bluuebunny/crossref_metadata_2025_split huggingface dataset:
Emission Mechanism of the InGaN MQW Grown by MOCVD
Authors: Narukawa Yukio , Kawakami Yoichi , Fujita Shizuo , Fujita Shigeo , Nakamura Shuji 
Published: April 2011

ABSTRACTDynamical behavior of radiative recombination has been assessed in the In0.20Ga0.80N (3nm)/In0.05Ga0.95N (6 nm) multiple quantum well (MQW) structure by means of transmittance (TR), electroreflectance (ER), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL) spectroscopy. The PL at 20 K was mainly composed of two emission bands whose peaks are located at 2.920 eV and 3.155 eV. The ER and PLE revealed that the transition at 3.155 eV is due to the excitons at quantized level between n=1 conduction and n=1 A(Γ9υ) valence bands, while the main PL peak at 2.920 eV is attributed to the excitons localized at the trap centers within the well. The TRPL features were well understood as the effect of localization where photo-generated excitons are transferred from the n=1 band to the localized centers, and then are localized further to the tail state. The origin of the localized centers were attributed to the In-rich region in the wells acting as quantum dots which could be observed by transmission electron microscopy (TEM) and energy-dispersive X-ray microanalysis (EDX).
Optical Dielectric Response of Gallium Nitride Studied by Variable Angle Spectroscopic Ellipsometry
Authors: Yao H. , Yan C. H. , Jenkinson H. A. , Zavada J. M. , Speck J. S. , Denbaars S. P. 
Published: April 2011

ABSTRACTVariable angle spectroscopic ellipsometry (VASE) and transmission measurements have been employed to study the dielectric response of gallium nitride (GaN) thin films — an important material for light emitting diodes (LEDs) and laser diodes applications. The GaN films were grown by atomsphere pressure metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates (α-AI2O3). Room temperature VASE measurements were made, in the range of 0.75 to 5.5eV, at the angle of incidence of 73, 75, and 77 degree, respectively. Evidence of anisotropy is observed especially in the spectral range under the band gap (∼3.4 eV), reflecting the nature of wurtzite crystal structure of GaN. The ordinary dielectric function ε⊥(ω) of GaN were obtained through the analysis of transmission and VASE data in the range below and above the band gap. The thickness of these GaN films is also determined via the analysis.
Growth of High Quality Poly-SiGe on Glass Substrates
Authors: Shiota Kunihiro , Inoue Daisuke , Minami Kouichirou , Yamamoto Masaji , Hanna Jun-ichi 
Published: April 2011

AbstractThe composition variation and strutural properties of poly-SiGe thin films were investigated by Reactive Thermal CVD with Si2H6 and GeF4. Deposition of the films was carried out at a low temperature of 450°C on oxidized silicon substrates using different growth parameters, i.e., the source gas flow ratio (Si2H6/ GeF4) and thegas flow rate. The structural profiles of as-deposited films were characterized by X-ray diffraction (XRD) and Raman scattering spectroscopies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).All these films show (220) preferential orientation. The mole fractions of Si in poly-SixGe1−x films were estimated to be from 0.95 to 0.05 for x by using Vegard's law for the XRD peaks. TEM observation revealed that high crystallinity was well established even in poly-Si0.95Ge0.05 films owing to the direct nucleation on the substrate surface.
Comparative Optical Studies of Chemically Synthesized Silicon Nanocrystals
Authors: Delgado Gildardo R. , Lee Howard W.H. , Kauzlarich Susan M. , Bley Richard A. 
Published: April 2011

AbstractWe studied the optical and electronic properties of silicon nanocrystals derived from two distinct fabrication procedures. One technique uses a controlled chemical reaction. In the other case, silicon nanocrystals are produced by ultrasonic fracturing of porous silicon layers. We report on the photoluminescence, photoluminescence excitation, and absorption spectroscopy of various size distributions derived from these techniques. We compare the different optical properties of silicon nanocrystals made this way and contrast them with that observed in porous silicon. Our results emphasize the dominant role of surface states in these systems as manifested by the different surface passivation layers present in these different fabrication techniques. Experimental absorption measurements are compared to theoretical calculations with good agreement. Our results provide compelling evidence for quantum confinement in both types of Si nanocrystals. Our results also indicate that the blue emission from very small Si nanocrystals corresponds to the bandedge emission, while the red emission arises from traps.
Recent Progress in Microcrystalline Semiconductor Thin Films
Authors: Tanaka K. 
Published: April 2011

