimura Kazumasa , Hoshino Katsuyuki 
Published: April 2011

ABSTRACTWe compared the photoluminescence (PL) mappings of epitaxial wafers for light emitting diodes (LEDs) by using a He-Cd laser (325 nm line) and a laser diode (LD) with peak wavelength of 405 nm as excitation sources and the electroluminescence (EL) mappings of the same wafers. The samples were epitaxial wafers for blue and green InGaN-LEDs obtained in commercial. The wafers were fabricated into LEDs with a Ni/Au transparent p-type electrode and a Ti/Ni n-type electrode after the PL mapping measurements. The He-Cd laser performed the band to band excitation of (Al)GaN cladding and contact layers (non-selective excitation). Because the photo-excited carriers at the cladding and contact layers diffused into the multi-quantum wells (MQWs) and contributed the PL emission by radiative recombination in the MQWs, the PL mapping under the influence of the (Al)GaN cladding and contact layers was obtained. On the other hand, the LD (405 nm) enable us to obtain the PL mapping under selective excitation of the MQWs without the influence of the cladding and contact layers. The PL mapping measurements were carried out at room temperature (RT) at the excitation power density of 310 W/cm2 under non-selective excitation (by the He-Cd laser) and at that of 11.5 W/cm2 under selective excitation (by the LD). The EL mapping was measured at a forward current of 20 mA at RT. The area of the wafer with high EL intensity was coincident with the area with the high PL intensity under selective excitation. Therefore, the PL mapping measurement under selective excitation of MQWs is recommended to characterize the epitaxial wafers and to estimate the device performance of InGaN-LEDs.
500 K operation AlGaN/GaN HFETs with a large current and a high breakdown voltage
Authors: Kambayashi Hiroshi , Li Jiang , Ikeda Nariaki , Yoshida Seikoh 
Published: April 2011

ABSTRACTIt is reported that we demonstrated a large current operation AlGaN/GaN HFET with a low-on state resistance and a high breakdown voltage operation at room temperature and 500 K. We developed our unique ohmic electrode using Ti/AlSi/Mo. In addition, we investigated the dependence between the distance from the gate electrode to the drain electrode and the off-state breakdown voltage. As a result, the breakdown voltage of a unit HFET was over 1100 V. Furthermore, on the large scale HFET with the gate width of 240 mm, the maximum drain current of over 50 A was obtained at room temperature and also, that of over 25 A was obtained at 500 K. The off-state breakdown voltage was obtained about 800 V at room temperature and about 600 V at 500 K, although Si-based FETs can not operate in such a high temperature.
Influence of Overheating Effect on Transport Properties of AlGaN/GaN Heterostructures
Authors: Kurakin Andriy , Vitusevich Svetlana , Petrychuk Mykhailo , Hardtdegen Hilde , Danylyuk Serhiy , Belyaev Alexander , Klein Norbert 
Published: April 2011

ABSTRACTAlGaN/GaN heterostructures can operate at very high power in dc and rf regimes due to the superior properties of III-nitride materials. At the same time, the higher level of dissipated power causes higher overheating in the structures. Considering the fact that the temperature is a driving force for aging processes in devices, it is clear that the investigations of thermal effects in 2DEG of GaN-based structures are of great importance. Additionally, such studies result in a better understanding of the transport features which affect device performance, and allow us to determine the conditions under which hot-electron effects play the dominant role in electron transport. In this report we focus on thermal effects in transmission line model (TLM) patterns. The experimental results and numerical simulations of self-heating effects in TLM patterns processed on heterostructures with different layer design (different Al mole fraction of barrier layer, different substrates) are analyzed. The GaN-based heterostructures were grown by MOCVD. One of the investigated samples consists of a nucleation layer of AlGaN (16% Al), an undoped GaN buffer layer and an AlGaN (33%) undoped barrier layer. This type of structure was formed on sapphire and SiC substrates. Additionally, another two types of structures were grown on sapphire substrate: with wide bandgap barrier layer (with Al mole fraction of 75%), and with a thin AlN spacer layer (5nm) in between the AlGaN barrier layer (23 nm) and the GaN buffer layer (1300 nm). All samples were processed with TLM patterns with standard Ti/Al/Ti/Au contact metallization and annealed for 40s at 800° C. The conducting channel of the devices has a width W = 100 μm with intercontact lengths L of 1, 5, 10, 15, 20, 25, 30, 35 μm. The average overheating temperature over the active channel area for each structure type is estimated. The strong influence of the Al mole fraction of the barrier layer and also the substrate type of the structure on noise and transport properties is revealed and analyzed. It was shown that the implementation of wide bandgap barriers and high thermal conductance substrates allows us to improve the overall performance of the structures significantly. Optimal conditions for the observation of hot-electron effects are determined.
Fully Unstrained GaN on Thick AlN Layers for MEMS Application
Authors: Tonisch Katja , Niebelschuetz Florentina , Cimalla Volker , Romanus Henry , Ambacher Oliver 
Published: April 2011

