bluuebunny/crossref_metadata_2025 huggingface dataset:
MBE Growth and Optical Characterization of InGaN/AlGaN Multiquantum Wells
Authors: Singh R. , Herzog W.D. , Doppalapudi D. , ÜnlÜ M.S. , Goldberg B.B. , Moustakas T.D.
Published: April 2011

We report the growth of InGaN/AIGaN MQWs on c-plane sapphire by electron cyclotron resonance assisted molecular beam epitaxy (ECR-MBE). Two types of structures were investigated; one employing a GaN and the other a A1GaN barrier layer. The first structure consists of five periods of 80 Å thick In0.09Ga0.91N wells separated by 90 Å thick GaN barriers. The second structure consists of|seven periods of 120 Å thick In0.35Ga0.65N wells and Al0.1Ga0.9N barriers. The substrate temperature was kept constant during the growth of both the wells and the barriers, thus avoiding the need for any temperature cycling during the growth, which may lead to interfacial contamination. The films were characterized by cross sectional transmission electron microscopy (TEM), room temperature photoluminescence (PL) and sub-micron resolution luminescence microscopy. TEM images show sharp and abrupt interfaces, thus confirming the high interfacial quality of the MQW structures. Both structures exhibit strong RT luminescence emission peaking at 387 nm (FWHM = 16nm) for the In0.09Ga0.91N/GaN structure and at 463 nm (FWHM = 28nm) for the In0.35Ga0.65N/A10.1Ga0 9N structure. The high resolution luminescence microscopy studies reveal that the radiative recombination for the InGaN quantum wells is 60–70 times more efficient than for the underlying GaN film.
Growth and Characterization of AlN on 6H-SiC Substrates
Authors: Lekova M. , Auner G. W. , Jin F. , Naik R. , Naik V.
Published: April 2011

AIN/SiC structures are of current importance for the development of high temperature electronic devices. A systematic study of the growth and structure of AIN on Lely grown 6H-SiC was performed. The SiC substrates were chemically etched followed by thermal desorption at 875 °C under UHV conditions. AFM and RHEED images show the relative improvement in surface morphology following the cleaning procedure. AIN films were grown on 6H-SiC substrates using plasma source molecular beam epitaxy (PSMBE). Substrate temperatures were varied from 400 °C to 800 °C and deposition energies from 1 eV to 25 eV during AIN growth. The AIN crystal structure and topology were analyzed by RHEED and AFM. The AIN/SiC film microstructure are correlated with the quality of the SiC surface and the parameters of the AIN deposition process. Epitaxial growth of AIN(0002) was strongly dependent on the SiC surface quality and deposition energy. The highest quality AIN was achieved at growth temperatures of approximately 600 °C and 15 eV ion energy. Higher ion energy resulted in amorphous film growth.
Low Pressure CVD of GaN from GaCl3 and NH3
Authors: Topf M. , Koynov S. , Fischer S. , Dirnstorfer I. , Kriegseis W. , Burkhardt W. , Meyer B.K.
Published: April 2011

We report on the heteroepitaxial growth of GaN from GaCl3 and NH3 on (0001) A12O3 and (0001) 6H-SiC substrates. In order to enable homogeneous growth within the entire deposition zone one has to use low process pressures in the 10-1 mbar range, where still a growth rate of ∼ 2 μm/h can be achieved. We present a simple model to describe our process and explain our observations. A comparison of GaN deposited on different substrates and with GaN buffer layers is given by low temperature Photoluminescence (PL). Furthermore, impurities are traced by secondary ion mass spectroscopy (SIMS).
Structural Characteristics of MOCVD Grown AlN Films with Different Carbon Concentration
Authors: Li Yun-Xin , Salamanca-Riba Lourdes , Spencer M. G. , Wongchigul K. , Zhou P. , Tang X. , Talyansky V. , Venkatesan T.
Published: April 2011

The structural characteristics of MOCVD AlN films with different carbon doping concentrations grown on sapphire were investigated by XRD (θ–2θ scans, ϕ scans and rocking curves ), HRTEM and Auger spectroscopy. The AlN:C films have very high crystalline quality and low resistivity. The epitaxial relationship between AlN and Sapphire is: (0001)AlN //(0001)Sap, [ 12 10]AlN//[0 110] Sap and [10 10]AlN //[ 2110]Sap. With increasing carbon concentration, the AlN films have higher carrier concentrations, and lower resistivities even though they have higher defect density. The resistivity decreases by 8 orders of magnitude with C doping. When the carbon concentration reaches 11%, an interfacial layer of ∼5nm was observed in HRTEM images. This layer suggests that some C in the film is diffusing into the sapphire substrate. However, optical diffractograms obtained from the negatives of the HRTEM showed no appreciable change in the structure of the interfacial layer compared to the pure substrate.
Spin Resonance Investigations of GaN and AlGaN
Authors: Reinacher N. M. , Angerer H. , Ambacher O. , Brandt M. S. , Stutzmann M.
Published: April 2011

Electron Spin Resonance (ESR) and related methods have been used to study defects in GaN and in AlxGa1-xN ternary alloys. In particular, Light-induced ESR of a thick MOCVD grown layer of GaN shows that the ESR signature of the deep defect appears for excitation energies > 2.6 eV and saturates above 2.7 eV. This information provides a direct measure of the energetic level position of the diamagnetic to paramagnetic transition of this center. Furthermore, standard ESR investigations of MBE-grown layers of AlxGa1-xN alloys were performed with emphasis on the effective-mass donor resonance. Composition and temperature-dependent measurements of the resonance position are presented.
Multiphoton Excitation Studies on GaN with PS Pulses
Authors: Libon I. H. , Voelkmann C. , Kim D. , Petrova-Koch V. , Shen Y. R.
Published: April 2011

