bluuebunny/crossref_metadata_2025 huggingface dataset:
Thresholds for dielectric breakdown in laser-irradiated diamond
Authors: Klein Claude A. , DeSalvo Richard
Published: July 2002

We report on picosecond laser-induced damage experiments that were carried out on a natural type-IIa diamond and a thick specimen of high-quality chemically vapor-deposited (CVD) diamond. In conjunction with earlier measurements performed elsewhere on an ‘‘optically thick’’ single crystal, it is shown that for spot sizes (2ω) ranging from 3 to 60 μm, the breakdown field strength (EBD) at the damage threshold of diamond obeys a pattern best described as follows: EBD≂A/√2ω, where A=30.7 and 38.7 MVμ1/2/cm at 532 and 1064 nm, respectively. The case of CVD diamond demonstrates that if problems arising from localized high absorption at the deposition surface can be avoided, this material should be of much promise for contemplated high-power free-electron laser window applications.
Ultrasonic force microscopy for nanometer resolution subsurface imaging
Authors: Yamanaka Kazushi , Ogiso Hisato , Kolosov Oleg
Published: July 2002

We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low-frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.
On the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films
Authors: Im James S. , Kim H. J.
Published: July 2002

This letter reports on the experimental findings, and provides a theoretical description of the super lateral growth (SLG) phenomenon observed in the pulsed laser-induced solidification of amorphous thin Si films on SiO2. Experimentally, we report and elaborate on the isolated single-crystal disk structure that is observed at the upper threshold of the SLG regime; the structure is revealed as an important microstructural feature for understanding the phenomenon. A theoretical discussion of the SLG phenomenon is provided in terms of the key factors that are suggested by our model—the interface response function of the solid, the nucleation kinetics of the solid, and a highly transient lateral-thermal profile near the solid-melt interface. Our model and analysis point out the important inadequacies associated with the vertical solidification rate/temperature gradient model, which is currently being utilized to explain the excimer laser crystallization of thin Si films on SiO2.
Characterization of a cesium surface ionization source with a porous tungsten ionizer. I
Authors: Alton G. D.
Published: July 2002

A cesium surface ionization source of the type and geometry customarily used in conjunction with sputter-type negative heavy ion sources has been characterized. Measurements have been made of positive-ion production and probabilities of ionization as functions of extraction voltage and cesium oven temperature for ionizer porosities of 0.7 and 0.8. The perveance P of the source, when operated in the space-charge-limited regime with the ρ=0.7ρ0 ionizer, is found from experiment to be 7.61×10−4 μP, while that for the ρ=0.8ρ0 ionizer is 1.92×10−4 μP. These values are lower by factors of 3.88 and 15.53, respectively, than those predicted by numerical solution to Poisson’s equation for full area emission from the source. Positive-ion current versus ionizer temperature data are also presented along with mechanical design features of the source.
Broadband fluctuation wave number measurement technique using a heavy ion beam probe
Authors: Heard J. W. , Crowley T. P. , Schoch P. M. , Ross D. W. , Hickok R. L. , Yang X. Z.
Published: July 2002

The heavy ion beam probe (HIBP) measures localized density fluctuations in the plasma interior. These measurements can be used to calculate two point estimates of interior poloidal wave numbers. The results so far show wave numbers significantly lower than FIR scattering wave numbers and drift wave expectations. Experiments were performed on TEXT to look for possible sources of error in our wave number calculations. The HIBP effects that are considered the possible dominant error terms are finite sample volume size and path effect fluctuations as well as the possible existence of counter-propagating modes. An aperture experiment showed the presence of a finite sample volume effect, but not large enough to correct the measured wave numbers significantly. Simulations using HIBP data show that our processing techniques detect counter-propagating modes. Path effects are discussed. The error analysis so far shows the HIBP calculations to be accurate wave number measurements, but more study on path effects is needed.
A capacitive system for three-dimensional imaging of fluidized beds
Authors: Fasching George E. , Smith Nelson S.
Published: July 2002

A novel capacitance imaging system (CIS) has been developed which has unique capabilities for providing real-time, three-dimensional monitoring of fluidized-bed density. While the system was developed primarily to serve as a high-speed, density-imaging tool in fluidization research and development, it could also be used in other applications, such as pipeline transfer of solids, where multiphase dielectric material density changes are to be monitored. The CIS uses 16 evenly spaced, circumferentially located electrodes at each of four levels evenly spaced along the axis of a 6-in. diam fluidized bed as the density-sensing elements. The electronic system utilizes interelectrode displacement current values to obtain bed density estimates at each of the 49 pixels defined at each of the four levels. The system provides a real-time display of each of the four density maps at a rate of 30 maps per second at each level. The system also stores displacement current data, updated 100 times per second, for post-run analysis of bed behavior. The CIS capabilities are demonstrated for a plastic bead bed material. Density maps show the progression of a low-density region upward through a fluidized bed. Experimental use of the system by others produced values of bubble dimension and velocity that are consistent with values predicted by accepted theory.
A development in the preparation of sharp scanning tunneling microscopy tips
Authors: Song J. P. , Pryds N. H. , Glejbo/l K. , Mo/rch K. A. , Thölén A. R. , Christensen L. N.
Published: July 2002

