bluuebunny/crossref_metadata_2025 huggingface dataset:
Focused ion-beam line profiles: A study of some factors affecting beam broadening
Authors: Templeton I. M. , Champion H. G.
Published: July 2002

The current–density profile of a focused ion beam (FIB) has a central peak accompanied by broader ‘‘wings’’ that, while unimportant in lithographic applications, can lead to unwanted effects during an implantation operation. The origin of the wings, and hence the best way to minimize them, is not clear and needs further study. We have measured the line profiles of several of the ions available in our FIB machine as a function of a number of variables, under ultrahigh vacuum (UHV) conditions. No effects are observed from changes in emission current or deliberate defocusing of the objective lens. There are some changes with beam aperture and/or current, but the biggest differences seem to be associated with a change of source type and hence, possibly, with a change in the source/extractor configuration or in the alloy and the emission process. The wing amplitudes are appreciably lower than many previously observed, and their profiles, at least for the lighter ions studied (Be++,  Be+, and B+), are Gaussian rather than exponential. It seems possible that our UHV conditions may have eliminated a scattering mechanism responsible for the larger, exponential wings previously observed. The corresponding beam and rectangle-edge profiles have been calculated.
Dose rate and thermal budget optimization for ultrashallow junctions formed by low-energy (2–5 keV) ion implantation
Authors: Craig M. , Sultan A. , Reddy K. , Banerjee S. , Ishida E. , Larson L.
Published: July 2002

High implant dose rates and two-step anneals have been used to optimize ultrashallow junctions (∼60 nm) formed by low-energy (2–5 keV) BF2 ion implantation. Variation in junction depth and dopant profile as a function of beam current density (dose rate) and damage/activation anneal is studied. Junction depths and transient enhanced boron diffusion, as seen from secondary ion mass spectroscopy, are reduced for increasing implant dose rate. In addition, transmission electron microscopy images indicate that implantation at a higher dose rate yields lower residual defect levels following anneal. Dependence of sheet resistance on dose rate seems to be a function of the amorphizing nature of the implant. At a dose of 1×1014 cm−2, use of a low-temperature, 600 °C furnace preanneal, prior to a high-temperature rapid thermal anneal, serves to reduce transient enhanced diffusion. At a dose of 1×1015 cm−2, subtle changes in residual damage are observed while no apparent changes in transient enhanced diffusion or sheet resistance are seen.
Film property comparison of Ti/TiN deposited by collimated and uncollimated physical vapor deposition techniques
Authors: Wang Shi-Qing , Schlueter J.
Published: July 2002

The film properties of single layers of Ti and TiN and bilayer Ti/TiN film stacks sputtered with and without a collimator on SiO2 substrates were studied and compared by Rutherford backscattering spectrometry, Auger electron spectroscopy, x-ray diffractometry with both glancing angle and θ–2θ coupled diffraction techniques, scanning electron microscopy, transmission electron microscopy of both cross section and plan view, and atomic force microscopy. The properties investigated include color, stoichiometry, density, electrical resistivity, grain and grain boundary structure, preferred orientation (texture), stress, surface morphology (roughness), reflectance, and impurity level. It was found that the density was higher, the surface morphology was less rough, and the electrical resistivity was lower for the collimated films than for the uncollimated films. The lower density and rougher surface of the uncollimated films are caused by shadowing effects.
Metal based single electron transistors operating at several Kelvin
Authors: Altmeyer S. , Hamidi A. , Spangenberg B. , Kurz H.
Published: July 2002

Metal based single electron transistors fabricated by the step edge cut off process will be presented. Although standard electron beam lithography, dry etching and lift-off techniques were used, devices with tunnel capacitances down to 1.5 aF are realized. They are characterized at different temperatures ranging from 50 mK up to 77 K. Transistor operation at 4.2 K is demonstrated. Traces of the Coulomb blockade are observed up to 77 K.
Shearing stress on the surface topography by scanning shearing stress microscopy
Authors: Iwata Futoshi , Sasaki Akira , Kawaguchi Makoto , Katsumata Akira , Aoyama Hisayuki
Published: July 2002

A scanning shearing stress microscopy has been developed in order to investigate shearing stress effect in the surface. This microscopy is based on scanning tunneling microscopy and makes it possible to measure shearing stress. The sample is coupled on the AT-cut quartz resonator which is oscillated at its resonance frequency, and the shift of the resonant frequency corresponding to the strength of the shearing stress in the sample can be measured. Gold thin films have been observed for the surface topography as the image of scanning tunneling microscopy and also the distribution of subsurface shearing stress is identified, simultaneously. The strong shearing stresses are generated at the hollow parts of the surface topography. These stresses are caused by the frictional force between the scanning tunneling microscope tip and the sample surface which is oscillated laterally.
Comparison of the physical and electrical properties of electron cyclotron resonance and distributed electron cyclotron resonance SiO2
Authors: Firon M. , Hugon M. C. , Agius B. , Hu Y. Z. , Wang Y. , Irene E. A.
Published: July 2002

