bluuebunny/crossref_metadata_2025 huggingface dataset:
Retarding-field analyzer for measurements of ion energy distributions and secondary electron emission coefficients in low-pressure radio frequency discharges
Authors: Böhm Christian , Perrin Jérôme
Published: July 2002

A four-grid electrostatic energy analyzer for measurements of the ion velocity distribution and the emission of secondary electrons on the electrodes of low-pressure radio frequency glow-discharge systems has been conceived. Problems arising from poor analyzer design are discussed and the performance of the presented analyzer is shown for measurements of the ion velocity distribution in pure hydrogen, helium, and argon discharges. Moreover, the secondary electron yields on aluminium, stainless steel, copper, and amorphous silicon exposed to radio frequency argon, helium, and hydrogen plasmas are determined in situ, for the first time to our knowledge. In parallel-plate radio frequency discharges secondary electron emission involves the contributions of ions, fast neutrals, metastables, and photons impinging on the electrode surfaces. Therefore, secondary electron emission must be considered as a global phenomenon. Global secondary electron emission can be up to ten times larger than only ion-induced secondary electron emission. Typically, the global emission coefficient is of the order of 0.1 (secondary electrons per ion) but can exceed 1 when the contribution of metastables is important. This is a noteworthy result since secondary electron emission is often neglected or underestimated in modeling of electrical discharges.
A novel AFM/STM/SEM system
Authors: Ermakov A. V. , Garfunkel E. L.
Published: July 2002

An atomic force/scanning tunneling (AFM/STM) microscope intended for operation inside a scanning electron microscope (SEM) is described. This AFM/STM/SEM system enables us to image a sample conventionally by SEM as well as to investigate the local surface topography by AFM or STM. This device incorporates a new method for monitoring AFM cantilever deflection that utilizes the focused electron beam of the SEM.
Quantitative electron-beam tester for defects in semiconductors (CL/EBIC/SDLTS system)
Authors: Sekiguchi Takashi , Sumino Koji
Published: July 2002

A pulsed electron beam testing system was developed for quantitative analysis of optical and/or electrical properties of defects in semiconductors. This system can control not only the electron beam but also detect and record the absolute values of light intensity: cathodoluminescence (CL), electron-beam-induced current (EBIC), and scanning deep level transient spectroscopy (SDLTS). Several improvements were made to enable accurate quantitative measurement. An optical system with an ellipsoidal mirror of low magnification was designed to make uniform collection efficiency of CL over a wide specimen area. A photon counting system and a lock-in amplifying system were implemented to measure absolute value of light intensity. A quantitative EBIC or SDLTS system was also incorporated to this system. The combined system of CL/EBIC/SDLTS can both image the distribution of defects and study their characteristics.
Multiple trim magnets, or ‘‘magic fingers,’’ for insertion device field integral correction
Authors: Hoyer E. , Marks S. , Pipersky P. , Schlueter R.
Published: July 2002

Multiple trim magnets (MTMs), also known as ‘‘magic fingers,’’ are an arrangement of magnets for reducing integrated magnetic-field errors in insertion devices. The idea is to use transverse arrays of permanent magnets, hence the name ‘‘multiple trim magnets,’’ above and below the midplane, to correct both normal and skew longitudinal magnetic-field integral errors in a device. MTMs are typically installed at the ends of an ID. Adjustments are made by changing either the size, position, or orientation of each trim magnet. Application of the MTMs to the ALS undulators reduced both the normal and skew longitudinal field integral errors, over the entire 20 mm×60 mm ‘‘good field region,’’ of the beam aperture by as much as an order of magnitude. The requirements included corrections of field and gradients outside the multipole convergence radius. Additionally, these trim magnet arrays provided correction of the linear component of the integrated field gradients for particles with trajectories not parallel to the nominal beam axis. The MTM concept, design, construction, tests that demonstrated feasibility, and magnetic-field integral reduction of ALS undulators are presented.
A differentially pumped harmonic filter on the Chemical Dynamics Beamline at the Advanced Light Source
Authors: Suits Arthur G. , Heimann Philip , Yang Xueming , Evans Matt , Hsu Chia-Wei , Lu Kuieh-tzu , Lee Yuan T. , Kung A. H.
Published: July 2002

A differentially pumped rare gas cell has been developed to suppress undulator harmonics on the Chemical Dynamics Beamline at the Advanced Light Source. Greater than 104 suppression of the harmonics has been demonstrated with no measurable (<5%) attenuation of the fundamental. The overall design is presented, and vacuum and optical performance are reported.
Ion probe for tightly bunched charged particle beams
Authors: Pasour John A. , Ngo Mai T.
Published: July 2002

A nonperturbing probe that utilizes a focused ion beam has been developed to measure the position and profile of a tightly bunched charged particle beam (CPB). The ions are injected across the path of the CPB and are deflected by the electromagnetic field of the CPB. Deflected ions are detected by a gated microchannel plate detector and video camera. From the magnitude of the deflections, one can determine the position, radial profile, and in some cases, the axial length of the CPB. Ion parameters can be adjusted to tailor the probe to a wide range of CPB parameters, from submicron, ps bunches to millimeter-sized, quasicontinuous beams. We present the theory of operation of the probe, provide a number of numerical examples to illustrate its capabilities, and describe a proof-of-principle version of the probe that we built and successfully tested at SLAC. The experimental results obtained with this prototype version have shown the ion probe diagnostic to be feasible, reliable, and well understood by the theory and models.
An ultraviolet nanosecond light pulse generator using a light emitting diode for test of photodetectors
Authors: Araki Tsutomu , Fujisawa Yasumitsu , Hashimoto Mamoru
Published: July 2002