We report the observation of UV photoluminescence (PL) from wurtzite GaN by multiphoton excitation. The dependence of the PL intensity on excitation intensity as well as PL excitation measurements with sub-bandgap photon energies indicate the existence of deep defect states centered at about 1.0 eV above the top of the valence band. This result was confirmed by a sum-frequency excitation spectrum. We correlate these measurements with the omnipresent yellow luminescence in GaN. Our two-photon PL excitation spectrum yields a two-photon absorption coefficient that agrees very well with theoretical predictions.
Depth Profile of the Excitonic Luminescence in Gallium-Nitride Layers
Authors: Siegle H. , Hoffmann A. , Eckey L. , Thomsen C. , Detchprohm T. , Hiramatsu K. , Davis T. , Steeds J. W.
Published: April 2011

We present results of spatially-resolved photoluminescence and Raman measurements on a 200 μm thick GaN layer grown on sapphire by hydride vapor phase epitaxy. Our microphotoluminescence measurements reveal that the peak position of the excitonic and donoracceptor-pair transitions strongly depends on the distance to the substrate interface. We observed a strong blue shift near the interface and discuss the influence of strain, which we quantified by micro-Raman experiments.
Subpicosecond Time-Resolved Raman Studies of Nonequilibrium Excitations in Wurtzite GaN
Authors: Tsen K. T. , Joshi R. P. , Ferry D. K. , Botchkarev A. , Sverdlov B. , Salvador A. , Morkoc H.
Published: April 2011

Non-equilibrium electron distributions as well as phonon dynamics in wurtzite GaN have been measured by subpicosecond time-resolved Raman spectroscopy. Our experimental results have demonstrated that for electron densities n ≥ 5 × l017cm−3, the non-equilibrium electron distributions in wurtzite GaN can be very well described by Fermi-Dirac distribution functions with the temperature of electrons substantially higher than that of the lattice. The population relaxation time of longitudinal optical phonons was directly measured to be τ ≅ 5 ± 1 ps at T = 25 K. The experimental results on the temperature dependence of the lifetime of longitudinal optical phonons suggest that the primary decay channels for these phonons are the decay into (1) one transverse optical phonon and one high energy, longitudinal or transverse acoustical phonons; and (2) one transverse optical phonon and one E2 phonon.
Intrinsic and Thermal Stress in Gallium Nitride Epitaxial Films
Authors: Ager J. W. , Suski T. , Ruvimov S. , Krueger J. , Conti G. , Weber E. R. , Bremser M. D. , Davis R. , Kuo C. P.
Published: April 2011

Strain in GaN epitaxial layers at room temperature is measured with three complementary methods: Raman spectroscopy (via shifts of phonon frequencies), low temperature photoluminescence (via shifts of band-edge luminescence), and X-ray diffraction (via shifts in lattice spacings). GaN films grown on the c-plane of sapphire tend to be in compression. Increasing the Si-dopant concentration (up to 1019 cm−3) is observed to add compressive strain to the layer. Axially resolved measurements obtained by micro-Raman in 4 μm thick Si-doped films reveal strain relaxation toward the sample surface at Si concentrations above 1018 cm−3. Mg- and Si-doped GaN films on SiC substrates are found to be in tension. An experimental methodology is presented that separates two contributions to the room temperature residual stress in GaN epilayers: (1) the thermal stress due to differences in the thermal expansion coefficients of the epilayer and substrate and (2) the intrinsic stress, which is influenced by the growth conditions. We measure stress as a function of temperature up to 325 C, about one-third of the growth temperature, by monitoring the frequency of the E2 phonon mode by Raman spectroscopy. A high-quality bulk single crystal of GaN is used as a strain-free standard. Over this temperature range, most layers behave elastically; the observed stress trends are well-fit by a thermal expansion model using previous reported values of the thermal expansion coefficients of GaN and the substrates. The intrinsic stress states at the growth temperature for films grown on sapphire and SiC are predicted to be tensile and compressive, respectively, in agreement with the a-plane lattice coefficient mismatch.
Yellow Luminescence and Associated Odmr in Movpe Gan: A Comparison of Defect Models
Authors: Mason P. W. , Dörnen A. , Härle V. , Scholz F. , Watkins G. D.
Published: April 2011

Two positive ODMR resonances are commonly observed on a luminescence band in GaN at 2.2 eV, one identified as a shallow donor, the other currently unidentified. We here report a study of their dependencies on a variety of experimental parameters, including microwave modulation frequency, microwave power, photoexcitation power and photoexcitation energy. ODMR simulations using two theoretical models are compared to experimental results which are consistent with spin-dependent recombination between the two defects, assuming the donor has a spin-lattice relaxation time shorter than the spin-dependent recombination lifetime. The photoexcitation energy dependence suggests that the spin-dependent recombination associated with the 2.2 eV band is not the same recombination that is responsible for the luminescence. This supports the two stage model put forth by Glaser et al. for the luminescence process.
A Model for Indium Incorporation in the Growth of InGaN Films
Authors: Piner E. L. , McIntosh F. G. , Roberts J. C. , Boutros K. S. , Aumer M. E. , Joshkin V. A. , El-Masry N.A. , Bedair S.M. , Liu S.X.
Published: April 2011

