bluuebunny/crossref_metadata_2025 huggingface dataset:
A Compact Nuclear Microprobe With a Liquid Metal Ion Source and a Toroidal Analyzer
Authors: Takai Mikio , Mimura Ryou , Sawaragi Hiroshi , Aihara Ryuso
Published: March 2011

A nondestructive three-dimensional RBS/channeling analysis system with an atomic resolution has been designed and is being constructed in Osaka University for analysis of nanostructured surfaces and interfaces. An ultra high-vacuum sample-chamber with a threeaxis goniometer and a toroidal electrostatic analyzer for medium energy ion scattering (MEIS) was combined with a short acceleration column for a focused ion beam. A liquid metal ion source (LMIS) for light metal ions such as Li+ or Be+ was mounted on the short column.

A minimum beam spot-size of about 10 nm with a current of 10 pA is estimated by optical property calculation for 200 keV Li+ LMIS. An energy resolution of 4 × 10-3 (AE/E) for the toroidal analyzer gives rise to atomic resolution in RBS spectra for Si and GaAs. This system seems feasible for atomic level analysis of localized crystalline/disorder structures and surfaces.
Low Energy Point Source Electron Microscopy
Authors: Kreuzer H. J.
Published: March 2011

The theory of the point source low energy electron microscope is reviewed. Images are calculated for a carbon fibre, a small cluster of MgO and a double helix of carbon atoms. A Kirchoff-Helmholtz transform is used for reconstruction. The importance of image size is stressed and the chemical specificity of the method is demonstrated
Dissociative Phase Transitions, Split Shock Waves, Rarefaction Shocks, and Detonations
Authors: White C. T. , Robertson D. H. , Elert M. L. , Mintmire J. W. , Brenner D. W.
Published: March 2011

We present a comparative study of two different chemically-sustained shock waves. One shows behavior expected from the Zel'dovich, von Neumann, and Doering (ZND) continuum theory of planar detonations. The other exhibits the complexity of a split shock wave resulting from the presence of a polymorphic phase transition. This comparative study demonstrates the importance of carefully considering the high-pressure characteristics of the model in developing potentials for simulating detonations. This comparative study also raises the fascinating possibility of a first-order phase transition accompanying a condensed-phase detonation.
Femtosecond Spectroscopy of Chemically Reactive Solids: a Methodology
Authors: Wang Weining , Wefers Marc M. , Nelson Keith A.
Published: March 2011

An experimental methodology for recording femtosecond time-resolved observations of irreversible change in solids is described. The central problem posed is that the trime-dependent evolution must be observed on a single-shot (i.e. real-time) basis since the sample may be permanently altered after each excitation event. Preliminary demonstrations of real-time femtosecond spectroscopic observations are presented. In addition, one-shot data acquisition techniques open up the possibility of excitation intensities that greatly exceed optical damage threshholds of most samples. Since only one excitation pulse is used, cumulative damage mechanisms may be circumvented. Even if the sample is damaged in a single shot, in some cases the events of interest may be observed before damage occurs. The use of timed sequences of high-intensity excitation pulses to drive large-amplitude, coherent lattice vibrations is discussed. If successful, such large-amplitude lattice vibrations could assist crystalline chemical reactions or structural phase transitions.
Explosion of Drops Impacting Non-Wetting Rigid Surfaces
Authors: Zener Clarence , Prieve Dennis
Published: March 2011

More than 100 years ago Worthington [1,2] reported his observations on drops exploding on impacting rigid surfaces they did not wet. Mercury was his favorite fluid, since mercury does not wet most solids. For water drops he had to especially “smoke” his surfaces to make them non-wetting. In his day high speed photography had not been developed, but he had learned to “stop” his falling drops by spark illumination. His drops had radii of typically 1 mm, fell from a height of ∼10 cm and ejected always an even number of spikes, typically ∼24. These spikes would shoot out from the impacted area at velocities several times higher than the impact velocity.
Molecular Level Study of Insensitive and Energetic Layered and Intercalated Materials
Authors: Bardo Richard D.
Published: March 2011

An important goal in the detonation physics and chemistry community is to design explosives with high-performance capabilities, yet sufficiently low impact and shock sensitivities. The traditional ways of developing increasingly energetic explosives have led to an increase in sensitivity. It is, therefore, of practical and theoretical importance to design, from molecular level considerations, polycrystalline explosive systems which clearly exhibit directional sensitivity and initiation properties, and which approach the insensitivity of homogeneous or single-crystal explosives. Theoretical studies indicate that such systems may be constructed from special materials such as high-quality pyrolytic, layered, hexagonal boron nitride (BN) crystals. Intercalation of the crystals with various oxidizing agents can yield an explosive system with the desired properties. Under ordinary laboratory conditions and with modest pressures, the intercalate molecules readily enter the host lattice by exploiting the weak bonding between the layers, resulting in denser, metastable crystals. High energy release is achieved when products containing B2O3 are formed.
Pressure/Temperature/Reaction Phase Diagrams for Several Nitramine Compounds
Authors: Russell T. P. , Miller P. J. , Piermarini G. J. , Block S.
Published: March 2011

Pressure/temperature/reaction phase diagrams for several nitramine compounds, including hexanitrohexaazaisowurtzitane (HNIW), 1,3,5-trinitrohexahydro-l,3,5-triazine (RDX), ammonium dinitramide (ADN), and p-nitroaniline (PNA) are presented. A diamond anvil cell was used in conjunction with optical polarizing light microscopy (OPLM), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray diffraction (EDXD), and micro FT-Raman spectroscopy to determine these diagrams. A description is given of the diamond anvil cell and the associated techniques employed.
The Growth and Perfection of Single Crystals of Trinitrotoluene (TNT)
Authors: Gallagher Hugh G. , Sherwood John N.
Published: March 2011

A study has been made of the crystal growth behaviour and perfection of TNT. Depending on the purity and the solvent used, crystallisation from solution yields material of monoclinic, twinned monoclinic or orthorhombic forms. Of these only the monoclinic form was suitable for detailed study by X-ray topographic techniques. The crystals of the orthorhombic form were too small and imperfect for examination.