An optical function pulse generator that emits (1) short pulse of 1 ns duration, (2) double pulse with variable time interval, and (3) square waveform pulse of variable width in nanosecond range is devised using an InGaN/AlGaN double heterostructure light emitting diode (LED). Although the LED emits a 450 nm (blue) light under conventional dc operation below 30 mA, 380 nm light due to the InGaN/AlGaN component appears when a current larger than 50 mA is applied. This phenomenon is used to realize a pulsed ultraviolet light source. Under large nanosecond current pulsing (peak current >1 A), an intense pulsed emission of 380 nm is obtained. Pulse waveform of the LED emission can be adjusted electrically by applying a shaped current to the LED. To evaluate the potential of the pulse generator as a test source of photodetectors, the response waveforms of photomultiplier tubes were measured.
A new system for two-dimensional analysis of hydrogen on solid surfaces
Authors: Ishikawa K. , Yoshimura M. , Ueda K. , Sakai Y.
Published: July 2002

This article reports the development of a two-dimensional analyzer, which enables us to observe the distribution of hydrogen on surfaces. A micro-focused electron beam with low primary electron energy (<1 keV) is scanned over a sample surface, in conjunction with a time-of-flight type electron-stimulated desorption spectroscope, to obtain clear H+ ion images of a specimen surface. A line scan analysis of H+ ions on an integrated circuit and a scanning electron-stimulated desorption image of H+ ions on a Cu mesh are presented as demonstrations.
Developments of widely tunable light sources for picosecond time-resolved resonance Raman spectroscopy
Authors: Uesugi Yuki , Mizutani Yasuhisa , Kitagawa Teizo
Published: July 2002

Two systems of widely tunable light sources for picosecond time-resolved resonance Raman (ps-TR3) spectroscopy have been constructed using a 1 kHz ps-Ti:sapphire laser/regenerative amplifier system. Performance of the systems was examined in terms of pulse duration, spectral width, pulse energy, and shot-to-shot stability. One system, consisting of white light continuum seeder and β-barium borate optical parametric amplifier, demonstrated tunability in the 500–660 nm range with 1.3–2.1 ps pulse duration and 15 μJ pulse energy. The other system, consisting of a lithium triborate optical parametric generator in type II noncritical phase matching configuration and β-barium borate optical parametric amplifier, was found to yield visible pulses as high as 18 μJ with 1.5–2.4 ps of pulse duration in the 520–630 nm region. The shot-to-shot stability of generated pulses is less than 10% for both systems. Capability of the present systems is demonstrated by observing ps-TR3 spectra of nickel octaethylporphyrin in coordinating and noncoordinating solvents.
Diagnosing hot electron production by short pulse, high intensity lasers using photonuclear reactions
Authors: Phillips T. W. , Cable M. D. , Cowan T. E. , Hatchett S. P. , Henry E. A. , Key M. H. , Perry M. D. , Sangster T. C. , Stoyer M. A.
Published: July 2002

Solid targets irradiated with 1019 W/cm2 or greater of 1 μm light in picosecond pulses are found to be radioactive. The strongest activities observed are the result of photonuclear reactions in which an energetic photon excites the nucleus sufficiently to produce particle emission leaving a radioactive daughter. The photoreaction cross sections are known for a wide range of nuclei and provide a quantitative measure of the photon flux produced in the target. Both the delayed daughter activities and measurements of the prompt particles emitted in the reaction can be used as diagnostics. Examples of these techniques applied in diagnosing experiments at the Nova laser facility adapted to generate petawatt pulses using chirped pulse amplification will be presented. These results will be compared with bremsstrahlung photon spectra calculated using electron spectra measured in a magnetic spectrograph.
Are electrical properties of an aluminum–porous silicon junction governed by dangling bonds?
Authors: Stievenard D. , Deresmes D.
Published: July 2002

Using an aluminum–porous p+ silicon junction, we have realized a sensor which dc current increases up to two orders of magnitude in the presence of ammonia, as for a series of various gases. To interpret quantitatively this phenomenon, we assume that the conductivity is governed by the width of a channel resulting from the partial depletion of silicon located between two pores. This depleted region is due to the charges trapped on surface states associated with the Si–SiO2 interface where SiO2 is the native silicon oxide. When some gas is adsorbed, mainly on Si–H bonds, we propose there is an electrical screening of the interface states (mainly dangling bonds located in the neighborhood of the Si–H bonds), leading to a decrease of the depleted region, i.e., an increase of the width of the channel and thus an increase of the current.
Tomography of the fast electron bremsstrahlung emission during lower hybrid current drive on TORE SUPRA
Authors: Peysson Yves , Imbeaux Frédéric
Published: July 2002