AbstractNanocrystalline/microcrystalline thin films prepared at relatively low temperatures by plasma-enhanced chemical vapor deposition (PECVD), in particular hydrogenated microcrystalline Si films (μc-Si:H), have attracted an increasing attention not only as potential materials for thin film solar cells, but also as active layers in thin film transistor arrays for flat panel displays. This paper reviews recent progress in the investigation of these materials; preparation methods, structural and optical properties, and electronic transports. Emphasis is placed on the understanding of the growth mechanism of μc-Si:H films as well as the microscopic characterization of the film structure.
X-Ray Diffraction and Reflectivity Studies of Thin Porous Silicon Layers
Authors: Buttard D. , Dolino G. , Bellet D. , Baumbach T. 
Published: April 2011

AbstractHigh resolution X-ray diffraction and reflectivity have been used for the structural characterization of thin porous silicon layers of p and p+ doping type. Thin porous silicon layers studied either by diffraction or reflectivity, in the range of 10–1000 nm, exhibit several thickness fringes, corresponding to a lateral homogeneity of the layer thickness. The comparison between the experimental results with simulations enables one to deduce structural information relative to the porosity, thickness, lattice parameter as well as interface thickness. For p+ type samples a double fringe system was observed, showing the existence of a surface film probably at the porous silicon layer top surface.
Optical Properties of Free-Standing Ultrahigh Porosity Silicon Films Prepared by Supercritical Drying
Authors: Von Behren J. , Fauchet P. M. , Chimowitz E. H. , Lira C. T. 
Published: April 2011

AbstractHighly luminescent free-standing porous silicon thin films of excellent optical quality have been manufactured by using electrochemical etching and lift-off steps combined with supercritical drying. One to 50 μm thick free-standing layers made from highly (p+) and moderately (p) Boron doped single crystal silicon (c-Si) substrates have been produced with porosities (P) up to 95 %. The Fabry-Pérot fringes observed in the transmission and photoluminescence (PL) spectra are used to determine the refractive index. At the highest P the index of refraction is below 1.2 from the IR to 2 eV. The absorption coefficients follow a nearly exponential behavior in the energy range from 1.2 eV and 4 eV. The porosity corrected absorption spectra of free-standing films made from p type c-Si substrates are blue shifted with respect to those prepared from p+ substrates. For P > 70 % a blue shift is also observed in PL. At equal porosities the luminescence intensities of porous silicon films made from p+ and p type c-Si are different by one order of magnitude.
Selection Rules in the Raman Spectrum of Porous Silicon
Authors: Agulló-Rueda F. , Moreno J. D. , Montoya E. , Guerrero-Lemus R. , Martín-Palma R. J. , Martínez-Duart J. M. 
Published: April 2011

AbstractWe have measured the Raman spectrum of porous silicon layers for different light polarizations. For the polarizations where the first-order Raman peak is forbidden in bulk silicon, porous silicon shows a band. This band is almost as intense as the one in the allowed polarizations but its lineshape is very different. The ‘forbidden’ band is usually wider and shifted to lower energies with respect to the allowed band. Both bands are analyzed in terms of sample characteristics, layer depth, and excitation wavelength.
Transient Photocurrent Response of Dye-Sensitized Porous Nanocrys1Alline TiO2 Electrodes
Authors: Goossens Albert , Boschloo G. K. , Schoonman J. 
Published: April 2011

AbstractIn order to investigate the fundamentals of electron migration in nanostructured metal-oxide semiconductors, the transient photocurrent response of dye-sensitized porous nanocrystalline TiO2 is studied. The time-resolved photocurrent response at light steps or pulses shows a faster transient upon increasing the light intensity. Intensity-modulated photocurrent spectroscopy (IMPS) reveals that the transient photocurrent is dominated by two time constants, i.e. the geometrical one and a characteristic time related to electron trapping. A theoretical model is derived in which the occupation dynamics of a single electron trap is considered using Shockley-Read-Hall kinetics. The geometrical RC time of the electrode is also included. Excellent agreement between this model and the measured IMPS spectra is obtained.
Visible Light Emitting Diode Employing Electrochemically Anodized Nanocrystalline Silicon Thin Film
Authors: Toyama T. , Yamamoto T. , Matsui T. , Okamoto H. 
Published: April 2011