ABSTRACTUsually, the fabrication of microelectromechanical systems (MEMS) requires unstrained or tensile strained active layers on a selectively removable sacrificial layer. Compressive strain would lead to instabilities due to buckling effects. For group III-nitride based MEMS, AlN is a promising material for sacrificial layers since it can be epitaxially overgrown and etched selectively to GaN. However, due to the larger lattice constants GaN is growing compressively strained on AlN. Nanoheteroepitaxy opens a way to yield unstrained, high quality epitaxial GaN layers on nanocrystalline AlN thin film by means of a 3D strain relaxation mechanism. For this purpose sputtered nanocrystalline AlN films were overgrown with single crystalline GaN and AlGaN/GaN layers by metalorganic chemical vapor deposition. The high quality of the layers is proven by an atomically flat surface and a 2D electron gas at the interface of the AlGaN/GaN heterostructure.
Assembling the Building Blocks for Diamond Electronics
Authors: Yost William Joseph 
Published: April 2011

ABSTRACTThe desire to exploit the extreme properties which differentiate diamond from other, more mature wide bandgap technologies has recently been given further impetus by the development of high quality single crystal CVD diamond material [1].To realise the significant potential of diamond devices over existing device technology depends on completing a number of key objectives, in particular providing:(a). access in volume to high quality, ultra-high purity, single crystal material,(b). the capability to provide carriers by doping the material in a controlled manner,(c). the ability to process thin layers and structures.Providing access to bulk single crystal diamond (albeit not electronic grade material) has already been largely achieved and plates are commercially available for cutting applications [2]. Routes to providing suitable charge carriers are being widely investigated. Although intrinsic diamond can have exceptional electronic properties [1], in reports of both p-type and n-type diamond [3,4] the dopants are very deep (0.37 eV and 0.6 eV for boron and phosphorous respectively), which limits the realisation of conventional electronic devices operating at ambient temperatures.A series of novel devices undergone preliminary experimental evaluation. Devices made up of boron and intrinsic layers, where the boron concentration exceeds the limit of metallic conduction (>1×1020 cm−3), offer carrier diffusion at room temperature from the highly doped regions with low mobility, into adjacent regions of intrinsic material with high carrier mobility [5]. To provide the required device performance, the interface between the doped and intrinsic layers needs to be defect free and to change doping levels by several orders of magnitude in a few atomic layers.Although progress over the last few years has been rapid, there remain substantial technical challenges ahead for the realisation of large scale diamond active electronics. This paper will identify and review progress against these key issues.
Color Centers in Diamond as Practical Single-Photon Source to Illustrate Quantum Complementarity
Authors: Jacques Vincent , Regnnie Steven , Chauvat Dominique , Roch Jean-François 
Published: April 2011

ABSTRACTA recent experiment performed by S. S. Afshar [reviewed in M. Chown, New Scientist183, 30 (2004)] has been interpreted as a possible violation of the complementarity principle of quantum mechanics. Starting from a single-photon wavefront-splitting interference experiment, we propose a new scheme for Afshar's experiment, and we show that Afshar's interpretation is incorrect. Furthermore, this design is well suited to illustrate the complementarity inequality in the interesting intermediate regimes with partial fringe visibility and partial which-path information.
Improving Diamond Device Performances for Pulsed Mode Detection
Authors: Hamrita Hassen , Tromson Dominique , Descamps Caroline , Mer Christine , Nesladek Milos , Bergonzo Philippe 
Published: April 2011

ABSTRACTCVD diamond combines attractive properties for the fabrication of detection devices operating in specific environments. One inherent problem however with diamond is the presence of defect levels that are altering the detection characteristics. They result in unstable responses and carrier losses. One of the issues can be to operate the devices in the pulsed mode regime, when transitory effects are less impacted by defective levels evolution than for steady state currents. This therefore implies the use of materials that are faster, but also of lower carrier lifetime, and therefore generally agreed to be of poorer quality. This has therefore to be compromised with respect to the desired performances.
High Efficiency Heat Sinks from Polycrystalline Diamond Grown by CVD Method
Authors: Voronov Oleg A. , Tompa Gary S. , Veress Veronika 
Published: April 2011

