bluuebunny/crossref_metadata_2025 huggingface dataset:
A compact micropositioner for use in ultrahigh vacuum
Authors: Curtis Robert , Pearson Chris , Gaard Peter , Ganz Eric
Published: July 2002

We have designed and built a low profile compact micropositioner for use in air or ultrahigh vacuum. The dimensions are 3.1×1.9×0.75 cm3 with a 1.4 cm range of motion. The device is actuated by two piezoelectric tubes, and utilizes both stick slip and sequential clamping to attain linear motion in one dimension. For motion in two dimensions the flat profile allows units to be stacked. The micropositioner is capable of climbing an incline of 30° while carrying a 17 g load. It is driven forward by staggered 0–250 V signals with a fast rise time and slow decay time. We obtained step sizes from 30 to 400 nm with standard deviation less than 15 nm, and have demonstrated a maximum average velocity of 0.14 mm/s.
A simple self-propelled two-dimensional micropositioner
Authors: Blackford B. L.
Published: July 2002

A quartz tube is propelled horizontally along a quartz track by the stick-slip inertial slider method, activated by the longitudinal mode of a piezoelectric tube mounted inside the quartz tube. Two-dimensional motion is achieved by rotation of the quartz tube, activated by the bending mode of the piezo. With a sample attached to the quartz tube, the device is an excellent replacement for the electrostatic-clamp type of walker used as the coarse approach mechanism in scanning probe microscopes (SPMs). Alternatively, by attaching the tip (or sample) to the piezo tube, the device can function as a very simple SPM in which the same piezo tube is used for coarse approach and for scanning. The device performs reliably in air and high vacuum, and is ultrahigh vacuum compatible. Step sizes from 10 to 1000 nm, and speeds up to 0.2 mm/s are possible.
Resonance EMAT system for acoustoelastic stress measurement in sheet metals
Authors: Hirao M. , Ogi H. , Fukuoka H.
Published: July 2002

A practical method of acoustoelastic stress measurement based on electromagnetic acoustic resonance is presented. This overcomes fundamental limitations of conventional procedures and exhibits the stress resolution to 0.1 MPa for thin aluminum plates and the spatial resolution to several millimeter square in a noncontacting operation. The proposed method successfully combines an electromagnetic acoustic transducer (EMAT) and a superheterodyne phase- sensitive detector. An EMAT is excited by a high-power rf burst in the 0.5–20-MHz range and generates ultrasonic oscillations in a plate through a Lorentz force mechanism. The signals in the plate are then received by this same EMAT and analyzed for the amplitude spectrum using swept-frequency phase-sensitive detection circuitry. The resonance frequencies can be determined for longitudinal and shear modes simultaneously. To illustrate the resonance spectrum technique, experimental results are shown for the measurement of a two-dimensional stress field in a thin aluminum plate. Liftoff sensitivity is also determined.
10 μs FWHM gas pulse generation using a pulsed solenoid valve
Authors: Huang Yuhui , Sulkes Mark
Published: July 2002

A General Valve Series 9 pulsed solenoid valve, when equipped with a 3.3 Ω coil and driven by 400 μs low voltage (∼22–65 V) pulses, can stably produce gas pulses as little as 10 μs FWHM. This capability can enhance the performance of a skimmed supersonic molecular beam system.
A modern and fully automated calibration system for space ion mass spectrometers
Authors: Steinacher M. , Jost F. , Schwab U.
Published: July 2002

Major parts of our proven calibration facility have been totally renewed and other equipment has been added to fulfill the actual requirements for efficient calibration of modern space ion mass spectrometers. The main change was the replacement of the old two axes turntable by a new four axes turntable that can be used in a wide range with high precision. An additional effort has been made to design and test a new expansion system with higher transmission and improved quality with respect to overall homogeneity, parallel trajectories, and angular divergence of the ion beam. A personal computer is used to survey the state of the whole vacuum system permanently and to vent, pump down, and bakeout the calibration chamber automatically. A main computer has been installed which controls the turntable, the ion beam, the beam diagnostics, experimental high voltages, and the data acquisition with our detectors. A general interface allows the full control of the whole system for the complete automated calibration of a flight instrument. As an example, the calibration of the toroidal imaging mass-angle spectrograph instrument for the POLAR mission is described.
Characterization of a complementary metal-oxide semiconductor operational amplifier from 300 to 4.2 K
Authors: Hastings J. Todd , Ng K.-W.
Published: July 2002

We report the first operation of a commercially available complementary metal-oxide semiconductor operational amplifier, at liquid helium temperature. In addition, we have characterized several factors important to the practical application of such a circuit from room temperature down to 4.2 K. The temperature dependence and measurement techniques for open-loop gain, input offset voltage, input referred noise voltage, and quiescent current are presented. We will discuss our observations of low temperature behavior of the opamp with respect to others’ previous results. This work represents an advancement over earlier studies which only reported opamp operation down to 77 or 30 K with measurements taken only at a limited number of temperatures instead of a broad range. Our data suggest that under special operating conditions the opamps can be effectively used with careful consideration of noise and gain performance. Input offset voltage levels and quiescent current (including power consumption) resemble normal room temperature operation.
Analysis of the enthalpy probe technique for thermal plasma diagnostics
Authors: Rahmane Mohamed , Soucy Gervais , Boulos Maher I.
Published: July 2002

