bluuebunny/crossref_metadata_2025 huggingface dataset:
Splitting of Guest Atom Sites and Lattice Thermal Conductivity in Ba-Ga-Ge Clathrate Compounds
Authors: Okamoto Norihiko L. , Tanaka Katsushi , Inui Haruyuki
Published: April 2011

The crystal structures of some type-I and -III clathrate compounds in the Ba-Ga-Ge system have been investigated by synchrotron X-ray powder diffraction at room temperature, paying special attention to the changes of the cage structure and the splitting behavior at the guest atom site upon alloying with Ga. For both types of the clathrate compounds, the split distance of the Ba(2) sites increases with the increase in the Ga content, corresponding to the change in the size and shape of the encapsulating polyhedral cage. Lattice thermal conductivity at room temperature has a positive correlation with the atomic displacement parameter (ADPsplit) based on the split-site model but has an inverse correlation with the split distance of the Ba(2) sites, indicating that a dominant factor of reducing the lattice thermal conductivity is not thermal vibration at the split sites but thermal jump among the split sites.
Effect of Ga-doping on the Thermoelectric Properties of Ba-Ge Type-III Clathrate Compounds
Authors: Kim Jung-Hwan , Okamoto Norihiko L. , Tanaka Katsushi , Inui Haruyuki
Published: April 2011

Effect of Ga addition on the thermoelectric properties of Ba-Ge type-III clathrate has been investigated as a function of Ga content and temperature. The substitution of Ga atom for Ge atom leads to the decrease of carrier (electron) concentration. Electrical conduction is of n-type for all clathrate compounds investigated and the values of electrical resistivity and Seebeck coefficient increase with the increase in the Ga content and in temperature. Both electronic and lattice thermal conductivity decrease with the increase in the Ga content because of the decreased carrier concentration and the increased extent of the rattling motion of Ba atoms encapsulated in open-dodecahedron, respectively. A very high ZT value of 1.25 is obtained at 670 °C for Ba24Ga15Ge85.
Application of Transmission Line Theory for Modeling of a Thermoelectric Module in Multiple Configurations for AC Electrical Measurements
Authors: Downey Adam Darwin , Timm Edward , Poudeu Pierre F. P. , Kanatzidis Mercouri G. , Shock Harold , Hogan Timothy P.
Published: April 2011

Measurements of assembled thermoelectric modules commonly include investigations of the module output power versus load resistance. Such measurements include non-ideal effects such as electrical and thermal contact resistances. Using an AC electrical measurement, two models for a thermoelectric module have been developed utilizing electrical circuits for both the thermal and electrical characteristics of the module.

Measurements were taken over the frequency range of 1mHz to 500Hz using lock-in amplifiers. We present data showing the extraction of ZT from such measurements on commercially available modules utilizing both the magnitude and phase of the measured impedance. Here we extend upon a simple RC equivalent circuit model by utilizing transmission line theory in electrical circuits to explain the thermal activity in a thermoelectric module. This model includes all components of a module such as nickel traces and ceramic end caps, and makes use of their corresponding thermal conductivities, thermal capacitance, and density. This model can then be applied to pn unicouples in either a standard or inline configuration, and to individual p or n legs of the module. Data is presented showing the advantages of both models. Measurements on new thermoelectric materials and modules are also presented.
Thermoelectric Properties of Bi2SrCo2O9 Tellurium-Doped Single Crystalline Whiskers
Authors: Bourne Dwayne , Tang Xiaofeng , Aaron Kelvin , Barnes Julius , Payne James , Tritt Terry M.
Published: April 2011

Long single crystalline whiskers (10-200 µm diameter) were synthesized using tellurium-doped precursors. The length of these whiskers varies from less than 1 mm up to 9 mm. The thermopower and resistivity were approximately 150 µV/K and 5 mΩ-cm respectively at 325K. The thermopower was measured using a differential technique, while the resistivity was measured using a standard four-probe method. The thermal conductivity of these small samples was measured using our parallel thermal conductance technique. The total thermal conductivity was on the order of 2 Wm−1K−1.
Effect of Rare-Earth Cation Doping on Enhancement of The Thermoelectric Power of Zinc Oxide
Authors: Fuda Kiyoshi , Sugiyama Shigeaki
Published: April 2011

We investigated the effects of doping of rare-earth ions (Ce, Pr, Nd, Sm, and Eu) on the thermoelectric properties of ZnO, especially Seebeck coefficient in the temperature range from 100 to 800 °C. The data were scattered more or less depending on the doping species, but it was commonly found that the rare-earth doped samples showed higher Seebeck coefficients with compared to those for Al-doped samples. The highest values were observed for Nd-doped one ranging from -360 to -400μVK−1 in the temperature range observed, the lowest ones for Pr-doped one ranging from -250 to -310 μVK−1, whereas those for Al-doped one ranging from -80 to -140 μVK−1. It should also be pointed out that the power factor at 100 °C for Pr-doped ZnO was twice or more as much as that for Al-doped one. From the results it seemed that rare earth doping is effective for enhancement of thermoelectric power of ZnO.
Thermoelectric Modules For High Temperature Waste Heat
Authors: Funahashi Ryoji , Mihara Toshiyuki , Mikami Masashi , Urata Saori
Published: April 2011