Seeded growth from ethyl acetate solution yielded lamellar twinned crystals in which the twin density was low. Under ideal conditions, relatively perfect samples could be obtained in which the dominant residual growth defects were stacking faults of Burgers vector b ½ [001] and dislocations.
Molecular Structure and Performance of High Explosives
Authors: Stine James R.
Published: March 2011

Empirical methods have been developed to predict density and heat of formation of proposed high explosives. These methods are parameterized in terms of molecular constituents consisting of atoms in particular bonding environments. These methods, along with the atomic composition of the molecule, are then used in an empirically based method to predict the performance of the explosive.

Together, these methods form a unified mechanism of relating molecular structure to explosive performance. Indeed, this effort, along with the synthesis effort at Los Alamos, have led to the identification of a new class of high-nitrogen molecules that may prove to be high performance explosives.
Energetic Materials in Ceramics Synthesis
Authors: Kingsley J. J. , Pederson L. R.
Published: March 2011

Combustion of a proper combination of an oxidizer and a fuel can produce the exothermicity required for the simultaneous synthesis of oxide ceramic powders. Oxidizers include metal nitrates, ammonium nitrate, and ammonium perchlorate, while urea, carbohydrazide, glycine and others have been used successfully as fuels. Combustion methods are particularly well-suited to producing multicomponent metal oxides, yielding compositionally homogeneous, fine particles with low impurity content. Organic fuels, particularly those containing nitrogen, also serve as a complexant in the precursor, which inhibits inhomogeneous precipitation from occurring prior to combustion. The exothermic redox decomposition of these oxidizer-fuel mixtures is initiated at low temperatures, usually <250°C. Properties of the products are influenced by the nature of the fuel and the oxidizer/fuel ratio. Many technologically important oxide ceramics have been produced by these methods.
A Novel Structure of a-SiN:H/a-Si:H Multilayer Avalanche Photodiode (MAPD)
Authors: Lihong Jiao , Zhiguo Meng , Zhonglin Sun
Published: March 2011

Because of the lower density of interface states in a-Si:H/a-SiN:H than that in a-Si:H/a-SiC:H, an a-Si:H/a-SiN multilayer reach-through avalanche photodiode is fabricated on an ITO/glass substrate by plasma-enhanced chemical vapor deposition (PECVD) . In order to improve the performance of the a-Si:H/a-SiN:H APD'S, a novel structure is used. By controlling the deposition ratio of silicon and nitrogen of amorphous SiN,the valence band top of a-Si:H is deeper than that of a-SiN:H, that is, the a-Si :H/a-SiN: H system has the electron potential well in a-Si:H, while the hole well is in a-SiN:H, thus we can successfully suppress the hole impact ionization, correspondingly enhance the electron impact ionization effectively.

The measurement of current versus voltage is employed to study the multiplication factors and the impact ionization coefficients. The characteristics of a-Si:H/a-SiN:H APD's,such as I-V curves, optical gains, impact ionization rates, excess noise factors, the relative response and the relationship between the breakdown voltage and wavelength, are studied. The electron multiplication factor is Mc=4.5 at reverse bias V=12v. An optical gain of 3.7 at reverse bias VR=12v and an incident light power Pin=3μw is obtained. Homo junction a-Si:H reach-through APD's and homojunction a-Si:H APD's are also fabricated for comparison.The results show that the novel a-Si:H/a-SiN:H APD's is promising in high-gain, low-noise photodetectors.
Microstructural and Vibrational Characterization of the Hydrogenated Amorphous Silicon Powders
Authors: Costa J. , Sardin G. , Campmany J. , Andújar J.L. , Canillas A. , Bertran E.
Published: March 2011

Hydrogenated nanophase silicon powders prepared by low pressure and low temperature rf plasma using pure silane gas, have been characterized by transmission electron microscopy (TEM), Fourier transform IR spectroscopy (FTIR) and thermal desorption spectrometry (TDS) of hydrogen. By means of these analysis, the evolution of the hydrogen bonds and the oxidation processes as a consequence of annealing of the silicon powders under vacuum or atmospheric conditions have been investigated. The TDS results reveal the fundamental differences between the concentrations of hydrogen weakly and strongly bonded in silicon powders as compared to amorphous silicon films, and the FTIR spectra evidence the oxidation process taking place in the silicon powders as a consequence of the annealing. These results along with the TEM analysis show that silicon powder particles present intergrain linkage. We also study the silicon powder particles considering their microstructure and vibrational characteristics as well as their degree of polymerization.
Microstructure and Integrity of Thin Silicon Films on Sio2 after Immersion in 10:1 Buffered HF
Authors: Hegde Rama I. , Chonko Mark A. , Tobin Philip J.
Published: March 2011