AbstractVisible electroluminescence (EL) has been achieved on the entirely solid state thin film light emitting diode (TFLED) employing electrochemically anodized nanocrystalline Si (nc-Si) as a light emitting active layer. The TFLED consisting of p-type nc-Si, and intrinsic and n-type amorphous layers was fabricated on a SnO2-coated glass substrate. The nc-Si was formed in HF aqueous solution from boron doped microcrystalline Si (μc-Si) deposited by rf plasma chemical vapor deposition (CVD). The TFLED exhibits clear rectification with a forward threshold voltage of about 1.5 V, whereas visible EL emission is observed upon applying reverse bias voltages. The diode ideality factor is more than 2, and the light output increases with the square of the diode current. The EL emission color is orange-red and the spectral peak energy is 1.8 eV.
Electroluminescence and Carrier Transport in Leds Based on Silicon-RICH Silicon Oxide
Authors: Tsybeskov L. , Hirschman K. D. , Duttagupta S. P. , Fauchet P. M. 
Published: April 2011

AbstractLEDs based on silicon-rich silicon oxide (SRSO) have been fabricated utilizing thermal oxidation of electrochemically etched c-Si and standard microelectronic processes including LPCVD, photolithography, ion implantation and metallization. The bipolar devices consist of heavily-doped polycrystalline Si, a transition layer made of oxidized mesoporous Si, an active SRSO layer (doped or intrinsic) and a crystalline Si substrate. The LED's electrical properties exhibit a low interface state density which is explained by the partial filling of the micropores within the transition layer by polycrystalline Si. The dominant carrier transport in SRSO is due to thermally-assisted tunneling at low fields and electric field-assisted (Fowler-Nordheim) tunneling at high fields. The electroluminescence (EL) is stable and the EL modulation is limited by the carrier transition time (not by the carrier lifetime) which explains the high modulation speed (≥ 10 MHz). We have fabricated a prototype alphanumeric 7-segment display with an isolated version of the LEDs and standard microelectronic processing techniques.
A Study of the Factors Which Determine the Modulation Speed of a Shallow PN Junction Porous Silicon Led
Authors: Simons A. J. , Cox T. I. , Loni A. , Calcott P. D. J. , Uren M. J. , Canham L. T. 
Published: April 2011

AbstractThe effect of the chemical thinning of the porous silicon structure on the speed and efficiency of electroluminescent devices, produced by the anodisation of a pn junction in bulk silicon is investigated. Thinning of the silicon wires results in an increase in the efficiency but at the expense of a reduction in operating speed. It is demonstrated that the operating speed is limited by the photoluminescence lifetime for small signal excitation. However, for large signals, the electroluminescence can be turned off more than 5 times faster than the photoluminescence lifetime, indicating that this need not necessarily limit device operating speed.
Grain-Size Control of Nanocrystalline Silicon by Pulsed Gas Plasma Process
Authors: Itoh A. , Ifuku T. , Otobe M. , Oda S. 
Published: April 2011

AbstractA new method for the formation of nanocrystalline Si (nc-Si) in the SiH4 plasma using pulsed-H2 supply with very-high-frequency (VHF;144MHz) excitation is proposed to control the size of nc-Si. Nanocrystalline Si is formed in the gas phase of SiH4 plasma cell by coalescence of radicals. The principle of size control is based on the separation of nucleation and growth process. Supplying H2 into a SiH4 plasma enhances nucleation of nc-Si and suppresses growth rate of nc-Si. The nucleated nc-Si grows larger in a SiH4 plasma during the off state of the H2 supply. As the newly supplied H2 forces nc-Si grown in the previous cycle out of the plasma cell into the deposition chamber, the next nucleation of nc-Si is enhanced simultaneously. Using this method, we fabricated 8 nm-diameter nc-Si with small dispersion (±1 nm) successfully.
Synthesis, Optical Properties, and Microstructure of Semiconductor Nanocrystals Formed by Ion Implantation
Authors: Budai J. D. , White C. W. , Withrow S. P. , Zuhr R. A. , Zhu J. G. 
Published: April 2011

