bluuebunny/crossref_metadata_2025 huggingface dataset:
Preliminary Characterization of GaN MBE Epitaxial Layers Grown on Nanoporous 6H-SiC Substrates
Authors: Wolan J. T. , Gopalkrishna A. , Saddow S. E. , Mynbaeu M. , Morkoç H. , Reshchikov M. , Yun F. , Dmitriev V. , Wood C.E.C.
Published: April 2011

We report on the growth and quality of plasma enhanced molecular beam epitaxy (PEMBE) of an n-GaN layer grown on a 6H-SiC substrate, which was prepared to be half nanoporous. A layer of nanoporous SiC was fabricated on a half section of n-type, on-axis (0001)Si face 41 mm commercial wafer by surface anodization. Following H-plasma surface cleaning and in-situ thermal annealing, a 0.7µm thick silicon doped GaN film was deposited at a growth rate of 0.35µm/hr. Substrate temperature during GaN growth was 709oC with a chamber pressure of 1.9 × 10−5Torr. Results of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) characterization suggest improved chemical quality of the film grown on the nanoporous section. Photoluminescence (PL) data indicate greatly reduced nonradiative recombination in the epi layer grown on the nanoporous as compared to the control surface.
Impact of the Growth Polar Direction on the Optical Properties of Gan Films Grown by Metalorganic Vapor Phase Epitaxy
Authors: Setoguchi A. , Yoshimura K. , Sumiya M. , Uedono A. , Chichibu S. F.
Published: April 2011

The growth polar direction during metalorganic chemical vapor phase epitaxy of wurtzite GaN films was shown to affect the optical properties in terms of impurity and vacancy-type defect incorporation during the growth. The GaN film grown towards the Ga- face (0001) (+c polarity) exhibited clear excitonic features in its optical absorption and luminescence spectra up to room temperature. Conversely, the film with the N-face (000-1) (-c polarity) exhibited a broad emission band, which is located in the broadened absorption tail. The Stokes shift remained even at 300 K. The difference between the two was explained in terms of the presence of impurity-induced band tail states in –c GaN due to increased impurity density and enhanced incorporation of large volume vacancy-type defects, which were confirmed by secondary ion mass spectrometry [Sumiya et al., Appl. Phys. Lett. 76, 2098 (2000)] and monoenergetic slow positron annihilation technique.
In Segregation Effects on Optical and Doping Properties of InAlGaN Quaternary for UV Emitting Devices
Authors: Hirayama Hideki , Kinoshita Atsuhiro , Yamanaka Takuya , Hirata Akira , Aoyagi Yoshinobu
Published: April 2011

We demonstrate room temperature intense ultraviolet (UV) emission wavelength ranging 300- 340 nm from InxAlyGa1-x-yN quaternary alloys grown by metal-organic vapor-phase-epitaxy (MOVPE). We found that the UV emission is drastically enhanced by introducing several percent of In into AlGaN. We fabricated single quantum well (SQW) consisting of InxAlyGa1-x-yN quaternary well and barrier, and clearly observed In segregation of sub-micron size from a cathode luminescence (CL) images. The intensity of 320nm-band emission from InAlGaN/InAlGaN QWs were as strong as those of 410nm-band emission from InGaN based QWs, at room temperature. The temperature dependence of photoluminescence (PL) emission for InAlGaN based QWs were much improved in comparison with GaN or AlGaN based QWs. We also grew Mg-doped InxAlyGa1-x-yN quaternary, and obtained hole concentration of 3×1017cm−3 by Hall measurement for high Al content (more than 50%) InxAlyGa1-x-yN quaternary.
Simulation of Stress Generation during GaN Lateral Epitaxial Overgrowth
Authors: Feng Zhaohua , Lovell Edward G. , Engelstad Roxann L. , Kuech Thomas F. , Babcock Susan E.
Published: April 2011

To facilitate an understanding of defect production in gallium nitride during lateral epitaxial overgrowth, computer models have been developed to simulate the complete mechanical stress and strain fields. The virtual process included the deposition of a GaN seed layer on a sapphire substrate followed by a silicon dioxide stencil mask through which, and over which, the GaN product layer evolved and then cooled down to room temperature. Lattice mismatch and thermal strain were continuously assessed. Shear stresses on different crystallographic planes were analyzed to predict dislocation generation.
Step Flow Surface Morphology in Plasma Assisted Molecular Beam Epitaxy Grown GaN
Authors: Kusakabe Kazuhide , Kikuchi Akihiko , Kishino Katsumi
Published: April 2011

The surface morphologies of homoepitaxial GaN films grown by molecular beam epitaxy (MBE) on metalorganic chemical vapor deposition (MOCVD) grown GaN template layers were investigated, using atomic force microscopy (AFM). Typical surface morphology of MBE-grown films on MOCVD-templates was dominated by spiral hillocks due to the high density of dislocations having a screw character and large driving force of MBE growth. Introduction of the AlN multiple interlayer (AlN -MIL) into MBE-GaN layers suppressed the formation of spiral hillocks. It was attributed to obstructing the dislocation propagation by AlN-MIL. Migration enhanced epitaxy (MEE) growth of GaN also reduced the density and tightness of spiral hillocks. This observation was attributed to that MEE growth technique decreased the driving force of growth.
A Study on the Growth of Cubic GaN Films Using an AlGaAs Buffer Layer Grown on GaAs (100) by Plasma-Assisted Molecular Beam Epitaxy
Authors: Kimura Ryuhei , Takahashi Kiyoshi , Grahn H. T.
Published: April 2011