A new tomography dedicated to detailed studies of the fast electron bremsstrahlung emission in the hard x-ray (HXR) energy range between 20 and 200 keV during lower hybrid (LH) current drive experiments on the TORE SUPRA tokamak [Equipe TORE SUPRA, in Proceedings of the 15th Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville (International Atomic Energy Agency, Vienna, 1995), Vol. 1, AIEA-CN-60 / A1-5, p. 105] is presented. Radiation detection is performed by cadmium telluride semiconductors, which have most of the desirable features for a powerful diagnosing of magnetically confined hot plasmas—compact size, high x-ray stopping efficiency, fast timing characteristics, good energy resolution, no sensitivity to magnetic field, reasonable susceptibility to performance degradation from neutron/γ-induced damages. This instrument is made of two independent cameras viewing a poloidal cross-section of the plasma, with respectively 21 and 38 detectors. A coarse spectrometry—8 energy channels—is carried out for each chord, with an energy resolution of 20 keV. The spatial resolution in the core of the plasma is 4–5 cm, while the time sampling may be lowered down to of 2–4 ms. Powerful inversion techniques based on maximum entropy or regularization algorithms take full advantage of the large number of line-integrated measurements for very robust estimates of the local HXR profiles as a function of time and photon energy. A detailed account of main characteristics and performances of the diagnostic is reported, as well as preliminary results on LH current drive experiments.
Negative-ion implanter for powders and its application to nanometer-sized metal particle formation in the surface of glass beads
Authors: Tsuji Hiroshi , Kido Shunsuke , Sasaki Hitoshi , Gotoh Yasuhito , Ishikawa Junzo
Published: July 2002

We have developed a negative-ion implanter for uniform implantation into each powder surface without particle scattering. It consists of a plasma sputter-type negative-ion source, a mass separator, an acceleration tube, a lens, X–Y deflectors, a 90° deflector, and a Faraday cup with an agitator. The electrostatic 90° deflector bends a horizontal beam to a vertical direction and leads it into the Faraday cup. The agitator is an electromagnetic vibrator at a frequency of 120 Hz, which mixes particles for whole surface treatment and uniform implantation. In this implanter, we obtained no scattering implantation for spherical oxide beads with diameters ranging from 5 to 1000 μm in an agitated state, and also obtained a good uniformity of implanted atoms among beads. For an application of the negative ion implantation into powders, copper ions were implanted into soda-lime glass beads and plates at conditions of 50 and 30 keV, respectively, with 1×1017 ions/cm2. In linear optical properties, both implanted samples show a clear absorption at a photon energy of 2.2 eV due to resonance absorption of copper surface plasmon. In addition, the implanted glass plate shows the large third-order nonlinear susceptibility, χ(3)=1.3×10−7 esu. These results suggest the existence of copper nanometer-sized particles in glass.
Atomic steps on a silicon (001) surface tilted toward an arbitrary direction
Authors: Zhong Lei , Hojo Akimichi , Aiba Yoshiro , Chaki Katuhiro , Yoshikawa Jun , Hayashi Kenro
Published: July 2002

In this letter, we show that the misorientation of a vicinal (001) surface annealed in hydrogen is accommodated by a pair of near-Sa steps which are evolved in two perpendicular {110} independently and coordinately. We demonstrate that the deviation of surface orientation in a tiny area can be readily evaluated by inspecting the local variation of the near-Sa step pair. We finally indicate that the atomic steps on a surface annealed in argon exhibit in contrast, no conformity with the misorientation towards an azimuth other than {110}.
Morphology and photoluminescence improvements from high-temperature rapid thermal annealing of GaN
Authors: Zolper J. C. , Hagerott Crawford M. , Howard A. J. , Ramer J. , Hersee S. D.
Published: July 2002

Rapid thermal annealing of GaN in an Ar or N2 ambient up to 1100 °C is shown to improve surface morphology and photoluminescence intensity. For both ambients the average rms surface roughness as determined by atomic force microscopy decreases from ∼4 nm on the as-grown material to ∼1 nm after a 1100 °C anneal. The band-edge luminescence intensity was increased by a factor of 4 after a 1100 °C anneal in a N2 ambient and a factor of 2 for annealing at 1100 °C in an Ar ambient as compared to as-grown material. The 1100 °C anneal improves the ratio of band edge to deep-level luminescence and also reduces the electron concentration and mobility. The reduction in mobility can be explained in terms of a two-band conduction mechanism where defect band conduction dominates at the lower carrier densities or an increase in the free-carrier compensation ratio.
A bilayer Ti/Ag ohmic contact for highly doped n-type GaN films
Authors: Guo J. D. , Lin C. I. , Feng M. S. , Pan F. M. , Chi G. C. , Lee C. T.
Published: July 2002

Ohmic contacts with low resistance are fabricated on n-type GaN films using Ti/Ag bilayer metallization. The GaN films are grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) with Si as the dopant. Ohmic characteristics are studied for films with carrier concentration range from 1.5×1017 to 1.7×1019 cm−3. The lowest value for the specific contact resistivity of 6.5×10−5 Ω cm2 is obtained without annealing. The barrier height of Ti on GaN is calculated to be 0.067 eV.
Solid-state dye lasers based on polymers incorporating covalently bonded modified rhodamine 6G
Authors: Costela A. , Garcia-Moreno I. , Figuera J. M. , Amat-Guerri F. , Sastre R.
Published: July 2002

We report on the lasing action of modified rhodamine 6G dyes copolymerized with methacrylic monomers. The laser samples were transversely pumped with a N2 laser at 337 nm. Lasing efficiencies similar to those found for rhodamine 6G in ethanol solution have been achieved. Important increases in photostability, with lifetimes in excess of 20 000 shots at 2 Hz, have been obtained. By using a rotating mechanism, a stable output with no sign of degradation after half a million shots has been demonstrated.
Ultrastable emission from a metal–oxide–semiconductor field-effect transistor-structured Si emitter tip
Authors: Itoh Junji , Hirano Takayuki , Kanemaru Seigo
Published: July 2002