AbstractHigh-dose ion implantation, followed by annealing, has been shown to provide a versatile technique for creating semiconductor nanocrystals encapsulated in the surface region of a substrate material. We have successfully formed nanocrystalline precipitates from groups IV (Si, Ge, SiGe), III-V (GaAs, InAs, GaP, InP, GaN), and II-VI (CdS, CdSe, CdSxSe1x, CdTe, ZnS, ZnSe) in fused silica, Al2O3 and Si substrates. Representative examples will be presented in order to illustrate the synthesis, microstructure, and optical properties of the nanostructured composite systems. The optical spectra reveal blue-shifts in good agreement with theoretical estimates of size-dependent quantum-confinement energies of electrons and holes. When formed in crystalline substrates, the nanocrystal lattice structure and orientation can be reproducibly controlled by adjusting the implantation conditions.
Fabrication of Polycrystalline Si Thin Film for Solar Cells
Authors: Tanaka M. , Tsuge S. , Kiyama S. , Tsuda S. , Nakano S. 
Published: April 2011

AbstractThe a-Si/poly-Si thin film tandem solar cell is a promising candidate for low-cost solar cells. We have conducted R&D on poly-Si thin film using the Solid Phase Crystallization (SPC) method from amorphous silicon (a-Si). To improve the film quality of SPC poly-Si, we have developed a new SPC method called the partial doping method. This method features two stacked starting a-Si layers, a P-doped layer and a non-doped layer. Nucleation occurs in the P-doped layer, and the non-doped layer is the crystal growth layer. For the nucleation layer, we developed a Si film with a unique structure, which features relatively large crystallites (-1000A) embedded in a matrix of amorphous tissue. By combining these technologies, a conversion efficiency of 9.2% was obtained for poly-Si thin-film solar cells. For further improvement in the conversion efficiency, based on the concept of “independent control of nucleation and crystal growth”, it is necessary to combine the best fabrication methods for each layer. A high conversion efficiency of more than 12% was found possible by using the CVD method and a new back surface reflection structure.
Visible Photoluminescence From Si Ion-Implanted and Thermally Annealed SiO2 Films
Authors: Kanemitsu Y. , Shimizu N. , Okamoto S. , Komoda T. , Hemment P. L. F. , Sealy B. J. 
Published: April 2011

AbstractWe have experimentally studied the photoluminescence (PL) properties of Si clusters in SiO2 glassy matrices. Si clusters in the SiO2 matrices were fabricated by Si+ ion implantation into SiO2 glasses and then thermally annealed in forming gas. Broad PL peaks are observed in the visible spectral region at room temperature. Resonantly excited PL spectra indicate that the strong coupling of excitons and stretching vibrations of the Si-0 bonds causes the broad luminescent spectra. It is concluded that the interaction between electronic and vibrational excitations controls the luminescent emission and the observed dynamics.
Properties of Nanophase Materials Synthesized by Mechanical Attrition
Authors: Fecht H.-J. , Moelle C. 
Published: April 2011

ABSTRACTMechanical attrition and mechanical alloying has been developed as a versatile alternative to other processing routes in preparing nanophase materials with a broad range of chemical composition and atomic structure. In this process, lattice defects are produced by “pumping” energy into initially single-crystalline powder particles of typically 50 μm particle diameter. This internal refining process with a reduction of the average grain size by a factor of 103– 104results from the creation and self-organization of small-angle and high-angle grain boundaries within the powder particles during the milling process. This microstructural evolution has been characterized by X-ray, neutron and electron scattering methods revealing the grain refinement and increase in internal stress. As a consequence, a change of the thermodynamic, mechanical and chemical properties of these materials has been observed with the properties of nanophase materials becoming controlled by the grain size distribution and the specific atomic structure and cohesive energy of the grain or interphase boundaries. An analysis of the thermal stability of attrited powder specimen gives the grain boundary energy of non-equilibrium and fully relaxed grain boundaries as well as their mobility. In summary, it is expected that the study of mechanical attrition processes in the future not only opens new processing routes for a variety of advanced nanophase materials but also improves the understanding of technologically relevant deformation processes, e.g. surface wear, on a nanoscopic level.
Preparation of Nanometer Sized Aluminum Powders
Authors: Johnson Curtis E. , Higa Kelvin T. 
Published: April 2011