The development of high quality indium based III-nitride compounds is lagging behind the corresponding aluminum and gallium based compounds. Potential problems confronting the growth of epitaxial and double heterostructure InGaN will be discussed. A mass balance model is presented describing the competing reaction pathways occurring during the growth of indium containing compounds. Atomic layer epitaxy and metalorganic chemical vapor deposition grown InGaN films will be used to explain this model. Also, the growth parameters leading to the attainment of high InN percentages, reduced indium metal formation, and improved structural and optical properties of indium containing nitrides will be discussed.
Theoretical Study of Group-III-Nitride Alloys
Authors: Kim Kwiseon , Limpijumnong Sukit , Lambrecht Walter RL , Segall B.
Published: April 2011

Band gap bowing, structural relaxations, and energies of formation were calculated for the three pseudobinary nitride zincblende alloy systems Al-Ga, In-Ga and In-Al using the full-potential linearized muffin-tin orbital method. The cluster expansion and Connolly-Williams approaches were used to relate calculated band structures and energies of formation of ordered compounds to the behavior of disordered alloys. Effects of bond length and volume variation on those properties are discussed. An interpolation formula for the gap of the full pseudoternary AlxGayInzN system is proposed and tested by separate calculations. Extension of the results to the wurtzite alloys is discussed.
Low-Temperature Metalorganic Chemical Vapor Deposition of Gallium Nitride on (0001) Sapphire Substrates Using a Remote rf Nitrogen Plasma
Authors: Sone Cheolsoo , Hong Kim Min , Hyung Yi Jae , Ok Heur Soun , Yoon Euijoon
Published: April 2011

We report the low-temperature growth of GaN layers on (0001) sapphire substrates by a remote plasma enhanced metal-organic chemical vapor deposition in the temperature range of 500 - 800 $C. Effects of process parameters on the growth of GaN were studied. The structural quality of GaN improved as the growth temperature increased and the rf power decreased. Highly oriented GaN layers could be deposited at fairly low temperatures such as 500 $C under low rf power with low growth rate conditions.
The Influence of Hydrogen Addition on the Chemical Properties of Hydrogenated Aluminum Nitride Films Prepared by RF Reactive Sputtering
Authors: Lee Jai-Young , Yong Yoon-Joong
Published: April 2011

Hydrogenated aluminum nitride (A1N:H) films have been deposited on the (100) silicon wafers by the RF reactive magnetron sputtering method with H2 gas in addition to an Ar-N2 gas mixture. Stoichiometric A1N films without oxygen impurities can be prepared by adding 10 % H2 to reactive gas, which is proven by Rutherford Backscattering Spectrometry (RBS). The bonding aspects of Al, N, O and H atoms in A1N:H films have been examined by X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared (FTIR) to understand the effects of H2 addition. The chemical shift of the binding energies of A1, N and O atoms in A1N:H films from XPS analysis and the change of N-H bonding in FTIR with respect to different partial pressures of H2 gas have been confirmed. The role of H atoms is suggested to facilitate bonding with unbound N atoms in A1N:H films and hinder N-O bonding, thus, reducing oxygen concentration in A1N:H films. Also, the activation energy for the evolution of H2 gas from A1N:H film has been determined to be 0.11 eV/atom through a Kissinger-type analysis by a thermal desorption test using Gas Chromatograph(GC). This result implies that the hydrogen atom in film forms the hydrogen bond.
Thermoelectric Properties of RhSb3 Crystals and Thin Films
Authors: Chen Baoxing , Xu Jun-Hao , Hu Siqing , Uher Ctirad
Published: April 2011

RhSb3 belongs to a class of solids called skutterudites which were recently identified as promising novel thermoelectric materials. Our intent was to explore thin film growth of RhSb3 and to assess the thermoelectric properties of such films. RhSb3 films were prepared by electron-beam deposition on silicon and sapphire substrates. Thermopower, electrical resistivity and the Hall effect were measured over the temperature range from 2K to 300K. The data are compared to measurements on single crystals of RhSb3.
UV and Blue Photoluminescence from Silicon Nanocolloids
Authors: Iwasaki Shingo , Kimura Keisaku
Published: April 2011

We have prepared silicon nanocolloids by trapping silicon nanocrystallites produced by a gas evaporation method into an organic liquid. Blue photoluminescence (PL) at a peak maximum around 480 nm from silicon nanocolloids has been observed at room temperature. The external quantum efficiency of blue PL is 0.16 %. A room temperature UV PL has been observed by dropping alkaline solids such as sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)2) into silicon nanocolloids. The mechanism of blue and UV PL from silicon nanocolloids is discussed.
Preparation and Characterization of Silicon Nanocrystals in a SiO2 Matrix and Study of Suboxide Stability
Authors: Hinds B. J. , Banerjee A. , Johnson R. S. , Lucovsky G.
Published: April 2011

The kinetics of the decomposition of silicon suboxides (SiOx, x<2) to Sic + SiO2 was studied as a function of composition and post-deposition annealing. Amorphous hydrogenated SiOx films (0.8<x<1.4) were deposited by remote plasma enhanced chemical vapor deposition (RPECVD) and rapid thermal annealed (RTA) at temperatures of 500–1000°C. By monitoring the Si-O infra-red (IR) bond-stretch mode frequency, it was found that at temperatures below 850°C, or at a oxygen poor composition near SiO0.8, the decomposition reaction only proceeded to a metastable form of SiO1.6 + Si. Characterization by Raman and spectroscopie ellipsometry confirm similar trends. Cross sectional transmission electron microscopy (TEM) confirms that Si nanocrystals (Sine) are formed with anneals at 900°C (30 sec). As deposited suboxides show band edge photoluminescence at 1.6 eV which disappears upon annealing at 900°C, indicating a sharp suboxide free interface between Sinc and SiO2 matrix.
The Characteristics and Oxidation of Vapor - Liquid - Solid Grown Si Nanowires
Authors: Westwater J. , Gosain D. P. , Tomiya S. , Hirano Y. , Usui S. , Ruda H.
Published: April 2011

The Vapor - Liquid - Solid (VLS ) technique allows the growth of high aspect ratio Si wires. The Si nanowires formed by this technique can be thinned down by oxidation. This approach allows the formation of very thin Si cores which may be used to research the properties of Si nanostructures. In this work the growth and oxidation of these wires is characterized.

