bluuebunny/crossref_metadata_2025 huggingface dataset:
Blue-emitting β-SiC fabricated by annealing C60 coupled on porous silicon
Authors: Wu X. L. , Siu G. G. , Stokes M. J. , Fan D. L. , Gu Y. , Bao X. M.
Published: July 2002

C 60 molecules were chemically coupled in the pores of porous Si through a coupling agent and then coated with a layer of Si, and followed by N2 annealing. X-ray diffraction results indicate that the fabricated samples contain β-SiC particles which may exist in the pores, in addition to Si, SiO2, and graphite. The photoluminescence (PL) spectra show an asymmetrical broadband, which can be Gaussian divided into two bands at 380 (3.26 eV) and 454 (2.73 eV) nm. Spectral analyses and the experimental results from infrared spectroscopy and PL excitation measurements suggest that the 380 nm PL band is related to oxygen-vacancy defects in the SiO2 matrix, whereas the blue PL band is closely connected with the β-SiC particles. Our experiments provide a way for fabricating stable blue-emitting β-SiC materials.
Imaging of charge transport in polycrystalline diamond using ion-beam-induced charge microscopy
Authors: Sellin P. J. , Breese M. B. H. , Knights A. P. , Alves L. C. , Sussmann R. S. , Whitehead A. J.
Published: July 2002

Studies have been made using a 1 μm spatial resolution ion-beam-induced charge (IBIC) technique of the charge transport distribution in polycrystalline diamond produced by chemical vapor deposition. The devices tested used a coplanar electrode structure fabricated only on the growth side of the diamond film, and were predominantly sensitive to charge transport close to the growth surface of the diamond film where the diamond crystallites are largest. Irradiation with 5.48 MeV alpha particles gave a pulse height spectrum with a broad full energy peak and a mean charge collection efficiency of 15%. IBIC images obtained using microfocus proton and alpha particle beams showed spatially resolved regions of high charge collection efficiency correlating to individual diamond crystallites with a typical width of 20 μm, as observed by secondary electron microscopy.
Self-assembled zinc blende GaN quantum dots grown by molecular-beam epitaxy
Authors: Martinez-Guerrero E. , Adelmann C. , Chabuel F. , Simon J. , Pelekanos N. T. , Mula Guido , Daudin B. , Feuillet G. , Mariette H.
Published: July 2002

Zinc blende (ZB) GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on AlN buffer layers using 3C-SiC(001) substrates. The two- to three-dimensional growth mode transition is studied by following the evolution of the reflection high-energy electron diffraction pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a density of 1.3×1011 cm−2. Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.
Nonequilibrium electron distributions in a three-subband InGaAs/InAlAs quantum well studied via double resonance spectroscopy
Authors: Serapiglia G. B. , Vodopyanov K. L. , Phillips C. C.
Published: July 2002

Midinfrared optical pumping of electrons from the ground (n=1) to the first excited (n=2) subband of the quantum well produces a strongly nonthermal electron distribution which is probed spectroscopically. Two sharp induced absorption peaks appear, associated with electrons which have scattered from the upper subband via longitudinal optical (LO) phonon emission and absorption. The presence of the phonon absorption channel evidences the importance of nonequilibrium LO phonon populations (nph∼1), and the impact on the nonradiative intersubband scattering rates in quantum cascade laser devices is explored.
Intersubband electroluminescence using X−Γ carrier injection in a GaAs/AlAs superlattice
Authors: Domoto C. , Ohtani N. , Kuroyanagi K. , Vaccaro P. O. , Takeuchi H. , Nakayama M. , Nishimura T.
Published: July 2002

We report midinfrared electroluminescence originated from an intersubband transition in a GaAs/AlAs superlattice consisting of asymmetric double quantum wells. It is confirmed from interband photoluminescence properties under applied bias voltages that electrons populate at the second Γ (Γ2) subband in the GaAs layer. The electron population results from the carrier injection into the Γ2 subband from the adjacent X1 subband in the AlAs layer, which is initiated by the X1–Γ2 resonance. The energy of the intersubband electroluminescence, 190 meV, agrees with the energy spacing between the Γ2 and Γ1 subbands. This demonstrates that the carrier injection into the higher Γ subband using X–Γ scattering is useful for the design of intersubband-emission devices.
Ultrabroadband AIGaAs/CaF2 semiconductor saturable absorber mirrors
Authors: Schön S. , Haiml M. , Keller U.
Published: July 2002

Ultrabroadband semiconductor saturable absorber mirrors (SESAMs) are required to support self-starting sub-10-fs-pulse generation with Ti:sapphire lasers. Conventional AlxGa1−xAs/AlAs SESAMs are limited by the reflection bandwidth of about 60 nm of the bottom Bragg mirror. In this letter, we demonstrate a GaAs saturable absorber which is epitaxially grown on CaF2 using molecular-beam epitaxy. Even though the difference of the thermal expansion coefficient is very large, we were able to demonstrate good modulation depth with small nonsaturable losses. This is interesting for ultrabroadband SESAMs because the large refractive-index difference between CaF2 and AlxGa1−xAs results in very broadband AlxGa1−xAs/CaF2 Bragg mirrors extending over about a 400-nm-wide reflection bandwidth for a center wavelength of 850 nm.
Selective growth of nanocrystalline Si dots using an ultrathin-Si-oxide/oxynitride mask
Authors: Miyata Noriyuki , Watanabe Heiji , Ichikawa Masakazu
Published: July 2002