A comparison was made of thin films of silicon dioxide deposited, at floating temperatures, using electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) and distributed electron cyclotron resonance plasma enhanced chemical vapor deposition (DECR PECVD). The refractive index, composition, and chemical bonding of the plasma oxides were determined by null and spectroscopic ellipsometry, nuclear reaction analysis, and Fourier transform infrared spectroscopy and were compared with thermal oxides. The damaged layer at the Si/SiO2 interface resulting from ECR and DECR techniques was evaluated by spectroscopic ellipsometry. Finally, high frequency and quasi-static capacitance voltage characteristics and ramped current voltage measurements were performed to determine the electrical properties of the ECR and DECR deposited silicon oxide. Device quality SiO2 thin films have been prepared using both deposition techniques: low interface state density [5×1010 eV−1 cm−2 (ECR) or 2.5×1010 eV−1 cm−2 (DECR)], and high critical field [5.2 MV/cm (ECR) or 6 MV/cm (DECR)] have been achieved.
Stressing and high field transport studies on device-grade SiO2 by ballistic electron emission spectroscopy
Authors: Ludeke R. , Wen H. J. , Cartier E.
Published: July 2002

The tip of a scanning tunneling microscope was used to inject hot electrons across the gate and into the oxide of a metal-oxide-semiconductor structure. This method, known as ballistic electron emission microscopy (BEEM), allows an arbitrary choice of the energy of the injected electrons, which may be further accelerated by the application of a gate bias. The high current densities and choice of energy make BEEM an attractive method to study hot electron transport and breakdown phenomena in dielectrics. The studies reported here were made on Pd/SiO2/Si(100) structures with a SiO2 layer thickness of 3.8 nm. Monte Carlo techniques were used to calculate the spreading of the electron beam as it traverses the oxide. A strong dependence of the spreading on the kinetic energy and oxide thickness were observed. Using the calculated beam spreads to determine current densities and injected charge densities, the charge-to-breakdown (Qbd) was measured for several breakdown sequences. The Qbd’s consistently exceeded by several orders of magnitude the values obtained by conventional Fowler–Nordheim (FN) tunnel injection under high field conditions. Most of the time, breakdowns could not be achieved for 3.8 nm oxides. It is concluded that impurity/defects still control all observed breakdowns; an intrinsic limit—although claimed to have been reached—has not yet been established. Changes in the BEEM spectra with injected electron charge are interpreted in terms of a three stage process to breakdown: (I) electron trap creation and filling at the SiO2–Si interface, (II) prebreakdown believed to occur through thinning of the oxide that starts at the oxide-Si interface, and (III) oxide punch-through, characterized by an injection threshold ≲1 V, close to that for Si. The role of hot hole injection into the SiO2 valence band was also assessed and deemed a negligible factor in the degradation process under the zero or low oxide biases used in the experiment reported here.
Investigation of GaN deposition on Si, Al2O3, and GaAs using in situ mass spectroscopy of recoiled ions and reflection high-energy electron diffraction
Authors: Taferner W. T. , Bensaoula A. , Kim E. , Bousetta A.
Published: July 2002

In situ mass spectroscopy of recoiled ions (MSRI) and reflection high-energy electron diffraction (RHEED) are used to monitor the surface structure, and the near surface layer composition during the pre-growth preparation step and the deposition of GaN thin films on Si (001), Al2O3 (0001), and GaAs (001) substrates by electron-cyclotron-resonance (ECR)-assisted molecular beam epitaxy. Due to the high lattice mismatch between GaN thin films and these substrates, the initial nucleation step is found to be critical on the resulting thin film properties. Nitridation of the clean Al2O3 (0001) and GaAs(001) surface is observed by RHEED, and the formation of an AlN/GaN overlayer is confirmed by MSRI. During GaN deposition on Si, Al2O3, and GaAs, MSRI and RHEED are used to follow the growth mode and coverage. If a RHEED pattern from the nucleating GaN overlayer has a cubic or wurtzitic symmetry and the MSRI spectra—which is sensitive to the first few monolayers—shows the presence of Ga and N, only then the growth is layer by layer. Scanning electron microscopy micrographs also show a smooth and continuous surface. If an amorphous RHEED pattern is observed and the presence of the Si (Al, O in the case of Al2O3 and As in the case of GaAs) signal is detected in addition to Ga and N, the growth is columnar. In conclusion, we show that RHEED and MSRI form a powerful combination of thin film surface characterization tools ideally suited for real-time optimization of substrate preparation and thin film growth parameters for the nitride materials system.
Bayard–Alpert vacuum gauge with microtips
Authors: Baptist R. , Bieth C. , Py C.
Published: July 2002

The Bayard–Alpert ionization gauge is one of the most commonly used gauges for the measurement and control of high and ultrahigh vacua (UHV). The electrons that ionize the residual gas are produced by a hot filament. The replacement of this source by field-effect microtips would overcome many problems caused by thermoionic emission, such as degassing, high power consumption, and mechanical weakness. The results of the experiments involving the replacement of the tungsten filament by an array of microtips fabricated at LETI in a classical Bayard–Alpert gauge are presented. They show that microtips could be a good substitute for filaments in this application, and also they improve some characteristics such as the sensitivity of the gauge and the width of the working scale. Moreover, new options such as fast pulsed operation or fast security cutout modes are possible due to the very short time needed to stabilize the electronic emission. However, an improvement of the tip’s material is necessary if the gauge is intended to work continuously at pressures above 10−5 mbar.
Benefits of the lateral resistor in a field effect display
Authors: Levine Jules D.
Published: July 2002

