bluuebunny/crossref_metadata_2025 huggingface dataset:
X-Ray Absorption Spectroscopy from H-Passivated Porous Si and Oxidized Si Nanocrystals
Authors: Schuppler S. , Friedman S. L. , Marcus M. A. , Adler D. L. , Xie Y.-H. , Ross F. M. , Harris T. D. , Brown W. L. , Chabal Y. J. , Szajowski P. J. , Chaban E. E. , Brus L. E. , Citrin P. H.
Published: April 2011

Quantum confinement in nanoscale Si structures is widely believed to be responsible for the visible luminescence observed from anodically etched porous silicon (por-Si), but little is known about the actual size or shape of these structures. Extended x-ray absorption fine structure data from a wide variety of por-Si samples show significantly reduced average Si coordination numbers due to the sizable contribution of surface-coordinated H. (The H/Si ratios, as large as 1.2, were independently confirmed by ir-absorption and α-recoil measurements.) The Si coordinations imply very large surface/volume ratios, enabling the average Si structures to be identified as crystalline particles (not wires) whose dimensions are typically <15 Å. Comparison of the size-dependent peak luminescence energies with those of oxidized Si nanocrystals, whose shapes are known, shows remarkable agreement. Furthermore, near-edge x-ray absorption fine structure measurements of the nanocrystals shows the outer oxide and interfacial suboxide layers to be constant over a wide range of nanocrystal sizes. The combination of these results effectively rules out surface species as being responsible for the observed visible luminescence in por-Si, and strongly supports quantum confinement as the dominant mechanism occurring in Si particles which are substantially smaller than previously reported or proposed.
Analysis of the Dimerized Sb/Si(001)-(2×l) Surface by X Ray Standing Waves
Authors: Lyman P. F. , Qian Y. , Bedzyk M. J.
Published: April 2011

X ray standing wave measurements were undertaken to study the bonding position of Sb adatoms on the Sb-saturated Si(001)−(2×1) surface. Using the (004) and (022) Bragg reflections, we find that the Sb atoms form dimers, and that the center of the Sb ad-dimers lies 1.64 Å above the bulk-like Si(004) surface atomic plane. This work describes the extraction of structural parameters for a dimerized surface from in-plane measurements. Our results are compared to two structural models consisting of dimers whose bonds are parallel to the surface plane and whose centers are either shifted or unshifted (parallel to the dimer bond direction) relative to the underlying substrate planes. We thus find two special cases consistent with our data: one with symmetric (unshifted) dimers having a dimer bond length of 2.81 Å, and the other with midpointshifted dimers, having a bond length of 2.88 Å and a lateral shift of 0.21 Å.
The Structure and Phases of a Relaxed Langmuir Monolayer of Methyl Eicosanoate as Determined by Grazing Incidence Xray Diffraction and Brewster Angle Microscopy
Authors: Foster W. J. , Shih M. C. , Pershan P. S.
Published: April 2011

The molecular structure and phase diagram of relaxed monolayer films of methyl eicosanoate at the air/water interface were studied using x-ray diffraction and Brewster Angle Microscopy. Six phases have been identified in relaxed films in a temperature range of 5 to 28 degrees Celsius.
High-Resolution X-Ray Diffraction Measurements of SiGe/Si Structures
Authors: Jordan-Sweet J. L. , Mooney P. M. , Stephenson G. B.
Published: April 2011

High-resolution x-ray diffraction is an excellent probe of strain relaxation in complex SiGe structures. The high flux provided by synchrotron sources enables us to make extensive reciprocal space map measurements and evaluate many samples. The diffraction peak positions of each layer in a step-graded structure, measured for two different reflections, yield quantitative values for the relaxation and alloy composition in the layer. Grazing-incidence diffraction allows us to determine the in-plane structure of very thin layers, which have thickness-broadened peaks at conventional diffraction geometries. We demonstrate the power of these techniques with two examples.
Exafs Study of Co/Cu Multilayers: Mbe Versus Sputtered
Authors: Foiles C. L. , Franklin M. R. , Loloee R. , Clarke Roy , Morrison T. I.
Published: April 2011

EXAFS data are used to probe the local structure of Co/Cu multilayers having comparable dimensions but prepared by two different techniques, MBE and sputtering. Significant differences are found. The nearest neighbor distance in all Co layers increases about the same amount relative to the bulk value and MBE Co layers are less disordered than sputtered Co layers. Nearest neighbor distances in thinner Cu layers are less than bulk values but approach bulk value for thicker Cu layers. Nearest neighbor distances in thin MBE Cu layers are more contracted and have a more rapid thickness dependence than in sputtered Cu layers.
X-Ray Absorption Studies of Uranium Sorption on Mineral Substrates
Authors: Hudson Eric A. , Terminello Louis J. , Viani Brian E. , Reich Tobias , Bucher Jerome J. , Shuh David K. , Edelstein Norman M.
Published: April 2011

UraniumL3-edge x-ray absorption spectra have been measured for uranium-mineral sorption systems. An expansible layer silicate, vermiculite, was treated to obtain a collapsed phase, thereby limiting access to the interior cation exchange sites. Samples were prepared by exposing the finely powdered mineral, in the natural and modified form, to aqueous solutions of uranyl chloride. EXAFS spectra of the encapsulated samples were measured at the Stanford Synchrotron Radiation Laboratory. Results indicate that the uranyl ion possesses a more symmetric local structure for the natural vermiculite than for the collapsed form, suggesting structural differences between uranyl species within the interlayer regions of vermiculite and on the external surfaces.
Element Specific Vector Magnetometry (Esvm) Via Soft X-Ray Magnetic Circular Dichroism
Authors: Chakarian V. , Lin H.-J. , Idzerda Y. U. , Chaban E. E. , Meigs G. , Park J.-H. , Gutierrez C. J. , Prinz G. A. , Chen C. T.
Published: April 2011

