bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear optical anisotropy and molecular orientational distribution in poly(p-phenylene benzobisthiazole) Langmuir–Blodgett films
Authors: Wang Liming , Wada Tatsuo , Yuba Tomoyuki , Kakimoto Masaaki , Imai Yoshio , Sasabe Hiroyuki
Published: July 2002

The orientational distribution and packing of polymer chains were investigated in poly(p-phenylene benzobisthiazole) (PBT) Langmuir–Blodgett (LB) films by nonresonant third-harmonic generation measurement at a wavelength of 1907 nm. The tensor components of the third-harmonic susceptibility on the PBT LB film with a surface pressure of 50 mN/m were determined to be χ(3)XXXX=(16.6±2.5)×10−12 and χ(3)YYYY=(2.0±0.3)×10−12. The large nonlinear optical anisotropy can be explained as a result of highly oriented packing of the polymer chains induced by a flow orientation. A Gaussian distribution function with a standard deviation of σ=0.40 gives a practical description of the orientational distribution of PBT polymer chains. A maximum χ(3) value of (26.8±4.4)×10−12 esu is predicted assuming a perfect alignment of polymer chains. The χ(3)XXXX value increased by factor of 2 with the surface pressure from 30 to 50 mN/m mainly due to the packing density of the polymer chains, while the orientational degree did not change.
Reabsorption, band-gap narrowing, and the reconciliation of photoluminescence spectra with electrical measurements for epitaxial n-InP
Authors: Sieg R. M. , Ringel S. A.
Published: July 2002

The effects of reabsorption and band-gap narrowing (BGN) on experimental photoluminescence (PL) spectra of n-InP grown by metalorganic chemical vapor deposition are analyzed. PL spectra show a pronounced widening of the main PL peak and a shift of that peak to higher photon energy with increasing doping due to band filling. However, the magnitude of these effects, both here and in earlier studies of n-type III–V semiconductors, is smaller than expected based upon band filling calculations and electrical measurements. Various explanations for these discrepancies between PL spectra and band filling calculations have been proposed, but little experimental support is currently available. In this article we demonstrate unambiguously that both the n-InP PL peak width and the peak position are significantly reduced by reabsorption, and that reabsorption completely explains the observed discrepancy between the measured PL peak width and the calculated band filling based on electrical measurements. In particular, we show that reabsorption must be accounted for when extracting the Fermi level from experimental n-InP PL spectra, otherwise the Fermi level value is severely underestimated. Since previous studies of the n-InP PL line shape have neglected reabsorption and instead attributed the unexpectedly low extracted Fermi level value to band-gap narrowing effects, we reinvestigate BGN in n-InP by considering only the low-energy tail of the PL spectra. The extent of the low-energy band tail below the intrinsic band-gap energy is observed to be only about half as large as n-InP BGN predicted theoretically. Very similar results have been reported in the literature for n-GaAs and is either due to an overestimation of the BGN by theory or a failure of PL to reflect the full extent of a highly nonrigid BGN shift. In regard to the latter, we demonstrate that a highly nonrigid BGN shift does indeed exist for n-InP, with the BGN shift near zone center being at least three times larger than the energy shift of states near the Fermi surface for n=4×1018 cm−3.
Laser heating of a cavity versus a plane surface for metal targets utilizing photothermal deflection measurements
Authors: Jeong S. H. , Greif R. , Russo R. E.
Published: July 2002

The effects of a cylindrical cavity in a metal surface on the energy coupling of a laser beam with the solid were investigated by using a photothermal deflection technique. The photothermal deflection of a probe beam over the cavity was measured while the bottom of the cavity was heated with a Nd–YAG laser with a wavelength of 1064 nm. Cavities in three different materials and with two different aspect ratios were used for the experiment. Temperature distributions in the solid and the surrounding air were computed numerically and used to calculate photothermal deflections for cavity heating and for plane surface heating. Reflection of the heating laser beam inside the cavity increased the photothermal deflection amplitude significantly with larger increases for materials with larger thermal diffusivity. The computed photothermal deflections agreed more closely with the experimental results when reflection of the heating laser beam inside the cavity was included in the numerical model. The overall energy coupling between a heating laser and a solid is enhanced by a cavity.
Preparation and characterization of CuAlxGa1−xSe2 alloy layers grown by low-pressure metalorganic vapor phase epitaxy
Authors: Chichibu S. , Nakanishi H. , Shirakata S. , Isomura S. , Harada Y. , Matsumoto S. , Higuchi H. , Kariya T.
Published: July 2002

CuAlxGa1−xSe2 alloy layers were successfully grown on GaAs(001) by low-pressure metalorganic vapor phase epitaxy. The distribution coefficient of Al was unity. All alloy layers had their c-axis normal to the substrate plane. Exciton resonance energies were determined as a function of x by means of photoreflectance measurements. A quadratic dependence of exciton energies on x was confirmed. The spin-orbit splittings of the epilayers were approximately the same as that of bulk crystals. The magnitudes of crystal-field splittings were larger than that of bulk crystals, and this was explained in terms of residual tensile biaxial strain in the epilayers. The color of the low-temperature photoluminescence (PL) changed from red to crimson, orange, yellow, green, and bluish-purple with increasing x. A peak due to a free-to-acceptor transition was dominant in the PL spectra of the alloy layers. The acceptor ionization energy increased with increasing x, and the result may reflect an increase of the hole effective mass.
The origin of aging in Al–SiO2–Si tunnel diodes
Authors: Chattopadhyay P. , Das K.
Published: July 2002