We demonstrate the selective growth of nanocrystalline Si (nc–Si) dots by using ultrathin-Si-oxide/oxynitride mask and low-pressure chemical vapor deposition. The oxynitride layer is selectively grown on the Si(001)–2×1 open window formed in the ultrathin oxide layer by electron-beam-induced selective thermal decomposition. The 10-nm-scale hemispherical nc–Si dots grow selectively on the oxynitride-covered window within the incubation period in which Si growth does not occur on the oxide-covered surface.
Microstructural evolution of laser-exposed silicon targets in SF6 atmospheres
Authors: Fowlkes J. D. , Pedraza A. J. , Lowndes D. H.
Published: July 2002

The microstructures formed at the surface of silicon during pulsed-laser irradiation in SF6-rich atmospheres consist of an array of microholes surrounded by microcones. It is shown that there is a dynamic interplay between the formation of microholes and microcones. Fluorine produced by the laser-induced decomposition of SF6 is most likely responsible for the etching/ablation process. It is proposed that silicon-rich molecules and clusters that form in and are ejected from the continually deepening microholes sustain the axial and lateral growth of the microcones. The laser-melted layer at the tip and sides of the cones efficiently collects the silicon-rich products formed upon ablation. The total and partial pressures of SF6 in the chamber play a major role in cone development, a clear indication that it is the laser-generated plasma that controls the growth of these cones.
SiGe heterojunction vertical p-type metal–oxide–semiconductor field-effect transistors with Si cap
Authors: Chen Xiangdong , Ouyang Qiqing , Onsongo David M. , Jayanarayanan Sankaran Kartik , Tasch Al , Banerjee Sanjay
Published: July 2002

SiGe source heterojunction p-type metal–oxide–semiconductor field-effect transistors (p-MOSFETs) have been used before to suppress the short channel effect for sub-100 nm devices. While the leakage is reduced, the drive current is also reduced due to the heterojunction. In this letter, we discuss a SiGe source heterojunction vertical p-MOSFET with a few nanometers thick Si cap. With this device structure, the absence of the heterojunction-induced potential barrier right below the oxide interface improves the drive current substantially while the drain induced barrier lowering (DIBL) effect and floating body effect are still suppressed. The electrical characterization of the device shows it exhibits higher drive current and less DIBL compared with a Si control device.
Layer-by-layer growth of GaN induced by silicon
Authors: Munkholm A. , Thompson Carol , Ramana Murty M. V. , Eastman J. A. , Auciello O. , Stephenson G. B. , Fini P. , DenBaars S. P. , Speck J. S.
Published: July 2002

We present in situ x-ray scattering studies of surface morphology evolution during metal–organic chemical vapor deposition of GaN. Dosing the GaN(0001) surface with Si is shown to change the growth mode from step-flow to layer-by-layer over a wide temperature range. Annealing of highly doped layers causes Si to segregate to the surface, which also induces layer-by-layer growth.
Scanning reflection electron microscopy study of surface defects in GaN films formed by epitaxial lateral overgrowth
Authors: Watanabe Heiji , Kuroda Naotaka , Sunakawa Haruo , Usui Akira
Published: July 2002

We have used scanning reflection electron microscopy (SREM) to detect surface defects in GaN films formed by facet-initiated epitaxial lateral overgrowth. SREM revealed individual threading dislocations and single atomic steps on the GaN surface, and provided images of crystallographic tilting near the surfaces. We found that one of the two tilted GaN crystals in the overgrown areas became dominant and that the surface changed to a single domain after 50-μm-thick GaN deposition. Our SREM results also showed that the deposition of thick (over 100 μm) GaN films significantly improves the crystallographic structures of the overgrown regions, and reduces the threading dislocations in the GaN films.
Effects of boundary roughness on the conductance of quantum wires
Authors: Csontos D. , Xu H. Q.
Published: July 2002

The generic effects induced by boundary roughness on the electron transport through quantum wires have been studied. It is found that the conductance of the rough quantum wires shows rapid fluctuations and strong, broad dips between adjacent plateaus at very low temperatures, and a recovery of the plateau structure at increased temperatures. It is also found that in the recovered plateau structure, the step values are suppressed and the conductance shows long transition regions between adjacent steps. These results agree with existing experiments and can be used as a guideline for the evaluation of the fabrication process of quantum wires.
Electric-field dependence of mobile proton-induced switching in protonated gate oxide field-effect transistors
Authors: Devine R. A. B.
Published: July 2002

The switching characteristics of protonated gate oxide field-effect transistors for use as integrated memory element devices have been studied. The rate of switching of the source drain current in devices having 20 and 40 nm gate oxide thicknesses which contain mobile protons is found to vary exponentially with the electric field applied across the oxide. The time t1/2 at which the current attains half the maximum value is found to vary as the oxide thickness to the power √3→2. Extrapolation to the case of devices with 4 nm thick gate oxide for fields ∼4 MV cm−1 indicates 3.2⩽t1/2⩽6.5 ms time scales should be attainable.
Aluminum-seeded growth of microcrystalline Si thin film onto SnO2 substrate
Authors: Lee Chang Hyun , Lim Koeng Su
Published: July 2002

To improve the crystallinity of thin boron-doped microcrystalline Si (p-μc-Si:H) films at an initial growth stage, we tried an Al-seeding method, which is to deposit an ultrathin (∼50 Å) Al layer onto a transparent conducting oxide before preparing the p-μc-Si:H film by using a mercury-sensitized photochemical vapor deposition method. It was found that the Al layer serves as seeds to facilitate the p-μc-Si:H with high crystallinity at an initial growth stage and the Al seeds stimulate the nucleation of Si crystallites. It was also found that the absorption loss in the ultrathin (∼50 Å) Al-seed layer is negligible in the visible wavelength region due to its natural oxidation. When used in superstrate-type p-i-n amorphous silicon solar cells, even the Al-seeded p-μc-Si:H film having a several hundred angstrom thickness acts as an excellent p layer compared to the conventional p-μc-Si:H.
Surface-emitting distributed feedback quantum-cascade lasers
Authors: Schrenk W. , Finger N. , Gianordoli S. , Hvozdara L. , Strasser G. , Gornik E.
Published: July 2002