Soft X-Ray Magnetic circular dichroism (SX-MCD) can be used to obtain element-specific magnetic hysteresis curves and to elucidate the two- and three-dimensional magnetization reversal processes for each constituent magnetic element of a heteromagnetic system. As a demonstration, two systems which exhibit in-plane magnetization reversal are studied: a thin Fe (100) singlecrystal film and a Fe1-xCox/Mn/Fel-xCox trilayer. The results for both systems show that the magnetic moment vector reverses via a combination of coherent rotation toward the nearest in-plane magnetically easy axis followed by the formation of orthogonal <100> domains which rapidly sweep across the sample. In the case of the trilayer, the moment reversal process is significantly more complex due to a strong ∼90° coupling between the magnetic moments of the two FeCo layers. By using element-specific vector magnetometry (ESVM), the details of this reversal process are revealed. Furthermore, the results of the SX-MCD for Mn show that Mn possesses a ferromagnetically aligned net magnetic moment which depicts a 2D magnetization behavior different than that for either Fe or Co.
Nonlinear Neutron and X-Ray Reflectance for Surface Films
Authors: Zhou Xiao-Lin
Published: April 2011

This paper presents an analysis of the relationship between Parratt's recurrence formula and several existing analytical formulations for the neutron and x-ray reflectance of a surface film. In particular, it was shown that the Born approximation and the weighted-superposition approximation (WSA) for the neutron and x-ray reflectance can both be derived directly from Parratt's recurrence formula. It was also shown that the well-known nonlinear differential equation for the reflectance at an arbitrary point inside a film can be obtained from Parratt's recurrence formula and the WSA was an approximate solution to this equation. Finally, the nonlinearity in the WSA was discussed through a comparison between the Born approximation and the WSA. It was pointed out that the exponential factor in the WSA correctly accounted for both the phase shift effect due to the scattering length density of the film and the amplitude attenuation effect due to evanescence.
In Situ Neutron Reflection from Electrochemically Produced Conducting Polymeric Systems
Authors: Roser Stephen J. , Hillman A.Robert , Richardson Robert M.
Published: April 2011

The technique of neutron reflection has been developed by us to investigate a variety of thin film electrochemical systems, including polymer bilayers, matrix-trapped active biological molecules and inorganic oxides. Measurements have been made with the electrodes in contact with the electrolyte, allowing studies of structural rearrangements after polymerisation, redox cycling and solvent exchange have taken place. In this paper we demonstrate these possibilities for the technique by concentrating on thin layers of an electrochemically produced polymer. polybithiophene.
Neutron Reflection Measurement of Near-Surface Structural Change in an Oil-Water-Surfactant System
Authors: Jeng U. , Esibov L. , Crow M. L. , Steyerl A.
Published: April 2011

The interfacial characteristics of an oil-water-surfactant sample were studied using the thermal neutron reflectometer at the Rhode Island Nuclear Science Center (RINSC). The sample was a mixture of deuterated octane with H2O and non-ionic surfactant (C10E4), spread onto an H2O substrate. A mixture of low surfactant concentration was used so that it could demix into a lamellar oil-water structure, stabilized by a monolayer of surfactant intervening at each oil-water interface. The reflectivity data showed a clearly visible difference between 25.2° C and 30.2°C, indicating that the significant increase of interfacial tension between these two temperatures induced a structural transition.
Reflectivity Studies on Adsorbed Block Copolymers Under Shear
Authors: Smith Gregory S. , Wages Stephen , Baker Shenda M. , Toprakcioglu Chris , Hadziioannou George
Published: April 2011

We report neutron reflectivity data on (poly)styrene-(poly)ethylene oxide (PS-PEO) diblock copolymers adsorbed onto quartz from the selective solvent deuterated cyclohexane (a non-solvent for PEO and a poor solvent for PS). The PEO "anchor block" adsorbs strongly to form a thin layer on the quartz substrate, while the PS chains dangle into the solvent. We find that under static conditions the density profile of the PS block in a poor solvent can be well described by a Schultz function which is indicative of a polymer "mushroom." Furthermore, we have studied the same system under shear at shear rates from 0-400s-l. We find that there is a dramatic increase in the thickness of the PS layer under shear in cyclohexane and that the relaxation time from the shear-on profile back to the static profile is on the order of several days.
Effects of Shear on the Structures in a System of Small Rodlike Micelles
Authors: Butler Paul D. , Magid L.J. , Hamilton William A. , Kreke P.J. , Hayter Johnb.
Published: April 2011

We have used small angle neutron scattering (SANS) to study solutions of cetyltrimethylammonium 2,6-dichlorobenzoate (2,6) at rest and as a function of shear. These solutions are Newtonian, and up to a concentration of at least 100mM contain spherical micelles. Comparison with viscoelastic solutions of the closely related, and much studied, cetyltrimethylammonium 3,5-dichlorobenzoate (3,5) is made. While spheres are not expected to align under shear, a shear induced sphere to rod transition has been postulated for this system. In the results presented here, no shear induced sphere to rod transition was observed even at the highest accessible shear rate. Addition of sufficient supporting electrolyte (up to 1M NaCl) does cause such a transition in the quiescent solution. Hoffmann et al. have seen what they refer to as the formation of a shear induced structure (SIS or type II micelles) on similar systems where short rods form in the absence of supporting electrolyte. Shearing of the salted system produces rapid though not spectacular alignment, with no evidence of a SIS forming.
SANS Study of Chirality and Order in Liquid Crystalline Cellulose Suspensions
Authors: Orts W.J. , Revol J.-F. , Godbout L. , Marchessault R.H.
Published: April 2011

