bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructure of AlN on Si (111) grown by plasma-assisted molecular beam epitaxy
Authors: Stevens K. S. , Ohtani A. , Kinniburgh M. , Beresford R.
Published: July 2002

We have grown monocrystalline AlN on Si (111) substrates over the temperature range 550–900 °C using electron cyclotron resonance plasma assisted molecular beam epitaxy. The best (0002) peak omega rocking curve full width at half-maximum value obtained was 26 min for a film deposited at 900 °C. All films nominally displayed the AlN[0001]∥Si[111] orientation. The exact angle between AlN[0001] and Si[111] decreased from 2.1° to 1.1° and the (0002) peak widths improved with increasing substrate temperature. Mosaic-type disorder was shown by high resolution x-ray diffraction to be the dominant cause of the ω rocking curve peak full widths.
Conduction-band engineering in piezoelectric [111] multiple quantum well p-i-n photodiodes
Authors: Sánchez-Rojas J. L. , Sacedón A. , Calle F. , Calleja E. , Muñoz E.
Published: July 2002

The influence of the design parameters on the conduction-band profile and optoelectronic properties of [111]-oriented InGaAs/GaAs p-i(MQW)-n diodes is presented. An analytical expression for the average electric field (AEF) in the p-i-n active region (MQWs within the intrinsic region) is obtained. The existence of two different potential envelopes, corresponding to a positive or to a negative sign of the AEF, and giving rise to clearly different optical and electronic properties, is demonstrated. In samples with negative AEF, as compared to structures with positive AEF, larger reverse voltages are needed to quench the photoluminescence and to enhance the p-i-n photocurrent. An analysis of both transition energies and intensities, versus bias, clearly indicates that in samples with a negative AEF carriers accumulate at the extremes of the active region, giving rise to a long-range screening effect of the field in the wells.
Suppression of strain relaxation and roughening of InGaAs on GaAs using ion-assisted molecular beam epitaxy
Authors: Millunchick J. Mirecki , Barnett S. A.
Published: July 2002

InxGa1−xAs films with x=0.67 and 0.75 were grown on GaAs (001) using molecular beam epitaxy (MBE) and ion-assisted MBE. The MBE films roughened and relaxed at 2–4 monolayers, after an initial coherently strained layer-by-layer growth stage, in good agreement with previous results. Ion energies <16 eV had little effect on growth. However, increasing the ion energy from 16 to 27 eV during ion-assisted MBE decreased the rate of strain relaxation and decreased the surface roughness. For x=0.75 and ion energies ≳27 eV, relaxation was eliminated to thicknesses of 50 monolayers, well beyond the Matthews–Blakeslee prediction. We infer from the results that surface roughening of large-mismatch MBE InGaAs films allows strain relaxation well before misfit dislocations are introduced.
Blue photoluminescence and local structure of Si nanostructures embedded in SiO2 matrices
Authors: Zhang Qi , Bayliss S. C. , Hutt D. A.
Published: July 2002

Strong and stable blue photoluminescence (PL), visible to the naked eye under 0.4 μW of 300 nm and 2.7 μW of 370 nm excitation, has been observed for samples of Si clusters embedded in SiO2 matrices, prepared by rf co-sputtering followed by N2 annealing at 800 °C. Si K-edge extended x-ray absorption fine structure (EXAFS) and near-edge x-ray absorption fine structure (NEXAFS) strongly suggest the existence of Si nanoclusters with crystalline cores in the efficient emitting material. The PL excitation dependence is explained by an increase in the conduction band density of states deep in the band, and the formation of a band tail.
Low-frequency measurement of leakage in enclosures
Authors: Sherman Max , Modera Mark
Published: July 2002

A wide variety of enclosed structures either require or cannot entirely prevent leakage from their interior space to the outside. Existing methods for measuring such leakage have important disadvantages. We have developed a device and technique that permits leakage areas to be measured from within or without the enclosure without causing unacceptable disturbance. The apparatus uses low-frequency (1 Hz) acoustic monopoles to generate an internal pressure signal which is then analyzed synchronously to provide a measurement of leakage area. We have successfully applied this technique to measuring air tightness in residential houses, and believe it can be easily adapted for use in field, laboratory, or classroom applications. We are currently evaluating why the values we obtained were, on average, 14% lower than those obtained through conventional methods and we are investigating the apparent inability of the device, as presently designed, to measure large leaks.
High-energy x-ray microscopy with multilayer reflectors (invited)
Authors: Underwood James H.
Published: July 2002

A knowledge of the spatial distribution of the x rays emitted by the hot plasma region is a key element in the study of the physical processes occurring in laser-produced plasmas and complements other diagnostics such as spectroscopy and temporal studies. X-ray microscopy with reflection microscopes offers the most direct means of obtaining this information. Until recently, the two types of microscopes that had been developed for this purpose, the Kirkpatrick–Baez and the Wolter, operated at relatively low energies (about 4–5 keV) and had very little spectral selectivity, relying on filters for coarse spectral resolution. With the development of x-ray reflecting multilayer mirrors, the energy response of such microscopes can be extended to 10 keV or higher, with good spectral selectivity. In addition, it is possible to reduce some of the optical aberrations to obtain improved spatial resolution. This paper describes some of the recent progress in making and evaluating x-ray reflectors, and outlines the optical design considerations for multilayer-coated microscopes. Results from a prototype multilayer K–B microscope are presented.
Simultaneous measurement of two orthogonal components of electric field using a Pockels device
Authors: Hidaka Kunihiko , Kouno Teruya , Hayashi Izumi
Published: July 2002

