bluuebunny/crossref_metadata_2025 huggingface dataset:
A probe for the investigation of the superconducting metastable state in YBa2Cu3O7−x step-edge junctions
Authors: Barbanera S. , Castellano M. G. , Torrioli G. , Cirillo M.
Published: July 2002

We have investigated the pair tunneling process in YBa2Cu3O7−x step-edge Josephson junctions by studying the lifetime of the metastable zero-voltage state determined by the thermal activation out of the potential well. The junctions have a hysteretic current–voltage characteristic at 4.2 K, which allows the recording of transitions and switching distribution from the Josephson current to the resistive state. By applying an external magnetic field, we have found a strong dependence of the activation energy on the field intensity. Striking similarities with analogous measurements performed on low Tc conventional tunnel junctions suggest that the high Tc counterparts can be described by a standard model for long junctions.
Ultrathin pseudomorphic Sn/Si and SnxSi1−x/Si heterostructures
Authors: Min Kyu Sung , Atwater Harry A.
Published: July 2002

Ultrathin, coherently strained Sn/Si and SnxSi1−x/Si alloy quantum well structures with substitutional Sn incorporation far in excess of the equilibrium solubility limit have been fabricated via substrate temperature and growth flux modulations in molecular beam epitaxy. Sn/Si single and multiple quantum wells with Sn coverage up to 1.3 ML, Sn0.05Si0.95/Si multiple quantum wells of up to 2.0 nm, and Sn0.16Si0.84/Si multiple quantum wells of up to 1.1 nm are determined to be pseudomorphic, and coverage-dependent Sn segregation dynamics are observed.
Electron field emission from SiC/Si heterostructures synthesized by carbon implantation using a metal vapor vacuum arc ion source
Authors: Chen Dihu , Wong S. P. , Cheung W. Y. , Wu W. , Luo E. Z. , Xu J. B. , Wilson I. H. , Kwok R. W. M.
Published: July 2002

A remarkably low turn-on field of about 1 V/μm has been observed in electron field emission from planar SiC/Si heterostructures formed by high dose C implantation into Si using a metal vapor vacuum arc ion source. An implant energy of 35 keV was used to a dose of 1.0×1018 ions/cm2 with subsequent annealing in nitrogen at 1200 °C for 2 h. X-ray photoelectron spectroscopy showed that a thin surface stoichiometric SiC layer, with a thickness of about 150 nm, had been formed. Atomic force microscopy showed that there are densely distributed small protrusions formed on the surface. The formation of a thin surface stoichiometric SiC layer and the formation of densely distributed small protrusions on the surface are believed to be the two factors responsible for the efficient electron field emission.
Analog measurements of long wavelength vertical-cavity lasers
Authors: Wesselmann Jon R. , Margalit Near M. , Bowers John E.
Published: July 2002

We report the measurements of nonlinearity effects on analog transmission of long wavelength vertical-cavity surface-emitting lasers. The experiments involved measuring single-mode fiber-optic transmissions of microwave signals (100–1000 MHz) at 1.54 μm to characterize the dynamic range, intermodulation distortion and frequency response. Spur free dynamic ranges up to 69 dB were demonstrated.
Angular dependence of the reflectance and transmittance of CdS films formed by laser ablation
Authors: Dushkina N. M. , Ullrich B.
Published: July 2002

The dependence of reflectance and transmittance on the angle of incidence and the polarization of visible light was measured for thin (≈2 μm) CdS films formed by laser ablation. The experiments were carried out at 300 K with argon and He–Ne lasers. In the blue and green spectral ranges, the experiments were described straightforwardly by the theoretical approach for highly absorptive materials, omitting the film thickness. For red light, the sample represents dielectric material and multiple reflections must be considered to fit the experiments. Furthermore, it is shown that the angular-dependent features of red light are an accurate tool to determine the film thickness.
Flashlamp pumped polymer dye laser containing Rhodamine 6G
Authors: Finlayson A. J. , Peters N. , Kolinsky P. V. , Venner M. R. W.
Published: July 2002

Polymeric rods incorporating the laser dye Rhodamine 6G in a copolymer of hydroxypropyl acrylate and methylmethacrylate have been shown to lase under flashlamp excitation. The rods exhibit high bulk transmission levels, coupled with positive lensing characteristics, that have enabled output energies up to 354 mJ/pulse with a 0.14% energy efficiency to be demonstrated in a preliminary study.
Minimum temperature sensitivity of 1.55 μm vertical-cavity lasers at −30 nm gain offset
Authors: Piprek J. , Akulova Y. A. , Babic D. I. , Coldren L. A. , Bowers J. E.
Published: July 2002

