bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct-current method for differentiating contact and bulk low frequency resistance fluctuations
Authors: West P. W.
Published: July 2002

Electronic conductance fluctuations, which have often been used as a detailed probe of electronic transport properties, are known to originate from the metal/material contact region as well as from the material itself. In this article, a measurement technique of conductance fluctuations is presented that differentiates between material bulk fluctuations and contact fluctuations. While it was developed for noise studies on high resistance materials (>100 k Ω) where other methods of eliminating or identifying contact noise may not be possible, the method is applicable for all sample resistance ranges. The measurement circuit is a variation of the well-known four-probe technique used to eliminate the influence of the direct-current contact resistance from the sample resistance measurement. A battery supplies the dc current but does not act as a constant current source, while two low-noise preamplifiers measure the current and voltage fluctuations simultaneously. By comparing the real time fluctuations in the current and voltage across pair-wise variations of the four probes, and in particular, determining whether the current fluctuations are symmetric or antisymmetric with the voltage fluctuations, the spatial region of the sample responsible for the noise can be directly identified. This technique does not eliminate contact noise but is a sensitive means of distinguishing contact from bulk fluctuations. Examples of systems for which this method will be useful include high impedance semiconductors, where an insulating or Schottky barrier at the metal/semiconductor interface may generate noise from a separate process that is comparable to or greater in magnitude than the bulk noise.
Electrically controlled broadband microwave attenuator with p-i-n diode switches
Authors: Nakano Hiroshi , Kato Yoshihiko
Published: July 2002

An electrically controlled broadband attenuator composed of two p-i-n diode switches and two quadrature hybrid couplers has been newly developed. Its attenuation can be precisely controlled by electrical pulses and also easily measured by counting its pulse repetitions. It can thus be used for broadband microwave measurements, especially noise measurement. When controlled with pulses whose width was 10 μs and repetition frequencies were between 10 and 90 kHz, this attenuator’s attenuation to repetition frequencies was linear for signal frequencies in the range of 2–12 GHz.
Transmission grating spectroscopy in the 10 keV range
Authors: Fill E. , Stephan K.-H. , Predehl P. , Pretzler G. , Eidmann K. , Saemann A.
Published: July 2002

We demonstrate the feasibility of transmission grating spectroscopy in the 10 keV range. The diffraction efficiency of a 5000 ℓ/mm free-standing gold transmission grating was measured at the three photon energies of 1.49, 5.4, and 8.04 keV (Kα of Al, Cr, and Cu). At 1.49 keV, the measured efficiencies agree well with efficiencies calculated by standard grating theory. At 5.4 and 8.04 keV, the measured efficiencies significantly exceed the theoretical ones. For Cu Kα, the absolute efficiency into first order is still as high as 3.3%. We present spectra of fs laser plasmas generated by irradiating Al, Cr, and Cu targets.
Three-dimensional optical trapping and evanescent wave light scattering for direct measurement of long range forces between a colloidal particle and a surface
Authors: Clapp A. R. , Ruta A. G. , Dickinson R. B.
Published: July 2002

A novel technique is described to measure weaker long range forces between a single colloidal particle and a flat surface as a function of separation distance. The technique uses a three-dimensional optical trap as a force transducer and evanescent wave light scattering to measure the particle-surface separation distance. The three-dimensional optical trap allows direct manipulation of the particle near the surface, and the measured displacement of the particle from the center of the optical trap determines the force of interaction between the particle and the surface. The particle position is determined by measuring the scattered light from the particle in an evanescent wave created by total internal reflection of a laser beam on the surface. This combination of optical techniques allows forces to be directly measured for smaller particles (∼0.25–5.0 μm) and over a range of forces (∼10−14–10−11 N) than has been previously reported for similar force measurement techniques such as atomic force microscopy and total internal reflection microscopy. The experimental results have been compared with DLVO theory predictions for 1.5 μm silica microspheres interacting with glass in NaCl solutions of known ionic strength.
A 400 kHz, fast-sweep Langmuir probe for measuring plasma fluctuations
Authors: Chiodini G. , Riccardi C. , Fontanesi M.
Published: July 2002

A novel, fast-sweep Langmuir probe has been constructed and successfully operated on “Thorello.” It is based on a novel, dual channel circuit that compensates for stray capacitance and permits sweep speeds up to 400 kHz. The circuit response has been tested by measuring the known current–voltage characteristics of resistors and diodes. In addition, the probe has been used to measure the electron temperature and density as well as the plasma potential of plasmas generated in Thorello. A method of three-parameter curve fitting is used to analyze the time-dependent data. The measurements compare favorably with those derived from other standard probe techniques.
Comprehensive diagnostic suite for a magnetically insulated transmission line oscillator
Authors: Haworth M. D. , Englert T. J. , Hendricks K. J. , Lemke R. W. , Luginsland J. W. , Shiffler D. S. , Spencer T. A.
Published: July 2002

The magnetically insulated transmission line oscillator (MILO) is a gigawatt-class cross-field microwave tube that requires no external magnetic field due to inherent self-magnetic insulation. The tube operates with a 500 kV, 60 kA electron beam which, along with high rf fields, poses quite a challenge for diagnosing the device. We report on the comprehensive set of experimental diagnostics (both beam and microwave) employed in the MILO experiment, and show how these diagnostics, teamed with particle-in-cell computer simulations, have been instrumental in discovering problems with the tube.
Acceptor-bound exciton recombination dynamics in p-type GaN
Authors: Smith M. , Chen G. D. , Lin J. Y. , Jiang H. X. , Asif Khan M. , Sun C. J.
Published: July 2002