A new optical method using a Pockels device is developed to simultaneously measure two directional components of an electric field. In this method, an output light beam from a Pockels device is split into two beams and then each of the beams is transmitted through a polarizing plate whose polarization direction is different, depending on the measured field component. The relation between two measured field components and the required polarization direction of the polarizing plate is theoretically derived for representative materials of the Pockels device and is also examined experimentally. A field sensor is made on the basis of this new idea and is used to investigate a nonuniform electric field. An electric field changing spatially by 20% per centimeter can be measured with an accuracy of 5% using the Pockels sensor. The sensor is successfully applied to measure an impulse electric field between a sphere-to-sphere gap.
JET high resolution bent crystal spectrometer
Authors: Bartiromo R. , Bombarda F. , Giannella R. , Mantovani S. , Panaccione L. , Pizzicaroli G.
Published: July 2002

In this paper we describe the bent crystal spectrometer of the Johann type built for the JET tokamak. This high resolution (λ/Δλ∼20 000) instrument is characterized by a very long focal length (25 m) allowing the crystal and the position sensitive detector to be placed outside the JET torus hall, in order to keep the noise induced by the background radiation to an acceptable level. By means of the shape analysis of x-ray lines emitted by medium Z, highly ionized impurities present in the plasma, the ion temperature can be evaluated. Relevant information about other plasma parameters such as the electron temperature, the impurity ionization equilibrium and concentration, and the plasma toroidal velocity can be obtained by means of the analysis of different lines present in the spectra and of their wavelengths. Examples of the results obtained during the first year of operation of this instrument are also illustrated.
Performance of a directly water-cooled silicon crystal for use in high-power synchrotron radiation applications
Authors: Oversluizen T. , Matsushita T. , Ishikawa T. , Stefan P. M. , Sharma S. , Mikuni A.
Published: July 2002

Results of measurements made using the multipole wiggler at beamline 16 of Photon Factory are reported. Double crystal rocking curves, Si(111) and Si(333) topographs, and thermal maps of the cooled-crystal surface were measured at total incident power levels up to 1600 W and power densities up to 0.7 W/mm2, for two different crystal designs. Significant crystal deformation occurred in the form of surface curvature for both designs. Finite element calculations provided a preview of the experimental results and proved very valuable in interpretation of the measurements.
ECR ion and free radical sources for MBE applications
Authors: Asmussen J. , Fritz R. , Mahoney L. , Fournier G. , Demaggio G.
Published: July 2002

A small 2.45 GHz ion/free radical source that has been specifically designed for MBE applications is described. The overall outside diameter of this cylindrical source is 5.8 cm (includes magnets and applicator) and the length is less than 15 cm. Thus, it fits through vacuum ports on existing MBE machines and can be placed near the substrate. The microwave discharge, which is created within a cylindrical quartz tube, is surrounded by rare-earth magnets and a tunable coaxial cavity applicator. Single or double accelerating grids can be placed on the 3.75-cm-diam discharge output producing an ion beam, or the discharge can be freely exhausted into the vacuum allowing the charged and excited species to diffuse onto the substrate for plasma and/or free radical processing. This device can operate at pressures as low as 10−5 Torr with A, O2, and N2 gases using only 50–150 W of incident power. The detailed description and the preliminary experimental test of this source is discussed.
Time-delayed luminescence from oxidized porous silicon after ultraviolet excitation
Authors: Kux A. , Kovalev D. , Koch F.
Published: July 2002

Oxidized porous silicon is known to luminesce efficiently and with a ns-fast response in the green-blue region of the spectrum (2.3–2.7 eV) at room temperature. For microporous Si processed by rapid thermal oxidation at 1100–1200  °C in a dry oxygen ambient, we observe that the blue light has a time-delayed component that indicates carrier trapping. In addition to the well-established ultrafast light (∼1 ns) a signal with a time delay of order 1 s is present under UV photoexcitation. The time-delayed blue light exists at low temperature and has an excitation onset edge at 4.3±0.1 eV, an energy that is usually associated with a band discontinuity at the Si–SiO2 interface.
Thickness-direction coefficient of thermal expansion measurement of thin polymer films
Authors: Tong H. M. , Hsuen H. K. D. , Saenger K. L. , Su G. W.
Published: July 2002

A highly sensitive capacitance-change technique, suitable for thin polymer films, has been developed for the measurement of the coefficient of thermal expansion (CTE) in the film-thickness direction. Using this technique, we have determined the thickness-direction CTE of Kapton polyimide to be about 81±4 ppm/°C, averaged over the temperature range 50–150 °C. This polymer was chosen because it is widely used in the electronics industry for the fabrication of flexible printed circuits. Two other techniques, a Fabry–Perot laser interferometric technique, and the more conventional but less sensitive thermomechanical analysis, were also investigated. The thickness-direction CTEs obtained from the three techniques were at least a factor of 2.4 greater than the in-plane CTE of about 34±2 ppm/°C. The in-plane CTE, found here via simple length measurements, was in good agreement with literature values. It is speculated that the CTE anisotropy observed might be due to the in-plane molecular orientation of the polymer chains.
Deflection of carbon dioxide laser and helium-neon laser beams in a long-pulse relativistic electron beam diode
Authors: Bosch R. A. , Ching H. , Gilgenbach R. M. , Ventzek P. L. G. , Menge P. R. , Choi J. J. , Spencer T. A.
Published: July 2002