A protective resistor in series with a microtip is known to homogenize field emission. It is not widely recognized, however, that this protective resistor must be fail-safe as well, since the microtips are exposed to rare but potentially destructive events during life. This article addresses two proposed protective resistor concepts: (a) a vertical resistor located locally underneath each microtip, and (b) a lateral resistor radially distributed from the center of each microtip. It is shown by electrical and thermal analysis that the lateral resistor is significantly more fail-safe than the vertical resistor by factors of about 10× and 100×, respectively.
Surface structure of 3C–SiC(111) fabricated by C60 precursor: A scanning tunneling microscopy and high-resolution electron energy loss spectroscopy study
Authors: Hu C.-W. , Kasuya A. , Suto S. , Wawro A. , Nishina Y.
Published: July 2002

The surface composition and structure of the 3C–SiC(111) surface prepared by thermal reaction of C60 molecules with the Si(111) substrate are investigated with scanning tunneling microscopy and high-resolution electron energy loss spectroscopy. Three distinct surface reconstructions are observed by STM on the SiC(111) surfaces as the reaction temperature varied from 800 to 1200 °C. The (2×2) and (2×3) reconstructions observed under low reaction temperature (<900 °C) are considered to be a C cluster-covered surface. The (3×3) structure yielded at the elevated temperature (1100 °C) is believed to be the Si-terminated 3C–SiC(111) surface. A transient (4×3) structure shows up at around 1000 °C during the annealing processing. The surface composition and reconstruction are further confirmed through the optical Fuchs–Kliewer surface phonon measured in situ by high-resolution energy loss spectroscopy. The diffusivity of Si atoms through SiC film at various temperatures is suggested as the main reason for the formation of different surface reconstructions.
Cluster size measurement using microtrench in a thermal plasma flash evaporation process
Authors: Takamura Yuzuru , Hayasaki Kei , Terashima Kazuo , Yoshida Toyonobu
Published: July 2002

Soft-vacuum plasma processes have been increasing in importance for high-quality, large-area, and high-rate fabrication of thin films for practical applications. In such processes, the characteristics of the deposited species (considered to be clusters) have been of interest. However, even their size has not been sufficiently investigated because of the many difficulties arising due to the high pressure conditions and the steep temperature and concentration gradients in the boundary layer, especially when the species change easily during measurements. In this work, we describe a new method of using a microtrench, to determine the mean cluster size dc, diffusivity D, and sticking probability η without any disturbances to the plasma or the boundary layer conditions. Using this method, the cluster size in the deposition of YBa2Cu3O7−x high-Tc superconductor by plasma flash evaporation is successfully measured as 1–9 nm, with the parameters of source powder feeding rate from 60 to 350 mg/min, torch substrate distance from 270 to 360 mm in 200 Torr, 50 kW 95%O2-5%Ar inductively coupled rf plasma.
Intensification of optical electric fields caused by the interaction with a metal tip in photofield emission and laser-assisted scanning tunneling microscopy
Authors: Hagmann Mark J.
Published: July 2002

Numerical simulations of the interaction of optical fields with metal tips, such as in photofield emission and laser-assisted scanning tunneling microscopy, show that the electric field at the apex of the tip is much greater than that in the incident optical beam. Calculations were made using the method of moments with two models of the tip; a conical frustrum with hemispherical end caps, and a paraboloid. A large number of subvolumes (850–1000) was used to decrease the errors caused by finite matrix size with the method of moments, and it is shown that these errors have no significant effect on the results. The ratio of the optical electric field at the apex of the tip to that in the incident beam (field ratio) varies directly with the curvature at the apex, and is increased by raising the tip length and the optical wavelength. For the range of parameters used in the examples field ratios of 3–15 dB were found for incidence normal to the tip axis, and values as large as 32 dB were found for an angle of approximately 10° between the incident laser beam and the tip axis. Differences between the results for the two models and the dependence on the radius of curvature show that the field ratio must vary greatly from tip to tip.
Modulation of the current in a field emitter caused by a continuous wave or pulsed laser: Simulations and experimental results
Authors: Hagmann Mark J. , Brugat Manuel
Published: July 2002

Numerical simulations suggest that a laser would increase field emission current due to a resonant interaction in which tunneling electrons exchange quanta with the laser, and this has been confirmed experimentally. Floquet methods are used to determine the steady-state response to a cw laser, and the transient response to a laser pulse is computed with a product formulation that does not require absorbing boundary conditions or wave packets. Resonances in the steady-state response typically result in a single broad peak when the full distribution of energies in a metal are considered. The transient solutions show delays equal to the semiclassical tunneling time, which is defined as the time for traversing the inverted barrier. A 1 MHz tunneling current of 0.4 nA was measured when a tungsten field emitter with a dc current of 5 μA was illuminated with a laser diode amplitude modulated at 1 MHz. The laser increases the dc tunneling current by 270 nA, which is attributed to tip heating, but the rf current has a period much less than estimates of the thermal relaxation time.
Neural network model for global alignment incorporating wafer and stage distortion
Authors: Ghazanfarian Amir A. , Pease R. F. W. , Chen Xun , McCord Mark A.
Published: July 2002