A new adhesive material has been developed in order to obtain practically usable thermoelectric modules composed of oxide thermoelectric legs. The thermoelectric module composed of 8-pair oxide legs has been fabricated. Both hot- and cold-sides of the module were covered by alumina plates. Open circuit voltage VO and maximum power Pmax reach 0.38 V and 0.30 W, respectively at 803 K of a hot-side temperature TH and 362 K of a temperature differential ΔT between TH and cold-side temperature TC. Generating power was repeated 11 times at 873-993 K of TH and at 200-290 K of ΔT. The module was cooled down to room temperature after each generation. At third measurement internal resistance RI of the module increased by 30 %. This is due to destruction of junctions because of thermal strain. No deterioration, however, was observed in thermoelectric properties for the oxide legs.
Mechanical Degradation of Carbon Nanotubes: ESR Investigations
Authors: Chipara Mircea , Kukovecz Akos , Konia Zoltan , Zaleski Jeffrey M. , Chipara Magdalena
Published: April 2011

ESR investigations on milled multiwalled carbon nanotubes are reported. The ESR spectra of pristine and milled nanotubes consists of two resonances, a wide line located at g>3.0 assigned to magnetic particles (catalysts residues) and a broad line with a peak to peak linewidth of about 10−2 T located at g=2.05 and assigned to the interaction between the conducting electrons delocalized over carbon nanotubes and magnetic ions. It was observed that this line depends on the milling time. For short milling times (up to 125 minutes) both the resonance linewidth and the g-factor increase as the milling time is increased. Longer milling times resulted in a decrease of both the g-factor and resonance linewidth as the milling time was increased. This behavior was assigned to the removal of the magnetic nanoparticles from nanotubes. No resonance line due to the destruction of carbon nanotubes was observed.
Photo-oxidation of Single-walled Carbon Nanotubes
Authors: Parekh B , Debies Thomas , Evans Chris , Landi Brian , Raffaelle Ryne , Takacs Gerald
Published: April 2011

UV- and vacuum UV-assisted photo-oxidation of single-walled carbon nanotube (SWNT) paper occurred with: (1) atmospheric oxygen pressure using low-pressure Hg lamps (λ = 253.7 and 184.9 nm), (2) low oxygen pressure employing emission downstream from an Ar microwave plasma (λ = 106.7 and 104.8 nm), and (3) high pressures of He in a rotating d.c. arc that was designed to produce a spectral continuum from He excimers (λ = 58 - 110 nm). The photo-oxidized materials were characterized by x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Raman spectroscopy. UV photo-oxidation was demonstrated to be a controlled dry procedure for introducing oxygenated functional groups (C-O-C, C=O, O-C=O and O=C-O-C=O) on SWNTs.
Fire Retardancy and Morphology of Nylon 6-Clay Nanocomposite Compositions
Authors: Shanmuganathan Kadhiravan , Razdan Sandeep , Dembsey Nick , Fan Quinguo , Kim Yong K , Calvert Paul D , Warner Steven B , Patra Prabir
Published: April 2011

We investigated the effect of organically modified clay on the thermal and flammability behavior of nylon 6 nanocomposites. We also used zinc borate along with layered silicate with an aim of achieving synergistic effect in flame retardancy. It is found that addition of 10 wt% clay reduced the onset decomposition (5% wt loss) temperature of nylon 6 by 20°C, while addition of 5 wt% zinc borate and 5 wt% clay in combination reduced it by around 10°C. Differential thermogravimetric analysis indicated that the peak decomposition temperature was not affected by the addition of clay, but the rate of weight loss decreased with increasing clay concentration. The horizontal burning behavior of the nanocomposite films of approximately 0.5mm thickness changed with additive concentration. The nanocomposites with 2.5 wt% and 5 wt% clay burned for almost the same duration as neat nylon 6 but dripping was reduced. The 10 wt% clay nanocomposite sample burned without any dripping and the flame spread rate was reduced by 25-30%. The burn rate of 5 wt % zinc borate/5 wt% clay nanocomposite sample was about 20% higher than that of 10 wt% clay nanocomposite sample, which could be attributed to varying char morphology. Scanning electron microscopy images of the 10wt% clay nanocomposite char surface and cross- section revealed an integrated layer of clay platelets with increasing density gradient from the center to the surface, while the 5 wt% zinc borate/5 wt% clay nanocomposite char appeared foamy and porous. The 5 wt% zinc borate and 5 wt% clay sample developed into a very good intumescent system in cone calorimeter test, swelling about 10-13mm height prior to ignition forming a cellular char structure. This was as effective as the 10wt% clay nanocomposite sample in reducing the heat release and mass loss rate of nylon 6 by around 65%. Fourier transform infrared spectroscopy of the 10 wt% clay nanocomposite char showed the presence of amides, indicating possible residual polymer within the shielded char.
Effects of ion-beam irradiation on the L10 phase transformation and their magnetic properties of FePt and PtMn films (Invited)
Authors: Lai Chih-Haung , Huang Sheng-Huang , Yang Cheng-Han , Chiang C.C. , Liou S. H. , Sellmyer D. J. , Yan M. L. , Yuan L. , Yokata T.
Published: April 2011

Ion-beam irradiation shows enhancement or degradation of magnetic properties on L10 phase of FePt and PtMn films. A highly ordered L10 FePt phase was directly achieved by using 2 MeV He-ion irradiation without conventional postannealing. The in-plane coercivity greater than 5700 Oe can be obtained after disordered FePt films were irradiated at the beam current of several μA/cm2 with the ion does of 2.4×1016 ions/cm2. The high beam-current-density results in direct beam heating on samples. In addition, the irradiation-induced heating process provides efficient microscopic energy transfer and creates excess point defects, which significantly enhances the diffusion and promotes the formation of the ordered phase. Consequently, the direct ordering of FePt took place by using ion-irradiation heating at temperature as low as 230°C. The comparison has been made on the [Fe/Pt]10/C films by RTA and high current-density He irradiation. Although RTA and ion irradiation both reach high coercivity, ion irradiation seems to suppress the (001) texture, leading to isotropic Hc. Ion-beam irradiation can also be applied to the transformation of PtMn. An ordered PtMn phase, a large exchange field and a high GMR ratio (11%) were obtained in PtMn-based spin valves by using 1.25 μA/cm2 He-ions. On the other hand, Ge-ion and O-ion irradiation completely destroyed the ferromagnetism of FePt and GMR of PtMn-based spin valves, respectively. We propose a novel approach to achieve magnetic patterning by using ion irradiation, which can be applied for patterned media and magnetic sensors.
Large magnetoresistance in oxide based ferromagnet / superconductor spin switches
Authors: Peña V. , Nemes N. , Visani C. , Garcia-Barriocanal J. , Bruno F. , Arias D. , Sefrioui Z. , Leon C. , Te Velthuis S. G. E. , Hoffmann A. , Garcia-Hernandez M. , Santamaría Jacobo
Published: April 2011

