bluuebunny/crossref_metadata_2025 huggingface dataset:
Kr excimers produced by soft x rays emitted from a laser-produced plasma
Authors: Kubodera Shoichi , Ikeda Masaki , Kawanaka Junji , Sasaki Wataru
Published: July 2002

We have demonstrated an excitation method to produce Kr2* by using soft x rays emitted from a laser-produced plasma. We have found that Kr2+ ions were directly produced by photoionization of Kr atoms by soft x rays. The behavior of Kr2* emission was similar to that of Kr2+, indicating fast relaxation processes to produce Kr2* from Kr2+ during the plasma producing laser pulse. Numerical results based on a photoionization model of Kr reproduced the experimental results, which verified the production of Kr2* initiated by the photoionization of Kr.
Edge emission observed in heavily boron-doped diamond films epitaxially grown on platinum
Authors: Yokota Yoshihiro , Tachibana Takeshi , Miyata Koichi , Kobashi Koji , Hatta Akimitsu , Ito Toshimichi , Hiraki Akio , Shintani Yoshihiro
Published: July 2002

Cathodoluminescence (CL) spectra of diamond films epitaxially grown on single crystal platinum (111) have been investigated at room temperature and 89 K. It was found that the CL spectra of the heavily boron-doped (>3×1020 cm−3) diamond films of more than 16 μm thickness consist only of a near-edge emission at 248±1 nm (5.00±0.02 eV), while any mid-gap emissions are absent. That result suggests that few defects are in the films. It was also found that the temperature dependence of the 248 nm band is very unique, since its intensity increases as temperature increases. The mechanism of the 248 nm band seems to involve quasi-direct transition.
High-temperature optically pumped lasing in ZnMgSSe/ZnSe heterostructures grown by metalorganic vapor phase epitaxy
Authors: Gurskii A. L. , Marko I. P. , Lutsenko E. V. , Yablonskii G. P. , Kalisch H. , Hamadeh H. , Heuken M.
Published: July 2002

Lasing and optical properties of ZnMgSSe/ZnSe-, ZnMgSSe/ZnSSe/ZnSe-, and ZnMgSSe/ZnMgSSe/ZnSe-based single- and multiple-quantum-well heterostructures grown by metalorganic vapor phase epitaxy were studied, and the characteristics were found to depend on the excitation intensity Iexc, temperature, and well width. Laser action under transverse optical pumping was achieved only for well widths Lz⩾4 nm and optical confinement factors Γ>0.04. In separate confinement heterostructures, lasing with the lowest threshold (Ithr=10–30 kW/cm2 at T=78 K) was achieved and device characteristics were studied up to T=577 K.
Manipulation of Ag nanoparticles utilizing noncontact atomic force microscopy
Authors: Martin M. , Roschier L. , Hakonen P. , Parts Ü. , Paalanen M. , Schleicher B. , Kauppinen E. I.
Published: July 2002

We have developed a scheme to manipulate metallic aerosol particles on silicon dioxide substrates using an atomic force microscope. The method utilizes the noncontact mode both for locating and moving nanoparticles of size 10–100 nm. The main advantage of our technique is the possibility of “seeing” the moving particle in real time. Our method avoids well sticking problems that typically hamper the manipulation in the contact mode.
Near-field scanning optical microscopy studies of V-grooved quantum wire lasers
Authors: Ben-Ami U. , Nagar R. , Ben-Ami N. , Scheuer J. , Orenstein M. , Eisenstein G. , Lewis A. , Kapon E. , Reinhardt F. , Ils P. , Gustafsson A.
Published: July 2002

Detailed near-field scanning optical microscopy studies of GaAs/AlGaAs V-grooved quantum wire lasers at room temperature were performed. We measured the spectrally resolved near-field intensity distributions, emitted from the complex epitaxially grown optical waveguide in the V groove with a spatial resolution of ∼0.1 μm. Distinct regions emitting at different wavelengths were identified, and a heart shaped modal distribution of less than 0.5 μm in diameter was measured and well matched the calculated light distribution of this structure.
Theoretical performance of wurtzite and zincblende InGaN/GaN quantum well lasers
Authors: Radtke R. J. , Waghmare U. , Ehrenreich H. , Grein C. H.
Published: July 2002

The theoretical gain, radiative and Auger recombination rates, and threshold current densities of ideal wurtzite (WZ) and zincblende (ZB) 20 Å In0.2Ga0.8N/70 Å GaN multiple quantum well laser diodes are compared. We obtain upper bounds on device performance, which are based on reliable calculations for both band structure and recombination dependent features and show (1) that the performance of present devices having the ZB and WZ structures are within 20% of each other in InGaN/GaN, and (2) that present performance of the best currently available devices is only a factor of 3–4 below the theoretical limit. Radiative recombination is far more important than Auger processes. The calculations are performed using a superlattice K⋅p formalism and density functional theory within the local-density approximation. The latter yields bulk zone-center energies, wave functions, and directly calculated momentum matrix elements.
Characterization of pitting corrosion in aluminum films by light scattering
Authors: Zhao Y.-P. , Cheng C.-F. , Wang G.-C. , Lu T.-M.
Published: July 2002

We report a detailed study of the morphology of pits formed by corrosion of aluminum thin films using an in-plane light scattering technique. We show that the corrosion front of the Al thin film can be treated as a quasi-two-level random rough surface. Based on an elastic diffraction theory, we are able to determine the average depth, the area, and the density of pits, as well as the fractal dimension of the surface. Using the advantages of light scattering, one can quantify the morphological parameters of corroded films in situ and nondestructively.
Interwell inhomogeneity of carrier injection in InGaN/GaN/AlGaN multiquantum well lasers
Authors: Domen K. , Soejima R. , Kuramata A. , Horino K. , Kubota S. , Tanahashi T.
Published: July 2002