Dynamics of the neutral-acceptor-bound exciton transition (the I1 line) in a Mg doped p-type GaN epitaxial layer grown by metalorganic chemical vapor deposition (MOCVD) have been studied by time-resolved photoluminescence emission spectroscopy. Two emission lines in the I1 transition region have been resolved in the time-resolved spectra, possibly due to the existence of two energy states of the Mg impurities after postgrowth thermal annealing. The recombination lifetimes of the acceptor-bound exciton transition have been measured under different conditions including temperature, excitation intensity, and emission energy. From these measurements, a value of about 450 ps for the radiative recombination lifetime has been obtained, which is an important physical quantity for optoelectronic device applications based on GaN.
A layer-by-layer metallic photonic band-gap structure
Authors: McCalmont J. S. , Sigalas M. M. , Tuttle G. , Ho K.-M. , Soukolis C. M.
Published: July 2002

A new photonic band-gap structure has been developed using a periodic array of metallic rods. Structures have been designed and built that operate in the 75–110 GHz frequency range. A periodic structure shows a high-pass transmission characteristic, while the addition of a defect to the structure adds a bandpass response. Measured responses show good agreement with theoretical simulations. A defect mode operated in the reflection mode showed a quality factor Q of 461. This new metallic structure is considerably smaller than comparable dielectric photonic band-gap structures, and should be useful for building compact, inexpensive filters with operating frequencies ranging from 1 GHz to 1 THz.
Stable electroluminescence from reverse biased n-type porous silicon–aluminum Schottky junction device
Authors: Lazarouk S. , Jaguiro P. , Katsouba S. , Masini G. , La Monica S. , Maiello G. , Ferrari A.
Published: July 2002

We report the realization of a bright and stable electroluminescent Schottky diode based on aluminum-porous silicon junction. White light, visible in normal daylight, is emitted when a reverse bias is applied to the device, promoting the junction breakdown. The device has a fast (100 ns) rise time of the light emission. An excellent stability, tested over more than one month of continuous operation at a high bias level, is achieved by the complete encapsulation of the active porous silicon under a transparent alumina layer. The external power efficiency of light emission is 0.01%.
Layer-by-layer atomic manipulation on Si(111)-7×7 surface
Authors: Komeda T. , Hasunuma R. , Mukaida H. , Tokumoto H.
Published: July 2002

Layer-by-layer removal of Si atoms from the Si(111)-7×7 surface was executed at room temperature by making a point contact of a biased W tip of scanning tunneling microscope (STM) to the sample surface. The adatom layer and the three layers were controllably removed by tuning the sample bias voltage. In the created holes, clear atomic images were obtained. The current between the STM tip and substrate exhibited characteristic structures during the tip excursion, which are closely related with the atom removal process and the nature of the Si nanoscale wire, respectively.
Surface photoabsorption study of the effect of substrate misorientation on ordering in GaInP
Authors: Murata H. , Ho I. H. , Hosokawa Y. , Stringfellow G. B.
Published: July 2002

Substrate orientation strongly affects Cu–Pt ordering in Ga0.5In0.5P layers grown by organometallic vapor phase epitaxy. In situ surface photoabsorption (SPA) measurements were used to measure the concentration of [1̄10]-oriented P dimers, characteristic of the (2×4) reconstructed surface, as a function of substrate misorientation from (001). For substrates misoriented toward either [110] [or (111)A] or [1̄10] [or (111)B], the P-dimer concentration is found to decrease systematically as the misorientation angle increases from 0° to 15.8° at 620 °C with tertiarybutylphosphine (TBP) partial pressures of 10, 50, and 200 Pa. The P-dimer concentrations on substrates misoriented toward [110] are higher than for those misoriented toward [1̄10]. The Ga0.5In0.5P layers were found to form the Cu–Pt structure during growth. The degree of order, determined from 20 K photoluminescence measurements, shows a strong correlation with the concentration of [1̄10]-oriented P dimers. The data also clearly show the effect of step structure on Cu–Pt ordering. They indicate that [110] steps formed by misorientation toward [1̄10] enhance ordering while [1̄10] steps formed by misorientation toward [110] retard ordering.
Dielectric apertures as intracavity lenses in vertical-cavity lasers
Authors: Coldren L. A. , Thibeault B. J. , Hegblom E. R. , Thompson G. B. , Scott J. W.
Published: July 2002

Dielectric current apertures are shown to be effective as intracavity lenses for reduced optical losses in vertical-cavity lasers. The Gaussian resonator theory is applied to obtain a mode radius, and this is shown to be in good agreement with experiment as well as a recently proposed uniform waveguide model.
Room-temperature, short-wavelength (400–500 nm) photoluminescence from silicon-implanted silicon dioxide films
Authors: Skorupa W. , Yankov R. A. , Tyschenko I. E. , Fröb H. , Böhme T. , Leo K.
Published: July 2002

Experiments are reported which explore the possibility of using low-temperature, multiple-energy Si+ ion implantation into thin SiO2 films on Si and subsequent short-time thermal processing to form silicon nanostructures capable of yielding a high-intensity emission in the short-wavelength part of the visible spectrum. A room-temperature short-wavelength PL band of high intensity was found after double implantation with energies of 200 and 100 keV at a temperature of −20 °C to a total dose of 4.8×10 16 cm−2 (atomic concentration about 2×1021 cm−3) and subsequent furnace annealing at 400 °C for 0.5 h in forming gas or by flash lamp annealing at 1050 °C for 20 ms.
Charge-free etching process using positive and negative ions in pulse-time modulated electron cyclotron resonance plasma with low-frequency bias
Authors: Ohtake Hiroto , Samukawa Seiji
Published: July 2002