One of the most crucial emerging challenges in lithography is achieving rapid and accurate alignment under a wide variety of conditions brought about by the different overlying films occluding the marks. The problem is exacerbated by planarizing processes such as chemical mechanical polishing that reduce the topographical contrast used to view the marks and by distortion of the wafer and of the stage. Thus, an effective learning process is needed to rapidly acquire the best possible positional information from an array of the marks across the wafer. In this article, a neural network model for global alignment is described. The wafer and stage distortions can be incorporated into the model. The algorithm finds the best fit for the wafer distortion and at the same time compensates for both stage and wafer distortion. The algorithm can also learn to identify and ignore any alignment marks that yield significantly erratic signals. A few common distortion functions will also be used to test the model. Preliminary simulation results show alignment errors <5 nm in the presence of Gaussian noise with σ=30 and 500 nm sinusoidal stage distortion. Also the simulations show that increasing the magnitude of stage distortion has no impact on the results. It will be shown that considering wafer distortion can reduce the alignment error more than 10 times in the case of bowed and distorted wafers.
Macroscopic and microscopic studies of electrical properties of very thin silicon dioxide subject to electrical stress
Authors: Daniel E. S. , Jones J. T. , Marsh O. J. , McGill T. C.
Published: July 2002

The electrical characteristics of various size tunnel switch diode devices, composed of Al/SiO2/n-Si/p+-Si layers, which operate with a range of parameters (such as current densities in excess of 104 A/cm2) that stress the oxide layer far beyond the levels used in typical thin oxide metal-oxide semiconductor research have been examined. It is found that the first time a large current and electric field are applied to the device, a “forming” process enhances transport through the oxide in the vicinity of the edges of the gate electrode, but the oxide still retains its integrity as a tunnel barrier. The device operation is relatively stable to stresses of greater than 107 C/cm2 areally averaged, time-integrated charge injection. Duplication and characterization of these modified oxide tunneling properties was attempted using scanning tunneling microscopy (STM) to stress and probe the oxide. Electrical stressing with the STM tip creates regions of reduced conductivity, possibly resulting from trapped charge in the oxide. Lateral variations in the conductivity of the unstressed oxide over regions roughly 20–50 nm across were also found.
Ion absorbing stencil mask coatings for ion beam lithography
Authors: Wasson J. R. , Torres J. L. , Rampersad H. R. , Wolfe J. C. , Ruchhoeft P. , Herbordt Martin , Löschner H.
Published: July 2002

The implantation of ions into the silicon membrane masks of ion beam lithography is known to create compressive stress which leads to very serious distortion after only a few tens of exposures. In this article, we describe a new, dimensionally stable, protective coating for silicon membranes which meets the exacting requirements of very large scale integrated manufacturing. The coating is formed by first depositing a low density, low stress graphitic carbon film which is subsequently amorphized by He+ ion bombardment with a dose of about 125 mC/cm2. We show that the stress of these bilayer membranes is stable to within experimental error (±1 MPa) for 20 keV He+ ion doses up to 0.53 C/cm2, corresponding to about 8.5 million exposures in a 4×-ion projector.
High-resolution damage depth profiles of unannealed sub-100 nm B+ implants in (100) silicon
Authors: Hartford C. L. , Hillard R. J. , Mazur R. G. , Foad M. A.
Published: July 2002

Si (100) wafers were boron implanted using an xRLEAP ion implanter at energies of 0.2, 1.0, and 3.0 keV all at a dose of 1×1014 cm−2. Advanced spreading resistance (SR) measurements were performed on the as-implanted samples to obtain high-resolution profiles of the damaged layer produced during implantation. The measured spreading resistance was highest at, or near, the surface on all samples. The 1.0 and 3.0 keV implants also showed minima in the SR near the substrate. The as-implanted samples were examined with secondary ion mass spectrometry to determine the dopant profile. Forward and reverse Schottky I–V measurements were also done on the same as-implanted samples with a mercury probe. The forward Schottky I–V curves for the 0.2 and 1.0 keV samples showed nonideal Schottky diode behavior. The 3.0 keV sample was similar to that of a good quality Schottky diode and had an ideality factor of 1.28. The reverse Schottky I–V data were processed to obtain the effective dielectric constants for each sample; a neutron transmutation-doped (NTD) wafer was used as a reference. The dielectric constants were 0.77, 1.467, 2.797, and 10.26 for the 0.2, 1.0, and 3.0 keV and NTD samples, respectively.
Generating ∼90 nanometer features using near-field contact-mode photolithography with an elastomeric phase mask
Authors: Rogers John A. , Paul Kateri E. , Jackman Rebecca J. , Whitesides George M.
Published: July 2002

This article describes a near-field photolithographic method that uses an elastomeric phase mask in conformal contact with photoresist. The method is capable of generating ∼90 nm lines in commercially available photoresist, using broadband, incoherent light with wavelengths between 330 and 460 nm. Transfer of these patterns into silicon dioxide and gold demonstrates the integrity of the patterned resist.
Hot-carrier effects on the scattering parameters of lightly doped drain n-type metal–oxide–semiconductor field effect transistors
Authors: Kwan W. S. , Deen M. J.
Published: July 2002