An experimental analysis of the enthalpy probe technique for thermal plasma diagnostics is presented. Emphasis is given to the probe sensitivity, its accuracy, and its effect on the plasma flow. The study was carried out using a direct current (dc) plasma torch operated with Ar/H2 mixture (1% H2) under two different reactor chamber pressures, 400 and 200 torr. The results show that when the probe tip is coated with a thermal barrier coating, the sensitivity is improved because of the reduction of the tare heat flux to the probe surface. Thus, even at low values of gas sampling flow rate, the probe can still provide measurements with acceptable accuracy while the isokinetic sampling condition continues to be respected. Emission spectroscopic results show that the probe has negligible effect on the temperature field of the plasma jet.
GaN thin films deposited via organometallic vapor phase epitaxy on α(6H)–SiC(0001) using high-temperature monocrystalline AlN buffer layers
Authors: Warren Weeks T. , Bremser Michael D. , Ailey K. Shawn , Carlson Eric , Perry William G. , Davis Robert F.
Published: July 2002

Monocrystalline GaN(0001) thin films, void of oriented domain structures and associated low-angle grain boundaries, have been grown via organometallic vapor phase epitaxy (OMVPE) on high-temperature monocrystalline AlN(0001) buffer layers predeposited on vicinal α(6H)–SiC(0001) wafers using TEG, TEA, and ammonia in a cold wall, vertical, pancake-style reactor. The surface morphology was smooth, and the PL spectrum showed strong near-band-edge emission with a full width at half-maximum (FWHM) value of 4 meV. The dislocation density within the first 0.5 μm was ≊1×109 cm−2; it decreased substantially with increasing film thickness. Controlled n-type Si doping of GaN has been achieved for net carrier concentrations ranging from ∼1×1017 to 1×1020 cm−3. Double-crystal XRC measurements indicated a FWHM value of 66 arcsec for the GaN(0004) reflection.
Crossed aperture lenses for the correction of chromatic and aperture aberrations
Authors: Baranova L. A. , Yavor S. Ya. , Read F. H.
Published: July 2002

The chromatic and aperture aberrations of crossed five-aperture lenses are analyzed by direct ray tracing. The apertures are rectangular and the voltages are applied in such a way that the first-order properties of the crossed lens are similar to those of a quadrupole doublet. It is shown that in astigmatic modes the chromatic and aperture aberrations of one of the linear images can be simultaneously eliminated or made negative. It is also shown that stigmatic modes exist in which the magnification is different in two perpendicular planes and in which the image blurring caused by the chromatic and aperture aberrations in the direction of smaller magnification is ten times smaller than that given by a round lens of the same focal length and the blurring in the other direction is at least two times smaller. The stigmatic crossed lens also gives a larger working distance than the equivalent round lens. The crossed lens will therefore be preferable for many probe forming systems.
A high sensitivity two-color interferometer for pulsed power plasmas
Authors: Weber B. V. , Fulghum S. F.
Published: July 2002

A high sensitivity, high bandwidth, two-color interferometer (1064 and 532 nm) has been tested on the Hawk pulsed power generator at the Naval Research Laboratory. The phase resolution is 10−5 waves with a rise time of 3 ns, a new capability for diagnosing plasmas, and neutrals in pulsed power experiments. The two-color feature is used to distinguish phase shifts from free (plasma) electrons and bound (neutral and ion) electrons. Simultaneous electron and neutral density measurements were demonstrated in a plasma opening switch (POS) experiment. The ability to measure small phase shifts with fast rise time were demonstrated in a plasma filled diode experiment. The high sensitivity and vibration isolation enable neutral gas distribution measurements from supersonic nozzles used in plasma radiation source experiments. Examples of these measurements and future applications are described.
Imprint of sub-25 nm vias and trenches in polymers
Authors: Chou Stephen Y. , Krauss Peter R. , Renstrom Preston J.
Published: July 2002

A nanoimprint process that presses a mold into a thin thermoplastic polymer film on a substrate to create vias and trenches with a minimum size of 25 nm and a depth of 100 nm in the polymer has been demonstrated. Furthermore, the imprint process has been used as a lithography process to fabricate sub-25 nm diameter metal dot arrays of a 100 nm period in a lift-off process. It was found that the nanostructures imprinted in the polymers conform completely with the geometry of the mold. At present, the imprinted size is limited by the size of the mold being used; with a suitable mold, the imprint process should mold sub-10 nm structures with a high aspect ratio in polymers. The nanoimprint process offers a low cost method for mass producing sub-25 nm structures and has the potential to become a key nanolithography method for future manufacturing of integrated circuits and integrated optics.
Vibration compensation for high speed scanning tunneling microscopy
Authors: Croft D. , Devasia S.
Published: July 2002

Low scanning speed is a fundamental limitation of scanning tunneling microscopes (STMs), making real time imaging of surface processes and nanofabrication impractical. The effective scanning bandwidth is currently limited by the smallest resonant vibrational frequency of the piezobased positioning system (i.e., scanner) used in the STM. Due to this limitation, the acquired images are distorted during high speed operations. In practice, the achievable scan rates are much less than 1/10th of the resonant vibrational frequency of the STM scanner. To alleviate the scanning speed limitation, this article describes an inversion-based approach that compensates for the structural vibrations in the scanner and thus, allows STM imaging at high scanning speeds (relative to the smallest resonant vibrational frequency). Experimental results are presented to show the increase in scanning speeds achievable by applying the vibration compensation methods.
Reduction of the C54–TiSi2 phase transformation temperature using refractory metal ion implantation
Authors: Mann R. W. , Miles G. L. , Knotts T. A. , Rakowski D. W. , Clevenger L. A. , Harper J. M. E. , D’Heurle F. M. , Cabral C.
Published: July 2002