A silicon field emitter tip with a dual-gate metal–oxide–semiconductor field-effect transistor (MOSFET) structure was fabricated and demonstrated. The present tip structure is just the same as an n-channel MOSFET whose drain was replaced by a cone-shaped Si tip. Two coplanar gates of 0.3-μm-thick Nb are made on a 0.6-μm-thick thermally oxidized SiO2 insulator between the source and the tip and make inversion layers in a p-type Si substrate under each gate. One of the gates has a 1.8-μm-diam aperture surrounding the tip for extraction of electrons from the tip. The other is 3 μm wide and 300 μm long and is separated by 2 μm from this gate. Ultrastable emission of about 0.3 μA was demonstrated with a single tip for one day.
Quantum cascade disk lasers
Authors: Faist Jerôme , Gmachl Claire , Striccoli Marinella , Sirtori Carlo , Capasso Federico , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

The fabrication and characterization of quantum cascade (QC) disk lasers at 5 μm wavelength is reported. The QC scheme yields several advantages for the disk geometry: an inherently transverse magnetic polarization, tailorable wavelength, and unipolar carrier transport. The low loss whispering gallery resonator allows small devices, 17–125 μm in diameter, and low threshold currents (2.85 mA for a 17 μm diameter disk) with a size independent pulsed threshold current density of 1.2 kAcm−2 at 15 K. From the threshold current density the quality factor of the disks is estimated at Q∼3500. The emission spectra show single and multiple whispering gallery modes.
Highly tetrahedral amorphous carbon films with low stress
Authors: Chhowalla M. , Yin Y. , Amaratunga G. A. J. , McKenzie D. R. , Frauenheim Th.
Published: July 2002

We have deposited boronated highly tetrahedral amorphous carbon (ta-C:B) films with low stress using a filtered cathodic vacuum arc. The sp3 fraction, hardness, and resistivity were measured as a function of the ion energy and were found to reach a maximum above 50 eV for B concentrations of 2% and 4%. The most significant result we found was that highly tetrahedral a-C:B film (sp3≊80%) with low stress (1–3 GPa) with B concentrations up to 4% could be obtained. The B in the films was found to be predominantly (≊75%) sp2 bonded. Additionally, the stress in the films did not vary with the ion energy or sp3 fraction unlike in regular ta-C films.
Passive pulse shaping of femtosecond pulses using birefringent dispersive media
Authors: Radzewicz C. , la Grone M. J. , Krasinski J. S.
Published: July 2002

Theoretical and experimental study of linear propagation of femtosecond laser pulses through a birefringent dispersive medium is presented. Ordinary and extraordinary components of the pulse propagate through the crystal with different group velocities and they acquire different chirp. We show that, under proper circumstances, the output pulse shows significant time dependent change of polarization. The effect can be used to shape such pulses yielding, for example, a series of equally spaced short pulses.
Possible origin for (110)-oriented growth of grains in hydrogenated microcrystalline silicon films
Authors: Sun Yong , Miyasato Tatsuro , Wigmore J. K.
Published: July 2002

We have measured the temperature dependence of preferentially oriented growth of hydrogenated microcrystalline silicon films, prepared by reactive hydrogen plasma sputtering of silicon. X-ray-diffraction measurements showed that the relative number of (110) grains increased rapidly at the expense of the (111), but not (311), directions with increasing substrate temperature between 60 and 400 °C. Over the same temperature range grain sizes for all orientations increased in scale with each other. Calculations of the step energy for grains of different sizes and orientations suggest that the changing number of dangling bonds with increasing grain size may be responsible for the observed temperature dependence of the preferential growth.
Thermal oxidation of free-standing porous silicon films
Authors: Salonen J. , Lehto V.-P. , Laine E.
Published: July 2002

We have studied the thermal oxidation of free-standing porous silicon films from room temperature to 730 °C with a differential scanning calorimeter and a thermogravimeter. We have observed three different thermal oxidation processes for the porous silicon. The change of enthalpy (ΔH) and activation energy (Ea) for the first reaction has been calculated. The oxidation of a fresh sample has been compared with those of aged samples, which were stored in dry relative humidity (RH 0%), humid (RH 100%) and normal (RH 25%–35%) laboratory air atmospheres. We also used Fourier transform infrared spectroscopy to clarify the bonds for each process.
Conformal oxides on Si surfaces
Authors: Tsai Vincent , Wang X.-S. , Williams Ellen D. , Schneir J. , Dixson R.
Published: July 2002

The characteristics of the Si–vacuum interface were compared with the characteristics of the oxide–air interface formed following room temperature oxidation for a variety of samples. Scanning tunneling microscopy was used to measure the surface structure following vacuum preparation, and atomic force microscopy was used to measure the oxide surface on the same samples following exposure to air. Samples investigated included nominally flat Si(111) with equilibrated and quenched surface configurations, Si(111) miscut by 1.25° toward the [2̄11] and equilibrated to yield the faceted structure, and nominally flat Si(001) wafers. In all cases, the step morphology of the clean surfaces was duplicated on the surface of the oxide.
Impurity contamination of GaN epitaxial films from the sapphire, SiC and ZnO substrates
Authors: Popovici Galina , Kim Wook , Botchkarev Andrei , Tang Haipeng , Morkoç Hadis , Solomon James
Published: July 2002