Charge build-up on a substrate is greatly reduced by using a pulse-time modulated electron cyclotron resonance generated plasma of more than 50 μs with 600 kHz radio frequency (RF) substrate bias. Negative ions are generated in the afterglow plasma due to decay in electron temperature. Low-frequency RF bias directly accelerates both negative and positive ions from the plasma onto the substrate and reduces self-bias voltage, thus reducing charge accumulation on the substrate. This method shows promise for charge-free etching processes in the production of ultra large scale integrated circuits.
Ion drag effects in inductively coupled plasmas for etching
Authors: Collison Wenli Z. , Kushner Mark J.
Published: July 2002

Ion drag refers to Coulomb momentum transfer collisions between a directed flux of charged particles and a second charged species. It is an important force in determining the motion of negatively charged dust particles in plasma processing reactors. The same ion drag mechanism acts between the directed flux of positive ions moving towards the boundaries of a plasma etching reactor and negative ions being accelerated by electrostatic forces towards the center of the plasma. In this letter, we discuss the parameter space for inductively coupled plasma etching reactors in which ion drag forces on negative ions influence their transport using results from a two-dimensional plasma equipment model. We find that ion drag forces on negative ions are important at high plasma densities and low ion temperatures. Under these conditions, the large positive ion flux, coupled with a large Coulomb cross section, creates an ion drag force which may dominate the electrostatic forces on negative ions. The end result is that negative ions may accumulate near the sheath edge.
The nonlinear transport regime of a T-shaped quantum interference transistor
Authors: Šordan R. , Nikolić K.
Published: July 2002

We present the current–voltage characteristics of a T-shaped semiconductor electron waveguide structure in the nonlinear regime. This system is interesting as a potential quantum interference transistor. Calculated zero-temperature I–V characteristics are with multiple peaks and exhibit regions of negative-differential resistance. The peak-to-valley ratio has maximum value of 3:1. The calculated values of the transconductance and the differential drain conductance are small, which suggests limited abilities for conventional applications of the transistor.
Investigations of the structural stability of highly strained [(Al)GaIn]As/Ga (PAs) multiple quantum wells
Authors: Marschner T. , Lutgen S. , Volk M. , Stolz W. , Göbel E. O.
Published: July 2002

The structural stability of highly strained [(Al)GaIn]As/Ga (PAs) strained layer superlattice (SLS) structures as a function of the average lattice mismatch is investigated. The SLS structures, grown by metalorganic vapor-phase epitaxy, are studied by using high-resolution x-ray diffraction. The range of stable SLS structures obtained experimentally is compared to a recent theoretical model assuming strain relaxation by formation of misfit dislocations [D. C. Houghton, M. Davies, and M. Dion, Appl. Phys. Lett. 64, 505 (1994)]. In particular, the influence of the surface migration length of the group-III species on the growth surface is investigated by varying the Al concentration in the compressively strained [(Al)GaIn]As layers. Structurally stable [(Al)GaIn]As/Ga(PAs) SLS structures having a total layer thickness of 1 μm and an average lattice mismatch of up to (Δd/d)⊥=1.2×10−2 have been realized. These values exceed by far the values of the misfit dislocation model. This behavior indicates that surface morphology changes rather than misfit dislocation formation are the primary cause for the instability of highly strained SLSs.
Time-resolved-spectrum studies of GaN light emitting diodes
Authors: Choa F. S. , Fan J. Y. , Liu P.-L. , Sipior J. , Rao G. , Carter G. M. , Chen Y. J.
Published: July 2002

Time-resolved-emission spectra of InGaN and GaN light emitting diodes with different device structure are studied. Result shows that the UV generation from bulk materials and blue and green emissions from single quantum well devices are band edge recombination.
Characteristics of the high density plasma production by m=0 helicon wave
Authors: Sakawa Y. , Koshikawa N. , Shoji T.
Published: July 2002

The characteristics of the high density plasma production by the m = 0 azimuthal mode helicon wave is investigated. By varying the power of the applied radio frequency Prf two discharge modes exist; one is the low density mode (plasma density np<1012 cm−3), at which the discharge is sustained not by a helicon wave but by the antenna induction field at low Prf; the other is the high density mode (np≳1013 cm−3), in which helicon wave is excited at high Prf; in between the two modes, np jumps more than an order of magnitude. The threshold Prf for the density jump is higher for the m = 0 mode than that of the m = +1 and −1 modes excited by the helical antenna.
Room temperature epitaxy of Pd films on GaN under conventional vacuum conditions
Authors: Liu Q. Z. , Lau S. S. , Perkins N. R. , Kuech T. F.
Published: July 2002

Pd films deposited at room temperature have been found to grow epitaxially on GaN grown by metalorganic vapor phase epitaxy (MOVPE). The Pd films were deposited on GaN substrates cleaned by chemicals only, and in a conventional e-beam evaporation system with a vacuum of ∼1×10−7 Torr. MeV 4He backscattering spectrometry and the Read x-ray camera were used to evaluate the Pd films. The effects of various chemical etchants—such as aqua regia, HCl:H2O, and HF:H2O—on the epitaxial quality of the Pd films have also been investigated. Ni and Pt films deposited on GaN in a similar manner were also found to be epitaxial.
Low temperature photoluminescence measurements on boron- and hydrogen-implanted 6H–SiC
Authors: Peppermüller C. , Helbig R. , Rottner K. , Schöner A.
Published: July 2002