ABSTRACTWhile absolute power levels in microelectronic devices are relatively modest (a few tens to a few hundred watts), heat fluxes can be significant (through 50 W/cm2 in current electronic chips and up to 2000 W/cm2 in semiconductor lasers). Diamond heat sinks enable heat transfer rates well above what is possible with standard thermal management devices. We have fabricated heat sinks using diamond, which has the highest temperature thermal conductivity of any known material. Polycrystalline diamonds manufactured by chemical vapor deposition (CVD) are machined by laser and combined with metallic or ceramic tiles. Cooling by fluid flow through micro-channels enhances heat removal. These unique attributes make diamond based heat sinks prime contenders for the next generation of high heat load sinks. Such devices could be utilized for efficient cooling in a variety of applications requiring high heat transfer capability, including semiconductor lasers, microprocessors, multi-chip modules in computers, laser-diode arrays, radar systems, and high-flux optics, among other applications. This paper will review test designs, heat flux measuring system, and measured heat removal values.
Low Threshold Field Emission from Amorphous Carbon Films Grown by Electrochemical Deposition
Authors: Kiyota Hideo , Higashi Mikiteru , Kurosu Tateki , Iida Masamori 
Published: April 2011

ABSTRACTElectrochemical deposition of amorphous carbon (a-C) film is performed by applying a direct-current potential to substrates immersed in methanol. Both scanning electron microscopy and Raman results indicate that smooth and homogeneous a-C films are grown on specific substrate materials such as Si, Ti, and Al. Field emission measurements demonstrate excellent emission properties such as threshold fields lower than 5 V/μm. Field enhancement factors are estimated to be 1300–1500; these are attributed to local field enhancements around sp2 carbon clusters that are embedded in the a-C films. Emission properties of a-C films grown on Si exhibit a current saturation under higher applied fields. These saturation behaviors are explained by an effect of a potential barrier formed at the interface between a-C films and substrates. Since the interface barrier is reduced by formation of the Ti interfacial layer, an approach to use carbide formation at the interface is verified as useful to improve the emission properties of a-C films.
Surface Bio-functionalization of boron doped diamond
Authors: Bonnauron Mathias , Agnes Charles , Mailley Pascal , Arnault Jean-Charles , de Sanoit Jacques , Mer Christine , Rocha Licinio , Bergonzo Philippe , Nesladek Milos , Omnes Frank , Elmazria Omar 
Published: April 2011

ABSTRACTBiotin was grafted onto boron doped nanocrystaline diamond electrode through a four-step-chemical process. First step of grafting was electro-chemical reduction of p-4,nitro diazonium salts. The grafting was characterized using X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and fluorescence analysis. XPS analysis and curve fitting procedure lead to the identification of different bond states of carbon present in the grafted film and highlights difference of coverage rate between samples. AFM highlights the non-mononuclear aspect of the diazonium electrografting. Fluorescence measurements with avidin recognition prove the efficient of the grafting.
Energetics, Electronic Properties, and Geometries of B-Doped Diamond: A First-Principles Study
Authors: Saito Susumu , Maeda Tomohisa , Miyake Takashi 
Published: April 2011

ABSTRACTWe study electronic and geometrical properties of B-doped diamond in the framework of the density functional theory. The systems studied are simple-cubic (sc) BC7 and face-centered-cubic (fcc) BC15 and B2C14. In the case of BC7, we also study the effect of the tetragonal and rhombohedral distortions of the unit cell to be introduced to the B-doped diamond grown homoepitaxially on the diamond (100) and (111) films, respectively. In the case of fcc B2C14, on the other hand, the inhomogeneous doping effects are discussed from the viewpoint of the energetics. It is confirmed that in the homogeneous doping case, B doping gives rise to hole doping into the valence band, which should be responsible for the metallic and superconducting transport properties of the material. In the case of the inhomogeneous doping, two B atoms on the first-nearest neighbor sites as well as on the second-nearest neighbor sites are found to lower the Fermi-level density of states, which is unfavorable for superconductivity. Although tetragonal and rhombohedral distortions are found to give a sizable change to the electronic states near the Fermi level, the value of the Fermi-level density of states itself is found to remain almost unchanged. Finally, in order to study the fundamental-gap value of the heavily B-doped diamond, we study the electronic structure of the sc BC7 and fcc BC15 in the GW approximation. A large reduction of the gap value upon doping is confirmed.
Simple and Effective Way to Improve the Stability of Titanium Based Boron Doped Diamond Film Electrode
Authors: Guo Liang , Chen Guohua 
Published: April 2011

ABSTRACTBoron-doped diamond film coated titanium (Ti/BDD) becomes increasingly attractive because of the combined properties of these two unique materials. The challenge for the composite material is the stability especially when it is used as an electrode. In order to meet this challenge, two temperature (2-temp) staged hot filament chemical vapor deposition method was employed in this study. The accelerated working life time was significantly increased to 804 hours for the 2-temp electrode, compared with 244 hours for the diamond film electrode fabricated under one temperature (1-temp) stage method. With the characterization of micro-Raman, XRD, and cross-sectional SEM, a multilayer of Ti/TiC/(Diamond+Amorphous Carbon)/Diamond can be found in the 2-temp sample and the structure of Ti/TiC/Diamond in the 1-temp sample. There was less void space observed in the interlayer of 2-temp sample. The multi-layered compact structure plays an important role in improving the adhesion of diamond film to the titanium substrate which in turn increases the electrode working life time by over 3 times.