Small angle neutron scattering, SANS, was used to describe the magnetic alignment and in situ shear ordering of polyelectrolytic, liquid crystalline cellulose microfibrils in aqueous (D2O) suspension. In a 2.4 Tesla magnetic field, microfibril suspensions exhibit anisotropic chiral nematic (cholesteric) ordering in which the distance between nematic planes along the cholesteric axis is shorter than between rods within a nematic plane. This is consistent with the hypothesis that cellulose microfibrils are helically twisted rods. During shear, the SANS interference peaks perpendicular to the flow direction sharpen with increasing shear rate. Yet, the highest degree of alignment (for microfibrils with axial ratios of ~45) was observed a short period after the cessation of shear flow.
Dissolution and Growth of Precipitates Under Electron Irradiation in an Al-11.8 at % Zn Alloy by Small Angle Neutron Scattering.
Authors: Baig M.R.
Published: April 2011

Dissolution and growth of precipitates in a room temperature aged Al-11.8 at % Zn alloy have been studied under electron irradiation using small angle neutron scattering (SANS). A series of electron irradiations were performed on each sample and SANS measurements were made on each irradiation. In general for low doses the results show an initial decrease in the magnitude of the scattering, but associated with an increase in the precipitate size. This is followed on prolonged irradiation by an increase in the magnitude of the scattering with a continued increase in precipitate size. It is believed, that at low doses some precipitate grow in size but others may dissolve in the matrix, which then becomes supersaturated. With the enhanced rate of diffusion as a result of the irradiation, the remaining precipitates grow rapidly. As the supersaturation reduces, a coarsening mechanism takes over, via a radiation enhanced diffusion mechanism.
Internal Strains Of A Creep Deformed Single Crystal Nickel-Based Superalloy
Authors: Glatzel Uwe , MÜller Andreas
Published: April 2011

Neutron scattering experiments have been carried out with undeformed and creep deformed samples of the superalloy CMSX-4. From the measured constrained misfit data, obtained from different reflecting planes with respect to the external load axis, two strain tensors for horizontal and vertical matrix plates can be extracted.

A careful data analysis is necessary since the technical manufactured single crystals show a mosaic spreading of 0.5° and the wavelength spreading of the incident neutron beam, Δλ/λ is about 410-3 This is in the same range as the unknown constrained misfit quantities δc. A method has been developed which allows a calculated intensity distribution to be fitted to the measured one.
Plastic Strain Estimation from Neutron Bragg Peaks
Authors: Swallowe G.M. , Osborn J.C. , Lukas P. , Mikula P. , Vrana M.
Published: April 2011

This paper describes experiments carried out at three neutron sources in which Bragg reflection peak profiles of samples of copper, nickel and iron were investigated as a function of plastic strain. All peaks were well fitted by Gaussian functions for true strains of upto 0.25. Peak widths of all samples were found to be simple power law functions of strain and independent of measurement direction. This suggests that use could be made of data gathered in standard residual elastic stress measurements to simultaneously estimate plastic strains.
Small Angle Neutron Scattering Studies of Asphaltic Materials
Authors: Storm D. A. , Sheu E. Y.
Published: April 2011

Asphaltic materials are used as binders for road asphalts. Such materials have been found to contain a dispersed phase of colloidal particles in the size range of 50-100 Å, according to small angle X-ray scattering(SAXS) techniques. The presence of this dispersed phase has a significant influence on the rheological properties of these materials. These colloidal particles can be isolated by precipitation with heptane, and so they are associated with the well-known asphaltene solubility class in some manner. In this work we review previously published results that suggest that at least part of the asphaltenes are surface active agents in certain solvents such as toluene, or pyridine, and can form micelles in these solvents. Small angle neutron scattering (SANS) indicates the micelles are spherical with average radii in the 50-100 Å range. There is a polydispersity of sizes. The micelles do not grow in these solvents with increasing concentration in the range of 1-10 wt.%, nor do they dissociate on heating to ~ 170 °C. At higher concentrations, however, a more complicated structure develops. The micelles aggregate to form a large cluster composed of micelles. The fractal dimension of the clusters is found to be 1.8.
Application of Neutron Scattering Methods to Cementitious Materials
Authors: Livingston Richard A. , Neumann D.A. , Allen Andrew , RUSH J.J.
Published: April 2011

A major part of the infrastructure is constructed of Portland cement concrete. Nevertheless, many aspects of this material are still not well understood. The main constituent of hydrated Portland cement is a hydrous gel, with significant features on the nanometer scale, that tends to be unstable under vacuum. It is difficult to investigate with conventional instrumental methods. Neutron scattering methods possess unique capabilities to characterize cementitious materials and thus to resolve a number of long-standing issues. Among the applications, quasi-elastic neutron scattering is used to follow non-destructively the progress of the main hydration reaction and to determine the distribution of the water content among the various states: i.e. free, chemically bound, or physically trapped in the gel network. Quasi-elastic neutron scattering can also be applied to investigate the process of ice formation in the pore spaces. Neutron diffraction has been used to observe the development of hydrous phases as a function of time. Small-angle neutron scattering (SANS) proves to be well-suited to the characterization of the microstructure. This method is being used to determine the surface and volume fractal dimensions of the gel as a function of time and as a function of additives such as fly ash or silica fume. In addition, SANS has been applied to characterize silica aggregates for the potential for deleterious reactions in highly alkaline concrete. Aside from their application to specific problems, the combination of neutron scattering techniques is leading to new interpretation of the fundamental structure of the Portland cement gel.
Small-Angle Neutron Scattering from Hydrated Cement Pastes
Authors: Aldridge L.P. , Bertram W.K. , Sabine T.M. , Bukowski J , Young J.F. , Heenan R.K.
Published: April 2011