Likely contamination of GaN films by impurities emanating from Al2O3, SiC, and ZnO substrates during growth has been studied by secondary ion mass spectrometry (SIMS) analysis. The defective near-substrate region allows impurities to incorporate more readily as compared to the more perfect crystal as evidenced by increased impurity levels in that region detected by SIMS. The SIMS measurements in GaN layers grown on SiC, ZnO, and sapphire showed large amounts of Si, Zn, and O, respectively, within a region wider than the defective near-substrate layer pointing to the possibility of impurity diffusion at growth temperatures. The qualitative trend observed is fairly clear and significant.
Is ball lightning “Anderson Localization”?: Localized and enhanced fields in a corridor with irregular-shaped metal walls
Authors: Tanaka Kazuo , Tanaka Masahiro
Published: July 2002

We propose that “Anderson Localization” is a possible explanation for ball lightning on the basis of computer simulation of electromagnetic wave propagation in a corridor with irregular-shaped metal walls. It was found that the electromagnetic wave could be strongly localized and its strength could be strongly enhanced in small regions in the corridor under specific conditions. Similar effects may create electric fields strong enough to cause electrical breakdown conditions in the open atmosphere, and this phenomenon may be observed as ball lightning.
Phase separation in InGaN grown by metalorganic chemical vapor deposition
Authors: El-Masry N. A. , Piner E. L. , Liu S. X. , Bedair S. M.
Published: July 2002

We report on phase separation in thick InGaN films with up to 50% InN grown by metalorganic chemical vapor deposition from 690 to 780 °C. InGaN films with thicknesses of 0.5 μm were analyzed by θ–2θ x-ray diffraction, transmission electron microscopy (TEM), and selected area diffraction (SAD). Single phase InGaN was obtained for the as-grown films with <28% InN. However, for films with higher than 28% InN, the samples showed a spinodally decomposed microstructure as confirmed by TEM and extra spots in SAD patterns that corresponded to multiphase InGaN.
A 3 kV Schottky barrier diode in 4H-SiC
Authors: Wahab Q. , Kimoto T. , Ellison A. , Hallin C. , Tuominen M. , Yakimova R. , Henry A. , Bergman J. P. , Janzén E.
Published: July 2002

High-voltage Schottky barrier diodes with low reverse leakage current were processed on hot-wall chemical vapor deposition grown 4H-SiC films. A metal overlap onto the oxide layer was employed to reduce electric field crowding at the contact periphery. By utilizing a 42–47 μm thick, high-quality epitaxial layers with doping in the range of 7×1014–2×1015 cm−3, a record blocking voltage of above 3 kV was achieved. The large diodes with 1.0 mm diameter showed breakdown at 2.1 kV. The reverse leakage current density at 1.0 kV was measured to be 7.0×10−7 A cm−2. Specific on-resistance of the diode with breakdown voltage at 3 kV was 34 mΩ cm2.
Optical characterization of GaN/SiC n-p heterojunctions and p-SiC
Authors: Torvik John T. , Qiu Chang-hua , Leksono Moeljanto , Pankove Jacques I.
Published: July 2002

Optical characterization of GaN/SiC heterojunctions and p-SiC has been performed to explain the current–voltage (I–V) characteristics in GaN/SiC n-p heterojunction diodes. The I–V characteristics exhibit tunneling-assisted current with low forward “turn-on” voltages around 1.15 V as opposed to the expected drift/diffusion current with a turn on around 2.5 V. Electroluminescence (EL) measurements on these diodes revealed an infrared peak at 1.25 eV and a red peak at 1.75 eV. Photoluminescence (PL) measurements on p-SiC yielded peaks at 1.25 and 1.80 eV. Since the band gap of 6H–SiC is 3.03 eV, we attribute the EL and PL peaks to radiative transitions from the conduction band edge to a defect level and subsequently down to the valence band edge of p-SiC. This defect level is located 1.25 eV above the valence band edge.
Deep level defects in n-type GaN grown by molecular beam epitaxy
Authors: Wang C. D. , Yu L. S. , Lau S. S. , Yu E. T. , Kim W. , Botchkarev A. E. , Morkoç H.
Published: July 2002

Deep-level transient spectroscopy has been used to characterize electronic defects in n-type GaN grown by reactive molecular-beam epitaxy. Five deep-level electronic defects were observed, with activation energies E1=0.234±0.006, E2=0.578±0.006, E3=0.657±0.031, E4=0.961±0.026, and E5=0.240±0.012 eV. Among these, the levels labeled E1, E2, and E3 are interpreted as corresponding to deep levels previously reported in n-GaN grown by both hydride vapor-phase epitaxy and metal organic chemical vapor deposition. Levels E4 and E5 do not correspond to any previously reported defect levels, and are characterized for the first time in our studies.
Direct observation of region by region suppression of the switchable polarization (fatigue) in Pb(Zr,Ti)O3 thin film capacitors with Pt electrodes
Authors: Colla E. L. , Hong Seungbum , Taylor D. V. , Tagantsev A. K. , Setter N. , No Kwangsoo
Published: July 2002