In the growth a very thin layer of Au is deposited on a Si (111) surface, silane gas is introduced into the chamber as the Si source gas and the temperature is raised to 300 – 600°C. Initially a catalytically active Au surface phase leads to the growth of a defective epitaxial Si layer. As Au / Si molten alloy balls nucleate and grow in size to approach the threshold size for VLS wire growth, which is determined by the Gibbs - Thomson effect, the epitaxial layer growth rate decreases and a transition to Si nanowire growth occurs. The morphology and width of the wires is strongly dependent on the growth temperature and pressure. At low pressure and high temperature relatively thick well-formed wires grow straight up from the substrate surface along the [111] direction. As the temperature is decreased and the pressure is increased thinner wires (as thin as 10 nm ) grow which tend to exhibit growth defects. A light oxidation yields Si cores which are of the order of 5 nm in diameter.
Limits and Properties of Size Quantization Effects in InAs Self Assembled Quantum Dots
Authors: Schmidt K. H. , Medeiros-Ribeiro G. , Cheng M. , Petroff P. M.
Published: April 2011

In this paper we report on the limits and properties of size quantization effects in InAs self assembled quantum dots (QDs). Size, density and character of the InAs islands are investigated by transmission electron microscopy. The electronic and optical properties of the islands in the coherent and dislocated growth regime are studied using capacitance, photoluminescence, photovoltage and photocurrent spectroscopy. In the data measured with the different techniques, the change in dot size and density as well as the transition from coherent to dislocated island growth is clearly observable. An increasing QD size causes a red shift in the energetic position of the QD features while the density of the islands is reflected in the intensity of the QD signal. The decrease in intensity at high InAs coverage is attributed to dislocated island formation.
Surface Derivatization of Nanocrystalline CdSe Semiconductors
Authors: Lee J.-K. , Kuno M. , Bawendi M. G.
Published: April 2011

CdSe quantum dots (QDs) with several different ligands were prepared by ligand exchange reactions of trioctylphosphine oxide (TOPO)/trioctylphosphine selenide (TOPSe) passivated QDs with an excess of new ligands, such as pyridine, 4-picoline, thiophenol, 4-(trifluoromethyl)-thiophenol, and tris(2-ethylhexyl)phosphate. We find that 85–90% of ligands passivating the surface of the QDs are the newly introduced capping species and 10–15% are native TOPO/TOPSe ligands, reflecting the incomplete exchange of the QD surface. Thermal gravimetric analysis (TGA) and quantitative proton NMR measurements show that on average only 30% of surface cadmium atoms are passivated by TOPO/TOPSe. Approximately 1/3 of these native ligands occupy sites too strongly bound to be replaced by ligand exchange reactions.
The Band Edge Luminescence of Surface Modified CdSe Nanocrystallites
Authors: Kuno M. , Lee J. K. , Dabbousi B. O. , Mikulec F. V. , Bawendi M. G.
Published: April 2011

We study the band edge luminescence of CdSe nanocrystallites to determine the origin of this emission. Previous studies have attributed the band edge emission to the recombination of photo-generated carriers trapped in localized surface states. Recently a number of “dark exciton” theories have been proposed which explain the luminescence in terms of recombination through internal core states. To address this issue we modify the surface of CdSe nanocrystallites with a number of organic/inorganic ligands and monitor the effect this has on the energetics of the resonant and non-resonant band edge luminescence. Our results for nanocrystallites passivated with trioctylphosphine oxide (TOPO), ZnS, 4-Picoline, 4-(trifluoromethyl)thiophenol, and tris(2-ethylhexyl)phosphate are in agreement with a dark exciton description of the band edge luminescence.
Nuclear Magnetic Resonance (NMR) of Porous Silicon
Authors: Chang W. K. , Liao M. Y. , Gleason K. K.
Published: April 2011

Porous silicon (PS) was characterized by 1H, 19F and 29Si solid-state nuclear magnetic resonance (NMR). On freshly prepared samples, hydrogen contents were between 3 × 1014 and 3 × 1015 per cm2 of PS surface area, while fluorine concentrations were below the detection limit. Cross-polarization (CP) was used to selectively observe the 29Si near the hydrogen passivation. The features of the 29Si NMR spectra are assigned as (O)2(Si)Si-H (-50 ppm), (O)3Si-H (-84 ppm), (Si)3Si-H (-91 ppm), (Si)2Si-H2 (-102 ppm) and (O)4Si (-109 ppm). Changes resulting from low temperature annealing in air and an HF soak were observed by both NMR and infrared spectroscopy. The 29Si NMR line widths for PS fall between those for crystalline silicon and those for amorphous hydrogenated silicon films (a-Si:H), suggesting disorder near the PS surface is intermediate between these extremes. However, comparison of the isotropie chemical shift values shows that the bonding in the disordered regions of PS differs from that found in a-Si:H. In addition, the sharp 29Si NMR resonance observed in the bulk single crystal starting material can not be resolved in the spectra of PS. Thus, well-ordered silicon nanocrystallites in the PS are either several bond-lengths removed from hydrogen or comprise only a small fraction of the PS layer.
Electrochemical and Chemical Deposition of II-VI Semiconductors in Porous Silicon
Authors: Herino R. , Gros-Jean M. , Montes L. , Lincot D.
Published: April 2011