Double-fused vertical-cavity surface-emitting lasers (VCSELs) have demonstrated the highest temperature performance of any 1.5 μm VCSEL, but further optimization is needed to reduce their temperature sensitivity. We present and analyze threshold current measurements of these devices between −90 °C and 30 °C stage temperature. Despite a zero gain peak offset from the emission wavelength at room temperature, the pulsed threshold current has its minimum near −50 °C corresponding to about −30 nm gain offset. This is in contrast to a common VCSEL design rule. Temperature effects on the optical gain of the strain-compensated InGaAsP/InP active region are found to be the main cause for the disagreement. A design rule modification is proposed. Numerical simulation of an optimized 1.55 μm VCSEL shows that gain offset improvements are counteracted by loss mechanisms.
Wavelength dependence of carrier-type in reduced BaTiO3:Rh
Authors: Chang J. Y. , Chinjen C. R. , Duan S. H. , Huang C. Y. , Sun C. C.
Published: July 2002

We report that in BaTiO3:Rh the carrier-type depends not only on the postprocessing condition but also on the incident wavelength in two beam coupling. Carriers are the holes for the as-grown sample, and change to electrons for the sample reduced in the atmosphere of 10−14 atm oxygen partial pressure. However, for the sample reduced in the atmosphere of 10−10 atm oxygen partial pressure, the carrier is an electron for the incident wavelength of 514 nm, and a hole for 633 nm. Its absorption, photoinduced absorption, and two beam coupling are investigated and discussed.
High-finesse AlxOy/AlGaAs nonabsorbing optical cavity
Authors: Shin Hyun-Eoi , Ju Young-Gu , Song Hyun-Woo , Song Dae-Sung , Han Il-Young , Ser Jung-Hoon , Ryu Han-Youl , Lee Yong-Hee , Park Hyo-Hoon
Published: July 2002

We report the measured finesse value of ∼390 in nonabsorbing AlxOy/AlGaAs cavities. The nonabsorbing cavity consisting of a bottom AlxOy/AlGaAs distributed Bragg reflector (DBR), an AlxOy spacer layer, and a top AlxOy/AlGaAs DBR is prepared by a wet-oxidation process. The measured resonance linewidth agrees well with that of calculation, indicating very small overall losses in the cavity. The wet-oxidation process does not seem to degrade the interface of the epitaxial layers significantly. The lower bound of maximum achievable reflectivity from the AlxOy DBR is estimated to be >99.95%, assuming an average interface roughness of about 0.6 nm. The maximum achievable finesse of this type of cavity is expected to be larger than that of the all-epitaxial counterpart assuming the same roughness.
Transient photocurrent spectroscopy: Electrical detection of optical absorption for supported semiconductor nanocrystals in a simple device geometry
Authors: Erley G. , Gorer S. , Penner R. M.
Published: July 2002

The optical absorption of supported cadmium sulfide nanocrystals is directly detected as an electrical signal using a method described in this letter. Cadmium sulfide nanocrystals were epitaxially deposited onto highly oriented, pyrolitic graphite surfaces using a hybrid electrochemical/chemical synthesis method. The resulting submonolayer of cadmium sulfide nanocrystals was then covered with a thin (10–100 nm) insulating Formvar layer and a semitransparent gold top electrode. We exposed this structure to monochromatic chopped light and detected the transient photocurrent phase-sensitively and without applied bias. In the resulting device spectra, the absorption edge of the cadmium sulfide nanocrystals was clearly discernible at room temperature. The device response spectrum is compared with photoluminescence spectra acquired for the quantum dots in these devices at 20 K.
Improved performance of a wavelength converter based on dual pump four-wave mixing in a bulk semiconductor optical amplifier
Authors: Tomkos I. , Zacharopoulos I. , Syvridis D. , Sphicopoulos T. , Caroubalos C. , Roditi E.
Published: July 2002

Very high conversion efficiency (6.5 dB for 23.4 nm down-wavelength shift and 4.5 dB for 16 nm up-wavelength shift) is reported, using four-wave mixing in a bulk semiconductor optical amplifier. Based on a dual pump configuration where wavelength conversion is achieved by shifting one of the pumps, superior performance has been achieved in terms of both efficiency and the converted signal-to-background amplified spontaneous emission noise ratio, especially for very large wavelength shifts. Simple theoretical considerations are also discussed for the explanation of the improved performance of the converter.
Thermally activated electron capture by mobile protons in SiO2 thin films
Authors: Vanheusden K. , Karna S. P. , Pugh R. D. , Warren W. L. , Fleetwood D. M. , Devine R. A. B. , Edwards A. H.
Published: July 2002

The annihilation of mobile protons in thin SiO2 films by capture of ultraviolet-excited electrons has been analyzed for temperatures between 77 and 500 K. We observe a strong increase in proton annihilation with increasing temperature, and derive an activation energy for electron capture of about 0.2 eV. Based on quantum chemical [(OH)3Si]2–O–H+ cluster calculations, we suggest photoexcitation of electrons from excited vibrational states of the ground electronic (valence band) state to a nearby excited electronic (SiO2 gap) state. It is argued that the latter excitation can result in H0 formation at elevated temperatures.
Quantitative determination of the local Kerr rotation by scanning near-field magneto-optic microscopy
Authors: Fumagalli P. , Rosenberger A. , Eggers G. , Münnemann A. , Held N. , Güntherodt G.
Published: July 2002