An improved and reliable method for making sharp scanning tunneling microscopy (STM) tips is described. It is based on the widely used drop-off electrochemical etching procedure, here modified to improve the control of the tip shape. A second etching is applied not only to remove the oxide layer from the tip surface, but also to sharpen the STM tips further. A tip radius less than 20 nm can be obtained reproducibly. The quality of the produced tips was inspected with transmission electron microscopy, and micrographs of tips produced with different times of second etching are shown. To produce tips which are all without an oxide layer an electronic phase control unit is necessary. Even without this etch controller more than half the tips are oxide free, and then only standard laboratory equipment is used for the tip production.
An energy dispersive x-ray absorption spectroscopy beamline, X6A, at NSLS
Authors: Lee P. L. , Beno M. A. , Jennings G. , Ramanathan M. , Knapp G. S. , Huang K. , Bai J. , Montano P. A.
Published: July 2002

An energy dispersive x-ray absorption spectroscopy instrument has been built at the X6A beam port of the x-ray ring at the National Synchrotron Light Source (NSLS). This instrument allows the collection of extended x-ray-absorption fine structure and/or x-ray absorption near-edge structure spectra for many elements on the millisecond time scale. The beamline employs a four-point crystal bender and a rectangular Si 220 crystal to access incident energies between 6.5 and 21 keV. Because the polychromator focuses the synchrotron beam to a narrow 100-μm line, this experimental apparatus is ideal for x-ray absorption spectroscopy experiments in special environments such as at high pressures, for in situ experiments, and/or for very small samples. In this manuscript we will describe the instrument design and present data with which to evaluate the instrument. This beamline is available through the NSLS user proposal system.
Portable instrument employing a surface acoustic wave sensor with a regenerable reagent coating for direct measurement of 1,3-butadiene and styrene
Authors: Zhang Guo-Zheng , Zellers Edward T.
Published: July 2002

The design and preliminary laboratory testing of a prototype portable instrument capable of near-real-time measurement of selected airborne olefins and dienes at low- and sub-ppm concentrations is described. The sensor employed is a 60-MHz surface-acoustic-wave (SAW) oscillator coated with a Pt-olefin coordination compound having the formula trans-PtCl2(olefin)(pyridine) (olefin=1-hexene or ethylene). The continual mass change that occurs upon replacement of the initially coordinated olefin by the olefin or diene analyte leads to a commensurate change in the output frequency of the SAW sensor. Analyte air concentrations can be determined therefore by measuring the rate of frequency change. In tests with 1,3-butadiene and styrene the instrument provided linear response isotherms from 0.050 to 3.0 and 0.60 to 50 ppm, respectively, and calculated detection limits of 0.043 and 0.28 ppm, respectively, based on a 10 s measurement interval. Repeated regeneration of the reagent sensor coating is accomplished automatically by brief exposure to a high concentration of the initially coordinated olefin generated from a reservoir that can be housed within the instrument.
Far-infrared electron-paramagnetic-resonance spectrometer utilizing a quasioptical reflection bridge
Authors: Earle Keith A. , Tipikin Dmitriy S. , Freed Jack H.
Published: July 2002

We describe a far-infrared electron-paramagnetic-resonance (EPR) spectrometer for broadband (100–300 GHz) use. The spectrometer is operated in the reflection mode and uses broadband quasioptical methods to separate the transmitted from the reflected radiation. We describe and illustrate its operation at 170 GHz (1.8 mm) and compare its performance to that of our transmission mode spectrometer operating at 250 GHz. We also discuss the advantages of the reflection bridge for performing EPR experiments over a broad range of frequencies, and we consider methods of improving the performance of the bridge. This includes a novel design for variable coupling of the reflection-mode Fabry–Pérot resonator.
Room-temperature inverse Faraday effect in terbium gallium garnet
Authors: Raja M. Yasin Akhtar , Allen David , Sisk Wade
Published: July 2002

The inverse Faraday effect (IFE) in terbium gallium garnet (TGG) is observed at room temperature. A circularly polarized 532 nm optical pulse from a frequency doubled Nd:YAG laser traverses a TGG crystal, resulting in an induced voltage in a surrounding detection coil. Variation of laser intensity and polarization direction leads to changes in the induced voltage in accordance with Faraday’s law of induction. The IFE is attributed to the optical Stark effect.
Quantitative Auger electron spectroscopic analysis of Ge surface segregation in Si/Ge/Si(100) heterostructures
Authors: Li Yun , Hembree G. G. , Venables J. A.
Published: July 2002

Germanium surface segregation in Si/Ge/Si(100) heteroepitaxial structures has been studied by quantitative Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Ge segregation was measured for 1–4 ML Ge, for slow Si growth rates (3×10−4 nm/s) at moderate deposition temperature (520 °C). A layer growth model is used to infer segregation probabilities between 90% and 98%, increasing with Ge layer thickness, especially between 1 and 2 ML. Using this model, the Ge profile in the cap layer is calculated. Surface roughness is just noticeable at 4 ML, and is incorporated into the model in an illustrative manner. A two-state model is used to determine the segregation energy, 0.24±0.02 eV, for 2–3 ML.
Atomic force microscopy improved resolution employing large scanning speeds: Effects of the double relaxation time
Authors: Teschke O. , Souza E. F.
Published: July 2002