An investigation of the growth mechanism for RF-plasma assisted molecular beam epitaxy of cubic GaN films using a nitrided AlGaAs buffer layer was carried out by in-situ reflection high energy electron diffraction (RHEED) and high resolution X-ray diffraction (HRXRD). It was found that hexagonal GaN nuclei grow on (1, 1, 1) facets during nitridation of the AlGaAs buffer layer, but a highly pure, cubic-phase GaN epilayer was grown on the nitrided AlGaAs buffer layer.
Approaches For Reduction Of The Defect Density In Group III Nitride Based Heterostructures
Authors: Zheleva T. S. , Karoui F. , Kirchner K. , Derenge M. , Jones K. A. , Vispute R. D. , Venkatesan T.
Published: April 2011

Lateral epitaxial overgrowth (LEO), pendeo-epitaxy (PE), and solid-phase epitaxial recrystallization (SPER) are discussed as three approaches for reducing the defect density in group III nitride based heterostructures. Studies of the LEO GaN and PE GaN revealed, that a major factor for the defect reduction in the laterally overgrown regions is the change of the dominant growth direction - from vertical in the window regions to lateral in the regions over the mask or over the trenches, and the related threading dislocations lines redistribution. The mechanisms of defect reduction in LEO GaN and PE GaN are similar, although they arise through different process routes, and are related to the free-standing (PE) or quasi-free-standing (LEO) growth of GaN, and the associated stress redistribution. The stress distributions in the LEO and PE GaN heterostructures are calculated and compared with finite element modeling. Another approach for reduction of the defects is the SPER process and the related thermal activation for dislocation reactions and grain boundary mobility and migration. This approach is shown in the example of annealed AlN films.
Characterization of the Mechanism of Activation for Visible Luminescence in Tb-Doped Crystalline and Amorphous AlN Thin Films
Authors: Spalding C. M. , Caldwell M. L. , Martin A. L. , Dimitrova V. I. , Kordesch M. E. , Richardson H. H. , Patten P. G. Van
Published: April 2011

High-temperature treatments are necessary for luminescence ‘activation’ of sputterdeposited, RE-doped, III-nitride phosphor materials. We report an investigation of the activation process in crystalline and amorphous Tb-doped AlN films. These films were characterized before and after thermal activation at temperatures up to 1000° C by cathodoluminescence (CL), static and time-resolved photoluminescence (PL), and secondary ion mass spectrometry (SIMS). The results suggest that the absence of luminescence in unactivated and in old activated samples is due to rapid quenching of the RE excited state. Furthermore, the quenching in the activated, aged samples appears to be due to sorption of ambient water vapor. Energy transfer to a harmonic of the O-H vibration is a likely quenching pathway in these samples. Unfortunately, this water-related quenching mechanism is implausible as a candidate in freshly-made, unactivated samples since water is excluded from the growth chamber.
Unusual Surface Undulations Observed in Ge/Si(100) Multilayers
Authors: Pai Woei Wu , Peng Y. H. , Chung W. F. , Wang S. Y. , Chen K. N. , Cheng H. H.
Published: April 2011

Multilayer Ge/Si quantum wells grown on Si(100) at lower temperatures (300 C for Ge and 435 C for Si) exhibit unexpected large surface undulations. These groove-like features along <110> directions are correlated to the underlying dislocation networks. Studies on the dependence of surface roughness versus Ge spacer thickness exclude kinetic limitation as the cause of the observed troughs. Further comparison of the surfaces for multilayer and single-layer samples indicates a combination of the multilayer structure and the inhomogeneous surface strain field greatly enhances the undulations.
Epitaxial growth of (001) and (111) Ni films on MgO substrates
Authors: Lukaszew Rosa Alejandra , Stoica Vladimir , Uher Ctirad , Clarke Roy
Published: April 2011

Metal-ceramic interfaces are important in applications as diverse as magnetic storage media and supported catalysts. It is very important to understand how the crystallography and microstructure of metallic films deposited onto ceramic substrates depend on growth and/or annealing conditions so that their physical properties (e.g. magnetic, electronic, etc.) can be tailored for specific applications. To this end, we have studied the epitaxial growth and annealing of (001) and (111) Ni films MBE grown on MgO substrates, where we have observed the evolution of the surface using correlated in- situ RHEED (reflection high-energy-electron diffraction) and STM (scanning tunneling microscopy) measurements.
Studies of Silicon Oxynitride Films Produced by Radio-Frequency Plasma Assisted Electron Cyclotron Resonance
Authors: Brewer J.D. , Raveh A. , Irene E.A.
Published: April 2011

Single crystal silicon wafers were treated by direct oxynitridation using electron cyclotron resonance (ECR) and radio-frequency (rf) plasma with separate N2 and O2 gas sources. Fabricated layers were characterized by ex situ spectroscopic ellipsometry (SE), Auger electron spectroscopy (AES), and atomic force microscopy (AFM).

