bluuebunny/crossref_metadata_2025 huggingface dataset:
Waveguide fabrication and high-speed in-line intensity modulation in 4-N,N-4′-dimethylamino-4′-N′-methyl-stilbazolium tosylate
Authors: Pan F. , McCallion K. , Chiappetta M.
Published: July 2002

We report the formation of thin-film waveguides of organic crystals by precision optical polishing and the fabrication of an electro-optic intensity modulation device based on a thin-film waveguide of N,N-4′-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST®) as an overlay on a side-polished fiber (SPF). Successful fabrication of single-crystal DAST® waveguides with thicknesses in the 20–25 μm range have been produced. The waveguides were investigated via an evanescent coupling technique using side-polished fibers rather than traditional end-firing methods. Surface quality is believed to have been sufficient for low-loss propagation. Electrodes were added to the SPF/DAST® overlay architecture and intensity modulation observed out to 18 GHz. The device frequency response is believed to extend beyond 100 GHz under optimum conditions.
Anisotropic nanoindentation of transcrystalline polypropylene by scanning force microscope using blade-like tips
Authors: Amitay-Sadovsky Ella , Cohen Sidney R. , Wagner H. Daniel
Published: July 2002

A scanning force microscope was fitted with an elongated, blade-like tip, with which nanoindentations were performed in the transcrystalline isotactic polypropylene phase grown from the surface of high-modulus carbon fibers. The anisotropic Young’s modulus was evaluated by measuring the force–penetration curve of the indentation and the tip topography, as a function of the indentation depth. The modulus is 1.6–3 times higher when the longer axis of the indenting tip is perpendicular to the transcrystalline growth direction than when it is parallel to that direction. We discuss possible options for the lamellar arrangement in a transcrystalline isotactic polypropylene layer and, based on the present experimental data, propose a most likely model.
Charge storage in GaAs metal insulator semiconductor field effect transistor metal nanodot memory structures
Authors: Sabri F. , Hasko D. G.
Published: July 2002

Charging effects in 3–4-nm-diam AuPd nanodots have been investigated by comparing the characteristics of depletion mode delta-doped GaAs metal insulator semiconductor field effect transistor with and without a nanodot floating gate layer. The nanodots were embedded in a 10-nm-thick Al2O3 gate insulator, formed by a self-limiting oxidation process. Charge transfer takes place by tunneling between the channel of the transistor and the nanodots through the Al2O3. In this experiment the total nanodot layer charge could be directly determined using capacitance measurements and compared with the number of nanodots. It was concluded that there was an average of about one excess electron per nanodot. The charging of the nanodots gives rise to a threshold voltage shift in the transistor that can be applied to information storage applications.
Low-threshold 7.3 μm quantum cascade lasers grown by gas-source molecular beam epitaxy
Authors: Slivken S. , Matlis A. , Rybaltowski A. , Wu Z. , Razeghi M.
Published: July 2002

We report low-threshold 7.3 μm superlattice-based quantum cascade lasers. The threshold current density is 3.4 kA/cm2 at 300 K and 1.25 kA/cm2 at 79 K in pulsed mode for narrow (∼20 μm), 2-mm-long laser diodes. The characteristic temperature (T0) is 210 K. The slope efficiencies are 153 and 650 mW/A at 300 and 100 K, respectively. Power output is in excess of 100 mW at 300 K. Laser far-field intensity measurements give divergence angles of 64° and 29° in the growth direction and in the plane of the quantum wells, respectively. Far-field simulations show excellent agreement with the measured results.
Application of nonalloyed PdGe ohmic contact to self-aligned gate AlGaAs/InGaAs pseudomorphic high-electron-mobility transistor
Authors: Oh Jung-Woo , Lee Jong-Lam
Published: July 2002

A self-aligned gate pseudomorphic high-electron-mobility transistor (PHEMT) was developed through nonalloyed PdGe ohmic contact on an n+-GaAs cap layer. The lowest contact resistivity obtained was 1.2×10−7 Ω cm2 at 300 °C. This allows us to change the sequence on the formation of source/drain and gate electrodes in the process of PHEMT fabrication, namely self-aligned gate PHEMT. Performance of the self-aligned gate PHEMT was remarkably pronounced with annealing temperature or the decrease of contact resistivity. This is due to the solid-phase regrowth of highly Ge-doped GaAs below the PdGe contact, which acts to reduce barrier height for electron tunneling.
Structural and electro-optic properties in lead magnesium niobate titanate thin films
Authors: Lu Ya-lin , Gaynor Brad , Hsu Christine , Jin Guanghai , Cronin-Golomb Mark , Wang Feiling , Zhao Jing , Wang S.-Q. , Yip Pearl , Drehman A. J.
Published: July 2002

The structural and quadratic electro-optic properties in lead magnesium niobate titanate thin films have been systematically studied as a function of the fraction x of lead titanate. (110)-oriented, pyrochlore-free PMN–PT films with PT contents around morphotropic phase boundary compositions, which have a propagation loss as low as 3 dB/cm and a high electro-optical (EO) coefficient of 1.32×10−16 (m/V)2, have been fabricated on (1012)-cut sapphire substrates. The EO measurements show that the PMN–PT is a highly promising electro-optic material.
Photoluminescence from silicon nanoparticles in a diamond matrix
Authors: Terranova M. L. , Piccirillo S. , Sessa V. , Botti S. , Rossi M.
Published: July 2002