The latest evolution in complementary metal–oxide–semiconductor technology has made the metal–oxide–semiconductor field effect transistor (MOSFET) a viable choice for rf applications, especially for frequencies in the low GHz region. However, hot-carrier effects should also be considered carefully when the devices are operating in the GHz regime. Here, we studied the effects of dc hot-carrier stress on lightly doped drain (LDD) n-type MOSFET (NMOSFET) high frequency performance by measuring and simulating its s parameters. This is the first time, to the author’s best knowledge, that such experiments are reported. We demonstrated clearly the effects of hot-carrier stressing on LDD NMOSFETs by giving representative s parameter and noise measurement results from a 0.8 μm long device. We showed that hot-carrier stress can significantly degrade both s parameters and noise of NMOSFETs, and thus can have considerable consequences for circuit designers. Therefore, these effects should be carefully considered when using MOSFETs in high frequency analog circuits. Unfortunately, both MEDICI and SPICE simulation could not satisfactorily model the LDD MOS structure after hot-carrier stress. Various results indicated that the current MEDICI platform is not very consistent for ac simulation, although dc simulation is very good. SPICE simulation showed very promising results when modeling the changes in S12 and S21 due to hot-carrier stress, yet the results for S11 and S22 were not very good. This deficiency implies that a better small-signal model for LDD MOS structures would be necessary for SPICE to be useful in modeling hot-carrier effects on MOSFET high frequency performance.
Molecular beam epitaxy grown InGaN multiple quantum well structures optimized using in situ cathodoluminescence
Authors: Van Hove J. M. , Chow P. P. , Wowchak A. M. , Klaassen J. J. , Hickman R. , Polley C.
Published: July 2002

In this work, in situ cathodoluminescence (CL) is presented as a technique to optimize the molecular beam epitaxy (MBE) growth conditions for InGaN films and structures. InGaN was grown at 1 μm/h using a reactive nitrogen rf plasma source at substrate temperatures ranging from 550 to 650 °C. The quick determination of the emission wavelength and quality from the peak position and width allowed various growth conditions and structures to be tried without removal of the sample from the MBE system. CL scans are presented from samples grown under varying Ga/In flux ratios, III/nitrogen flux ratios, and substrate temperatures showing the usefulness of in situ monitoring for MBE InGaN growth.
Temperature stabilized 1.55 μm photoluminescence in strained GaxIn1−xAs quantum wire heterostructures
Authors: Wohlert D. E. , Moy A. M. , Chou L. J. , Cheng K. Y. , Hsieh K. C.
Published: July 2002

We present two techniques for manipulating the peak photoluminescence wavelength towards ∼1.55 μm, from the usual 1.61 μm, of strained GaxIn1−xAs quantum wire (QWR) heterostructures. The QWR samples have been prepared by the strain-induced lateral-layer ordering process during molecular beam epitaxy utilizing short-period superlattices of (GaAs)m/(InAs)n. The subscripts m and n refer to the number of deposited monolayers of GaAs and InAs, respectively. In the first approach, for some cases of m>n, the QWRs will contain more Ga thereby decreasing the 300 K wavelength towards 1.55 μm provided the strain is not too great. The second approach relies on post-growth annealing to shift the 300 K peak emission. For anneals performed at 650 °C for 3–5 h, 300 K wavelengths from 1.55 to 1.59 μm have been attained. Moreover, all of these samples display a unique behavior of peak PL with respect to temperature. Some samples show no net shift in wavelength over a range of 77–380 K. Other samples have 77 K wavelengths longer than their 300 K wavelengths. It is believed that these structures have the potential to be processed into temperature stabilized Fabry–Pérot lasers emitting at 1.55 μm.
Optical probing of flip chip packaged microprocessors
Authors: Paniccia Mario , Rao R. M. , Yee Wai Mun
Published: July 2002

A method to optically probe flip chip packaged complementary metal–oxide–semiconductor (CMOS) microprocessors is described. The technique utilizes an infrared laser with a wavelength of 1064 nm to probe the electric field and the free carrier induced absorption modulation that occurs in a reverse biased P-N junction. This absorption modulation is related directly to the voltage across the junction. The mode-locked laser, which generates a train of 35 ps optical pulses at a repetition rate of 100 MHz, is focused through the heavily doped silicon onto the diffusion regions of the CMOS chip. The small modulations in laser power riding on the reflected optical beam are detected and recovered to measure the voltage across the junction. Time resolution is achieved by making the measurement stroboscopically, by phase locking the mode-locked laser to the tester driving the chip. Results from the flip chip packaged microprocessor have been measured and are presented.
Microdisk laser structures for mode control and directional emission
Authors: Backes S. A. , Cleaver J. R. A. , Heberle A. P. , Köhler K.
Published: July 2002

GaAs microdisk laser structures with modified resonator geometry have been investigated. Device fabrication employs electron-beam lithography and etch procedures for generating disks with notches, projections, and holes, and a novel ring-laser structure. Directional emission from the notched microdisk lasers has been investigated using external scattering structures, which may also be used as couplers for the laser emission.
Strain in AlGaN layer studied by Rutherford backscattering/channeling and x-ray diffraction
Authors: Wu M. F. , Yao Shude , Vantomme A. , Hogg S. M. , Langouche G. , Li J. , Zhang G. Y.
Published: July 2002