We report large magnetoresistance (in excess of 1000%) in ferromagnet / superconductor / ferromagnet structures made of La0.7Ca0.3MnO3 and YBa2Cu3O7 in the current in plane (CIP) geometry. This magnetoresistance has many of the ingredients of the giant magnetoresistance of metallic superlattices: it is independent on the angle between current and magnetic field, depends on the relative orientation of the magnetization in the ferromagnetic layers, and takes very large values. The origin is enhanced scattering at the F/S interface in the anti parallel configuration of the magnetizations. Furthermore, we examine the dependence of the magnetoresistance effect on the thickness of the superconducting layer, and show that the magnetoresistance dies out for thickness in excess of 30 nm, setting a length scale for the diffusion of spin polarized quasiparticles.
Diamond/Carbide Nano-Composite Gradient Films: a Route to solve the Adhesion Issues of Diamond Films
Authors: Staedler Thorsten , Vadali Srikanth , Jiang Xin
Published: April 2011

Due to their outstanding mechanical properties diamond films are ideal candidates for many cutting and machining applications. However, industrial applications of these films are limited due to poor adhesion. Two main reasons causing this poor adhesion, which are based on the extrinsic physical and chemical properties of diamond, can be identified: High mechanical stresses induced by a difference of the thermal expansion coefficient between the diamond film and the substrate as well as a catalytic effect in case of metallic substrates containing iron-, cobalt- and nickel that, in combination with a methane atmosphere during deposition, leads to soot formation. One option to overcome these difficulties is to provide an interfacial layer that acts as adhesion layer as well as barrier layer to prevent the catalytic effect of the substrate elements. Even though some successful examples exist, this approach usually requires a time consuming and expensive multi-step process.

In this paper, the synthesis of nanocrystalline diamond/carbide composite films with a compositional gradient will be reported. Focusing on the example of diamond/ß-SiC the possibility to create a gradient layer ranging from ß-SiC to diamond in a controlled manner will be shown. The films are prepared by a Microwave Assisted Plasma Chemical Vapour Deposition process (MWCVD) using H2, CH4 and Tetramethylsilane (TMS) as reactive gases. The structure, grain sizes, and volume fractions of the components of these composite films, which consist of a mixture of diamond and carbide phase, can be controlled by adjusting the concentrations of the reactive gases in the gas mixture. This strategy, which handles all depositions in one process step, should allow for an improved diamond film adhesion on tools. The preparation and characterization of the composite films with special emphasize on their mechanical and tribological properties will be discussed and a short outlook on other diamond/carbide systems will be given.
The Effect of Surface Interactions on the Viscosity of Polymer Thin Films
Authors: Li Chunhua , Jiang Jun , Rafailovich Miriam J. , Sokolov Jonathan C.
Published: April 2011

Previously, we reported that the viscosity of a polymer film can be measured in situ by observing the liquid-liquid dewetting of polymer bilayer films. In this study, we use the technique to investigate the effect of film thickness and surface interactions on the effective viscosity of polymer thin films. We found that the effective viscosity increases dramatically with decreasing the film thickness. We attribute this to the pinning of the polymer chains at the strongly interacting polymer/Silicon interface.
Thermal stability of Cr1−xAlxSiyN coatings with medium and high aluminium content prepared by arc evaporation
Authors: Karvankova Pavla , Karimi Ayat , Coddet Olivier , Cselle Tibor , Morstein Marcus
Published: April 2011

The thermal stability of Cr1−xAlxN and Cr1−xAlxSiyN nanocomposite coatings with different composition x, Si content, and phase structure has been investigated. The coatings were deposited in an industrial unit by the cathodic arc evaporation method, using rotating cylindrical cathodes. Further sets of coated WC/Co plates were subjected to an annealing step in a forming gas (92% N2 / 8% H2) atmosphere for several hours at a constant temperature (maximum 1000°C). Chemical composition, microstructure and mechanical properties of the coatings were investigated by the RBS, XRD, TEM, SEM, XPS and nanoindentation techniques.