We report on the realization of second-order distributed feedback quantum-cascade lasers at λ=9.35 μm, where the active region consists of GaAs, AlGaAs, and strained InGaAs grown on GaAs. A metal-stripe surface grating structure allows a high surface emission efficiency for the TM-polarized light. The emitted power via the surface is in the range of 100 mW and exceeds the emitted power from one facet. A double-lobed surface-emission far-field pattern is obtained for the lasing mode. The single-mode emission wavelength is continuously tunable by the heat sink temperature.
Far-infrared reflectivity of Cd3−xZnxAs2 monocrystals
Authors: Belogorokhov A. I. , Zakharov I. S. , Kochura A. V. , Knjazev A. F.
Published: July 2002

Reflectivity spectra of Cd3−xZnxAs2 solid solutions with compositions x=1.2, 1.8, 2.25, and 2.55 were measured at room temperature within the range of 50–700 cm−1. Analysis based upon Kramers–Kronig relation has helped to calculate the basic optical functions (real and imaginary parts of dielectric constant, reflection, and absorption idecis) in the measured energy range. Dispersion analysis has been applied to find the parameters of phonons within the range of 50–300 cm−1. As a result a curve has been made to characterize the dependency of the most strong oscillator upon their composition features.
Dual-band infrared metallodielectric photonic crystal filters
Authors: Oswald J. A. , Wu B.-I. , McIntosh K. A. , Mahoney L. J. , Verghese S.
Published: July 2002

We have designed a metallodielectric photonic crystal with planar metallic scatterers to produce high reflectance in both the 3–5 and 8–12 μm atmospheric windows over a wide range of angles of incidence. We have fabricated similar crystals using photolithography with Al scatterers in a polymer dielectric medium and measured their optical transmission, confirming the results of our design simulations. A fabricated filter produced a reflection band with >30 dB rejection and a fractional bandwidth of 0.38. Our results show that it is possible to fabricate infrared photonic crystal filters comparable in performance to those with thick metal elements, but with the ease of manufacture of thin metal layers.
Ultraviolet lithography of self-assembled monolayers for submicron patterned deposition
Authors: Friebel Susanne , Aizenberg Joanna , Abad Silvia , Wiltzius Pierre
Published: July 2002

We report on a lithographic technique that uses self-assembled monolayers (SAMs) as a resist to fabricate patterned, chemically functionalized surfaces. Large area line, square, and triangular patterns with a periodicity of 532 nm were generated exposing SAMs of hydrophobic or hydrophilic alkanethiolates to an ultraviolet laser interference pattern at 193 nm for only a few minutes (corresponding to ∼16 J/cm2) followed by the immersion into an alternating thiol. Patterned films of CaCO3, Zn(OH)2, and polymers were directly deposited on these templates. Using substrates patterned with oppositely charged SAMs, large periodic arrays of charged colloids were fabricated.
Enhanced-response pyroelectric heterostructures
Authors: Tipton C. Wesley , Kirchner K. , Godfrey R. , Cardenas M. , Aggarwal S. , Li H. , Ramesh R.
Published: July 2002

We have observed enhanced pyroelectric responses in sub-100 nm, epitaxial Pb–Zr–Ti–O films contacted with conducting perovskite oxide top and bottom electrodes. These enhancements are obtained in capacitors where the bottom electrode is processed under reducing conditions. This leads to an asymmetric, temperature-dependent internal electric field that is produced within the ferroelectric capacitor and manifests itself as a strongly shifted ferroelectric hysteresis loop. Because the shifted coercive voltage lies near the unbiased operating point, the pyroelectric film has a large value of dP/dE. The product (dP/dE)/(dE/dT) gives rise to an enhanced pyroelectric response. Our data show that a 10–30 times increase in the pyroelectric response can be obtained over symmetric devices, with a concomitant improvement of the sensing figure-of-merit by three times.
Backside-illuminated photoelectrochemical etching for the fabrication of deeply undercut GaN structures
Authors: Stonas A. R. , Kozodoy P. , Marchand H. , Fini P. , DenBaars S. P. , Mishra U. K. , Hu E. L.
Published: July 2002

A photoelectrochemical (PEC) wet-etching technique (backside-illuminated PEC) is described that utilizes the dopant or band-gap selectivity of PEC etching to fabricate deeply undercut structures. Lateral etch rates exceeding 5 μm/min have been observed, producing cantilevers in excess of 100μm in length. Dramatically different etch morphologies were noted between the frontside- and backside-illuminated etching, though dopant-dependent etch selectivities were maintained.
Role of inversion layer formation in producing low effective surface recombination velocities at Si/liquid contacts
Authors: Royea William J. , Michalak David J. , Lewis Nathan S.
Published: July 2002