The microstructure and integrity of thin silicon films on SiO2 after immersion in 10:1 buffered HF solution were investigated by AES, AFM and TEM measurements. Silicon films of 30 nm - 50 nm thicknesses were deposited on 15 nm of thermal oxide by LPCVD using SiH4 at 550 °C, 580 °C and 625 °C. These films were then treated with the HF for 15 seconds and characterized systematically. The data provided direct physical evidence for etching of gate oxide by the HF solution through defects (holes) in the 580 °C Si film.
Charge Carrier Transport in c-Si/a-Si:H Heterojunctions
Authors: Sanders A. , Neitzert H.-C. , Swiatkqwski C. , Kunst M.
Published: March 2011

Contactless transient photoconductivity measurements in a- Si:H/c-Si heterojunctions are presented. The experimental data are decomposed in the contribution of excess electrons in a-Si:H, the contribution of excess carriers in c-Si and the contribution of excess carriers injected from a-Si:H into c-Si. A value for the electron drift mobility of in a-Si:H of 1cm2 V-1 s-1 is determined. At low excitation densities a high injection efficiency is observed, which is quenched by bias illumination.
Amplitude Dependence of Metastable Defect Formation
Authors: Jackson W.B. , Nebel C. , Street R.A.
Published: March 2011

The kinetics of small changes from thermal equilibrium are measured and simulated for thermally induced metastable changes. We find that only a distribution of rate constants is consistent with the data, and therefore we can eliminate certain types of microscopic models for metastability which depend on a single variable such as the carrier density.
Dependence of Thermally Induced Metastability on Hydrogen Content in Amorphous Silicon
Authors: Pietruszko Stanislaw M.
Published: March 2011

The thermally induced metastability was observed in unhydrogenated doped amorphous silicon prepred by Low Pressure Chemical Vapour Deposition (LPCVD). The equilibration temperature (T*) was found to be 380°± 20°C. The preliminary study of the influence of hydrogen content on thermally induced metastability in amorphous silicon is reported. In the hydrogenated doped films the equilibration temperature changes from 380° ± 20°C to 170° ± 10°C, as H content increases to 12 at.%.
Hydrogen Dynamics in a-Si:H
Authors: Santos Paulo V.
Published: March 2011

The interaction between electronic carriers and hydrogen migration in a-Si:H was investigated by diffusion experiments in the intrinsic (i−) layer of p-i-n a-Si:H photo-diodes. The carrier concentration in the i-layer was controlled by varying the bias applied to the devices. Hydrogen migration (i) is enhanced when the carrier population is increased by illumination and (ii) is suppressed when it is reduced below the thermal equilibrium value by the application of a reverse bias to the diodes. The effect is attributed to the dependence on carrier density of the dissociation rate of hydrogen from Si-H bonds into the diffusion path consisting of interstitial sites. In addition, the migration length in the diffusion path increases under reverse bias. The enhanced migration is associated with a decrease in the effective density of traps for hydrogen in a carrier-depleted layer. Possible mechanisms for the interaction between hydrogen migration, carriers and defects are discussed.
1 / F Noise Measurements of Interacting Current Filaments in Hydrogenated Amorphous Silicon
Authors: Parman C.E. , Israeloff N.E. , Fan J. , Kakalios J.
Published: March 2011

The coplanar current in n-lype doped hydrogenated amorphous silicon (a-Si:H) displays random telegraph switching noise, indicating the presence of inhomogeneous current filaments whose conductance varies with time. There are strong correlations of the 1/f noise power spectra across differing frequency octaves which are much larger than expected if the magnitudes of the fluctuators are varied in parallel. The scale invariant second spectra and the temperature dependence of the spectral slope indicate that hydrogen motion is involved in the cooperative dynamics between noise sources. A model is described wherein the properties of the current filaments are modulated by hydrogen-hydrogen interactions which are mediated by the Si strain fields.
Odesr Studies of a-Si:H with Subgap Excitation
Authors: Mao D. , Gu S.Q. , Taylor P.C.
Published: March 2011

We report studies of the spin-dependent recombination processes in device-quality a-Si:H by optically detected ESR (ODESR) with the excitation energy (Ex) varied from above-gap to sub-gap. The photoluminescence (PL) transients induced by the chopping of the microwaves are recorded as functions of the magnetic field to yield the lineshapes. This scheme, to some extent, circumvents the problem of interferences between "enhancing" and "quenching" signals encountered in conventional phase-sensitive detection. The PL below 1.1 eV is monitored. For Ex above 1.5 eV, we find all the lineshapes are similar: a broad, slightly asymmetric enhancing line peaking at g = 2.008 and an asymmetric quenching line peaking at g = 2.005. Lineshape changes are noticeable for Ex below 1.5 eV. The relative signal intensity AI/I changes dramatically with varying excitation energy. Light-soaking of the sample induces an additional enhancing signal which peaks at g = 2.006.
Accurate Reconstruction of the Density of States in a-Si:H by Photothermal and Photoconductive Spectra.
Authors: Amato G. , Giorgis F. , Manfredotti C.
Published: March 2011

The distribution of occupied states in a-Si:H has been inferred by applying a new self- consisting iterative method to the absorption coefficient spectra. This procedure does not require any assumption about the localized states below the Fermi level, and provides a more accurate insight with respect to the simple derivative method. Numerical simulations have been made in order to probe the reliability of our method. The optical spectra have been obtained by means of Photothermal Deflection Spectroscopy (PDS) and Constant Photocurrent Method (CPM); the comparison between the results as obtained by the two techniques suggests that different sensitivities to electronic transitions are involved; this can be used to infer information about the unoccupied defects.
Depletion Fraction of Silane and Dominant Neutral Radical in RF Glow Discharge in Silane
Authors: Lin Quixun , Lin Xuanying , Xu Zeng , Yu Yunpen , Peng Shaoqi
Published: March 2011