The origin of aging of Al–SiO2–Si tunnel diodes is investigated. A theoretical model is proposed based on the assumption that aluminum undergoes a chemical reaction with the SiO2 layer and produces silicon which introduces active centers for trapping electrons in the oxide layer. The model shows that the negative charge density that develops at the interface is responsible for the time dependence of the barrier height observed in Al–SiO2–Si tunnel devices.
Dependence of sensitivity of SnOx thin-film gas sensors on vacancy defects
Authors: Vlachos D. S. , Papadopoulos C. A. , Avaritsiotis J. N.
Published: July 2002

Oxygen flow during dc reactive sputtering of SnOx thin films affects film conductivity in zero grade air and gas sensitivity to carbon monoxide and ethanol. The experimental results show that an increase in oxygen flow during film deposition produces films exhibiting higher conductivity in zero grade air and lower gas sensitivity. A theoretical model is presented that explains this behavior. The proposed model takes into account both the dependence of conductivity on the potential barrier height at grain boundaries of the film and the dependence of chemisorption rate of oxygen on free-electron availability. The theoretical analysis is in good qualitative agreement with experiment.
Optical absorption and photoluminescence studies of β-FeSi2 prepared by heavy implantation of Fe+ ions into Si
Authors: Katsumata Hiroshi , Makita Yunosuke , Kobayashi Naoto , Shibata Hajime , Hasegawa Masataka , Aksenov Igor , Kimura Shinji , Obara Akira , Uekusa Shin-ichiro
Published: July 2002

Mass-separated 56Fe+ ions were implanted into Si(100) at 350 °C using three different energies and doses of 140 keV (1.32×1017 cm−2), 80 keV (6.20×1016 cm−2), and 50 keV (3.56×1016 cm−2). Their optical properties were investigated as a function of subsequent annealing temperature and its duration time. X-ray diffraction analysis revealed that polycrystalline semiconducting β-FeSi2 layers are formed in the as-implanted and annealed samples. From Rutherford backscattering spectrometry analysis, the formation of β-FeSi2 up to the surface was confirmed, and the average thickness and composition of the layers at peak concentration were estimated to be 70–75 nm and Fe:Si=1:2.0–2.2, respectively. The types of optical transition and the inverse logarithmic slope (E0) of the Urbach tail were investigated using room temperature optical absorption measurements. All the synthesized β-FeSi2 layers exhibited a direct transition with direct band-gap energies (Egdir) of 0.802–0.869 eV and with high optical absorption coefficients extending to 105 cm−1 at photon energy above 1.0 eV. The E0 value characteristic of the Urbach tail was observed to decrease from 260 to 100 meV with elevating annealing temperature. Some of the materials having E0<160 meV showed two strong photoluminescence (PL) emissions peaked at 0.805–0.807 eV (No. 1) and 0.840–0.843 eV (No. 2) at 2 K, whereas those with E0≳250 meV exhibited only weak emissions. From the results of the temperature- and excitation power-dependent PL measurements, emissions Nos. 1 and 2 were attributed to the trap-related recombinations related to β-FeSi2, with thermal activation (quenching) energies of 54.7 and 46.7 meV, respectively.
A kinetic Monte Carlo simulation of fiber texture formation during thin-film deposition
Authors: Smith Richard W.
Published: July 2002

A modified kinetic Monte Carlo computer simulation has been developed and applied to the investigation of fiber (out-of-plane) texture formation in polycrystalline thin films grown in the absence of high-energy bombarding particles. Unlike high-energy deposition, in which texture formation appears to result from a combination of preferential resputtering and self-shadowing at the film surface, the simulation results described herein indicate that texture formation in low-energy deposition is caused by a bias in surface diffusion and adsorption energies on different crystallographic faces. A net flux of atoms toward grains with high-binding-energy faces oriented along the film surface coupled with a greater probability that an atom near the interface between two grains will become attached to the high-binding-energy face produces a preferential in-plane expansion of these grains at the expense of their neighbors. The rate of texture development is shown to increase with increases in the binding energy of the preferred faces and the deposition temperature, and to decrease with increasing deposition rate and initial grain size. Voids and vacancies incorporated into the film form as a result of incomplete layer-by-layer growth induced by conditions of low surface diffusion and high deposition rate. The density of voids and vacancies, as well as the local surface roughness, varies from grain to grain within the material due to the differences in surface diffusion on the crystallographic faces exposed to the deposition flux as each layer of the various grains is formed.
Microstructure, heat treatment, and oxidation study of porous silicon formed on moderately doped p-type silicon
Authors: Zheng D. W. , Huang Y. P. , He Z. J. , Li A. Z. , Tang T. A. , Kwor R. , Cui Q. , Zhang X. J.
Published: July 2002