We report that the ion implantation of a small dose of Mo into a silicon substrate before the deposition of a thin film of Ti lowers the temperature required to form the commercially important low resistivity C54–TiSi2 phase by 100–150 °C. A lesser improvement is obtained with W implantation. In addition, a sharp reduction in the dependence of C54 formation on the geometrical size of the silicided structure is observed. The enhancement in C54 formation observed with the ion implantation of Mo is not explained by ion mixing of the Ti/Si interface or implant-induced damage. Rather, it is attributed to an enhanced nucleation of C54–TiSi2 out of the precursor high resistance C49–TiSi2 phase.
New nonlinear optical crystal: Cesium lithium borate
Authors: Mori Yusuke , Kuroda Ikuo , Nakajima Satoshi , Sasaki Takatomo , Nakai Sadao
Published: July 2002

A new nonlinear optical crystal CsLiB6O10 (CLBO), is described that can be grown from either stoichiometric melt or from solution. A large, high quality single crystal with dimensions of 14×11×11 cm3 was obtained by the top-seeded Kyropoulos method. Fourth harmonic and fifth harmonic generations of the 1.064 μm Nd:YAG laser radiation with type-I phase matching were realized in the CLBO crystal. Output pulse energies obtained were 110 mJ at 266 nm and 35 mJ at 213 nm.
ArF and KrF excimer laser deposition of yttria-stabilized zirconia on Si(100)
Authors: Delgado J. C. , Sánchez F. , Aguiar R. , Maniette Y. , Ferrater C. , Varela M.
Published: July 2002

Yittria-stabilized zirconia films have been deposited on Si(100) substrates by ArF (193 nm) and KrF (248 nm) laser ablation. By using KrF radiation, high-quality (h00) epitaxial films are obtained on bare silicon. Epitaxy, although with slightly worse properties, is also obtained on substrates covered with a native oxide. Films have been deposited by ArF laser ablation over a wide range of substrate temperature and oxygen partial pressure and with variation in substrate surface cleaning. These films are polycrystalline and texture or epitaxy have not been obtained in any case. These results reveal that laser wavelength is a crucial factor in determining film properties.
Electron and hole transport in poly(p-phenylene vinylene) devices
Authors: Blom P. W. M. , de Jong M. J. M. , Vleggaar J. J. M.
Published: July 2002

The transport properties of electrons and holes in poly(dialkoxy-p-phenylene vinylene) (PPV) are investigated by current–voltage measurements using Ca as an electron and indium-tin-oxide as a hole injecting contact. Both the electron and hole currents are dominated by the bulk conduction properties of the PPV, in contrast to previous reports. The hole current is governed by bulk space-charge limited conductivity and a hole mobility of 0.5×10−6 cm2/V s is determined. The electron current is strongly reduced by the presence of traps with a total density of 1018 cm−3.
Fundamental optical transitions in GaN
Authors: Chen G. D. , Smith M. , Lin J. Y. , Jiang H. X. , Wei Su-Huai , Asif Khan M. , Sun C. J.
Published: July 2002

A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time-resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.
Indium contamination from the indium–tin–oxide electrode in polymer light-emitting diodes
Authors: Schlatmann A. R. , Floet D. Wilms , Hilberer A. , Garten F. , Smulders P. J. M. , Klapwijk T. M. , Hadziioannou G.
Published: July 2002

We have found that polymer light-emitting diodes (LEDs) contain high concentrations of metal impurities prior to operation. Narrow peaks in the electroluminescence spectrum unambiguously demonstrate the presence of atomic indium and aluminum. Rutherford backscattering spectroscopy (RBS) and x-ray photoelectron spectroscopy (XPS) depth profiling data corroborate this result. An average indium concentration of 5×1019atoms/cm3 originating from the indium–tin–oxide (ITO) electrode has been found in the polymer layer.
A superconducting bolometer with strong electrothermal feedback
Authors: Lee Adrian T. , Richards Paul L. , Nam Sae Woo , Cabrera Blas , Irwin K. D.
Published: July 2002

We present a theoretical analysis and experimental evaluation of a transition-edge superconducting bolometer for detecting infrared and millimeter waves. The superconducting film is voltage biased and the current is read by a superconducting quantum interference device ammeter. Strong electrothermal feedback maintains the sensor temperature within the transition, gives a current responsivity that is simply the inverse of the bias voltage, and reduces the response time by several orders of magnitude below the intrinsic time constant C/G. We evaluated a voltage-biased bolometer that operates on the Tc∼95 mK transition of a tungsten film with a thermal conductance of G∼1.2×10−9 W/K. As expected, the electrical noise equivalent power of 3.3×10−17/W√Hz is close to the thermal fluctuation noise limit and is lower than that of other technologies for these values of G and temperature. The measured time constant of 10 μs is ∼100 times faster than the intrinsic time constant.
Neutron diffraction measurement of stress-induced transformation in superelastic NiTi
Authors: Bourke M. A. M. , Vaidyanathan R. , Dunand D. C.
Published: July 2002