An AlGaN layer with good crystalline quality (χmin=2.1%) was grown by metalorganic vapor phase epitaxy on a sapphire (0001) substrate using a GaN intermediate layer. The Al composition, which cannot be unambiguously determined by x-ray diffraction (XRD) or by photoluminescence, was determined by Rutherford backscattering (RBS). The perpendicular and parallel elastic strain of the Al0.28Ga0.72N layer, e⊥=−0.16% and e∥=+0.39%, respectively, were derived using XRD and RBS/channeling. The small ratio |e⊥/e∥|=0.41 indicates that the Al0.28Ga0.72N lattice is much stiffer in the c-axis direction than in the a-axis direction. A comparison of the strain data for GaN, InGaN, and AlGaN layers is presented.
Extreme ultraviolet emission of laser-produced plasmas using a cryogenic xenon target
Authors: Schriever G. , Bergmann K. , Lebert R.
Published: July 2002

In this article we show that laser-produced plasmas using a cryogenic xenon target are emitters of pulsed broadband extreme ultraviolet radiation with an intensity maximum around 11 nm. The photon flux and conversion efficiency (CE) of this source is comparable to the emission of laser-produced plasmas of solid state targets using elements with high atomic numbers. The CE of the laser-produced xenon plasma in a 2% wavelength band at the beryllium absorption K edge at 11.1 nm is about two times higher compared to the CE at the silicon absorption L edge at 12.4 nm.
Scanning tunneling microscope study of diamond films for electron field emission
Authors: Rakhimov A. T. , Suetin N. V. , Soldatov E. S. , Timofeyev M. A. , Trifonov A. S. , Khanin V. V. , Silzars A.
Published: July 2002

An experimental method has been developed for measuring the emission parameters of diamond-like materials using a scanning tunneling microscope modified for both tunneling and emission regimes. With this technique, nanodiamond films with emission site densities as high as 106 cm−2 were investigated. The emission sites were found to be located on the grain slopes or in the areas between grains and consist of multiple smaller emission points.
Electron emission performance of nitrogen-doped hydrogen-free diamond-like carbon coating on Mo-Tip field emitter arrays
Authors: Jung Jae Hoon , Lee Nam Yang , Jang Jin , Oh Myung Hwan , Ahn Saeyoung
Published: July 2002

Nitrogen gas phase doped diamond-like carbon (DLC) coating on molybdenum-tip field emitter arrays (FEAs) has been used to improve electron emission characteristics. The conventional plasma enhanced chemical vapor deposition method using a layer-by-layer technique was applied to deposit N-doped hydrogen-free DLC films. Electron emission characteristics were measured and the possibility of applying these type of emitters to high frequency field emission displays was reported. The turn-on voltage of Mo-tip FEAs was decreased from 75 to 55 V by DLC coating and emission current was more stable than that of conventional Mo-tip FEAs. The transconductance of Mo-tip FEAs was increased by N-doped hydrogen-free DLC coating with lowering effective work function of emitters.
Imaging of soft structures: Dependence of contrast in atomic force microscopy images on the force applied by the tip
Authors: Teschke O. , Ceotto G. , de Souza E. F.
Published: July 2002

Forces acting on atomic force microscope tips during scanning of films of ionic surfactant molecules adsorbed from aqueous solutions onto hydrophilic substrates are measured. Near critical micellar concentration images of mica substrates show aggregate regions at the interface. Force versus distance curves indicate that aggregates are the thickest adsorbed structures on the substrate. However, topographic images registered at low scanning speed (15 μm/s) show that these aggregates appear as holes, consequently observed as inverted in contrast images. In atomic force microscope imaging of soft structures such as surfactants or biological material, inverted images may be observed when the tip penetrates the scanned layers. This penetration can be adjusted by changing the force applied by the tip, which results in different images. In order to obtain the conventional atomic force microscope contrast in scanned images the applied force set point is determined by the analysis of the force versus distance curves.
Arrays of field emission cathode structures with sub-300 nm gates
Authors: Bernhardt A. F. , Contolini R. J. , Jankowski A. F. , Liberman V. , Morse J. D. , Musket R. G. , Barton R. , Macaulay J. , Spindt C.
Published: July 2002

A novel field emission cathode process has been developed to produce cathode arrays with individual emitter structures having gates with <300 nm diameters. Ion tracking lithography was utilized to pattern submicron features, which can be controlled over the range 30–300 nm, and to create self-aligned and nanosized, gated emitter structures. Nanocone emitter tips were deposited into the gate structure using a variation of the Spindt process. Field emitter arrays having ∼300  nm gate diameters and an emitter density of 108/cm2 exhibited a current density of 4  mA/cm2 for a 45 V gate bias. This ion tracking lithographic approach is suitable and scalable for large flat panel video displays and appears to be commercially viable.
Growth and nonlinear optical properties of GaAs absorber layers for AlGaAs/CaF2 semiconductor saturable absorber mirrors
Authors: Schön S. , Haiml M. , Achermann M. , Keller U.
Published: July 2002