Composite polycrystalline diamond layers containing light-emitting Si nanoparticles (mean diameter: 3.6 and 6.2 nm) have been prepared by using a chemical-vapor deposition technique coupled with a powder-flowing apparatus. The room-temperature photoluminescence behavior of such material in the 1.6–2.5 eV energy range is found characterized by a strong increase of the emission efficiency from the Si nanostructures embedded in the diamond matrix. Moreover, the studies reveal that it is possible to tune the luminescence frequency in the yellow-green spectral region by selecting the size of the inserted Si grains.
GHz bandwidth GaAs light-emitting diodes
Authors: Chen C. H. , Hargis M. , Woodall J. M. , Melloch M. R. , Reynolds J. S. , Yablonovitch E. , Wang W.
Published: July 2002

Double-heterostructure GaAs/GaAlAs light-emitting diodes (LEDs) have been fabricated with the emitter regions beryllium doped to 2×1019 and 7×1019 cm−3. The 7×1019 cm−3 doped emitters have an internal quantum efficiency of 10% and an optical modulation bandwidth of 1.7 GHz. The steady-state optical output power versus the input current shows an external efficiency of 2.5 μW/mA. The 2×1019 cm−3 emitters have internal quantum efficiencies as high as 80%, but a reduced cutoff frequency. The external quantum efficiency reaches 10 μW/mA. These high-speed LEDs are produced by reducing the radiative lifetime to 100–250 ps without significantly degrading internal quantum efficiency. The current results on heavily beryllium-doped LEDs exhibit, to the best of our knowledge, the highest external efficiencies to date for such high doping and efficiencies close to that observed for lower-doped LEDs.
Pauli-blocking imaging of single strain-induced semiconductor quantum dots
Authors: Obermüller C. , Deisenrieder A. , Abstreiter G. , Karrai K. , Grosse S. , Manus S. , Feldmann J. , Lipsanen H. , Sopanen M. , Ahopelto J.
Published: July 2002

The photoluminescence (PL) of InP strained-induced quantum dots in a GaInAs/GaAs quantum well is measured at low temperature (4.2 K) using near-field scanning optical microscopy. The PL originating from the first three confined levels of eight individual dots is mapped out over an area of 1.4×1.4 μm. The spatial resolution of the PL of the lowest energy level is found to be limited to about 0.5 μm. In contrast, the mapping of the PL of the higher excited state shows a much improved spatial resolution of the order of 150 nm which is the instrument resolution. This effect is understood in terms of Pauli-blocking of the dot level filling.
Response of 1.6 μm Er:Y3Al5O12 fiber-optic temperature sensor up to 1520 K
Authors: Henry D. M. , Herringer J. H. , Djeu N.
Published: July 2002

Fiber-optic temperature sensors based on fluorescence decay of the 1.6 μm transition in Er:Y3Al5O12 (Er:YAG) have been operated up to 1520 K. The probes are of a monolithic construction consisting of a single-crystal YAG lead fiber and an Er:YAG tip. The temperature-dependent fluorescence decay data show the dominance of the phonon mode at 220 cm−1 in mediating multiphonon relaxation in YAG at high temperatures. Such a sensor was used in the direct measurement of temperature in the interior of molten aluminum.
Gas-phase nanoparticle formation and transport during pulsed laser deposition of Y1Ba2Cu3O7−d
Authors: Geohegan D. B. , Puretzky A. A. , Rader D. J.
Published: July 2002

The gas-phase growth and transport of nanoparticles are characterized at the low background oxygen pressures used for pulsed laser deposition of high-Tc Y1Ba2Cu3O7−d superconducting films. Onset times and pressures for gas-phase nanoparticle formation were determined by intensified charge-coupled device imaging and optical spectroscopy of laser-induced fluorescence from diatomic oxides and Rayleigh scattering from gas-suspended nanoparticles. Nanoparticles are detected for oxygen pressures above 175 mTorr at room temperature, with growth continuing during seconds within the cloud of stopped vapor near the heater surface. Elevated heater temperatures create background density gradients which result in reduced resistance to the initial plume expansion. The temperature gradient also moves nanoparticles away from the heater surface as they grow, effectively limiting the time and spatial confinement necessary for continued growth or aggregation, and inhibiting deposition by thermophoresis.
Light-induced electron spin resonance in amorphous hydrogenated germanium
Authors: Marques F. C. , de Lima M. M. , Taylor P. C.
Published: July 2002

We report the observation of light-induced electron spin resonance (LESR) in amorphous hydrogenated germanium. Two new lines with zero crossings near g=2.01 and g=2.03 were detected and ascribed to electrons and holes in the conduction- and valence-band-tail states, respectively. The ratio between the LESR spin densities of both lines is approximately one, suggesting the absence of spin pairing, charge defect creation, or LESR of dangling bonds. The growth and decay spectra exhibit dispersive behavior with a dispersion parameter ∼0.5. The decay spectrum is best fit assuming bimolecular recombination. The LESR spin density depends weakly on the photogeneration rate as a sublinear power law.
Optical properties of a high-quality insulating GaN epilayer
Authors: Zeng K. C. , Lin J. Y. , Jiang H. X. , Yang Wei
Published: July 2002

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of an insulating GaN epilayer grown by metalorganic chemical vapor deposition on a sapphire substrate. Two emission lines at 3.503 and 3.512 eV in the continuous wave (cw) PL spectra observed at 10 K under a low excitation intensity (∼23 W/cm2) were identified as the band-to-band transitions involving the A and B valence bands, respectively. A third emission line at 3.491 eV was identified as a band-to-impurity transition involving a shallow donor. The PL decay behavior can be well understood with a model taking into account both the free carriers and impurities. The effective recombination lifetime of the band-to-band transition in GaN was found to be about 3.7 ns. Possible mechanisms for the band-to-band transition being dominant in this high quality insulating GaN epilayer have also been discussed.
Half-symmetric cavity microelectromechanically tunable vertical cavity surface emitting lasers with single spatial mode operating near 950 nm
Authors: Wang Peidong , Tayebati Parviz , Vakhshoori Daryoosh , Lu Chih-Cheng , Azimi Masud , Sacks Robert N.
Published: July 2002