A porous Si (PS) layer with a spongy microstructure on top of a dendritic microstructure was fabricated on a moderately doped p-type Si wafer using a two-step anodization process. This illustrates that in addition to substrate doping, anodization current density also has an effect on the porous Si microstructure. A preoxidation heat treatment of the spongy-type porous Si was found to change the porous structure significantly, making it more difficult to fully oxidize the layer at low temperatures. However, dendritic porous Si can better withstand the heat treatment without suffering noticeable changes in structure. X-ray photoelectron spectroscopy, infrared spectroscopy, and electrical breakdown tests were used to analyze the oxidized porous Si samples. The oxidation process and the resultant oxide were found to depend on several factors, including the porosity, the microstructure itself (e.g., spongy or dendritic-type), and the heat treatment history prior to oxidation. With similar porosity, dendritic PS is easier to oxidize compared to spongy PS.
A polarized Raman study of the relaxor and ferroelectric states of La-modified lead zirconate titanate ceramics
Authors: El Marssi M. , Farhi R. , Viehland D.
Published: July 2002

Lanthanum modified lead zirconate titanate ceramics with La/Zr/Ti ratios of 6/40/60, 12/40/60, 5/65/35, and 9/65/35 have been studied by polarized Raman scattering. In the two former (tetragonal-type) compositions, selection rules were rather well defined and allowed an assignment of the Raman lines at lower temperature. These selection rules disappeared for 6/40/60 but remained for 12/40/60 even at higher temperatures. The behavior of the two latter (rhombohedral-type compositions) was quite different. No assignment of lines could be done. In addition, the spectra recorded in parallel and crossed polarizations were almost identical for 5/65/35 regardless of temperature. A very smeared paraelectric to ferroelectric transition was evidenced by Raman intensity measurements for this composition. On the contrary, selection rules were always present for the relaxor composition 9/65/35 which were similar to those for 12/40/60 and 6/40/60. This suggests that both tetragonal and rhombohedral-type local polar order is present for 9/65/35. The spectra of the field induced ferroelectric phase for 9/65/35 revealed no selection rules, similar to 5/65/35. This is attributed to the onset of only medium range ferroelectric order. Finally, Raman spectra recorded on powders of every composition did not reveal any selection rules, suggesting that intergranular effects might play a significant role in the behavior of these materials.
Factors affecting the optical properties of metal-semiconductor composites
Authors: Bates Clayton W. , Chen Quark Y.
Published: July 2002

The variation in the optical properties of metal-semiconductor composites (MSC) thin films due to particle-size distribution, metal volume fraction, film thickness, semiconductor dielectric constant, and microstructure are treated in a unified manner. These parameters are treated within the dynamic effective medium approximation which has been shown to accurately describe the optical properties of experimentally prepared MSC. The results of this treatment are successfully applied to Ag–Si composite systems.
Expansion dynamics of the plasma produced by laser ablation of BaTiO3 in a gas environment
Authors: Gonzalo J. , Afonso C. N. , Madariaga I.
Published: July 2002

The expansion dynamics of the plasma produced by excimer laser ablation of BaTiO3 has been studied by spatially resolved optical emission spectroscopy over a broad gas pressure range (10−7–40 mbar), the gas being either reactive (oxygen) or inert (argon). The results obtained in both environments are qualitatively similar and they show that there is a distance-related pressure threshold above which the expansion dynamics differ from that of the free-expansion regime observed in vacuum. Analysis of the results in the framework of the drag model as well as the shock wave model show that the plasma expansion can be well described by the drag model for the initial stages of the expansion, whereas a delayed shock wave model is required for long distances to the target or high gas pressure.
Theoretical investigation of near-field optical properties of tapered fiber tips and single molecule fluorescence
Authors: Chang Railing , Wei Pei-Kun , Fann W. S. , Hayashi Michi-toshi , Lin S. H.
Published: July 2002

We investigate herein the propagation and tunneling of monochromatic light through a tapered fiber tip, modeled as a tapered perfectly conducting waveguide. The opening aperture of the waveguide is much smaller than the wavelength of light. The transmission rate through the tip as well as the profiles of electric fields are obtained. A detailed formulation for calculating the fluorescent spectra of a single molecule under near-field excitation is also presented. In addition, the limitations of fluorescence measurement for obtaining near-field pattern is discussed on the basis of the present formulation.
Nanostructure, interfaces, and magnetic properties in giant magnetoresistive NiO-Co-Cu-based spin valves
Authors: Chopra Harsh Deep , Hockey B. J. , Chen P. J. , McMichael R. D. , Egelhoff W. F.
Published: July 2002

Giant magnetoresistive NiO-Co-Cu-based top and symmetric spin valves were studied with emphasis on the role of oxygen partial pressure, p(O2), in affecting the magnetic pinning of Co by NiO. Results show a correlation between the underlying nanostructure and pinning strength as a function of p(O2). High-resolution transmission electron microscopy show that at progressively higher oxygen partial pressures, NiO-Co interface becomes crystallographically disordered, in part, due to the oxidation of Co layer on which NiO is deposited; an amorphous film is formed at the interface. An optimum p(O2) interval for the deposition of NiO is identified (∼1×10−5–3.0 ×10−5 Torr), in which a sharp and crystallographically well-defined NiO-Co interface can exist. Finally, a simple model suggest that defect structures at the NiO-Co interface may play an important role in the magnetization reversal of a NiO-pinned Co layer in an applied field.
Kinetics of thin-film reactions of Cu/a-Ge bilayers
Authors: Wang Z. , Ramanath G. , Allen L. H. , Rockett A. , Doyle J. P. , Svensson B. G.
Published: July 2002