Small angle neutron scattering (SANS) has been used to examine hydrated cement pastes with water-to-cement ratios between 0.25 and 0.8 and cured for 28 days. Various methods of data analysis are applied to the pastes.

Fitting the entire SANS spectra of the hydrated cement paste has suggested that there are two scattering entities in the paste. There is some evidence to suggest that one scattering entity is globules of calcium silicate hydrate formed in the hydrated paste.
A Method for Interpreting Small Angle Neutron Scattering Data from Quasi-Spherical Objects
Authors: Sabine T.M. , Bertram W.K. , Aldridge L.P.
Published: April 2011

Small angle scattering data are traditionally interpreted in terms of scattering patterns at the Porod and the Guinier limits. It is difficult to fit the entire scattering pattern to account for scattering from spheres because Rayleigh's formula contains oscillatory terms which are smeared out in practice by perturbations in the sizes of the scattering agents.

It is proposed that a Lorenztian type function be used instead of Rayleigh's function. By using this equation it is possible to fit the entire small angle scattering pattern and obtain the correct forms of the Guinier and Porod limits.

The effects of correlation and multiple scattering are also explored.
Neutron scattering of H.T.S.C. Materials : Zn substituted Versus Pure YBCO, Nuclear and Magnetic Structures, Spin Excitations.
Authors: Collin G. , Villeneuve R. , Sidis Y. , Bourges Ph. , Hennion B. , Onufrieva F. , Gillon B. , Bouree F. , Mirebeau I. , Mendels P. , Alloul H. , Marucco F. , Schweiss P.
Published: April 2011

The Zn substitution on Cu(2) sites in YBa2(Cu1-yZny)3O6+x materials is most probably limited to yzn ≈ 0.04. Results of neutron diffraction associated with NMR and μ+SR results reveal ; i) a lowering of TN in the pure AF region x≤0.20 due to a dilution effect of the antiferromagnetic lattice by a static non magnetic impurity, ii) the existence of a non conventionnal magnetic behavior within the intermediate region x≥0.35 characterized by an unusual temperature dependance of magnetic intensities, a reentrancy and the existence of diffuse magnetic scattering (this region is much broader in oxygen composition for Zn-doped than for pure YBCO),iii) an unusual spin dynamics in the "doped A-F phase" which corresponds to a coupling of magnetic excitations to the hole dynamics.
Small-Angle Neutron Scattering from YBa2Cu3O7 and Bi2Sr2CaCu2O8 Near the Critical Temperature
Authors: Frost C. D. , Squires G.L.
Published: April 2011

One of the 30 meter small-angle neutron scattering spectrometers at NIST has been used to measure the scattering from YBa2Cu3O7 and Bi2Sr2CaCu2O8 at a range of temperatures from 35 K to 135 K with wavevector transfers Q from 0.01 Å-1 to 0.08 Å-1. Considerable attention was paid to sample preparation in order to reduce the background scattering from crystalline defects and stoichiometric variations in the sample. For both materials, the difference in the differential scattering cross-sections at the critical temperature Tc and at temperatures of Tc ± 20 K showed considerable structure in Q, corresponding to spatial variations in the neutron scattering potential with Fourier components of wavelength of the order of 100 Å.
The Oxidation State and Magnetic Behaviour of Tb in High-Tc Related Materials
Authors: Soderholm L. , Staub U. , Skanthakumar S. , Antonio M. R.
Published: April 2011

L3-x-ray absorption spectroscopy is used together with inelastic neutron scattering and magnetic susceptibility measurements to characterize the Tb oxidation state and bonding in the high-Tc related materials Y1-xTbxBa2Cu3O7 and Pb2Sr2TbCu3O8. The Tb is found to be essentially trivalent in both compounds with no indications of significant hybridization. However, there is evidence of significant Tb-Tb magnetic interactions in Pb2Sr2TbCu3O8 that persist to temperatures much higher than the Tb long-range ordering temperature.
Structural Chemistry of Some Phases in The YC-Ni-B System
Authors: Chakoumakos B. C.
Published: April 2011

NiB, monoclinic Ni4B3, Ni2B and Ni3B were prepared by arc-melting and their roomtemperature crystal structures were refined by Rietveld analysis of neutron powder diffraction data. The NiB refinement is altogether new data. Although the B atoms in NiB form characteristic zigzag chains, the primary coordination of each atom by atoms of the other kind is similar and distinctively sevenfold, with one short (2.117 Å), two intermediate (2.152 Å), and four long (2.163 Å) bonds. Other samples with stoichiometries (YC)nNi2B2, n = 3, 4, did not yield single-phase material, but both x-ray and neutron powder diffraction suggest that the n = 4 structure is present in both of these samples. Phase-pure samples of these homologues may require non-stoichiometry and a more controlled thermal history than is attainable by arc melting.
The Ferroelastic Transformation in Yttria- and Ceria-Stabilised Tetragonal Zirconia Via Neutron Diffraction
Authors: Cain Markys G. , Lewis Mike H. , Backshall Matthew , Bennington Stephen M. , Hull Steve
Published: April 2011