A simulation of current flow for an InGaN/GaN/AlGaN multiquantum well (MQW) laser showed that generation of the optical gain is inhomogeneous across the MQW due to inhomogeneous carrier injection. Laser diodes with three and five wells in MQW were compared to experimentally investigate inhomogeneous carrier injection. We found that external quantum efficiency was significantly improved by a reduction in the number of wells from five to three. The result was explained quantitatively by the fact that the five-well laser had a larger internal loss and a poorer internal quantum efficiency due to the inhomogeneous carrier injection.
Millimeter wave generation by a self-sustained current oscillation in an InGaAs/InAlAs superlattice
Authors: Brandl S. , Schomburg E. , Scheuerer R. , Hofbeck K. , Grenzer J. , Renk K. F. , Pavel’ev D. G. , Koschurinov Yu. , Zhukov A. , Kovsch A. , Ustinov V. , Ivanov S. , Kop’ev P. S.
Published: July 2002

We report millimeter wave generation by a self-sustained current oscillation in a doped InGaAs/InAlAS wide miniband superlattice. The superlattice (miniband width 160 meV) showed, at room temperature, a current voltage characteristic with negative differential conductance. Coupled to a high-frequency circuit, the superlattice generated millimeter waves, at a frequency (55 GHz) which was tunable by half a percent by changing the bias voltage. The power (0.3 mW) corresponded to an efficiency (i.e., ratio of microwave power to dc power input) of 0.3%. We attribute the microwave generation to a current oscillation caused by traveling dipole domains.
Low-temperature homoepitaxial growth on Si(111) through a Pb monolayer
Authors: Evans P. G. , Dubon O. D. , Chervinsky J. F. , Spaepen F. , Golovchenko J. A.
Published: July 2002

A monolayer of Pb mediates high-quality homoepitaxial growth on Si (111) surfaces at temperatures where growth with other overlayer elements or on bare surfaces leads to amorphous or highly defective crystalline films. Nearly defect-free epitaxy proceeds for film thicknesses up to 1000 Å with no sign that this is an upper limit. The minimum temperature for high-quality epitaxy depends on the substrate miscut. For a 0.2° miscut, the minimum temperature is 340 °C. Films grown on substrates miscut 2.3° towards [112̄] show good crystalline quality down to 310 °C.
Effect of well number on organic multiple-quantum-well electroluminescent device characteristics
Authors: Huang Jingsong , Yang Kaixia , Xie Zhiyuan , Chen Baijun , Jiang Hongjin , Liu Shiyong
Published: July 2002

A doping technique for fabricating organic multiple-quantum-well electroluminescent (EL) devices is demonstrated. This device consists of N,N′-Bis(3-methyphenyl)-N,N′-diphenylbenzidine used as a hole transporter, undoped tris(8-quinolinolato) aluminum (Alq) as a barrier potential or electron transporter, and Alq doped with 5,6,11,12-tetraphenylnaphthacene as a potential well and an emitter. Our experimental results suggest that the double-quantum-well EL devices show the optimum emission characteristics. The efficiency and the luminance of the device achieve 15.7 lm/W and 7500 cd/m2, respectively.
Quantum Monte Carlo simulation of resonant tunneling diodes based on the Wigner distribution function formalism
Authors: Garcı́a-Garcı́a J. , Martı́n F. , Oriols X. , Suñé J.
Published: July 2002

A tool for the simulation of resonant tunneling diodes (RTDs) has been developed. This is based on the solution of the quantum Liouville equation in the active region of the device and the Boltzman transport equation in the regions adjacent to the contacts by means of a Monte Carlo algorithm. By accurately coupling both approaches to current transport, we have developed a quantum simulation tool that allows the use of simulation domains much larger and realistic than those previously considered, without a significant increase in computational burden. The main characteristics expected for the considered devices are clearly obtained, thus supporting the validity of our tool for the simulation of RTDs.
Engineering of the magnetic properties of strained quantum dots
Authors: Khurgin Jacob B. , Jin F. , Obeidat A.
Published: July 2002

Exchange interactions between the holes confined in arrays of strained quantum dots subject to compressive and tensile strain are shown to lead to ferrimagnetic arrangement of magnetic moments. Using example of strained In1−xGaxAs quantum dots on InP substrate, it is shown how magnetic properties of a semiconductor material can be engineered without resorting to doping with magnetic ions, and the Curie temperature is estimated to be of the order of 25 K. Potential applications in information storage and processing are considered.
Experiments on the depolarization near-field scanning optical microscope
Authors: Adelmann Ch. , Hetzler J. , Scheiber G. , Schimmel Th. , Wegener M. , Weber H. B. , Löhneysen H. v.
Published: July 2002

We demonstrate the operation of an apparatus which we call the depolarization near-field scanning optical microscope. It delivers subwavelength resolution with uncoated optical fiber tips without the need for additional modulation techniques. We show that—in the near field—the edges perpendicular to the incident optical polarization are imaged. This dependence on the orientation of the linear polarization as well as the influence of small ellipticities of the polarization state on the imaging process are measured on a well-defined test sample. The transition from near- to far-field imaging as a function of the tip height is demonstrated. The results are in good agreement with recent theoretical predictions.
Single-electron capacitance spectroscopy of vertical quantum dots using a single-electron transistor
Authors: Koltonyuk M. , Berman D. , Zhitenev N. B. , Ashoori R. C. , Pfeiffer L. N. , West K. W.
Published: July 2002