The kinetics of the Cu3Ge phase formation during reactions between 600 nm polycrystalline Cu (poly-Cu) and 600 nm amorphous Ge (a-Ge) layers on Si (100) substrates have been studied as a function of annealing conditions. Monoclinic Cu3Ge nucleated rapidly, resulting in smooth Cu3Ge layers. The room-temperature resistivity of the Cu3Ge was ∼8 μΩ cm (∼4.5 times that of pure Cu). The real-time resistance versus temperature [R(T)] characteristics were nearly identical for heating rates of 0.1–5 °C/min. Modeling of the R(T) data indicates that the reaction was predominantly diffusion controlled with a rate of (4×10−3 cm2/s) exp [−0.85±0.01 eV/kT] where k=8.617×10−5 eV/K. Secondary ion mass spectrometry profiles and R(T) data for the films annealed to various temperatures indicate that the Cu3Ge/Ge interface is stable for T<300 °C.
Anomalous temperature quenching of fluorescence in Pr3+ doped sulfide glass
Authors: Quimby R. S. , Aitken B. G.
Published: July 2002

Fluorescence from the 1D2 level of Pr3+-doped AsGaGeS glass is found to be strongly quenched with increasing temperature. A model for this quenching is proposed which involves energy transfer from the Pr3+ ions to near-band gap defect states in the host glass. Energy transfer from these defect states back to Pr3+ in the 1G4 state with an estimated efficiency of ≈10−3 is also observed. The occurrence of these energy transfer processes complicates quantum efficiency measurements made using a self-calibrating branching ratio technique. After accounting for this energy transfer, the quantum efficiency for the 1300 nm emission in Pr3+ doped AsGaGeS glass is found to be 0.48 rather than the previously obtained value of 0.55. No temperature quenching of the 1G4 level of Pr3+ is observed when this level is pumped directly. Nd3+ impurities in the same glass are much less strongly quenched than Pr3+ when pumping around 600 nm.
Temperature dependence of electron mobility in Bi12GeO20 and Bi12SiO20 using the time-of-flight technique
Authors: Bloom D. , McKeever S. W. S.
Published: July 2002

We have measured the temperature dependence of the electron mobility between ∼200 and 300 K in undoped and 0.3% Fe-doped Bi12GeO20 (BGO) and undoped Bi12SiO20 (BSO) using a time-of-flight technique. We found that mobilities calculated from the observed transit times were independent of sample thickness and applied voltage, but depended approximately exponentially on temperature. The hole current transients were very weak and featureless, and consequently we were unable to measure mobilities for holes. The measured drift mobilities varied from 6.1×10−4 to 0.10 cm2/Vs for undoped BGO, 6.1×10−4 to 0.0170 cm2/Vs for 0.3% Fe-doped BGO, and ∼5×10−4 to 0.014 cm2/Vs for undoped BSO. Values of the microscopic mobility were estimated to be 5.9, 2.3, and 1.4 cm2/Vs for undoped, 0.3% Fe-doped BGO, and undoped BSO, respectively. From the temperature dependence of the electron mobility we obtained an activation energy for the dominant trap of Et=0.31 eV for undoped BGO, Et=0.34 eV for 0.3% Fe-doped BGO, and Et=0.31 eV in the undoped BSO sample. These values are interpreted as the trap depth of the main traps dominating the mobility and agree well with values obtained from the analysis of thermally stimulated conductivity measurements performed previously.
Iron precipitation in float zone grown silicon
Authors: Henley Worth B. , Ramappa Deepak A.
Published: July 2002

Temperature dependent iron precipitation in float zone grown silicon wafers has been experimentally investigated. Results of iron precipitation experiments over a wide thermal process temperature range and time are presented. Precipitation of iron in silicon was analyzed by a quantitative assessment of change in interstitial iron using a surface photovoltage minority carrier lifetime analysis technique. Contamination levels of iron in the range 1011–1013 atoms/cm3 are investigated. It is concluded that maximum iron precipitation occurs in the temperature range of 500–600 °C. Iron precipitation is rapid in this region where more than 90% of the interstitial iron precipitates in a period of 30 min.
The stress assisted evolution of point and extended defects in silicon
Authors: Chaudhry Samir , Law Mark E.
Published: July 2002

As semiconductor devices are scaled to sub-micron dimensions, they are becoming more complex in geometry and materials. Stress related problems are therefore pervasive and critical in ultralarge scale integrated technology. High levels of stress can cause severe degradation of device characteristics by generating and propagating dislocations in the silicon substrate. Furthermore, stress in the silicon substrate can cause dopant redistribution to an extent it can no longer be neglected when designing scaled devices. In this study the effect of stress, generated from patterned nitride stripes on silicon, on the diffusion of phosphorus and evolution of ion-implanted dislocation loops is investigated. Phosphorus displays retarded diffusion, while the dislocation loops are smaller and less dense in the compressive regions under the nitride film.
Influence of Al content on formation of InAlGaAs quantum dots grown by molecular beam epitaxy
Authors: Baklenov O. , Huffaker D. L. , Anselm A. , Deppe D. G. , Streetman B. G.
Published: July 2002