The introduction of II-VI semiconductor compounds into porous silicon layers has been investigated in order to obtain transparent and conducting contacts with the inner surface of the material. CdTe and ZnSe have been electrodeposited cathodically on n type nanoporous electrodes from acidic solutions containing the metallic cations and dissolved oxides of selenium or tellurium. CdS incorporation into p-type porous silicon has been achieved by chemical bath deposition, from solutions containing cadmium complexes and thioacetamid as a sulfur donor. Characterization of the deposits has been performed by SEM observations, X-ray analysis and RBS. Results confirm the penetration of the compounds into the porous films, with small to strong concentration gradients in thickness depending on the deposition method. After deposition and sample drying, the luminescence of CdTe embedded layers has almost disappeared, whereas those containing ZnSe and CdS show a photoluminescence efficiency which is not severely degraded.
Photo-Irradiation-Induced Narrowing of Photoluminescence Spectra from Porous Silicon
Authors: Okamoto M. , Nagao T. , Ooiwa T. , Hatta A. , Ilo T.
Published: April 2011

Porous silicon (PS) has been prepared by varying photo-irradiation time in the latter period of anodization, and spectral intensity and width of photoluminescence (PL) have been measured as well as their dependences on current density during the photo-irradiation. Significant differences in PL spectra have been found when the timing of photo-irradiation is varied during the anodization. Sufficient numbers of photo-excited holes are responsible for the formation of PS layers yielding bright and sharp PL. The narrowest FWHM obtained was 0.182eV for a PL peak energy of 1.71eV while the highest peak energy of PL with FWHM < 0.30 eV was 2.16 eV (575 nm).
Quantum Dots as Inorganic DNA-Binding Proteins
Authors: Murphy Catherine J. , Brauns Eric B. , Gearheart Latha
Published: April 2011

Semiconductor quantum dots of cadmium sulfide, CdS, are approximately the size of proteins and are photoluminescent in the red, yellow, or green, depending on surface preparation. This photoluminescence is very sensitive to the Nature and amount of adsorbates. We have found that DNAs with intrinsic curvature adsorb more strongly to the surface of 47 Å CdS quantum dots, as judged by concentration-dependent changes in photoluminescence. The binding constants we obtain are similar to those found for nonspecific protein-DNA interactions. The surface groups of the CdS substrate also influence DNA adsorption. Thus these protein-sized colloidal particles can be used in chemical sensing applications for curved or kinked DNA; DNA with unusual structures is thought to influence biological function such as transcription.
D.C Current-Voltage Characteristics and Admittance Spectroscopy of an Al-Porous Si Barrier
Authors: Khirouni K. , Bourgoin J. C. , Borgi K. , Maaref H. , Deresmes D. , Stiévenard D.
Published: April 2011

We present the temperature and frequency dependence of the current-voltage (I-V) characteristics of Al barriers deposited on porous Si grown on p-type Si substrates. These barriers exhibit a rectifying behaviour when the temperature is higher than 250 K. The I-V characteristics can be understood by a conduction in porous Si taking place via free electrons thermally excited from Pb centers associated with the existence of a SiO2-Si interface and via hopping between these Pb centers.
Elaboration and Light Emission Properties of Low Doped p-Type Porous Silicon Microcavities
Authors: Lérondel G. , Ferrand P. , Romestain R.
Published: April 2011

We account for the elaboration of Bragg reflectors and microcavities based on efficiently luminescent porous silicon. A characterisation of very thin porous silicon layers obtained with current densities of formation varying from 1.5 mA to 300 mA is presented. The resulting refractive index variation (typically from 1.37 to 1.86 at 700 nm) enables the elaboration of high quality Bragg reflectors and Fabry-Perot filters from the yellow to the near infrared. Although low doped p-type porous silicon develops rougher interfaces than highly doped p-type porous silicon, its better luminescence efficiency has enabled us to elaborate microcavities with a strong emission in a narrow band.
Microcrystalline Silicon Solar Cells at Higher Deposition Rates by the VHF-GD
Authors: Torres P. , Meier J. , Goetz M. , Beck N. , Kroll U. , Keppner H. , Shah A.
Published: April 2011

A 7.7 % single junction cell efficiency for an entirely microcrystalline silicon (μc-SiH) device has recently been reported by our group [1]. This was achieved by applying the purifier technique, a technique which is indeed easier to handle than the earlier used “microdoping” approach. The purpose of the present paper is twofold: First to show in detail the impact on device performance when a gas purifier is used; and second to illustrate that the deposition rate of the active, absorbing i-layer can be increased from the former 1.55 Å/s up to 4.3 Å/s while still maintaining reasonable device performances. In the latter case a first n-i-p solar cell structure on an aluminium sheet could be fabricated with an efficiency of 4.9 %.
Low-Temperature Formation of Device-Quality Polysilicon Films by cat-CVD Method
Authors: Matsumura Hideki , Heya Akira , Iizuka Ritsuko , Izumi Akira , He An-Qiang , Otsuka Nobuo
Published: April 2011

