bluuebunny/crossref_metadata_2025 huggingface dataset:
Error analysis for fast scintillator-based inertial confinement fusion burn history measurements
Authors: Lerche R. A. , Ognibene T. J.
Published: July 2002

Plastic scintillator material acts as a neutron-to-light converter in instruments that make inertial confinement fusion burn history measurements. Light output for a detected neutron in current instruments has a fast rise time (<20 ps) and a relatively long decay constant (1.2 ns). For a burst of neutrons whose duration is much shorter than the decay constant, instantaneous light output is approximately proportional to the integral of the neutron interaction rate with the scintillator material. Burn history is obtained by deconvolving the exponential decay from the recorded signal. The error in estimating signal amplitude for these integral measurements is calculated and compared with a direct measurement in which light output is linearly proportional to the interaction rate.
Inexpensive but accurate driving circuits for quartz crystal microbalances
Authors: Bruschi L. , Delfitto G. , Mistura G.
Published: July 2002

The quartz crystal microbalance (QCM) is a common technique which finds a wide variety of applications in many different areas like adsorption, catalysis, analytical chemistry, biochemistry, etc., and more generally as a sensor in the investigation of viscoelastic films. In this article we describe some driving circuits of the quartz which we have realized and tested in our laboratory. These can be assembled with standard components which can be easily found. Their performance, in some cases, is as good as that of the much more expensive frequency modulation technique employed in very precise QCM measurements and which requires high-quality commercial radiofrequency generators and amplifiers.
High-voltage subnanosecond pulse transformer composed of parallel-strip transmission lines
Authors: Homma Akira
Published: July 2002

A subnanosecond pulse isolation transformer composed of a parallel-strip transmission line was designed, constructed, and tested. The insertion loss and effective time constant of the transformer were calculated with transmission line analysis. To facilitate the analysis, the reflection diagram method was applied. The experiments indicate that the transformer can transmit subnanosecond rise time pulses across a potential difference of 100 kV with the low insertion loss of −2 dB. The measured time constant was about 17 ns which corresponded to calculated values. The validity of the analytical method is also discussed.
High magnetic field corrections to resistance thermometers at low temperatures
Authors: Zhang B. , Brooks J. S. , Perenboom J. A. A. J. , Han S.-Y. , Qualls J. S.
Published: July 2002

We present a reliable method to account for the magnetoresistance of resistance sensors which are used as thermometers in many low temperature (T⩽20 K) experiments carried out in high magnetic fields (to 31 T). To apply the method, a set of isothermal magnetoresistance data, and a zero magnetic field temperature calibration are first necessary. A simple algorithm, which uses this data set, can then be applied to compute the temperature from the measured resistance at any field. The method is particularly useful for temperature dependent measurements at fixed field, or where, in cases where the temperature may change unpredictably during a change in magnetic field. We apply this method to the treatment of data in two separate experiments with the two different thermometers, RuO2 (below 1 K) and Cernox (above 1 K) sensors, respectively.
Characterization and stabilization of fiber-coupled laser diode arrays
Authors: Phillips D. F. , Wong G. P. , Bear D. , Stoner R. E. , Walsworth R. L.
Published: July 2002

We have characterized the spectra and performance of an ensemble of 11 fiber-coupled laser diode arrays (LDAs) manufactured by Opto Power Inc. These high-power LDAs operate near 795 nm and are of a type commonly used for spin-exchange optical pumping of noble gases. We find the Opto Power LDAs to vary significantly in output power, spectral width, and other important characteristics, in a manner not correlated with age, operating lifetime, or information supplied by the manufacturer. In addition we have developed a two-loop feedback technique for use with LDAs that stabilizes the Rb magnetization in an optical pumping cell to better than one part in a thousand.
Deconvolution and measurement of bulk and surface optical absorptions in Ti:Al2O3 crystals using photopyroelectric interferometry
Authors: Wang Chinhua , Mandelis Andreas
Published: July 2002

The extension of our earlier single-layer (monolithic) photopyroelectric (PPE) interferometric theory to include surface and bulk optical absorptions has allowed the measurement of both bulk absorption coefficient and surface absorptance in one single experiment. Based on purely thermal-wave interferometry, the thermal-wave cavity lengths of a PPE interferometer were scanned using pairs of Ti: sapphire crystals with appropriate combinations of figure of merit, surface polish, and thickness. In the conventional single-ended (noninterferometric) PPE technique, the surface reflectivity, surface absorptance, and bulk absorption coefficient are always coupled together. However, PPE destructive interferometry provides a method for extracting highly precise values of one of these optical parameters, without the requirement of equally precise knowledge of the values of the others.
Gated time-of-flight plasma composition analyzer for space physics research
Authors: Keller John W. , Chornay Dennis J. , Hunsaker Floyd H. , Ogilvie Keith W. , Coplan Michael A.
Published: July 2002

We have constructed and tested a prototype instrument for space plasma physics research. The instrument is capable of measuring total ion velocity distribution functions as well as velocity distribution functions for individual mass-per-charge (m/q) species. The instrument consists of a deflection system, a toroidal energy-per-charge (E/q) analyzer, and a hemispherical condenser that serves either as the second stage of E/q analysis or as a gated time-of-flight velocity filter. The deflection system subtends a full 360° in azimuth and allows one to select ions for analysis in the toroidal section according to their elevation angle with respect to the axis of the instrument. The hemispherical condenser accepts a fraction of the ions transmitted by the toroidal analyzer through an electronic shutter at its entrance plane. With the shutter open, total ion distribution functions are measured. By appropriately gating the shutter and measuring the time of flight of ions introduced into the hemisphere, ion velocities are measured and m/q determined. The dimensions of the instrument have been chosen for measurements of the interplanetary solar wind at distances of 4–20 solar radii (Rsun) from the surface of the sun, consistent with the requirements of a solar probe mission. It can also be adapted to make measurements at 1 a.u.
High-speed (104 °C/s) scanning microcalorimetry with monolayer sensitivity (J/m2)
Authors: Lai S. L. , Ramanath G. , Allen L. H. , Infante P. , Ma Z.
Published: July 2002