Data are presented on the dependence of Al content on the formation of InAlGaAs quantum dots grown by strained-layer molecular beam epitaxy. Atomic force microscopy and reflection high-energy electron diffraction patterns show that the addition of Al both slows the formation of the quantum dots in the brief time following layer deposition, and results in smaller dots of higher density. The effects might be qualitatively explained by reduced surface migration of the column III atoms due to the stronger Al bond strength.
Above-threshold longitudinal profiling of carrier nonpinning and spatial modulation in asymmetric cavity lasers
Authors: Bennett A. J. , Clayton R. D. , Xu J. M.
Published: July 2002

Longitudinal variations of photon and carrier density along the length of a Fabry–Perot quantum well laser are simulated and compared to experimental carrier density measurements performed using a side spontaneous emission scanning technique. Both simulation and measurement demonstrate the existence of carrier nonpinning along the cavity of the laser, with increasing carrier density from the low reflectivity facet toward the high reflectivity facet. Measured carrier densities increased monotonically above their threshold value with injection current, at all points along the laser, seemingly contradicting the condition of net round-trip gain pinning at threshold. This behavior is attributed to current spreading into the low-gain regions aside the active region causing a current-dependent increase in spontaneous emission, and thus an apparent increase in measured active region carrier density. Periodic spatial modulation of the carrier density profile, consistent with the hypothesis of beam steering induced by beating and locked lateral modes, was also observed.
Magnetic properties and growth texture of high-coercive Nd–Fe–B thin films
Authors: Parhofer S. , Kuhrt C. , Wecker J. , Gieres G. , Schultz L.
Published: July 2002

We have investigated the growth texture and magnetic properties of sputtered Nd–Fe–B thin films with thicknesses from 5 to 350 nm. Films deposited directly onto a quartz substrate grow with a pronounced c-axis texture perpendicular to the film plane and have a coercivity of about 160 kA/m. If the films thickness is reduced below 150 nm, a significant part of the Nd is oxidized by substrate oxygen and the coercivity decreases to values below 20 kA/m. The deposition of a 80 nm Cr buffer layer between substrate and Nd–Fe–B film prevents oxidation of the film and decreases the growth texture of the films. This enables us to fabricate Nd–Fe–B films as thin as 20 nm with good hard magnetic properties [Hcj=800 kA/m, Jr=1.1 T, and a maximum energy product (BH)max=190 kJ/m3 perpendicular to film plane]. An inverse relationship between growth texture and coercivity is found which can be understood in terms of domain wall motion. The deposition of a Cr buffer leads to an island type of growth and a high surface roughness, while films without Cr buffer are very smooth. By depositing a protective Cr layer on top of the film, the corrosion resistance can be enhanced.
Experimental determination of the N-p-partial density of states in the conduction band of GaN: Determination of the polytype fractions in mixed phase samples
Authors: Katsikini M. , Paloura E. C. , Moustakas T. D.
Published: July 2002

Hexagonal, cubic, and mixed phase GaN samples grown by molecular beam epitaxy are studied using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The spectra were recorded at the N-K-edge, at several angles of incidence. It is shown that the N-K-edge NEXAFS spectra, which are proportional to the p-partial density of states in the conduction band, are characteristic of the cubic or hexagonal structure of the examined crystal. The spectra of the cubic sample do not depend on the angle of incidence (θ), contrary to the spectra of the hexagonal sample in which the areas under the NEXAFS resonances depend linearly on cos2 θ. From the fitting of the lines Ai=A+B cos2 θ, where Ai are the areas under the resonances and A and B are constants, the directions of maximum electron charge density with respect to the normal to the surface are determined for the hexagonal sample. The energy positions of the absorption edge and the NEXAFS resonances in the spectra from the cubic sample are different from those of the hexagonal sample and in either case are independent of θ. Contrary to that, the energy positions of the NEXAFS resonances in a mixed phase sample have a characteristic angular dependence and shift between the energies corresponding to the cubic and hexagonal polytypes. Based on this observation, we propose that the spectrum of the mixed-phase sample can be approximated as the weighted average of the spectra from the pure cubic and hexagonal samples. From this approximation the coexisting fractions of α- and β-GaN in a mixed-phase sample are determined and they are found in good agreement with results from high resolution transmission electron microscopy and x-ray diffraction.
Polarization effects in reflection-mode scanning near-field optical microscopy
Authors: Durkan C. , Shvets I. V.
Published: July 2002

It is shown that the polarization state of light emitted from an aperture-type scanning near-field optical microscopy (SNOM) tip may be significantly altered on reflection from a metal surface in close proximity (5 nm) to the tip, while remaining unchanged at distances of several microns. Proximity to dielectric surfaces produces no discernible change. This effect is discussed and explained theoretically. We demonstrate that optical image contrast of metal samples may be enhanced by using this effect. The mechanism of the enhancement is based on selectively detecting the light emitted from the SNOM aperture and filtering out the light emitted through the sidewalls of the probe. It is also shown that images of a metal grating pattern on glass show strong polarization contrast, which may be understood in terms of simple dipole–image dipole schemes and transmission-line concepts.
Ion-induced room temperature synthesis of low resistive nickel germanide phase
Authors: Dhar Sankar , Som Tapobrata , Kulkarni V. N.
Published: July 2002