We have incorporated an aluminum single-electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single-electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single-electron capacitance peaks.
Spontaneous self-embedding of three-dimensional SiGe islands
Authors: Mateeva E. , Sutter P. , Lagally M. G.
Published: July 2002

It is shown that, under appropriate conditions, high-Ge-concentration coherent three-dimensional SiGe islands grown on Si(100) self-embed in a matrix of a low-Ge-concentration alloy. The process may be more generally useful for preserving the shape of self-assembled “quantum dot” islands during embedding in a matrix material.
Supersoft elastic parameters and low melting temperature of the C49 phase in TiSi2 by Brillouin scattering and molecular dynamics
Authors: Miglio Leo , Iannuzzi M. , Celino M. , Pastorelli R. , Bottani C. , Sabbadini A. , Pavia G.
Published: July 2002

In this letter, we show that the polymorphic C49 form of TiSi2 has much smaller elastic constants than those of the C54 bulk-stable structure and that its melting temperature is about 300 °C lower. These issues supply intriguing hints in explaining the kinetic advantage of the C49 over the C54 phase and in understanding the role of the elastic energy in the phase transformation from C49 to C54.
Evidence of discrete interface traps on thermally grown thin silicon oxide films
Authors: Cai Jin , Sah Chih-Tang
Published: July 2002

A very-low density (<1010 cm−2) residual discrete energy-level interface trap at 228 meV from the Si midgap is detected on crystalline silicon covered by thermally grown oxide. Three signatures pointing to discreteness are delineated in the experimental data of the gate voltage modulation of the dc steady-state electron-hole recombination rate. Using the Shockley-Read–Hall recombination kinetics, the energy level is shown to have a linewidth less than 20 meV. From the present and previous data, the dopant impurity origin of this residual interface trap species is speculated rather than the amphoteric silicon dangling bonds.
Quantum grid infrared photodetectors
Authors: Rokhinson L. P. , Chen C. J. , Tsui D. C. , Vawter G. A. , Choi K. K.
Published: July 2002

In this letter we introduce a quantum well infrared photodetector (QWIP) structure, which we refer to as the quantum grid infrared photodetector (QGIP). In an ideal structure, a grid pattern with very narrow linewidth is created in the QWIP active region to achieve lateral electron confinement, thereby improving its absorption as well as transport characteristics. In order to realize this detector structure, we have fabricated QGIPs with line patterns of lithographical linewidths wl ranging from 0.1 to 4 μm, allowing for possible sidewall depletion. Low-damage reactive ion beam etching was employed to produce vertical sidewalls. From the experimental data, although the best detector performance occurs at wl≈1.5 μm, the detector starts to improve when wl<0.5 μm, indicating a possible quantum confinement effect.
Unusual picosecond dynamics of a semiconductor vertical-cavity-surface-emitting laser under resonant perturbation
Authors: Hense S. G. , Wegener M.
Published: July 2002

A stationary electrically pumped semiconductor vertical cavity surface emitting laser held at T=300 K is perturbed by a femtosecond optical pulse, which is resonant with its cavity mode. For low to moderate intensities, the semiconductor laser reacts with a switch-off. Surprisingly, for yet higher pulse energies, this behavior disappears and a direct recovery of the original emission level is observed. Such behavior cannot be understood within standard or extended rate equations. We speculate that Rabi flops are important under these conditions.
Optical anisotropy of uniaxially drawn and silver-dispersed polyimide films
Authors: Sawada Takashi , Ando Shinji , Sasaki Shigekuni
Published: July 2002

Distinct anisotropy in optical transmittance in the visible and near-infrared region for uniaxially drawn and silver-dispersed polyimide films was observed. The films were prepared in a one-step operation that involves thermal curing and simultaneous uniaxial drawing of poly(amic acid) (PAA) films, which were made by dissolving silver nitrate in the PAA solution at a 1:4 mol ratio. The polyimide molecular chains with a rod-like structure were oriented along the drawing direction during curing, and this orientation accompanied the generation of silver nanoparticles with elongated shapes. An anisotropy in the optical transmittance of 5:1 was obtained for a 22-μm-thick film at 850 nm with transmittance of 68% perpendicular to the drawing direction. The optical and mechanical properties of this film were retained after annealing at 300 °C for 1 h.
Long-wavelength InGaAs quantum wells grown without strain-induced warping on InGaAs compliant membranes above a GaAs substrate
Authors: Jones A. M. , Jewell J. L. , Mabon J. C. , Reuter E. E. , Bishop S. G. , Roh S. D. , Coleman J. J.
Published: July 2002

Long-wavelength photoluminescence at 1.35 μm has been measured from an InGaAs quantum-well heterostructure deposited on disk-shaped InGaAs (xIn=0.05) compliant-film membranes. Strain-induced warping is avoided by utilizing a single pedestal to suspend each compliant-film disk over a GaAs substrate. Cathodoluminescence (CL) spectra verify the long-wavelength emission, and panchromatic CL images reveal that strong emission occurs only on compliant film structures supported by 1-μm-diam pedestals.
Mode-discriminating photoconductor and coplanar waveguide circuit for picosecond sampling
Authors: Zamdmer N. , Hu Qing , Verghese S. , Förster A.
Published: July 2002

We introduce a circuit for ultrafast measurements with coplanar waveguide transmission lines. The circuit contains photoconductive switches that allow tunable generation and reception of a coplanar waveguide’s two propagating modes. The circuit has fewer discontinuities than other circuits with similar capabilities and does not require airbridges. We show how the photoconductive switch can be biased to compensate for pump laser beam misalignment, and that a circuit composed of two chips making up a hybrid has only 4% broader temporal resolution than a circuit integrated on a single chip. We measured a minimum temporal resolution of 2.4 ps.
Contributions of individual Fe sites to magnetocrystalline anisotropy of Y2Fe17−xGax compounds
Authors: Cheng Zhao-hua , Shen Bao-gen , Wang Fang-wei , Kronmüller H.
Published: July 2002