We introduce a high sensitivity (1J/m2) scanning microcalorimeter that can be used at high heating rates (104 °C/s). The system is designed using ultrathin SiN membranes that serve as a low thermal mass mechanical support structure for the calorimeter. Calorimetry measurements of the system are accomplished via resistive heating techniques applied to a thin film Ni heating element that also serves as a thermometer. A current pulse through the Ni heater generates heat in the sample via Joule heating. The voltage and current characteristics of the heater were measured to obtain real-time values of the temperature and the heat delivered to the system. This technique shows potential for measuring irreversible heat of reactions for processes at interfaces and surfaces. The method is demonstrated by measuring the heat of fusion for various amounts of thermally evaporated Sn ranging from 50 to 1000 Å.
Holey-plate ion source
Authors: Yoshida Yoshikazu
Published: July 2002

A low-pressure and high-density microwave ion source, created and sustained by evanescent waves emitted from a holey plate (HP) has been studied. This source is called a HP ion source. Microwave power at 2.45 GHz is supplied from a rectangular waveguide and then converted into an evanescent mode through the use of a HP placed on a H plane located at one end of the rectangular waveguide. The argon plasma density in the ion source is 1×1011 cm−3 with an electron temperature of 3 eV. A 150 mm×150 mm argon ion beam is extracted from the ion source. The ion current densities are in excess of 2 mA/cm2 at an extraction voltage of 1 kV.
Electron cyclotron resonance negative ion source
Authors: Fukumasa Osamu , Matsumori Masanori
Published: July 2002

Production and control of electron cyclotron resonance (ECR) plasmas for negative ion sources have been studied. A new production method using permanent magnets is proposed as one possibility for a large diameter high density uniform microwave plasma. The microwave power is launched by an annular slot antenna into the circumference of a chamber with a line-cusp or a ring-cusp type permanent magnets, where magnetic field can be applied in a local region and plasmas can be efficiently produced if the ECR condition is satisfied. In this article, we report the structure of the ECR negative ion source, the characteristics of the ECR plasmas, and comparison of the ECR plasmas with dc discharge plasmas from the viewpoint of a negative ion source for neutral beam injector. H− volume production is confirmed in ECR plasmas although the effect of the magnetic filter for controlling plasma parameters is different from that in dc discharge plasmas.
Roughness in Nb/Cu multilayers determined by x-ray diffraction and atomic force microscopy
Authors: Temst K. , Van Bael M. J. , Wuyts B. , Van Haesendonck C. , Bruynseraede Y. , de Groot D. G. , Koeman N. , Griessen R.
Published: July 2002

Nb/Cu multilayers grown by molecular beam epitaxy have been studied by x-ray diffraction and atomic force microscopy. X-ray diffraction provides the average interface roughness while atomic force microscopy shows the roughness and topology of the upper surface. Comparison of both methods shows that high-angle diffraction averages over a lateral length which is in good agreement with the typical grain size.
Synthesis and ferroelectric properties of c-axis oriented Bi4Ti3O12 thin films by sol-gel process on platinum coated silicon
Authors: Gu Haoshuang , Kuang Anxiang , Wang Shimin , Bao Dinghua , Wang Lianshan , Liu Jianshe , Li Xingjiao
Published: July 2002

c-axis oriented Bi4Ti3O12films have been prepared on Pt/Ti/Si(111) substrates by sol-gel process. Crack-free films of 0.5 μm thickness were fabricated using a multilayer spinning technique and calcination at 600 °C for 30 min. The average grain size of the film is about 0.15 μm. The film exhibits ferroelectric hysteresis with remanent polarization Pr=4.8 μc/cm2 and coercive field Ec=70 kV/cm. The measured dielectric constant and loss factor at a frequency of 100 kHz are 170 and 0.02 respectively.
Strain-sensing cryogenic field-effect transistor for integrated strain detection in GaAs/AlGaAs microelectromechanical systems
Authors: Beck R. G. , Eriksson M. A. , Westervelt R. M. , Campman K. L. , Gossard A. C.
Published: July 2002

We have fabricated a strain-sensing cryogenic field-effect transistor (FET) from a GaAs/AlGaAs heterostructure containing a near-surface two-dimensional electron gas. The FET has transconductance 100 μS and a small signal drain-source resistance 10 MΩ. The charge noise has a flat spectrum at high frequencies with magnitude 0.2e/√Hz and 1/f noise corner less than 300 Hz. The piezoelectric effect couples stress in the substrate to the electron density in the FET channel giving an electrical response to applied strain. Strain sensitivity was measured to be 2×10−9/√Hz, limited by FET noise. Integrated strain-sensing FETs offer advantages for detecting small forces in GaAs/AlGaAs microelectromechanical systems.
High-modulation-depth and short-cavity-length silicon Fabry–Perot modulator with two grating Bragg reflectors
Authors: Liu Mark Y. , Chou Stephen Y.
Published: July 2002