We describe tunable vertical cavity surface emitting lasers with a half-symmetric cavity structure. The cavity is realized by inducing a curvature (R∼300 μm) in the top movable dielectric mirror. This half-symmetric microcavity forces lasing oscillations in a single fundamental spatial mode with a side-mode suppression ratio of >25 dB. Continuous wavelength tuning of 44 nm was achieved microelectromechanically with a tuning voltage of 14 V. The device operates near 950 nm with maximum power output of ∼0.9 mW.
Depletion layer imaging using a gaseous secondary electron detector in an environmental scanning electron microscope
Authors: Phillips M. R. , Toth M. , Drouin D.
Published: July 2002

We present a method for imaging depletion layers using the gaseous secondary electron detector (GSED) employed in environmental scanning electron microscopes. GSED images of a p-n junction were obtained from a Si P+PN power diode. Behavior of the junction contrast as a function of imaging conditions is unrelated to reported GSED contrast formation mechanisms [ A. L. Fletcher, B. L. Thiel, and A. M. Donald, J. Phys. D 30, 2249 (1997)]. Optimum imaging conditions are presented, and the contrast behavior is interpreted in terms of a previously unreported induced current component in GSED images. The presented technique is unique as it will enable imaging of depletion layers in uncoated semiconductor/oxide devices in controlled gaseous environments at elevated specimen temperatures.
Amorphization of single-crystalline silicon by thermal-energy atomic hydrogen
Authors: Kang J. H. , Jo S. K. , Gong B. , Parkinson P. , Brown D. E. , White J. M. , Ekerdt J. G.
Published: July 2002

Si (100)-(2×1) was exposed to gas-phase atomic hydrogen, H(g), at various substrate temperatures Ts between 115 and 300 K. No low-energy electron diffraction patterns could be obtained from such hydrogenated surfaces. In temperature-programmed desorption measurements, SiHx(x=1–3) radical species as well as SiH4 desorbed at Ts between 600 and 1000 K, in addition to β1- and β2–H2 desorption peaks. Combined together, the results indicate that amorphous hydrogenated silicon (a-Si:H) films are formed. While surface etching competes, a-Si:H formation dominates. Once formed, a-Si:H further suppresses etching.
Fabrication of lateral single-electron tunneling structures by field-induced manipulation of Ag nanoclusters on a silicon surface
Authors: Park Kang-Ho , Shin Mincheol , Ha Jeong Sook , Yun Wan Soo , Ko Young-Jo
Published: July 2002

Nanostructures composed of Ag clusters on an Sb-terminated Si surface were designed in a highly controlled manner and the electric conduction through Ag nanoclusters to the silicon substrate was investigated by using a scanning tunneling microscopy/spectroscopy. It was found that the lateral conduction between neighboring Ag clusters significantly contributed to the tunneling current–voltage characteristics, and the metallic single-electron tunneling structures employing the lateral conduction channels at room temperature can be fabricated via a field-induced manipulation of Ag clusters.
High exchange bias field induced in ferrimagnetic bilayers of amorphous rare earth–transition-metal alloys
Authors: Sbiaa R. , Le Gall H.
Published: July 2002

The magnetic reversal processes of exchange-coupled double layers (ECDL) and corresponding single layers of rare-earth–transition metal were studied by magneto-optical Faraday rotation (FR). The dependence of FR on the temperature shows a transition of DyFeCo/TbFeCo ECDL from one type to another near the compensation temperature of single layers. Minor and major hysteresis loops allowed us to measure the exchange bias field induced by DyFeCo, which has also been calculated with an analytical model. We have also observed that an unusual hystersis loop appears few celsius degrees before this transition occurs. In this letter, we analyze and describe different magnetization states, and various kinds of magnetization processes, including the usual and unusual FR loops, are explained in detail.
Excitation of bright and dark microwave magnetic envelope solitons in a resonant ring
Authors: Kalinikos Boris A. , Kovshikov Nikolai G. , Patton Carl E.
Published: July 2002

The excitation of both bright and dark microwave magnetic envelope solitons has been realized for one and the same resonant ring containing a yttrium–iron–garnet film magnetostatic wave (MSW) delay line. The resonant ring modifies the MSW character to produce a series of frequency intervals with alternating regions of positive and negative dispersion. These alternating regions support bright and dark soliton production, respectively.
Mechanical nanomanipulation of single strain-induced semiconductor quantum dots
Authors: Obermüller C. , Deisenrieder A. , Abstreiter G. , Karrai K. , Grosse S. , Manus S. , Feldmann J. , Lipsanen H. , Sopanen M. , Ahopelto J.
Published: July 2002

We report on low temperatures (4 K) in situ nanomanipulation of the confining potential of single strain-induced Ga0.9In0.1As quantum dots. This was achieved by scanning a metal coated tapered optical fiber tip over the self organized InP stressor islands that are responsible for the localized strain field in the GaInAs/GaAs quantum well. By scanning the tip with a shear force contact of the order of 1 nN, we thinned down the InP stressor islands in an unexpectedly reproducible and controlled way. The modification of the confining potential was directly monitored by measuring in situ the photoluminescence of each manipulated dot using a near-field scanning optical microscope.
The grain growth blocking effect of polycrystalline silicon film by thin native silicon oxide barrier during the excimer laser recrystallization
Authors: Park Kee-Chan , Jeon Jae-Hong , Park Cheol-Min , Lee Min-Cheol , Han Min-Koo , Choi Kwon-Young
Published: July 2002