At present, individual Fe sites anisotropies of Y2Fe17 compound cannot be directly measured by ordinary magnetization measurement. Alternatively, they are derived from the relationship between occupancy factors of Fe atoms at different sites and anisotropy constants of Y2Fe17−xGax compounds. It is found that only Fe atoms at 9d site have a significantly positive contribution to the uniaxial anisotropy, while those at 6c and 18h sites have strongly negative contributions; the contribution of Fe atoms at 18f site is slightly negative and less important. The planar anisotropy is rapidly weakened and finally a uniaxial anisotropy is induced by the Ga substitution. These results can be explained by the preferential occupancies of Ga atoms at 18h site for low Ga concentration as well as 6c and 18f sites for high Ga concentration.
Nonlinear 1/f noise characteristics in luminescent porous silicon
Authors: Bloom I. , Balberg I.
Published: July 2002

We present noise characteristics of luminescent porous silicon and show that they shed light on the transport mechanism in this system. The 1/f fluctuations show non-Gaussian and nonlinear behavior, and they give a high Hooge factor, typical of disordered conductors. By carrying out the measurements under various bias conditions, we found a bias-dependent redistribution of the percolating current paths. The close resemblance between the present results and those found in granular metals suggests that a tunneling process controlled by the electrostatic energy determines the conduction paths between the nanocrystallites in luminescent porous silicon.
Modeling and experimental observation of parametric processes in segmented KTiOPO4 channel waveguides
Authors: Sundheimer Michael , Aschieri Pierre , Baldi Pascal , Bierlein John
Published: July 2002

This letter presents a modeling technique for quasiphase-matched parametric processes in segmented channel waveguides and the results of measurements of infrared parametric fluorescence in segmented KTiOPO4 channel waveguides. We demonstrate both theoretically and experimentally a strong dependence of optical parametric oscillator tuning curves and parametric conversion efficiency (parametric gain) on the segmentation duty cycle. We show that the optimum conversion efficiency for parametric interactions in segmented waveguides is obtained for duty cycles greater than 50%. Our results validate the use of an effective continuous waveguide model for segmented channel waveguide calculations over a wide range of wavelengths and duty cycles.
Characterization of electronic states in molecular beam epitaxy grown GaN by optical admittance spectroscopy: Comparison of different nitrogen plasma sources
Authors: Krtschil A. , Witte H. , Lisker M. , Christen J. , Birkle U. , Einfeldt S. , Hommel D.
Published: July 2002

Optical transitions between the bands and electronic states in n-type GaN layers grown by molecular beam epitaxy on sapphire substrates using an electron cyclotron resonance (ECR) or a radio frequency (rf) nitrogen plasma source were investigated by means of optical admittance spectroscopy. The spectra of all layers similarly consist of a band gap region, a blue and a yellow band, and several defect-to-band transitions. However, in rf grown layers distinct transitions are separable, whereas ECR grown samples reveal broad bands, originating from potential fluctuations due to structural inhomogeneities induced by the ECR source. A defect at 0.82 eV is found characteristic for all ECR samples.
Imaging of acoustic surface wave slowness
Authors: Xiang D. , Hsu N. N. , Blessing G. V.
Published: July 2002

An experimental method has been devised for imaging the acoustic surface wave slowness (inverse of the phase velocity) in anisotropic solids. This technique utilizes a specially designed broadband, line-focus ultrasonic transducer to sense the leaky surface waves as well as leaky pseudosurface waves emanating from a solid immersed in water. By rotating such a transducer about its symmetric axis normal to the solid surface, the orientation-dependent time wave forms can be obtained. These wave forms are readily transformed into a surface wave slowness image with a simple algorithm.
Clamping of the linewidth enhancement factor in narrow quantum-well semiconductor lasers
Authors: Hader J. , Bossert D. , Stohs J. , Chow W. W. , Koch S. W. , Moloney J. V.
Published: July 2002

The linewidth enhancement factor in single quantum-well graded index separate confinement heterostructure semiconductor lasers is investigated theoretically and experimentally. For thin wells, a small linewidth enhancement factor is obtained which clamps with increasing carrier density, in contrast to the monotonous increase observed for thicker wells. Microscopic many-body calculations reproduce the experimental observations attributing the clamping to excitation dependent gain shifts and the influence of the population of off-resonant states on the refractive index.
Red organic electroluminescence devices with a reduced porphyrin compound, tetraphenylchlorin
Authors: Sakakibara Y. , Okutsu S. , Enokida T. , Tani T.
Published: July 2002

We constructed red organic electroluminescence (EL) devices with a reduced porphyrin compound, tetraphenylchlorin, that was doped at various concentrations (0.5, 1.7, and 3.7 wt %) within a tris(8-hydroxyquinoline) aluminum (Alq3) host layer. We measured their EL properties and found that all three devices emitted a red EL band at 660 nm with a width of 20 nm. Emission color of the 1.7 and 3.7 wt % devices was red (chromaticity coordinates x=0.67, y=0.29 and x=0.78, y=0.21) and the luminance maximum was 100 and 24 cdm−2, respectively. The 0.5 wt % device emitted a green Alq3 EL band as well, and showed an increase in relative intensity of the Alq3 emission with increasing applied voltage.
Light-controlled erasure of induced χ(2) in thermally poled glasses
Authors: Montant S. , Le Calvez A. , Freysz E. , Ducasse A. , Nazabal V. , Fargin E. , Le Flem G.
Published: July 2002