Photoconductivity decay lifetimes have been obtained for NH4F(aq)-etched Si(111) and for air-oxidized Si(111) surfaces in contact with solutions of CH3OH or tetrahydrofuran (THF) containing either ferrocene+/0 (Fc+/0), bis(pentamethylcyclopentadienyl) Fe+/0, or I2. Si surfaces in contact with electrolytes having Nernstian redox potentials >0 V versus the standard calomel electrode exhibited low effective surface recombination velocities regardless of the different surface chemistries, whereas those exposed only to N2(g) ambients or to electrolytes containing mild oxidants showed differing rf photoconductivity decay behavior depending on their different surface chemistry. The data reveal that formation of an inversion layer, and not a reduced density of electrical trap sites on the surface, is primarily responsible for the long charge-carrier lifetimes observed for Si surfaces in contact with CH3OH or THF electrolytes containing I2 or Fc+/0.
Variable frequency photoconductive grating method
Authors: Nicholson J. P.
Published: July 2002

The theoretical treatment of the steady-state photocarrier grating method is extended to include the frequency dependence of the chopping frequency, ω. It is shown that measurements of the characteristic parameter, β, as a function of ω yield both the ambipolar diffusion length, L, as well as the carrier lifetime, τ, for a fixed angular setting. An alternative experimental setup using a Pockel cell and polarizing cube as beam splitter is suggested to facilitate measurements at higher frequencies.
Giant magnetoimpedance near a metal–insulator transition: Study of Fe in a V2O3 matrix
Authors: Kale Sangeetá , Lofland S. E. , Bhagat S. M. , Sebastian Litty , Ramesha K. , Gopalakrishnan J. , Ogale S. B. , Li Y. H. , Garrison J.
Published: July 2002

We report microwave absorption measurements as a function of temperature (from 290 to 125 K) and magnetic field (from 0 to 0.3 T) in mm-thick parallelepipeds of sintered V2O3 and V2O3 containing micron-size Fe precipitates. As before, it turns out that near the metal–insulator (MI) transition, the loss exhibits a sharp peak as a function of temperature. On application of a magnetic field, the peak temperature for (V2O3+Fe) changes by a few kelvin, causing a giant magnetoimpedance (≈ 200% in 0.1 T field) in the neighborhood of the MI transition.
Self-enhanced nonvolatile holographic storage in LiNbO3:Fe:Mn crystals
Authors: Liu De’an , Liu Liren , Liu Youwen , Zhou Changhe , Xu Liangying
Published: July 2002

The self-enhancement and self-depletion phenomena originated from the beam-coupling effect are observed during both the recording and fixing phases in LiNbO3:Fe:Mn nonvolatile holographic storage experiments. A numerical simulation and a steady-state analysis are developed to provide a mathematical evaluation to the phenomena. From the experimental and theoretical studies, it is seen that the highest diffraction efficiency available from the self-enhancement may be twice as large as the lowest one from the self-depletion, both during the recording and fixing phases. Therefore, the use of the self-enhancement is necessary for the practical applications.
Single-electron transistor strongly coupled to an electrostatically defined quantum dot
Authors: Lu W. , Rimberg A. J. , Maranowski K. D. , Gossard A. C.
Published: July 2002

A system is described in which an Al-based superconducting single-electron transistor (S–SET) is fabricated directly above an electrostatically defined quantum dot formed in a two-dimensional electron gas. This arrangement allows the coupling capacitance between the S–SET central island and the dot to be comparable to the tunneling capacitances of the dot. As a result, the measured sensitivity of the S–SET referred to charge on the dot is 1.2×10−4 e/Hz, about an order of magnitude better than previously reported results. The increased sensitivity makes this system ideally suited for broadband measurements such as study of individual tunneling events.
Structural and compositional variations in ZnSnP2/GaAs superlattices
Authors: Lita B. , Beck M. , Goldman R. S. , Seryogin G. A. , Nikishin S. A. , Temkin H.
Published: July 2002

We have investigated the structural and compositional uniformity of a set of ZnSnP2/GaAs superlattices grown by gas-source molecular-beam epitaxy. Cross-sectional scanning tunneling microscopy reveals an asymmetry in interface abruptness, with the ZnSnP2 on GaAs interfaces apparently much smoother than the GaAs on ZnSnP2 interfaces. The increased roughness of the GaAs on ZnSnP2 interface occurs simultaneously with the apparent surface segregation of Sn. High-resolution x-ray diffraction and photoluminescence spectroscopy suggest that the ZnSnP2 regions consist of a mixture of ZnSnP2 and ZnSnAs2. This is further confirmed by cross-sectional scanning tunneling microscopy and spectroscopy, which reveal the presence of nanometer-scale ZnSnP2 and ZnSnAs2-rich regions. Interestingly, these lateral compositional variations are not correlated with observed growth front undulations.
Magnetic-field-induced anomalous microwave response in superconducting YBa2Cu3O7−δ thin films
Authors: Rao X. S. , Ong C. K. , Feng Y. P.
Published: July 2002

The magnetic-field-induced decrease of microwave surface impedance, Zs=Rs+jXs, of superconducting YBa2Cu3O7−δ thin films is investigated using a microstrip resonator technique. Through measuring Zs as the direct current (dc) magnetic field is increased from zero to a certain value Hmax and subsequently decreased back to zero, it is found that Zs(Hdc) is totally reversible if Hmax does not exceed the threshold field H*, at which Rs shows a minimum, while it is irreversible if Hmax is larger than H*. It is confirmed that the anomalous effect happens in a vortex-free state. The threshold field H* is identified with the lower critical field for the vortices penetrating into the superconducting thin films. The anomalous effect is believed to be mainly contributed by the imperfect microstructure of the superconducting thin films.
High-dose Al-implanted 4H-SiC p+-n-n+ junctions
Authors: Kalinina E. , Kholujanov G. , Solov’ev V. , Strel’chuk A. , Zubrilov A. , Kossov V. , Yafaev R. , Kovarski A. P. , Hallén A. , Konstantinov A. , Karlsson S. , Adås C. , Rendakova S. , Dmitriev V.
Published: July 2002