By using a polarization-modulation technique in combination with reflection-mode scanning near-field optical microscopy we are able to determine quantitatively the local polar Kerr rotation on thin magnetic films. As an example, the magneto-optic contrast of magnetic domains thermomagnetically written on a Co/Pt sample is discussed. The size of the magnetic domains is 3×0.5 μm2 and the angular resolution of the local Kerr rotation is better than 0.1°. This measurement technique will make it possible to investigate, e.g., the fine structure of domain walls by scanning near-field optical microscopy.
GaN microcavities: Giant Rabi splitting and optical anisotropy
Authors: Kavokin Alexey , Gil Bernard
Published: July 2002

Numerical simulation of light reflection from a λ/2 GaN microcavity with Ga0.8Al0.2N/Ga0.5Al0.5N Bragg mirrors grown on the A surface of Al2O3 revealed a Rabi splitting of the order of 50 meV and remarkable optical anisotropy. These effects are originated from the giant exciton oscillator strength in GaN and a pronounced uniaxial strain in the structure.
Electrical properties of PdGe ohmic contacts to GaAs/AlxGa1−xAs heterostructures at liquid helium temperature
Authors: Morozov S. V. , Dubrovskii Yu. V. , Abrosimova V. N. , Würfl J.
Published: July 2002

Nonlinear features in the electrical behavior of PdGe-based sintered ohmic contacts to the GaAs/AlxGa1−xAs heterostructures with two-dimensional electron gas (2DEG) were investigated both in magnetic field and at the liquid helium temperature. It was shown that current flow under the contacts has multilayer structure. The 2DEG exists under contact metallization with the decreased electron concentration as compared with the 2DEG under uncovered surface. The main contribution to the resistance between the metallization and the 2DEG is due to the heterojunction between the cap layer and the 2DEG and has tunneling nature.
Experimental evidence of photoinduced expansion in hydrogenated amorphous silicon using bending detected optical lever method
Authors: Gotoh T. , Nonomura S. , Nishio M. , Nitta S. , Kondo M. , Matsuda A.
Published: July 2002

Photoinduced structural change in hydrogenated amorphous silicon (a-Si:H) has been studied by a sensitive bending detection method using an optical lever. We observed that a-Si:H films show not only thermal expansion due to a photothermal effect but also residual and persistent expansion after light soaking. The volume change is recovered by thermal annealing at 200 °C. A dehydrogenated sample annealed at 550 °C and a microcrystalline sample, in which photoinduced defects are not created, show little photoinduced expansion. The photoinduced expansion and photoinduced defect density show identical time evolution. These results suggest that the photoinduced expansion is related to the photoinduced defect creation. A quantitative evaluation of the photoinduced expansion indicates that the photoinduced structural change is spread over several molecular volumes around a photocreated defect.
Electrical characterization of hole traps in p-type ZnSe and ZnSSe grown by molecular beam epitaxy
Authors: Seghier D. , Hauksson I. S. , Gislason H. P. , Brownlie G. D. , Prior K. A. , Cavenett B. C.
Published: July 2002

Using deep-level transient spectroscopy (DLTS) and admittance spectroscopy we investigated nitrogen doped ZnSe and ZnSSe layers grown on p-type GaAs substrates by molecular beam epitaxy. Three major hole traps denoted T1, T2, and T3 were observed with energy levels at 0.11, 0.46, and 0.56 eV from the valence band, respectively. Similar energy levels were observed in nitrogen doped ZnSSe except that T1 was at 0.12 eV from the valence band. A crude estimation of the 0.11 eV trap concentration obtained from DLTS data shows correlation with the free carrier concentration due to nitrogen. We attribute T1 to a nitrogen acceptor which controls the p-type conduction in the materials. No other direct observations of this important acceptor level have been reported in the literature so far. The two remaining levels may originate from the nitrogen doping process. We also point out the effect of the series resistance observed in this kind of material.
Atomic force measurement of low-frequency dielectric noise
Authors: Walther L. E. , Vidal Russell E. , Israeloff N. E. , Gomariz H. Alvarez
Published: July 2002

Using noncontact scanning probe microscopy techniques, dielectric properties were studied on 50-nm-length scales in poly-vinyl-acetate (PVAc) and poly-methyl-methacrylate films. Low-frequency (1/f ) fluctuations observed in the measurements, peaked in intensity near the glass transition temperature in PVAc. The noise is shown to arise from thermal dielectric polarization fluctuations. Analysis of this noise provides a noninvasive method of probing equilibrium nanometer-scale dynamical processes in dielectric materials and devices.
Dynamics of infrared absorption caused by hydroxyl groups and its effect on refractive index evolution in ultraviolet exposed hydrogen loaded GeO2-doped fibers
Authors: Araújo F. M. , Joanni E. , Marques M. B. , Okhotnikov O. G.
Published: July 2002

The growth dynamics of UV induced IR absorption and related refractive index change in hydrogen loaded GeO2-doped fibers have been studied. We report a higher initial rate and strong saturation for Ge–OH generation compared with Si–OH formation under UV exposure. A close correlation was found between the Ge/Si–OH groups concentration and the induced index change as a function of the UV exposure time.
Nondestructive investigation of microstructures and defects at a SrTiO3 bicrystal boundary
Authors: Jiang Q. D. , Huang Z. J. , Brazdeikis A. , Dezaneti M. , Chen C. L. , Jin P. , Chu C. W.
Published: July 2002