A quadripole mass spectrometer was installed on the line of a PCVD system to measure the depletion fraction of silane and the monosilicon radicals in the discharges. The paper presents a method of measuring the depletion fraction of silane when setting the mass spectrometer at the high ionization voltage. The experimental results were compared and accordant with those reported before [1,2,3].
Effects of In-Depth Inhomogeneities on the Optical Absorption Spectra of a-Si:H Films: Theory and Quantitative Evaluations
Authors: Amato G. , Giorgis F. , Manfredotti C.
Published: March 2011

The effect of the non-uniform defect profile in a-Si:H films on the subgap photothermal spectra is investigated. A comprehensive theoretical model is suggested. By means of this model it is possible to extract quantitative information about the defects inhomogeneity in the film.
Undoped and Phosphorus Doped a-Sic:H Films: Investigation of Electrical Properties and Hall Effect
Authors: Pisarkiewicz T. , Stapinski T. , Demichelis F. , Pirn C.F. , Tresso E. , Rava P.
Published: March 2011

We have deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD), under particular conditions of high H2 dilution and high power density, undoped and phosphorus doped (µc‐SiC:H films. We have obtained n‐doped films with a high band‐gap (E04 up to 2.1 eV) and a high dark conductivity (σ4 up to 10 S cm‐1) which are promising materials for window layers in solar cells. Thermo Electric Power (TEP) measurements allowed to identify the type of majority carriers. The dark conductivity and the Hall mobility have been obtained as a function of temperature in the range 80‐480 K.
Dual-Beam Photocurrent Spectra in Undoped a-Si:H
Authors: Liu J.Z , Lewen G. , Conde J.P. , Cabarrocas P. Rocai
Published: March 2011

It is shown that the anomalous band in dual-beam CPM (DBCPM) spectra results from a combination of two processes: photocurrent enhancement due to excitations by the probe light from the valence baud to theD+ defect states, and photocurrent reduction due to excitations by the probe light from the D° defect states to the conduction band. From the DBCPM spectra, the electron correlation energy for the (filled)D - defects is determined to be 0.25 eV.
Mechanism of High Rate a-Si:H Deposition in a VHF Plasma
Authors: Heintze M. , Zedlitz R. , Bauer G.H.
Published: March 2011

Very high frequency (VHF) glow discharges are employed for high rate a-Si:H deposition while maintaining good optoelectronic materials properties. A more efficient radical generation, either due to higher electron densities or an enhanced high energy electron tail is generally assumed as the mechanism. We have investigated a VHF a-Si:H deposition plasma between 40 and 250MHz by optical emission spectroscopy (OES), mass spectroscopy (MS), ion energy measurements and electrical impedance analysis. The present study shows that the increase of the deposition rate with frequency is essentially due to an enhanced ion flux to the growth surface.
Critical Laser Fluence Observed in (111) Texture, Grain Size and Mobility of Laser Crystallized Amorphous Silicon
Authors: Johnson R.I. , Anderson G.B. , Boyce J.B. , Fork D.K. , Mei P. , Ready S.E. , Chen S.
Published: March 2011

This paper describes new results on the relationship between the grain size, mobility, and Si (111) x-ray peak intensity of laser crystallized amorphous silicon as a function of the laser fluence, shot density, substrate temperature, and film thickness. These observations include an unexpected narrow peak found in the silicon (111) x- ray peak intensity, which occurs at a specific laser fluence for a given film thickness and substrate temperature. Amorphous silicon materials processed at laser energy densities defined by this peak exhibit exceptionally large grain sizes and electron mobilities that cannot be obtained at any other energy and shot density combination above or below the energy at which the Si (111) x-ray peak intensity maximum occurs.
Temperature Dependence of the Hall Mobility in Polycrystalline Silicon
Authors: Ready S.E. , Boyce J.B. , Fork D.K. , Mei P. , Anderson G.B. , Johnson R.I.
Published: March 2011

Crystallization of amorphous silicon thin films by various methods has fostered enhancements in the electrical characteristics over their amorphous counterparts. For example, carrier mobilities ranging from 10 to >100 cm2/V-sec have been reported for laser crystallized films. The rather large variability of the transport characteristics with crystallization processing conditions is not well understood and, as a result, greatly complicates device process debugging. In addition, while it is generally believed that defects inherent in the grain boundaries provide the primary barriers degrading transport properties relative to single crystal silicon, the specific nature of these defects is not known. In this paper, we present data on the temperature dependence of the Hall mobility of thin silicon films crystallized by thermal and excimer laser processing. Hall data for the laser-crystallized phosphorus-doped material show a temperature dependence which differs dramatically from that for thermally crystallized materials, while the effects of hydrogenation are similar, reducing the barriers at the grain boundaries.
A Comparative Study of Defect States in Light-Soaked and High-Temperature-Annealed a-Si:H
Authors: Saleh Z. M. , Tarui H. , Tsuda S. , Nakano S. , Kuwano Y.
Published: March 2011

Our previous results of light-induced electron spin resonance (LESR) indicate that, in hydrogenated amorphous silicon (a-Si:H), light-induced defects differ from those formed during deposition or high-temperature annealing. A plausible interpretation, in which light-induced defects occupy higher-energy states, was proposed to explain these differences. In this study, the constant photocurrent method (CPM), dark conductivity and steady-state (SS) LESR are used to supply new evidence for the difference and conduct two important tests on our hypothesis. In striking agreement with our predictions, we find that the light-induced changes in the SS-LESR lineshape (a decrease in the narrow component relative to the broad one upon light exposure) become indeed more dramatic as the demarcation energies move closer to the midgap by increasing temperature or decreasing bias-light intensity for SS-LESR.
Thermal and Optical Stretched Exponentials in Defect Kinetics in a-Si:H
Authors: Redfield David , Bube Richard
Published: March 2011