The thickness and nitride/oxide ratio of silicon oxynitride layers were found to be dependent on the main process variables, namely gas pressure, bias voltage, and N2/O2 flow rate ratio. It was also observed that ECR/rf plasma is more efficient for the formation of layers with higher nitrogen content at relatively low pressure (≤ 200 mTorr) and bias voltage (−40 V) when compared to that of rf plasma alone. SE modeling provided good fitting to experimental data, while AES and AFM analyses supported SE results. The effects of fabrication conditions on the thickness and composition of the layers will be presented.
Growth Front Roughening of Room Temperature Deposited Oligomer Thin Films
Authors: Tsamouras D. , Palasantzas G. , De Hosson J. Th. M. , Hadziioannou G.
Published: April 2011

Growth front scaling aspects are investigated for PPV-type oligomer thin films vapor- deposited onto silicon substrates at room temperature. For film thickness d~15-300 nm, commonly used in optoelectronic devices, correlation function measurement by atomic force microscopy yields roughness exponents in the range H=0.45±0.04, and an rms roughness amplitude which evolves with film thickness as a power law σ∝ dβ with β=0.28±0.05. The non-Gaussian height distribution and the measured scaling exponents (H and β) suggest a roughening mechanism close to that described by the Kardar-Parisi-Zhang scenario.
Investigation of GaN epilayers growth mechanisms using in-situ reflectance in MOCVD
Authors: Moret Matthieu , Briot Olivier , Ruffenach-Clur Sandra , Aulombard Roger-Louis
Published: April 2011

GaN is a wide gap semiconductor which is now used to produce blue and green light emitting diodes, blue laser diodes and which has numerous other potential applications, like high frequency HEMT transistors, UV sensors, etc. A complicated two step process, using a low temperature buffer layer, subsequently annealed and followed by the deposition of the monocrystalline semiconductor was developed, and due to the excellent results obtained following this method, a rush towards applications resulted. There is now a need to investigate in more detail the growth mechanisms, and the influence of the growth parameters, in order to ensure a better reproducibility of the results. In this paper, we report an investigation of the growth mechanisms and the influence of the growth parameters using in-situ reflectance experiments. The reflectance measurements allow us to follow the growth rates, and the changes in the surface morphology (transitions between islands growth and 2D growth). Additional exsitu measurements (AFM ) were performed at different stages of the growth process to ensure additional information. As a result, we demonstrate that the recrystallization of the low temperature buffer layer is a critical step, which is drastically influenced by the composition of the annealing atmosphere (amount of ammonia present in the gas phase), while the deposition temperature and buffer thickness have a moderate effect. We will discuss here the growth mechanisms which may be involved to explain such a behavior.
Investigation of The Oxidation Behaviour of A TiAlCrYN Pvd Hard Coating
Authors: Lembke Mirkka I. , Titchmarsh John M. , Lewis D. Brian , Münz W.-Dieter
Published: April 2011

For dry high speed cutting the oxidation resistance of the protective hard coating of the cutting tool surface is very important. Therefore the effects of heat treatment on a TiAlN based hard coating deposited by the combined cathodic arc unbalanced magnetron sputtering technique have been studied using cross sectional transmission electron microscopy (XTEM) and energy dispersive X-ray analysis (EDX). The combination of these analytical techniques revealed the diffusion paths and preferences in diffusion of various coating and substrate elements in a physical vapour deposited (PVD) type coating after heat treatment. The structure comprises a ~2 µm thick TiAlCrYN coating on top of a 0.25 µm thick TiAlCrN base layer deposited on a stainless steel substrate. In the as-deposited sample Y was distributed in a fine layered structure (1.7 nm) throughout the coating. The coating was heat treated at temperatures between 600 °C and 900 °C in air for 10 hrs duration. With increasing temperature the microstructure changed gradually from interrupted columnar growth to a fully columnar structure at 900 °C as observed with XTEM. EDX analysis after heat treatment at 700 °C showed the presence of substrate elements Fe and Cr mainly at column boundaries in the base layer. In contrast no evidence of substrate elements could be observed in the TiAlCrYN coating, thus showing a sharp change in elemental composition concerning Cr and Fe between base layer and coating. This indicates that Y segregation in the TiAlCrYN coating along column boundaries inhibited column boundary diffusion of the substrate elements Cr and Fe. Energy dispersive X-ray distribution maps recorded after 800 °C annealing showed distinct segregation of Y along the column boundaries. The substrate elements, Fe and Cr, were observed through the coating along column boundaries up to 0.95 µm from the base layer/coating interface. After heat treatment at 900 °C the substrate elements had diffused from the substrate/coating interface to the coating surface. Y out-diffused, too and was concentrated adjacent to TiO2 crystals in the oxide layer.
Surface defects on MgO thin films: Their detection using metastable impact electron spectroscopy and interaction with probe molecules
Authors: Stultz Jeffrey A. , Kolmakov Andrei , Lai Xiaofeng , Kim Young Dok , Goodman D. Wayne
Published: April 2011

MgO thin films having different defect densities are explored in this study using metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), and scanning tunneling microscopy (STM). Surface point defects on MgO exhibit themselves in both the MIES and UPS spectra as a feature approximately 2 eV above the valance band, whereas extended defects are only observed spectroscopically as a broadening of the O 2p band. The interaction of NO and N2O with the MgO surface as a function of surface defect density is explored. Upon adsorption on MgO thin films at 100K, both NO and N2O show the development of three features which coincide with a standard gas phase N2O spectrum. The saturation coverage of N2O from NO adsorption increases with increasing defect density, indicating that defect sites are mainly responsible for N2O formation. STM images confirm the increase of thin film defect density upon thermal quenching.
Synchrotron X-ray Studies of Molecular Ordering in Confined Liquids
Authors: Kim Hyunjung , Seeck O. H. , Lee D. R. , Kaendler I. D. , Shu D. , Basu J. K. , Sinha S. K.
Published: April 2011