We investigated the low temperature (T<2 K) photoluminescence (LTPL) emission of 6H–SiC samples after implantation of either boron or boron together with hydrogen. After implantation of boron and annealing at 1700 °C, we detected new LTPL emission at 4205 Å. After implantation of hydrogen into the boron-implanted and annealed sample, another LTPL emission at 4183 Å appeared. We interpret two additional peaks as vibrational modes of the 4183 Å emission with energies of 86 and 118 meV.
Observation of magnetic domains using a reflection-mode scanning near-field optical microscope
Authors: Durkan C. , Shvets I. V. , Lodder J. C.
Published: July 2002

It is demonstrated that it is possible to image magnetic domains with a resolution of better than 60 nm with the Kerr effect in a reflection-mode scanning near-field optical microscope. Images taken of tracks of thermomagnetically prewritten bits in a Co/Pt multilayer structure magnetized out-of plane showed optical features in a track pattern whose appearance was determined by the position of an analyzer in front of the photomultiplier tube. These features were not apparent in the topography, showing this to be a purely magneto-optic effect.
Interfacial hardness enhancement in deuterium annealed 0.25 μm channel metal oxide semiconductor transistors
Authors: Devine R. A. B. , Autran J.-L. , Warren W. L. , Vanheusdan K. L. , Rostaing J.-C.
Published: July 2002

Interfacial degradation induced by hot-electron injection has been studied in n-channel metal oxide semiconductor transistors with channel lengths down to 0.2 μm. The devices were annealed in either H2 or D2 containing atmospheres. The channel transconductance and threshold voltage variations induced by hot-electron injection into the gate are consistent with interface state generation. Charge pumping experiments confirm this conclusion. The lifetime for a 10% reduction in the transconductance is enhanced by ∼10 times for devices annealed in D2 containing atmospheres as compared to those annealed in H2.
Voltage tunable plasma resonances in induced-base hot-electron transistors
Authors: Ryzhii V.
Published: July 2002

The excitation of the standing plasma waves in induced-base hot-electron transistors (IBHETs) and its impact on the IBHET high-frequency operation are considered. It is shown that the plasma waves result in resonant behavior of the IBHET performance at high frequencies. The frequency dependent small-signal transconductance of the IBHET is calculated using an analytical model. The resonant frequencies and the sharpness of the resonant peaks are found as functions of structural parameters and bias voltage. The resonant frequencies can correspond to the terahertz range. The resonant transconductance can significantly exceed the steady-state transconductances of the IBHET.
Stability and evolution of stacking faults in melt textured YBa2Cu3O7−x
Authors: Sandiumenge F. , Vilalta N. , Maniette Y. , Obradors X.
Published: July 2002

The annihilation process of large 16〈301〉 stacking faults in melt textured YBa2Cu3O7−x is in situ monitored in the transmission electron microscope. The process is activated at the onset of the orthorhombic to tetragonal transition induced in the electron microscope, thus evidencing the instability of such defects in the deoxygenated YBa2Cu3O7−x matrix. The observations suggest that the annihilation (and growth) process is limited by surface diffusion along Y2BaCuO5/YBa2Cu3O7−x interfaces. The propagation mechanism of such stacking faults is addressed in the light of the present findings.
Transient ion drift detection of low level copper contamination in silicon
Authors: Heiser T. , McHugo S. , Hieslmair H. , Weber E. R.
Published: July 2002

The transient ion drift (TID) method was used to measure quenched interstitial copper concentrations in both copper plated and copper implanted silicon. Comparison with existing literature data allows one to conclude that, contrary to the general expectation, it is possible to quench in most of the Cu dissolved at temperatures of 600 °C and below. This result suggests that the TID technique could be an excellent means to detect copper contamination in p-type silicon. The expected detection limit, on the order of 1011 cm−3, makes the method a potentially interesting tool to use in gettering or in-diffusion barrier studies.
Effect of illumination on the subband electronic structure of Si δ-doped GaAs
Authors: Li G. , Jagadish C.
Published: July 2002

The subband electronic structure of Si δ-doped GaAs grown by metal organic vapor phase epitaxy was investigated using magnetotransport measurements. The work focused on the effect of illumination. We found that illumination leads to a slight increase of the quasi-two dimensional electron gas density in the well. This increase does not depend on the illumination intensity. The illumination-generated electron density weakly increases with an increase in the Si δ-doping concentration. We also experimentally confirmed that illumination only slightly alters the electron densities of the occupied subbands and the illumination-generated electrons populate one previously empty subband. Only a fraction of the illumination-generated electrons persist but the newly occupied subband under illumination remains populated in the dark after removal of the illumination. The experimental results suggest that the DX centers are unlikely to be populated in our Si δ-doped GaAs regardless of the doping concentration. The weak and partially persistent photoconductivity effect observed in Si δ-doped GaAs may arise from ionization of other Si localized states.
Etching and passivation effects on boron-doped amorphous silicon carbide p layer of amorphous silicon solar cell by hydrogen treatment using a mercury-sensitized photochemical vapor deposition method
Authors: Jang Jae Hoon , Lim Koeng Su
Published: July 2002