Polycrystalline silicon (poly-Si) films are deposited at temperatures lower than 300–400°C by the cat-CVD method. In the method, a SiH4 and H2 gas-mixture is decomposed by catalytic cracking reactions with a heated tungsten catalyzer placed near substrates. Carrier transport, optical and structural properties are investigated for this cat-CVD poly-Si. The films show both large carrier mobility and large optical absorption for particular deposition conditions. The cat-CVD poly-Si films are found to be one of the useful materials for thin film transistors and thin film solar cells.
Molecular Dynamic Computer Simulation of Elastic and Plastic Behavior of Nanophase Ni
Authors: Van Swygenhoven H. , Caro A.
Published: April 2011

Molecular dynamics computer simulations of low temperature elastic and plastic deformation of Ni nanophase samples with several mean grain size in the range 3–5 nm are reported. The samples are polycrystals nucleated from different seeds, with random locations and orientations. Bulk and Young modulus are calculated from stress-strain curves and the onsett of plastic deformation is discussed. At higher loads substantial difference in the plastic behaviour with respect to the coarse grain counterpart is observed: among the mechanism responsible for the deformation, grain boundary sliding and motion, as well as grain rotation are identified. An interpretation in terms of grain boundary viscosity is proposed and a linear dependence of strain rate with the inverse of the grain size is obtained.
Optical Properties of Metal Wire Array Composites
Authors: Luo Laura , Huber T. E.
Published: April 2011

According to effective medium theories, electrically conducting composites consisting of parallel metal wires embedded in a transparent dielectric can propagate light in the direction of the wire length. We have prepared densely packed arrays (76% volume fraction) of 10-μm diameter indium wires by high pressure injection of glass microchannel plates. For wavelengths longer than 100 μm (k<100 cm−1) the absorption of the wire array is almost three orders of magnitude smaller than that of an indium foil of equal thickness. The measured absorption increases as k0.45±0.07 and can be accounted for by including magnetic dipole effects.
Heat Treatment of Nanocrystalline Al2O3-Zr02
Authors: Smyser Bridget M. , Connelly Jane F. , Sisson Richard D. , Provenzano Virgil
Published: April 2011

The effects of grain size on the phase transformations in nanocrystalline ZrO2-Al2O3 have been experimentally investigated. Compositions from 10 to 50 vol% Al2O3 in ZrO2 were obtained as a hydroxide gel. The powders were then calcined at 600 °C for 17 hours and heat treated at 1100 °C for 24 and 120 hours and at 1200 °C for 2 hours. The phase distribution and grain size were determined using x-ray diffraction and transmission electron microscopy. The initial grain size after calcining was 8–17 nm. It was determined that the critical ZrO2 grain size to avoid the tetragonal to monoclinic phase transformation on cooling from 1100 °C was between 17 and 25 nm. Samples containing 50% Al2O3 maintained a grain size below the critical size for all times and temperatures. The 30% Al2O3 samples showed the same behavior in all but one heat treatment. The remainder of the samples showed significant grain growth and at least partial transformation to the monoclinic phase.
Embedded Gold Clusters: Growth in Glass and Optical Absorption Spectra
Authors: Rao P. G. N. , Doremus R. H.
Published: April 2011

Optical absorption of nanosized particles in glass was measured. The plasma absorption band was completely spread out for particles below about 1.5 ran in diameter, as also has been found by gold particles in water and a polymer. The growth kinetics suggested growth of spherical particles controlled by diffusion of gold from the matrix to the particles, and a constant number of growing particles, giving a narrow size distribution of particles. For particles below about one ran in diameter (31 gold atoms), the optical absorption was proportional to λ−4, as expected if the absorption results from free electrons in the particles.
Influence of Si and W Additions on High Temperature Oxidation of γ-α2 Ti-Al Alloys
Authors: Tomasi A. , Noseda C. , Nazmy M. , Gialanella S.
Published: April 2011

Titanium aluminides have potential interest for high temperature applications because of their low density and high temperature strength. In this study the isothermal oxidation behavior in air and in the temperature range 700–850°C of γ-α2Ti-Al bulk alloys with different additions of W (0–9.5 wt.%) and Si (0–5.0 wt.%) was investigated. The samples were prepared by arc-melting starting from pure element powders (99.99%). After thermal treatments, for homogenisation and phase stabilisation, the samples were tested using a thermal analysis apparatus in order to evaluate their oxidation resistance. The oxidation rates show the beneficial effect of the W and Si additions. The growth and adherence of the protective scale on alloys have been investigated in conjunction with detailed oxide scale characterisation using the techniques of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results of the study are used for critical assessment of the oxidation mechanisms leading to the formation of surface layers of different compositions.
Effect of interstitial concentration and heat treatment on microstructure and primary creep of investment cast Ti-47Al-2Nb-2Mn with 0.8v% TiB2
Authors: Seo Dong Yi , Bieler T. R. , Larsen D. E.
Published: April 2011

Most research about creep has focused on minimum creep rate or stress rupture properties, but primary creep is important for practical applications. In this paper, we compare how interstitial content and heat treatments of shorter duration than used in past studies affect the microstructure and primary creep resistance of a particular alloy containing TiB2 particles. More time and temperature in heat treatment homogenizes the microstructure and reduces the scatter of creep times to 0.5% strain. Increasing interstitial content increases the time to 0.5% creep, and it stabilizes the a2 phase. Preliminary evidence for large mechanical twinning strains parallel to lamellar interfaces shortly following loading is provided.
Phase Stability in Processing of High Temperature Intermetallic Alloys
Authors: Perepezko J. H. , Nuñes C. A. , Yi S.-H. , Thoma D. J.
Published: April 2011