As-deposited Cr1−xAlxSiyN coatings with Al contents x in the range of 0.4 ≤ x ≤ 0.6 showed a single-phase cubic CrAlN structure and a maximum hardness of 38 GPa. With increasing the Al content to x = 0.95 – 0.96, two-phase films consisting of hexagonal AlN and cubic CrAlN structure with hardness of about 31 GPa were obtained. The hardness of Cr1−xAlxSiyN coatings increases during annealing at temperatures in the range of 800 – 1000°C by about 1 – 4 GPa, even though the residual compressive stress in the coatings was found to relax considerably during this procedure. The addition of Si into Cr1−xAlxN delays the appearance of hcp-AlN and thereby improves the thermal stability of the coatings.
A theoretical approach to the energetic stability and geometry of hydrogen and oxygen terminated diamond (100) surfaces
Authors: Petrini Daniel , Larsson Karin
Published: April 2011

The thermodynamic stability of diamond (100) surfaces as a function of degree of hydrogen and oxygen-related termination coverage has been theoretically studied using DFT techniques. The results show that an exchange of the hydrogen atoms with hydroxyl groups is disfavored, whereas a corresponding exchange with oxygen atoms (in the ketone or ether position) is energetically preferred. The adsorption of up to about 50 % oxygen coverage (ether position) is, however, largely disfavored compared to a fully hydrogen-terminated surface. However, this oxygen termination will be energetically improved as the coverage increases above the 50 % level. The adsorption energy per terminating species (at 100% surface coverage) is −4.13 eV, −4.30 eV, −5.95 eV and 6.21 eV for H, OH, O(ketone) and O(ether) species, respectively.
Interdiffused InGaAsP Quantum Dots Lasers on GaAs by Metal Organic Chemical Vapor Deposition
Authors: Arif Ronald A. , Kim Nam-Heon , Mawst Luke J. , Tansu Nelson
Published: April 2011

Self-assembled InGaAs quantum dots (QD) grown by metal organic chemical vapor deposition (MOCVD) have a natural peak emission wavelength around 1150-1200-nm due to its specific composition, shapes, and sizes. In this work, a new method to engineer the emission wavelength capability of MOCVD-grown InGaAs QD on GaAs to ∼1000-nm by utilizing interdiffused InGaAsP QD has been demonstrated. Incorporation of phosphorus species from the GaAsP barriers into the MOCVD-grown self-assembled InGaAs QD is achieved by interdiffusion process. Reasonably low threshold characteristics of ∼ 200-280 A/cm2 have been obtained for interdiffused InGaAsP QD lasers emitting at 1040-nm, which corresponds to blue-shift of ∼ 85-90-nm in comparison to that of unannealed InGaAs QD laser.
Cathodoluminescence study of orientation patterned GaAs films for non linear optics
Authors: Avella Manuel , Jiménez Juan , Bliss David , Lynch Candance , Weyburne David
Published: April 2011

Zincblende semiconductor materials such as GaAs and ZnSe are very promising for the generation of tunable IR signals by quasi phase-matched nonlinear optical frequency conversion. Orientation patterned GaAs crystals (OP-GaAs) were grown by low-pressure hydride vapor phase epitaxy (HVPE) on lithographically prepared templates.. Cathodoluminescence (CL) imaging was used to study these epitaxial films, which consist of periodic domains of inverted crystallographic orientation. The challenge is to achieve thick, vertical domains and to minimize sources of optical loss. Both the domain walls and crystal defects may contribute to transmission losses, therefore, a characterization of both is necessary to improve the OP-GaAs crystal performance. The stress and non radiative recombination activity of the domain walls were investigated. The presence of dislocation glide was revealed and CL spectral imaging was used to observe the stress distribution around both the domain walls and the dislocations.
Thin Multiwall Carbon Nanotube Field Emitters with Microchannel Plate for High Current Emission
Authors: Seelaboyina Raghunandan , Huang Jun , Choi Won Bong
Published: April 2011

We report a method to amplify the electron current of carbon nanotube field emitter cathode using an electron multiplier and synthesis of thin multiwall carbon nanotubes by thermal chemical vapor deposition. A commercial microchannel plate which is an array of microscopic electron multipliers was used for this purpose. A microchannel plate placed between the cathode and anode amplified the current and also acted as protection shield for nanotubes during vacuum arcing. The turn-on field was approximately 1 V/µm for both cases, but current obtained was 3 times higher and the field was also lower. The increase in current is attributed to the electron multiplication, and to the increase in field enhancement factor from 9296 to 12815 (37%) with microchannel plate. The thin multiwall carbon nanotubes were synthesized using Mo/Fe catalyst solution. The inner and outer wall diameters of the nanotubes were in the range of 2.5-5 nm and 7-15 nm respectively, determined by Raman spectroscopy and transmission electron microscopy. The nanotubes had excellent field emission property which is attributed to their thin wall diameters ∼10 nm, high aspect ratio of 1500 and the high field enhancement factor of 9296. The emission current was stable with an average fluctuation of 2%.
Silicon Thin Film Growth by Pulsed Plasma CVD under Near-Atmospheric Pressure
Authors: Kitabatake Hirotatsu , Suemitsu Maki , Nakajima Setsuo , Uehara Tsuyoshi , Toyoshima Yasutake
Published: April 2011

Si Plasma-enhanced chemical vapor deposition (PECVD) at a near-atmospheric pressure (NAP) of 500 Torr has been conducted by using a pulsed-electric-field based NAP-PECVD system. At a growth temperature of 180°C, poly-Si films with a high Raman ratio of 7.4 are obtained on glass substrates, while epitaxial-like growth occurs when Si(100) substrates are employed, as confirmed by Raman-scattering spectroscopy, X-ray diffraction, and a cross-sectional transmission-electron microscopy.
Growth of high quality Ge epitaxial layer on Si(100) substrate using ultra thin Si0.5Ge0.5 buffer
Authors: Nakatsuru Junko , Date Hiroki , Mashiro Supika , Ikemoto Manabu
Published: April 2011

Methods for forming Ge epitaxial layer on Si (100) substrate have been vigorously sought due to potential applications of such structure as a virtual substrate for III-V devices on Si. Various methods were proposed to realize low threading dislocation density and smooth surface. To date, such methods involve more than one of thick (micrometer order) SiGe buffer growth process, high temperature annealing steps and CMP process, which could compromise reliability and suitability for production.