We propose a silicon Fabry–Perot planar waveguide modulator structure consisting of two Bragg reflectors to form the cavity. The Bragg reflectors are nanoscale trenches in the waveguide fabricated using electron beam lithography and reactive ion etching. Compared to conventional waveguide modulator designs, large modulation depth can be achieved with much smaller modulator length using high-finesse Fabry–Perot cavity, leading to much less loss and higher speed. This modulator design can also be utilized as an externally tunable spectral filter.
Electroluminescence from poly(phenylene vinylene) in a planar metal-polymer-metal structure
Authors: Lemmer U. , Vacar D. , Moses D. , Heeger A. J. , Ohnishi T. , Noguchi T.
Published: July 2002

We report on electroluminescence (EL) from a surface emitting (planar) metal-polymer-metal structure. Two gold electrodes (separated by 20 μm) were evaporated on free-standing films of poly(phenylene vinylene), PPV. Polarized light emission was observed from tensile drawn PPV films. The planar structure enables spatial resolution of the emission from devices based on conjugated polymers. Spatially resolved EL measurements show that the recombination zone extends over about 10 μm. The emitting region is asymmetric with the maximum closer to the electron injecting contact.
Solid-phase epitaxial growth of amorphized GaAs: The influence of microscopic nonstoichiometry
Authors: Belay K. B. , Llewellyn D. J. , Ridgway M. C.
Published: July 2002

The solid-phase epitaxial growth of amorphized GaAs has been characterized with a variety of techniques to investigate the influence of microscopic nonstoichiometry on the onset of twinning and subsequent interfacial nonplanarity. Nonstoichiometry on a local, microscopic scale (or equivalently, chemical disorder) can result from the statistical nature of the ion recoiling and stopping processes. For the present study, microscopic nonstoichiometry was produced by implanting samples with equal doses of both Ga and As ions. The onset of twinning and subsequent interfacial nonplanarity were not correlated with the energy deposited in vacancy production – the latter considered a first estimate of relative differences in microscopic nonstoichiometry between samples. Such observations are potentially indicative of the saturation of microscopic nonstoichiometry over the given dose range.
Time-resolved measurements of the photoluminescence of Cu-quenched porous silicon
Authors: Rigakis N. , Yamani Z. , AbuHassan L. H. , Hilliard J. , Nayfeh M. H.
Published: July 2002

We have performed time-resolved photoluminescence measurements in the submicrosecond to microsecond time regime on porous silicon samples under several diffusion-based chemical treatments with copper ion solutions that produce varying crystallite surface conditions. Our results for short emission wavelengths emanating from high lying states indicate that Cu acts largely on the population process commencing from the top of the well at short relaxation time scales immediately after excitation, and to less extent on the radiating states in the microsecond regime.
High quality GaN–InGaN heterostructures grown on (111) silicon substrates
Authors: Yang J. W. , Sun C. J. , Chen Q. , Anwar M. Z. , Asif Khan M. , Nikishin S. A. , Seryogin G. A. , Osinsky A. V. , Chernyak L. , Temkin H. , Hu Chimin , Mahajan S.
Published: July 2002

We report on the low pressure metal organic chemical vapor deposition of single crystal, wurtzitic layers of GaN and GaN/InGaN heterostructures on (111) GaAs/Si composite substrates. The structural, optical, and electrical properties of the epitaxial layers are evaluated using x-ray diffraction, transmission electron microscopy, photoluminescence, and measurements of minority carrier diffusion length. These measurements demonstrate high quality of GaN grown on the composite substrate.
Charge trapping, isolated Ge defects, and photosensitivity in sputter deposited GeO2:SiO2 thin films
Authors: Warren W. L. , Simmons-Potter K. , Potter B. G. , Ruffner J. A.
Published: July 2002

The nature of the defects in sputter-deposited GeO2:SiO2 thin films and their relationship to charge trapping and enhanced photosensitivity have been studied using electron paramagnetic resonance, capacitance–voltage, and optical bleaching and absorption spectroscopies. We find a good qualitative agreement between the density of isolated Ge dangling bonds measured magnetically, the density of charge trapping sites measured electrically, and the density of absorbing centers measured optically. Collectively, all observations can be modeled by assuming that a change in spin state and charge state of isolated paramagnetic neutral Ge dangling bonds, to form either diamagnetic positively or negatively charged Ge sites, are largely responsible for the charge trapping and photosensitivity in these thin films.
Highly luminescent Eu3+ or Tb3+ doped and ZnO sensitized optical fibers drawn from silicon compatible sealing glasses
Authors: Rüter D. , Bauhofer W.
Published: July 2002

A strongly enhanced luminescence efficiency of zincborate glasses and glass fibers doped with trivalent europium or terbium is reported. The enhancement is attributed to a high content of direct gap ZnO nanocrystallites which operate as sensitizer for the rare-earth ions. The glasses have a low softening point and are designed to be compatible with the silicon technology. Applications as scintillating devices integrated on silicon are suggested.
High-carrier-density electron dynamics in low-temperature-grown GaAs
Authors: Sosnowski T. S. , Norris T. B. , Wang H. H. , Grenier P. , Whitaker J. F. , Sung C. Y.
Published: July 2002

Pump-probe differential transmission measurements examine high-carrier-density phenomena in as-grown and annealed GaAs samples grown at temperatures from 210 to 270 °C. We observe trap saturation and Auger recombination, and accurately model the measurements on annealed samples with a simple two level rate equation, allowing us to extract the trapped-electron lifetimes.
The interaction of H2O with an electron paramagnetic resonance center in oxidized, heat treated SiC
Authors: Macfarlane P. J. , Zvanut M. E.
Published: July 2002