Neutron diffraction studies of tetragonal zirconia polycrystals (TZP) subjected to a varying applied uniaxial stress indicates that a ferroelastic crystalline transformation occurs within the bulk of the material. The critical coercive stress was determined to be (1.65±0.03) GPa for yttria stabilised TZP and (0.70±0.05) GPa for ceria stabilised TZP. The results agree well with previous reports concerning the ferroelasticity of tetragonal zirconia, and provide direct evidence indicating that the anomalies observed using the first X-ray diffraction studies originate from the bulk of the ceramic and not solely from the surface. The preferred orientation observed in the tetragonal crystallites was measured as a function of applied uniaxial stress and interpreted via a resolved stress criterion. It is believed that the ferroelastic transformation can act as an energy absorbing mechanism and hence toughen zirconia or ceramics containing dispersed zirconia.
Static and Dynamic Displacements in α-Phase FeCr
Authors: Robertson J. L. , Reinhard L. , Neumann+* D. A. , Moss#* S. C.
Published: April 2011

A single crystal of α-Fe0.47Cr0.53 was annealed at 1100K, 5K above the a-phase transition temperature, for four days and then water quenched. A detailed investigation using X-ray synchrotron radiation was previously carried out to study the short-range order and static displacements in this alloy. The phonon dispersion curves, studied here by inelastic neutron scattering, appear typical of bcc alloys and the phonon groups are observed to be significantly broadened in the vicinity of the dip at ξ=2/3 in the longitudinal [ξ ξ ξ] phonon dispersion. In this paper we explore the possibility that nearly random concentration fluctuations and an extremely small difference in the atomic sizes, together with an appropriate lattice response function, can nonetheless produce elastic diffuse scattering indicative of a new phase.
Preparation of Long-Alkane Monola Yers Adsorbed On Exfoliated Graphite For Neutron Diffraction Studies
Authors: Herwig K. W. , Matthies B. , Taub H.
Published: April 2011

A classic problem in polymer science is the preferential adsorption at a solid interface of one polymer species from a multi-component fluid mixture. Prototype systems for studying this selective adsorption process are solutions consisting of alkane molecules of two different lengths in the presence of a solid interface. Before neutron diffraction could be used to probe structural questions related to the selective adsorption process in such systems, a significant effort was required to develop methods for fabricating suitable samples. We describe techniques for preparing samples of long-alkane monolayers adsorbed on exfoliated graphite by adsorption from solution and by deposition from the vapor phase.
Polarized Cathodoluminescence Study of (InP)2/(GaP)2 Bilayer Superlattice Structures
Authors: Tang Y. , Rammohan K. , Lin H.T. , Rich D.H. , Colter P. , Vernon S.M.
Published: April 2011

Linearly polarized cathodoluminescence (LPCL) imaging and spectroscopy techniques have been employed to examine the optical properties and homogeneity of (InP)2/(GaP)2 bilayer superlattice (BSL) structures which exhibit a lateral composition modulation that leads to the formation of quantum wires. LPCL spectra were measured for various temperature and electron beam excitation densities. The magnitude of the polarization anisotropy and spectral lineshape are found to depend sensitively on the excitation conditions, revealing large nonlinear optical effects in these samples. CL images reveal that defects in the bilayer superlattice structure originate from the GaAs substrate or the initial stages of InGaP growth.
Preparation and Time Resolved Photoluminescence of Nanoscale InP Islands in In0.48Ga0.52P
Authors: Eberl K. , Kurtenbach A. , HÄusler K. , Noll F. , RÜhle W.W.
Published: April 2011

Nanoscale InP islands are formed during InP/In0 48Ga0.52P heteroepitaxy due to the lattice mismatch of about 3.7%. The samples are prepared by solid source molecular beam epitaxy on (001) GaAs substrate. Atomic force microscopy measurements show that the size of the islands is typically 15 to 50 nm in diameter and about 5 to 10 nm high depending on the nominally deposited InP layer thickness, which is between 1 and 7.5 monolayers. Transmission electron micrographs show the coherent incorporation into the In0.48Ga0.52P matrix for InP islands with 2.5 monolayers. Resonantly excited time-resolved photoluminescence (PL) measurements of the self assembling InP dots are performed for optical characterisation. The decay times are typically 400 ps. The dependence on excitation power and temperature indicates the quantum dot nature of the InP islands. Finally a pronounced alignment of the InP islands is obtained on strained In0.61Ga0.39P buffer layers.
Dielectric Function and Band Gaps of Si1−xCx AND Si0.924−xGe0.076Cx (0≤x≤0.014) Semiconductor Alloys Grown on Si
Authors: Lee Hosun , Floro J.A. , Strane J. , Lee S. R. , Jones E. D. , Mayer T. , Picraux S. T.
Published: April 2011

We have characterized the optical properties of heteroepitexial Si1−xCx and Si0.924−xGe0.076Cx (0≤x≤0.014) alloys grown on Si substrates by solid phase epitaxy using spectroscopic ellipsometry. The measured dielectric function confirms that the samples are of good crystalline quality. We determined the E1 and E2 band gaps by lineshape-fitting the features in the second derivative spectra of the dielectric functions. Also, we discuss the shift of the band gaps with C concentration arising from strain and chemical alloying.
Structural and Compositional Study of Sil-xGex Multilayer Structures Using Medium Energy Ion Scattering
Authors: Hucknall P.K. , Sugden S. , Sofield C.J. , Noakes T.C.Q , Mcconville C.F.
Published: April 2011