The minimum scanning speed of atomic force microscopes for improved atomic resolution has been measured in liquid media, and shown to be equal to 100 nm/s for mica immersed in water corresponding to the time spent scanning the distance between two neighbor ions (∼0.52 nm) of ∼5 ms. The scanning velocity dependence of the force acting on the tip in the double-layer region (∼135 nm) when it approaches the surface was also measured. The stationary component of this force, for scanning speeds up to 30 μm/s, was identified as the exchange of the liquid media with ε≈80 by the tip with ε≈6. As the tip approaches the surface and as well as when the tip images atomic features, this repulsive force shows a relaxation time of a few milliseconds, corresponding to the shielding of the surface charge by the solution, i.e., the double-layer relaxation time. Scanning surfaces at speeds higher than the ratio of the atomic features distance and this relaxation time results in a variable repulsive force acting on the tip, as a function the scanning speed, which might be used to improve the atomic imaging resolution.
Multipurpose 10 in. manipulator-based optical telescope for Omega and the Trident laser facilities
Authors: Oertel J. A. , Murphy T. J. , Berggren R. R. , Faulkner J. , Schmell R. , Little D. , Archuleta T. , Lopez J. , Velarde J. , Horton R. F.
Published: July 2002

We have recently designed and are building a telescope which acts as an imaging light collector relaying the image to an optical table for experiment dependent analysis and recording. The expected primary use of this instrument is a streaked optical pyrometer for witness plate measurements of the hohlraum drive temperature. The telescope is based on the University of Rochester’s 10 in. manipulator (TIM) which allows compatibility between Omega, Trident, and the NIF lasers. The optics capture a f/7 cone of light, have a field of view of 6 mm, have a spatial resolution of 5–7 μm per line pair at the object plane, and are optimized for operation at 280 nm. The image is at a magnification of 11.7×, which is convenient for many experiments, but can be changed using additional optics that reside outside the TIM.
ac self-field loss measurement system
Authors: Jansak Lubomil
Published: July 2002

The accuracy and reproducibility of a self-field ac loss measurement technique for high-temperature superconducting tapes using a digital lock-in amplifier are discussed. The influence of the reference phase setting on the error was calculated and verified experimentally for different degrees of inductive component compensation. We estimated that for precise ac loss measurements of uncompensated specimens it is necessary set the reference phase with the precision on the order of 0.001°. For a compensated sample a phase error of 0.1° is tolerable. The measurement error due to common-mode voltage was studied and its minimization is proposed. A test procedure for common-mode voltage error evaluation was also experimentally verified. Ideas for better performance of the measurement system are proposed.
Observation of the transverse second-harmonic magneto-optic Kerr effect from Ni81Fe19 thin film structures
Authors: Crawford T. M. , Rogers C. T. , Silva T. J. , Kim Y. K.
Published: July 2002

We report second-harmonic magneto-optic Kerr measurements on air-exposed, polycrystalline Ni81Fe19 thin films, ranging in thickness from 1 nm to 2 μm, on Al2O3 coated Si (001). For samples thicker than 20 nm, in the transverse Kerr geometry, we observe a factor of 4 change in second-harmonic intensity upon magnetization reversal. For thin samples, we observe interference between second-harmonic fields from the various interfaces and deterioration of ferromagnetism in the 1 and 2 nm films. Modeling suggests that the Ni81Fe19/Al2O3 interface has a larger second-order susceptibility than the air/Ni81Fe19 surface.
Self-organization processes of InGaAs/GaAs quantum dots grown by metalorganic chemical vapor deposition
Authors: Heinrichsdorff F. , Krost A. , Grundmann M. , Bimberg D. , Kosogov A. , Werner P.
Published: July 2002

In0.5Ga0.5As/GaAs (001) quantum dots (QDs) were grown by metalorganic chemical vapor deposition (MOCVD) on exactly (001) oriented substrates using the Stranski–Krastanow growth mode. The dot density and their relative geometrical arrangement are found to be strongly dependent on the substrate temperature. The dots have identical square shaped bases oriented along 〈100〉. For high densities a preferential relative alignment of the dots along the 〈110〉 directions is found. These dots tend to be arranged in a chainlike pattern with decreasing dot size towards the ends of the chains. From these observations the dot formation process for In0.5Ga0.5As quantum dots is suggested to be driven by energetics whereas the relative orientation is governed by kinetic effects.
Radio-frequency-magnetron-sputtered CdS/CdTe solar cells on soda-lime glass
Authors: Shao M. , Fischer A. , Grecu D. , Jayamaha U. , Bykov E. , Contreras-Puente G. , Bohn R. G. , Compaan A. D.
Published: July 2002

We report the fabrication of an 11.6% efficient, polycrystalline thin-film CdS/CdTe solar cell in which both semiconductor layers were deposited by planar-magnetron-radio-frequency sputtering at 380 °C on commercially available soda-lime float-glass substrates coated with SnO2:F. We show that the magnetron magnetic field is critical to obtaining high cell efficiency. Much stronger photoluminescence and higher electrical conductivity are found in films and cells grown with unbalanced-field magnetrons. The magnetic field dependence is interpreted as arising from the enhanced electron and ion bombardment of the film growth interface when unbalanced magnetrons are used.
Nature of Mg impurities in GaN
Authors: Li J. Z. , Lin J. Y. , Jiang H. X. , Salvador A. , Botchkarev A. , Morkoc H.
Published: July 2002