We examine a defect in heat treated, oxidized 6H-SiC with electron paramagnetic resonance spectroscopy. Samples are examined before treatment, after oxidation, after dry (<10 ppm H2O) N2 heat treatment, and after standard (approximately 50 ppm H2O) Ar annealing. A center is observed following dry heat treatment at temperatures greater than 800 °C. This center is passivated by standard Ar annealing at temperatures greater than 700 °C and regenerated after subsequent dry heat treatment. We suggest that this defect is related to an unpaired electron on a carbon atom created by the release of a hydrogen-related species.
Aluminum single-electron nonvolatile floating gate memory cell
Authors: Chen C. D. , Nakamura Y. , Tsai J. S.
Published: July 2002

We have developed an aluminum single-electron transistor device which was modified to incorporate an additional floating node in between the gate and the electrometer. At high gate voltages, Fowler–Nordheim type emission occurred between the gate and the floating node where the charges were stored. The emission events were evidenced by the oscillatory electrometer current which allowed estimation of the number of storage electrons. We demonstrated experimentally the nonvolatile memory function of this device and the results were justified by numeric simulations.
Carrier density dependence of the ambipolar diffusion coefficient in GaAs n-i-p structures
Authors: Streb D. , Klem G. , Fix W. , Kiesel P. , Döhler G. H.
Published: July 2002

We report on investigations of the ambipolar diffusion coefficient in GaAs n-i-p structures in which the electron density in the n channel is varied. The ambipolar diffusion coefficient was determined by time-resolved diffusion experiments of the Shockley–Haynes type. The dependence of the ambipolar diffusion coefficient on the varying carrier density is in excellent agreement with the theory of ambipolar diffusion in n-i-p-i systems. In the investigated sample, the ambipolar diffusion coefficient reaches a maximum value of 16 300 cm2/s. Although the ambipolar diffusion constant declines with decreasing carrier density, it is in a wide range of the carrier density more than two orders of magnitude enhanced compared to bulk materials.
High coercivity, epitaxial Sm–Co films with uniaxial in-plane anisotropy
Authors: Fullerton Eric E. , Jiang J. S. , Rehm Christine , Sowers C. H. , Bader S. D. , Patel J. B. , Wu X. Z.
Published: July 2002

Epitaxial Sm–Co(11̄00) and (112̄0) films have been grown by magnetron sputtering onto Cr(211) and (100) buffer layers, respectively. The Sm–Co(11̄00) films exhibit uniaxial in-plane anisotropies of ≈20–25 T and room-temperature coercive fields that increase to 4.1 T as the film thickness decreases to 75 Å. The 3 T coercivities of the (112̄0) films are independent of thickness.
Solution deposition and heteroepitaxial crystallization of LaNiO3 electrodes for integrated ferroelectric devices
Authors: Cho Chae Ryong , Payne David A. , Cho Seong Lae
Published: July 2002

Electrically conducting LaNiO3 (LNO) thin layers were deposited on (100)SrTiO3 (STO) and (100)LaAlO3 (LAO) substrates and crystallized at temperatures between 700 and 800 °C. The chemical method of deposition was solution sol-gel processing. X-ray 2θ and rocking curve measurements, φ scans, and atomic force microscopy studies revealed the degree of crystallinity and heteroepitaxy for the integrated electrodes. The full-width at half-maximum for (100) LNO deposited on LAO and STO was 0.16° and 0.25°, respectively. Surface bonding states and chemistry were examined by x-ray photoelectron spectroscopy. The room temperature resistivity of LNO electrodes deposited on STO and LAO was 460 and 340 μΩ cm, respectively.
Dielectric properties of lithium triborate single crystals
Authors: Kim Ji Won , Yoon Choon Sup , Gallagher H. G.
Published: July 2002

From the impedance analysis, we report the dielectric constants of lithium triborate single crystals for the three principal axes in the frequency range of 100 Hz–1 MHz and in the temperature range of 303–773 K. The observed highly anisotropic behavior of dielectric dispersions is mainly related to the anisotropy of the activation energy for Li+ ion hopping, which is constituted by the B3O7 channel framework lain along the c axis.
Orientation control of ex situ (Hg1−xRex)Ba2CaCu2Oy (x≈0.1) thin films on LaAlO3
Authors: Gasser C. , Moriwaki Y. , Sugano T. , Nakanishi K. , Wu X.-J. , Adachi S. , Tanabe K.
Published: July 2002

( Hg 1−x Re x ) Ba 2 CaCu 2 O y  (x≈0.1) thin films have been prepared ex situ on LaAlO3 substrates by a two-step process in which an HgO/Re0.1Ba2CaCu2Oy multilayer precursor film is annealed in a quartz tube using an unreacted monoxide pellet containing HgO as an Hg vapor source. By varying the composition of the monoxide pellet, the film orientation can be changed from c-axis to the predominantly a-axis one. The a-axis growth of the grains directly on the substrate is confirmed by x-ray diffraction and transmission electron microscopic observation, although the grains are twinned with 90° orientation. The a-axis films exhibit a Tc onset and Tc zero of approximately 110–127 and 55 K, respectively, suggesting substantial influence of weak links at the grain boundaries on their transport properties.
Sample temperature measurement in a scanning near-field optical microscope
Authors: Kazantsev D. , Guttroff G. , Bayer M. , Forchel A.
Published: July 2002