ABSTRACTNanometer aluminum powders have been prepared from the catalytic thermal decomposition of aluminum hydride adducts in organic solvents. This process provides excellent control of particle size and uniformity with batch particle sizes ranging from 50 to 600 nm. Variables affecting particle size were explored, including reaction temperature, catalyst concentration, and adducting amine concentration. Passivation techniques of the reactive aluminum powders were established for safe handling in air. Samples were mainly characterized by scanning electron microscopy and thermogravimetric analysis.
Three-Dimensional Superlattice Packing of Faceted Silver Nanocrystals
Authors: Harfenist S. A. , Wang Z. L. , Alvarez M. M. , Vezmar I. , Whetten R. L. 
Published: April 2011

ABSTRACTOrientational ordering of faceted nanocrystals in nanocrystal arrays has been directly observed for the first time, by use of transmission electron microscopy imaging and diffraction to resolve the structure of thin molecular-crystalline films of silver nanocrystals passivated by alkylthiolate self-assembled monolayers. The type of ordering found is determined by the nanocrystals faceted morphology, as mediated by the interactions of surfactant groups tethered to the facets on neighboring nanocrystals. Orientational ordering is crucial for the understanding of the fundamental properties of quantum-dot arrays, as well as for their optimal utilization in optical and electronic applications.
Preparation of Metal Nanosuspensions by High-Pressure DC-Sputtering on Running Liquids
Authors: Wagener M. , Murty B. S. , Günther B. 
Published: April 2011

ABSTRACTA modified VERL-process (Vacuum evaporation on running liquids) employing high pressure magnetron sputtering has been used for the preparation of suspensions with metal nanoparticles. The method has been tested for Ag- and Fe-suspensions by varying the pressure of the Argon sputtering atmosphere in the range of 1 to 30 Pa. A narrow particle size distribution with a mean particle size ranging from 5–18 nm has been found. The mean particle size increases with increasing Argon pressure in the pressure range under investigation. A descriptive model for the process of particle formation as a function of sputtering gas pressure is given.
Characterization of Consolidated Rapidly Solidified Cu-Nb Ribbons
Authors: Ebrahimi F. , Henne M. L. C. 
Published: April 2011

ABSTRACTCopper-niobium ribbons produced by melt-spinning were compacted by swaging and consolidated using HIPping. Final processing to obtain in-situ composites was done by swaging. The strength of the composite is discussed in terms of the composition and morphology of the niobium phase as evaluated using electron microscopy techniques.
Commercialization Status of Ni3Al-Based Alloys
Authors: Sikka V. K. 
Published: April 2011

ABSTRACTThe Ni3Al-based alloys have been under development at the Oak Ridge National Laboratory (ORNL) and other research institutions in the United States and around the world for the last ten years. The incremental developments of composition, melting process, casting methods, property data, corrosion data, weldability development, and prototype component testing under production-like operating conditions have pushed the ORNL-developed Ni3Al-based alloys closer to commercialization. This paper will present the highlights of incremental technical developments along with the approach and current status of commercialization. It is concluded that cast components are the primary applications of Ni3Al-based alloys, and applications range from heat-treating fixtures to forging dies. It is also concluded that the commercialization process is accelerated when technology is licensed to an organization that can produce the alloy, has component manufacturing capability, and is also a user.
TEM Characterization of Planar Defects in A Massively Transformed TiAl Alloy
Authors: Zhang X. D. , Wiezorek J. M. K. , Kaufman M. J. , Loretto M. H. , Fraser H. L. 
Published: April 2011

ABSTRACTThe microstructure of a massively transformed Ti-49at.%Al alloy has been studied by conventional transmission electron microscopy (CTEM) and high resolution TEM (HREM). A high density of planar defects, namely complex anti-phase domain boundaries (CAPDB) and thermal micro-twins (TMT) have been observed. CTEM images and diffraction patterns showed that two anti-phase related γ-matrix domains were generally separated by a thin layer of a 90°-domain, for which the c-axis is rotated 90° over a common cube axis with respect to those of the γ-matrix domains. HREM confirmed the presence of two crystallographically different types of 90°-do-mains being associated with the CAPDB. Furthermore, interactions between the CAPDB and TMT have been observed. Local faceting of the generally wavy, non-crystallographic CAPDB parallel to the {111}-twinning planes occurred due to interaction with the TMT. The relaxation of the CAPDB onto {111} required diffusion which is proposed to be enhanced locally in the presence of the dislocations associated with the formation of TMT during the massive transformation.