p + -n-n + junctions were fabricated by ion implantation with Al of low-doped epitaxial n layers of 4H-SiC grown by chemical vapor deposition on commercial 4H-SiC wafers both with and without reduction of micropipe densities. It was shown that, using high levels of Al ion doping (5×1016 cm−2) in combination with rapid thermal anneal, single-crystal p+-4H-SiC layers can be obtained. These layers do not form barriers at the contact metal–semiconductor interface and do not introduce additional resistance into structures with p+-n junctions. This significantly reduces the forward voltage drop across the structure in a wide range of current densities up to 104 A cm−2.
Ideal quantum gases in D-dimensional space and power-law potentials
Authors: Salasnich Luca
Published: July 2002

We investigate ideal quantum gases in D-dimensional space and confined in a generic external potential by using the semiclassical approximation. In particular, we derive density of states, density profiles and critical temperatures for Fermions and Bosons trapped in isotropic power-law potentials. From such results, one can easily obtain those of quantum gases in a rigid box and in a harmonic trap. Finally, we show that the Bose–Einstein condensation can set up in a confining power-law potential if and only if D/2+D/n>1, where D is the space dimension and n is the power-law exponent.
Selective regrowth of Al0.30Ga0.70N p–i–n photodiodes
Authors: Collins C. J. , Li T. , Lambert D. J. H. , Wong M. M. , Dupuis R. D. , Campbell J. C.
Published: July 2002

We report on the device performance of selective-area regrown Al0.30Ga0.70N p–i–n photodiodes. Tensile strain, induced by the lattice mismatch between AlxGa1−xN and GaN, leads to cracking above the critical thickness in layers with high aluminum concentration. Selective-area regrown devices with ⩽70 μm diameters were fabricated without signs of cracking. These devices show low dark current densities with flat photoresponse and a forward turn-on current of ∼25 A/cm2 at 7 V. A quantum efficiency greater than 20% was achieved at zero bias with a peak wavelength of λ=315 nm. A differential resistance of R0=3.46×1014 Ω and a detectivity of D*=4.85×1013 cm Hz1/2 W−1 was demonstrated.
Interface corrugation in GaAs/AlAs (311)A superlattices
Authors: Vorob’ev A. B. , Gutakovsky A. K. , Prinz V. Ya. , Preobrazhenskii V. V. , Putyato M. A.
Published: July 2002

GaAs/AlAs  (311)A superlattices are studied by cross-sectional high-resolution transmission electron microscopy X-HRTEM. An anisotropic relief on GaAs/AlAs (311)A interfaces with a height up to 6 ML is observed, predominantly aligned along the [2̄33] direction. A strong (up to 12 ML) highly anisotropic modulation of the thickness of the GaAs layers is found, which can explain the in-plane anisotropy of carrier transport previously observed in this system. At the same time, the thickness of the AlAs layers appeared to be nearly constant. The X-HRTEM studies have revealed no 32 Å periodicity of the GaAs/AlAs (311)A interfacial structure, although a distinct (8×1) surface reconstruction of both GaAs and AlAs surfaces was observed by reflection high-energy electron diffraction during growth.
Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points
Authors: Henkelman Graeme , Jónsson Hannes
Published: July 2002

An improved way of estimating the local tangent in the nudged elastic band method for finding minimum energy paths is presented. In systems where the force along the minimum energy path is large compared to the restoring force perpendicular to the path and when many images of the system are included in the elastic band, kinks can develop and prevent the band from converging to the minimum energy path. We show how the kinks arise and present an improved way of estimating the local tangent which solves the problem. The task of finding an accurate energy and configuration for the saddle point is also discussed and examples given where a complementary method, the dimer method, is used to efficiently converge to the saddle point. Both methods only require the first derivative of the energy and can, therefore, easily be applied in plane wave based density-functional theory calculations. Examples are given from studies of the exchange diffusion mechanism in a Si crystal, Al addimer formation on the Al(100) surface, and dissociative adsorption of CH4 on an Ir(111) surface.
Noninvasive investigation of defects in multicrystalline silicon and photovoltaic devices by photomagnetic detection using superconducting quantum interference device magnetometers
Authors: Beyer J. , Zhong Q. , Schurig TH.
Published: July 2002

An approach for the localization and characterization of electrically active defects in multicrystalline silicon and of artefacts in photovoltaic devices is presented. The essential idea of the method is the noninvasive magnetic detection of photocurrents generated in the sample by local illumination. Magnetic imaging with high spatial and magnetic resolution is performed by scanning the sample under investigation and synchronously measuring the magnetic field of the photocurrents using highly sensitive superconducting quantum interference device magnetometers. The technique enables the nondestructive and nonpolluting detection and investigation of electrically active grain boundaries in multicrystalline silicon wafers. Results obtained from photovoltaic devices show that typical, performance-limiting defects can be localized, as they cause distinct features in the magnetic field topography.
Thermoelectric properties of junctions between metal and strongly correlated semiconductor
Authors: Rontani Massimo , Sham L. J.
Published: July 2002

We propose a junction of metal and rare-earth compound semiconductor as the basis for a possible efficient low-temperature thermoelectric device. If an overlayer of rare-earth atoms differing from the bulk is placed at the interface, very high values of the figure of merit ZT can be reached at low temperature. This is due to sharp variation of the transmission coefficient of carriers across the junction at a narrow energy range, which is intrinsically linked to the localized character of the overlayer f orbital.
Development of far-infrared Ge:Ga photoconductor having a longitudinal configuration
Authors: Fujiwara M.
Published: July 2002