The formation of stress-induced martensite in superelastic NiTi was studied by neutron diffraction during uniaxial compressive loading and unloading. The respective phase fractions were determined as a function of the applied stresses using a Rietveld refinement with a March–Dollase texture formulation. Before loading, the specimen was fully austenitic. At the highest applied stress of −625 MPa, about 90% of the austenitic phase had transformed to martensite, with a concomitant macroscopic strain of −2.8%. Upon unloading, all of the stress-induced martensite reverted to austenite and the totality of the macroscopic strain was recovered. The propensity for various austenitic crystallographic orientations to transform at different stresses was determined and qualitative observation of this incipient texture in the austenite and of the inherent texture in the nascent martensite are reported.
Effect of oxygen incorporation at AlGaAs/GaAs interfaces on the electrical properties of two-dimensional electron gas
Authors: Kadoya Y. , Noge H. , Someya T. , Sakaki H.
Published: July 2002

We have investigated the carrier concentration and mobility of the two-dimensional electron gas (2DEG) in AlGaAs/GaAs selectively doped heterojunction structures that involve oxygen impurities at the heterointerfaces. The oxygen impurities were incorporated during growth interruption of molecular beam epitaxy at the interfaces for several tens of minutes. It is shown that the carrier concentration of the 2DEG does not change for the areal density of the oxygen impurity up to 6×1011 cm−2, but it decreases drastically when the oxygen density exceeds 3×1012 cm−2. In contrast, the decrease of the mobility is observable for a much smaller oxygen density of the order of 1010 cm−2. We show that these influences are well explained by the formation of charged states at the interface.
Mg-doped green light emitting diodes over cubic (111) MgAl2O4 substrates
Authors: Sun C. J. , Yang J. W. , Lim B. W. , Chen Q. , Anwar M. Zubair , Khan M. Asif , Osinsky A. , Temkin H. , Schetzina J. F.
Published: July 2002

We report on the fabrication and characterization of Mg-doped green light emitting diodes (LEDs) over cubic (111) MgAl2O4 substrates with a very strong impurity band electroluminescence. A 0.2-micron-thick Mg-doped In0.13Ga0.87N layer was used in the active region of mesa type LED structures. The emission spectrum was centered at 510 nm (green) with a full-width at half-maximum of 60 nm. At a forward bias current of 20 mA the output power and the external quantum efficiency were about 200 μW and 0.3%. The origin of green light in the Mg-doped In0.13Ga0.87N layer has also been studied by time resolved photoluminescence. Our study demonstrates impurity band LEDs to be a viable alternative to band edge emission green LEDs with high indium mole fractions in the active region.
Reduction of InGaAs/GaAs laser facet temperatures by band gap shifted extended cavities
Authors: Piva P. G. , Fafard S. , Dion M. , Buchanan M. , Charbonneau S. , Goldberg R. D. , Mitchell I. V.
Published: July 2002

Reflectance modulation measurements have been used to determine facet temperatures of InGaAs/GaAs double quantum well (QW) graded-index separate-confinement heterostructure ridge-waveguide lasers possessing band gap tuned passive cavity sections. We find that the incorporation of transparent extended cavities, produced by ion-implantation enhanced QW intermixing, significantly decreases the laser facet temperatures. The reduced photoabsorption occurring at the facets, achieved by the QW intermixing process, should lead to increases in both the maximum optical power levels and device longevity prior to the onset of catastrophic failure.
Dynamic growth effects during low-pressure deposition of diamond films
Authors: Gilbert Donald R. , Singh Rajiv , Clarke Roy , Murugkar S.
Published: July 2002

Diamond films were deposited in a modified electron–cyclotron-resonance plasma system operating at pressures between 1.0 and 2.0 Torr. This system provides the advantage of efficient plasma generation due to magnetic enhancement and high diffusion rates due to relatively low-pressure operation. Films were formed from preexisting seed layers providing high “nucleation” densities to promote rapid coalescence. Raman analysis of grown films showed a quality dependence on both deposition pressure and nucleation density. We speculate that the increased presence of amorphous carbon and larger film stresses is the result of grain-boundary impurity effects in the seeded films. Oxygen addition improved film quality by reducing nondiamond carbon incorporation.
Fabrication of ZnSe quantum dots under Volmer–Weber mode by metalorganic chemical vapor deposition
Authors: Liao M. C. Harris , Chang Y. H. , Chen Y. F. , Hsu J. W. , Lin J. M. , Chou W. C.
Published: July 2002

The possibility of fabricating quantum dots under Volmer–Weber growth mode is investigated. Layers of ZnSe/ZnS were grown by metalorganic chemical vapor deposition on both Si and GaAs substrates. The images of surface morphology, taken by atomic force microscopy, showed that the layers were grown in three-dimensional islands. Blueshift was observed in the photoluminescence spectra up to room temperature for these samples. This blueshift was shown to originate from the ZnSe islands. The effect of ZnSe growth duration was investigated. It was found that the blueshift increased with shorter ZnSe growth durations, but this near band gap emission disappeared when the ZnSe growth duration was longer than 5 s. Effects of quantum confinement and strain were considered to resolve the origin of the blueshift. Our result suggests that carrier confinement plays a dominant role.
Blue luminescence in films containing Ge and GeO2 nanocrystals: The role of defects
Authors: Zacharias M. , Fauchet P. M.
Published: July 2002