The luminescence of GaAs quantum-well-based microstructures was investigated with a scanning near-field optical microscope at different ambient temperatures. The scanning tip was fabricated from an optical fiber. The excitation laser beam (514 nm, 100–300 mW) was focused onto the opposite end of the fiber. The carrier distribution in the sample was investigated using the high-energy tail of the luminescence peak. Temperature fits show no overheating of the sample despite the expected high temperatures of the tip.
Efficient production of silicon-on-insulator films by co-implantation of He+ with H+
Authors: Agarwal Aditya , Haynes T. E. , Venezia V. C. , Holland O. W. , Eaglesham D. J.
Published: July 2002

We have investigated the process of thin film separation by gas ion implantation and wafer bonding, as well as the more basic phenomenon of blistering, on which the technique is based. We show that when H and He gas implants are combined they produce a synergistic effect which enables thin-film separation at a much lower total implantation dose than that required for either H or He alone. By varying the H and He implantation doses we have been able to isolate the physical and chemical contributions of the gases to the blistering processes. We find that the essential role of H is to interact chemically with the implantation damage and create H-stabilized platelet-like defects, or microvoids. The efficiency of H in this action is linked to its effective lowering of the silicon internal surface energy. The second key component of the process is physical; it consists of diffusion of gas into the microvoids and gas expansion during annealing, which drives growth and the eventual intersection of the microvoids to form two continuous separable surfaces. He is more efficient than H for this process since He does not become chemically trapped at broken bonds and thus segregates into microvoids more readily. In particular, we have demonstrated that a 1×1016 cm−2 He dose in combination with a 7.5×1015 cm−2 H dose are sufficient to shear and transfer a thin silicon film onto a handle wafer after bonding the two wafers together.
Doping-density dependence of scanning tunneling spectroscopy on lightly doped silicon
Authors: Lin H.-A. , Jaccodine R. , Freund M. S.
Published: July 2002

The doping-density dependence of scanning tunneling spectroscopy on lightly doped hydrogen-terminated Si(100) (resistivities in the range of 0.2–12 Ω cm) was investigated in air with and without illumination. The observed doping-density dependence is consistent with a generation model in which the changes in the three-dimensional depletion region, induced by a scanning tunneling microscopy tip, contributes to changes in the concentration of thermally and/or photogenerated carriers in lightly doped samples. These results suggest that scanning tunneling spectroscopy can be used to image variations in dopant density in lightly doped samples.
Observation of quantum confined excited states of GaN nanocrystals
Authors: Leppert Valerie J. , Zhang Christina J. , Lee Howard W. H. , Kennedy Ian M. , Risbud Subhash H.
Published: July 2002

GaN nanocrystals with an average diameter of 4.5 nm±1.6 nm were synthesized by pulsed laser ablation of a gallium metal target in a nitrogen atmosphere. Transmission electron microscopy and selected area electron diffraction confirmed the hexagonal structure and size of the nanocrystals. Optical absorption and photoluminescence spectroscopy revealed quantum confined excited states in the nanocrystalline samples with features at 4.43 eV (280 nm) and 5.47 eV (227 nm).
Low-frequency noise in GaN/GaAlN heterojunctions
Authors: Levinshtein M. E. , Pascal F. , Contreras S. , Knap W. , Rumyantsev S. L. , Gaska R. , Yang J. W. , Shur M. S.
Published: July 2002

We report on the results of the measurements of the low-frequency nitride/gallium aluminum nitride (GaN/GaAlN) heterojunctions grown on sapphire substrates. The noise spectra have the form of the 1/f noise with the Hooge parameter of approximately 10−2. The effects of band-to-band and impurity illumination on the low-frequency noise show that the nature of the 1/f noise in GaN might be a result of the occupancy fluctuations of the tail states near the band edges. This mechanism of the 1/f noise is similar to that in GaAs and Si.
Spin-coated ferroelectric SrBi2Nb2O9 thin films
Authors: Watanabe Koji , Tanaka Masahiro , Sumitomo Eiko , Katori Kenji , Yagi Hajime , Scott J. F.
Published: July 2002

The electrical and microstructural properties of pure strontium bismuth niobate (SBN) prepared by spin coating have been examined. Randomly ordered films have larger dielectric constants and remanent polarization values (maximum of Pr=12.5 μC/cm2) than do c-axis oriented films. The heretofore unexplained dependences of Pr upon Sr content in SBN and SBT is shown to be entirely due to the sensitivity of grain orientation to Sr content in SBN. Vertical shifts (P axis) in the hysteresis curves are eliminated via ozone anneals.
Oxygen-free dry etching of α-SiC using dilute SF6:Ar in an asymmetric parallel plate 13.56 MHz discharge
Authors: Scofield J. D. , Bletzinger P. , Ganguly B. N.
Published: July 2002

Etch rates of up to 2200 Å/min have been achieved on hexagonal silicon carbide (SiC) using dilute mixtures of SF6:Ar in a standard 13.56 MHz asymmetric parallel plate discharge. Furthermore, these etch rates have been realized with excellent pattern anisotropy profiles of approximately 1 at pressures in the range of 100–350 mTorr and the SF6 fraction at or below 50%. An understanding of the mechanisms responsible for the high etch rates in this simple, dilute, gas mixture can be achieved by considering the electrical characteristics of the radio frequency plasma. The conditions defining maximum etch rates are associated with peak fluorine ion and/or radical production, and can be defined entirely in terms of the relative current–voltage phase shift leading to optimal plasma impedance conditions and ultimately to maximal power deposition into the plasma. In addition, this study shows that the pervasive practice of utilizing oxygenated gas chemistries for SiC etching is not required, as previously thought, for carbon saturation and removal in order to obtain high etch rates, good surface morphology, and reliable pattern definition on SiC.
Formation of CoTi barrier and increased thermal stability of CoSi2 film in Ti capped Co/Si(100) system
Authors: Sohn Dong Kyun , Park Ji-Soo , Lee Byung Hak , Bae Jong-Uk , Byun Jeong Soo , Kim Jae Jeong
Published: July 2002