Dispersive description of defect generation in a-Si:H that leads to stretched-exponential transients is extended by relaxing the assumption that light-induced processes and thermally induced processes have the same dispersive character. This is done by separating the rate equation for the defect density into two parts, one thermal and one optical, each with its own dispersion parameter. The solutions of this new equation — which must be obtained numerically — generally have two distinct parts: there may be a two-part rise or a peak, depending on the relative values of the two stretch parameters. Using this formulation we have readily simulated the recently observed peak in relaxation of a previously heavily degraded solar cell while exposed to a weak light. We find no way to explain other reports in similar two-part experiments that relaxation is faster under weak excitation than without.
Effect of Light Soaking on the Spectrally and Spatially Resolved Collection Efficiency of a-Si:H Solar Cells
Authors: Linden M. B. VON DER , Schropp R. E. I. , Lekkerkerker O. P. , Ouwens J. DAEY , Van Der Weg W. F.
Published: March 2011

Light soaking of a-Si:H solar cells gives rise to a decrease in the spatially resolved collection efficiency. In the internal collection efficiency distributions measured under bias light conditions two crossing points appear during light soaking. The existence and the shift of these points could be explained by a combined bulk i-layer light soaking effect and a more pronounced collection loss at the p+/i-interface. This spatially non-uniform degradation was confirmed by the Dynamic Inner Collection (DICE) method.
Bandgap Engineering in Hydrogenated Silicon Films Made by Combined Hydrogen Dilution and Atomic Hydrogen Treatments
Authors: Hsu K.C. , Hwang H.L.
Published: March 2011

Effective bandgap modifications of hydrogenated silicon are demonstrated in this work. These films were obtained by (combined) hydrogen dilution and atomic hydrogen treatments during the deposition. The Si—H bonding configurations were characterized by NMR measurements. The hydrogen dilution tends to create a very sharp line-shape in the NMR spectra as the dilution ratio is increased and the addition of atomic hydrogen treatment can produce the same NMR spectra even at a lower temperature. The optical bandgap of these Si:H samples showed the same tendency that the bandgap narrowed as both the dilution ratio was increased and the addition of atomic hydrogen treatment.
The Density of States in a-Si:C:H Revealed by Electrophotography
Authors: Van Swaaij R.A.C.M.M. , Willems W.P.M. , Bezemer J. , Von Der Linden M.B. , Van Der Weg W.F.
Published: March 2011

Electrophotographic dark decay measurements have been used to determine the surface density of states (SDOS) of a-Si:C:H. Injection of trapped charge from these deep states into the conduction band governs the dark discharge of a photoconductor, provided bulk generation and bulk space charge are negligible. It is found that the SDOS profiles peak around 0.60 eV below the conduction band for materials with different carbon concentration. This observation implies that the energy position of these states is fixed with respect to the conduction band edge, even though the optical band gap of these materials increases with increasing carbon concentration. The nature of these states may be ascribed to D− states, whose density is strongly enhanced by filling D° states when the material is charged negatively. Furthermore, we observed that the SDOS around 0.60 eV below the conduction band edge is approximately the same for materials with up to 8 at.% carbon. From temperature dependent measurements a value of 2·108 s−1 was obtained for the attempt-to-escape frequency.
Parallel Conduction in a-Si:H/a-Si1−xCx:H Multilayers
Authors: Bertomeu J. , Puigdollers J. , Asensi J.M. , Delgado J.C. , Andreu J.
Published: March 2011

This paper deals with the electrical properties in the parallel direction of compositionally modulated amorphous silicon/amorphous silicon-carbon multilayers. Conductivity of three series of samples with varying well and barrier thicknesses is studied. The results show that dark conductivity decreases when reducing a-Si:H layer thickness. This is interpreted as an alloy effect at interfaces. The role of the a-Si1-xCx:H layers in the photoconductivity decrease observed in series with variable mean composition and constant well thickness is discussed
Conduction and Valence Band Offsets at the Hydrogenated Amorphous Silicon-Carbon/Crystalline Silicon Interface Via Capacitance Techniques
Authors: Essick John M. , Mather Richard T. , Bennett Murray S. , Newton James
Published: March 2011

Heterostructure Schottky diode samples each composed of a sub-micron thick layer of intrinsic hydrogenated amorphous silicon-carbon (a-Si1−xCx:H) deposited on an n-type crystalline silicon (c-Si) substrate are used to measure the a-Si1−xCx:H/c-Si band offsets via junction capacitance techniques. The samples range in carbon concentration from x=0.0−0.3. First, a thermally activated capacitance step due to the response of defects at the amorphous/crystalline interface is evident in capacitance vs. temperature spectra taken on all these samples. The bias-dependence of this step’s activation energy provides a direct measure of the a-Si1−xCx:H/c-Si interface potential as a function of c-Si depletion width in each sample. By application of Poisson’s equation, we find that the a-Si1−xCx:H/c-Si conduction band offset ΔEc. increases from 0.00 to 0.10 eV as x increases from 0.00 to 0.26. Second, while under reverse-bias at low temperature, we optically pulsed each sample with c-Si band-gap light to create trapped holes at the a-Si1−xCx:H/c-Si valence band offset ΔEV. By noting the threshold for the subsequent optical release of these trapped holes by sub-band gap light, we found that ΔEV increases from 0.67 to ≥0.83 eV as x increases from 0.00 to 0.26.
Doping and Alloying of Hydrogenated Amorphous Germanium Films Prepared by DC-Magnetron Sputtering
Authors: DrÜsedau T. , Annen A. , SchrÖder B.
Published: March 2011