X-ray measurements show that GeO2 and Ge nanocrystals with a radius from 25 down to 1.5 nm are formed in an oxide matrix after annealing. Under ultraviolet excitation, both types of films luminesce around 3.1 eV, with identical photoluminescence (PL) line shapes and subnanosecond PL dynamics. The strongest PL intensity was found for the films containing GeO2 crystals and for the largest nanocrystals. These results are a clear indication that although the blue luminescence is without a doubt correlated with the formation of Ge (or GeO2) nanocrystals, it is not produced by the radiative recombination of excitons confined in the nanocrystals. Possible mechanisms for the luminescence include defects at the nanocrystal/matrix interface or in the matrix itself.
Preparation and properties of triple perovskite La3−3xCa1+3xMn3O10 ferromagnetic thin films
Authors: Asano H. , Hayakawa J. , Matsui M.
Published: July 2002

The ferromagnetic compound La3−3xCa1+3xMn3O10 with a triple perovskite structure can be successfully synthesized by using a thin-film growth method. Sputtered a-axis thin films with x=0.3 have been examined with respect to their magnetotransport properties. For the triple perovskite compound, we have observed the features, including the enhanced magnetoresistene (MR) effect and the characteristic low-temperature MR effect resulting from intragrain spin-polarized tunneling, which were reported for the double perovskite manganites. A comparison of the magnitude of these features in triple and double perovskite manganites suggests that the features are actually determined by the c-axis Mn–O bond configuration in a layered-perovskite ferromagnet.
Growth of AlN and GaN on 6H–SiC(0001) using a helium supersonic beam seeded with ammonia
Authors: Torres V. M. , Stevens M. , Edwards J. L. , Smith David J. , Doak R. B. , Tsong I. S. T.
Published: July 2002

We have grown AlN and GaN layers on 4° off-axis 6H–SiC (0001) substrates using He supersonic beams seeded with NH3. The AlN films were used as buffer layers for GaN growth at 800°C. We estimate 39% incorporation of the NH3 molecules impinging on the substrate surface during GaN film growth. High structural quality of the epitaxial GaN layers was confirmed by transmission electron microscopy and electron channeling patterns. The GaN films, which had a thickness of ∼105 nm, contained a defect density of ∼2×1010 cm−2.
Direct writing of conductive aluminum line on aluminum nitride ceramics by transversely excited atmospheric CO2 laser
Authors: Yabe Hiroyuki , Takahashi Akihisa , Sumiyoshi Tetsumi , Obara Minoru , Ishii Katsunori
Published: July 2002

The fabrication of very low resistance aluminum conductor line on the aluminum nitride (AlN) by transversely excited atmospheric (TEA) CO2 laser writing was demonstrated. A minimum electrical resistance was measured to be less than 0.1 Ω/mm with a linewidth of 500 μm. The electrical resistance measured is lower than those of conventional conductor lines used for electronic circuits. The x-ray photoelectron spectroscopy depth profile analysis of aluminum layers can explain the resistance of the metallic aluminum layer formed. Compared to the experimental data by KrF laser, TEA CO2 laser writing is found to be a more effective process for fabricating a low resistance conductor line on AlN than the KrF excimer laser.
The effect of applied strain on the resistance of VO2 thin films
Authors: Gregg J. M. , Bowman R. M.
Published: July 2002

Vanadium dioxide thin films have been synthesized by pulsed laser deposition from pure vanadium metal targets. The electrical characteristics of these films were measured as a function of externally applied strain, based on three-point bend geometry. It is observed that strain significantly affects the resistance of the VO2 thin films below their metallic-semiconducting transformation temperature (0.04% in-plane tensile strain causing ∼35% reduction in resistance). Moreover, the levels of strain required to cause such resistance changes are well within those which can be produced by many strictive materials.
High performance InAs/Ga1-xInxSb superlattice infrared photodiodes
Authors: Fuchs F. , Weimer U. , Pletschen W. , Schmitz J. , Ahlswede E. , Walther M. , Wagner J. , Koidl P.
Published: July 2002

The optical and electrical properties of infrared photodiodes diodes based on InAs/(GaIn)Sb superlattices grown by molecular beam epitaxy were investigated. The diodes, with a cut-off wavelength around 8 μm show a current responsivity of 2 A/W. By proper adjustment of the p-doping level above the n-background concentration the depletion width exceeds a critical size of about 60 nm, leading to the suppression of band-to-band tunneling currents. Above that critical width the dynamic impedance R0A at 77 K reaches values above 1 kΩ cm2 leading to a Johnson-noise-limited detectivity in excess of 1×1012 cm√Hz/W.
Induction mapping of Nd2Fe14B magnetic domains by electron holography
Authors: McCartney M. R. , Zhu Yimei
Published: July 2002

The induction of 90° magnetic domains in a Nd2Fe14B hard magnet has been mapped at high spatial resolution using off-axis electron holography. Line profiles from the reconstructed phase image established an upper limit for domain wall width of 10 nm. Thermal annealing resulted in domain wall movement, magnetization rotation, and evidence for pinning at a structural defect.
Improved reliability of red GaInP vertical-cavity surface-emitting lasers using bias-induced annealing
Authors: Herrick Robert W. , Petroff Pierre M.
Published: July 2002

We have used a bias-induced annealing process to improve the initial performance and the reliability of red (680 nm) Ga.46In.54P vertical-cavity surface-emitting lasers. Measurements showed improved cathodoluminescence efficiency and increased current collection efficiency after the anneal. Trap concentration is believed to be reduced by the bias-induced annealing process.
High-resolution imaging of contact potential difference with ultrahigh vacuum noncontact atomic force microscope
Authors: Kitamura Shin’ichi , Iwatsuki Masashi
Published: July 2002