Deflection of carbon dioxide and helium-neon laser beams has been used to measure plasma and neutral density gradients during the operating mode and after the shorting time of a long-pulse field-emission electron beam diode. Plasma density gradients of (1014–1015) cm−4 were observed throughout the diode during the final microsecond of the 2–3 μs electron beam pulse. The neutral density gradient was less than 1×1018 cm−4 during the electron beam pulse. Upon diode shorting, neutral density gradients increased to (1018–1019) cm−4 over ∼1 μs, and decayed over many microseconds. Plasma density gradients of ∼1015 cm−4 were also observed after shorting. These experiments demonstrate the value of carbon-dioxide laser and helium-neon laser deflection for diagnosing plasma and neutral particles in long-pulse electron beam diodes.
New technique for the measurement of adsorbed moisture concentration on a solid surface
Authors: Ohmi T. , Nakagawa Y. , Aomi H. , Takano J.
Published: July 2002

We have established a new measurement method to evaluate the adsorbed moisture concentrations on solid surfaces exposed to the atmosphere having high moisture content ranging from 100 ppm to a few percent. The technique is based on the variation of the electric conductivity of anhydrous hydrogen fluoride. It has been found that the amount of adsorbed moisture on electropolished 316L stainless steel surface in an atmosphere with moisture concentration of 1% and 3% at 25 °C (relative humidity of 32% and 96% respectively) are 2.5× 1016 and 1.25×1017 molecules/cm2, respectively. This represents 25 and 125 molecule layers of water. The adsorbed moisture has been confirmed to reach an equilibrium level within an exposure time of 1 s.
Large-bore copper vapor laser with a double thyratron pulse modulator
Authors: Singh B. , Nakhe S. V. , Mittal J. K. , Dixit S. K. , Vora H. S. , Bhatnagar R.
Published: July 2002

Use of a double thyratron pulse modulator in a large-bore copper vapor laser is demonstrated. Design of the laser consisting of a discharge tube of 70-mm diameter and 1500-mm length is discussed. The laser gave 55-W output power at 4.8-kHz repetition rate using neon as the buffer gas with 9.1-kW modulator switching of electrical power. The efficiency of the laser was about 0.6%.
A novel technique for scintillator light collection in neutron measurements
Authors: Choi P. , Aliaga-Rossel R.
Published: July 2002

In time resolved measurements of neutron emission, the most common practice is to use a plastic scintillator coupled to a fast photomultiplier. There are different ways of coupling the light emitted by the scintillator to the photomultiplier (PM), according to the size and the shape of the scintillator. Some of the more usual coupling geometries are light pipes, Plexiglas cones with silvered walls or mirror ‘‘huts’’ facing the PM window.
Micromechanical sensors for chemical and physical measurements
Authors: Wachter E. A. , Thundat T.
Published: July 2002

The advent of inexpensive, mass-produced microcantilevers promises to bring about a revolution in the field of chemical and physical sensor design. In this paper, a novel class of highly sensitive sensors are discussed that are based on commercially available microcantilevers, such as those used in atomic force microscopy. When coated with a sensitizing overlayer, these microcantilevers show significant changes in two independent analyte-induced signals, resonance frequency and static bending, as the result of exposure to various chemical and physical phenomena. Resonance frequency shift has the particular advantage of being relatively insensitive to interference from external factors such as thermal drift. Examples of micromechanical sensors based on this approach that are capable of detecting mercury vapor (with a sensitivity of 1.25 Hz/pg and linear correlation of 0.998), relative humidity (55 Hz/%R.H., correlation=0.999), or optical irradiation (10 Hz/nJ response) are discussed in detail, along with the effects of coatings on sensitivity, linearity, and reversibility of response. Further, extension of this tremendously flexible concept into a universal detection paradigm for chemical and physical phenomena is examined.
The x-pinch as a point source of x rays for backlighting
Authors: Kalantar D. H. , Hammer D. A.
Published: July 2002

The fine metal wire x-pinch is a compact source of soft-x-ray emission. Under certain conditions, x rays in the 3–7 keV range are emitted in <1 ns bursts from source regions that are <10 μm in size. The small size and high intensity of these 3–7 keV sources makes the x-pinch useful as a point source for x-ray backlighting. This capability has been demonstrated by using an x-pinch to produce radiographic images of separate x-pinches and z-pinches.
Chemical bonding and structure of the sulfur treated GaAs(111)B surface
Authors: Moriarty P. , Murphy B. , Roberts L. , Cafolla A. A. , Hughes G. , Koenders L. , Bailey P. , Woolf D. A.
Published: July 2002