Absorber layers of semiconductor saturable absorber mirrors are required to show high absorption modulation with recovery times on the order of 100 fs and low nonsaturable losses. While to provide such fast recovery times, defect states for carrier trapping need to be incorporated into the absorber material, defects and surface roughness can cause additional losses in nonsaturable absorption. A specially designed GaAs/fluoride multilayer stack was grown to study the relation between growth conditions, surface roughness, nonsaturable losses, and absorption modulation. The growth of the multilayer stack included three epitaxial growth regimes: (a) homoepitaxial growth of GaAs on GaAs (111) B, (b) heteroepitaxy of CaF2 on GaAs, and (c) heteroepitaxy of GaAs on CaF2. While the homoepitaxial and first CaF2 layer growth proceeded two dimensional, island nucleation was obtained for the GaAs absorber and top CaF2 layer. The CaF2 surface was exposed to an electron beam of different doses to increase free surface energy for subsequent GaAs overgrowth and the surface roughness of the absorber layer was found to decrease with increasing electron dose. Nonlinear properties and light scattering were measured and correlated to the growth parameters. Linear reflectivity and absorption modulation were close to the theoretical values of the designed multilayer stack for the region exposed to the highest electron dose. With a recovery time of about 500 fs, the grown GaAs absorber layer is an excellent choice for an all-optical switching application in broadband AlGaAs/CaF2 semiconductor saturable absorber mirrors.
Destruction of the Global Phase Coherence in Ultrathin, Doubly Connected Superconducting Cylinders
Authors: Liu Y. , Zadorozhny Yu. , Rosario M. M. , Rock B. Y. , Carrigan P. T. , Wang H.
Published: July 2002

In doubly connected superconductors, such as hollow cylinders, the fluxoid is known to be quantized, allowing the superfluid velocity to be controlled by an applied magnetic flux and the sample size. The sample-size–induced increase in superfluid velocity has been predicted to lead to the destruction of superconductivity around half-integer flux quanta. We report transport measurements in ultrathin Al and Au
0.7
In
0.3
cylinders verifying the presence of this destructive regime characterized by the loss of the global phase coherence and reveal a phase diagram featuring disconnected phase coherent regions, as opposed to the single region seen in larger superconducting cylinders studied previously.
Can This Marriage Be Saved?
Authors: Gibbons Ann
Published: July 2002

A decade ago, a linguist, an archaeologist, and a geneticist pooled their evidence and proposed that there are three distinct groups of native Americans, and that each group came over from Asia in a separate migratory wave. But now, as the genetic evidence shifts, the interdisciplinary union is beginning to fray, as researchers from each side grow disenchanted with the others' data.
Ice-Age Rain Forest Found Moist, Cooler
Authors: Kerr Richard A.
Published: July 2002

What happened to the tropical forests when an ice age gripped the world 18,000 years ago? This question has spurred two distinct debates for years: Climatologists have gathered conflicting data on tropical temperatures, and ecologists have squabbled about whether the Amazon rain forest dried out and fragmented. Now a pollen record from a lake deep in Amazonia, reported on page 85, suggests that the forest was definitely cooler than today, but still moist, and that tree cover was as dense as ever.
Search Narrows for Gamma-Ray Bursts
Authors: Schilling Govert
Published: July 2002

For more than 30 years, astronomers have been unable to explain the mysterious bursts of gamma rays that occur approximately once a day in random positions across the sky. One problem is that gamma-ray cameras can't pin down the direction of a burst to less than a few degrees. But now a simultaneous sighting by an x-ray and a gamma-ray camera on board the Italian-Dutch Beppo-SAX satellite has narrowed the search. Because x-ray cameras offer much sharper angular resolution than do gamma-ray detectors, the 20 July double sighting pinned down the position of the burst to an area smaller than the full moon. Astronomers are now scouring this patch of sky, hoping to find an optical or radio "counterpart."
Dole, Clinton Lay Out Opposing Views
Authors: Lawler Andrew
Published: July 2002

In responses to questions posed by
Science
(
page 361
), Bob Dole and Bill Clinton both say they want to strengthen basic research, improve science education, and encourage technology transfer. But they offer different strategies for carrying out their policies
ELSI's Cystic Fibrosis Experiment
Authors: Marshall Eliot
Published: July 2002

When a group of biologists identified a genetic defect that causes cystic fibrosis in 1989, many observers warned of a tidal wave of genetic testing by doctors too ill-informed or hurried to provide good counseling. The tidal wave never struck, and a set of studies funded by the program in Ethical, Legal, and Social Implications (ELSI) at the National Center for Human Genome Research suggests it's not about to. Although the early results indicate that worries about counseling and informed consent were justified, the investigators also found that people offered the test don't take it unless they are already planning to have children who may be affected.
Research ‘Summit’ Ponders Health Funding Shortfall
Authors: Cohen Jon
Published: July 2002

OTTAWA
—In what was informally referred to as the G-14 summit, health officials from 14 countries met here earlier this month for an unprecedented gathering. For 2 days, these health mandarins sat around a giant horseshoe-shaped table with leading economists, pharmaceutical industry titans, and heads of the world's largest biomedical philanthropies, volleying ideas about innovative ways to raise money for research and how to lobby for more annual governmental appropriations.
The Moon Looms Large in Japan's Plans
Authors: Normile Dennis
Published: July 2002

KYOTO—Moon viewing is an autumn tradition in this country, and last week the world's space scientists were given a close look at Japan's plans for lunar exploration. Japan is pressing ahead with its lunar program—backing two of the three international missions now scheduled to land on or orbit the moon in the next several years—at a time when most other nations are focusing their space efforts elsewhere.
Both Sides Point Finger in Tiff Over China Dig
Authors: Mervis Jeffrey
Published: July 2002