Mg doped GaN epilayers grown by reactive molecular beam epitaxy (MBE) exhibit clear persistent photoconductivity (PPC) whose manifestation has been used to probe the nature of Mg impurities in GaN. PPC buildup and decay transients and the dependence of the PPC decay time constant on the PPC buildup time have been systematically measured and formulated in the context of lattice relaxed Mg impurities (or AX centers). Our results have demonstrated that there is an energy barrier of about 129 meV which prevents free hole capture by ionized Mg impurities and that there is a lattice relaxation associated with Mg impurities in GaN. We also present a detailed comparison for Mg impurities in p-type GaN epilayers grown by MBE (hydrogen-free) and metalorganic chemical deposition (hydrogen rich).
Large photoinduced ferroelectric coercive field increase and photodefined domain pattern in lithium-tantalate crystal
Authors: Chao Shiuh , Hung Chien-Chung
Published: July 2002

We found that the ferroelectric coercive field of LiTaO3, both in forward and reverse directions, can be increased in large amount by high power multiline argon laser illumination. This phenomenon is especially profound when the coercive field measurement is performed closer in time to the previous domain inversion operation. We also demonstrated that the domain in lithium-tantalate crystal can be patterned in accordance with the argon laser light distribution on the crystal plate surface.
Deep-level defects and n-type-carrier concentration in nitrogen implanted GaN
Authors: Haase D. , Schmid M. , Kürner W. , Dörnen A. , Härle V. , Scholz F. , Burkard M. , Schweizer H.
Published: July 2002

We analyzed the intrinsic defects and the n-type-carrier concentration generated by nitrogen ion implantation in n-type GaN by deep-level-transient spectroscopy and by capacitance–voltage measurements, respectively. The samples were grown on sapphire by metalorganic vapor-phase epitaxy. Nitrogen implantation with different ion doses and postimplantation rapid-thermal annealing (RTA) were investigated. We observed a growing n-type-carrier concentration and increasing defect concentration with increasing nitrogen ion implantation doses. After RTA the concentration of free carriers and deep levels as found in the as-grown state are restored. We also address contrarily seeming results from measurements of sheet resistance after N implantation published recently.
Electrostatic trapping of single conducting nanoparticles between nanoelectrodes
Authors: Bezryadin A. , Dekker C. , Schmid G.
Published: July 2002

For molecular electronics, one needs the ability to electrically address a single conducting molecule. We report on the fabrication of stable Pt electrodes with a spacing down to 4 nm and demonstrate a new deposition technique, i.e., electrostatic trapping, which can be used to bridge the electrodes in a controlled way with a single conducting nanoparticle such as a conjugated or metal–cluster molecule. In electrostatic trapping, nanoparticles are polarized by an applied electric field and are attracted to the gap between the electrodes where the field is maximum. The feasibility of electrostatic trapping is demonstrated for Pd colloids. Transport measurements on a single Pd nanoparticle show single electron tunneling coexisting with tunnel-barrier suppression.
Two-photon pumped partially cross-linked polymer laser
Authors: He Guang S. , Kim Kie-Soo , Yuan Lixiang , Cheng Ning , Prasad Paras N.
Published: July 2002

Two-photon pumped (TPP) frequency-upconverted lasing properties of a partially cross-linked fluorescent polymer are reported. This lasing medium is a cross-linked homogeneous copolymer with two major compounds: the chromophore, 4-[bis(2-hydroxyethyl)amino]-N-methylstilbazolium iodide, and the monomer, 2-hydroxyethyl methacrylate. The spectral, temporal, and spatial structures as well as the output/input characteristics of the TPP cavity lasing were measured using a 13 mm long polymer rod. The net conversion efficiency from the absorbed 1064 nm pump pulse energy to the ∼606 nm upconverted cavity lasing energy was found to be ∼4%.
Significantly enhanced flux pinning in the Bi-substituted TlBa2Ca2Cu3Oy system
Authors: Badri V. , Wang Y. T. , Hermann A. M.
Published: July 2002

We have found that substitution of bismuth into the TlBa2Ca2Cu3O9 system leads to a high temperature superconductor with significantly greater low temperature flux pinning than that of the other copper oxide superconductors. Bi substitutes at the Tl site up to 20 at. %. The superconducting transition temperature Tc increases from 110 K for x=0.0 to 116 K for x=0.2. Scanning electron microscope (SEM) images reveal a platelike morphology which is highly suitable for powder-in-tube processing. The magnetization–hysteresis loop shows that the Bi-substituted phase has significantly better low temperature flux pinning properties than the pristine Tl cuprate, and exhibits only a weak dependence on temperature and field when compared to that of unsubstituted phase. However, at 77 K the compound has lower Jc, determined magnetically, compared to that of the (Tl,Pb,Bi)Sr1.6Ba0.4Ca2Cu3Oy and Y-123 systems. The enhancement of superconducting properties is attributed to an increased coupling between the CuO2 planes along the c axis. Substitution of 20% of Bi in TlBa2Ca2Cu3O9 makes the compound an excellent choice for magnet wire applications at relatively low temperatures.
Topography dependent doping distribution in selectively regrown InP studied by scanning capacitance microscopy
Authors: Hammar M. , Rodrı́guez Messmer E. , Luzuy M. , Anand S. , Lourdudoss S. , Landgren G.
Published: July 2002