X-ray specular and off-specular reflectivity studies have been carried out to study the density modulations in liquids confined between two smooth silicon mirrors. The special technique as well as the advantages of using high energy and high brilliance synchrotron x-ray beams for carrying out such experiments will be discussed. Results will be presented on the ordering of octamethyl-cyclotetrasiloxane (OMCTS) as a function of the confining pressure, where we find evidence of layering as the gap is decreased from macroscopic down to a few nanometers.
Spectroscopic Studies of Liquid Crystals Confined in Sol-gel Matrices
Authors: Fehr Carlos , Dieudonne Philippe , Bac Christophe Goze , Gaveau Philippe , Sauvajol Jean-Louis , Anglaret Eric
Published: April 2011

Thermotropic liquid crystals (5CB and 8CB) were confined in silica porous matrices (xerogels and xero-aerogels) with different pore sizes. The structure and dynamics of confined liquid crystals were studied by Raman spectroscopy and Nuclear Magnetic Resonance. In Raman, the frequency of the CN stretching peak is a good probe of the smectic_A-crystal phase transition. The CN peak is observed to split upon quenching. This suggests a coexistence of crystalline and supercooled liquid phases for confined LC which was not observed in the bulk. In NMR, strong differences in both chemical shifts and linewidths are observed in confinement with respect to the bulk. We present a model which analyses the changes in the spectra in terms of changes of the order parameter and molecular dynamics for the confined LC.
Surface and Volume Diffusion of Water and Oil in Porous Media by Field Cycling Nuclear Relaxation and Pgse NMR
Authors: Godefroy S. , Korb J.-P. , Petit D. , Fleury M. , Bryant R. G.
Published: April 2011

The microdynamics of water and oil in macroporous media with SiO2 or CaCO3 surfaces has been probed at various temperatures by magnetic field-cycling measurements of the spin-lattice relaxation rates. These measurements and an original theory of surface diffusion allowed us to obtain surface dynamical parameters, such as a coefficient of surface affinity of the liquid molecules and the surface diffusion coefficient. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones. Complementary information on the nature of the solid-liquid interface was given by NMR chemical shift experiments at high magnetic field.
The Formation of Striped Phases by Charge Localization in Vanadium Phosphates
Authors: Bontchev Ranko P. , Do Junghwan , Jacobson Allan J.
Published: April 2011

The compound (H3O)1.5{[VO(H2O)]PO4}3·2.5H2O is a new member of a series of vanadium phosphates containing vanadium in both plus four and five oxidation states. Charge ordering occurs in these mixed vanadium phosphate layered compounds forming striped phases with different periodicities. The VOPO4 layers undulate as a consequence of differences in the O-O atom distances in the ordered V(V)O6 and V(IV)O6 octahedra.
A New Three-dimensional Lanthanide Framework Constructed by Oxalate and 3,5-pyridinedicarboxylate
Authors: Lawandy Michael A. , Pan Long , Huang Xiaoying , Li Jing , Yuen Tan , Lin C. L.
Published: April 2011

Hydrothermal reactions of rare-earth metal acetates and nitrates with oxalic acid (H2C2O4) and 3,5-pyridinedicarboxylic acid (H2pddc) in triethylamine/water solutions have yielded five isostructural polymers with the general formula: [Ln(pddc)(C2O4)1/2(H2O)2]·H2O. [Ln = La(1), Pr(2), Nd(3), Eu(4) and Er(5)]. These compounds crystallize in monoclinic crystal system, space group P21/n, Z = 4, with only slight variations in their unit cell parameters: 1 a = 7.747(2), b = 9.954(2), c = 15.134(3) Å, = 98.64(3)°, V = 1153.8(4) Å3; 2 a = 7.707(2), b = 9.895(2), c = 15.006(3) Å, β = 98.54(3)°, V = 1131.7(4) Å3; 3 a = 7.688(2), b = 9.897(2), c = 14.955(3) Å, β = 98.43(3)°, V = 112 5.6(4) Å3; 4 a = 7.638(2), b = 9.842(2), c = 14.809(3) Å, β = 98.42(3)°, V = 1101.2(4) Å3; 5 a = 7.573(2), b = 9.761(2) Å, c = 14.630(3) Å, β = 98.10(3)°, V = 1070.7(4) Å3. The structure of these compounds is composed of 2D Ln(pddc) layers that are interconnected by chelating oxalate. Within the layer, each rare-earth metal forms a monodentate bond with each of the four pddc groups. The metal centers in the neighboring layers are bridged through µ4-oxalate, resulting in a three-dimensional framework. The remaining two sites around the eight-coordinate Ln are occupied by water molecules. Compounds 2, 3, and 5 exhibit paramagnetic behavior and 1 is diamagnetic. They are thermally stable up to 250°C.
Two Pillared Three-Dimensional Inorganic/Organic Hybrid Structures Composed of Bimetallic Layers Bridged by Exo-bidentate Ligands
Authors: Pan Long , Huang Xiaoying , Li Jing
Published: April 2011

The hydrothermal reactions of Cu(NO3)2.3H2O, K2CrO4 and exo-bidentate ligand bpy/bpe, (bpy = 4,4'-bipyridine, bpe = trans-1,2-bis(4-pyridyl)ethylene) result in two pillared 3D organic-inorganic hybrid compounds. Both contain bimetallic layers of copper(II) cations and chromate anions, which are directly connected by different organic ligands, leading to pillared structures with varying pillar length and thickness of layers. Crystal data for [Cu3(CrO4)2(OH)2(bpy)2] (1): triclinic, P1, a = 5.358(1), b = 5.603(1), c = 13.515(3) Å, α = 81.78(3), γ = 86.56(3), Å3 = 79.75(3)°, V = 394.91(13) Å3, Z = 1; [CuCrO4(bpe)] (2): monoclinic, C2/c, a = 23.282(5), b = 11.917(2), c = 9.726(2) Å, β = 112.99(3)°, V = 2484.2(8) Å3, Z = 8.
ESCA and Vibrational Spectroscopy of Alkali Cation Exchanged Manganic Acids with the 2×2 type Tunnel Structure Synthesized via Different Chemical Routes
Authors: Tsuji Masamichi , Kanoh Hirofumi , Ooi Kenta
Published: April 2011