We have investigated the atomic structure and chemical bonding present at sulfur exposed GaAs(111)B-(2×2) surfaces using both scanning tunneling microscopy (STM) and synchrotron radiation core-level photoemission. Exposure of the (2×2) surface to a molecular beam of sulfur leads to the appearance of a (1×1) low-energy electron diffraction pattern which becomes increasingly well defined as the sample is annealed. However, at no stage of the annealing process does the surface display an ordered (1×1) ideal termination. Both the photoemission data and STM images show that a large proportion of the As trimer units of the clean (2×2) surface remain after sulfur exposure and annealing to 450 °C with strong evidence of sulfur substituting for As in atomic layers below the surface. The effect of these reactions is to increase the surface band-bending from that of the clean (2×2) surface.
A long path length pulsed slit valve appropriate for high temperature operation: Infrared spectroscopy of jet-cooled large water clusters and nucleotide bases
Authors: Liu Kun , Fellers Raymond S. , Viant Mark R. , McLaughlin Ryan P. , Brown Mac G. , Saykally Richard J.
Published: July 2002

We report the design and performance of a pulsed slit valve for generation of supersonically cooled species in a long path length planar expansion. Utilizing three commercial solenoids driven synchronously by an economic power transistor circuit, the valve produces pulses adjustable in width from 500 to 1000 μs with a repetition rate up to 80 Hz. The pulsed valve can be operated continuously for 12 h, and operation over one month is typical before major maintenance is required. The path length × density product attained by this pulsed source is sufficient for observing large cluster species, such as the water hexamer, on our far-infrared spectrometer. With the addition of a detachable sample oven, it can also be heated up to 230 °C to inject nonvolatile molecules into planar supersonic expansions.
Dynamics of laser ablation plume penetration through low pressure background gases
Authors: Geohegan David B. , Puretzky Alexander A.
Published: July 2002

The dynamics of laser-ablated yttrium plume propagation through background argon have been investigated with fast time- and spatially-resolved plasma diagnostics in order to characterize a general phenomenon believed to be important to film growth by pulsed laser deposition (PLD). During expansion into low-pressure background gases, the ion flux in the laser ablation plasma plume is observed to split into fast and slow components over a limited range of distances including those typically utilized for PLD. Optical absorption and emission spectroscopy are employed to simultaneously identify populations of both excited and ground states of Y and Y+. These are correlated with intensified-CCD (ICCD) photographs of visible plume luminescence and ion fluxes recorded with fast ion probes. These measurements indicate that plume-splitting in background gases is consistent with scattering of target constituents by ambient gas atoms. The momentum transfer from these collisions produces a transition from the initial, ‘‘vacuum’’ velocity distribution into a velocity distribution which is significantly slowed in accordance with shock or drag propagation models.
Vacuum compatible high-sensitive Kelvin probe force microscopy
Authors: Kikukawa Atsushi , Hosaka Sumio , Imura Ryo
Published: July 2002

A vacuum compatible Kelvin probe force microscopy (KPFM) is presented. Difficulties in operating KPFM in a vacuum were overcome by utilizing the direct cantilever resonance frequency detection in the tip height control whereas the indirect resonance frequency detection scheme was used in primordial KPFM. The potential measurement sensitivity was improved by 14 dB compared to that in air. It is due to the increased cantilever Q value and the reduction in the interference from the tip height detection signal because potential measurement is conducted using the cantilever’s second resonance while tip height control was conducted using the first resonance. A silicon wafer whose surface is partially doped with arsenic by ion implantation was observed, and surface potential difference at the junctions were clearly imaged.
Thickness and conductivity of metallic layers from pulsed eddy-current measurements
Authors: Tai Cheng-Chi , Rose James H. , Moulder John C.
Published: July 2002

We describe a time-domain (pulsed) eddy-current technique for determining the thickness and conductivity of conductive coatings on metal plates. The pulsed eddy-current instrument records the transient current induced in an absolute, air-cored coil placed next to a layered sample and excited with a step-function change in voltage. Signals are digitized with 16-bit resolution at a sampling rate of 1 megasamples per second, and the excitation is repeated at a rate of 1 kHz. The instrument displays the difference in the transient current measured on the substrate and on the substrate plus coating. We measured pulsed eddy-current signals for a series of metal foils of varying thickness placed over 1 cm thick metal plates. Seven combinations of foil and substrate metals were studied including pure aluminum, copper, and titanium foils over substrates of aluminum, titanium alloy, and stainless steel. We report results for three types of samples: aluminum foils on Ti–6Al–4V substrate, titanium foils on 7075 aluminum alloys, and aluminum foils on AISI 304 stainless steel. Foil thickness ranged from 0.04–1.00 mm. We found that three features of the signal—the peak height, the time of occurrence of the first peak, and a characteristic zero-crossing time—depend sensitively upon the thickness of the layers and the relative electrical conductivity of coating and substrate. Theoretical calculations were compared to the measurements. Absolute agreement between calculated and measured signals was, in most cases, within 3%. No calibration with respect to artifact standards was used. Finally, a feature-based rapid inversion method was developed and used to infer the thickness and conductivity of the layers. The accuracy of the inversion depends upon the thickness of the layer and the contrast in conductivity between layer and substrate. For the materials studied the thickness could be determined within 13%, while the error in determining conductivity was 20%–30%. The time-domain method is much simpler and hundreds of times faster than the frequency-domain method previously reported by Moulder et al. [Rev. Sci. Instrum. 63, 3455 (1992)].
Thermal conductivity of thin metallic films measured by photothermal profile analysis
Authors: Langer G. , Hartmann J. , Reichling M.
Published: July 2002