Low resistivity Ni2Ge phase, a promising candidate as a contact and interconnect material for very large scale integrated circuit applications, has been synthesized by MeV ion beam mixing using Kr and Ar ions at and above room temperatures. Thin films of this phase produced by ion beam mixing show resistivity comparable to the one produced by thermal annealing and is much lower than that of many important silicides. The critical temperature which marks the beginning of the temperature dependent mixing is 326 K. The room temperature mixing efficiency for this system is found to be about an order of magnitude higher than the value predicted by any of the existing ballistic and spike models. The crystalline nature of the mixed region and the occurrence of the Ni2Ge phase have been discussed.
High concentration diffusivity and clustering of arsenic and phosphorus in silicon
Authors: Solmi S. , Nobili D.
Published: July 2002

The As diffusion coefficient as a function of its concentration was determined by Boltzmann–Matano analysis of the profiles of the dopant diffusing out of its conjugate phase precipitates during furnace annealing at 900 and 1050 °C of samples heavily doped by ion implantation. This method allowed to assure a constant diffusion source of As and to investigate a doping range attaining 3×1021 cm−3. Along the same lines, the diffusivity versus concentration of specimens heavily implanted with P was determined at 900 and 1000 °C. Dopant profiles were determined by secondary neutral mass spectroscopy. The diffusivity of both As and P increases with dopant content, attaining a maximum at a concentration which closely corresponds to the saturation value of the carrier density, ne, which we previously determined by equilibration annealing of specimens with excess dopant. This finding demonstrates that ne represents the limiting value of the concentration of unclustered dopant at the diffusion temperature. On the contrary, a diffusivity monotonically increasing with dopant concentration up to its solubility limit, was observed in the case of B and Sb, which do not cluster. Finally, we report the results of a simulation model which can accurately describe the evolution of the As profile upon annealing, by using our diffusivity data and taking into account both the precipitation and clustering phenomena.
Narrowband laser produced extreme ultraviolet sources adapted to silicon/molybdenum multilayer optics
Authors: Schriever G. , Bergmann K. , Lebert R.
Published: July 2002

The extreme ultraviolet radiation emitted from a plasma generated by a pulsed Nd:yttrium aluminum garnet laser is investigated around 13 nm wavelength for several low Z elements (lithium, nitrogen, oxygen, fluorine). A narrowband EUV source can be designed by using the narrowband line emission of low Z elements in combination with the broadband reflection characteristic of silicon/molybdenum (Si/Mo) multilayer mirrors. Experimental results are discussed within a theoretical model, which allows a deduction of an optimization criterion for a maximum conversion efficiency. The Lyman-α line of hydrogenlike lithium ions fulfills the demands for high intense, free-standing narrowband emission at the long wavelength side of the silicon absorption L edge.
An angle-resolved time-of-flight mass spectrometric study of pulsed laser ablated Ta2O5
Authors: Qin Qi-Zong , Han Zhen-Hui , Dang Hai-Jun
Published: July 2002

Pulsed laser ablation of a Ta2O5 target at 532 nm has been investigated using an angle-resolved time-of-flight quadrupole mass spectrometric method. Mass and velocity distributions of the ionic and neutral ablated species were measured. The major ablated species, O (m/e=16), O2 (m/e=32), Ta (m/e=181), TaO (m/e=197), and TaO2 (m/e=213) were observed at a laser fluence of 0.7 J cm−2, and the intensities of neutral species were found to be higher than those of the ionic species. The time-of-flight spectra of ablated species were fitted by a Maxwell–Boltzmann distribution with a center-of-mass velocity. The angular distributions of the ionic and neutral ablated species can be fitted with cosn θ (n=15) and 0.25 cos θ+0.75 cosn θ (n=15), respectively. The laser fluence had no obvious influence on the translational energies of the ablated species. In addition, a continuous wave oxygen molecular beam was introduced into the ablated plume, and the enhancement of the intensities of the TaO and TaO2 signals was observed. This result implies that the reactions of ablated Ta-containing species with ambient oxygen molecules in the gas phase play an important role in the pulsed laser deposition process.
Remanence enhancement in mechanically alloyed isotropic Nd(Fe, Mo)12Nx compounds
Authors: Xiao Yiaofu , Zeng Qi , Dong Shengzhi , Wang Run
Published: July 2002

The structure and magnetic properties of Nd(Fe, Mo)12Nx compounds prepared by mechanical alloying have been studied using x-ray diffraction, transmission electron microscopy, and magnetic measurements. After heat treatment and nitriding, it is found that almost single-phase Nd(Fe, Mo)12Nx can be obtained. Electron microscopy revealed a nanometer grain size of 10–30 nm and the magnets have a remanence enhancement of approximately 60% of the saturation.
Local and global demagnetization process: Is there any self-organized critical behavior?
Authors: Chubykalo O. A. , González J. M.
Published: July 2002

The behavior of local and global demagnetization processes (avalanches and Barkhausen jumps) is analyzed in terms of a micromagnetic model. Our study of the statistics of these events as a function of exchange correlation length parameter, system size, and system driving rate shows the absence of self-organized critical behavior if a realistic model of a magnetic system is considered.
Nonlinear self-localized magnetoelastic surface waves in antiferromagnetic media
Authors: Dikshtein Igor E. , Salk Sung-Ho Suck
Published: July 2002