We investigated the formation of CoSi2 for Ti capped Co on (100) Si substrate with emphasis on the Co–Ti interaction and its effect on thermal stability. A 15 nm thick Ti capping layer is shown to improve the interfacial roughness and thermal stability of CoSi2 film grown on Si substrate compared with TiN capping. The increased uniformity of silicide/Si(100) interface is speculated to result from retarded Co–Si reaction by the formation of CoTi binary phase. And the high thermal stability can be explained by the fact that the amount of Ti atoms in CoSi2 film for Ti capping is much higher than what is in TiN capping. It is likely that the surface Ti diffuses rapidly into CoSi2 grain boundaries and slows down the agglomeration process, thereby increasing thermal stability while Ti in TiN capping did not.
Wide viewing angle, homeotropic nematic liquid-crystal display controlled by effective field
Authors: Lee S. H. , Kim H. Y. , Lee Y. H. , Park I. C. , Rho B. G. , Galabova H. G. , Allender D. W.
Published: July 2002

The electro-optic properties of a liquid-crystal display associated with a homeotropic to multidomainlike transition for a liquid crystal with a positive dielectric anisotropy have been investigated. The cell plates are prepared in such a way that, in the absence of an electric field, the liquid-crystal alignment is homeotropic. an electric field created by interdigitated electrodes on both substrates causes a director deformation of a multidomain type. The display shows wide viewing angle, excellent color characteristics, and a fast response time. The molecular director configuration, together with the electro-optic characteristics of the device, are discussed in this letter.
“Contact epitaxy” observed in supported nanoparticles
Authors: Yeadon M. , Ghaly M. , Yang J. C. , Averback R. S. , Gibson J. M.
Published: July 2002

We have observed the formation of heteroepitaxial interfacial layers between silver nanoparticles and a single crystal copper surface by a phenomenon we term “contact epitaxy.” Upon depositing Ag nanoparticles (5–20 nm diameter) onto clean (001) Cu in an ultrahigh vacuum in situ transmission electron microscope, a thin (111)-oriented layer of Ag was detected at the interface between the substrate and particles. Molecular dynamics simulations reveal that the epitaxial layers form within picoseconds of impact, with rapid alignment arising from mechanical relaxation of the highly stressed interface formed upon initial contact. The simulations also show that multiple grains form in the nanoparticle as a consequence of this relaxation process. The unique structure of the nanoparticles, induced by contact epitaxy, is expected to significantly influence physical properties such as interfacial bonding, diffusion, chemical activity, and electrical transport, as well as forming a nucleus for grain growth and epitaxy which we also observe. Due to its simple origin, the phenomenon should also apply to materials systems beyond the field of nanoparticles with implications for cluster deposition, adhesion, rheology, and catalysis.
Spectroscopy of a silicon quantum dot
Authors: Khoury M. , Rack M. J. , Gunther A. , Ferry D. K.
Published: July 2002

We have fabricated a silicon quantum dot embedded in a metal–oxide–semiconductor field-effect transistor structure. Two side gates deplete the quasi-two-dimensional electron gas created by a top inversion gate. We have tested devices ranging in size from 40 to 200 nm. By varying the density with the top gate, the conductance peaks reveal the details of the energy-level structure within the dot and their interactions with one another.
Deactivation and diffusion of boron in ion-implanted silicon studied by secondary electron imaging
Authors: Castell Martin R. , Simpson Todd W. , Mitchell Ian V. , Perovic D. D. , Baribeau J.-M.
Published: July 2002

Secondary electron (SE) imaging in a scanning electron microscope is used to map electrically active dopant distributions of B-doped superlattices in Si. By comparing SE contrast profiles with secondary ion mass spectroscopy data, it is shown that B is electrically deactivated when the damage caused during Si implantation falls onto a doped region. Following a 450 °C anneal, the effect of the implantation damage is severely reduced in the SE profiles and the B is partially reactivated. An 815 °C anneal results in transient enhanced diffusion of some of the B with the remainder trapped in an inactive immobile peak.
Rapid evaluation of dislocation densities in n-type GaN films using photoenhanced wet etching
Authors: Youtsey C. , Romano L. T. , Molnar R. J. , Adesida I.
Published: July 2002

We describe a technique based on photoelectrochemical wet etching that enables efficient and accurate evaluation of dislocation densities in n-type GaN films. The etching process utilizes dilute aqueous KOH solutions and Hg arc lamp illumination to produce etched GaN “whiskers” by selectively etching away material around threading dislocations. The etched whiskers, each corresponding to a single threading dislocation, can be effectively imaged by plan-view scanning electron microscopy. The distribution and density of dislocations are then readily observed over very large sample areas. Transmission electron microscope and atomic force microscope studies of the GaN samples confirm the accuracy of the dislocation density obtained by the wet etching.
Single-electron memory using carrier traps in a silicon nitride layer
Authors: Sunamura H. , Sakamoto T. , Nakamura Y. , Kawaura H. , Tsai J. S. , Baba T.
Published: July 2002