Thermal conductivity of nickel and gold films on quartz (thickness 0.4–8 μm) was measured by a modulated thermoreflectance technique recording the surface temperature profile. Model calculations predict an optimum frequency for measuring thermal transport within the film. Measurements on films with various thicknesses reveal a thermal conductivity close to the bulk value for nickel while gold films exhibit a reduced conductivity with decreasing film thickness.
Unipolar arc simulation device
Authors: Wang S. G. , Brown I. G.
Published: July 2002

We describe a simple laboratory device for establishing a vacuum arc plasma discharge that can serve to simulate a unipolar arc. The technique makes use of a triggered vacuum arc plasma gun to generate a plasma plume that in turn causes breakdown of a secondary discharge. The device is in fact a secondary vacuum arc discharge that is triggered by a primary vacuum arc discharge, with some of the features of the secondary plasma discharge having similarities with a unipolar arc configuration. Here we describe the experimental setup and outline how the device can be used for some areas of unipolar arc materials research.
Efficient, compact power supply for repetitively pulsed, “triggerless” cathodic arcs
Authors: Anders André , MacGill Robert A. , McVeigh Thomas A.
Published: July 2002

A power supply for “triggerless,” repetitively pulsed cathodic arcs has been developed. It is based on a thyristor-switched, high-voltage, high-current, pulse-forming network (PFN). It can provide high pulsed currents (up to 2 kA), with duration of 600 μs, and pulse repetition rate of up to 10 Hz. Higher repetition rates are possible at lower current. The rectangular pulse shape and amplitude are reproducible to within a few percent. Cathodic arc initiation is extremely reliable because the charging voltage is much higher than the minimum starting voltage for the triggerless arc initiation method. The energy utilization efficiency is very high by intentionally mismatching load and PFN impedances and by using an efficiency-enhancing diode; the stored energy is dissipated primarily in the arc.
A frequency division multiplexed low-finesse fiber optic Fabry–Perot sensor system for strain and displacement measurements
Authors: Liu T. , Fernando G. F.
Published: July 2002

We report a novel frequency multiplexing method for addressing low finesse fiber Fabry–Perot sensors using a white light source and a CCD based monochromator. The absolute optical path length imbalance of the sensors can be determined using this method. The concept was demonstrated with three fiber Fabry–Perot interferometers for strain and displacement measurements. The accuracy is better than 0.01 μm and the maximum range is approximately 1 mm.
Nondestructive measurement of interfacial SiO2 films formed during deposition and annealing of Ta2O5
Authors: Devine R. A. B.
Published: July 2002

It is demonstrated that infrared absorption spectroscopy of the transverse optic and longitudinal optic vibrational modes of the asymmetric bridging oxygen stretch in amorphous SiO2 may be used to conveniently estimate film thickness in sandwiched structures. Measurements have been made on the SiO2 film formed at the substrate/dielectric interface during deposition of Ta2O5 on Si substrates and the evolution of its thickness during subsequent annealing. The accuracy of the film thickness determined is compared with electron microscopy observations.
Partial screening of internal electric fields in strained piezoelectric quantum well lasers: Implications for optoelectronic integration
Authors: Pabla A. S. , Woodhead J. , Khoo E. A. , Grey R. , David J. P. R. , Rees G. J.
Published: July 2002

The spectral electroluminescence characteristics of broad-area (Al)GaAs/In0.23Ga0.77As/(Al)GaAs single quantum well separate confinement heterostructure lasers grown on (111)B GaAs have been studied under forward biased current injection. A room-temperature threshold current density of 750 A/cm2 is measured for a 1000 μm laser. The subthreshold electroluminescence spectrum blue shifts with increasing current up to the point of lasing threshold. Our measurements reveal that lasing is achieved while there is a strong residual or ‘‘unscreened’’ electric field across the quantum well. Based on these observations we outline how piezoelectric quantum wells can be used to monolithically integrate a quantum well laser with a blue-shifting electroabsorption modulator.
Selectivity of growth on patterned GaAs (311)A substrates
Authors: Nötzel Richard , Menniger Johann , Ramsteiner Manfred , Ruiz Ana , Schönherr Hans-Peter , Ploog Klaus H.
Published: July 2002

We report on the selectivity of growth on patterned GaAs (311)A substrates by solid-source molecular beam epitaxy. For mesa stripes oriented along the [01-1] direction, the selectivity of growth is qualitatively different from that on patterned GaAs (100) substrates with a higher growth rate on one of the side facets of the stripes. This growth mode develops a convex curved surface profile enclosing thicker wirelike regions of GaAs due to preferential migration of Ga atoms from both sides toward the sidewall leaving behind thinner regions on the adjacent mesa top and bottom areas. A mechanism for the formation of the surface profile is proposed.
Formation of nanocrystals based on decomposition in the amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy
Authors: Schneider S. , Thiyagarajan P. , Johnson W. L.
Published: July 2002

Primary crystallization and decomposition in the bulk amorphous alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 have been studied by small angle neutron scattering (SANS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at q=0.46 nm−1 after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals.
Magnetothermopower in La0.67Ca0.33MnO3 thin films
Authors: Jaime M. , Salamon M. B. , Pettit K. , Rubinstein M. , Treece R. E. , Horwitz J. S. , Chrisey D. B.
Published: July 2002