The ability to determine structural and compositional information from the sub-surface region of a semiconductor material has been demonstrated using a new time-of-flight medium energy ion scattering spectroscopy (ToF-MEISS) system. A series of silicon-silicon/germanium (Si/Sil-xGex) hetero-structure and multilayer samples, grown using both solid source molecular beam epitaxy (MBE) and gas source chemical vapour deposition (CVD) on Si(100) substrates, have been investigated. These data indicate that each individual layer of Sil-xGex can be uniquely identified with a depth resolution of approximately 3 nm. A comparison of MBE and CVD grown samples has also been made using layers with similar structures and composition and the results compared with conventional Rutherford back-scattering spectrometry (RBS).
Calculation of Optical Characteristics for Si1-xGExAlloy
Authors: Litovchenkoe V. G. , Frolov S. I. , Shkarban B. N. , Kruger D.
Published: April 2011

In the framework of virtual crystal approximation using the empirical pseudopotential theory optical transitions of Si1 -xGexhave been calculated. The effects of hydrostatic and uniaxial stress, and alloy stoichiometry on the optical gaps were investigated. Since the mechanical stress and material composition affect the distinctive behaviour of different optical transitions it allows to put forward a spectral measurement technique for determination of crystal lattice strain and alloy parameter x simultaneously.
A New Approach for Determining Epilayer Strain Relaxation and Composition Through High Resolution X-Ray Diffraction
Authors: Matney K. M. , Goorsky M.S.
Published: April 2011

We developed a new method of determining epilayer relaxation (along one direction) and composition using a symmetric and any single asymmetric high resolution x-ray diffraction scan. The previous use of small angle approximations can be very detrimental to calculated results and should be avoided. This new method does not employ small angle approximations or first order Taylor approximations, producing accurate results. The effect of x-ray geometry (glancing incident versus glancing exit) on the analysis of epilayer composition and strain is also reviewed. It is also shown that the glancing exit geometry is generally less susceptible to experimental error.
Effects of Strain-Induced Defects on Excess Carrier Lifetime and Ambipolar Diffusion in nipi-Doped In0.2Ga0.8As/GaAs Mqws
Authors: Lin H.T. , Rich D.H. , Larsson A.
Published: April 2011

The effects of strain-induced defects on excess carrier lifetime and transport in a nipi-doped In0.2Ga0.8As/GaAs multiple quantum well (MQW) structure were examined with a new method called electron beam-induced absorption modulation (EBIA) in which the kinetics of carrier transport and recombination are examined with a high-spatial, -spectral and -temporal resolution. The excess carrier lifetime and ambipolar diffusion were found to be reduced by factors of ∼1013 and ∼103 compared to theoretical values, respectively, and this is attributed to the presence of strain-induced defects. The MQW excitonic absorption coefficient sensitively depends on the carrier density in the QWs, as a result of screening of the electron-hole (e-h) Coulombic interaction. Likewise, ambipolar diffusion is found to depend on the excess carrier density in a nonlinear fashion, as a result of the e-h plasma-induced changes in the local depletion widths in the vicinity of structural defects.
Effects of Silicon Misorientation Angle on the Rf and Dc Characteristics of GaAs-on-Si Mesfets
Authors: Papavassiliou Christos , Constantinidis G. , Kornilios N. , Georgakilas A. , LÖchterman E. , Panayotatos P.
Published: April 2011

A systematic experimental investigation has been undertaken for the optimization of the wafer parameters and processing for silicon wafers intended for use as substrates for MBE growth, with emphasis on heteroepitaxial growth of GaAs-on- Si. Within this investigation, results are presented of an initial study focused on the optimization of the magnitude of the misorientation angle towards a <110> direction for the growth of GaAs on (001) Si wafers. This angle controls the structure of the stepped (001)Si surface and can influence the defect density and surface smoothness of the GaAs-on-Si layers. Silicon substrates misoriented from 0 deg. up to 9 deg. were cut to specification and subsequently used for the epitaxial growth of GaAs MESFET structures. MESFETs were fabricated and their dc and RF characteristics compared. The resistivity of the GaAs-on-Si buffer layers was evaluated and correlated to the results from device characterization. This work presents the effects of the magnitude of the angle of misorientation in the range from 0 to 9 deg.
Degradation and Recovery of Si1−xGex Devices by Irradiation
Authors: Ohyama H. , Vanhellemont J. , Takami Y. , Hayama K. , Kudo T. , Hakata T. , Kobayashi K. , Sunaga H. , Hironaka I. , Poortmans J. , Caymax M.
Published: April 2011

Results are presented of a study on the degradation and recovery behavior of strained Si 1−xGex diodes and heterojunction bipolar transistors (HBTs) by electron and neutron irradiation. The degradation of device performance and the generation of lattice defects are reported as a function of germanium content and radiation source. Isochronal annealing is performed to study the recovery behavior of the irradiated devices. The radiation source dependence of the degradation is discussed taking into account the absorbed energy dunng irradiation.
Boron Diffusion in Si and Si1−xGex
Authors: Kuo P. , Hoyt J. L. , Gibbons J. F. , Turner J. E. , Lefforge D.
Published: April 2011