In this paper we present a detailed study of doping and alloying effects in a-Ge:H due to the incorporation of phosphorus, nitrogen, oxygen, boron, carbon and silicon. The film characterization was performed by means of quantitative analysis , optical absorption in the band and subband range and conductivity measurements. N and O were found to form donor states resulting in an increase of the dark conductivity up to five orders of magnitude. The maximum value of 0.3 (Ωcm)-1 has been obtained by N-doping of about 1 at% solid phase concentration, which exceeds the effect of P doping. B doping causes a decrease of the dark conductivity to a minimum of 10-6 (Ωcm)-1 at 150 ppm solid phase concentration. Addition of Si and C up to concentrations of about 5 at % does not influence the subgap absorption, where best values of E0<50 meV and α(0.6 eV)<10 cm-1 were obtained for the Urbach energy and midgap absorption, respectively. A Si content of 1 at % decreases the dark conductivity, whereas the normalized photoconductivity remains at a high level of ŋμτ=10-5 cm2V-1.
Optoelectronic Properties of a-Si:H Films Deposited From He-Diluted Silane
Authors: Swiatkowski C. , Cabarrocas P.Rocai , Kunst M.
Published: March 2011

In-situ and ex-situ transient photoconductivity measurements of intrinsic a-Si:H films deposited from He-diluted SiH4 are presented. It is shown that material with good optoelectronic properties can be deposited at high deposition rates. The films can already be characterized during the deposition. It is shown that material with fairly different properties can be deposited with a relative low defect density.
Observation of Deep Defect Relaxation Processes in Hydrogenated Amorphous Silicon-Germanium Alloy
Authors: Zhong Fan , Cohen J.David
Published: March 2011

Modulated photocurrent (MPC) spectroscopy has been used to investigate the energy distribution of deep defect states in photo-CVD grown a-Si1 -xGex:H alloys. We observe a distinct electron trapping feature at a thermal energy of 0.55eV below Ec for higher Ge concentration (x = 0.62) alloys. For those samples with Ge concentration between 30-50 at. %, an anomalously large phase shift was observed within a temperature window between 150K to 240K. The modulated photocurrent exhibits strong quenching at the edges of this window. We suggest that these MPC spectra are controlled by deep defect relaxation processes in this temperature region, which causes electron thermal emission processes to be greatly suppressed. This effect can be reduced by increasing the reverse bias. Under such conditions, the energy position of these defect states can be roughly estimated to lie near Ec - 0.33eV. This result supports the recent result obtained from transient photocapacitance and photocurrent measurement.
Hydrogenated Amorphous Silicon-Nitrogen, a-Si,N:H ALLOYS: An Alternative to A-SI,C:H for the Wide Band Gap Photo-Active Material in Tandem PV Cells
Authors: Williams M.J. , Cho S.M. , Lucovsky G.
Published: March 2011

We have investigated a-Si,N:H alloys as an alternative wide band-gap, photo-active material. The entire alloy range between a-Si:H and a-Si3N4:H can be formed by a remote plasma-enhanced chemical-vapor deposition (PECVD) process. Other studies have demonstrated that a-Si,N:H alloys could be doped to form window materials for p-i-n devices. This paper focuses on alloy materials with E04 bandgaps to about 2.2 eV. We have prepared these a-Si,N:H alloys, characterized their microstructure, and studied their photoconductivity, sensitivity to light-soaking and transport properties. For example, with increased alloying we show that i) the white-light photoconductivity and ii) the kinetics and magnitude of the decay of photoconducitivity under intense illumination (the Staebler-Wronski effect), are about the same as for PV-grade a-Si:H.
Major Improvement of Intrinsic a-Si:H Films for Solar Cell Applications
Authors: Xi Jianping , Liu Tongyu , Nugent Martin , Si Kevin , Iafelice Vincent , Cueto Joe Del , Ghosh Malathi , Kampas Frank
Published: March 2011

The combination of a carefully-designed deposition system, process optimization, and a proprietary processing technique, has resulted in reduced structural defects and photo-stable, high quality intrinsic a-Si films. The improvement in material properties is further evidenced by successful incorporation of the material into single junction devices with a record stabilized device efficiency of 7.5%.
Correlation of Small-Angle X-Ray Scattering and Solar Cell Performance of a-SiGe:H Alloys
Authors: Jones S.J. , Chen Y. , Williamson D.L. , Xu X. , Yang J. , Guha S.
Published: March 2011

Small-angle x-ray scattering (SAXS) measurements were made on a-SiGe:H alloys to study microstructure on the nanometer scale as a function of Ge content, and the results were compared with representative single-junction solar cell properties. Samples consisting of only the i-layer were used for SAXS. Above a Ge content of 20 %, a significant increase in SAXS was seen. From measurements made with the samples tilted relative to the incident x-ray beam, the increase in scattering is attributed to the appearance of elongated low density regions in the film, modeled as ellipsoidal microvoids, which are preferentially oriented perpendicular to the film surface and may be related to columnar-like microstructure. Flotation density measurements support the presence of low density regions. Initial and light-degraded measurements on corresponding solar cell structures do not show a correlation between SAXS and initial cell properties; there is, however, some evidence that the light-induced degradation is higher for cells with larger amounts of SAXS-detected microstructure and this needs further investigation.
Measurement and Modeling of a-Si,Ge:H Solar Cell Performance After High-Intensity Light Soaking
Authors: Vasanth K. , Nakata M. , Wagner S. , Bennett M.
Published: March 2011