A method to control the recrystallization depth of amorphous silicon (a-Si) film during the excimer laser annealing is presented. By irradiating the XeCl excimer laser on the triple film structure of a-Si/thin native silicon oxide (∼20 Å)/thick a-Si layer, only the upper a-Si film is recrystallized, whereas the lower thick a-Si film remains amorphous. The thin native silicon oxide layer blocks the grain growth and prevents the upper grains from growing into the lower a-Si. As a result, the thin oxide film sharply defines the boundary between polycrystalline silicon (poly-Si) and the a-Si layer. The poly-Si/a-Si double layer is useful for the fabrication of high-performance poly-Si thin film transistors.
Positive ions as growth precursors in plasma enhanced chemical vapor deposition of hydrogenated amorphous silicon
Authors: Hamers E. A. G. , Bezemer J. , van der Weg W. F.
Published: July 2002

We present a straightforward method to determine the contribution of ions to the growth rate in capacitively coupled plasma deposition systems. Ions and radicals from the plasma can be spatially separated by a small aperture in front of the substrate. Separation is caused by the different angular distribution of the velocity of these types of particles. From measured thickness profiles of deposited hydrogenated amorphous silicon, we deduce a surface reaction probability of the radicals of 0.30. It is concluded that the ions contribute about 10% of the silicon atoms to the growth process under typical deposition conditions.
Effect of polyvinylcarbazole barrier layer on charge storage in amorphous selenium films
Authors: Chand Suresh , Sharma G. D. , Dwivedi S.
Published: July 2002

Effect of polyvinylcarbazole (PVK) barrier layer on charge storage in amorphous selenium (a-Se) films (∼100 μm) in the temperature range of 295–385 K has been studied as a function of barrier layer thickness (∼2400–7500 Å) using thermally stimulated discharge current (TSD) technique. TSD spectra of a-Se films show a significant reduction in the charge stored under relaxation peaks upon incorporation of PVK interface layer. These results have been attributed to blocking and field enhanced mobility role of PVK barrier layer film.
A monolithic field-effect-transistor-amplified magnetic field sensor
Authors: Schaadt D. M. , Yu E. T. , Sankar S. , Berkowitz A. E.
Published: July 2002

We propose and demonstrate the operation of a monolithic field-effect-transistor-amplified magnetic field sensor device, in which a tunnel-magnetoresistive (TMR) material is incorporated within the gate of a Si metal–oxide–semiconductor–field-effect transistor. A fixed voltage is applied across the TMR layer, which leads charge to build up within the gate. Applying or changing an external magnetic field causes a change in the charge within the TMR layer and, consequently, a shift in the transistor threshold voltage, which leads to an exponential change in subthreshold current IDS sub and a quadratic change in saturation current IDS sat. The application of a 6 kOe magnetic field at room temperature leads in our device to an absolute change in IDS sub three times as large and in IDS sat 500 times as large as the corresponding change in current through the TMR layer alone. The relative change in IDS sub is a factor of four larger than that in the current through the TMR layer.
Matrix effects in particle induced x-ray emission channeling measurements of ZnSe/GaAs heterostuctures
Authors: Seppälä A. , Räisänen J.
Published: July 2002

The significance of the matrix effects, i.e., of self-absorption and secondary fluorescence, in particle induced x-ray emission (PIXE) channeling measurements was studied in epitaxial ZnSe layers on a GaAs substrate. The effect of self-absorption on Zn and Se channeling minimum yields was found to be significant only with thick ZnSe layers. The importance of secondary fluorescence was estimated by applying an introduced correction formula to the measured Zn minimum yields for ZnSe/GaAs heterostructures. The corrections in Zn minimum yields were significant at high proton energies (>2 MeV). Thus, errors for instance in impurity atom lattice location determinations by PIXE channeling occur unless these effects are correctly taken into account.
High-efficiency piezoelectric motor combining continuous rotation with precise control over angular positioning
Authors: Glazounov A. E. , Wang S. , Zhang Q. M. , Kim C.
Published: July 2002

The letter describes a piezoelectric motor that combines the merits of piezoelectric materials, such as high power density generated at electromechanical resonance, and a precise control of displacement. The motor utilizes a direct coupling mechanism between the stator and rotor, where a clutch drives the rotor via locking it. The direct coupling makes it possible to transmit the whole power generated in the piezoelectric element to the rotor, and thus achieve the high efficiency of the motor. It also allows the combining of two regimes of operation: continuous rotation and a stepwise motion within a 360° interval with a high resolution of angular displacement.
Charge density wave ratchet
Authors: Visscher Mark I. , Bauer Gerrit E. W.
Published: July 2002

We propose to operate a locally gated charge density wave as an electron pump. Applying an oscillating gate potential with frequency f causes equally spaced plateaus in the sliding charge density wave current separated by ΔI=2eNf, where N is the number of parallel chains. The effects of thermal noise are investigated.
A piezoelectric micromotor based on acoustic precessional waveguide
Authors: Carotenuto Riccardo , Iula Antonio , Pappalardo Massimo , Lamberti Nicola
Published: July 2002