The effects of thermal annealing on the microstructure at the grain boundary of a 36.8° symmetric [100] tilt SrTiO3 bicrystal were studied. Scanning tunneling microscopy and atomic force microscopy were used for nondestructive observation of the boundary structures. Annealing the bicrystalline substrates at temperatures as low as 780 °C led to the formation of grooves at their boundaries. This provides direct evidence that the thickness depression of YBa2Cu3O7−δ films at the bicrystal boundaries originates from the underlying grooved substrates. Defects characterized as holes with diameters ranging from ∼30 nm to ∼200 nm were also observed.
Coupled-power theory of nonlinear distributed-feedback lasers, yielding reduced longitudinal spatial hole burning
Authors: Wang Jing-Yi , Cada Michael , Makino Toshihiko
Published: July 2002

An analysis of the nonlinear distributed-feedback (DFB) laser action is presented. A set of nonlinear coupled-power equations is derived. A general condition for a nonlinear DFB laser with reduced spatial hole burning (SHB) is obtained. It is shown that the elimination of SHB in DFB lasers can be achieved by introducing properly-chosen variations of the coupling strength along the longitudinal direction; a novel complex-coupling AR-coated DFB laser diode is proposed as a result. It is also shown that the optical nonlinearity must be considered for the design of high-power laser structures with reduced SHB.
Catastrophic optical damage in GaInN multiple quantum wells
Authors: Cohen Daniel A. , Margalith Tal , Abare Amber C. , Mack Michael P. , Coldren L. A. , DenBaars S. P. , Clarke David R.
Published: July 2002

We have observed progressive damage due to reabsorption of stimulated emission in optically pumped laser-quality GaInN–GaN multiple quantum wells. The degradation occurred on a time scale consistent with the lifetime of electrically pumped lasers incorporating the same active region, suggesting that the failure mechanism was in part catastrophic optical damage, and not just heating in the p contact and p cladding as is often assumed.
Long rectangular islands of β-Ga2O3 on CoGa(001)—studied by electron energy loss spectroscopy and scanning tunneling microscopy
Authors: Eumann M. , Schmitz G. , Franchy R.
Published: July 2002

At 700 K, the oxidation with 1 L O2 of CoGa(001) was investigated by means of high-resolution electron energy loss spectroscopy (EELS), low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy (STM). Oxidation with 1 L O2 at 700 K leads to the formation of long, rectangular islands of β-Ga2O3 oriented in the [100] and [010] directions of the substrate. EEL spectra of the islands of β-Ga2O3 show intense Fuchs–Kliewer (FK) modes at 305, 455, 645, and 780 cm−1. The β-Ga2O3 islands are well ordered and show a (2×1) structure with two domains, oriented perpendicular to each other. The two-dimensional lattice parameters of β-Ga2O3 are determined to be a=2.8±0.1 Å and b=5.8±0.1 Å.
Small signal electron charge centroid model for quantization of inversion layer in a metal-on-insulator field-effect transistor
Authors: King Ya-Chin , Hu Chenming , Fujioka Hiroshi , Kamohara Shiroo
Published: July 2002

A simulator using a coupled Schrödinger equation, Poisson equation and Fermi–Dirac statistics to analyze inversion layer quantization is shown to match the measured capacitance versus voltage data of thin oxide gate metal-on-insulator capacitance closely. The effects of bias voltage, oxide thickness and doping concentration on the charge centroid are presented. A simple empirical model for the alternating current charge centroid of the inversion layer is proposed. This model predicts the in-version layer capacitance or charge centroid in terms of Tox (oxide thickness), Vt (threshold voltage), and Vg (gate voltage) explicitly.
Origin of the charge to breakdown distributions in thin silicon dioxide films
Authors: Okhonin S. , Fazan P.
Published: July 2002

This letter shows that in thin silicon dioxide films the charge to breakdown distribution can have two origins. It can be related to the oxide thickness variation across the wafer or to the statistical nature of the breakdown event. The oxide nonuniformity is a major factor in the case of stress-induced-bulk-charge enhanced breakdown.
Directionally sensitive photorefractive interferometric line receiver for ultrasound detection on rough surfaces
Authors: Tuovinen Hemmo , Krishnaswamy Sridhar
Published: July 2002

An adaptive heterodyne interferometer is configured as a line receiver using wave mixing in photorefractive bismuth silicate crystal. The line receiver is directionally most sensitive to ultrasound impinging normal to the line, and is significantly less sensitive to ultrasound impinging in other directions. Such a system is attractive in situations where the ultrasonic scatter from a specific direction is to be selectively pulled out in the presence of scatter from other “noise” sources. It is also demonstrated that the line probe system can be used to bridge the sensitivity gap that optical detection thus far has suffered vis-à-vis piezoelectric detection.
Linear and nonlinear optical properties of stepped InxGa1−xAs/GaAs quantum wells
Authors: Tomassini N. , D’Andrea A. , Righini M. , Selci S. , Calcagnile L. , Cingolani R. , Schiumarini D. , Simeone M. G.
Published: July 2002