The size, shape, and polarization orientation of fatigued areas formed during the suppression of the switchable polarization (Prs) (fatigue) in Pt–PZT–Pt ferroelectric capacitors (FECAPs), were observed by means of atomic force microscopy and by imaging the phase of the piezoelectric vibration induced by a low ac field applied between the top and bottom electrodes. In the virgin state (FECAP as prepared), the pattern of the polarization domains with opposite orientation was randomly distributed with typical sizes of 1–3 μm. The application of a dc field larger than the coercive field (Ec) enabled to fully orient the polarization of the regions in either directions. During the initial fatigue (<35% of suppressed Prs), polarized regions with frozen orientation and size ranging between 100 nm and 1 μm became visible. In the fatigued state (>65% of suppressed Prs), two main configurations of the frozen polarization domains were distinguished. One was characterized by a strong preferential direction (top to bottom electrode) and the other by randomly distributed regions of opposite oriented frozen polarization. The degrees of fatigue obtained by analyzing the vibration phase images are in good agreement with those obtained by standard polarization measurements. It is concluded that the Prs suppression (fatigue) is due to “region by region” or “grain by grain” freezing of Prs and that the frozen Prs can have a preferential orientation.
Tuning the emission wavelength of Si nanocrystals in SiO2 by oxidation
Authors: Brongersma M. L. , Polman A. , Min K. S. , Boer E. , Tambo T. , Atwater H. A.
Published: July 2002

Si nanocrystals (diameter 2–5 nm) were formed by 35 keV Si+ implantation at a fluence of 6×1016 Si/cm2 into a 100 nm thick thermally grown SiO2 film on Si (100), followed by thermal annealing at 1100 °C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O2 at 1000 °C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO2 film surface and later at greater depths. Upon oxidation for 30 min the photoluminescence peak wavelength blueshifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The room temperature luminescence lifetime measured at 700 nm increases from 12 μs for the unoxidized film to 43 μs for the film that was oxidized for 29 min.
Structure and photoluminescence of single AlGaAs/GaAs quantum dots grown in inverted tetrahedral pyramids
Authors: Hartmann Arno , Ducommun Yann , Loubies Laurent , Leifer Klaus , Kapon Eli
Published: July 2002

Arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition in inverted tetrahedral pyramids on {111}B GaAs substrates are investigated. Cross-sectional atomic force microscopy images evidence a pronounced thickening of the GaAs quantum well layer at the tip of the pyramid, giving rise to a lens-like QD structure. Low-temperature photoluminescence and cathodoluminescence spectra show distinct luminescence from the dots, exhibiting filling of QD states separated by 33 meV at increased carrier densities. Luminescence linewidths of 15 meV and line energy variations of less than 5 meV are obtained across mm2 sample areas.
Transmission electron microscopy study of Si nanowires
Authors: Zhou Guang Wen , Zhang Ze , Bai Zhi Gang , Feng Sun Qi , Yu Da Peng
Published: July 2002

Microstructures of Si nanowires (SiNW’s) synthesized using laser ablation were investigated by transmission electron microscopy. The SiNW’s have a high density of structural defects, which may play an important role in the formation of SiNW’s and in the determination of the morphology of the nanowires. A model for the growth mechanism of the SiNW’s was discussed on the basis of the observation.
Confocal Raman spectroscopic observation of hexagonal diamond formation from dissolved carbon in nickel under chemical vapor deposition conditions
Authors: Nishitani-Gamo Mikka , Sakaguchi Isao , Loh Kian Ping , Kanda Hisao , Ando Toshihiro
Published: July 2002

We have studied nucleation and growth of diamond on nickel substrate by microwave plasma assisted chemical vapor deposition followed by in situ carbon–hydrogen solution treatment. The Raman depth profiles of isolated diamond crystals on the (100) and (111) surfaces were measured by confocal Raman spectroscopy. In the case of the crystals having the [111] growth direction, the 1332 cm−1 diamond peak was observed near the diamond–nickel interface and it shifted 1323 cm−1 towards the surface. The 1323 cm−1 peak may be assigned to “hexagonal diamond.” The in-depth spectral change from cubic to hexagonal was observed in chemical vapor deposited diamond crystals. The chemical and/or structural effects of nickel substrate for dissolved carbon are essential for specific diamond crystallization.
Native defect engineering of interdiffusion using thermally grown oxides of GaAs
Authors: Cohen R. M. , Li Gang , Jagadish C. , Burke Patrick T. , Gal Michael
Published: July 2002

Interdiffusion can be either increased or decreased when annealing epitaxial layers covered by an oxide of GaAs. AlGaAs/GaAs quantum wells (QWs) of different widths were grown by organometallic vapor phase epitaxy. A top layer of GaAs was thermally oxidized at 450 °C, and rapid thermal annealing (RTA) was performed at 950 °C under Ar. Photoluminescence showed that an order of magnitude increase in interdiffusion occurred in the oxide-covered QWs compared to uncovered QWs. However, when a thin layer of Al was evaporated over the oxide layer prior to RTA, the rate of interdiffusion was reduced by more than order of magnitude compared to that of uncovered QWs. Interdiffusion slows because the oxide, composed primarily of Ga2O3, is quickly reduced by Al metal during RTA to form atomic Ga and Al2O3. The Al2O3 layer formed over the QWs traps the free Ga as interstitials in the GaAs. The excess Ga interstitial concentration reduces the group III vacancy concentration, and it is this reduction in group III vacancy concentration which slows the interdiffusion rate. The results show that metallurgical reactions may be used as a tool for engineering native defect concentrations and associated diffusivities.
Photoluminescence investigation of InGaN/GaN single quantum well and multiple quantum wells
Authors: Wang T. , Nakagawa D. , Wang J. , Sugahara T. , Sakai S.
Published: July 2002