In order to efficiency obtain a two-dimensional (2D) image in the 3 THz range, it is important to develop a 2D far-infrared detector array. We have developed a gallium-doped germanium (Ge:Ga) far-infrared photoconductor that has a longitudinal configuration. This structure is suitable for a large format monolithic 2D array. In this letter, we show that a transparent electrode that is responsive in the far-infrared range can be formed by ion implantation, and this layer contributed to increasing quantum efficiency. We obtained a high responsivity (16.2 A/W) and good noise equivalent power (2.6×10−17 W/Hz1/2) of Ge:Ga photoconductor in longitudinal configuration by combination of ion-implanted layers and Ge:Ga bulk material.
Observation of asymmetric Bloch walls in epitaxial Co films with strong in-plane uniaxial anisotropy
Authors: Prejbeanu I. L. , Buda L. D. , Ebels U. , Ounadjela K.
Published: July 2002

Combined studies involving magnetic force microscopy and micromagnetic simulations are used to investigate the domain wall structure in epitaxial Co(101̄0) thin films with strong in-plane uniaxial magneto-crystalline anisotropy. This letter shows experimental evidence that, for such a system, the domain wall structure transforms from an asymmetric Bloch wall into an asymmetric Néel wall upon decreasing the film thickness from 100 to 20 nm. This transition occurs without cross-tie wall formation. Furthermore, it is found that from the four possible energetically equivalent asymmetric Bloch wall configurations, only two are stabilized along a single domain wall. For a given wall, the transition from one configuration to the other involves the simultaneous reversal of the polarity of the Bloch core and the Néel cap.
Near-room-temperature continuous-wave operation of multiple-active-region 1.55 μm vertical-cavity lasers with high differential efficiency
Authors: Kim J. K. , Nakagawa S. , Hall E. , Coldren L. A.
Published: July 2002

We present completely monolithic, single-step grown, bipolar cascade vertical-cavity surface-emitting lasers at 1.55 μm with a greater-than-unity differential quantum efficiency. A typical device had a threshold current density of 1 kA/cm2, a threshold voltage of 3.2 V, and demonstrated continuous wave operation up to 8 °C. Devices smaller than 10 μm in diameter lased single mode. Active regions in our device were epitaxially stacked in three stages. This technique of multiple-active regions enabled the greater-than-unity differential quantum efficiency operation, which is essential in constructing high-efficiency microwave optical links with gain. We report the device characteristics and a model on the scaling properties of active region stacking in multiple-active-region vertical-cavity lasers.
Scanning tunneling microscopy investigation of truncated InP/GaInP2 self-assembled islands
Authors: Ballet P. , Smathers J. B. , Yang H. , Workman C. L. , Salamo G. J.
Published: July 2002

We present an investigation of the morphology of InP/GaInP three-dimensional (3D) islands obtained by molecular beam epitaxy. This material system should represent the counterpart of the InGaAs/GaAs system for the visible range. The islands are found to be truncated pyramids with observable phosphorous-rich surface reconstruction on top. The investigation of the effect of P overpressure reveals a path to achieve extremely homogeneous 3D islands through an island shape transition. These results help us understand the emerging issue of 3D island shape transition.
Terahertz optical pulse generation with a simple encoding scheme using spatial slicing technique
Authors: Lee Ka-Suen , Shu Chester
Published: July 2002

We propose and experimentally demonstrate a spatial slicing technique for the generation and encoding of a terahertz optical pulse train. The configuration consists simply of a group of optical delay lines arranged in a two-dimensional array. Optical pulse trains at repetition rates of 0.6 and 1.0 THz are achieved. With the use of spatial masks, different output signals can also be produced. The spatial slicing technique provides a feasibility to switch off selected optical pulses in a terahertz pulse train using low-speed switches.
Nonlinear wave interaction in a magnetron plasma
Authors: Gopalraja Praburam , Forster John
Published: July 2002

Oscillations in the frequency range 10<f<300 MHz were measured using a Langmuir probe in a magnetron plasma. Dual probes were used to obtain wave number information. Three distinct peaks were observed that obeyed the nonlinear selection rule ω=ω0−ω1, where ω0 is the pump frequency, and ω and ω1 are the decay frequencies. The phase velocity of the highest frequency wave (f=262 MHz) was consistent with beam plasma oscillations driven by secondary electrons from the target. The nonlinearly driven lower frequency wave had a phase velocity enabling efficient Landau damping. The resulting transfer of energy from the secondary electrons to the waves to the bulk electrons may explain the enhanced ionization of sputtered material observed in some high power density magnetron discharges.
Nanoindentation hardness and atomic force microscope imaging studies of pressure-quenched zirconium metal
Authors: Catledge Shane A. , Spencer Philemon T. , Vohra Yogesh K.
Published: July 2002

We have carried out mechanical property measurements on zirconium metal compressed in a diamond anvil cell to 19 GPa at room temperature with subsequent quenching to room pressure. The irreversible transformation from the ambient hexagonal-close-packed phase to the simple hexagonal ω phase (AlB2 structure) is confirmed by synchrotron energy dispersive x-ray diffraction followed by nanoindentation of the pressure-quenched sample. We document an 80% increase in hardness as a consequence of the pressure-induced transformation to the ω phase at room temperature. This is a large increase for a metallic phase transformation and can be attributed to the presence of sp2-hybrid bonds forming graphite-like nets in the (0001) plane of the AlB2 structure. Atomic force microscopy of the indents shows that a plastic deformation of 2 μm in depth was achieved with a force of 200 mN.
A climbing image nudged elastic band method for finding saddle points and minimum energy paths
Authors: Henkelman Graeme , Uberuaga Blas P. , Jónsson Hannes
Published: July 2002