An ultrahigh vacuum scanning Kelvin probe force microscope (UHV SKPM) utilizing the gradient of electrostatic force, was developed based on an ultrahigh vacuum noncontact atomic force microscope (NC-AFM) capable of atomic level imaging, and used for simultaneous observation of contact potential difference (CPD) and NC-AFM images. CPD images of a Si(111) surface with Au deposited, clearly showed the potential difference in phases between 7×7 and 5×2 structures. When Ag was deposited as a submonolayer on the Si(111) 7×7 reconstructed surface, the atomic level lateral resolution was observed in CPD images as well as in NC-AFM topographic images.
Room-temperature ultraviolet laser emission from self-assembled ZnO microcrystallite thin films
Authors: Tang Z. K. , Wong G. K. L. , Yu P. , Kawasaki M. , Ohtomo A. , Koinuma H. , Segawa Y.
Published: July 2002

Room-temperature ultraviolet (UV) laser emission of ZnO microcrystallite thin films is reported. The hexagonal ZnO microcrystallites are grown by laser molecular beam epitaxy. They are self-assembled and parallelly arrayed on sapphire substrates. The facets of the hexagons form natural Fabry–Pérot lasing cavities. The optical gain for the room-temperature UV stimulated emission is of an excitonic nature and has a peak value an order of magnitude larger than that of bulk ZnO crystal. The observation of room-temperature UV lasing from the ordered, nano-sized ZnO crystals represents an important step towards the development of nanometer photoelectronics.
Contrast mechanisms in scanning ion microscope imaging for metals
Authors: Sakai Y. , Yamada T. , Suzuki T. , Sato T. , Itoh H. , Ichinokawa T.
Published: July 2002

The contrast of the secondary electron images in scanning ion microscopy (SIM) is compared with that in scanning electron microscopy (SEM) with ultrahigh vacuum for Al, Cu, Ag, and Au metals deposited on the Si(100) clean surface. The order of the secondary electron yields as a function of the atomic number (Z2) for ion bombardment is opposite to that for electron bombardment. The brightness of the secondary electron images observed by a focused Ga+ ion beam at 30 keV decreases with increasing Z2, while that by the electron beam increases with Z2. On the other hand, the order of the total secondary ion yields in SIM increases with Z2. The secondary electron image observed by a focused Ar+-ion beam at 3 keV shows the similar contrast to that of the Ga+-ion beam. The different Z2 dependence of the secondary electron yields between SEM and SIM was quantitatively confirmed by the total secondary electron spectra and is discussed based on the range profile below the surface, and it is concluded that the decrease of the secondary electron yields in SIM is attributed to the increase of the reflected ions at the surface with increasing Z2.
Adatom diffusion at GaN (0001) and (0001̄) surfaces
Authors: Zywietz Tosja , Neugebauer Jörg , Scheffler Matthias
Published: July 2002

The diffusion of Ga and N adatoms has been studied for the technologically relevant wurtzite (0001̄) and (0001) surfaces employing density-functional theory. Our calculations reveal a very different diffusivity for Ga and N adatoms on the equilibrium surfaces: While Ga is very mobile at typical growth temperatures, the diffusion of N is by orders of magnitude slower. These results give a very detailed insight of how and under which growth conditions N adatoms can be stabilized and efficiently incorporated at the surface. We further find that the presence of excess N strongly increases the Ga diffusion barrier and discuss the consequences for the growth of GaN.
Diffusion of iron in the silicon dioxide layer of silicon-on-insulator structures
Authors: Kononchuk O. , Korablev K. G. , Yarykin N. , Rozgonyi G. A.
Published: July 2002

The redistribution of iron implanted into the oxide layer of silicon-on-insulator structures has been measured using the secondary ion mass spectroscopy technique after annealing at 900–1050 °C. Iron diffusion has been found to be much faster in the oxide prepared by the separation-byimplantation-of-oxygen (SIMOX) procedure compared to the thermally grown oxide in the bonded and etched-back structures. In the latter case, the Fe diffusivity exhibits a thermal activation with an energy of 2.8 eV, confirming the literature data on silica glass. In the SIMOX oxide, the diffusivity depends only weakly on temperature, indicative of an essentially activation-free diffusion mechanism. Gettering of Fe at below-the-buried-oxide defects in SIMOX wafers has been observed. No iron segregation has been detected at the SiO2–Si interfaces.
Two-dimensional carrier profiling of InP structures using scanning spreading resistance microscopy
Authors: De Wolf P. , Geva M. , Hantschel T. , Vandervorst W. , Bylsma R. B.
Published: July 2002

Scanning spreading resistance microscopy (SSRM) is an analytical technique originally developed for measuring two-dimensional carrier distribution in Si device structures with high spatial resolution. It is in essence an atomic force microscope equipped with a conducting tip that is biased relative to the sample. The spreading resistance value derived from the measured electrical current is a function of the local carrier concentration at the surface region surrounding the probe’s tip. In this letter, we report the successful application of SSRM to the analysis of InP semiconductor device structures. We imaged a multilayer epitest structure, and a cross section of a three-dimensional structure in which we observed lateral Zn-dopant diffusion. Comparison of the SSRM profiles with one-dimensional secondary ion mass spectrometry depth profiles shows good qualitative agreement. SSRM analysis of InP-based device structures was found to be much simpler than that of Si structures: there is no need for surface preparation of the cleaved surface, a lower tip force is required, and metal tips, rather than doped diamond can be used.
Multicolor multilayer light-emitting devices based on pyridine-containing conjugated polymers and para-sexiphenyl oligomer
Authors: Wang Y. Z. , Sun R. G. , Meghdadi F. , Leising G. , Epstein A. J.
Published: July 2002