A single-electron memory that utilizes carrier traps in a silicon nitride layer is proposed and experimentally demonstrated. The proposed device consists of an insulating three-layered memory node structure formed on a silicon substrate and a highly sensitive aluminum single-electron transistor that detects the written information. Successful memory operation is demonstrated with two types of write modes (slow/rapid) that depend on the state of the Si channel underneath. Carrier retention time is estimated to be around 45 min. Possibilities for both destructive and nondestructive readout are discussed.
Enhanced flux pinning in (Nd0.33Sm0.67)Ba2Cu3O7−δ melt-processed superconductors by Ar postannealing
Authors: Hu A. , Schätzle P. , Bieger W. , Verges P. , Fuchs G. , Krabbes G.
Published: July 2002

High-quality binary (Nd0.33Sm0.67)Ba2Cu3O7−δ monodomain superconductors were fabricated by the combination of a compositional-controlled melt-processing technique in air and postannealing in Ar. Critical current densities over the whole field range were observed strongly dependent on Ar postannealing while holding superconducting transition temperatures as high as 96 K. The suppressed chemical variation between the rare-earth element (RE: Nd, Sm) and Ba associated with disperse RE2BaCuOy (RE211) precipitate due to the decomposition occurring in Ar postannealing was attributed to the modified flux pinning. This makes Ar postannealing promising to process superconductors with desired properties for various applications.
A near-field scanning optical microscope with a high Q-factor piezoelectric sensing element
Authors: Davydov Dmitri N. , Shelimov Konstantin B. , Haslett Thomas L. , Moskovits Martin
Published: July 2002

A tapping-mode near-field scanning optical microscope utilizing a piezoelectric microtuning fork as its height-sensing element is described. We have developed a method for modifying and attaching an optical fiber to the tuning fork that allows the assembly to retain a Q factor of up to 9000, substantially higher than the Q factors described so far in the literature for tuning-fork-based instruments. The method involves reducing the diameter of the cladding of the optical fiber down to 17–25 μm using several chemical etching steps, before the fiber is attached to the tuning fork. A sharp upturn in the Q factor is observed when the fiber diameter d drops below ∼25 μm. An analysis showing that the stretching force constant of a bent fiber is proportional to d4 accounts for the great sensitivity of the Q factor to the fiber diameter. The high Q factors result in improved force sensitivity and allow us to construct a tapping-mode instrument without the use of additional dithering piezoelements.
Near-infrared laser pumped intersubband THz laser gain in InGaAs–AlAsSb–InP quantum wells
Authors: Liu Ansheng , Ning C. Z.
Published: July 2002

We investigate the possibility of using InGaAs–AlAsSb–InP coupled quantum wells to generate THz radiation by means of intersubband optical pumping. We show that large conduction band offsets of these quantum wells make it possible to use conventional near-infrared diode lasers around 1.55 μm as pump sources. Taking into account the pump–probe coherent interaction and the optical nonlinearity for the pump field, we calculate the THz gain of the quantum well structure. We show that resonant Raman scattering enhances the THz gain at low and moderate optical pumping levels. When the pump intensity is strong, the THz gain is reduced by pump-induced population redistribution and pump–probe coherent interactions.
Fatigue study of metalorganic-decomposition-derived SrBi2Ta2O9 thin films: The effect of partial switching
Authors: Wu Di , Li Aidong , Ling Huiqin , Yu Tao , Liu Zhiguo , Ming Naiben
Published: July 2002

Fatigue characteristics of metalorganic-decomposition derived SrBi2Ta2O9 (SBT) ferroelectric thin films were investigated with varying film thickness, capacitor size, switching pulse amplitude, and pulse width. The effect of partial switching on fatigue was studied. An increase of nonvolatile polarization (Pnv) was observed over the initial period of fatigue when the capacitor was fully switched by the applied bipolar cycles. Fatigue increases with decreasing applied field and pulse width. These support the idea that fatigue in SBT may be viewed as a competition between domain-wall pinning and field-assisted unpinning. Partial switching caused by small field and short pulse width enhances charge trapping at domain walls, thus increasing fatigue.
Effects of nitridation and annealing on interface properties of thermally oxidized SiO2/SiC metal–oxide–semiconductor system
Authors: Lai P. T. , Chakraborty Supratic , Chan C. L. , Cheng Y. C.
Published: July 2002

The effects of N2O nitridation and subsequent annealing in different conditions on thermally oxidized n-type 6H–silicon carbide (SiC) metal–oxide–semiconductor (MOS) interface properties were investigated. Influence of high-field stress on the MOS system was also studied. The nitrided device annealed in dry or wet O2 is found to have lower interface-state density compared to the device annealed in N2 because the reoxidation can reduce nitridation-induced interface damage. Furthermore, significantly less shift of flatband voltage during high-field stress for all nitrided devices indicates much better oxide reliability by replacing strained Si–O bonds with stronger Si–N bonds during nitridation. This is further supported by the fact that annealing of the nitrided device in dry or wet oxygen slightly reduces the robustness of the oxide. In summary, the O2-annealing conditions have to be optimized to deliver a proper tradoff between interface quality and reliability.
Low resistance ohmic contacts to n-GaN and n-AlGaN using NiAl
Authors: Ingerly D. B. , Chen Y. , William R. S. , Takeuchi T. , Chang Y. A.
Published: July 2002