We have used scanning capacitance microscopy (SCM) to study the dopant distribution in regrown InP with high sensitivity and spatial resolution. Sulfur or iron doped InP was selectively regrown around n-doped InP mesas using hydride vapor phase epitaxy, and the resulting structure was imaged in cross section by SCM. For calibration purposes, reference layers with known doping levels were grown directly on top of the region of interest. Dramatic variations in the carrier concentration around the mesa, as well as pronounced differences in the behavior of S and Fe are observed. We correlate these findings to the growth and doping incorporation mechanisms.
Effect of magnetic field on the superparamagnetic relaxation in granular Co-Ag samples
Authors: Zhang Y. D. , Budnick J. I. , Hines W. A. , Chien C. L. , Xiao J. Q.
Published: July 2002

In order to study the effect of applied magnetic field on the superparamagnetic relaxation behavior of small Co particles, magnetization measurements were carried out on as-prepared and annealed granular samples of Co20Ag80 and Co25Ag75. Values of the superparamagnetic blocking temperature TB− were obtained from the characteristic peak in the zero-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease of TB− with applied magnetic field is quadratic. An estimate of the magnetic anisotropy “energy density” Ku yielded a value which is two orders of magnitude greater than the value for bulk cobalt. The results reported here underscore the importance of considering the effect of superparamagnetic relaxation on the performance of nanostructured magnetic materials.
Accurate measurement of laser-driven shock trajectories with velocity interferometry
Authors: Celliers P. M. , Collins G. W. , Da Silva L. B. , Gold D. M. , Cauble R.
Published: July 2002

We describe a velocity interferometer used to measure the velocity and trajectory of laser driven shocks in liquid deuterium accurately and continuously. This demonstration of velocity interferometry to measure shock velocity and shock trajectory in condensed matter shows strong potential for future studies of laser-driven shocks in transparent media. Accuracy of this technique can be better than 1% in velocity and ±0.2 μm in position during a 10 ns interval.
Study of impurity states in p-type Hg1−xCdxTe using far-infrared spectroscopy
Authors: Li Biao , Gui Yongsheng , Chen Zhanghai , Ye Hongjuan , Chu Junhao , Wang Shanli , Ji Rongbin , He Li
Published: July 2002

This letter reports the far-infrared (FIR) transmission spectra of undoped and Sb-doped p-type Hg1−xCdxTe films grown by a liquid-phase epitaxy (LPE) or molecular-beam epitaxy (MBE) technique. The activation energies of cation vacancy acceptor are found to be ∼10–12 meV and are almost independent on Cd composition. The absorption strength per Hg vacancy, useful for evaluating the cation vacancy density from the absorption spectra, is derived as 3.4×10−12 cm. Further, Zeeman splitting resulting from two different acceptors is observed from magnetotransmission measurement for the Sb-doped LPE sample, and light hole effective mass is estimated. FIR transmission seems to be a powerful tool for nondestructive characterization of impurity states in Hg1−xCdxTe.
Enhanced electro-optic poling in guest–host systems using conductive polymer-based cladding layers
Authors: Drummond James P. , Clarson Stephen J. , Zetts John S. , Hopkins F. Kenneth , Caracci Stephen J.
Published: July 2002

We report on the development of conductive polymer-based cladding layers for use in multilayered electro-optic (EO) device structures. The waveguide claddings used in these studies were based on blends formed from the conductive polymer poly (ethylene dioxythiophene) with poly (vinylalcohol). Thin films of the blended materials were spun to form cladding layers having tunable conductivity and optical absorption. We tested these materials using a well-known guest–host core layer to determine their effect on the polar alignment of second order nonlinear optical chromophores. The claddings tested were shown to provide a large effective poling field across the active layer, and thus produced larger EO coefficients than systems having comparable, but more highly resistive, claddings.
Damage threshold of HgCdTe induced by continuous-wave CO2 laser
Authors: Zhao Jianhua , Li Xiangyang , Liu Hua , Jiang Runqing , Liu Zhaopeng , Hu Zhihan , Gong Haimei , Fang Jiaxiong
Published: July 2002

We studied the interaction of p-type HgCdTe with a continuous-wave (cw) carbon dioxide (CO2) laser experimentally and theoretically and obtained an apparent damage threshold temperature, 670–680 K, which corresponds to the temperature of the solid–solid phase transition of HgCdTe. We proposed a two-dimensional thermal conducting model dealing with the thermal diffusion of cw laser processing of materials in this letter. The temperature distribution during the interaction between HgCdTe and the cw CO2 laser is also presented and discussed. All the theoretical calculations are in good agreement with the experimental results.
Effect of surface proximity on end-of-range loop dissolution in silicon
Authors: Raman R. , Law M. E. , Krishnamoorthy V. , Jones K. S. , Herner S. B.
Published: July 2002