The photoluminescence investigation at a low temperature was carried out in In0.13Ga0.87N/GaN single quantum well (SQW) and multiple quantum wells with 10 (10QW) or 5 periods. With decreasing number of wells, the emission peak shows a redshift. In the case of a low excitation power, the emission intensity is enhanced by an increase in the number of wells while it decreases in the case of a high excitation power. With increasing excitation power, the emission peak of the SQW exhibits a blueshift and its linewidth decreases, but the emission peak of the 10QW remains unchanged and its linewidth increases. Based on the theory of the quantum confined Stark effect, the behavior of the SQW and the 10QW can be well explained. This result should be highly emphasized in designing InGaN/GaN based optical devices.
Direct evidence of micropipe-related pure superscrew dislocations in SiC
Authors: Huang X. R. , Dudley M. , Vetter W. M. , Huang W. , Wang S. , Carter C. H.
Published: July 2002

A set of powerful x-ray imaging techniques using white-beam synchrotron radiation have been developed and applied to clearly reveal and map micropipes in SiC crystals at a “magnified” level. The experimental results and the corresponding simulations demonstrate explicitly that the micropipes are pure superscrew dislocations (SSDs). Moreover, these techniques provide accurate descriptions of the detailed structure of the SSDs, including the spatial distribution of the strain fields, the magnitudes of the Burgers vectors, the dislocation senses, and the surface relaxation effects.
Noninvasive absorption and scattering spectroscopy of bulk diffusive media: An application to the optical characterization of human breast
Authors: Cubeddu R. , Pifferi A. , Taroni P. , Torricelli A. , Valentini G.
Published: July 2002

A time-resolved reflectance and transmittance spectrophotometer was developed and used to evaluate noninvasively and simultaneously the absorption and transport scattering spectra of bulk turbid media. The experimental setup, based on mode-locked dye and titanium:sapphire lasers and on an electronic chain for time-correlated single photon counting, was fully automated to make in vivo measurements on biological tissues feasible. The optical characterization of breast, of interest for the development of optical mammography, was performed in vivo on a wide spectral range (610–1010 nm). In particular, the absorption spectral features of the main tissue components (hemoglobin, water, and lipids) were recognized with different relative weight, depending on the age and the measurement position. Best fit with the absorption spectra of the main constituents was performed to evaluate their relative concentration in tissues.
The effect of Al–Ta2O5 topographic interface roughness on the leakage current of Ta2O5 thin films
Authors: Kim Y. S. , Lee Y. H. , Lim K. M. , Sung M. Y.
Published: July 2002

Amorphous Ta2O5 thin films were formed by radio-frequency magnetron sputtering at the substrate temperature of 200 °C. The electrical properties of Ta2O5 thin films were investigated as a function of the film thickness. The dominant conduction mechanism transited from the electrode-limited conduction (Schottky emission current) at low field to the bulk-limited conduction (Poole–Frenkel current) at high field. With increasing thickness of the thin films, the surface roughness increased, whereas the transition fields from the electrode-limited to the bulk-limited conduction process decreased. To verify the effect of this surface roughness on the electric conduction mechanism, a two-dimensional numerical simulator, MEDICI, was used to simulate the electric-field distribution at the bulk region of the thin film and the interface region between the thin film and the electrode.
Imaging of acoustic charge transport in semiconductor heterostructures by surface acoustic waves
Authors: Streibl M. , Wixforth A. , Kotthaus J. P. , Govorov A. O. , Kadow C. , Gossard A. C.
Published: July 2002

We demonstrate room-temperature acoustic charge transport of electrons and holes in an InGaAs/GaAs heterostructure. The carriers are optically generated by interband absorption and then separated, stored, and transported in the piezoelectric potential superlattice of a surface acoustic wave. The charge distribution is detected with a spatial resolution of a few acoustic wavelengths by a second orthogonal probe beam, genererated by so-called tapered transducers. The image information is given as a phase shift signal in frequency space and allows for the direct comparison of the number of generated and transported carriers. Regions of mere carrier drag and full carrier capture and transport are observed simultaneously.
Self-assembly of floating magnetic particles into ordered structures: A promising route for the fabrication of tunable photonic band gap materials
Authors: Golosovsky M. , Saado Y. , Davidov D.
Published: July 2002

We report stable ordered arrays of millimeter-size magnetic particles floating on a liquid surface. Self-assembly into a regular two-dimensional lattice results from lateral magnetic interactions between the particles. The lattice constant may be easily tuned by the application of external magnetic field. The array symmetry is designed by using different particle shapes, magnets, and magnet position inside the particle, so that complex symmetries may be achieved. Three-dimensional ordered arrays are obtained in a stack of troughs containing floating magnets. Computer simulations of electromagnetic wave propagation in such three-dimensional structures suggest an opening of a tunable photonic band gap in the microwave range.
Electrical characterization of isoelectronic In-doping effects in GaN films grown by metalorganic vapor phase epitaxy
Authors: Chung H. M. , Chuang W. C. , Pan Y. C. , Tsai C. C. , Lee M. C. , Chen W. H. , Chen W. K. , Chiang C. I. , Lin C. H. , Chang H.
Published: July 2002