We demonstrate that the induced second order nonlinearity χ(2) generated in thermal poled doped borosilicate glasses can be erased by a prolonged exposure to near IR laser radiation. The evolution of the second harmonic signal with respect to laser time exposure, intensity and wavelength is reported. The IR laser radiation induced relaxation of the χ(2) is mainly related to two-photon absorption of the glass. The nonexponential time dependence of the second harmonic signal agrees with a simple model involving diffusion of photoexcited carriers. The reversible erasure process is also observed when the sample is exposed to near ultraviolet laser radiation.
Temperature- and field-dependent electron and hole mobilities in polymer light-emitting diodes
Authors: Bozano L. , Carter S. A. , Scott J. C. , Malliaras G. G. , Brock P. J.
Published: July 2002

We have studied the transport properties of electron- and hole-dominated MEH-PPV, poly(2-methoxy,5-(2′-ethyl-hexoxy)-p-phenylene vinylene), devices in the trap-free limit and have derived the temperature-dependent electron and hole mobilities (μ=μ0eγ√E) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility μ0 and electric-field coefficient γ are temperature dependent with an activation energy of the hole and electron mobility of 0.38±0.02 and 0.34±0.02 eV, respectively. At 300 K, we find a zero-field mobility μ0 on the order of 1±0.5×10−7 cm2/V s and an electric-field coefficient γ of 4.8±0.3×10−4 (m/V)1/2 for holes. For electrons, we find a μ0 an order of magnitude below that for holes but a larger γ of 7.8±0.5×10−4 (m/V)1/2. Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap-free limit, applicable to thin films of MEH-PPV.
Quasipermanent photochemical gratings in a dual use photorefractive polymer composite
Authors: Strutz Shane J. , Hayden L. Michael
Published: July 2002

The dual nature of (3-(2-(4-(N,N-diethylamino)phenyl)ethenyl)-5,5-dimethyl-1,2-cyclohexenylidene)propanedinitrile (Lemke-e) allows us to write both quasipermanent and erasable holographic gratings in the same storage volume. In the presence of a triplet excited sensitizer, Lemke-e undergoes a photochemical reaction allowing the creation of quasipermanent photochemical holographic gratings. In addition, applying an electric field to the composite allows the storage of erasable photorefractive holograms in the same location as previously stored permanent photochemical holograms. Photochemical gratings (η>10%) can be written in less than 1 min while photorefractive gratings (η>50%) can be written in less than 1 s. The photochemical gratings have a dark lifetime of several days.
Epitaxial lateral overgrowth of GaN over AlOx surface formed on Si substrate
Authors: Kobayashi Nobuhiko P. , Kobayashi Junko T. , Zhang Xingang , Dapkus P. Daniel , Rich Daniel H.
Published: July 2002

An approach by which single crystal α-GaN can be grown laterally over oxidized AlAs (AlOx) formed on Si substrates is demonstrated. Regular α-Ga2-O3 stripe templates, spatially separated by AlOx, on which subsequent GaN growth is selectively seeded are formed. Since the boundary between the stripe template and AlOx is nominally planar, two pyramidal planes on separated GaN can merge by growing laterally over the AlOx (referred to as planar epitaxial lateral overgrowth). Transmission electron microscopy reveals that the number of structural defects in GaN laterally grown over the AlOx is remarkably reduced compared to that in GaN grown on the stripe templates, and accordingly cathodoluminescence reveals a strong band edge emission from GaN laterally grown over the AlOx, suggesting that this approach allows us to grow GaN on Si substrates with fewer defects.
Substrate effects on GaN photoconductive detector performance
Authors: Smith G. M. , Redwing J. M. , Vaudo R. P. , Ross E. M. , Flynn J. S. , Phanse V. M.
Published: July 2002

GaN photoconductive detectors were fabricated on three substrates: sapphire, SiC, and GaN-on-sapphire substrates. The undoped GaN was deposited on each substrate by metalorganic vapor phase epitaxy. The structural properties, as measured by transmission electron microscopy, x-ray diffraction, and atomic force microscopy, of the layers grown on GaN-on-sapphire and SiC were superior to those of the layers grown on sapphire. A corresponding improvement in optical response and sharpness of optical response of the photoconductive detectors was observed with improved material quality.
Superconducting tunneling as a probe of sputtered oxide barriers
Authors: Platt C. L. , Katz A. S. , Dynes R. C. , Berkowitz A. E.
Published: July 2002

The tunneling properties of sputtered oxide barriers were studied in Pb/oxide/ferromagnet junctions. The initial oxide/ferromagnet bilayer was made without breaking vacuum. The bilayer was exposed to atmosphere before the deposition of a Pb counterelectrode in a separate vacuum chamber. I–V curves and conductance measurements at 1.5 K confirmed the presence of single-step, elastic tunneling in these structures. Separate experiments involving gold bottom electrodes, variation of exposure times in air between bilayer and Pb depositions, and plasma oxidation proved that the oxide layer is permeable, allowing for oxidation of the ferromagnetic base electrode. This revealed that the tunneling channels were due to the contribution of small-area junctions at thermally oxidized “pinhole” sites on the bottom electrode.
High-quality highly strained InGaAs quantum wells grown on InP using (InAs)n(GaAs)0.25 fractional monolayer superlattices
Authors: Jourba S. , Gendry M. , Marty O. , Pitaval M. , Hollinger G.
Published: July 2002