There is increased interest in developing multicolor light-emitting devices. We report here the fabrication and study of multilayer color-variable/white light-emitting devices based on pyridine-containing conjugated polymers and para-sexiphenyl (6P) oligomer. Voltage-dependent multicolor emission was observed in both bilayer and trilayer configurations. The emission colors of single devices cover a wide range of visible spectra whose Commission International de I’Eclairage (CIE) color coordinates vary from blue to white to green with increasing voltages. The color coordinate traverses along a straight line in the CIE chromaticity diagram. The role of interfaces in the operation of devices is discussed.
Imposed layer-by-layer growth by pulsed laser interval deposition
Authors: Koster Gertjan , Rijnders Guus J. H. M. , Blank Dave H. A. , Rogalla Horst
Published: July 2002

Pulsed laser deposition has become an important technique to fabricate novel materials. Although there is the general impression that, due to the pulsed deposition, the growth mechanism differs partially from continuous physical and chemical deposition techniques, it has hardly been used. Here, we will introduce a growth method, based on a periodic sequence: fast deposition of the amount of material needed to complete one monolayer followed by an interval in which no deposition takes place and the film can reorganize. This makes it possible to grow in a layer-by-layer fashion in a growth regime (temperature, pressure) where otherwise island formation would dominate the growth.
Decay dynamics of near-infrared photoluminescence from Ge nanocrystals
Authors: Takeoka Shinji , Fujii Minoru , Hayashi Shinji , Yamamoto Keiichi
Published: July 2002

Photoluminescence (PL) decay dynamics of Ge nanocrystals (nc-Ge) 1.2–3.2 nm in average diameter embedded in SiO2 matrices was studied. The samples showed a PL peak in the near-infrared region with strong size dependence. A very fast component (≪1 μs) was found in decay curves for all the samples. For the samples containing relatively large nc-Ge, a slow component of the order of microseconds was also observed. With decreasing the size, the slow component gradually faded out, and the PL intensity increased significantly. The transition from indirect to direct recombination of carriers with decreasing the size is considered to be responsible for the observed PL decay dynamics.
Roles of the first atomic layers in growth of SrTiO3 films on LaAlO3 substrates
Authors: Kim Dong-Wook , Kim Dae-Ho , Kang Bo-Soo , Noh T. W. , Lee D. R. , Lee K.-B.
Published: July 2002

Growth behaviors of SrTiO3/LaAlO3 films could be varied substantially by controlling terminating atomic layers of the substrates. In a film on the LaO-terminated substrate, strain-induced roughening was observed. In a film on the AlO2-terminated substrate, the first atomic layer of the interface seemed to have lots of defects. However, the stress became quickly relaxed, so a SrTiO3 film could be grown in a layer-by-layer mode after a few monolayers. All these observations could be explained in terms of chemical matching between the atomic layers at the interface.
Thermal crystallization of amorphous Si/SiO2 superlattices
Authors: Zacharias M. , Bläsing J. , Veit P. , Tsybeskov L. , Hirschman K. , Fauchet P. M.
Published: July 2002

Annealing of amorphous Si/SiO2 superlattices produces Si nanocrystals. The crystallization has been studied by transmission electron microscopy and x-ray analysis. For a Si layer thinner than 7 nm, nearly perfect nanocrystals are found. For thicker layers, growth faults and dislocations exist. Decreasing the a-Si layer thickness increases the inhomogeneous strain by one order of magnitude. The origin of the strain in the crystallized structure is discussed. The crystallization temperature increases rapidly with decreasing a-Si layer thickness. An empirical model that takes into account the Si layer thickness, the Si/SiO2 interface range, and a material specific constant has been developed.
Infrared properties of bulk GaN
Authors: Hao M. , Mahanty S. , Qhalid Fareed R. S. , Tottori S. , Nishino K. , Sakai S.
Published: July 2002

Transmission and reflection measurements in the frequency range (700–2000 cm−1) have been carried out on high-quality bulk GaN grown by the sublimation technique. The transmission spectra are polarization dependent and show combination bands arising from anharmonic forces. The combination absorption is much stronger for E∥c polarization than that for E⊥c polarization. A high value of 350 cm−1 for the absorption coefficient in the combination band can be explained by the strong anharmonic forces in GaN.
Optical investigation of InGaN/GaN multiple quantum wells
Authors: Wang T. , Nakagawa D. , Lachab M. , Sugahara T. , Sakai S.
Published: July 2002

Optical investigation was performed on InGaN/GaN multiple quantum well (MQW) structures with different well thicknesses. At low temperature, the excitation power dependence of the photoluminescence (PL) emission energy of a MQW with 5 nm well thickness was found to be different from that of a MQW with 2.5 nm well thickness. Their temperature dependence of the optical behaviors including the PL line shapes and the internal quantum efficiencies also showed distinct features. The optical behaviors of the quantum well with a thickness above 2.5 nm can be explained by a model based on the formation of self-organized small In-rich regions, rather than by the piezoelectric field-induced quantum-confined Stark effect.
Indications of antiferromagnetic interlayer coupling in La2/3Ba1/3MnO3/LaNiO3 multilayers
Authors: Nikolaev K. R. , Bhattacharya A. , Kraus P. A. , Vas’ko V. A. , Cooley W. K. , Goldman A. M.
Published: July 2002