We have measured the thermopower S(T,H) and resistivity ρ(T,H) of laser ablated La0.67Ca0.33MnO3 films, as a function of temperature and external magnetic field. On heating, a metal–insulator transition occurs at temperatures below the resistivity peak, observed via a jump between a low T regime, where S∝T, and a high T regime, where S∝1/T. An applied magnetic field shifts the transition in a postannealed sample to higher temperatures, causing a giant magnetothermopower effect 100×(S8T−S0T)/S8T=−1400%. Both S(T) and ρ(T) are activated at high temperatures, but with significantly different activation energies. We interpret this as an evidence of small polarons at high temperatures.
Room-temperature lasing operation of a quantum-dot vertical-cavity surface-emitting laser
Authors: Saito Hideaki , Nishi Kenichi , Ogura Ichiro , Sugou Shigeo , Sugimoto Yoshimasa
Published: July 2002

Self-assembled growth of quantum dots by molecular-beam epitaxy is used to form the active region of a vertical-cavity surface-emitting laser (VCSEL). Ten layers of InGaAs quantum dots are stacked in order to increase the gain. This quantum-dot VCSEL has a continuous-wave operating current of 32 mA at room temperature. Emission spectra at various current injections demonstrate that the lasing action is associated with a higher-order transition in the quantum dots.
Structural and optical properties of InAs–GaAs quantum dots subjected to high temperature annealing
Authors: Kosogov A. O. , Werner P. , Gösele U. , Ledentsov N. N. , Bimberg D. , Ustinov V. M. , Egorov A. Yu. , Zhukov A. E. , Kop’ev P. S. , Bert N. A. , Alferov Zh. I.
Published: July 2002

Annealing at higher temperature (700 °C) of structures with two-dimensional and three-dimensional arrays in InAs–GaAs quantum dots (QDs) results in an increase in the size and in a corresponding decrease in the indium composition of the QDs. The change in the In composition is monitored by the contrast pattern in the plan-view transmission electron microscopy (TEM) images viewed under the strong beam imaging conditions. Increase in the size of the QDs is manifested by the plan-view TEM images taken under [001] zone axis illumination as well as by the cross-section TEM images. We show that the dots maintain their geometrical shape upon annealing. Luminescence spectra demonstrate a shift of the QD luminescence peak toward higher energies with an increase in the annealing time (10–60 min) in agreement with the decrease in indium composition revealed in TEM studies. The corresponding decrease in the QD localization energy results in an effective evaporation of carriers from QDs at room temperature, and the intensity of the QD luminescence decreases, and the intensity of the wetting layer and the GaAs matrix luminescence increase with the increase in the annealing time.
Ultralow-loss broadened-waveguide high-power 2 μm AlGaAsSb/InGaAsSb/GaSb separate-confinement quantum-well lasers
Authors: Garbuzov D. Z. , Martinelli R. U. , Lee H. , York P. K. , Menna R. J. , Connolly J. C. , Narayan S. Y.
Published: July 2002

Broadening the waveguides of 2 μm AlGaAsSb/InGaAsSb separate-confinement multiquantum-well lasers decreases their internal losses to 2 cm−1, while threshold current densities remain as low as 300 A/cm2. The consequently high cw differential efficiency of 0.36 results in output powers of 1.2 W from 100 μm aperture lasers at 15 °C.
Two-dimensional electron gas properties of AlGaN/GaN heterostructures grown on 6H–SiC and sapphire substrates
Authors: Redwing J. M. , Tischler M. A. , Flynn J. S. , Elhamri S. , Ahoujja M. , Newrock R. S. , Mitchel W. C.
Published: July 2002

High quality Al0.15Ga0.85N/GaN heterostructures have been fabricated on 6H–SiC and sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A temperature independent mobility, indicative of the presence of a two-dimensional electron gas (2DEG), was observed in all samples below 80 K. The highest low temperature 2DEG mobility, 7500 cm2/V s, was measured in AlGaN/GaN grown on 6H–SiC; the sheet carrier density was 6×1012 cm−2. Strong, well resolved, Shubnikov–de Haas oscillations were observed in fields as low as 3 T and persisted to temperatures as high as 15 K. Hall effect measurements also revealed the presence of well-defined plateaus in the Hall resistance. The high quality 2DEG properties of the AlGaN/GaN heterostructures grown on 6H–SiC are attributed to the absence of significant parallel conduction paths in the material.
Selective oxidation of buried AlGaAs versus AlAs layers
Authors: Choquette Kent D. , Geib K. M. , Chui H. C. , Hammons B. E. , Hou H. Q. , Drummond T. J. , Hull Robert
Published: July 2002

We report significant differences between the properties of buried oxides converted from AlGaAs and AlAs layers using selective wet oxidation. Layers of AlxGa1−xAs with x≥0.96 exhibit crystallographic dependent oxidation rates, while for layers with x≤0.92 the oxidation rate is isotropic. Mesas containing partially oxidized layers of AlAs are unstable to rapid thermal cycling and exhibit excessive strain at the oxide terminus, while mesas containing partially oxidized layers of AlGaAs are robust and lack evidence of strain. Finally, the oxidation of AlGaAs layers, rather than AlAs, is found to provide robust oxide apertures for reliable vertical-cavity surface emitting lasers.
Laser diodes in piezoelectric quantum-well structures
Authors: Cooper Craig , Westwood David I. , Blood Peter
Published: July 2002