( InAs ) n /( GaAs ) m  (n=1.5–2, m=0.25 monolayer) fractional monolayer superlattices (FMS) have been used to grow highly strained InGaAs quantum wells (QWs) on InP by molecular beam epitaxy. We show that FMS quantum wells have better structural and optoelectronic properties compared to equivalent QWs grown using standard procedures. In addition, the onsets of the three-dimensional growth mode and plastic relaxation are delayed, which allows the highest emission wavelength in the InxGa1−xAs/InGaAlAs/InP system to be extended up to 2.35 μm at high growth temperatures (500 °C).
Doping dependence of low-frequency noise in polycrystalline SiGe film resistors
Authors: Chen X. Y. , Salm C.
Published: July 2002

Study on low-frequency noise in polycrystalline SiGe material is conducted. We use poly-Si0.7Ge0.3 resistors that were deposited by low-pressure chemical vapor deposition and were doped with different concentrations of boron by ion implantation. The doping dependence of low-frequency noise of poly-SiGe is similar to that of poly-Si. However, the value of α for poly-SiGe is one order smaller than that for poly-Si. We found that decreasing boundary scattering at higher doping concentration results in increased mobility, and decreased 1/f noise parameter α.
Temperature dependence of the relaxation time of photoconductivity and non-steady-state photocurrents in photorefractive Bi12SiO20 grown in an oxygen-free atmosphere
Authors: Bryushinin M. A. , Petrov A. A. , Sokolov I. A.
Published: July 2002

We present temperature measurements of the non-steady-state photocurrent and photoconductivity relaxation time in photorefractive sillenite crystal Bi12SiO20 grown in an oxygen-free atmosphere. The obtained dependencies are explained using a conventional model of semiconductor crystal with shallow energy level. The activation energy of shallow level obtained from photocurrent measurements was found to be ET=(0.45±0.11) eV. Independent measurements of the temperature dependence of the relaxation time of photoconductivity are in reasonable agreement with this estimation and give the value for ET equal to (0.56±0.01) eV.
Nonlinear optical characterization of a generalized Fibonacci optical superlattice
Authors: Qin Yi-qiang , Zhu Yong-yuan , Zhu Shi-ning , Luo Gui-peng , Ma Jing , Ming Nai-ben
Published: July 2002

Nonlinear optical characterization in a generalized Fibonacci-type optical superlattice, which can be generated by the concurrent inflation rule A→AmB and B→A (where m=positive integer), has been studied both theoretically and experimentally. Quasi-phase-matched second-harmonicgeneration spectrum and direct third-harmonic-generation with high efficiency have been measured in a LiTaO3 superlattice structure with m=2. Two different structures in real space have shown a similarity in reciprocal space.
Comparison of epitaxial growth of PbZr0.53Ti0.47O3 on SrRuO3 and La0.5Sr0.5CoO3
Authors: Cho J. H. , Park K. C.
Published: July 2002

We report structural and ferroelectric properties of epitaxial PbZr0.53Ti0.47O3 thin films on SrRuO3 and La0.5Sr0.5CoO3 grown on (100) LaAlO3 single crystal substrates, where all the films were deposited by the pulsed-laser deposition method. While the PbZr0.53Ti0.47O3 film on La0.5Sr0.5CoO3/LaAlO3 shows (00l)-oriented heteroepitaxial growth with a rhombohedral structure, the PbZr0.53Ti0.47O3 film on SrRuO3/LaAlO3 shows a tetragonal mixture of (00l) and (h00) heteroepitaxial growth and the (00l)-oriented rhombohedral heteroepitaxial growth. The mixture of the rhombohedral and the tetragonal structures of the PbZr0.53Ti0.47O3 film on SrRuO3/LaAlO3 influences the ferroelectric properties by domain pinning.
Temperature characteristics of threshold currents of columnar-shaped self-assembled InGaAs/GaAs quantum-dot lasers: Influence of nonradiative recombination centers
Authors: Sugawara Mitsuru , Mukai Kohki , Nakata Yoshiaki
Published: July 2002

We examined the temperature characteristics of threshold currents in columnar-shaped self-assembled InGaAs quantum-dot lasers. The threshold currents started to increase above 200 K, leading to a low characteristic temperature of 67 K above room temperature. The relationship between the spontaneous emission intensity, P, and the current, I, varied from P∝I below 160 K to P∝I2 above 298 K. Nonradiative carrier recombination centers are identified as the origin of the low characteristic temperature. A strong correlation between the increase in the threshold current and the shape of power-current curves was found, giving a hint about the origin of the homogeneous broadening of the single-dot optical gain.
Temperature dependence of the stimulated emission in a conjugated polymer
Authors: Spiegelberg Ch. , Peyghambarian N. , Kippelen B.
Published: July 2002

We report measurements of photoluminescence and stimulated emission of poly(2,5bis(2′-ethylhexyloxy)-1,4-phenylenevinylene for temperatures between 15 and 300 K. We show that the threshold for spectral line narrowing depends only slightly on the temperature. The observed decrease of the threshold intensity at lower temperatures can be mainly attributed to an increase of the quantum efficiency. We assign this deviation from the expected behavior for a four-level system to the dominant inhomogeneous broadening.
Depth-resolved microspectroscopy of porous silicon multilayers
Authors: Manotas S. , Agulló-Rueda F. , Moreno J. D. , Martı́n-Palma R. J. , Guerrero-Lemus R. , Martı́nez-Duart J. M.
Published: July 2002

We have measured depth-resolved microphotoluminescence (PL) and micro-Raman spectra on the cross section of porous silicon multilayers to sample different layer depths. The PL emission band gets stronger, blueshifts, and narrows at the high porosity layers. On the contrary, the Raman band weakens and broadens. This band is fitted to the phonon confinement model. With the bulk silicon phonon frequency and its linewidth as free parameters, we obtain crystallite size, temperature, and stress as a function of depth. Sizes are larger than those estimated from PL. Laser power was reduced to eliminate heating effects. Compressive stresses in excess of 10 kbar are found in the deepest layer due to the lattice mismatch with the substrate.
Surface sensitivity effects with local probe scanning Auger–scanning electron microscopy
Authors: van Agterveld D. T. L. , Palasantzas G. , De Hosson J. Th. M.
Published: July 2002