Types of self-localized, magnetoelastic (ME) surface waves in antiferromagnets (AFs) are predicted, the existence of which is determined entirely by the nonlinear properties of the AFs. The frequency spectrum of the self-localized ME surface waves is shown to lie in a gap between the upper quasimagnon and lower quasiphonon branches of the harmonic ME spectrum. A nonlinear Schrödinger equation for the envelope of ME waves is derived and its solitonic solutions are obtained. When spin pinning or capillary effects on the surface of the crystal exist, we find that the nonlinear surface ME wave excites the volume ME wave at fundamental harmonics carrying energy into the interior of a crystal.
Negative remanent magnetization of fine particles with competing cubic and uniaxial anisotropies
Authors: Geshev J. , Viegas A. D. C. , Schmidt J. E.
Published: July 2002

The magnetic properties of noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and a uniaxial components were investigated. Various directions of the uniaxial anisotropy were considered and the dependencies of the reduced remanence as a function of the ratio between the two anisotropies were obtained. It was found that for sufficiently strong uniaxial anisotropy and random arrangement of the particle orientations, reduced remanence lower than 0.5 is, in general, an intrinsic property of the system due to the negative remanent magnetization of some of the particles.
Effect of the silicon/oxide interface on interstitials: Di-interstitial recombination
Authors: Law M. E. , Haddara Y. M. , Jones K. S.
Published: July 2002

Interstitials can recombine at an oxide/silicon interface. Previous experimental work produces contradictory results. Transient enhanced diffusion experiments suggest a nearly infinite surface recombination rate, while oxidation enhanced diffusion suggests a much weaker recombination rate. A di-interstitial mechanism is investigated, and analytic solutions are developed. This is compared to the more commonly used interstitial mechanism. The di-interstitial mechanism can account for most of the discrepancy in the data.
Direct visualization of the dynamic behavior of a water meniscus by scanning electron microscopy
Authors: Schenk Michael , Füting Manfred , Reichelt Rudolf
Published: July 2002

Scanning probe microscopic imaging can be complicated by the capillary force of a water meniscus formed in air between the tip and the sample. Water menisci between a tungsten tip and Pt/C-coated mica and their dynamic behavior have been directly visualized by environmental scanning electron microscopy. Rapid scan secondary electron micrographs give information in the 100 nm range. We found that static models are not appropriate to describe the shape of a meniscus when the tip is moving across the sample. The surface structure and its properties influence the affinity of the meniscus thus causing a varying capillary force that may exhibit a vertical and a lateral component as well. Our experimental data indicate that the Kelvin equation also holds for microscopically small water menisci.
Electrical conduction in exploded segmented wires
Authors: Hollandsworth Clinton E. , Powell John D. , Keele Michael J. , Hummer Charles R.
Published: July 2002

Recent work by Me-Bar and Harel, intended to characterize the conduction characteristics of exploding segmented wires, is extended. A set of experiments on conductors composed of various materials and in various geometries is undertaken. The experimental work is complemented by two-dimensional calculations in which the magnetic diffusion equation is solved for the problems at hand. Results from these calculations are used to explain the behavior observed in the experiments. It is also observed from the calculations that different results for segmented wires might be obtained if the plasma, resulting from partial explosion of the wires, is confined radially around the wire. A set of experiments to test this contention is undertaken, and produces results in good qualitative agreement with the theoretical predictions. Physical reasons for the observed behavior, as well as possible methods for ameliorating undesired effects, are indicated.
Plasma processing of the silicon surface: A novel method to reduce transient enhanced diffusion of boron
Authors: Mannino Giovanni , Priolo Francesco , Privitera Vittorio , Raineri Vito , Spinella Corrado , Napolitani Enrico , Carnera Alberto , Arena Giuseppe , Messina Alberto , Rapisarda Cirino
Published: July 2002

We investigate in detail the effect of plasma processing on the transient enhanced diffusion of implanted boron in silicon. Thermally oxidized silicon wafers were first processed with CHF3/CF4 plasma and subsequently implanted with boron, with energies ranging from 3 to 20 keV and a dose of 1×1013/cm2. Chemical profiles were measured by secondary ion mass spectrometry while lattice extended defects induced in silicon by plasma processing were characterized by transmission electron microscopy. Secondary ion mass spectrometry measurements reveal that the transient enhanced diffusion of boron after rapid thermal annealing is strongly reduced in plasma processed samples with respect to unprocessed samples. Defects induced by plasma processing are responsible for the reduction by acting as very efficient traps for the interstitial atoms generated during the implant. We note that the trapping efficiency is critically dependent on the projected range of the boron implant, being extremely evident at low energies and less marked as the energy is increased (i.e., when the interstitials generated by the B implant are far away from the trapping sites). By varying the plasma conditions (an argon plasma is used instead of a CHF3/CF4 plasma), we are able to establish a general correlation between trapping defect centers and transient enhanced diffusion reduction. Finally, spreading resistance measurements reveal that the amount of electrically active boron in plasma processed pure epitaxial Si is almost equal to that obtained in samples not exposed to plasma bombardment, thus demonstrating that the plasma processing has no detrimental effect on the boron electrical activation.
Incorporation of optically active erbium into GaAs using the novel precursor tris(3,5-di-tert-butylpyrazolato)bis(4-tert-butylpyridine)erbium
Authors: Cederberg J. G. , Culp T. D. , Bieg B. , Pfeiffer D. , Winter C. H. , Bray K. L. , Kuech T. F.
Published: July 2002