The intermetallic compound NiAl (50:50 at. %) has been shown to be a low-resistance ohmic contact to n-GaN and n-AlGaN. NiAl contacts on n-GaN (n=2.5×1017 cm−3) had a specific contact resistance of 9.4×10−6 Ω cm2 upon annealing at 850 °C for 5 min. NiAl contacts annealed at 900 °C for 5 min in n-Al0.12Ga0.88N (n=2.4×1018 cm−3) and n-Al0.18Ga0.82N (n=2.7×1018 cm−3) had specific contact resistances of 2.1×10−5 Ω cm2 and 4.7×10−5 Ω cm2, respectively. Additionally, these contacts were subjected to long-term annealing at 600 °C for 100 h. On n-GaN, the contact specific contact resistance degraded from 9.4×10−6 Ω cm2 to 5.3×10−5 Ω cm2 after the long-term anneal. Contacts to n-Al0.18Ga0.82N showed only slight degradation with a change in contact resistance, from 4.7×10−5 Ω cm2 to 9.2×10−5 Ω cm2. These results demonstrate the NiAl has great promise as a stable, low-resistance contact, particularly to n-AlGaN used in high-temperature applications.
Dependence of the carrier lifetime on acceptor concentration in GaAs grown at low-temperature under different growth and annealing conditions
Authors: Stellmacher M. , Nagle J. , Lampin J. F. , Santoro P. , Vaneecloo J. , Alexandrou A.
Published: July 2002

Using the transient reflectivity technique, we have measured the carrier lifetime in low-temperature-grown GaAs (LT-GaAs) samples as a function of growth temperature and annealing conditions. We confirm the role of the deep donor as the dominant nonradiative recombination center, but we show here that the acceptor concentration is equally crucial for the determination of the carrier lifetime as the deep donor concentration. Using the number of acceptors as the only adjustable parameter in our model, we are able to simulate the carrier lifetime for the growth and annealing conditions used in our experiments and to reproduce all the characteristics of the carrier recombination dynamics in LT-GaAs, such as nonexponential transients and the influence of the illumination intensity. The implications for the use of LT-GaAs for optoelectronic applications are discussed.
Rigorous solutions of diatomic molecule oscillator with empirical potential function in phase space
Authors: Li Qian-Shu , Lu Jun
Published: July 2002

Within the framework of the quantum phase-space representation established by Torres-Vega and Frederick, the rigorous solutions of the Schrödinger equation of the diatomic molecule oscillator with an empirical potential function are solved and discussed, and the Heisenberg uncertainty principle is interpreted in this physical system.
Analytic spherical torus plasma equilibrium model
Authors: Weening R. H.
Published: July 2002

An analytic spherical torus plasma equilibrium model is developed from a general solution to the Grad–Shafranov equation. The analytic model allows the calculation of axisymmetric plasma equilibria with arbitrary aspect ratio, elongation, triangularity, and diamagnetism. Using a numerical method, examples of optimized analytic equilibria are presented with plasma profiles similar to those of bootstrapped spherical tokamaks. Numerical results obtained from analytic equilibrium solutions for plasmas with aspect ratios A=1.4, elongations κ=3.0, and Troyon factors g=6.7 suggest that spherical tori can achieve toroidal beta values as large as βt≈35% at poloidal beta values βp≈1.53, with stable magnetic wells, W⩾0.
Measurement of areal density modulation of laser-imploded shells through K-edge imaging
Authors: Yaakobi B. , Smalyuk V. A. , Delettrez J. A. , Marshall F. J. , Meyerhofer D. D. , Seka W.
Published: July 2002

A new method for studying the modulations in the ρΔR of imploded shells is introduced: using a recently developed pinhole-array x-ray spectrometer, core images are obtained at energies below and above the K-edge energy of a titanium dopant. The ratio of these images reflects the nonuniformity of the cold shell around the time of peak compression, independently of modulation in core emission. The two-dimensional images of ρΔR modulations are of interest in laser fusion because they show the final result of unstable implosion evolution. The measured average ρΔR of the cold shell and the amplitude and spectrum of its modulations are in agreement with the results of two-dimensional code simulations that include imprinting of laser nonuniformity on the target.
Fragmentation of small sodium clusters
Authors: Rytkönen A. , Manninen M.
Published: July 2002

Fragmentation of small sodium clusters was studied by performing both ab initio and classical molecular dynamics simulations. In ab initio calculations at 1200–2400 K, neutral sodium clusters with 10 and 13 atoms ejected both monomers and dimers. The observed behavior is in agreement with previous calculations stating that the electronic shell oscillations diminish strongly as a function of temperature. The fragmentation rates obtained with the ab initio method are consistent with the Kelvin equation for the equilibrium vapor pressure of small clusters. The differences between the results obtained using different models reflect the differences between the dissociation energies calculated correspondingly.
Energy coupling in a diode with a dielectric-gridded cathode
Authors: Lahav A. , Berezovsky V. , Schächter L.
Published: July 2002

It is shown experimentally that electrostatic coupling between the ferroelectric “capacitor” and the anode-cathode gap controls the emission process in ferroelectric cathode. Three main quantities were demonstrated to be directly correlated to the energy stored in the ferroelectric: the energy in the diode, the diode impedance, and the delay of the peak (anode) current relative to the triggering time of the ferroelectric. The polarity of the triggering voltage does not seem to have a significant impact on the performance of the diode.
Optimal bounds on electric-field fluctuations for random composites
Authors: Lipton Robert
Published: July 2002

The electric field inside a two-phase composite is studied when the composite sample is subjected to a constant applied electric field. Upper and lower bounds on the covariance tensor of the electric field are found in terms of the effective dielectric properties of the composite. The lower bounds are shown to be optimal for two well-known families of microgeometries. Lower bounds on the covariance tensor are found when only the phase area fractions and the two-point correlation function are available. For statistically isotropic composites optimal lower bounds are derived when only the phase area fractions are known.