Manganese dioxides have received much attention over the last two decades as ion exchangers. Actually these are typically mixed-valence compounds and the terminology of ‘dioxide’ is not appropriate. The mechanisms for their variety of chemical reactivities are still open for study. On the cation uptake mechanism there are strong claims that a redox process is involved in cation uptakes by manganic acids synthesized by substituting H+ for alkali cations incorporated in ‘hydrous manganese dioxides’. The present work was carried out to physically demonstrate the alkali cation exchange mechanism on tunnel-structured manganic acids and to study the ion exchange with lattice vibrational spectroscopy. Manganic acids were prepared through the redox process using KMnO4 and MnSO4, and thermal decomposition of (CH3)3COK and MnCO3 at 530°C. ESCA spectra of their alkali cation exchanged forms indicated no evidence of redox process and supported the ion exchange mechanism on these materials. Their infrared absorption spectra strongly depended on their preparation routes and are closely related to their ion-exchange selectivity of each material. Thus, the vibrational spectra of manganic acids take an important role as a synthesis index together with XRD patterns.
Study of Calcium/Lead Apatite Structure Type for Stabilising Heavy Metals
Authors: Dong Z.L. , Wei B. , White T.J.
Published: April 2011

(CaxPb10−x)(VO4)6F2 apatites were synthesised and their microstructures were studied using powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy before and after leach testing. X-ray diffraction showed that the apatites were hexagonal with a ≍ 10Å and c ≍ 7Å. During the leach test, Pb was released into the solution more slowly than Ca, which is desirable as the immobilisation of Pb is of importance. The experimental results also showed that V was almost undetectable in the leaching test solutions. In the (Ca7Pb3)(VO4)6F2 pellet, Ca and Pb distributions were not homogenous from one grain to another. Microstructural evidence from scanning electron microscopy revealed that the dissolution via development of etch pits began at grain boundaries and inside grains, and progressed faster in Ca rich regions. These results suggest that apatites of high Pb to Ca ratio are more durable.
Nickel Determination by Complexation Utilizing a Functionalized Optical Waveguide Sensor
Authors: Carter Erin S. , Dahmen Klaus-H.
Published: April 2011

This research focuses on the design of chemically functionalized optical waveguide sensors. The waveguide is an optically transparent sol-gel coated onto a glass substrate chip. By having a higher refractive index than the substrate, the waveguide internally reflects a laser beam to photodetectors at both ends of the chip. The adsorption of any species onto the waveguide surface changes the light propagation, and therefore its effective refractive index, N. The change in N is dependent upon the amount of analyte present. By covalently bonding specific chemical receptors onto its surface, it can be designed to target a particular analyte. This research involves functionalizing the surface of the waveguide with ED3A in order to complex out of solution Ni2+. The change in N and the thickness of the adlayer will allow the concentration to be determined.
In situ XAFS study on cathode materials for lithium-ion batteries
Authors: Nonaka Takamasa , Okuda Chikaaki , Ukyo Yoshio , Okamoto Tokuhiko
Published: April 2011

Ni and Co K-edge X-ray absorption spectra of LiNi0.8Co0.2O2 have been collected using in situ coin cells. To investigate the electronic and structural changes accompanied by the capacity fading during electrochemical cycling and keeping batteries at high temperatures, the cells with different cycling states and keeping conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted towards higher energy, and the good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. From quantitative analysis of EXAFS data, it was revealed that the capacity fading is closely related to the Jahn-Teller distortion of the NiO6 octahedron.
Relation between Erbium Photoluminescence and Dangling-Bond Defects in a-Si:H
Authors: Kumeda Minoru , Itoh Hiroshi , Shitakata Norio , Shimizu Tatsuo
Published: April 2011

In order to study a role of Si dangling bonds in the Er photoluminescence mechanism in a-Si:H, we prepared Er-doped a-Si:H films with various densities of Si dangling bonds using rf magnetron sputtering method. The density of Si dangling bonds was changed by three ways: (1) change of Ar-to-H2 gas ratio in the sputtering ambient, (2) light soaking and (3) annealing. The intensity of the Er photoluminescence at 1540 nm tends to decrease with increasing the density of Si dangling bonds in the first two cases. In the case of annealing, the intensity of the Er photoluminescence first increases with a decrease in the density of Si dangling bonds due to the structural relaxation up to 250 °C and continues to increase up to 350 °C even though the density of Si dangling bonds increases due to hydrogen evolution. This increase in the photoluminescence intensity is probably caused by a change of the environment of Er ions.
Light-induced Annealing of Deep Defects in Low Ge Fraction a-Si,Ge:H Alloys: Further Insights into the Fundamentals of Light-induced Degradation
Authors: Cohen J.David , Heath Jennifer , Palinginis Kimon C. , Yang Jeffrey C. , Guha Subhendu
Published: April 2011