The piezomotor is composed of a stator, a steel axle, and a rotor. In the stator, which consists of a piezoelectric membrane, a flexural traveling wave is excited via piezoelectric effect. The rotating flexural displacement of the membrane excites a wide precessional wave in the axle; this wave propagates as in a waveguide, producing a precessional motion in the terminal surface of the axle. The rotor consists of a cylindrical permanent magnet, pressed in contact with the top surface of the axle by means of the magnetic force. In this way, a continuous slipping takes place between the terminal surface of the axle and the rotor. We demonstrate that the acoustic waveguide can transfer a good mechanical power (max torque 1.5×10−5 Nm, max rotational velocity 3500 rpm) with unique design flexibility in a variety of different microsystem applications.
Observation of abnormal capacitance-frequency behavior in In0.12Ga0.88As/GaAs p-i-n superlattice grown at low temperature
Authors: Chen J. F. , Wang J. S. , Wang P. Y. , Wong H. Z.
Published: July 2002

Capacitance-frequency measurement is used to study In0.12Ga0.88As/GaAs p-i-n superlattice, with superlattice layer grown at 300 °C by molecular-beam epitaxy. Three traps are observed, and their parameters are consistent with those obtained from deep-level transient spectroscopy. Among these three traps, the trap (Ea=0.45±0.02 eV, σ=6±4×10−17 cm2) shows an abnormal increase of capacitance with increasing frequency, similar to that observed from the 0.66 eV trap in low-temperature grown GaAs p-i-n structure, suggesting that it is created by the low-temperature growth and is a generation-recombination center. This result also shows that the capacitance-frequency measurement is effective in studying the generation-recombination centers.
Rotating-mode content of gyrotron output
Authors: Brand G. F.
Published: July 2002

Previous work has investigated a variety of diffraction grating capable of creating beams with phase singularities from an incident plane wave. The grating is used here in reverse to analyze the fractions of oppositely rotating modes present in the output of a gyrotron.
Wavelength-dependent optical degradation of green II–VI laser diodes
Authors: Vogelgesang R. , Liang J. J. , Wagner V. , Lugauer H. J. , Geurts J. , Waag A. , Landwehr G.
Published: July 2002

In this letter we report on optical degradation studies on BeMgZnSe separate confinement quantum well laser structures for the blue-green spectral region. The wavelength of the incident light has been tuned from 3.81 down to 2.10 eV, corresponding to an energy range from above the band gap of the cladding layers down to below the band gap of the quantum well. The dominant degradation mechanism is initiated when electron hole pairs are created in the quantum well. Absorption of light in deep defect bands, e.g., of the p-type cladding material is negligible in these structures. The strain state of the quantum well is one possible driving force for the degradation. In this context, point defect propagation as well as a structural phase transition of the ZnCdSe quantum well are discussed.
Ultrafast low-temperature grown AlGaAs/GaAs photorefractive quantum wells using point defects as capture centers
Authors: Zhang M. H. , Huang Q. , Zhang Y. F. , Zhou J. M. , Li Q. , Xu Z. Y.
Published: July 2002

At a medium substrate temperature of 400 °C and a lower As flux, we have grown an ultrafast AlGaAs/GaAs photorefractive multiple quantum well (MQW) structure by molecular beam epitaxy. The as-grown sample exhibits strong photorefractive effect under the transverse Frantz–Keldysh geometry. A peak electroabsorption of 2100 cm−1 is measured in the as-grown sample in an 11 kV/cm dc electric field, and the peak photorefractive diffraction efficiency can be 1.2%. After postgrowth annealing, the photorefractive effect becomes weak and disappears in samples annealed above 700 °C. Using optical transient current spectroscopy, deep levels are measured in these samples. It is found that deep levels are stable against annealing until 700 °C. Using a pump-probe technique, carrier lifetimes are measured at room temperature. We find that the as-grown sample has a lifetime of 20 ps, while the 700 °C annealed sample has a lifetime of more than 200 ps. The ultrafast lifetime in the as-grown sample is caused by point defects, not by As clusters. Our result show that AlGaAs/GaAs MQW structure grown around 400 °C has better performance of the photorefractive effect.
Coplanar alignment of mesogenic moieties in a photocrosslinked liquid crystalline polymer film containing cinnamoyl groups
Authors: Kawatsuki Nobuhiro , Takatsuka Hirofumi , Yamamoto Tohei
Published: July 2002

Coplanar alignment of mesogenic groups using polymer films of a methacrylate comprising a photocrosslinkable 4′-(2-cinnamoyloxyethoxy)-biphenyl group and a photoinactive 4-cyanobiphenyl group was realized by slantwise ultraviolet (UV) irradiation. Slantwise absorption measurement revealed that an orientational plane of the mesogenic groups was perpendicular to direction of the UV light propagation. The orientational angle and the ellipsoid of refractive index of the film could be regulated by changing the irradiation angle and degree of polarization of the UV exposure, respectively.
A mesoscopic terahertz pulse detector
Authors: Orellana P. , Claro F.
Published: July 2002

We show that under the passage of an electromagnetic terahertz pulse an asymmetric double-barrier device may act as an on/off current switch, depending on the bias. The time-dependent response of the device is discussed.
A numerical study of optical second-harmonic generation in a one-dimensional photonic structure
Authors: Kiehne G. T. , Kryukov A. E. , Ketterson J. B.
Published: July 2002