The effect of surface proximity on the dissolution of end-of-range dislocation loops in silicon was investigated by transmission electron microscopy (TEM). A layer of dislocation loops was formed at a depth of 2600 Å by annealing a Si wafer amorphized by a 1015 cm−2, 120 keV, and a 1015 cm−2, 30 keV dual Si+ implant for 30 min at 850 °C. The wafer was diced into 1 cm×1 cm pieces and polished by a chemical–mechanical polishing technique to decrease the loop depth to 1800 and 1000 Å. The samples were then furnace annealed at 900 and 1000 °C in N2 gas. Quantitative TEM analysis revealed that the density of small loops decreases as the loop band is brought closer to the surface. The flux of interstitials to the surface varied inversely with loop depth, indicating that the loop dissolution is diffusion limited. Assuming that the loops maintain a supersaturation of interstitials (CIL) around them, and by integrating the measured interstitial flux from the loop layer to the surface, the relative supersaturation of interstitials near the loop layer (CIL/CI*) was extracted 900 and 1000 °C.
Two-photon absorption in semiconductor saturable absorber mirrors
Authors: Thoen E. R. , Koontz E. M. , Joschko M. , Langlois P. , Schibli T. R. , Kärtner F. X. , Ippen E. P. , Kolodziejski L. A.
Published: July 2002

The nonlinear reflectivity of semiconductor saturable absorber mirrors has been investigated as a function of incident energy fluence. The presence of two-photon absorption in commonly used structures was confirmed via time-resolved differential reflectivity measurements. Theoretical calculations predict that two-photon absorption will expand the continuous-wave mode-locking stability regime against Q-switched mode-locking, yet may simultaneously induce multiple pulses in a laser cavity.
Mechanism for low-temperature photoluminescence in GaNAs/GaAs structures grown by molecular-beam epitaxy
Authors: Buyanova I. A. , Chen W. M. , Pozina G. , Bergman J. P. , Monemar B. , Xin H. P. , Tu C. W.
Published: July 2002

The mechanism for low-temperature photoluminescence (PL) emissions in GaNAs epilayers and GaAs/GaNxAs1−x quantum well (QW) structures grown by molecular-beam epitaxy is studied in detail, employing PL, PL excitation, and time-resolved PL spectroscopies. It is shown that even though quantum confinement causes a strong blueshift of the GaNAs PL emission, its major characteristic properties are identical in both QW structures and epilayers. Based on the analysis of the PL line shape, its dependence on the excitation power and measurement temperature, as well as transient data, the PL emission is concluded to be caused by a recombination of excitons trapped by potential fluctuations in GaNAs.
Time-resolved Raman spectroscopy of polytetrafluoroethylene under laser-driven shock compression
Authors: Wakabayashi Kunihiko , Nakamura Kazutaka G. , Kondo Ken-ichi , Yoshida Masatake
Published: July 2002

Nanosecond time-resolved Raman spectroscopy has been performed to study polymer films, polytetrafluoroethylene, under laser-driven shock compression at about 2.3 GPa. A vibrational line at 1895 cm−1, which is attributed to the symmetric stretching mode of a C2F4 monomer produced by depolymerization, appears under the shock compression, and its intensity increases along the propagation of the shock wave.
High-quantum-efficiency 0.5 eV GaInAsSb/GaSb thermophotovoltaic devices
Authors: Wang C. A. , Choi H. K. , Ransom S. L. , Charache G. W. , Danielson L. R. , DePoy D. M.
Published: July 2002

We report high-performance lattice-matched GaInAsSb/GaSb thermophotovoltaic (TPV) devices with a 0.5 eV band gap. The TPV structures were grown on GaSb substrates by organometallic vapor phase epitaxy at a lower temperature (525 °C compared to 550 °C) to improve the quality of the metastable GaInAsSb alloy. The 0.5 eV TPV devices exhibit external quantum efficiency as high as 60%, which corresponds to an internal quantum efficiency of 90%, assuming 35% reflection losses. This efficiency is comparable to the value measured for 0.53 eV devices. The ratio of the open circuit voltage to band-gap energy ratio decreases from 0.57 for 0.53 eV devices to 0.48 for 0.5 eV devices.
Transition from small interstitial clusters to extended {311} defects in ion-implanted Si
Authors: Coffa Salvatore , Libertino Sebania , Spinella Corrado
Published: July 2002

We have investigated the transition from small interstitial clusters to {311} defects in ion-implanted Si. Czochralski Si wafers were implanted with 1.2 MeV Si ions to fluences in the range 1012–5×1013/cm2 and annealed at temperatures of 600–750 °C for times as long as 15 h. Photoluminescence and transmission electron microscopy analyses allowed us to analyze the transition of small interstitial clusters, formed by the agglomeration of the excess interstitials introduced by the beam, into {311} defects. It is found that {311} defects form only at fluences ⩾1013/cm2 and at temperatures above 600 °C. When {311} are observed in transmission electron microscopy, the luminescence spectrum is dominated by a sharp signal at 1376 nm which has been correlated with optical transitions occurring at or close to these defects. At lower temperatures or at lower fluence, no extended defects are observed in transmission electron microscopy and the luminescence spectrum present two broad signatures arising from carrier recombination at interstitial clusters. These data strongly indicate that a severe structural transformation occurs in the evolution from small interstitial clusters to extended {311} defects.
Control of color and efficiency of light-emitting diodes based on polyfluorenes blended with hole-transporting molecules
Authors: Sainova D. , Miteva T. , Nothofer H. G. , Scherf U. , Glowacki I. , Ulanski J. , Fujikawa H. , Neher D.
Published: July 2002