Modulated photocurrent (MPC) was employed to study metastable deep defect creation and annealing in low Ge fraction a-Si,Ge:H alloys (Ge fractions below 10at.%). These MPC spectra reveal two distinct bands of deep defects identified as neutral Si and neutral Ge dangling bonds. Upon heating in the dark from a strongly light degraded state, these are reduced in direct competition to each other, therefore implying a global reconfiguration mechanism. We have extended our studies to compare purely thermal annealing with light-induced annealing in these alloys. We have found that the relative anneal rate for the two types of defects differs from the case of purely thermal annealing. This therefore tends to rule out models in which the rate limiting step during annealing comes from the release of the mediating entity from a remote site.
Network Rebonding Model for Metastability in Amorphous Silicon
Authors: Biswas R. , Pan B.C. , Ye Y.
Published: April 2011

We propose a new network rebonding model to describe light-induced metastability in hydrogenated amorphous silicon. This model involves global changes in both the silicon and hydrogen bonding sites. The first step in the process is the breaking of a weak silicon bond generating a dangling bond and floating bond pair. The mobile floating bond diffuses away leaving behind an isolated dangling bond in agreement with ESR measurements. Tight-binding molecular dynamics simulations show clear evidence for each of these processes. Floating bonds are an intermediate transient species that do not remain in the light-soaked steady state. Floating bonds annihilate by reacting with SiH bonds and locally displacing hydrogen atoms. This model provides a new explanation for several major experimental features of the Staebler-Wronski effect, including the t1/3 kinetics for the growth of defects, and the anticorrelation of dangling bonds with H. This new model provides a new platform for understanding the atomistic origins of light-induced degradation.
Tailoring the Structure of Low-Temperature-Deposited Microcrystalline Silicon Films by Biasing the Substrate
Authors: Birkholz Mario , Selle Burkhardt , Fuhs Walther , Williamson Don L.
Published: April 2011

Biasing the substrate during deposition and the substrate's surface morphology may both have major effects on the structural properties of thin films. We present the results of structural investigations (Raman and FTIR spectroscopy, XRD, SAXS) of thin silicon films that were prepared at low temperatures by electron-cyclotron resonance (ECR) chemical-vapor deposition. The effect of substrate bias during the deposition was investigated for positive DC susceptor biases VB ranging from 0 to 45 V. For stainless steel substrates with an artificially enlarged surface roughness (smart substrates), an increase of the crystallinity could be observed with Raman spectroscopy. Films prepared under a susceptor bias of +15 V exhibited a texture inversion of preferential (220)- to (111)- oriented grains, which was accompanied by an increase in grain size from 18 to 42 nm. Small-angle X-ray scattering (SAXS) revealed the films as deposited on Al foil to exhibit significant free volume fractions (microvoids). The ability of tailoring the structure of thin Si films by applying a bias is discussed in terms of controlling the energy and intensity of ion flux to the surface of the growing film. This can efficiently be achieved in an ECR system, where the mean free path of gas particles exceeds the thickness of the plasma sheath.
Microscopic Aspects Of Charge Transport In Hydrogenated Microcrystalline Silicon
Authors: Fejfar Antonín , Mates Tomáš , Koch Christian , Rezek Bohuslav , Švřek Vladimír , Fojtík Petr , Stuchlíková Ha , Stuchlík Jiř́ , Kočka Jan
Published: April 2011

Charge transport in hydrogenated microcrystalline silicon (µc-Si:H) is determined by structure on several size scales: i) local atomic arrangement (<1 nm), ii) crystalline grains and their boundaries (1-10 nm), iii) grain aggregates or columns (0.1-1 µm) and finally iv) features comparable to layer thickness (0.1-10 µm). We first summarize our experimental results concerning these effects: differences of conductivities of grains and amorphous tissue measured locally by conductive AFM, transport anisotropy observed by comparing dark conductivity and ambipolar diffusion length parallel and perpendicular to the substrate, and finally thickness dependence of transport parameters (e.g. dark conductivity activation energy and prefactor). Most of these phenomena can be described by using a novel model of the µc-Si:H growth leading to a structure known as Voronoi adjacency network. The model is based on the nearest neighbor constrained growth. To our knowledge, the Voronoi structure is the first structural model able to predict structure and transport properties of the µc-Si:H and it may become a basis for the future predictive model of µc-Si:H based solar cells.
P-channel Polycrystalline Silicon Thin Film Transistors on Steel Foil Substrates
Authors: Wu Ming , Wagner Sigurd
Published: April 2011

We fabricated self-aligned polycrystalline silicon (polysilicon) thin film transistors on flexible steel substrates. The polysilicon was formed by furnace crystallization of hydrogenated amorphous silicon at 950°C/20sec or 750°C/2min. The TFTs made from these polysilicon films have hole field effect mobilities in the linear regime of 22 cm2·V−1s−1 (950°C) and 14 cm2·V−1s−1 (750°C). The OFF current at 10 V drain-source voltage is 10−10A and the drain current ON/OFF ratio is ∼106.
Microcrystalline silicon solar cells prepared by 13.56 MHz PECVD at high growth rates: Solar cell and material properties
Authors: Roschek Tobias , Rech Bernd , Beyer Wolfhard , Werner Peter , Edelman Felix , Chack Albert , Weil Raoul , Beserman Robert
Published: April 2011