In this study, we report feasibility of a thin (in the order of 10nm) SiGe buffer layer to realize pure Ge epitaxial layer with good crystalinity, low threading dislocation density, and smooth surface without high temperature annealing steps and CMP process.

As a result, we achieved shorter time for growth of practical thickness of crystalline Ge on Si (100) substrate, and also get the high quality Ge epitaxial layer which has low threading dislocation density with very smooth surface.

Ge epitaxial layer and underlying thin SixGe1−x buffer layer were grown on Si (100) substrate using a cold wall UHV-CVD system. Source gases of Si and Ge were Si2H6 and GeH4, respectively. No carrier gas was used for this process. SiGe buffer layer was grown on Si(100) substrate at 450 – 520˚C. Two-step growth process was employed to grow Ge epitaxial layer on the buffer layer. Ge seed layer was grown at a low temperature (350–400˚C), followed by Ge thick layer growth at a high temperature (550–650˚C). XRD, TEM, EPD, and AFM were used for characterization of Ge epitaxial layer.

Optimization of growth temperature and source gas flow rate ratio enabled to obtain an effective buffer layer thinner than 10nm. The thin buffer layer realizes process time shortening, which is within 10min, and the smooth surface is realized without crosshatch structure. The buffer thickness is 1/160 to 1/1000 than that of previously known methods using thick SiGe buffer layers. Thin SiGe buffer also enabled process time shortening for Ge seed layer growth as two-dimensional Ge layer was formed faster on the thin SiGe buffer layer than on Si. XRD of the Ge seed layer showed 97% relaxation as grown and fully relaxed at 550˚C. The threading dislocation density of the top Ge layer was estimated below 1E7counts/cm2 by TEM and EPD.

The misfit dislocations oriented along [110] at the interface between the SiGe buffer layer and the Ge layer and the distances are 9.5nm constantly in cross-sectional TEM image. We are checking the surface roughness of the Ge epitaxial layer by using AFM.
Growth of a-plane ZnO Thin Films on r-plane Sapphire by Plasma-assisted MBE
Authors: Xie Junqing Q. , Dong J. W. , Osinsky A. , Chow P. P. , Heo Y. W. , Norton D. P. , Pearton S. J. , Dong X. Y. , Adelmann C. , Palmstrøm C. J.
Published: April 2011

ZnO thin films have been epitaxially grown on r-plane sapphire by RF-plasma-assisted molecular beam epitaxy. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies indicate that the epitaxial relationship between ZnO and r-plane sapphire is (1120)ZnO // (1102)sapphire and [0001]ZnO // [1101]sapphire. Atomic force microscopy measurements reveal islands extended along the sapphire [1101] direction. XRD omega rocking curves for the ZnO (1120) reflection measured either parallel or perpendicular to the island direction suggest the defect density anisotropy along these directions. Due to the small lattice mismatch along the ZnO [0001] direction, few misfit dislocations were observed at the ZnO/Al2O3 interface in the high-resolution cross-sectional TEM image with the zone axis along the ZnO [1100] direction.
Structural and optical properties of Zn1−xCoxO and ZnCo2O4 thin films
Authors: Samanta Kousik , Bhattacharya Pijush , Katiyar Ram S. , Iwamoto W. , Urbano R. R. , Pagliuso P. G. , Rettori C.
Published: April 2011

Thin films of Co substituted ZnO and ZnCo2O4 were deposited on c-axis (0001) oriented Al2O3 substrates using pulsed laser deposition. The XRD results showed all the films were highly (002) oriented with a less intense peak of (311) for ZnCo2O4 thin film. Micro-Raman spectra of ceramic targets showed the modes related to wurtzite ZnO and spinel ZnCo2O4 structures. In thin films of Zn1−xCoxO no modes corresponding to ZnCo2O4 were detected. The intensity of E1(LO) and multiphonon peak at 584 and 540 cm−1 respectively, increased with increase in Co substitution. The optical absorption of the films showed that the band gap decreased with increase of Co concentrations at room temperature along with the sub-bandgap absorptions due to d-d transitions of Co2+. Similar sub-bandgap d-d transition was also observed in the absorption spectra ZnCo2O4 thin films. The highest saturated magnetization (0.2μB/Co) was obtained for 5%Co substituted ZnO.
Concentration and Defect Dependent Ferromagnetism above Room Temperature in Co Doped ZnO Films Prepared by Metalorganic Decomposition
Authors: Kharel P. , Sudakar C. , Lawes G. , Suryanarayanan R. , Naik R. , Naik V. M.
Published: April 2011

Zn1−xCoxO (x = 0.0 – 0.047) thin films (thickness ∼0.5 – 1 μm) have been prepared on sapphire substrates using metalorganic decomposition (MOD) method. The X-ray diffraction and Raman scattering studies indicate films to be polycrystalline ZnO with wurtzite structure. The optical absorption spectra show an expected bandgap of ∼3.2 eV. The magnetization studies show that the as prepared films lack the room temperature ferromagnetic order, whereas the films when vacuum annealed at a temperature 500 – 600 °C acquire ferromagnetic ordering at room temperature. Further, the observed ferromagnetism (FM) appears only for a limited range of Co concentration, 0.03 < x < 0.10 (after heat treating in vacuum at 550 °C), and it reversibly disappears upon re-annealing in air. The data presented here seem to suggest that the appearance of ferromagnetic order is dictated by both the oxygen defects and the critical concentration of Co, and thus may lend support to a recent model proposed by Coey et al. [Nature Materials4, 173 (2005)].
Electrical Properties of Diamond MISFETs with Submicron-Sized Gate on Boron-Doped (111) Surface
Authors: Saito Takeyasu , Park Kyung-ho , Hirama Kazuyuki , Umezawa Hitoshi , Satoh Mitsuya , Kawarada Hiroshi , Liu Zhi-Quan , Mitsuishi Kazutaka , Okushi Hideyo
Published: April 2011