Etching and passivation effects of hydrogen treatment of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p layer of p-i-n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated. For the hydrogen treatment of the p-layer film, longer p-layer deposition time was needed to obtain the same thickness as for no hydrogen treatment because of hydrogen etching effect. However, the cell performance was improved by ∼7% due to an increase in the open circuit voltage (Voc) and fill factor (FF) although the p-layer thickness was nearly identical in both cases. The increase in the Voc and FF could be explained by an increase in the built-in potential due to a decrease in the film activation energy. Moreover, the electrical property improvement of the film was well explained by the passivation effect of a SiH2/SiH ratio decrease and a hydrogen content increase calculated from Fourier transformed infrared absorption measurements.
Extremely low excitation threshold, superradiant, molecular aggregate lasing system
Authors: Özçelik Serdar , Akins Daniel L.
Published: July 2002

An extremely low threshold for lasing from a specially formulated system involving self-assembled, aggregated molecules enables the system to function as a mirrorless laser. Superradiant energy states created in coherently coupled aggregated molecules emit photons that stimulate emission from other spatially distributed superradiant states. This study focuses on a cyanine dye (specifically, 1,1′-3,3′-tetraethyl-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine iodide) adsorbed onto silica colloid on which the dye aggregates, forming excitonic states involving coherent domains in which a finite number of molecules act cooperatively in the process of emitting photons. It is found that lasing from such a system is induced at a threshold of ca. 39 pJ/pulse, which corresponds to a factor of 3×10+4 times smaller than the lowest lasing threshold reported in the literature.
Theory of ripple topography inhibition in depth profiling with sample rocking
Authors: Carter G.
Published: July 2002

A theory is developed which explains how sample rocking during ion beam sputtering erosion can inhibit ripple formation, observed with monodirectional ion incidence, on radiation amorphisable materials. The model assumes curvature dependent sputtering yield and random ion arrival and sputtering as roughening processes and radiation mediated viscous flow and ballistically driven effective surface diffusion as smoothing processes.
Resonant interband tunneling current in InAs/AlSb/GaSb/AlSb/InAs diodes with extremely thin AlSb barrier layers
Authors: Kitabayashi H. , Waho T. , Yamamoto M.
Published: July 2002

We have investigated the resonant interband tunneling current in InAs/AlSb/GaSb/AlSb/InAs double-barrier resonant interband tunneling diodes with extremely thin AlSb barriers. Although no negative differential resistance (NDR) was observed for the diode without AlSb barrier layers, NDR appeared when 0.5-monolayer(ML)-thick AlSb barrier layers were inserted. As the thickness of AlSb barriers (Lb) increased from 0.5 to 2 ML, the difference between the peak current density and the valley current density increased. This result indicates the crucial role of the extremely thin AlSb barrier layers that are responsible for the resonance level and move it up toward the GaSb valence-band edge with an increase in Lb.
Quantum interference in nanometric devices: Ballistic transport across arrays of T-shaped quantum wires
Authors: Goldoni Guido , Rossi Fausto , Molinari Elisa
Published: July 2002

We propose that recently realized T-shaped semiconductor quantum wires (T wires) could be exploited as three-terminal quantum interference devices. T wires are formed by intersecting two quantum wells (QWs). By use of a scattering matrix approach and Landauer–Büttiker theory, we calculate the conductance for ballistic transport in the parent QWs and across the wire region as a function of the injection energy. We show that different conductance profiles can be selected by tailoring the widths of the QWs and/or by combining more wires on the scale of the Fermi wavelength. Finally, we discuss the possibility of obtaining spin-dependent conductance of ballistic holes in the same structures.
Current distribution effects in magnetoresistive tunnel junctions
Authors: van de Veerdonk R. J. M. , Nowak J. , Meservey R. , Moodera J. S. , de Jonge W. J. M.
Published: July 2002

The influence of an inhomogeneous current density on the (magneto)resistance of a ferromagnet–insulator–ferromagnet tunnel junction in the cross-strip geometry is analyzed using a finite element approach. The four-probe resistance is smaller than the actual resistance for electrode resistances (in the junction area) comparable to or higher than the junction resistance. Even negative four-probe resistances can be obtained. The apparent resistance change due to the junction magnetoresistive effect also decreases, but always remains positive. This results in unrealistically large apparent magnetoresistance ratios which can even approach infinity, which explains some recent experiments.
Electric field poling of flux grown KTiOPO4
Authors: Karlsson H. , Laurell F.
Published: July 2002

A method to periodically pole conductive ferroelectric materials has been developed. With this method we have fabricated high quality domain inverted crystals of flux grown KTiOPO4 suitable for efficient quasi-phase matched frequency conversion. Samples have been evaluated by second harmonic generation and normalized conversion efficiencies up to 6.9%/W cm2 have been obtained.
The role of second-neighbor effects in photoemission: Are silicon surfaces and interfaces special?
Authors: Zhang K. Z. , Litz K. E. , Banaszak Holl M. M. , McFeely F. R.
Published: July 2002

A widely used assignment scheme for Si 2p core-level photoemission studies of silicon oxidation relies solely on the formal oxidation state of the silicon. The tacit assumption of this assignment methodology is that second-neighbor effects have no measurable effect on observed Si 2p binding energies. In this letter, new experiments are combined with literature precedents to make the case that the second-neighbor effects play an important role in determining binding energy shifts.
Characterization of the negative ion fraction in high-density SF6 magnetoplasmas using ion acoustic waves
Authors: St-Onge L. , Margot J. , Chaker M.
Published: July 2002