Boron diffusion in in-situ doped Si and strained Si1−xGex (x < 0.20) epitaxial layers, subjected to inert-ambient furnace annealing, was investigated as a function of temperature (T = 750 °C - 850 °C). Boron diffusivity parameters were extracted from SUPREM IV, a process simulation program. We observed slower B diffusion in strained Si1−xGex relative to that in Si for B concentration levels ranging from 2×1017 to 3×1019 cm−3. Using relaxed graded Si1−xGex as “substrates”, we also characterized B diffusion in relaxed Si1−xGex (x < 0.60) at T = 800 °C. We propose a reaction of mobile B atoms pairing with Ge atoms to model the slower B diffusion in both fully strained and relaxed Si1−xGex.
Low Temperature SiGe Heteroepitaxy by Ultrahigh Vacuum Electron Cyclotron Resonance Chemical Vapor Deposition
Authors: Joo Sung-Jae , Hwang Ki-Hyun , Hwang Seok-Hee , Yoon Euijoon , Whang Ki-Woong
Published: April 2011

Dislocation-free Si1−xGex epilayers are successfully grown on (100) silicon at 440°C by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition (UHV-ECRCVD). The effects of process parameters on the crystallinity of Si1−xGex epitaxial layers were studied. As the GeH4 flow rate increases and consequently Ge fraction increases above 20%, Si1−xGex epilayers become damaged heavily by ions. When Ge fraction is larger than 20%, process parameters like total pressure need to be adjusted to reduce the ion flux for high quality Sil−xGex. Growth rate of Si1−xGex epitaxial layers increases at 440°C with Ge content in the film. It is presumed that the hydrogen desorption from the surface is the rate-limiting step, however, the enhancement in growth rate is comparatively suppressed and delayed.
Defect-Free Band-Edge Photoluminescence in SiGeC Strained Layers Grown by Rapid Thermal Chemical Vapor Deposition
Authors: Liu C. W. , Amour A. St. , Sturm J. C. , Lacroix Y. R. J. , Thewalt M. L. W.
Published: April 2011

The defect-free band-edge photoluminescence at both 30K and 77K was observed for the first time in Si/SiGeC/Si quantum wells. The SiGeC samples were prepared by rapid thermal chemical vapor deposition (RTCVD) by using methylsilane as carbon source added in a dichlorosilane and germane mixture. Deep photoluminescence around 0.8 eV, previously reported by Boucaud et al., was no longer observed under any excitation conditions. Compared to control Si/SiGe/Si quantum wells, the initial effect of adding the C is to decrease the bandgap of the host SiGe layers, despite the fact that the diamond has a large bandgap.
The Effects of Rapid Recrystallization and Ion Implanted Carbon on The Solid Phase Epitaxial Regrowth of Si1−xGex Alloy Layers On Silicon
Authors: Antonell M.J. , Haynes T.E. , Jones K.S.
Published: April 2011

Transmission electron microscopy has been combined with time-resolved reflectivity and ion channeling to study the effects of regrowth temperature and carbon introduction by ion implantation on the solid phase epitaxial regrowth (SPER) of strained 2000Å, Sio.88Ge0.12/Si alloy films grown by molecular-beam epitaxy (MBE). Relative to the undoped layers, carbon incorporation in the MBE grown SiGe layers prior to regrowth at moderate temperatures (500- 700°C) has three main effects on SPER; these include a reduction in SPER rate, a delay in the onset of strain-relieving defect formation, and a sharpening of the amorphous-crystalline (a/c) interface, i.e., promotion of a two-dimensional (planar) growth front.1 Recrystallization of amorphized SiGe layers at higher temperatures (1 100°C) substantially modifies the defect structure in samples both with and without carbon. At these elevated temperatures threading dislocations extend completely to the Si/SiGe interface. Stacking faults are eliminated in the high temperature regrowth, and the threading dislocation density is slightly higher with carbon implantation.
Influence of Hydrogen On The Solid Phase Epitaxial Regrowth of Strained Layer Silicon Germanium Alloys
Authors: Love D. , Endisch D. , Simpson T.W. , Lowes T.D. , Mitchell I.V. , Baribeau J.-M.
Published: April 2011

Strained layer Si/Si0.79Ge0.21 superlattices consisting of 16 alternating 19.0 nm Si0.79Ge0.21 / 18.5 nm Si layers have been amorphized by Si ion irradiation, then implanted with H ions to nominal atomic concentrations of 1%, 0.1% and 0.05% within the amorphized region. Subsequent solid phase epitaxy (SPE) at a regrowth temperature of 575°C was monitored in situ by time resolved reflectivity (TRR) measurements, while changes in the H distribution were measured by elastic recoil detection analysis (ERDA). Analysis was supplemented by Rutherford backscattering spectrometry (RBS), x-ray double crystal diffraction and reflectivity (DCD/XRF) and transmission electron microscopy (TEM). TRR data reveals a decrease in the initial SPE rate in the Si substrate from 4.9 Å/sec (no H) to 2Å/sec for 1% H concentration as well as a rate decrease as the interface enters the Si/SiGe layers. TRR also indicates an increased roughness in the crystal/amorphous interface with increasing H concentration. ERDA reveals that a significant fraction of the implanted H is stable in the amorphous region for the anneal times (10-30 min) at 575°C, while in the regrown lattice the H concentration has dropped below 20 ppm, near the detection limit of the ERDA. DCD shows almost no strain in the regrown structures. TEM and RBS channeling techniques reveal degradation in the crystal quality of epitaxially regrown structures and a large concentration of strain relieving defects originating near the second deepest of eight SiGe layers in all regrown structures. XRF indicates decreasing sharpness of the regrown Si/SiGe interfaces with increasing H concentration.
A New type of Graded Buffer Layer for Gas-Source Molecular Beam Epitaxial Growth of Highly Strained INxGA1−XP/GAP Multiple Quantum Wells on Gap
Authors: Bi W. G. , Tu C. W.
Published: April 2011