Multilayers consisting of the perovskite metallic oxides, ferromagnetic La2/3Ba1/3MnO3 (LBMO) and paramagnetic LaNiO3 (LNO) have been grown by ozone-assisted molecular beam epitaxy. Structural characterization using in situ reflection high-energy electron diffraction and high-resolution x-ray diffraction reveal a very high degree of crystalline order. Magnetization and hysteresis measurements show evidence of antiferromagnetic coupling between LBMO layers when the LNO spacer is 15 Å or less in thickness.
Coherent two-dimensional lasing action in surface-emitting laser with triangular-lattice photonic crystal structure
Authors: Imada Masahiro , Noda Susumu , Chutinan Alongkarn , Tokuda Takashi , Murata Michio , Sasaki Goro
Published: July 2002

Lasing action of a surface-emitting laser with a two-dimensional photonic crystal structure is investigated. The photonic crystal has a triangular-lattice structure composed of InP and air holes, which is integrated with an InGaAsP/InP multiple-quantum-well active layer by a wafer fusion technique. Uniform two-dimensional lasing oscillation based on the coupling of light propagating in six equivalent Γ−X directions is successfully observed, where the wavelength of the active layer is designed to match the folded (second-order) Γ point of the Γ−X direction. The very narrow divergence angle of far field pattern and/or the lasing spectrum, which is considered to reflect the two-dimensional stop band, also indicate that the lasing oscillation occurs coherently.
Thermoelectromechanical refrigeration based on transient thermoelectric effects
Authors: Miner A. , Majumdar A. , Ghoshal U.
Published: July 2002

This letter introduces the concept of a thermoelectromechanical cooler (TEMC), which modifies a traditional thermoelectric cooler (TEC) by using intermittent contact of a mechanical element synchronized with an applied pulsed current. Using Bi2Te3 as the thermoelectric material, it is predicted that the maximum temperature drop across a TEMC operated under zero applied heat flux is about 35% higher than that of a TEC. This effectively increases the thermoelectric figure of merit for maximum temperature differential applications by a factor of 1.8.
Epitaxially grown Si/SiGe interband tunneling diodes with high room-temperature peak-to-valley ratio
Authors: Duschl R. , Schmidt O. G. , Eberl K.
Published: July 2002

Room-temperature current–voltage characteristics of Si/Si1−xGex/Si p+-i-n+ interband tunneling diodes are presented. Special attention is paid to the peak current density (PCD) and the peak-to-valley current ratio (PVCR) of the devices. A variation of the SiGe layer width between 4 and 2 nm changes the PCD from 0.4 to 12.6 kA/cm2, at the same time conserving a PVCR of more than 4.1. The optimization of the Ge concentration x in the Si1−xGex layer results in a PVCR of 5.1 for x=48%. Tuning the position of the Si0.52Ge0.48 layer within the intrinsic zone culminates in a structure with a PCD of 8 kA/cm2 and improves the PVCR to the value of 5.45.
Selective enhancement of 1540 nm Er3+ emission centers in Er-implanted GaN by Mg codoping
Authors: Kim S. , Rhee S. J. , Li X. , Coleman J. J. , Bishop S. G.
Published: July 2002

The ∼1540 nm I13/24 to I15/24 Er3+ photoluminescence (PL) and photoluminescence excitation (PLE) spectra of Er-implanted Mg-doped GaN reveal a selective enhancement of one of the nine different Er3+ centers observed previously in PL and PLE studies of Er-implanted undoped GaN. These Er3+ PL spectra are excited selectively by pump wavelengths that correspond to broadband, below-gap absorption bands associated with different Er3+ centers. In the Er-implanted, Mg-doped GaN, both the 1540 nm PL spectrum characteristic of the so-called violet-pumped Er3+ center and the ∼2.8–3.4 eV (violet) PLE band that enables its selective excitation are significantly enhanced by Mg doping. In addition, the violet-pumped PL center dominates the above-gap-excited Er3+ PL spectrum of Er-implanted Mg-doped GaN, whereas it was nearly unobserveable under above-gap excitation in Er-implanted undoped GaN. These results confirm our hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitation of Er3+ emission in Er-implanted GaN.
Geometry dependence of magnetization vortices in patterned submicron NiFe elements
Authors: Shi Jing , Tehrani S. , Scheinfein M. R.
Published: July 2002

When the magnetization is switched in patterned submicron Ni80Fe20 thin-film elements, magnetization vortices, local micromagnetic structures with a core size of 10 nm, are often trapped. This lowers the magnetostatic self-energy of the elements. Trapped vortices can cause anomalous switching during spin reversal between stable magnetization states. To expel trapped vortices, a large external magnetic field, the vortex exit field, is required. The dependence of the vortex exit field on lateral dimensions of the patterned elements is measured. In the limit where the lateral dimensions of the patterned elements are much larger than the vortex core size, smaller structures are more prone to the formation of trapped vortices.
Optically pumped InAs quantum dot microdisk lasers
Authors: Cao H. , Xu J. Y. , Xiang W. H. , Ma Y. , Chang S.-H. , Ho S. T. , Solomon G. S.
Published: July 2002

We have achieved lasing in InAs quantum dot embedded GaAs microdisks under optical pumping. Above the lasing threshold, a drastic increase of emission intensity is accompanied by a decrease of the spectral linewidth of the whispering gallery modes. The laser light is linearly polarized. The polarization direction is parallel to the disk plane. The wide gain spectrum of quantum dots allows simultaneous lasing in several whispering gallery modes of a microdisk.