Systematic studies of polaritons in single stepped quantum wells are performed, and the presence of forbidden transitions in optical spectra are taken as a fingerprint of nonhomogeneous indium concentration. The large intensities of such transitions for well thicknesses in the range of quasi-two-dimensional exciton behavior are pointed out. Finally, second harmonic generation excited at half gap is found to be enhanced with respect to the bulk contribution as suggested by the theory.
Intracavity white-light continuum generation in a femtosecond Ti:sapphire oscillator
Authors: Likforman J.-P. , Alexandrou A. , Joffre M.
Published: July 2002

We report on intracavity white-light continuum generation in a Ti:sapphire oscillator at an average repetition rate of 200 kHz. A spectrally resolved pump-probe experiment is performed in order to demonstrate the potentialities of these continuum femtosecond pulses.
Surface dynamics studied by perturbing the surface with the tip of a scanning tunneling microscope—Si(100) at 80 K
Authors: Hata K. , Ishida M. , Miyake K. , Shigekawa H.
Published: July 2002

We have utilized the tip of a scanning tunneling microscope to perturb a specific local structure (the C defect) of Si(100) at 80 K, and observed the dynamical symmetric⇔buckled transition of the surrounding dimers. The observed large-scale transition implies that the configuration of the dimers is determined by a detailed balance among many elastic long-range forces generated by the surrounding C defects.
Strain effects in lattice-mismatched InxGa1−xAs/InyAl1−yAs coupled double quantum wells
Authors: Kim T. W. , Jung M. , Lee D. U. , Lim Y. S. , Lee J. Y.
Published: July 2002

Transmission electron microscopy (TEM) and Raman scattering spectroscopy measurements were performed to investigate strain effects in lattice-mismatched InxGa1−xAs/InyAl1−yAs modulation-doped coupled double quantum wells. The high-resolution TEM images showed that a 100-Å In0.8Ga0.2As deep quantum well and a 100-Å In0.53Ga0.47As shallow quantum well were separated by a 30-Å In0.25Ga0.75As embedded potential barrier. The selected-area electron-diffraction pattern obtained from TEM measurements on the InxGa1−xAs/InyAl1−yAs double quantum well showed that the InxGa1−xAs active layers were grown pseudomorphologically on the InP buffer layer. The values of the strain and the stress of the InxGa1−xAs layers were determined from the electron-diffraction pattern. Based on the TEM results, a possible crystal structure for the InxGa1−xAs/InyAl1−yAs coupled double quantum well is presented.
Scattering rates due to lineal dislocations in heterostructures for the Monte Carlo charge transport simulation
Authors: Abou-Khalil Michel , Matsui Toshiaki , Bougrioua Zahia , Maciejko Roman , Wu Ke , Wu K. , Maciejko R. , Bougrioua Z.
Published: July 2002

We derived expressions for the scattering rates due to dislocations in pseudomorphic heterostructures. We gave explicit formulas for those scattering rates in the free and the confined states, depending on the initial carrier energy and on the component of the total wave vector parallel to the dislocations. We demonstrated that this new scattering process could affect the electron mean velocity by more than 30% from the ideal case treated commonly by the Monte Carlo simulators.
Quantum transmission in narrow ballistic wires containing a superconductor island
Authors: Takagaki Y. , Ploog K. H.
Published: July 2002

We investigate the transmission properties in quantum wires with a superconducting island placed in the center of the channel. The conductance quantization is destroyed, unless the Fermi energies in the quantum wire and the superconductor are identical as a consequence of the single-particle reflection from the normal-conductor–superconductor interface. The conductance exhibits fluctuations due to the quantum interference effect in the island. The fluctuation amplitude is found to depend strongly on the pair potential amplitude in the superconductor.
Excitonic luminescence in CuInSe2
Authors: Schön J. H. , Bucher E.
Published: July 2002

Band-edge luminescence of CuInSe2 single crystals was studied in the temperature region between 2 and 300 K. Sharp emission lines were attributed to the decay of free and bound excitons and their phonon replica. Accurate analysis of the peak position revealed values of 4.4 meV and 1.044 eV for the binding energy of the free exciton and the band gap at 2 K, respectively. Investigations on n- and p-type material showed different emission lines, which are ascribed to excitons bound to the different dominating intrinsic defects in these materials. Furthermore, the results are compared to thin-film studies.
The effect of surface segregation on the light-emission intensity of Si/SiGe/Si heterostructures
Authors: Kimura Yoshinobu , Nakagawa Kiyokazu , Miyao Masanobu
Published: July 2002

The correlation between the light-emission intensity from SiGe mixed crystals and Ge surface segregation during molecular beam epitaxy has been investigated. Atomic-hydrogen-assisted molecular beam epitaxy was used to vary the surface-segregation length of Ge. Results show that the photoluminescence (PL) intensity was very strong in the region where the surface-segregation length was less than 7 nm. However, when the surface-segregation length exceeded 7 nm, the PL intensity decreased sharply. A one-to-one correspondence between the PL intensity and the Ge segregation length was obtained. This is attributed to the dependence of the surface segregation on the degree of randomness in the SiGe alloy.
Optimized ZnSe:N/ZnTe:N contact structure of ZnSe-based II–VI laser diodes
Authors: Kijima Satoru , Okuyama Hiroyuki , Sanaka Yumi , Kobayashi Takashi , Tomiya Shigetaka , Ishibashi Akira
Published: July 2002