We have determined the carrier density and internal electric field as a function of drive current in InGaAs quantum-well laser structures grown on GaAs (111)B substrates by fitting the spontaneous emission spectra observed through a top contact window using a self-consistent solution of Poisson’s and Schrödinger’s equations. At room temperature at threshold the internal field is still 50% of its initial value and as a result the quasi-Fermi levels are closer to the band edges of the well than in a square well, causing significant carrier loss through the barrier/waveguide material. We determine the loss by this route to be twice that for a similar device if the internal field was completely screened.
Controlled manipulation of ethen molecules and lead atoms on Cu(211) with a low temperature scanning tunneling microscope
Authors: Meyer Gerhard , Zöphel Sven , Rieder Karl-Heinz
Published: July 2002

We present results of manipulation experiments for single C2H4 molecules and single Pb atoms and buildup of small nanostructures on a Cu(211) substrate employing our temperature-variable scanning tunneling microscope. We further discuss novel ‘‘indirect’’ lateral manipulation experiments with C2H4 chains and Pb dimers. The latter experiments prove the special stability of Pb dimers. Co-adsorption experiments with CO allow determination of the adsorption sites of C2H4 molecules as well as Pb atoms and Pb dimers.
Influence of light on the coercive field of repoled strontium barium niobate: The role of secondary repoling
Authors: Smith Peter G. R. , Eason Robert W.
Published: July 2002

We have found that the application of light to strontium barium niobate (SBN) during electrical repoling stabilizes the newly formed domains. This stabilization becomes apparent when repoling the crystal back into its original domain direction as a change in the distribution of displacement current as a function of voltage. This appears to be the process underlying the other recent work in the area of optical control of domain structures for quasi-phase-matching of nonlinear processes. We present an explanation for this effect in terms of the microdomain structure of SBN. This model should aid in the search for new materials for optical periodic poling.
Near-field scanning microwave microscope with 100 μm resolution
Authors: Vlahacos C. P. , Black R. C. , Anlage S. M. , Amar A. , Wellstood F. C.
Published: July 2002

We describe the operation of a simple near-field scanning microwave microscope with a spatial resolution of about 100 μm. The probe is constructed from an open-ended resonant coaxial line which is excited by an applied microwave voltage in the frequency range of 7.5–12.4 GHz. We present images of conducting structures with the system configured in either receiving or reflection mode. The images demonstrate that the smallest resolvable feature is determined by the diameter of the inner wire of the coaxial line and the separation between the sample and probe.
Optical studies of Ge islanding on Si(111)
Authors: Persans P. D. , Deelman P. W. , Stokes K. L. , Schowalter L. J. , Byrne A. , Thundat T.
Published: July 2002

We report an experimental study of the optical properties of island layers resulting from molecular beam epitaxial deposition of Ge on Si(111) substrates. The combination of electroreflectance spectroscopy of the E1 transition and Raman scattering allows us to separately determine the strain and composition of the islands. For deposition at 500 °C a deposited layer of 1.36 nm of Ge assembles into 80 nm diameter islands 11 nm thick. The average Si impurity content in the islands is 2.5% while the average in-plane strain is 0.5%. Both strain and Si impurity content in islands decrease with increasing Ge deposition.
High-index proton-exchanged waveguides in Z-cut LiNbO3 with undegraded nonlinear optical coefficients
Authors: Rams J. , Olivares J. , Cabrera J. M.
Published: July 2002

A method is described for producing high quality optical waveguides on Z-cut LiNbO3 substrates by proton exchange in pure benzoic acid vapor. Either α- or β-phase guides are obtained by varying the exchange temperature (within the 250–375 °C range) and/or the treatment duration (between 15 min and 24 h). The method presents definitive advantages with regards to common immersion methods. For both α- and β-phase guides the d33 nonlinear optical coefficient is 90% of the substrate value, and their optical losses are below 0.35 dB/cm. This is the first report on β-phase guides with almost undegraded nonlinear optical coefficients.
Overpressurized bubbles versus voids formed in helium implanted and annealed silicon
Authors: Fichtner P. F. P. , Kaschny J. R. , Yankov R. A. , Mücklich A. , Kreißig U. , Skorupa W.
Published: July 2002

The formation of helium induced cavities in silicon is studied as a function of implant energy (10 and 40 keV) and dose (1×1015, 1×1016, and 5×1016 cm−2). Specimens are analyzed after annealing (800 °C, 10 min) by transmission electron microscopy (TEM) and elastic recoil detection (ERD). Cavity nucleation and growth phenomena are discussed in terms of three different regimes depending on the implanted He content. For the low (1×1015 cm−2) and high (5×1016 cm−2) doses our results are consistent with the information in the literature. However, at the medium dose (1×1016 cm−2), contrary to the gas release calculations which predict the formation of empty cavities, ERD analysis shows that a measurable fraction of the implanted He is still present in the annealed samples. In this case TEM analyses reveal that the cavities are surrounded by a strong strain field contrast and dislocation loops are generated. The results obtained are discussed on the basis of an alternative nucleation and growth behavior that allows the formation of bubbles in an overpressurized state irrespective of the competition with the gas release process.
Analysis of transfer function of metal-semiconductor-metal photodetector equivalent circuit
Authors: Gvozdić Dejan M.
Published: July 2002