An H-terminated-surface conductive layer of B-doped diamond on a (111) surface was used to fabricate a metal insulator semiconductor field effect transistor (MISFET) using CaF2, SiO2 or Al2O3 gate insulators and a Cu-metal stacked gate. For a CaF2 gate, the maximum measured drain current (Idmax) was 240 mA/mm and the maximum transconductance (gm) was 70 mS/mm, and the cut-off frequency of 4 GHz was obtained. For a SiO2 gate, Idmax and gm were 75 mA/mm and 24 mS/mm, respectively, and for an Al2O3 gate, these characteristics were 86 mA/mm and 15 mS/mm, respectively. These values are among the highest reported DC and RF characteristics for a diamond homoepitaxial (111) MISFET.
Optimization of Ga(In)NAs thin film growth by atomic hydrogen-assisted molecular beam epitaxy
Authors: Shimizu Yukiko , Miyashita Naoya , Okada Yoshitaka
Published: April 2011

The effect of growth temperature on the crystal quality and optical properties of Ga(In)NAs films was investigated over a range of 340 ∼ 520 °C. We found that Ga(In)NAs films fabricated at lower growth temperatures generally result in an improved crystal quality. An XRD linewidth of as low as 45 arcsec was obtained for a 1 µm-thick Ga0.94In0.06N0.01As0.99 thin film grown at 380 °C. This is ∼1/2 of that grown at the conventionally-adopted growth temperature of 480 °C. After annealing, an improved optical property represented by a higher PL intensity compared to the conventional growth method (annealed, Tgrowth = 480 °C) was also obtained in the 1 µm-thick Ga0.94In0.06N0.01As0.99 thin film grown at low temperature of 380 °C.
Potentially Modulated GaAs/GaNAs/InGaAs Quantum Wells for Solar Cell Applications
Authors: Kobayashi Naoto , Sasaki Naoyuki , Okada Yoshitaka
Published: April 2011

We have fabricated GaAs/GaNAs and GaAs/GaNAs/InGaAs MQWs introduced into the intrinsic region of GaAs p-i-n solar cells by atomic H-assisted RF-molecular beam epitaxy. Compared to the more conventional GaAs/InGaAs MQW cells, GaAs/GaNAs/InGaAs MQW cells exhibited improved external quantum efficiencies in the low energy region. This is due to the extension of band edge into longer wavelengths as achieved by use of GaNAs materials.
Chemical Vapor Deposition and Defect Characterization of Silicon Carbide Epitaxial Films
Authors: Chen Yi , Dhanaraj Govindhan , Chen Hui , Vetter William , Dudley Michael , Zhang Hui
Published: April 2011

SiC homo-epitaxial layers have been grown using a halide chemical vapor deposition (HCVD) process. The thermodynamic process of SiC CVD in SiCl4-C3H8-H2 gas system was studied using equilibrium model. The predicted growth rate decreases gradually with the increase in growth temperature, and this trend is consistent with our experimental results. Good quality epitaxial layers with low density of basal plane dislocations could be grown. Some elementary screw dislocations present in the substrate do not seem to be propagating into the epitaxial layer.
X-ray standing wave investigations of Si dopant incorporation in GaN
Authors: Siebert Michael , Schmidt Th. , Flege J. I. , Zegenhagen J. , Lee T.-L. , Figge S. , Hommel D. , Falta J.
Published: April 2011

The synchrotron radiation technique of x-ray standing waves (XSW), which allows to directly obtain structural and element-specific data, was successfully used for the investigation of the dopant site distribution in Si doped GaN films grown on (0001) sapphire substrates for the first time. The Si dopant concentration was chosen to 5×1018 cm-3 and 5×1019 cm-3. The measurements were performed on 300 nm thin doped films deposited on high-quality thick undoped GaN films. With this sample structure, influences of the Si dopant induced decrease of the crystalline quality on the XSW signal are suppressed. The XSW data are compared to those obtained from thick homogeneously doped GaN films. All XSW measurements were performed in (002) backscattering geometry. Independent of the dopant concentration, the results indicate that Si atoms are solely incorporated on substitutional Ga sites.
The Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature
Authors: Lee Kyoungnae , Schires Eric D , Myers Thomas H
Published: April 2011

The Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature Accurate measurement of the substrate temperature during growth of gallium nitride by molecular beam epitaxy is crucial. Typically, thermocouples are usually used to measure the temperature of the back side of block which is holding the substrate. Alternatively, pyrometers are often used. However, there is a big range of an error. In-situ cathodoluminescence (CL) occurring during reflection high energy electron diffraction is a strong candidate to determine the growth temperature. The electron beam supplied by our RHEED gun has an energy of 13keV which was used for each measurement. CL was easily detected up to and beyond typical growth temperatures. The CL was directed into a monochromator using fiber optics. The final signal was detected with a photomultiplier tube. This technique appears quite useful to accurately and reproducably determine substrate temperature during growth.

The CL could also be observed using a ccd camera. Thus, we investigated using the CL to image the sample during growth. This could be used to see temperature inhomogenaities, and potentially to map alloy composition fluctuations. We calibrated the wavelength vs. growth temperature by using narrow band-pass interference filters. Background subtraction with blanking the e-beam could be used to remove black body radiation and other undesired sources of light. For gallium nitride, the photon energy at the growth temperature of 750°C is about 3.0eV. Using different filters, we can take a picture of e-beam on the surface of substrate for each filter and analyze the peak intensity using the line profile. We will present CL images of various samples at differing temperatures.