Most plasmas used in the microelectronics industry are electronegative, i.e., they contain a large number of negative ions. One simple way to characterize the negative-to-positive ion density ratio (n−/n+) in such plasmas is to measure the velocity of ion acoustic waves (IAWs). In this letter, a detailed study of the propagation of IAWs in high-density SF6 magnetoplasmas is given. Results (n−/n+) obtained by way of this technique as functions of different parameters (gas pressure, SF6 content in SF6/Ar mixture, and radial position in the reactor) are compared to those obtained with a more sophisticated technique based on laser photodetachment.
Formation of the precursor for diamond growth by
in situ
direct current glow discharge pretreatment
Authors: Gouzman I. , Lior I. , Hoffman A.
Published: July 2002

A direct current (dc) glow discharge—surface interaction process for in situ substrate surface pretreatment, for the promotion of diamond growth on mirror polished Si(100) is reported. A key step of the pretreatment is a generation of a stable dc-glow discharge between a grounded substrate and a positively biased electrode using a CH4/H2 gas mixture. During this stage, no additional gas phase activation by a hot filament is carried out. For subsequent diamond deposition, the dc-glow discharge is switched off and the standard hot filament CVD growth conditions are applied. The nature of the deposits formed by the dc-glow discharge process and after standard diamond deposition conditions, are investigated by Raman spectroscopy and high resolution scanning electron microscopy. It is found that the properties of the films, deposited following the dc-glow discharge pretreatment, strongly depend on the pretreatment conditions: methane concentration, substrate temperature, and pretreatment time. It is suggested that the nature of the diamond precursor formed during the dc-glow discharge pretreatment is similar to that one observed after bias enhanced nucleation process.
Alloying effects on the critical layer thickness in InxGa1−xAs/InP heterostructures analyzed by Raman scattering
Authors: Pizani P. S. , Boschi T. M. , Lanciotti F. , Groenen J. , Carles R. , Maigné P. , Gendry M.
Published: July 2002

Raman scattering has been used to estimate the critical layer thickness and to analyze the alloying effect on strain relaxation in InxGa1−xAs layers grown by molecular beam epitaxy on InP [001]-oriented substrate, for x ranging from 0.0 to 1.0. Measurements of longitudinal optical GaAs-like phonon frequency and Raman linewidth showed that the indium/gallium ratio contents greatly influences the strain relaxation. A comparison between Raman and x-ray diffraction measurements of relaxation ratios as a function of layer thickness is presented. The results can be explained in terms of the combined effect of strain and chemical and structural disorder.
Explanation for carrier removal and type conversion in irradiated silicon solar cells
Authors: Yamaguchi T. , Taylor S. J. , Watanabe S. , Ando K. , Yamaguchi M. , Hisamatsu T. , Matsuda S.
Published: July 2002

Heavy doses of radiation in space can cause the failure of n+/p/p+ silicon solar cells due to the gradual introduction of compensating defects into the base layer of the diode. In this letter, we show that the radiation-induced defects, which play the most important role in this process, referred to as “carrier removal,” are probably minority-carrier traps at an energy level approximately 0.18 eV below the conduction band. We conclude that these defects must be positively charged before electron capture, and therefore, act as donor centers which compensate the p-type base layer.
Hydrogen-decorated lattice defects in proton implanted GaN
Authors: Weinstein Marcie G. , Song C. Y. , Stavola Michael , Pearton S. J. , Wilson R. G. , Shul R. J. , Killeen K. P. , Ludowise M. J.
Published: July 2002

Several vibrational bands were observed near 3100 cm−1 in GaN that had been implanted with hydrogen at room temperature and subsequently annealed. Our results indicate that these bands are due to nitrogen-dangling-bond defects created by the implantation that are decorated by hydrogen. The frequencies are close to those predicted recently for VGa–Hn complexes, leading us to tentatively assign the new lines to VGa defects decorated with different numbers of H atoms.
Large positive magnetoresistance in thin films of silver telluride
Authors: Chuprakov I. S. , Dahmen K. H.
Published: July 2002

Thin films of Ag2Te were obtained by vapor transport and electron beam evaporation methods. The film phase and chemical compositions were analyzed by x-ray diffraction and x-ray photoelectron spectroscopy analyses, respectively. Film composition was close to stoichiometric with a slight excess of tellurium. Strong (1̄21) texture was found in the films obtained by the electron beam evaporation. Magnetoresistance (MR) measurements show different temperature dependencies for oriented and nonoriented films. MR of the oriented films has a strong peak up to 390% at 90 K and 5 T, whereas MR of the nonoriented films is almost temperature independent in the 10–100 K range. Some analogies can be found between these materials and colossal MR and nonmagnetic multilayer MR materials.
The special magnetoresistive effect in trilayered films made of manganite perovskites
Authors: Cheng Rongsheng , Li Kebin , Wang Shouguo , Chen Zhixiang , Xiong Caoshui , Xu Xiaojun , Zhang Yuheng
Published: July 2002