We have established the basic stability of ZnSe:N/ZnTe:N superlattice (SL) contact needed for 1000 h of operation. We have fabricated ZnCdSe/ZnSSe/ZnMgSSe separated confinement heterostructure laser diodes with the ZnTe:N contact layer/ZnSe:N/ZnTe:N SL/ZnSe:N cap layer to achieve a low voltage (<5 V) at 500 A cm−2 constant current for 1000 h (electrode test) by the reduction of the thickness of the ZnTe:N contact layer and by the optimization of the N concentration in the ZnSe:N cap layer. We found that, when the thickness of the ZnTe:N contact layer becomes 4 nm, the reduction of NA–ND in the ZnSe:N cap layer is prevented, and the defect density in the vicinity of the ZnSe:N/ZnTe:N SL is reduced. It is assumed that the reduction of NA–ND in ZnSe:N is caused by the stress in the ZnSe:N cap layer induced by a large lattice mismatch between ZnTe and ZnSe. We confirmed that the reduction of stress in the ZnSe:N layer and the reduction of the structural defect density were achieved by the reduction of the thickness of ZnTe to 4 nm.
Electronic properties of GaAs surfaces etched in an electron cyclotron resonance source and chemically passivated using P2S5
Authors: Glembocki O. J. , Tuchman J. A. , Dagata J. A. , Ko K. K. , Pang S. W. , Stutz C. E.
Published: July 2002

Photoreflectance has been used to study the electronic properties of (100) GaAs surfaces exposed to a Cl2/Ar plasma generated by an electron cyclotron resonance source and subsequently passivated by P2S5. The plasma etch shifts the Fermi level of p-GaAs from near the valence band to midgap, but has no effect on n-GaAs. For ion energies below 250 eV, post-etch P2S5 chemical passivation removes the surface etch damage and restores the electronic properties to pre-etch conditions. Above 250 eV, the etch produces subsurface defects which cannot be chemically passivated. Auger electron spectroscopy shows that etching increases As at the GaAs/oxide interface, while passivation reduces it.
High field behavior of artificially engineered boundaries in melt-processed YBa2Cu3O7−δ
Authors: Doyle R. A. , Bradley A. D. , Lo W. , Cardwell D. A. , Campbell A. M. , Vanderbemden Ph. , Cloots R.
Published: July 2002

Artificial bulk “zero-angle” boundaries parallel to the c axis have been engineered between large melt-processed YBa2Cu3O7−δ (YBCO) grains and observed to carry a transport supercurrent at fields up to at least 5 T at 77 K. The temperature and angular dependencies of the boundary resistance have exactly the same form as those of the grains, which is evidence that the grains are intimately coupled. The limiting mechanism for current transfer across these boundaries is, therefore, not a simple weak link or Josephson effect. This joining technique is extremely promising for production of macroscopic engineering artifacts.
Gating of InAs/GaSb quantum wells using a silicon monoxide gate insulator
Authors: Rahman F. , Gallagher B. L. , Behet M. , De Boeck J.
Published: July 2002

We report on a technique we have recently developed to fabricate very high quality gates and gated structures on InAs/AlxGa1−xSb quantum wells. The low thermal budget process leads to highly stable gates with extremely low leakage currents. Both electron and hole concentrations can be changed over a wide range by the application of modest gate voltages. We obtain a dn/dV value of 5×1011 cm2/V for electrons and 1.6×1012 cm2/V for holes at 1.2 K.
High conductance small area magnetoresistive tunnel junctions
Authors: Wong P. K. , Evetts J. E. , Blamire M. G.
Published: July 2002

Fe–Al 2 O 3 –CoFe mesa junctions with micron scale areas have been fabricated from dc sputtered whole wafer heterostructures by standard photolithographic processing. The junctions showed magnetoresistances (JMRs) of 6.2% at room temperature and 9.2% at 77 K. Resistance-area (RA) products as small as 9.6×10−10 Ω m2, suitable for device applications, were obtained. A JMR/RA ratio of 6.5×109% Ω−1 m−2 was also achieved. The fabrication processes we used provide a useful basis for manufacturing ferromagnetic tunnel devices for MR sensor applications.
Radio frequency photoinjector using LaB6 cathode and a nitrogen drive laser
Authors: O’Shea P. G. , Lancaster J. A. , Sachtschale R. , Jones C. R.
Published: July 2002

We report the successful operation of an inexpensive, simple, and reliable 2.856 GHz radio frequency photoinjector using a rugged LaB6 cathode and a nitrogen drive laser operating at a wavelength of 337 nm. The cathode was operated at a vacuum of ≈10−8 Torr, and produced excellent beam quality. The device produces a 1 ns long pulse containing 0.1 nC of charge. The photoelectrons have been accelerated to 270 MeV in traveling-wave linear accelerator.
Epitaxial structuring of blue light-emitting p-phenylene oligomers
Authors: Mikami Tatsuo , Yanagi Hisao
Published: July 2002