This work was supported by the AFOSR MURI F49620-03-1-0330 monitored by Todd Steiner and Gerald Witt and by ONR Grants N00014-02-1-0974 and N00014-01-1-0571, both monitored by Colin E. C. Wood.
III-N Epitaxial Growth for Nitride Devices
Authors: Dupuis Russell , Chung Theodore , Lee Wonseok , Li Peng , Limb Jae , Ryou Jae-Hyun , Yoo Dongwon
Published: April 2011

Various GaN-based device structures were grown on (0001) sapphire and 6H-SiC substrates by metalorganic chemical vapor deposition. The device structures of this study include a variety of p-n junction-based devices, such as InGaN/GaN multiple-quantum-well green light emitting diodes, GaN p-i-n vertical rectifiers, and GaN/InGaN heterojunction bipolar transistors. This paper describes the epitaxial growth and device performance characteristics of these device structures. We have developed state-of-of-the-art growth techniques for the materials that are critical for high-performance electronic and optoelectronic devices. High-performance InGaN HBTs, high-voltage GaN rectifiers and long-wavelength green LEDs have been epitaxially grown, fabricated, and characterized. The details of the material growth, device fabrication, and device characterization will be presented.
Al mole fraction dependence of deep levels in AlGaN/GaN-HEMT structures estimated by CV profiling
Authors: Kikawa Junjiroh , Katsuhiro Imada , Tomoyuki Yamada , Tadayoshi Tsuchiya , Yuichi Hiroyama , Masayuki Iwami , Tsutomu Araki , Akira Suzuki , Yasushi Nanishi
Published: April 2011

Current-voltage (IV) measurements and capacitance-voltage (CV) measurements have been carried out to investigate electrical properties of AlGaN/GaN-HEMT structures. By CV measurements of Schottky barrier diodes (SBDs) with large leak currents, we observed a distinct peak in CV profiling at low frequencies. The integral of this peak was found to have a correlation with a leak current. The behavior of this peak might be described by the Shockley-Read-Hall (SRH) model if we assume this peak is due to a phenomenon of an electron emission and capture by deep levels. Then Quasi-Fermi Level (Imref) at the bias point where this peak appears in CV profiling corresponds to energy depth of deep levels. That energy level can be approximated by Imref of two-dimensional (2D) electron gas. The result of our samples showed that the energy depth of deep levels from the conduction band is distributed from 320meV to 470meV for Al mole fraction from 0.19 to 0.30, respectively.
3D Simulations on Realistic GaN-Based Light-Emitting Diodes
Authors: Li Simon , Li Z.Q. , Shmatov O. , Xia C.S. , Lu W.
Published: April 2011

Comprehensive multiscale models have been employed to simulate the realistic GaN based light-emitting diodes (LED) using 3D finite-element analysis. The advanced features include drift-diffusion model for carrier transport, self-consistent Poisson-Shrodinger and K·P models for multi-quantum well band structure, quantum tunneling model for heterojunction, spontaneous and piezoelectric polarization models for built-in electric field, heat flow model for self-heating and ray-tracing model for photon extraction. All the advanced capabilities have been integrated into our software APSYS[1]. In this paper, we present the 3D simulations on the InGaN/InGaN LEDs. Based on the detailed simulation results, we were able to analyze the impact of micro- and nano-scale physical effects such as current crowding, carrier leakage, built-in interface charge and self-heating on the internal efficiency of the device. The macro-scale effect of the geometry on photon extraction was analyzed using 3D ray-tracing technique. Results of different structures will be given to demonstrate the power of the software in handling complicated realistic LED geometries. The simulation results can be used to optimize the design of quantum well layers, blocking layer materials and electrode geometries etc.
III-Nitride Epitaxial Material on Large-Diameter Semi-Insulating SiC Substrates for High Power RF Transistors
Authors: Saxler Adam , Hutchins Edward L. , Jenny Jason , Blew Austin
Published: April 2011

Metalorganic chemical vapor deposition was employed to deposit high quality, highly uniform III-Nitride transistor structures on 100 mm diameter semi-insulating 4H-SiC substrates. Electron mobility was over 2000 cm2/Vs at room temperature. Sheet resistivity uniformity was as low as 0.75%. Typical standard deviations were about 1% in most properties including sheet resistivity, carrier concentration, mobility, and AlGaN composition. Additionally, wafers maintained their flat shape after deposition of these structures. Wafer bow and warp were typically less than 20 μm for optimized structures and <5 μm for the best wafers.
Metal Organic Chemical Vapor Deposition of Zinc Oxide
Authors: Fenwick William E. , Woods Vincent T. , Pan Ming , Li Nola , Kane Matthew H. , Gupta Shalini , Rengarajan Varatharajan , Nause Jeff , Ferguson Ian T.
Published: April 2011

Thin films of ZnO were grown by metal organic chemical vapor deposition (MOCVD) in a vertical injection rotating disk reactor (RDR) system on sapphire substrates. Kinetics of ZnO growth by MOCVD were studied and an optimal growth window for a RDR tool was determined. Experimental growth conditions were chosen based on calculations of Reynolds Number (Re) and mixed convection parameter in order to select a growth window with stable gas flow and uniform heat transfer. Growth parameters were systemically varied within this window to determine the optimal growth conditions for this MOCVD tool and to study how these parameters affect film growth and quality. Properties of ZnNiO films grown by MOCVD were also studied to determine the effects of Ni incorporation on structural, optical, and magnetic properties.
A semipolar (10-1-3) InGaN/GaN green light emitting diode
Authors: Sharma Rajat , Pattison P Morgan , Baker Troy J , Haskell Benjamin A , Farrell Robert M , Masui Hisashi , Wu Feng , DenBaars Steven P , Speck James S , Nakamura Shuji
Published: April 2011

The performance of conventional c-plane GaN-based optoelectronic devices suffers from the effects of strong polarization-induced electric fields along the conduction direction, which result in a reduced overlap between electron and hole wavefunctions. These devices consequently demonstrate low radiative recombination rates, and a blue-shift in peak emission wavelength with increasing bias. There have been several recent demonstrations of light emitting diodes (LEDs) fabricated on non-polar a- and m- plane GaN that show greatly reduced to zero blue-shift of peak emission wavelength, and other recent work on non-polar GaN has yielded hole concentrations that are almost an order of magnitude higher than for c-plane GaN.