The Dutch research community is reacting angrily to a plan to overhaul the way basic research is funded in the Netherlands. The proposal would strip the Dutch Research Council, or NWO, of its responsibility for running labs and turn it into a granting agency, much like the U.S. National Science Foundation.
The Vatican's Position Evolves
Authors: Holden Constance
Published: July 2002

The theory of evolution got a papal blessing last week. But in fact, the Pope's pronouncement doesn't go much beyond the Vatican's 1950 endorsement of evolution as a theory meriting further investigation.
Weaponeers Cultivate Academics
Authors: Weiss Peter
Published: July 2002

Livermore, California—The $3.9 billion weapons program is reaching out to universities in an initiative that could funnel as much as $100 million next year to basic researchers. To bring basic science into the “stockpile stewardship” program, which aims to maintain the nuclear stockpile without testing, the Department of Energy plans to fund a total of five university computation centers and underwrite research in disciplines such as plasma physics and materials science.
Early Birds Rise From China Fossil Beds
Authors: Gibbons Ann
Published: July 2002

A rich bone bed in northeastern China has yielded rare bird fossils, including what may be the oldest modern-looking bird. On
page 1164
, a team of Chinese and American paleontologists argues that the antiquity of the lineage leading to modern birds upsets the prevailing view of the stages in bird evolution.
Developmental Neurobiology
Authors: Hines Pamela J. , Marx Jean
Published: July 2002

T
he nervous system, once thought to be so complicated that it would defy our efforts to understand it, has begun to yield its secrets with accelerating speed. Indeed, researchers' efforts to understand how the nervous system develops have succeeded to the point that Martin Raff of London's University College can aptly describe the progress as "staggering" in the guest
Editorial
that opens this Special Issue on Developmental Neurobiology. The articles in this issue describe some of that staggering progress.

Two deal with the earliest stages of vertebrate neurodevelopment, from the first formation of the neural ectoderm, the tissue that will give rise to the brain and spinal cord, to the later division of the central nervous system into the regions that will go on to acquire specific functions. Using the hindbrain and midbrain as examples,
A. Lumsden and R. Krumlauf
focus on how the overall anterior-posterior pattern of the nervous system is specified. Both the Hox genes and a variety of signaling molecules, such as retinoic acid, are important players in establishing the fates of these early cells. In their article,
Y. Tanabe and T. M. Jessell
Y. Tanabe and T. M. Jessell take up dorsal-ventral patterning. Here the spinal cord serves as a useful example. Signals, such as Sonic Hedgehog, emanate from the floor plate of the spinal cord and the underlying notochord to direct the fate of the spinal neurons.

As neurons acquire particular identities, they begin forming the interconnections that wire the mature nervous system. M. Tessier-Lavigne and C. S. Goodman describe axon guidance, the process by which a growing axon finds a reasonable route toward a likely target. The signposts along the way include both soluble and nondiffusible molecules, which give instructions that can either attract or repel the elongating axon.

Once the neural circuits are roughly sketched in early development, their final structure is refined by neuronal activity. L. C. Katz and C. J. Shatz discuss how both sensory experience and spontaneous neural activity can polish the final development of functional neural circuits as well as enable the mature central nervous system to respond to changing inputs.

To complement these articles, the News report offers two stories.
One
describes the human and mouse mutations that are helping reveal the molecules that guide newborn cortical neurons to their final destinations in the brain;
another
examines new developments in understanding the plasticity of the neuromuscular junction, findings that may also shed light on learning and memory.

And finally,
F. E. BlooM
provides an analysis of the neuroscience resources available on the World Wide Web, including those devoted to the intersection of neuroscience and developmental biology.
Trio of Programs Seeks Economic Payoff
Authors: Normile Dennis
Published: July 2002

Tokyo—The Ministry of Education, Science, Sports, and Culture and the Ministry of International Trade and Industry have launched innovative programs to link academic researchers with industrial partners and to foster a more entrepreneurial spirit on campus.
First Blush for Integrated Light Emitter
Authors: Seife Charles
Published: July 2002

Researchers at the University of Rochester in New York have just consummated an improbable marriage long sought by the microelectronics industry: They have combined light-emitting components with conventional microcircuitry on a single chip. The technology is far from being turned into a workable device, because it puts out too little light and too much heat, but it is being hailed as a research advance.
Found: Jupiter's Missing Water
Authors: Kerr Richard A.
Published: July 2002

Tucson, Arizona—The Galileo probe found a surprising lack of water in the depths of Jupiter's atmosphere last December, forcing researchers to rethink their ideas on the giant planet's formation. Now, new calibrations of probe data plus other observations presented at the recent American Astronomical Society Division for Planetary Sciences meeting here suggest that the dryness doesn't extend to the whole planet but is a meteorological phenomenon limited to deep pockets in the atmosphere.
Green Education Under Fire
Authors: Schmidt Karen F.
Published: July 2002

Conservative critics contend that teachers and texts are bombarding students in the United States with biased and incomplete scientific information on environmental issues. But advocates say the charges are exaggerated.
Fly Gene Discovery Gets at Root of Branching Structures
Authors: Roush Wade
Published: July 2002

A group led by Stanford University biochemist Mark Krasnow has found the signal that tells cells in embryos of the fruit fly
Drosophila melanogaster
to develop into branching structures: a network of respiratory tubes called trachea that supply oxygen directly to interior tissues. The finding may hold clues to what promotes branching in mammals, a problem that has long left biologists stumped.