The response of GaAs metal-semiconductor-metal photodetectors (MSM-PD) is simulated using a phenomenological model of transport equations, which takes into account the electron intervalley transfer. Calculations are performed for intrinsic devices with 1 μm and 0.5 μm interdigital spacing with the corresponding equivalent (parasitic) circuit of the photodetector. It is pointed out that parasitic effects may induce a significant distortion, a delay of intrinsic response, and increase bit error rate. Analysis of the equivalent or parasitic circuit of the MSM-PD transfer function shows that the distortion and delay of the intrinsic signal can be minimized by choosing the optimal photodetector capacitance.
Surface modification of indium tin oxide by plasma treatment: An effective method to improve the efficiency, brightness, and reliability of organic light emitting devices
Authors: Wu C. C. , Wu C. I. , Sturm J. C. , Kahn A.
Published: July 2002

We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from >20 to <10 V, the external electroluminescence quantum efficiency (backside emission only) increases by a factor of 4 (from 0.28% to 1%), a much higher drive current can be applied to achieve a much higher brightness (maximum brightness ∼10,000 cd/m2 at 1000 mA/cm2), and the forward-to-reverse bias rectification ratio increases by orders of magnitude (from 102 to 106–107). The lifetime of the device is also enhanced by two orders of magnitude.
High magnetic field effects on the ultrafast transport transient of hot electrons in InSb
Authors: Mendes E. A. , Caetano E. W. S. , Freire V. N. , da Costa J. A. P.
Published: July 2002

The ultrafast transient behavior of the transport parameters in InSb subjected to high parallel electric and magnetic fields is studied. The filling of high Landau subbands is considered in the numerical calculations. It is shown that the high magnetic field reduces the electron drift velocity and energy. An overshoot in the electron drift velocity can occur, but the magnetic field does not favor its existence and can even eliminate it.
Effects of aquaregia treatment of indium–tin–oxide substrates on the behavior of double layered organic light-emitting diodes
Authors: Li F. , Tang H. , Shinar J. , Resto O. , Weisz S. Z.
Published: July 2002

The effects of a controlled aquaregia treatment of indium–tin–oxide (ITO) substrates on the behavior of highly efficient vacuum evaporated double layered 8-tris-hydroxyquinoline Al (Alq3)-based light-emitting diodes are described. It is found that in suitably treated devices, both current injection and the electroluminescence (EL) are significantly enhanced. The enhancement is believed to result from the greater ITO/hole transporting layer contact areas and the contact conditions. The observed dependence of I(V), the EL output, and the EL efficiency on the ITO surface morphology indicates that space-charge-limited currents dominate the behavior of the devices.
Selective thermal decomposition of ultrathin silicon oxide layers induced by electron-stimulated oxygen desorption
Authors: Watanabe Heiji , Fujita Shinobu , Maruno Shigemitsu , Fujita Ken , Ichikawa Masakazu
Published: July 2002

The mechanism of electron-beam-induced selective thermal decomposition of ultrathin oxide layers on Si surfaces was studied by scanning reflection electron microscopy, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. We found that the change in the oxide layer composition caused by electron-stimulated oxygen desorption accounted for the selective thermal decomposition, where nanometer-scale voids were densely generated at a low heating temperature (720 °C). This implies that oxygen desorption from the oxide layers promotes the formation of a volatile oxide (SiO), and generates void nucleation sites.
Study of contact formation in AlGaN/GaN heterostructures
Authors: Liu Q. Z. , Yu L. S. , Deng F. , Lau S. S. , Chen Q. , Yang J. W. , Khan M. A.
Published: July 2002

The contact formation of Ti/Al and Ti metallization on AlGaN/GaN heterojunction field effect transistors (HFET) was investigated. It was found that ohmic contact formation is related to the low work function of the Ti contacting layer and the formation of a TiN phase at the Ti/nitride interface. Contact resistance as low as 1 Ω mm or less can be obtained on HFET samples with a nsμ product of ∼0.8×1016/V s and on n-GaN with a carrier concentration of 1.5×1018/cm3. Ti/Al bilayer contact scheme is superior to Ti-only contact due to a surface Al3Ti layer in the bilayer contact, which may reduce the oxidation problem when annealed in N2 at high temperatures. Preannealing the HFET samples at 850 °C for 1 h in N2 appears to improve the ohmic contact in general, but not always observed. Our results indicate that Ti/Al contact scheme yields sufficiently low contact resistance on HFET structures for microwave applications.
Transient electroluminescence: Mobility and response time in quinquethiophene Langmuir–Blodgett films
Authors: Pal A. J. , Österbacka R. , Källman K.-M. , Stubb H.
Published: July 2002

Transient characteristics of organic light-emitting diodes with Langmuir–Blodgett films of quinquethiophene as the emitting material have been studied. Field and thickness dependence of the response time have been studied. From the time lag between the voltage pulse and the first appearance of electroluminescence emission, the mobility parallel to the chain of the molecules has been calculated. In thicker films, anisotropy in mobility has been observed when compared with previous results obtained from field-effect studies. The interfaces have been found to play an important role in thinner films.