The transport and magnetic properties of La2/3Ca1/3MnO3(LCMO), (La1−yNdy)Ca1/3MnO3 (y≈0.7, LNCMO), Pr0.7Ca0.3MnO3(PCMO) thin films and LCMO/LNCMO/LCMO (LNL), PCMO/LNCMO/PCMO (PNP) trilayered films were studied. The LCMO thin films exhibit a paramagnetic semiconductor (PS)–ferromagnetic metal (FM) transition around their Curie temperatures (TC∼240 K), but both PCMO and LNCMO thin films display paramagnetic semiconducting behavior down to 60 K, below which the films seem to be ferromagnetic insulators. The zero-field resistance in a trilayered LNL film has an additional peak around 130 K besides the one at 250 K, corresponding to two PS–FM transitions. A similar PS–FM transition is also observed around 110 K in a trilayered PNP film. More than 30% (ΔR/R0) has been obtained in the temperature range 5–300 K for a LNL film at 7 T. These special transport and magnetic properties of trilayers made of manganite perovskites are likely due to the PS–FM transition occurring in the middle LNCMO layer, which is induced by the magnetic coupling between the top and bottom ferromagnetic layers.
Local disorder effects on the pressure dependence of the metal–insulator transition in manganese perovskites
Authors: Fontcuberta J. , Laukhin V. , Obradors X.
Published: July 2002

We address the role of the local disorder created by substitutional ions in L2/3A1/3MnO3 oxides, on the pressure dependence of the Curie temperature dln TC/dP. A number of manganites having distinct averaged lanthanide R0 radii and size variance σ2 have been prepared and their resistivity measured up to 11 kbar. It is found that the measured dln TC/dP is mainly determined by R0. However, materials having larger local disorder display larger pressure sensitivity. It is proposed that this results from partial suppression of the local disorder under pressure, and a model is developed to account for the experimental data.
Experimental investigation of physical mechanisms underlying lateral current injection laser operation
Authors: Sargent Edward H. , Xu J. M. , Caneau Catherine , Zah ChungEn
Published: July 2002

By comparing theoretical prediction with experimental performance, we gain insight into the physical operation of lateral current injection lasers. By studying the temperature dependence of lasing stimulated efficiency, we demonstrate the influence of lateral heterobarriers (achieved in this experiment by quantum well intermixing) on the confinement of carriers to the active region. By comparing the evolution of threshold current with temperature for devices of differing contact separations with a self-consistent model of lateral current injection laser operation, we reveal the importance of the interplay between the lateral material gain profile and the optical mode. Understanding these mechanisms, unique to the lateral injection family of lasers, is key to realizing the tremendous potential of this class of lasers to enable optoelectronic integration and novel functional devices.
Enhanced thermal stability of C49 TiSi2 in the presence of aluminum
Authors: Zhang S.-L. , d’Heurle F. M. , Lavoie C. , Cabral C. , Harper J. M. E.
Published: July 2002

The introduction of a thin layer of Al at the interface between Ti films and Si substrates enhances the formation of C49 TiSi2 and retards the transition from C49 to C54. An Al interlayer, 0.64 nm thick, reduces the time required to form C49 TiSi2 isothermally at 500 °C from 14 to 7 min. The C49–C54 transformation temperature is increased from 767 to 853 °C, when heating the samples at a constant ramp rate of 3 K/s. Most of the Al is found toward the interface between a Ti-rich silicide at the surface and TiSi2, rather than at the interface between TiSi2 and the Si substrate. The grain size of the C49 TiSi2 formed in the presence of Al is about five times smaller than that formed on a control sample with pure Ti, indicating that the increased density of grain boundaries in C49 TiSi2 in the presence of Al does not help the C49–C54 transformation. Therefore, the improved thermal stability of C49 TiSi2 is likely to be caused by other factors such as a reduced electron/atom ratio when replacing Si with Al in the disilicide.
Determination of frontier orbital alignment and band bending at an organic semiconductor heterointerface by combined x-ray and ultraviolet photoemission measurements
Authors: Schlaf R. , Parkinson B. A. , Lee P. A. , Nebesny K. W. , Armstrong N. R.
Published: July 2002

The alignment of the highest occupied molecular orbitals (HOMO) at the tris (8-hydroxy quinoline) aluminum (Alq3)/N,N′-di-(3-methylphenyl)-N,N′diphenyl-4,4′-diaminobiphenyl (TPD) heterojunction, used in organic light-emitting diodes (OLED), was determined by growing a TPD layer in several steps on a thick Alq3 substrate layer. After each growth step the sample was characterized in situ by x-ray and ultraviolet photoemission spectroscopy. The offset of the HOMO maxima at the interface was determined to be −0.13 eV from Alq3 to TPD. By including the known HOMO–lowest occupied molecular orbital (LUMO) gaps for both molecules into the evaluation, the offset of the LUMO minima was determined to be −0.33 eV from Alq3 to TPD. These values are consistent with previous assumptions that this interface represents a higher barrier for electron injection from Alq3 to TPD than for hole injection from TPD to Alq3.
Observation of the phase inversion in photoreflectance spectra from ZnSe/GaAs(001) heterostructures
Authors: Song J. H. , Sim E. D. , Lee S. H. , Chang S. K.
Published: July 2002

The pump photon energy dependence of photoreflectance (PR) of a ZnSe/GaAs heterostructure has been measured at 77 K. The phase inversion in the PR signal is observed for the pump photon energy when it decreases from above to below the excitonic absorption edge of ZnSe. The observation of the phase inversion in PR is explained in terms of the modulation of the built-in electric field at the interface of the ZnSe/GaAs heterojunction, not at the ZnSe surface. It provides evidence of a built-in triangular-well potential and of hole traps at the ZnSe/GaAs interface. This argument is confirmed by photoreflectance excitation spectroscopy.
Characterization of heavily carbon-doped InGaAsP layers grown by chemical beam epitaxy using tetrabromide
Authors: Sugiura Hideo , Mitsuhara Manabu , Kondo Susumu
Published: July 2002