A modification of the nudged elastic band method for finding minimum energy paths is presented. One of the images is made to climb up along the elastic band to converge rigorously on the highest saddle point. Also, variable spring constants are used to increase the density of images near the top of the energy barrier to get an improved estimate of the reaction coordinate near the saddle point. Applications to CH4 dissociative adsorption on Ir(111) and H2 on Si(100) using plane wave based density functional theory are presented.
Yarn ball polymer microstructures: A structural transition phenomenon induced by an electric field
Authors: Kossyrev Pavel A. , Crawford Gregory P.
Published: July 2002

We report on an unusual polymer “yarn ball” microstructure that is created through a template-based synthetic process involving reactive mesogens. We present experimental results of a unique structural transition of the yarn ball when subjected to an electric field, and describe this transition with a modified elastic theory. We model the interactions of the mesogenic thread segments that comprise the yarn ball in terms of mean-field theory by introducing an intersegment potential.
Room-temperature spin glass and its photocontrol in spinel ferrite films
Authors: Muraoka Yuji , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

Room-temperature spin-glass states have been found in (Mg,Fe){Mg,Fe,Ti}2O4 spinel ferrite thin films formed on Al2O3(0001) and SrTiO3(111) substrates. This elevated-temperature spin-glass state has been achieved by the fine tuning of spin states through the control of composition, random oxygen deficiencies, and the stress induced by the film/substrate lattice mismatch in the ferrite films. We have demonstrated the change of magnetic state by means of light irradiation from spin glass to a ferrimagnet over a wide range of temperatures below 290 K. The direct photoexcitation of spins with photon energy of 2.0 eV, which corresponds to the spin excitation energy for Fe ions, is the most effective for realizing photoinduced magnetization.
Controlling the noncentrosymmetry of azodye-doped polymers by nonresonant dual-frequency coherent excitation
Authors: Si Jinhai , Qiu Jianrong , Hirao Kazuyuki
Published: July 2002

All-optical control of the noncentrosymmetry of azodye-doped polymer films was demonstrated by nonresonant dual-frequency coherent excitation. The tensor analysis of the photoinduced noncentrosymmetry was performed by monitoring second harmonic generation (SHG) of the samples. The quadratic dependence of SHG on the film thickness showed that a χ(2) grating satisfying the phase-matching condition for SHG was optically induced in the polymer films, indicating this nonresonant all-optical poling can be used to tailor the symmetry of bulk samples.
Resonant tunneling diodes made up of stacked self-assembled Ge/Si islands
Authors: Schmidt O. G. , Denker U. , Eberl K. , Kienzle O. , Ernst F. , Haug R. J.
Published: July 2002

Multiple layers of self-assembled Ge/Si islands are used for resonant tunneling diodes (RTDs). The extremely closely stacked Ge nanostructures form vertical channels with energetically deep thermalization layers and high Si double barriers. Two resonances are found in the RTD current–voltage curve, which are attributed to the heavy-heavy hole (hh) and heavy-light hole (lh) transition. The lh resonance shows negative differential resistance up to 50 K. With increasing magnetic field, the lh resonance slightly shifts to higher voltages.
Field-induced dielectric properties of laser ablated antiferroelectric (Pb0.99Nb0.02)(Zr0.57Sn0.38Ti0.05)0.98O3 thin films
Authors: Bharadwaja S. S. N. , Krupanidhi S. B.
Published: July 2002

Niobium-modified lead zirconate stannate titanate antiferroelectric thin films with the chemical composition of (Pb0.99Nb0.02)(Zr0.57Sn0.38Ti0.05)0.98O3 were deposited by pulsed excimer laser ablation technique on Pt-coated Si substrates. Field-induced phase transition from antiferroelectric to ferroelectric properties was studied at different fields as a function of temperature. The field forced ferroelectric phase transition was elucidated by the presence of double-polarization hysteresis and double-butterfly characteristics from polarization versus applied electric field and capacitance and voltage measurements, respectively. The measured forward and reverse switching fields were 25 kV/cm and 77 kV/cm, respectively. The measured dielectric constant and dissipation factor were 540 and 0.001 at 100 kHz, respectively, at room temperature.
Inversion domains triggering recovery of luminescence uniformity in epitaxially lateral overgrown thick GaN film
Authors: Kim Chinkyo , Yi Jaehyung , Yang Min , Kim Minhong , Jeon Jina , Khym Sungwon , Cho Meoungwhan , Choi Yoonho , Leem Shi-Jong , Kim Seon Tai
Published: July 2002

A 150 μm-thick GaN layer was grown by halide vapor phase epitaxy utilizing selective lateral overgrowth on a SiO2-prepatterned sapphire substrate. A series of optically active regions above the SiO2 mask was observed in cross sectional monochromatic cathodoluminescence images taken at 367 nm. These bright regions were, however, consistently terminated by triangular shaped domains at 60 to 80 μm thickness, leaving no sign of luminescence nonuniformity beyond the thickness. In conjunction with the recent results on the characteristics of inversion domains in GaN, we proposed that these triangular regions might be inversion domains.
Study of gain mechanisms in AlGaN in the temperature range of 30–300 K
Authors: Lam J. B. , Bidnyk S. , Gainer G. H. , Little B. D. , Song J. J. , Yang W.
Published: July 2002