A new type of graded buffer layer, called graded short-period superlattice (GSSL) buffer layer, is proposed, and its effect on improving the material quality is investigated by comparing the qualities of 50-period InGaP/GaP multiple quantum wells (MQWs) grown on top of it and on a linearly graded (LG) buffer layer. Unlike the linearly graded buffer layer, in which the In composition is changed 1% per step size with a constant composition InGaP, the new buffer uses InGaP/GaP short-period superlattice within each step, while maintaining the average In composition of the short-period superlattice being changed the same amount per step size. Characterization of these structures by high-resolution X-ray diffraction and low-temperature photoluminescence indicates that, while the MQWs grown on a GSSL buffer show comparable structural properties as compared with those on an LG buffer, they exhibit higher PL intensity and narrower PL line width.
Layer Tilt and Relaxation in InGaAs/GaAs Graded Buffer Layers
Authors: Matney K.M. , Eldredge J.W. , Goorsky M.S.
Published: April 2011

We investigated the effect of substrate inclination and direction on the structural properties of an InGaAs linearly compositionally graded buffer layer with a AlGaAs/InGaAs superlattice grown by molecular beam epitaxy on 2° offcut GaAs substrates. Reciprocal space maps were used to determine the relaxation and tilt of the buffer layer and superlattice with respect to each other and to the substrate. From (004) reciprocal space maps, a linear relationship between tilt and In mole fraction was observed for the buffer layer. This tilt was greatly reduced near the top of the buffer which was found to be completely strained. Interestingly, the tilt along a <110> direction was greater than that observed along the miscut axis. This may be due to the miscut axis not being parallel to a low index plane. Reciprocal space maps of asymmetric diffraction planes were used to determine the relaxation of the buffer layer as a function of In mole fraction. Along a <110> direction in which no tilt was seen in the (004), the majority of the buffer layer was found to be completely relaxed. However, the top of the buffer layer was found to be completely strained, corresponding to a denuded zone observed in cross section transmission electron microscopy.
Simulation of the Growth of Lattice Mismatched Semiconductors
Authors: Rouhani M. Djafari , Malek R. , Gue A.M. , Esteve D.
Published: April 2011

We have studied the growth of lattice mismatched semiconductors through the association of the Monte Carlo technique and the Valence Force Field (VFF) approximation. The Monte Carlo technique monitors the atomic motion in the deposited layer using the Arrhenius law and taking into account the impingment of atoms from the gas phase, intralayer and interlayer migrations of atoms and evaporation from the surface. The VFF approximation is used as an energy model to determine the local strain and stress inside the deposited layer by minimizing the total energy. This is performed after each single atomic motion. The strain is assumed to enhance the atomic motion by lowering the activation energy barrier related to the particular event. Results concerning the case of large lattice mismatches are presented. It is observed that the growing surface becomes rapidly rough, showing grooves with (111) facets. The strain relaxation occurs as a result of this roughening and allows the determination of the critical thickness. At higher lattice mismatches, it is seen that the layer orientation changes from(100) to (111) from the beginning.
Etching Reactions at Solid Surfaces
Authors: Winters Harold F. , Coburn J. W.
Published: April 2011

An understanding of etching reactions in a plasma environment requires a knowledge of: (1) the types of gas phase particles which react at the surface, (2) the etch products formed, and (3) the processes which lead from reactants to products. Experimental data relavant to these topics are reviewed in this paper. A conceptual framework for understanding the etching reaction is reviewed and it is shown that the experimental data presently available is consistent with this framework. The influence of ion bombardment on etching reactions is extensively discussed.
Non-Stoichiometry and the Electronic Structure of Amorphous Silicon Nitride
Authors: Ley L. , Kaercher R. , Johnson R. L.
Published: April 2011

We have measured core-, valence-, and conduction band densities of states of amorphous hydrogenated (a-SiNx:H) and unhydrogenated (a-SiNx) silicon nitride with x varying between 0 and 2. From an analysis of the Si 2p core level spectra in terms of five chemically shifted components the number of Si-N bonds is calculated and compared to the total nitrogen concentration. Above x ≈ 0.8 the average silicon coordination of nitrogen starts to deviate from three. The addition of hydrogen increases this deviation because N-H bonds are favored over N-Si bonds accounting thus for the excess nitrogen concentration (x ≥ 1.33) found in hydrogenated samples. A band of N2p lone pair states is identified at the top of the valence bands in stoichiometric Si3N4. This band determines the character and position of the valence band maximum (VBM) above x = 1.1. Below x = 1.1 Si-Si bonding states mark the VBM. The conduction band minimum (CBM) is determined by Si-Si antibonding states up to x = 1.25 and its position relative to the core levels is virtually unaffected by the presence of nitrogen or hydrogen. Above x = 1.25 a percolation-like transition to Si-N antibonding states occurs which is accompanied by a sharp recession of the CBM. The position of the Fermi level within the gap is investigated as a function of x and hydrogen content. Si-H and N-H bonding states are identified at 6.3 and 9.8 eV below the VBM in nearly stoichiometric a-Si3N4. Si-Si bonding defect states lie 0.5 to 1.0 eV above the VBM and the corresponding antibonding states (3.0 ± 0.3) eV above the VBM.