Carbon doping in InGaAsP films using tetrabromide (CBr4) in chemical beam epitaxy has been studied, focusing on the relationship between the composition and the carrier concentration. The films with an equivalent wavelength of more than 1.4 μm have p-type conduction while the films with less than 1.3 μm have n-type conduction. Specifically, the hole concentration decreases from 7×1019 to 7×1018 cm−3 with decreasing wavelength in the 1.4–1.66 μm range, while electron concentration increases from 5×1018 to 2×1019 cm−3 with decreasing wavelength in the 1.1–1.3 μm range. Secondary-ion mass spectroscopy reveals that the incorporated carbon atom concentration is the same between an InGaAs film and a 1.4 μm equivalent wavelength InGaAsP film under a fixed CBr4 flux. Increase in gallium mole fraction monotonously increases hole concentrations under a fixed group-V mole fraction. We have found that the In1−xGaxAs1−yPy conduction type is mainly determined by two parameters: the Ga and P mole fractions, i.e., p type if x>y.
The effect of strain-induced polarization fields on impact ionization in a multiquantum-well structure
Authors: Doshi Bhautik , Brennan Kevin F. , Bicknell-Tassius Robert , Grunthaner Frank
Published: July 2002

We present a mechanism for enhancing the electron impact ionization rate based on the strain-induced polarization fields in a strained multiquantum-well system. To illustrate the concept, the electron ionization rate is calculated for a strained GaN and Al0.3Ga0.7N multiquantum-well device. The presence of the polarization fields within the Al0.3Ga0.7N layers provides an additional mechanism for carrier heating to the conduction band edge discontinuity of earlier simple multiquantum-well avalanche photodiode designs. It is found that the ionization rate is substantially enhanced over both its bulk GaN value and that for an unstrained multiquantum-well structure.
Giant magnetoresistance in a low-temperature GaAs/MnAs nanoscale ferromagnet hybrid structure
Authors: Wellmann P. J. , Garcia J. M. , Feng J.-L. , Petroff P. M.
Published: July 2002

We report the observation of a giant magnetoresistance effect in a low-temperature (LT-)GaAs/MnAs nanoscale ferromagnet hybrid structure. The MnAs nanomagnets are formed by ion implantation of Mn into LT GaAs and subsequent annealing. We have studied the magnetotransport using a vertically biased p+-GaAs/LT-GaAs:MnAs/p+-GaAs structure. A negative magnetoresistance (Δρ/ρ=[ρ(B)−ρ(0)]/ρ(0)) of up to −80% (B=7 T) is observed at low temperatures (T<20 K), which changes its sign from negative to positive between T=15 K and T=20 K. The value of the positive magnetoresistance decreases with increasing temperature from +115% (20 K) to +1.4% (300 K). The magnetoresistance variations with B and T are correlated with the nanomagnet spacing in the structure.
Intraband absorption in Ge/Si self-assembled quantum dots
Authors: Boucaud P. , Le Thanh V. , Sauvage S. , Débarre D. , Bouchier D.
Published: July 2002

We have observed intraband absorption in Ge/Si self-assembled quantum dots. The self-assembled quantum dots are grown at 550 °C by chemical vapor deposition. Atomic force microscopy shows that the quantum dots have a square-based pyramidal shape (≈100 nm base length) and a density ≈2×109 cm−2. Intraband absorption in the valence band is observed around 300 meV (4.2 μm wavelength) using a photoinduced spectroscopy technique. The intraband absorption is in-plane polarized. It is attributed to bound-to-continuum transitions since the intraband energy corresponds to the energy difference between the Si band gap and the photoluminescence energy of the quantum dots. The magnitude of the intraband absorption saturates when the ground level of the quantum dots is filled. This feature allows the measurement of the in-plane absorption cross section of the intraband transition which is found as large as 2×10−13 cm2.
Measurement of spontaneous emission from a two-dimensional photonic band gap defined microcavity at near-infrared wavelengths
Authors: Lee R. K. , Painter O. J. , D’Urso B. , Scherer A. , Yariv A.
Published: July 2002

An active, photonic band gap-based microcavity emitter in the near infrared is demonstrated. We present direct measurement of the spontaneous emission power and spectrum from a microcavity formed using a two-dimensional photonic band gap structure in a half wavelength thick slab waveguide. The appearance of cavity resonance peaks in the spectrum correspond to the photonic band gap energy. For detuned band gaps, no resonances are observed. For devices with correctly tuned band gaps, a two-time enhancement of the extraction efficiency was demonstrated compared to detuned band gaps and unpatterned material.
Ferromagnetism at room temperature in La0.8Ca0.2MnO3 thin films
Authors: Shreekala R. , Rajeswari M. , Srivastava R. C. , Ghosh K. , Goyal A. , Srinivasu V. V. , Lofland S. E. , Bhagat S. M. , Downes M. , Sharma R. P. , Ogale S. B. , Greene R. L. , Ramesh R. , Venkatesan T. , Rao R. A. , Eom C. B.
Published: July 2002

Anomalously high metal–insulator transition temperature Tp and ferromagnetic transition temperature Tc have been observed in thin films of La0.8Ca0.2MnO3. Ferromagnetic resonance signals from microwave studies are observed at room temperature (well above Tc) suggesting the presence of multiple magnetic spin systems. The Tp and Tc values are higher than that of La0.67Ca0.33MnO3 which have the highest Tp and Tc according to the bulk phase diagram. These results suggest that properties well beyond those seen in the bulk phase diagram may be achievable in thin films of the manganites.