We present a band-gap engineering approach for obtaining a finite photonic-band-edge-resonant stratified periodic dielectric structure in the presence of material dispersion. When the structure contains second-order nonlinear optical materials, enhanced phase-matched optical second-harmonic (SH) generation may be obtained. Using a transfer-matrix technique, we computed the transmitted and reflected SH power for a GaAs/AlAs Bragg grating and found that a substantial enhancement of the SH power is obtained for a modest number of periods. Asymptotically for a large number of periods, we found the SH power to scale as the eighth power of the total structure thickness.
Factors influencing the interfacial roughness of InGaAs/GaAs heterostructures: A scanning tunneling microscopy study
Authors: Chao Kuo-Jen , Liu Ning , Shih Chih-Kang , Gotthold D. W. , Streetman B. G.
Published: July 2002

Using cross-sectional scanning tunneling microscopy, we have investigated factors which influence interfacial roughness in InGaAs/GaAs heterostructures and have found that the roughness of the growth front and In segregation are two major factors influencing the interfacial roughness. In addition, we noticed no preferential clustering of indium atoms along the [001] growth direction as previously reported by others. Furthermore, a growth procedure which combines substrate temperature ramping with a growth interruption results in an atomically smooth interface.
Magnetoresistivity and metamagnetism of the Nd33Fe50Al17 alloy
Authors: Ding J. , Si L. , Li Y. , Wang X. Z.
Published: July 2002

A ternary phase has been identified in the rare-earth transition metal Nd–Fe–Al system. This phase has a composition close to Nd5(Fe3Al)12 and is antiferromagnetic with a Neel temperature of approximately 260 K. A clear step appears in magnetization curves of the isotropic ribbon at temperatures below 140 K, indicating metamagnetism. Magnetoresistivity (MR) has been observed in this compound. MR increases with decreasing temperature and is estimated to be 7.2% at 4.2 K. This compound exhibits MR of 1% in the paramagnetic state at room temperature.
Effects of broad-waveguide structure in 0.8 μm high-power InGaAsP/InGaP/AlGaAs lasers
Authors: Hayakawa T. , Wada M. , Yamanaka F. , Asano H. , Kuniyasu T. , Ohgoh T. , Fukunaga T.
Published: July 2002

Systematic study on the effects of the waveguide thickness Wg has been carried out for 200-μm-wide stripe separate-confinement-heterostructure lasers in the range of Wg=0.22–1.2 μm while the width of single quantum well is kept constant at 10 nm. The internal loss αi is reduced from 1.7 to 1 cm−1 when Wg is increased from 0.22 to 1.2 μm. It is shown that αi is not determined by the free-carrier absorption of clad layers, but primarily by Γ, the optical confinement factor, most probably due to scattering at the quantum well/waveguide interfaces. The external differential quantum efficiency ηd monotonically increases with Wg for pulsed operation. By contrast, ηd is maximum at Wg=0.8 μm for continuous-wave (cw) operation. Both the threshold carrier density and the threshold temperature sensitivity increases with Wg for Wg⩾0.8 μm, which decreases ηd in cw operation. When 200-μm-wide devices (20%/97% coated) were life tested at 2 W and 30 °C, the median degradation rate shows a minimal value of 3×10−6 h−1 at Wg=0.8 μm, which is 7 times smaller than that at Wg=0.22 μm. The facet temperature measured by the modulation reflectance is also minimized at Wg=0.8 μm. In broad-waveguide lasers with increasing Wg, the increase in carrier overflow competes with the reduction of optical power density, and thus self-absorption in the quantum well, which determines the optimal Wg.
Use of magnetocrystalline anisotropy in spin-dependent tunneling
Authors: Lukaszew R. A. , Sheng Y. , Uher C. , Clarke R.
Published: July 2002

Epitaxial growth techniques are used to impose in-plane magnetocrystalline anisotropy on a spin-polarized tunneling configuration. A Cu(100) buffer layer grown on a Si(100) substrate stabilizes epitaxial face-centered-cubic cobalt as one of the ferromagnetic electrodes. The negative magnetocrystalline constant of this metastable phase favors easy axes along Co 〈110〉 and, due to the single crystal nature of this layer, the coercivity is more than an order of magnitude larger than in the polycrystalline layers which form the second electrode. Our approach provides a way to access the high degree of spin polarization characteristic of the 3d transition metals.
Film formation by motion control of ionized precursors in electric field
Authors: Adachi Motoaki , Fujimoto Toshiyuki , Nakaso Koichi , Okuyama Kikuo , Shi Frank G. , Sato Hideki , Ando Toshio , Tomioka Hideki
Published: July 2002

A chemical vapor deposition (CVD) method, called ionization CVD, in which ionized source molecules were deposited on a substrate by Coulombic force, was developed to control gas-phase reaction and film morphology. This method was applied to the tetraethylorthosilicate (TEOS)/ozone-atmospheric pressure chemical vapor deposition process by using the surface corona discharge. TEOS/O3 films deposited on SiN and SiO2 films by this CVD method showed good properties for the flow shape, the gap filling and the surface morphology. In Fourier-transform infrared spectra of gas-phase intermediates collected in the vapor condenser, the intensity of the absorption peak at 600 cm−1 was different between ionized intermediates and nonionized intermediates.
Acoustic generation in liquid crystals
Authors: Kim Young Jin , Patel Jay S.
Published: July 2002

In this letter, we report on the acoustic emission from liquid crystals during the electric switching of these materials. The initial observation of the sound was in a bistable nematic liquid crystal. This investigation revealed that a faint, but audible, sound is generated when a liquid crystal is switched from one state to another. We have undertaken a systematic study to investigate the origin of acoustic generation in these materials. By investigating the intensity and frequency of the sound emission, we have concluded that the sound originates as a result of molecular motion, which results in the excitation of the cavity containing the liquid crystal.
Topographic and electronic studies of wedge-shape surface defects on AlGaAs/GaAs films grown on Ge substrates
Authors: Xu Q. , Hsu J. W. P. , Carlin J. A. , Sieg R. M. , Boeckl J. J. , Ringel S. A.
Published: July 2002