We have investigated the use of an alternative erbium precursor, tris(3,5-di-tert-butyl- pyrazolato)bis(4-tert-butylpyridine)erbium, to dope erbium into GaAs. The incorporated erbium forms an optically active center identified as Er–2O. The GaAs:Er formed using this precursor exhibits sharper and more intense optical emission, attributed to the Er–2O center, than that previously found with cylcopentadienyl-based erbium sources. Codoping GaAs:Er with shallow donors results in a quenching of the erbium-related luminescence, while codoping with shallow acceptors results in no significant change in the spectrum. Mechanisms for the observed luminescence-quenching behavior are discussed. Deep level transient spectroscopy performed on silicon or selenium codoped GaAs:Er showed the presence of several electron traps in the upper half of the band gap. The origins of these electron traps are considered.
Reciprocal space mapping of phase transformation in epitaxial PbTiO3 thin films using synchrotron x-ray diffraction
Authors: Lee K. S. , Baik S.
Published: July 2002

Phase transformation in epitaxial PbTiO3 thin films grown on MgO(001) substrates was studied and quantified by two-dimensional reciprocal space mapping technique using synchrotron x-ray diffraction equipped with an in situ high temperature stage. Just below the Curie temperature, a twin-like domain structure was formed with an initial value of c-domain abundance, α∼0.3, and the value increased continuously during cooling, and eventually the c-domain dominant structure (α∼0.72) was achieved at room temperature through continuous expansion of the c domains. By investigating the intensity distribution of contour maps, domain tilting and mosaicity were characterized along the q[h00] direction and the presence of a strain gradient along the growth direction q[001] was also confirmed from the asymmetric distribution of the contour maps.
Femtosecond transient absorption and luminescence decay studies of spectrally sensitized photographic emulsions
Authors: Gretchikhine A. , Schweitzer G. , Van der Auweraer M. , De Keyzer R. , Vandenbroucke D. , De Schryver F. C.
Published: July 2002

Fast electronic processes taking place immediately after excitation of sensitized silver halide emulsions were studied with picosecond time-correlated single-photon counting (SPC) and femtosecond transient absorption (TA). The fluorescence decays of the J aggregate obtained by SPC measurements were also analyzed as a sum of three exponentials. For the J aggregates of a benchmark dye, a dimethyl-μ-ethyl-thiacarbocyanine, a component with a 20 ps decay time, which was absent in samples containing no AgBr crystals, was attributed to J aggregates of the sensitizer adsorbed on the AgBr microcrystals. The fluorescence decay results were compared with photographic quantum yields in an attempt to show the relevance of the spectroscopic measurements. The second component (150 ps) is a combination of fluorescence decays of the unaggregated sensitizer molecules not adsorbed on the AgBr microcrystals and monomers of the dye on AgBr surface. The fluorescence decay was found to be independent of the crystal coverage. Prolonged illumination bleached the fluorescence of the fast component, which is consistent with depletion of the J aggregates by electron injection. The decay time of this fast component shows strong dependence on both the structure of the dye and the composition of the silver halide. Excitation in the narrow absorption band of the J aggregates with a femtosecond pump pulse resulted in ground state bleaching followed by a recovery that was analyzed as a sum of three exponentials. The fast (170 fs and 2 ps) components of the recovery are related to the relaxation of bi- or multiexciton states and to singlet-singlet annihilation processes. The remaining component (1 ns) can be attributed to the radiative relaxation of dye molecules not adsorbed on the silver halide crystals. The observed bleaching recovery demonstrated a complex dependence on both the excitation flux and the total illumination dose.
Electrical and optical properties of dense composites
Authors: Doyle William T.
Published: July 2002

Effective cluster models have been developed that treat disordered suspensions of monodisperse metal spheres as mixtures of isolated spheres and compact clusters of spheres using the Clausius–Mossotti equation [Phys. Rev. B 42, 9319 (1990); J. Appl. Phys. 71, 3926 (1992)] and the Bruggeman equation [J. Appl. Phys. 78, 6165 (1995)]. These effective cluster models are adapted to suspensions of dielectric particles with arbitrary complex permittivity. The models are compared with the coated sphere model of Sheng [Phys. Rev. Lett. 45, 60 (1980); J. Opt. Soc. Am. B 15, 1022 (1998)]. Model calculations are compared with the measurements of the optical transmission spectra and low frequency electrical conductivity of Au–SiO2 films by Cohen, Cody, Coutts, and Abeles [Phys. Rev. B 8, 3689 (1973)], and with the low frequency permittivity measurements on suspensions of Ag spheres in KCl of Grannan, Garland, and Tanner [Phys. Rev. Lett. 46, 375 (1981)]. The models accurately predict the percolation thresholds seen in the electrical measurements and are in good agreement with all of the experiments over the entire range of volume loading.