Epitaxial structuring of blue light-emitting molecules, p-phenylene oligomers (p-nP, n=3,4,5,6), was investigated in thin crystals vapor deposited on the (001) surface of a KCl single crystal. Their molecular orientation and morphology changed depending on the molecular length and substrate temperatures during deposition. As the length of p-nP was longer and the substrate temperature was lower, the orientation of the molecular axis changed from standing to lying on the KCl surface due to an increase of molecule–substrate interactions. p-terphenyl (3P) hardly adsorbed on the KCl (001) surface, but a portion of the standing molecules stuck along the step edge of KCl and formed a walllike structure. Long needlelike crystals of p-hexaphenyl (6P) epitaxially grew in two orthogonal KCl [110] directions taking lying orientation even at a higher substrate temperature. In these epitaxial needles, the transition dipoles parallel to the molecular axis gave rise to blue fluorescence sharply polarized along the KCl [110] directions.
Distance measurements using a noisy white light interferometer
Authors: Ben-Chorin Moshe , Chuartzman Silvia , Prior Yehiam
Published: July 2002

We describe a method for interferometric distance measurements in the presence of phase noise. The method is based on the beating between white light and a reference beam that travel along the same path through the interferometer. Since both the reference and the white light suffer the same phase noise, the envelope of the high frequency fringes is not affected by the noise. By measuring the signal variance, we recover the envelope while averaging out the high frequency fringes. We demonstrate the usefulness of the method for surface profilometers.
Voltage-controlled lasing pixels for projection display
Authors: Firehammer Joel A. , Crawford Gregory P. , Lawandy Nabil M.
Published: July 2002

A lasing pixel device that implements a low-voltage addressable spatially patterned variable loss element placed inside an optically pumped high-gain laser cavity is studied experimentally. A polymer dispersed liquid crystal (PDLC) was used as the loss element. It was found to have high output (600 lumens/cm2 at the device faceplate), very high on-screen contrast (1000:1), and narrow spectral linewidth (∼3 nm), which make it potentially useful for a nonscanned laser projection display. A grayscale model of pixel output versus element loss was developed and compared favorably to measurements.
Bond-orientational order in sheared dense flows of inelastic hard spheres
Authors: Zamankhan Piroz , Polashenski William , Vahedi Tafreshi Hooman , Sarkomaa Pertti J. , Hyndman Caroline L.
Published: July 2002

Three-dimensional bond-orientational order is studied using computer simulations with 4296 hard, monodisperse inelastic spheres flowing in a Couette geometry at a high shear rate. At an average volume fraction close to 0.6, a state with extended correlations in the orientations of particle clusters starts to develop for rough particles after sufficiently long run times. However, no clear evidence of crystallization is found in the system. Further tests of a sheared system comprised of smooth, inelastic spheres reveal crystallization consistent with the previous experimental observations.
Optical creation of a metastable two-dimensional electron gas in a ZnSe/BeTe quantum structure
Authors: Lovisa S. , Cox R. T. , Baron T. , Keim M. , Waag A. , Landwehr G.
Published: July 2002

A microstructure has been designed in which an electron gas is created optically with variable density. ZnSe and BeTe quantum wells are separated by a BeZnMgSe alloy with band gap 3 eV. When electron-hole pairs are created by ultraviolet light in the barriers, electrons accumulate in the ZnSe quantum well, and holes in the BeTe quantum well. This is because the BeTe valence band lies 900 meV higher than that of ZnSe. Reflectivity and luminescence spectra of the ZnSe well show the destabilization of excitons by an electron gas and formation of trions. The metastable charge-separated state has a very long lifetime (2 s at 2 K).
Carbon-induced germanium dots: Kinetically-limited islanding process prevents coherent vertical alignment
Authors: Schmidt O. G. , Schieker S. , Eberl K. , Kienzle O. , Ernst F.
Published: July 2002

Pregrowth of a small amount of C on a Si(001) substrate induces small Ge quantum dots. We present a structure where an initial layer of these C-induced Ge dots is followed by five layers of four monolayers Ge, each layer separated by a 2 nm thick Si spacer. Although the critical thickness for planar growth of a single Ge layer is not exceeded, a vertically aligned stack of Ge islands is formed. If we substitute the pure Ge layers by a fivefold stack of C-induced Ge dots, no vertical island correlation is observed. The phenomenon is explained by the strongly kinetically limited process of C-induced Ge island formation, itself. Photoluminescence experiments on stacks of 0.16 monolayers C/2.2 monolayers Ge dots, where we systematically varied the Si spacer thickness and the number of dot layers, suggest that the average dot size increases if the Si spacer is kept thinner than 10 nm.
Magnetostriction in (CexTb1−x)0.5Pr0.5Fe2 compounds
Authors: Tang Chengchun , Du Jiang , Li Yangxian , Wang Fangwei , Wu Guanghen , Zhan Wenshan , Qu Jingping
Published: July 2002