Topographic changes and surface contact potential variations near defects on the surface of AlGaAs/GaAs double heterojunction structures grown on Ge substrates are studied using scanning force microscope and electrostatic force microscope. Comparison with transmission electron microscopy results indicates that these surface defects are directly related to stacking faults originated from the GaAs/Ge interface. The surface contact potential inhomogeneities near these defects are consistent with variations in Si dopant concentration.
Electrostatic mechanism for cooling semiconductor heterostructures
Authors: Rego Luis G. C. , Kirczenow George
Published: July 2002

We introduce the physical concepts of an electrostatic mechanism for cooling semiconductor heterostructures. The cooling results from an adiabatic expansion that redistributes electrons among the quasi-two-dimensional subbands of a quantum well, and can be controlled by an external electric field perpendicular to the plane of the well. Under ideal conditions, the final temperature is half of the original and the dynamics of the process is completely universal and reversible. The applicability of this mechanism is verified by self-consistent calculations for AlGaAs/GaAs quantum wells at temperatures below 1 K.
Red electroluminescence from ZnGaS:Mn thin films
Authors: Dimitrova V. , Draeseke A. , Tate J. , Yokoyama T. , Clark B. L. , Keszler D. A.
Published: July 2002

We demonstrate color shifting from the yellow to the red in the electroluminescence from (ZnGa)S:Mn films. We observe threshold voltages down to about 35 V, extremely low for such devices. We discuss the materials characteristics of the phosphor films, and the potential for improvement of the luminous intensity of the devices.
Damage nucleation and vacancy-induced structural transformation in Si grain boundaries
Authors: Maiti A. , Pantelides S. T. , Chisholm M. F. , Pennycook S. J.
Published: July 2002

Atomic resolution Z-contrast scanning transmission electron microscopy reveals preferential nucleation of electron-beam-induced damage in select atomic columns of a Si tilt grain boundary. Atomic scale simulations find that the region of initial damage nucleation corresponds to columns where the formation energies of vacancies and vacancy complexes are very low. The calculations further predict that vacancy accumulation in certain pairs of columns can induce a structural transformation to low-density dislocation “pipes” with all atoms fourfold coordinated.
Characterization of silicon-silicon bonds on the Si(100) surfaces
Authors: Alfonso Dominic R.
Published: July 2002

The bonding nature of the surface atoms in the (2×1) and c(4×2) reconstruction of the Si(100) surface has been characterized using local analysis technique in the context of nonorthogonal tight binding approximation. We demonstrate the capability of this method to yield a real-space picture of the bonding character of the surface atoms for these systems. We also report our analysis of the surface atom bonds on the Si(100) substrate with single-dimer vacancy.
The electronic passivation properties of Si nanoislands on GaAs(110) step defects induced by cleaving
Authors: Teng K. S. , Dunstan P. R. , Wilks S. P. , Williams R. H.
Published: July 2002

Scanning tunneling microscopy and spectroscopy (STS) have been used to investigate submonolayer Si depositions on clean cleaved GaAs(110). These studies focus on the effect of cleaving-induced step defects on GaAs(110), the resultant Si overlayer formation, and the electrical properties of the substrate/overlayer system formed at 280 °C. STS measurements at a clean step edge on GaAs(110) show that the Fermi level shifts towards midgap for both n and p type, while measurements performed on Si nanoislands at step edges show that the Fermi level reverts back towards its “ideal” position. Results indicate the passivation properties of Si on GaAs(110) when deposited under these conditions. Implications for use in laser facets are discussed.
Low-field recombination in SrS:Cu thin-film electroluminescent devices
Authors: Neyts K. , Stuyven G.
Published: July 2002

Thin-film electroluminescent devices with a SrS:Cu phosphor layer emit bright saturated blue light, useful for display applications. Whereas SrS:Ce-based devices normally emit two light pulses for each voltage polarity, the studied SrS:Cu devices yield an additional light peak at lower voltages. Optoelectronic measurements combined with numerical simulations indicate that the third light peak is due to low-field recombination of electrons, which are released from shallow interface states at low fields.
Continuous wave laser radiation at 1314 and 1386 nm and infrared to red self-frequency doubling in nonlinear LaBGeO5Nd3+ crystal
Authors: Capmany J. , Jaque D. , Garcı́a Solé J.
Published: July 2002

We report efficient generation of continuous wave laser radiation in the nonlinear LaBGeO5:Nd3+ crystal around 1.3 μm. The F43/2→I413/2 laser channel of Nd3+ ions in this crystal leads to efficient generation of laser radiation at 1314 nm (π polarization) and at 1386 nm (σ polarization). Under resonant pumping with a Ti:sapphire laser at 800 nm in the GaAlAs laser diode emission spectral region, an absorbed power threshold of 26 mW is obtained for 1% output coupling. A slope efficiency of 40% is obtained for an output coupling of 3%. Visible red laser radiation at 657 nm is obtained by self-frequency doubling in a type I phase matched crystal.
In situ high-resolution transmission electron microscopy of direct bonding processes between silicon tips with oxide surfaces at room temperature
Authors: Kizuka Tokushi , Hosoki Kazue
Published: July 2002