In the development of high temperature intermetallics involving various aluminides, suicides and Laves phases, it has become evident that it is essential to consider the strong influence of materials processing throughout all stages. The underlying basis for alloy synthesis, processing and the assessment of thermal stability is established by the relevant phase equilibria, the characteristic diffusivities and the possible solidification reaction pathways. In almost all cases the microstructures of the most useful metallic alloys are multiphase assemblies in which the relative phase fractions, compositions and morphologies play key roles in optimizing the performance under high temperature conditions. The microstructure designs are usually tailored for strength, toughness, creep resistance and environmental stability and involve a balance of features derived from mixtures of a ductile phase and intermetallic phases. There is a clear experience that the level of materials processing can only be as sophisticated as the level of knowledge of the phase equilibria and the underlying kinetics. In many of the contemporary intermetallic alloys the phase stability must be considered in terms of multicomponent equilibria and non-stoichiometric intermetallic compositions. Recent developments in several important intermetallic alloy classes illustrate the guidance into alloy design and processing options provided by systematic studies of phase stability.
On the Mechanism of Milling Induced Disordering in AlFe
Authors: Clavaguera-Mora M. T. , Zhu J. , Meyer M. , Mendoza-Zelis L. , Sanchez F. H. , Clavaguera N.
Published: April 2011

The evolution of the B2-AlFe phase during mechanical grinding in Ar has been examined as a function of milling time by X-Ray diffraction, transmission Mössbauer spectroscopy and differential scanning calorimetry. Short and long range disorder was observed to increase with the mechanical treatment up to the attainment of a steady state. The evolution of the long range order parameter and of the local atomic configurations at Fe sites were analyzed in terms of possible mechanisms for milling induced disordering. The kinetics of the thermal reordering was studied under continuous heating and isothermal calorimetrie regimes. Modeling of the reordering processes by diffusion controlled growth of pre-existing ordered grains is presented as well as the estimated values of both the enthalpy and the activation energy of the reordering process. The results are consistent with a non uniform distribution of disorder throughout the sample and will be compared with preceding information on related systems.
Effect of Initial Microstructure on the Compatibility of Rapidly Solidified Ti-Rich TiAl Powder
Authors: Nishida M. , Chiba A. , Morizono Y. , Kai T. , Sugimoto J.
Published: April 2011

Initial microstructure dependence of compactibility at elevated temperature in rapidly solidified Ti-rich TiAl alloy powders produced by plasma rotating electrode process (PREP) has been investigated. There were two kinds of powders with respect to the microstructure. The first one had a surface relief of a martensitic phase, which was referred as M powder. The second one had a dendritic structure, which was referred as D powder. α2+γ microduplex and α2/γ lamellar structures were formed in Mand D powders of the Ti-40 at%Al alloy by heat treatment at 1273 K, respectively. The microduplex structure consisted of γ precipitate in the twin related α2 matrix with the usual orientation relationship. It was difficult to compact the D powder by hot pressing at 1273 K under 50 MPa for 14.4 ks. On the other hand, the M powder was compacted easily by hot pressing with the same condition. The twin related α2 and α2 boundary changed to random ones and the α2 and γ phases were lost the usual orientation relationship in the duplex structure during the hot pressing. In other words, the low energy boundaries were changed to the high energy ones suitable for grain boundary sliding. Dislocations were scarcely observed inside of both the α2 and γ crystal grains. It was concluded that the grain boundary sliding was a predominant deformation mode in the M powder during the hot pressing. D and M powders in Ti-45 and 47 at%Al alloys showed the same tendency as those in Ti-40 at%Al alloy during hot pressing.
Stacking Faults on (001) in Transition-Metal Disilicides with The Cllb Structure
Authors: Ito K. , Nakamoto T. , Inui H. , Yamaguchi M.
Published: April 2011

Stacking faults on (001) in MoSi2 and WSi2 with the Cllb structure have been characterized by transmission electron microscopy (TEM), using their single crystals grown by the floating-zone method. Although WSi2 contains a high density of stacking faults, only several faults are observed in MoSi2. For both crystals, (001) faults are characterized to be of the Frank-type in which two successive (001) Si layers are removed from the lattice, giving rise to a displacement vector parallel to [001]. When the displacement vector of faults is expressed in the form of R=l/n[001], however, their n values are slightly deviated from the exact value of 3, because of dilatation of the lattice in the direction perpendicular to the fault, which is caused by the repulsive interaction between Mo (W) layers above and below the fault. Matching of experimental high-resolution TEM images with calculated ones indicates n values to be 3.12 ± 0.10 and 3.34 ± 0.10 for MoSi2 and WSi2, respectively.
Elastic Constants of A Laves Phase Compound: C15 NbCr2
Authors: Ormeci Alim , Chu F. , Wills John M. , Chen S. P. , Albers R. C. , Thoma D. J. , Mitchell T. E.
Published: April 2011

The single-crystal elastic constants of C15 NbCr2 have been computed by using a first-principles, self-consistent, full-potential total energy method. From these single-crystal elastic constants the isotropie elastic moduli are calculated using the Voigt and Reuss averages. The calculated values are in fair agreement with the experimental values. The implications of the results are discussed with regards to Poisson's ratio and the direction dependence of Young's modulus.
Relating Mechanical Properties with Dislocation Cores in Ni3Ge-Fe3Ge Intermetallic Alloys
Authors: John Balk T. , Kumar Mukul , Hemker Kevin J.
Published: April 2011