This letter concentrates on a quantitative description of surface roughness effects on Auger peak-line profiles for pure and alloyed specimens. The nanometer lateral electron probe size of the order of 10 nm yielded peak-line profiles that capture surface topology variations down to nanometer-length scales. Surface roughness leads to peak-intensity fluctuations, which are described within the weak roughness limit by a simple form, I(r)≈Iav[1+βh(r)]. Iav is the average peak intensity, h(r) represents the roughness fluctuation along a lateral in-planar distance r, and β is a constant (<1). In addition, analyses of the peak-difference correlation function Iz(r)=〈|I(r)−I(0)|2〉1/2 showed a power-law behavior Iz(r)∝rα with α ranging between 0.7 and 1 at small-length scales, i.e., for r≪ξ, with ξ a peak correlation length that was comparable to average specimen cluster sizes.
Cycling endurance of silicon–oxide–nitride–oxide–silicon nonvolatile memory stacks prepared with nitrided SiO2/Si(100) interfaces
Authors: Habermehl S. , Nasby R. D. , Rightley M. J.
Published: July 2002

The effects of nitrided SiO2/Si(100) interfaces upon cycling endurance in silicon–oxide–nitride–oxide–silicon (SONOS) nonvolatile memory transistors are investigated. Analysis of metal–oxide–silicon field-effect transistor subthreshold characteristics indicate cycling degradation to be a manifestation of interface trap generation at the tunnel oxide/silicon interface. After 106 write/erase cycles, SONOS film stacks prepared with nitrided tunnel oxides exhibit enhanced cycling endurance over stacks prepared with non-nitrided tunnel oxides. If the capping oxide is formed by steam oxidation, rather than by deposition, SONOS stacks prepared with non-nitrided tunnel oxides exhibit endurance characteristics similar to stacks with nitrided tunnel oxides. For this case, a mechanism for latent nitridation of the tunnel oxide/silicon interface is proposed.
Integrated micromechanical cantilever magnetometry of Ga1−xMnxAs
Authors: Harris J. G. E. , Awschalom D. D. , Matsukura F. , Ohno H. , Maranowski K. D. , Gossard A. C.
Published: July 2002

We have developed a technique for fabricating submicron GaAs micromechanical cantilevers into which lithographically patterned samples grown by molecular beam epitaxy or evaporative deposition are integrated. The torque sensitivity of the 100-nm-thick cantilevers makes them ideal for torsional magnetometry of nanometer-scale, anisotropic samples. We present measurements on samples of the ferromagnetic semiconductor Ga1−xMnxAs at temperatures from 350 mK to 65 K and in fields from 0 to 8 T. By measuring the shift in the resonant frequency of the cantilevers, we demonstrate a moment sensitivity of 3×106 μB at 0.1 T, an improvement of nearly five orders of magnitude upon existing torsional magnetometers.
Defects in thermal oxide studied by photoluminescence spectroscopy
Authors: Nishikawa Hiroyuki , Stathis James H. , Cartier E.
Published: July 2002

Defects in as-grown thermal oxide were investigated by photoluminescence (PL) spectroscopy using synchrotron radiation as a light source. A PL band at 3.3 eV was observed under excitation at 5 eV for dry thermal oxide at room temperature. The PL band was also observed for forming-gas annealed oxide, in which the 3.3 eV PL was either enhanced by subsequent vacuum annealing or suppressed by hydrogen exposure. The PL measurements on oxynitride films show that effect of nitrogen incorporation on the 3.3 eV PL is less significant than that of hydrogen. Wide observability of the 3.3 eV PL band for oxide films prepared under various conditions indicates the intrinsic nature of the defects in thermal oxide introduced during thermal oxidation of silicon.
High-confinement SiGe low-loss waveguides for Si-based optoelectronics
Authors: Pogossian S. P. , Vescan L. , Vonsovici A.
Published: July 2002

The realization of two-dimensional SiGe/Si strained-layer low-loss waveguide (1.7 db/cm) is reported. The waveguide geometry is grown by selective epitaxy. It ensures loosened cutoff and critical thickness conditions. This geometry could be applied for waveguide active devices like light emitting diodes, photodetectors, or modulators. Because of the high cross section of the guided mode, these devices could be easily interfaced with optical fibers.
Attenuated total reflection Fourier transform infrared spectroscopy study of the adsorption of organic contaminants on a hydrogen-terminated Si(111) surface in air
Authors: Ye Shen , Ichihara Taro , Uosaki Kohei
Published: July 2002

The adsorption of organic contaminants on a hydrogen-terminated Si(111) surface was investigated using attenuated total reflection Fourier transform infrared spectroscopy. When the hydrogen-terminated Si(111) surface was exposed to dry air, the sharp Si–H monohydride peak became weaker and a broad component became visible in the lower wave-number region. Furthermore, a number of bands within the C–H stretching region were observed. The intensity of the sharp Si–H band was recovered to a certain extent and those of the broad component and the C–H bands decreased after the sample was rinsed in hexane. These results suggest that the contamination by organic adsorbates is not accompanied by a chemical bond formation.
The Bloch–Grüneisen mobility of two-dimensional electron gas in AlGaN/GaN heterostructures
Authors: Zakhleniuk N. A. , Bennett C. R. , Babiker M. , Ridley B. K.
Published: July 2002