For a-Si:H solar cells a clear correlation exists between the defect density in the i-layer and cell performance. For cells with a-Si,Ge:H alloy, i-layers, on the other hand, the effect of the properties of the i-layer and its interfaces on cell performance is under debate. To address these questions we light-soaked to saturation and measured a-Si,Ge:H films and cells, and numerically modeled the cell performance using film properties as model inputs. We study three cells with the structure Glass/textured SnO2/p+ a-Si,C:H/i a-Si,Ge:H/n+ a-Si:H/ZnO/Ag. The i-layers are of uniform composition, 150 nm thick and have Tauc gaps of .1.40, 1.46 and 1.50 eV. We modeled the cells with AMPS, a numerical model solving the Poisson equation and the continuity equations for electrons and holes. The defect density in the 1.40 eV film stays constant during light soaking, while the densities in the 1.46 eV and 1.50 eV films rise. The efficiency of the 1.40 eV i-layer cell stays constant, while it drops in the cells with the 1.46 eV and 1.50 eV i-layers. Using the currently accepted input parameters to the model, we obtain cell performances substantially above the experimentally observed values.
Thickness Dependence of Electroluminescence in a-Si:H P-I-N Devices
Authors: Wang Keda , Han Daxing , Silver M.
Published: March 2011

Electroluminescence studies in a-Si:H p-i-n devices have been made as a function of i-layer thickness (0.4 - 10 (μm), temperature (80 -300 K) and applied electric field (2xl03 – lx105 V/cm field across the i-layer). In particular, the spectral emission and lifetime distributions have been determined. With increasing temperature, there is a shift toward defect luminescence (0.9 eV) in all samples as found in photoluminescence. However, thicker samples (> 2.0 μm) show significantly more main band emission (1.2 eV) than thin samples. This trend is particularly true at higher applied voltages. Further, in thin samples, the lifetime distribution peaks at 10-6 sec in whole temperature range while in thick samples at lower temperature (< 250 K) the lifetime distribution is double peaked at 10-6 sec and at 10-3Sec.
Exploration of Amorphous Silicon Thin Film Transistor Degradation with Thermal Anneal
Authors: Kwasnick R.F. , Possin G.E. , Hill II W.L.
Published: March 2011

We have measured the device characterisics of short and long channel inverted- staggered hydrogenated amorphous silicon thin film transistors (TFTs) with either Mo or Cr source/drain metal after annealing at temperatures from 225 C to 275 C. The TFT deposition temperature at the substrate surface was about 270 C. From the slope of the transfer characteristic an effective mobility is extracted. Devices with Mo source/drain metal exhibit an initial effective mobility increase at short times (within about 30 min), while those with Cr do not. At long times the mobility of all devices decreases. The mobility changes are greatest for short channel length devices because of contact effects. The channel length dependence of the behavior permits a separation of the device behavior into contact and intrinsic mobility components.
Amorphous-Silicon TFTs with Self-Aligned Poly-Silicon Source and Drain
Authors: Kim Chang-Dong , Sugiura Osamu , Matsumura Masakiyo
Published: March 2011

A new self-aligned a-Si TFT has been proposed. The TFT has the highly conductive n+ poly-Si source and drain with self-aligned manner. The key process technique employed to this TFT is of excimer-laser selective recrystallization. Two ways have been successfully demonstrated to acheive the selectivity of recrystallization. The fabricated TFT had the field- effect mobility of 0.8cm2/Vs and high on/off current-ratio more than 107.
Photoluminescence from Nanocrystalline Silicon Prepared by Plasma CVD and Oxidation
Authors: Veprek S. , Rückschloβ M. , Landkammer B. , Ambacher O.
Published: March 2011

Light emitting nanocrystalline silicon has been prepared by a completely dry processing which uses standard silicon technology. This enables us to prepare compact films on various substrates and to control the crystallite size. Dependence of the photoluminescence intensity and its peak energy on the crystallite size is reported and compared with current theoretical models.
Reducing Dislocation Density by Sequential Implantation of Ge and C in Si
Authors: Im Seongil , Washburn Jack , Gronsky Ronald , Cheung Nathan W. , Yu Kin Man , Ager Joel W.
Published: March 2011

Carbon implantation was performed after high dose(5 x 1016/cm2) Ge implantation into [100] oriented Si substrates to study the effect of sequential implantation on dislocation nucleation. When the nominal peak concentration of implanted C is over 0.55 at%, Dislocations in the SiGe layer containing C are considerably reduced in density after solid phase epitaxial(SPE) annealing at 800°C for 1 hour, compared to the SiGe layer without C. These results suggest that during annealing, C atoms compensate the Geinduced misfit strain which causes dislocation generation in the region of peak Ge concentration. Channeling spectra obtained by RBS analysis show only 5% to 6% minimum back scattering yield as C atoms suppress the dislocation generation.
The Structure and the Physical Properties of Si/C Grown by Laser Pyrolysis of Si(CH3)2Cl2
Authors: Chang Hua , Lee Long Jeng , Hwang Rong Li , Lin M. S. , Lou J. C.
Published: March 2011