Indium isoelectronic doping was found to have profound effects on electrical properties of GaN films grown by metalorganic chemical vapor deposition. When a small amount of In atoms was introduced into the epilayer, the ideality factor of n-GaN Schottky diode was improved from 1.20 to 1.06, and its calculated saturation current could be reduced by 2 orders of magnitude as compared to that of the undoped sample. Moreover, it is interesting to note that In isodoping can effectively suppress the formation of deep levels at 0.149 and 0.601 eV below the conduction band, with the 0.149 eV trap concentration even reduced to an undetected level. Our result indicates that the isoelectronic In-doping technique is a viable way to improve the GaN film quality.
Composition-control of magnetron-sputter-deposited (BaxSr1−x)Ti1+yO3+z thin films for voltage tunable devices
Authors: Im Jaemo , Auciello O. , Baumann P. K. , Streiffer S. K. , Kaufman D. Y. , Krauss A. R.
Published: July 2002

Precise control of composition and microstructure is critical for the production of (BaxSr1−x)Ti1+yO3+z (BST) dielectric thin films with the large dependence of permittivity on electric field, low losses, and high electrical breakdown fields that are required for successful integration of BST into tunable high-frequency devices. Here, we present results on composition-microstructure-electrical property relationships for polycrystalline BST films produced by magnetron-sputter deposition, that are appropriate for microwave and millimeter-wave applications such as varactors and frequency triplers. Films with controlled compositions were grown from a stoichiometric Ba0.5Sr0.5TiO3 target by control of the background processing gas pressure. It was determined that the (Ba+Sr)/Ti ratios of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar+O2) process pressure, while the O2/Ar ratio did not strongly affect the metal ion composition. Film crystalline structure and dielectric properties as a function of the (Ba+Sr)/Ti ratio are discussed. Optimized BST films yielded capacitors with low dielectric losses (0.0047), among the best reported for sputtered BST, while still maintaining tunabilities suitable for device applications.
Smoothening of Cu films grown on Si(001)
Authors: Lukaszew R. A. , Sheng Y. , Uher C. , Clarke R.
Published: July 2002

We report an in situ study of the molecular-beam epitaxy growth and annealing of Cu(001) films grown on hydrogen-terminated Si(001) substrates, resulting in a promising approach to achieve smooth epitaxial morphology. Using correlated reflection high-energy electron diffraction and scanning tunneling microscopy data, we find a temperature interval below the onset of silicide formation where a dramatic smoothening of the epitaxial Cu surfaces occurs. Our measurements indicate that a reduction in roughness is possible in this regime because the annealing is controlled by lateral diffusion kinetics.
dc-electric-field-induced microwave loss in ferroelectrics and intrinsic limitation for the quality factor of a tunable component
Authors: Tagantsev Alexander
Published: July 2002

An effect of dc electric field on the intrinsic microwave loss in incipient ferroelectrics is theoretically studied. The contribution of the so-called field-induced quasi-Debye mechanism, which is related to the relaxation of phonon distribution function, is evaluated. It is found that the dc-field-induced loss is controlled by the relaxation of the acoustic-phonon distribution function rather that the distribution function of the soft-mode phonons, as was earlier assumed. The theory gives qualitative explanation for a dramatic field-induced increase of the loss factor recently reported for KTaO3 and SrTiO3 bulk resonators with superconductive electrodes. Numerical estimates performed for KTaO3 are in a good agreement with the experimental data. The field and frequency dependence of the figure of merit of a ferroelectric-based tunable microwave component is evaluated.
Bubble formation in organic light-emitting diodes
Authors: Liao L. S. , He J. , Zhou X. , Lu M. , Xiong Z. H. , Deng Z. B. , Hou X. Y. , Lee S. T.
Published: July 2002

Bubbles in organic light-emitting diodes can be formed from gas release due to Joule heating effect at localized electrical shorts during operation, which could be simulated by a rapid thermal annealing. The gases in the bubbles consist of not only adsorbed moistures but also the decomposed organic species, which are detected in situ in an ultrahigh vacuum chamber. In the device of Al/tris-(8-hydroxyquinoline) aluminum (Alq/N,N′-diphenyl-N.N′-bis-{3-methylphenyl}{1,1′biphenyl}-4,4′-diamine/indium tin oxide (ITO), the gases released from ITO surface were mainly of adsorbed moistures, while those released from the organic layers were of both the decomposed products from Alq and the trapped moistures. The decomposition of Alq could not be easily avoided if there were severe localized electrical shorts in the devices.
Three-dimensional instability of a liquid layer flowing down a heated vertical cylinder
Authors: Dávalos-Orozco L. A. , You Xueyi
Published: July 2002

The three-dimensional thermocapillary instability of a fluid film coating the outside or the inside of a cylinder is investigated in the absence and in the presence of gravity. In the absence of gravity (pure thermocapillarity), it is found that it is possible to excite high azimuthal modes as the most unstable. In the presence of gravity the thermocapillary instability of a thin fluid film flowing down a vertical cylinder is investigated. It is shown that thermocapillarity also promotes the instability of high azimuthal modes, even higher than those found in the pure thermocapillary case. This is in contrast with the isothermal case where only the zeroth mode (the longitudinal one) is the most unstable and also with previous work on flow down a rotating cylinder where, in some conditions, rotation may promote the first azimuthal mode as the most unstable. Curves of criticality and those corresponding to the maximum growth rate are given for different values of the parameters.