Microcrystalline silicon (μc-Si:H) solar cells were prepared in a wide range of deposition parameters using 13.56 MHz plasma-enhanced chemical vapour deposition (PECVD). The best μc-Si:H solar cells were prepared close to the transition to amorphous silicon (a-Si:H) growth at very high deposition pressures (∼10 Torr) showing solar cell efficiencies up to 8.0 % at a deposition rate of 5ÊÅ/s. Investigations of the solar cells were performed by Raman spectroscopy and transmission electron microscopy (TEM). TEM measurements revealed similar structural properties with similar high crystalline volume fractions for these cells although they showed distinctly different efficiencies. However, an increased amorphous volume fraction was detected by Raman spectroscopy for the low efficiency cells prepared at low deposition pressures. This result is attributed to an increased ion bombardment at low pressures.
Solar Cell Performance Under Different Illumination Conditions
Authors: Gemmer Christian , Schubert Markus B.
Published: April 2011

We numerically simulate performance data of hydrogenated amorphous silicon (a-Si:H) and copper indium gallium diselenide (CIGS) based solar cells for various illumination conditions. For ease of comparison, we model typical single junctions with the very same software. The study allows us to evaluate the cell feasibility in different hybrid electronic systems like smart cards, wrist watches, transponder systems, and mobile sensors. At an illumination intensity of 1 sun, the optical bandgap of the absorber material and the series resistances determine the spectral sensitivity of the solar cell to particular illumination spectra. For intensities of 10-2 suns and so-called D65 spectrum, which represents daylight under cloudy skies, the efficiency of a-Si:H solar cells nearly equates the CIGS cell performance although the AM 1.5 efficiency of the CIGS diode exceeds the one of our a-Si:H cell by more than a factorof two. Infrared-weighted black body radiation leads to superior performance of the CIGS type, whereas for ultraviolet-weighted illumination the a-Si:H cell shows better performance. For intensities below 10-4 suns theexternal shunt resistance dominates the current-voltage characteristics of both cell types, resulting in poor performance independent of the incident spectrum. We complete our study by simulating the solar-powered charging process of a gold capacitor, which serves us as a model for the energy storage within a hybrid electronic system. The charging behavior under various realistic illumination conditions shows particular cellcharacteristics: high open circuit voltages qualify a-Si:H solar cells for electronic systems that require increased voltages and CIGS cells are suited for applications with higher current need.
Hydrogenated Amorphous Silicon / ZnO Shottky Heterojunction for Position Sensitive Detectors
Authors: ǵuas H. , Nunes P. , Fortunato E. , Silva R. , Silva V. , Figueiredo J. , Soares F. , Martins R.
Published: April 2011

In this work a new structure is proposed for position sensitive detectors consisting of glass/Cr/a-Si:H(n+)/a-Si:H(i)/ZnO, where the ZnO forms an heterojunction with the a-Si:H(i). The results show that this structure works with success in the fabrication of linear position sensitive detectors. The devices present a good nonlinearity of ͌ 2% and a good sensitivity tothe light intensity. The main advantages of this structure over the classical p-i-n are an easier to built topology and a higher yield due to a better immunity to the a-Si:H pinholes, since the ZnO does not diffuse so easily into a-Si:H as the metal does, which are the cause of frequent failure in the p-i-n devices due to short-circuits caused by the deposition of the metal over the a-Si:H. In this structure the illumination is made directly on the ZnO, so a transparent substrate is not needed and a larger range of substrates can be used.
Basic mechanisms of structural relaxation and diffusion in amorphous silicon
Authors: Barkema G. T. , Mousseau Normand , Vink R.L.C. , Biswas Parthapratim
Published: April 2011

The low-temperature dynamics in amorphous silicon occurs through a sequence of discrete, activated events that reorganize the topology of the network. In this review, we present some recent work done to understand better the nature of these events and the associated dynamics ina-Si. Using the activation-relaxation technique (ART), we generated more than 8000 events in a 1000-atom model ofa-Si, providing an extensive database of relaxation and diffusion mechanisms. The generic properties of these events, such as the number of involved atoms and the activation energies, were investigated and foundto be in agreement with experimental data. As it turns out, the bond-transposition mechanism proposed by Wooten, Winer and Weaire (WWW) some time ago plays an important role in the events generated by ART. We have therefore turned to an optimized version of the WWW algorithm to generate the best overall configurations ofa-Si available today. We discuss the details of the optimization and present the structural and electronic properties of the resulting models.
Properties of Large Grain-Size poly-Si Films by Catalytic Chemical Sputtering
Authors: Masuda Atsushi , Kamesaki Koji , Izumi Akira , Matsumura Hideki
Published: April 2011

Large grain-size polycrystalline Si (poly-Si) films are obtained on glass substrate by newly developed catalytic chemical sputtering method at low temperatures around 400 C. Si films are also epitaxially grown on (100) single-crystalline Si substrates. In the method Si films are deposited by the chemical transport of SiH4 species generated by the reaction between solid Si target and catalytically generated H atoms. Efficient deposition is realized using the remarkable difference in the etch rate depending on Si target temperatures. That is, SiH4 species are efficiently generated on cooled Si target by atomic-H etching and deposited on substrates with suppressed etching phenomena by heating. Full-width at half maximum of transverse-optical Raman signals originating from crystalline phase for the obtained poly-Si films is narrower than that for poly-Si prepared by excimer-laser annealing. It was noticeable that the grain size exceeds 1 m for the films with a thickness of about 1 m. Growth mode of poly-Si films especially in the initial stage is remarkably changed with a difference in the substrate material. It was found that formation of seed layer enhances the growth of poly-Si films on glass substrate.
Phase Diagrams for the Optimization of rf Plasma Enhanced Chemical Vapor Deposition of a-Si:H: Variations in Plasma Power and Substrate Temperature
Authors: Ferlauto Andre S. , Koval Randy J. , Wronski Christopher R. , Collins Robert W.
Published: April 2011