Adding low-molecular-weight hole-transporting molecules (HTM) with different oxidation potentials to the polyfluorene emission layer of single-layer light-emitting diodes causes significant changes in the device properties. The pronounced increase in luminance efficiency combined with a decrease in current is attributed to significant hole trapping, as further suggested by thermoluminescence experiments. Using a oligo-triphenylamine HTM with an ionization potential of ∼4.9 eV, light-emitting diodes with stable blue emission, a brightness of 800 cd/m2 and an efficiency of 0.87 cd/A were realized. Further, the red-emitting contribution to the spectra as observed in the pure polymer devices could be fully suppressed.
Underwater shock measurements using a ruby pressure gauge
Authors: Pangilinan G. I. , Russell T. P. , Baer M. R. , Namkung J. , Chambers P.
Published: July 2002

The measurement of the temporal profile of the pressure in a shock wave propagating underwater and interacting with a nearby metal plate is presented. Pressures are deduced from the time-resolved fluorescence of a submillimeter ruby crystal mounted on an optical fiber tip. The measured pressures agree with finite-element code predictions, which model the production and propagation of the shock wave. These measurements are useful to evaluate shock-induced phenomena that are strongly dependent on the temporal profile of a shock wave, in small-scale testing, and in substantiating finite-element code predictions.
Cavity expansion resistance of brittle materials obeying a two-curve pressure–shear behavior
Authors: Satapathy Sikhanda S. , Bless Stephan J.
Published: July 2002

We derived a closed-form solution for the pressure required to open a spherical or a cylindrical cavity in brittle materials which demonstrate a two-curve pressure–shear behavior. The material is allowed to crack under tension and fail under shear; only both failure modes result in comminution. Since the cavity expansion pressure is closely related to the penetration resistance of a target material, this solution identifies the material parameters that are important in impact and penetration problems. It is found that cracking and comminution can be prevented when a large enough confinement pressure is present, and the resulting high cavity expansion resistance could explain the intriguing phenomenon of interface defeat. The effects of dilatancy, and shear strength of comminuted ceramic on cavity expansion pressure are explicitly revealed.
In situ measurement of mechanical stress in polycrystalline zinc-oxide thin films prepared by magnetron sputtering
Authors: Hinze J. , Ellmer K.
Published: July 2002

The generation of stress in magnetron-sputtered undoped and aluminum-doped zinc-oxide (ZnO:Al) films has been measured by an in situ technique. Radio-frequency magnetron sputtering from ceramic targets in an argon atmosphere was used for film deposition. The stress was measured by optically detecting the deflection of a thinned silicon cantilever on which the films were deposited. This in situ stress measurement setup was tested by investigating the sputtering pressure dependence of stress in molybdenum films. For zinc-oxide films it was found that by increasing the argon-sputtering pressure during deposition the intrinsic compressive stress decreased by about a factor of 2, both for undoped and doped zinc-oxide films. The undoped films exhibit significantly higher compressive stresses (1–2.5 GPa) compared to the aluminum-doped films (0.7–1.5 GPa). This fact was ascribed to a surfactant-like effect of the dopant aluminum, which improved the crystallinity of the doped films. In contrast to magnetron-sputtered metallic films (like molybdenum) the zinc-oxide films did not show a transition from compressive to tensile stress when the argon-sputtering pressure was increased. It is assumed that this behavior of the zinc-oxide films is due to an additional incorporation of oxygen into the films. By adjusting the oxygen partial pressure during the deposition at a fixed argon pressure, a minimum of the compressive stress in ZnO: Al films was found which coincided with a minimum of the film resistivity. This observation stresses the role of even small changes in deposition parameters on the mechanical and electrical properties of zinc-oxide films. The possibilities of the in situ stress measurement are demonstrated during the deposition of zinc-oxide multilayers.
Optical properties and laser characteristics of highly Nd3+-doped Y3Al5O12 ceramics
Authors: Shoji Ichiro , Kurimura Sunao , Sato Yoichi , Taira Takunori , Ikesue Akio , Yoshida Kunio
Published: July 2002

Diode-pumped laser oscillation in highly Nd3+-doped polycrystalline Y3Al5O12 (YAG) ceramics has been demonstrated. The Nd:YAG ceramics are highly transparent; the loss of a 2.3 at. % neodymium-doped ceramic is as low as that of a 0.9 at. % Nd:YAG single crystal. The high doping of Nd3+ ions realizes large pump absorption; a 6.6 at. %-doped ceramic has an absorption coefficient of 60.4 cm−1 at 808 nm. The same concentration quenching parameter is obtained between the Nd:YAG ceramics and Nd:YAG single crystals. A laser using an 847-μm-thick 3.4 at. % Nd:YAG ceramic as a gain medium operates at 2.3 times higher output power than the same laser with a 719-μm-thick 0.9 at. % Nd:YAG single-crystal gain medium.
Measurements of the electron energy distribution function in molecular gases in a shielded inductively coupled plasma
Authors: Singh Harmeet , Graves David B.
Published: July 2002