We report the results of an experimental study of the stimulated emission (SE) properties of AlGaN epilayers grown by metalorganic chemical vapor deposition under high optical excitation conditions in the temperature range of 30–300 K. The band gap and energy position of spontaneous and SE peaks were measured over the entire temperature range studied. Through an analysis of the temperature dependence of the relative energy positions and the SE threshold, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density at threshold to be ∼1019 cm−3 throughout the temperature range studied. Such a high carrier density indicates that an electron-hole plasma is responsible for the generation of gain in this material system from 30 to 300 K. Issues related to the development of short-wavelength AlGaN-based light emitting devices are discussed.
Characterizing carrier-trapping phenomena in ultrathin SiO2 films by using the x-ray photoelectron spectroscopy time-dependent measurements
Authors: Hagimoto Y. , Fujioka H. , Oshima M. , Hirose K.
Published: July 2002

We have characterized the carrier-trapping phenomena in ultrathin (1.3–3.5 nm) SiO2 films (practical used thermal oxide and oxynitride) by using x-ray photoelectron spectroscopy time-dependent measurements. It was found that the net amount of hole traps in the ultrathin oxynitride is smaller than that in the ultrathin thermal oxide. This result is consistent with the previously reported results for the thick thermal oxide and oxynitride using conventional electrical measurements. We consider what is responsible for the contribution to the formation of hole traps.
Planar degenerated anchoring of liquid crystals obtained by surface memory passivation
Authors: Dozov I. , Stoenescu D. N. , Lamarque-Forget S. , Martinot-Lagarde Ph. , Polossat E.
Published: July 2002

Weak anchoring of liquid crystals on solid substrates is usually dominated by the surface memory. We show that the memory can be removed by surface passivation by silane molecules. We realize planar, azimuthally degenerated, and memory-free anchorings without pretilt.
Low density of defect states in hydrogenated amorphous carbon thin films grown by plasma-enhanced chemical vapor deposition
Authors: Krishna K. M. , Ebisu H. , Hagimoto K. , Hayashi Y. , Soga T. , Jimbo T. , Umeno M.
Published: July 2002

The density of electronic defect states in most forms of amorphous carbon deposited at room temperature is found so far to be very high (1018–1022 spins cm−3). In this letter, we demonstrate that the radio-frequency plasma-enhanced chemical vapor deposited hydrogenated amorphous carbon (a-C:H) thin film exhibits the lowest spin density of the order of 1016 cm−3, investigated by using electron spin resonance (ESR) spectroscopy, a very promising reproducible result comparable with high-quality a-Si:H. In addition, the optical gap of a-C:H has been tailored between a wide range, 1.8–3.1 eV. The ESR spectra of all the films reveal a single Lorentzian line whose linewidth ΔHpp varies strongly with the optical gap. Also, there is a strong dependence of spin density on the optical gap, and we show that this dependency is a direct result of structural changes due to sp3/sp2 carbon bonding network.
Two-photon excited photon echo in CdS
Authors: Hillmann F. , Voigt J. , Redlin H.
Published: July 2002

Time-resolved and integrated degenerated four-wave mixing experiments with two-photon excitation at a wavelength of 600–710 nm on a 30–40 μm thick cadmium sulfide crystal are presented for a temperature range from 5 to 300 K. A photon echo is observed showing quantum beats both at positive and negative delay times τ with a period of 0.7–0.8 ps. The signal decays exponentially with time constants of ≈170 fs at τ>0 and half as large at τ<0, independent of temperature. The results are discussed in terms of the decay of a high-density gas of e–h pairs, excitons and excitonic molecules highly out of equilibrium.
In situ electron energy-loss spectroscopy on carbon nanotubes during deformation
Authors: Suenaga K. , Colliex C. , Iijima S.
Published: July 2002

Local modifications to the electronic structure during the bending deformation of carbon nanotubes are probed by in situ electron energy-loss spectroscopy. Reversible changes in the carbon K(1s) absorption near-edge fine structures are detected at the inner bending side for a multiwalled carbon nanotube and attributed to the curvature-induced electronic structure alteration of the graphite layers. More intensive changes in the carbon K edge across an abrupt kink of a bundle of single-walled carbon nanotubes are observed over a region of 4–5 nm around the kink. These results clearly demonstrate the unique deformation behavior of the carbon nanotube, namely, a high elastic deformability and variable electronic properties associated with bending.
Dimer-flipping-assisted diffusion on a Si(001) surface
Authors: Zi J. , Min B. J. , Lu Y. , Wang C. Z. , Ho K. M.
Published: July 2002

The binding sites and diffusion pathways of Si adatoms on a c(4×2) reconstructed Si(001) surface are investigated by a tight-binding method with an environment-dependent silicon potential in conjunction with ab initio calculations using the Car–Parrinello method. A new diffusion pathway along the trough edge driven by dimer flipping is found with a barrier of 0.74 eV, comparable to that of 0.68 eV along the top of the dimer rows.
X-ray phase-contrast imaging with submicron resolution by using extremely asymmetric Bragg diffractions
Authors: Kobayashi Kenji , Izumi Koichi , Kimura Hidekazu , Kimura Shigeru , Ibuki Takashi , Yokoyama Yoshiyuki , Tsusaka Yoshiyuki , Kagoshima Yasushi , Matsui Junji
Published: July 2002

We have obtained x-ray phase-contrast images with high spatial resolution by using extremely asymmetric Si 111 Bragg diffractions near the critical angle of the total reflection. The x-ray image could be magnified to 294 times in both vertical and horizontal directions. By using this x-ray microscopy system, we have observed clear phase-contrast images of a 0.7-μm-wide gold-line pattern.