Real time spectroscopic ellipsometry (RTSE) has been applied to characterize surface roughening transitions during the growth of undoped hydrogenated silicon (Si:H) thin films by rf plasma-enhanced chemical vapor deposition (PECVD). In particular, the amorphous–to–amorphous surface roughening transition [→] and the amorphous–to–(mixed-phase-microcrystalline) roughening transition [a→(a+µc)] observed during Si:H growth have been studied under different PECVD conditions of hydrogen dilution ratio R=[H2]/[SiH4], rf plasma power P, and substrate temperature T. For Si:H growth on crystalline Si substrates under the different conditions, phase diagrams have been constructed by plotting the bulk film thicknesses at which these transitions occur as a function of R. The effects of the substrate (c-Si wafers versus amorphous Si:H films) on the a→(a+µc) transition of the phase diagram are also explored. The results provide deeper insights into recent attempts to improve the material properties and solar cell performance for a-Si:H i-layers prepared by rf PECVD at higher rates.
Cluster-less Plasma CVD Reactor and Its Application to a-Si:H Film Deposition
Authors: Shiratani Masaharu , Koga Kazunori , Watanabe Yukio
Published: April 2011

Even under the conventional device-quality discharge conditions, large amount of clusters are found to exist in silane radio frequency (RF) discharges. The size and density of clusters near the substrate on the grounded electrode are 2 nm and 1011 cm-3, respectively. In order to prevent such clusters from depositing on hydrogenated amorphous silicon (a-Si:H) films, we have developed a cluster-less plasma CVD reactor which can significantly suppress cluster amount by using gas viscous and thermophoretic forces exerted on clusters, and by reducing gas stagnation region. Using the reactor, we have demonstrated deposition of ultra high quality a-Si:H films which have a significantly less concentration of Si-H2 bonds compared to conventional device-quality films.
A Novel Back-Reflecting UV-Assisted Metal-Induced Crystallization of Silicon on Glass
Authors: Rezaee Leila , Mohajerzadeh Shamsoddin , Khakifirooz Ali , Haji Saber , Soleimani Ebrahim Asl
Published: April 2011

A novel method of UV-assisted metal-induced-crystallization is introduced to grow polysilicon films on ordinary glass at temperatures as low as 400°C. Annealing is accomplished in the presence of an ultra-violet exposure, leading to high crystallinity of the silicon film as confirmed by XRD, TEM and SEM analyses. A back-reflecting chromium layer is incorporated to further trap UV photons and enhance their absorption in the silicon film. This results in a significant increase in the crystallization rate as studied by XRD spectroscopy. A growth rate of 2 µm/hr is observed at 400 °C, when employing this method for lateral crystallization. Thin-film transistors fabricated using the proposed UV-assisted MILC show a threshold voltage of 1V and hole mobility of about 50 cm2/V.s.
Large Red Shift of PL Peak Energy in High Growth Rate a-Si:H Prepared by Hot-Wire CVD
Authors: Han Daxing , Yue Guozhen , Weinberg-Wolf Jennifer , Owens Jessica M. , Xu Yueqin , Wang Qi
Published: April 2011

We characterized the electronic states and microstructure of high-growth-rate a-Si:H films by employing photoluminescence (PL) and Raman spectroscopies. The growth rate was from 50 to 115 Å/s compared to the standard rate of less than 10 Å/s. For the high-growth-rate a-Si:H films, we observed typical a-Si:H features in Raman but new features in PL. The new PL features are: a) the PL peak energy is as low as ∼1.15 eV compared to the standard ∼1.4 eV at 80 K; and b) the total intensity is more than one order of magnitude higher then the standard. We suggest that the nano-scale microstructure may be responsible for the anomalous PL features.
Stress Induced Lateral Concentration Profiles in SiGe Layers Grown on Si(001) Non-Planar Substrates
Authors: Eshed Anat , Beserman Robert , Dettmer Klauss
Published: April 2011

In this paper we report lateral variation of Si and Ge concentration profiles measured in SiGe layers grown on mesa shaped Si(001) substrates. Si(001) substrate was patterned using conventional lithography process in forms of mesa with two crystal planes: (001) top plane and the etched (111) side plane. 50 nm thick Si0.7Ge0.3 strained epitaxial layer was grown using Molecular Beam Epitaxy (MBE), on top of the substrate. Thickness on the center of both crystal planes agrees with the expected values. Lateral image of composition profiles has been obtained from Micro Raman spectra taken at various locations on the mesa. Lower concentration of Ge was found at the vicinity of the edges of the (001) planes relative to the centers. We introduce a diffusion model using stress enhanced activation energy to explain our data, taking into account stress profiles along the mesas, by modifying the activation energy for planar diffusion model. The planar diffusion length was used as a fitting parameter in the model, with good agreement to known values from the literature.
Microscopic Analysis of Residuals on Polycrystalline CdTe Following Wet CdCl2 Treatment
Authors: Gessert Timothy A. , Romero Manuel J. , Perkins Craig L. , Asher Sally E. , Matson Rick , Moutinho Helio , Rose Doug
Published: April 2011

In this study we report on the spatial distribution and composition of residuals on the CdTe surface following a typical wet CdCl2 treatment, and the effect that our ion-beam milling has on this residual-coated surface. Results show that residuals are spatially discrete, located primarily along grain boundaries, and are likely a cadmium oxychloride. Results also show that the residuals may penetrate deep into the CdTe surface such that typical ion-beam milling procedures do not produce complete residual removal.