The chemical kinetics of laser pyrolysis of Si(CH3)2Cl2 was studied. Besides Si/C deposited on substrate as a needle the main gaseous products were H2 and HCI. The decomposition rate constants of Si(CH3)2Cl2 and the formation rate constants of H2 and HCl were measured. The chemical and physical properties of the Si/C needles were studied by various methods, such as their outer features, elemental analysis, Auger depth profile spectra, X-ray diffraction. Other useful properties, such as electric resistivity, photovoltaic effect and hardness, were also reported. A model for the pyrolysis and the growth process of this Si/C needles was proposed. It should be emphasized that the samples with desired physical properties could be grown by controlling the growth conditions properly -- the laser power and the pressure of Si(CH3)2Cl2.
Can Oxidation and Other Treatments Help Us Understand the Nature of Light-Emitting Porous Silicon?
Authors: Fauchet P.M. , Ettedgui E. , Raisanen A. , Brillson L.J. , Seiferth F. , Kurinec S.K. , Gao Y. , Peng C. , Tsybeskov L.
Published: March 2011

Using a careful analysis of the properties of light-emitting porous silicon (LEpSi), we conclude that a version of the “smart” quantum confinement model which was first proposed by F. Koch et al [Mat. Res. Soc. Symp. Proc. 283, 197 (1993)] and allows for the existence of surface states and dangling bonds, is compatible with experimental results. Among the new results we present in support of this model, the most striking ones concern the strong infrared photoluminescence that dominates the room temperature cw spectrum after vacuum annealing above 600 K.
Ic Compatible Processing of Si:Er for Optoelectronics
Authors: Ren F. Y. G. , Michel J. , Sun-Paduano Q. , Zheng B. , Kitagawa H. , Jacobson D.C. , Poate J.M. , Kimerling L. C.
Published: March 2011

We have fabricated the first Si:Er LED, operating at 300 K, based on an understanding of the Si-Er-O materials system. Er-doped Si (Si:Er) provides an exciting opportunity for the monolithic integration of Si based opto-electronics. In this paper, Er-Si reactivity, and Er diffusivity and solubility have been studied to establish Si:Er process compatibility with a silicon IC fabline. Er3Si5 is the most stable silicid formed; and it can be oxidized into Er2O3 at high temperature under any oxidizing conditions. Among Er compounds, Er2O3 luminesces and Er3Si5 and ErN do not. The diffusivity of Er in Si is low and SIMS analysis yields a diffusivity D(Er) ∼ 10−12cm2/s at 1300 C and ∼ 10−15cm2/s at 900 C, and a migration enthalpy of ΔHm(Er) ∼ 4.6 eV. The equilibrium solubility of Er in Si is in the range of 1016 cm−3 at 1300 C. The Si:Er LED performance is compared with GaAs LEDs to demonstrate its feasibility.
Photoluminescence of Thin SI1-xGEx Quantum Wells
Authors: Lenchyshyn L.C. , Thewalt M.L.W. , Houghton D.C. , Noel J.-P. , Rowell N.L. , Sturm J.C. , Xiao X.
Published: March 2011

Well-resolved band edge photoluminescence spectra were obtained from SiGe quantum wells of various widths. In addition to the usual shallow bound exciton features, we observed a highly efficient deeper luminescence process, under conditions of low excitation density, in thick SiGe quantum wells. This luminescence band can be attributed to excitons localized by fluctuations in alloy concentration. The binding energy of the localized exciton feature is found to decrease with decreasing well width. In the thinnest quantum well samples only a single luminescence feature is observed at all power levels, while in several other thin quantum well samples having very sharp lines the localized exciton feature appears at higher energy than the bound exciton. Despite these changes in the spectra, the localized exciton luminescence could be identified in all cases by its characteristic intensity saturation at low excitation power density, as well as its slow decay time (∼ 1 ms). The mechanism behind the changes in the localized exciton luminescence may originate from limiting the exciton motion to two dimensions in thin wells, which at low temperatures would hinder migration to the lowest energy alloy fluctuation centers.
Catalysts Examined by Electron Spin Resonance-Examples from Hydrodesulfurization Catalysis
Authors: Silbernagel Bernard G.
Published: March 2011

The utility of electron spin resonance (ESR) for catalyst characterization is illustrated for the desulfurization catalysts used to remove sulfur, nitrogen, and organically bound metals from petroleum. These catalysts consist of active metals (Mo, W, Co, Ni) in mixed oxide and sulfide phases on high surface area alumina supports. Coordinated studies of unsupported sulfide and oxide model systems determine the chemical form and number of defect sites on the actual catalysts. Parallel catalysis studies correlate ESR defects and catalytic activity for many reactions.
The Mössbauer Effect and Some Applications in Materials Research
Authors: Shenoy G. K.
Published: March 2011

A brief introduction to the Mössbauer effect is presented. The hyperfine interactions associated with the electric monopole, magnetic dipole and electric quadrupole moments of the nuclear states involved in the Mössbauer transition are described. Their use in materials research is illustrated through examples dealing with phase analysis, binary solubility, defect interaction and surface properties.
Some Applications of Spin Precession Methods to Problems in Materials Science
Authors: Kaufmann E. N.
Published: March 2011

Nuclear and electron resonance and the Mössbauer effect are techniques which observe the interaction of moments with fields directly in the energy domain. An energy splitting, however, also implies the precession of the moment in the field. When a means exists to determine the orientation of the moment then the precession can be observed in the time domain. The direction of radiation emitted in a nuclear, muonic or atomic decay is correlated to the direction of corresponding moments (spins) and can thus act as a detector of spin precession. In the language of nuclear, muonic and atomic physics, these methods are called perturbed angular correlations (PAC), muonic spin rotation, and quantum beats, respectively. Below, these methods will be illustrated by displaying some examples of the application of perturbed angular correlations to a variety of materials systems.