A cylindrical Langmuir probe has been used to measure the electron energy distribution function (EEDF) in atomic and molecular gases in a shielded inductively coupled plasma. We report the EEDFs in these gases as a function of pressure. While the electron properties in a discharge depend on the product of the neutral number density (N0) and the effective discharge dimension (deff) for a given gas, this dependence is different for different gases. We find that pressure is a convenient parameter for comparison of the EEDFs in these gases. The EEDFs in inert (Ar, Kr, Xe) and molecular gases (H2,N2,O2,H2O,CO2,CF4) in the low pressure limit (below 1 mTorr) show a “three-temperature” structure. Since this wide range of gases display similar EEDF shape, we propose this structure to be common to all gas discharges in this limit. The EEDF in all of the gases shows a two-temperature structure with apparent tail depletion at 3 mTorr. The similarity of the EEDFs in all of the above gases is probably due to nonlocality of the electrons at these low pressures. The molecular gases exhibit a nearly Maxwellian EEDF between about 10 and 30 mTorr, while the EEDF in argon is non-Maxwellian in this range. At pressures above 30 mTorr, the EEDFs in molecular gases show deviations from a Maxwellian distribution, reflecting the electron-neutral collision cross sections of each gas. The EEDFs in molecular gases at 100 mTorr show significant deviations from a Maxwellian distribution. We find that the EEDF in molecular gases can be approximated by a Maxwellian distribution over a fairly large pressure range of 3–50 mTorr for the purposes of modeling these discharges.
Critical currents in YBa2Cu3O7−x high-temperature superconducting thin films irradiated by 4-MeV electrons
Authors: Fedotov Yu. V. , Ryabchenko S. M. , Shakhov A. P.
Published: July 2002

A decrease in the critical current density Jc of a YBa2Cu3O7−x thin film is observed after its irradiation by 4-MeV electrons. It is shown that the temperature dependence of Jc agrees with the idea of a granular structure of the film, with intergranular contacts of the superconductor–metal–insulator–superconductor type.
SiO
2
-passivated lateral-geometry GaN transparent Schottky-barrier detectors
Authors: Adivarahan V. , Simin G. , Yang J. W. , Lunev A. , Khan M. Asif , Pala N. , Shur M. , Gaska R.
Published: July 2002

We report on a transparent Schottky-barrier ultraviolet detector on GaN layers over sapphire substrates. Using SiO2 surface passivation, reverse leakage currents were reduced to a value as low as 1 pA at 5 V reverse bias for 200 μm diameter device. The device exhibits a high internal gain, about 50, at low forward biases. The response time (about 15 ns) is RC limited, even in the internal gain regime. A record low level of the noise spectral density, 5×10−23 A2/Hz, was measured at 10 Hz. We attribute this low noise level to the reduced reverse leakage current.
A theoretical study of the chemical vapor deposition of (100) diamond: An explanation for the slow growth of the (100) surface
Authors: Kang Jeung Ku , Musgrave Charles B.
Published: July 2002

In this investigation we use B3LYP density functional theory (DFT) to investigate the CVD growth mechanism of (100) diamond. Our results are consistent with the Garrison mechanism in which the dimer-opening step involves simultaneous formation of a surface olefin and dissociation of the dimer. We calculate this step to have a barrier of 9.6 kcal/mol. The olefin is then attacked by a surface radical to form a six-membered ring. We find this reaction to be the rate-limiting step with an activation energy of 13.6 kcal/mol. This is in excellent agreement with the experimental value of 15 kcal/mol obtained by the selective growth method and XPS. The direct ring-opening and ring-closing reaction from adsorbed CH2 radical has an activation energy of 49.4 kcal/mol and does not contribute significantly to the growth rate. The barrier on larger clusters that include the effects of neighboring adsorbed hydrogen increases to 15.6 kcal/mol. Additionally, our calculated vibrational frequencies agree within 2% of experimental IR and HREELS spectra.
Detection of volatile organic compound vapors by using a vapochromic material on a tapered optical fiber
Authors: Bariáin C. , Matı́as I. R. , Romeo I. , Garrido J. , Laguna M.
Published: July 2002

A low-cost optical fiber sensor to detect volatile organic compounds has been developed. Changes of up to 13.5 dB in the transmitted optical power have been detected with different concentrations of acetone and dichloromethane vapors. The device uses a standard single-mode fiber. The sensing mechanism relies on a vapor-induced refractive index change in a film of a vapochromic material deposited on the thinner region of a tapered fiber.
Properties of (Nb0.35, Ti0.15)xNi1−x thin films deposited on silicon wafers at ambient substrate temperature
Authors: Iosad N. N. , Mijiritskii A. V. , Roddatis V. V. , van der Pers N. M. , Jackson B. D. , Gao J. R. , Polyakov S. N. , Dmitriev P. N. , Klapwijk T. M.
Published: July 2002

We have studied the properties of (Nb0.35, Ti0.15)xN1−x films deposited by reactive magnetron sputtering at ambient substrate temperature, focusing in particular on the dependence of film properties on the total sputtering pressure. As the pressure increases we observe a transition in the film structure from the ZT to the Z1 structural zone according to the Thornton classification. In general, the superconducting transition temperature (Tc) and residual resistance ratio have a very moderate dependence on total sputtering pressure, while the film resistivity increases an order of magnitude as the sputtering pressure increases. A wide spectrum of material science techniques is used to characterize the films and to explain the relationship between the sputtering conditions and film properties. Transmission electron microscopy and x-ray diffraction analysis show that 160-nm-thick (Nb0.35, Ti0.15)xN1−x films consist of 20–40 nm grains with good crystallinity. Films sputtered under low pressures have a weak [100] texture, while films sputtered under high pressures have a distinct [111] texture. A stable chemical composition and reduction in film density as the sputtering pressure increases indicate that the change of resistivity in the ZT structural zone is due to a variation in the quenched-in vacancy concentration. In contrast voids on the grain boundaries and vacancies together produce the high film resistivities in the Z1 structural zone.