We present calculations of the Bloch–Grüneisen electron mobility in zincblende (ZB) and wurtzite (WZ) AlGaN/GaN quantum-well heterostructures. Within the Boltzmann equation approach, we derive an expression for the momentum relaxation time which explicitly takes into account the Pauli principle restrictions, and show that these are comparable in importance to a screening effect at temperatures up to 150 K provided that the electron density is high. This is of particular importance for GaN-based quantum wells for which very high electron densities initiated by the strain-induced and spontaneous polarization fields have been recently reported. Dependences of the mobility on the lattice temperature and the electron density for both ZB and WZ GaN are presented, and it is shown that the WZ mobility is higher than the ZB mobility.
Effects of deep levels on transconductance dispersion in AlGaAs/InGaAs pseudomorphic high electron mobility transistor
Authors: Choi Kyoung Jin , Lee Jong-Lam , Yoo Hyung Mo
Published: July 2002

The effects of deep levels on the transconductance dispersion in an AlGaAs/InGaAs pseudomorphic high electron mobility transistor was interpreted using capacitance deep level transient spectroscopy (DLTS). Transconductance was decreased by 10% in the frequency range of 10 Hz–10 kHz at the negative gate bias, but it was increased at the positive one. In the DLTS spectra, two hole trap-like signals corresponding to surface states were only observed at the negative pulse bias, whereas the DX-center with the activation energy of 0.42±0.01 eV were observed at the positive one. The activation energy agrees well with that obtained from the temperature dependence of the positive transconductance dispersion, 0.39±0.03 eV. These provide evidence that the positive and negative transconductance dispersions are due to the DX center and surface states, respectively.
Linear electro-optic effect due to the built-in electric field in InGaN/GaN quantum wells
Authors: Jiang Hongtao , Singh Jasprit
Published: July 2002

A strong piezoelectric effect and large lattice mismatch allow one to incorporate high built-in electric fields in InGaN/GaN quantum wells. This letter examines the implications of these fields on the absorption spectra and refractive index changes induced by an external perpendicular electric field. We find that InGaN/GaN quantum wells show linear electro-optic effect due to quantum confined Stark effect. Our results suggest application of InGaN/GaN quantum wells in Mach–Zehnder type modulators and in electroabsorption modulators in the blue light region.
Field and potential around local scatterers in thin metal films studied by scanning tunneling potentiometry
Authors: Ramaswamy Geetha , Raychaudhuri A. K.
Published: July 2002

We report the direct observation of electrochemical potential and local transport field variations near scatterers like grain boundaries, triple points, and voids in thin platinum films studied by scanning tunneling potentiometry. The field is highest at a void, followed by a triple point and a grain boundary. The local transport field near a void can even be four orders of magnitude higher than the macroscopic field, indicating that the void is the most likely place for an electromigration induced failure. The field build up for a particular type of scatterer depends on the grain connectivity. We estimate an average grain boundary reflection coefficient for the film from the temperature dependence of its resistivity.
Harmonic frequency mixing in Bi2Sr2CaCu2O8+X intrinsic Josephson junctions
Authors: Wang H. B. , Aruga Y. , Tachiki T. , Mizugaki Y. , Chen J. , Nakajima K. , Yamashita T. , Wu P. H.
Published: July 2002

In a c-axis junction stack (10 μm×10 μm in a-b plane) patterned on a Bi2Sr2CaCu2O8+X single crystal, we performed harmonic mixings between a 100 GHz signal and up to the 98th harmonic of a local oscillator at about 1 GHz. The dependence of PIF on V, where PIF is the mixing output at the intermediate frequency and V is the dc voltage across the junction stack, was strongly affected by the local oscillator power levels. For low values of local oscillator power, the optimum operation point where PIF became maximum was around zero bias, and typical multi-branch structure of PIF vs V curves were observed. For high values of local oscillator power, the optimum operation point moved to the high-voltage region. In both cases, interesting oscillations occurred on the PIF vs V curves. Our experimental results led us to believe that intrinsic Josephson junctions can be good candidates for high frequency applications and the harmonic mixing may be a useful probe to investigate plasma phenomena in the layered superconductors.
Time-resolved soft x-ray absorption spectroscopy of silicon using femtosecond laser plasma x rays
Authors: Nakano Hidetoshi , Goto Yoshinori , Lu Peixiang , Nishikawa Tadashi , Uesugi Naoshi
Published: July 2002

We measured time-resolved soft x-ray absorption of photoexcited silicon by means of pump-probe spectroscopy, using a picosecond soft x-ray pulse from femtosecond laser-produced plasma as a probe. We observed a 5% increase in the absorption caused by 1010 W/cm2 intensity laser pulse irradiation near the LII,III edge at 100 eV. The change was observed only when the laser and the soft x-ray pulses overlapped on the sample both in time and space.
A low-voltage, low-capacitance, vertical ruthenium–lithium–ruthenium sandwich layer thin-film-edge dispenser field emitter electron source
Authors: Hsu David S. Y. , Gray Henry F.
Published: July 2002

A low-voltage, low-capacitance, field emitter array (FEA) electron source has been developed for applications such as field emitter displays, high-voltage switches, and high frequency power amplifiers. The FEA cell has a horizontal gate and a vertical edge emitter made by sandwiching a low-work-function lithium dispenser thin film between two ruthenium films. We measured a low turn-on voltage (several nanoamps at 27 V) and high collected current (16 μA at 62 V) from 1 to 3 cells. Based on chemical beam deposition, the fabrication method allows all the FEA cell dimensions to be independently adjustable. Our FEA has many potential advantages in performance such as low capacitance, low voltage, high transconductance, high current, and resistance to oxidation and to erosion by back-ion bombardment. Furthermore, much lower manufacturing cost should result from the significantly fewer processing steps.