The effects of the strong polarization-induced electric fields may, conceivably, also be mitigated or potentially eliminated by growing films on so-called ‘semipolar’ planes. A semipolar plane is any plane that may not be classified as a c-, a- or m- plane, and has at least two non-zero h, i, or k Miller indices and a nonzero l Miller index (, and planes, for example). It is expected that devices grown on these semipolar planes should also demonstrate a reduced blue-shift in peak emission wavelength and higher hole concentrations. Further, recent work also suggests that the indium incorporation efficiency for growth on semipolar planes is comparable to that for growth on the c-plane.

We demonstrate the first green InGaN/GaN LED grown on a planar semipolar GaN template. The LED structure is grown by metalorganic chemical vapor deposition (MOCVD), and the 20 μm-thick, specular and optically transparent template is grown by hydride vapor phase epitaxy (HVPE). The fabricated devices have a peak emission wavelength of ∼525 nm and demonstrate rectifying behavior, with a low turn-on voltage of 3.2 V. We observe a small ∼7 nm blue-shift in the peak emission wavelength during electroluminescence measurements, over the range 20 to 250 mA. We also see an almost linear increase in the output power from 5 mA to 200 mA, with no appreciable decrease in the external quantum efficiency over the same range. We also observe evidence of polarization anisotropy in the emission from the semipolar green LEDs.
Oxidation of Aluminum Nitride for Defect Characterization
Authors: Edgar James , Gu Z. , Taggart K. , Chaudhuri J. , Nyakiti L. , Lee R.G. , Witt R.
Published: April 2011

The thermal oxidation of aluminum nitride was developed as a means to study defects in bulk aluminum nitride crystals. The oxidation kinetics was established for the dry oxidation of highly textured AlN polycrystals produced by sublimation-recombination crystal growth in a tungsten furnace. Despite seeding on polycrystalline tungsten, the grains were predominantly [0001] oriented as verified by electron backscattering diffraction (EBSD). The oxidation rate is dependent on the crystal’s orientation, polarity, stress, and surface condition, thus oxidation decorates grain boundaries, polishing scratches, and inversion domains by producing oxide layers of different thicknesses. The initial oxidation rate of nitrogen polar (0001) AlN is approximately 25% faster than on aluminum polar crystals. Low temperature (800 °C) dry oxidation produced an amorphous oxide layer and generated a high density of defects (vacancies, stacking faults, and dislocations) in the nitride near the oxide/nitride interface, as observed by cross-sectional transmission electron microscopy. In contrast, high temperature oxidation (1000 °C) produced a crystalline oxide layer, and left the nitride free of observable defects.
Effects of GaN passivation with SiO2 and SiNx studied by photoluminescence and surface potential electric force microscopy
Authors: Chevtchenko Serguei , Reshchikov M A , Zhu K , Moon Y-T , Baski A A , Morkoç H
Published: April 2011

The influence of passivation with SiO2 and SiNx on optical properties and surface band bending in unintentionally doped GaN has been studied by steady-state photoluminescence (PL) and surface potential electric force microscopy (SP-EFM). For both types of passivation we observed a significant increase of PL intensity in air ambient at room temperature. The measured surface potential was the same for control and passivated samples within the experimental error. The value of the surface band-bending was determined as 1.0±0.2 eV in all cases. We suggest that the strong enhancement of PL is caused by reduction of contribution of the surface states to recombination of photogenerated carriers after passivation.
LCA-based evaluation of ecological impacts and external costs of current and new electricity and heating systems
Authors: Dones Roberto , Heck Thomas
Published: April 2011

A systematic study of current European electricity and heat systems performed in the frame of the Swiss LCA project ecoinvent was extended to a few new technologies and used as a basis for comparison and ranking using External Costs Assessment and one selected Life Cycle Impact Assessment (LCIA) method. The energy systems include full process chains from extraction of resources through waste disposal. The external costs from airborne emissions were estimated using the most recent findings of the ExternE series on the average damage factors for Europe.

Current fossil electricity systems exhibit the highest LCIA scores as well as the highest external costs, unless greenhouse gas emissions (GHG) are valued very low (sensitivity) and advanced technologies are applied. Alpine hydropower always exhibits the lowest score. Environmental performance of current renewables is generally better than fossil but LCIA ranking for wind and PV may worsen when increased importance is attributed to abiotic resource depletion. Wood cogeneration has a relatively poor score compared to other renewables. Nuclear shows generally good environmental performance, unless the high radioactive wastes are given subjectively high negative value. For heating systems, oil has higher external costs than natural gas, with conventional wood in between. External costs of heat pumps strongly depend on the origin of the electricity supplied.

Sensitivity analyses were performed for external costs to reflect uncertainties of impacts and variations in monetary valuation. Fossils remain worst performers. External costs of nuclear remain low. Using allocation by exergy, electricity by diesel and natural gas cogeneration ranks worse than oil and natural gas combined cycle, respectively, and never better than renewables or nuclear.