The transition from positive to negative temperature dependence of 0.2% yield stress is investigated m the model pseudo-binary Ni3Ge-Fe3Ge system. Ni3Ge and Fe3Ge, both Ll2 intermetallic alloys, show complete solid solubility as Fe is continuously substituted for Ni across the composition range. However, Ni3Ge exhibits the yield stress anomaly, whereas the yield stress of Fe3Ge shows a normal decline with temperature. Mechanical testing has verified this behavior, with the anomalous behavior gradually disappearing with increasing Fe content. It is proposed that this transition results from changes in the structure of dissociated superdislocation cores. Alloys with anomalous behavior from this system are characterized by the presence of screw superdislocations locked in the Kear-Wilsdorf (KW) configuration. Conversely, alloys with normal yield stress dependence are observed to contain curvilinear superdislocations that glide on the cube planes. Results from mechanical testing are presented and correlated with TEM observations of deformation structures. These results are discussed in light of planar fault energies determined through computer simulations of images.
Mechanical Behavior of Molybdenum-Modified Cr3Si/Cr5Si3 Intermetallics
Authors: Nazmy M. , Noseda C. , Augustin S. , Lipetzky P. , Stoloff N. S.
Published: April 2011

Two molybdenum-modified chromium suicide base intermetallics have been prepared by powder metallurgy and investigated with respect to the hardness, flexural strength and fracture toughness at different temperatures. As toughening phase, tungsten, Pt-6%Rh and Saphikon fibres were tried to improve the toughness of these two intermetallics. The tungsten fibers generated microcracks in the base material due to the large mismatch in the coefficients of thermal expansion (CTE) between the fiber and the intermetallic. The Pt-6%Rh and Saphikon fibers did not exhibit such cracking.

An improvement in the fracture toughness was observed especially at elevated temperatures. The Saphikon fibers did not show interfacial reaction. Some limited diffusion of silicon was detected in the Pt-6%Rh phase after 1400°C treatment.
Electric Field Induced Control of Thin Film Diblock Copolymer Domain Orientation
Authors: Morkved T. L. , Lopes W. A. , Lu M. , Urbas A. M. , Jaeger H. M. , Mansky P. , Russell T. P.
Published: April 2011

Local control of domain orientation in diblock copolymer thin films is demonstrated through the use of external electric fields. Thin films of a polystyrene-polymethylmethacrylate diblock copolymers, denoted P(S-b-MMA), were spin coated onto silicon nitride membrane substrates with prefabricated in-plane electrodes, forming cylindrical PMMA microdomains. Films annealed under an applied electric field (E ≤ 37V/μm) at 250°C for 24h under an argon atmosphere showed an alignment of the cylindrical microdomains parallel to the electric field lines. A quantitative measure of the degree of alignment was obtained by correlating the local field strength, E, and direction with the observed cylinder orientation. The alignment was found to saturate above E≈30V/μm, and to decrease rapidly as E falls below this value.
The Distribution of SIS Compatibilizer in Melt-Mixed Polystyrene-Polyethylene Homopolymer Blends
Authors: Kim G. , Libera M. , Potluri R. , Gogos C. G.
Published: April 2011

This paper presents the results of transmission electron microscopy (TEM) studies on the distribution of polystyrene/polyisoprene/polystyrene (SIS) triblock copolymer compatibilizer in polystyrene-polyethylene homopolymer blends. The principal independent variable studied is the major-phase rheological properties (PSmajor/PEminor and PEmajor/PSmimor blends). The polyisoprene block was preferentially stained by OSO4 and could be clearly identified in cryo-ultramicrotomed TEM sections. The microstructural data show that when the Theologically strong PS phase is the major constituent, the compatibilizer follows the classical model localizing itself at the PS/PE interface. In the PEmajor/PSminor blends, however, the SIS compatibilizer is not localized at the PE/PS interface. Instead microdispersions of PS in SIS form which in turn are dispersed in the PE matrix. While the various microstructures are affected by the mixing protocol and polymer rheology, quiescent annealing after mixing indicates that the compatibilizer distribution is in part driven by thermodynamic effects.
Where is All the Amalgam Silvering?
Authors: Anheuser Kilian
Published: April 2011

Amalgam gilding was one of the most common techniques for the gilding of metalwork before the invention of electroplating. Its silver equivalent, amalgam silvering, was executed in the same manner, by applying a paste of silver amalgam and mercury to a copper alloy object and subsequent heating to form a diffusion bond between substrate and plating and to evaporate the excess mercury, followed by burnishing of the surface. The technical know-how was readily available. However, even though many medieval and post-medieval European metal objects were silver plated, only a few coins and small objects like medieval decorative mounts and horse trappings, and post-medieval astronomic instruments have been reported as examples of amalgam silvering. The technique was also used in China in the 3rd-1st century BC. Should we assume that there are many more amalgam silvered objects waiting to be discovered, as has been suggested? This paper investigates silver plated objects from 3rd century BC Chinese belt hooks to 18th century French decorative art and compares the analytical evidence with original silvering recipes and replication experiments, which showed that with amalgam silvering satisfactory results were far more difficult to achieve than with fire-gilding. It was found that the presence of mercury in XRF analyses of silver plated surfaces did not necessarily identify amalgam silvering, and that some allegedly amalgam silvered objects may in fact have been plated with silver chloride based electrochemical replacement silvering pastes.