Nanometer-sized silicon (Si) tips were mechanically bonded and retracted by piezo driving inside a high-resolution transmission electron microscope. The process was directly observed in situ at a spatial resolution of 0.2 nm. It was found that crystalline-Si/amorphous-Si oxide/crystalline-Si boundaries were produced by contact at room temperature. The deformation and strength of the boundaries were investigated.
Control of surface states in GaSb/AlxGa1−xAsySb1−y/GaxIn1−xSb/AlxGa1−xAsySb1−y quantum well structures
Authors: Johnstone D. K. , Yeo Y. K. , Hengehold R. L. , Turner G. W.
Published: July 2002

GaSb-based lasers show advanced capabilities over other material for the far-infrared wavelength range. However, for quantum-well (QW) laser structures of AlxGa1−xAsySb1−y/GaxIn1−xAsySb1−y/AlxGa1−xAsySb1−y grown on GaSb, each component element etches at a different rate, making it difficult to achieve quality surfaces sufficiently free of defects. Thus, in this letter, the effects on the surface quality of GaSb-based laser materials have been studied using phosphoric acid etching, boron trichloride reactive ion etching (RIE), and a postetch sulfur treatment. In addition, the quality of the surface is compared for two barriers of AlAs0.07Sb0.93 and Al0.9Ga0.1As0.07Sb0.93, based on current–voltage measurements as a function of temperature. The best surfaces were produced by RIE and by using as much Ga in the barriers as the device operation allows. A generation center in the Ga0.81In0.19As0.12Sb0.88 was found at 0.14 eV for QW diodes with low surface conduction.
Si field-effect transistor with doping dipole in buffer layer
Authors: Wu San Lein , Chang Shoou Jinn
Published: July 2002

An Si field-effect transistor with a doping dipole layer obtained by delta-n (δn) layer and delta-p (δp) layers in the buffer layer is proposed and fabricated. The dipole creates a large barrier between the channel and the buffer layer, and consequently more holes are confined in the upper undoped region (the channel layer) rather than around the δp layer by the influence of the transverse electric field coming from the δn barrier layer. This simple homoepitaxial grown Si structure exhibits an excellent property, i.e., not only high hole mobility (1350 cm2 V−1 s−1) but also enhanced extrinsic transconductance (50 mS/mm), which is expected to provide an additional degree of freedom for Si-based device applications.
Elimination of transient vertical-cavity surface-emitting laser oscillations using photoactive feedback
Authors: Riyopoulos Spilios
Published: July 2002

It is demonstrated numerically that photoactive layers placed in a vertical-cavity surface-emitting laser (VCSEL) cavity eliminate the generic transient oscillations caused by the increasing rate of carrier depletion with increasing photon flux ∂(dN/dt)/∂P<0. Reversing the sign ∂(dN/dt)/∂P>0, using appropriately chosen photoactive layers, makes the cavity relaxation frequency purely imaginary; the laser asymptotes to steady state without modulation as an overdamped oscillator. The sign reversal depends mainly on the layer doping parameters, but not on the laser power, thus suppression holds over the laser operation range.
Limiting current from a metallic ideal edge attached to a dielectric edge
Authors: Schächter Levi , Nation John A.
Published: July 2002

An analytic expression for the limiting current in the vicinity of metallic and dielectric edges is developed. It is shown that the basic V3/2 law remains valid and, for large dielectric coefficients (εr>5), the average limiting current density is not explicitly dependent on the angle of the metallic edge. Moreover, the limiting current depends primarily on the typical curvature of the emitter and not only on the diode gap. In general, the field curvature is reduced by the space charge and, in many cases, the singularity associated with the edge under vacuum conditions is eliminated. The azimuthal curvature parameter is identical to that in a planar configuration (p=4/3), whereas for the radial curvature parameter, an analytic expression is developed.
Strain-imaging observation of the polarization freezing of the domains under the electrode of a Pb(Zr, Ti)O3 film
Authors: Takata K. , Miki H. , Torii K. , Kushida-Abdelghafar K. , Fujisaki Y.
Published: July 2002

Strain imaging is used to measure and image piezoelectric properties by detecting electric-field-induced strains using a scanning probe microscope. In this letter, we present ferroelectric domain imaging under an electrode in a lead-zirconate-titanate (PZT) film. The imaging has been achieved by detecting the surface force modulation caused by the electrode displacements. Observation through the electrode suppresses the effect of space charges and enables us to investigate the PZT film in the same situation in devices. We observed the freezing of polarization of a PZT film deoxidized by atomic hydrogen produced by catalysis of platinum upper electrodes due to hydrogen annealing.
Conduction and mechanical properties of atomic scale gold contacts
Authors: Jarvis S. P. , Lantz M. A. , Ogiso H. , Tokumoto H. , Dürig U.
Published: July 2002

Using a dynamic atomic force microscopy (AFM) technique in ultrahigh vacuum, we have directly measured the stiffness and conduction characteristics of a gold tip and sample. The method involves the application of a small sinusoidal oscillating force to the tip at a frequency well below the primary resonance frequency of the cantilever. By measuring the change in amplitude during the approach and retraction of the sample we have a continuous and accurate measure of the contact stiffness. The high sensitivity of this technique has enabled us to measure the mechanical properties of the junction during its initial formation. The most interesting observations are made in the region of initial contact formation where it is not possible to obtain high mechanical sensitivity from the commonly used static force measurement technique. In this region, as the contact is compressed, the contact softens continuously while the conductance remains constant prior to discrete conductance jumps. These are accompanied by simultaneous jumps in stiffness, as predicted by molecular dynamic simulations. Furthermore, the jumps show a strong tendency to half integer values of the conductance quantum.