The structural and magnetostrictive properties of (CexTb1−x)0.5Pr0.5Fe2 were investigated. The Ce concentration must exceed x=0.6 in order to obtain the pure Laves phase. The magnetostriction and anisotropy constant increase with the Tb content in this system. The anisotropy value of PrFe2 is lower than that of isostructural DyFe2. From the x-ray step-scanned data, we find that λ111 of PrFe2 is 1310 ppm. A large increase can be observed under a small prestress of 6 MPa for Ce0.5Pr0.5Fe2.
Mode selectivity study of vertical-cavity surface-emitting lasers
Authors: Liu G. , Seurin J.-F. , Chuang S. L. , Babic D. I. , Corzine S. W. , Tan M. , Barnes D. C. , Tiouririne T. N.
Published: July 2002

Mode selectivity of an air-post index-guided vertical-cavity surface-emitting laser structure operating at 1550 nm is investigated using a full-vector Maxwell-equation solver with a finite-difference time-domain method. The resonance wavelengths, quality factors, and spatial field distributions are calculated for the three lowest-order modes. Transverse-mode competition is quantitatively described as a function of the cavity size and the pillar etch depth.
Frequency doubling in gallium–lanthanum–sulphide glass with surface crystallized Ga6La10/3S14 thin films
Authors: Ding Yong , Kondoh Atsuhiro , Miura Yoshinari , Nanba Tokuro
Published: July 2002

The reason for the optical frequency doubling in the 70Ga2S3–30La2S3 glass is investigated. The tetragonal Ga6La10/3S14 phase due to crystallization of the glass is responsible for the frequency doubling. Dense and transparent Ga6La10/3S14 crystalline thin films have been prepared by means of the controlled surface treatment of the glass due to an ultrasonic surface treatment. The effective second-order nonlinear optical susceptibilities of the crystalline thin films are analyzed.
Differential conductance of the ferromagnet/superconductor interface of DyBa2Cu3O7/La2/3Ba1/3MnO3 heterostructures
Authors: Vas’ko V. A. , Nikolaev K. R. , Larkin V. A. , Kraus P. A. , Goldman A. M.
Published: July 2002

The voltage bias, temperature, and magnetic field dependence of the differential conductance of the interface between a half-metallic ferromagnet and a high-Tc superconductor have been studied in DyBa2Cu3O7/La2/3Ba1/3MnO3 heterostructures. Below the superconducting transition the differential conductance exhibits a dip at zero bias whose amplitude is a decreasing function of temperature and/or magnetic field. These results are interpreted qualitatively using a picture based on the suppression of Andreev reflection as a consequence of the high spin polarization of the carriers in the half-metallic ferromagnet.
Coherent x-ray scattering from an optical grating
Authors: Lin Binhua , Schlossman Mark L. , Meron Mati , Williams Scott M. , Huang Zhengqing , Viccaro P. James
Published: July 2002

X-ray speckles due to scattering of partially (transverse) coherent x rays from an optical reflection grating are observed. The speckles indicate the presence of surface inhomogeneities of the grating that are otherwise undetectable with either visible laser light or transversely incoherent x-ray scattering. Qualitative analysis of the speckle patterns provide information on the surface morphology of the grating. The underlying order due to the periodicity of the grating enhances the detection of the speckles.
Strong excitonic recombination radiation from homoepitaxial diamond thin films at room temperature
Authors: Watanabe Hideyuki , Hayashi Kazushi , Takeuchi Daisuke , Yamanaka Sadanori , Okushi Hideyo , Kajimura Koji , Sekiguchi Takashi
Published: July 2002

We have observed strong emission lines at 5.27 and 5.12 eV at room temperature in cathodoluminescence spectra from homoepitaxial diamond films grown by step-flow mode prepared with microwave plasma assisted chemical vapor deposition. The temperature dependence of integrated intensity of the emission lines indicates that they are attributed to free-exciton recombination radiation associated with a transverse optical phonon and its replica. These results have demonstrated the superior crystalline quality of the homoepitaxial diamond films grown by the step-flow mode.
Dark current and infrared absorption of p-doped InGaAs/AlGaAs strained quantum wells
Authors: Zhang D. H. , Shi W.
Published: July 2002

The dark current as a function of temperature and infrared absorption of the p-doped In0.15Ga0.85As/Al0.45Ga0.55As multiple quantum well structures grown by molecular beam epitaxy are investigated. The dark current Id of the structure is found to be basically symmetrical over a voltage range from −10 to +10 V. It is about 10−9 A at a bias of 1 V at 80 K, more than two orders of magnitude lower than that reported for p-doped GaAs/AlGaAs QW structures with the same size. It is also found that Id is proportional to T exp[−(EC–EF)/kT] at 70 K and above while at temperatures below 30 K it does not change significantly. The EC–EF decreases with the increase in bias in an exponential form, due likely to energy bandgap bending. A strong infrared absorption peaked at a wavelength of 10.7 μm is in excellent agreement with the estimated value of 10.4 μm.
Linewidth of the infrared absorption spectra due to bound-to-continuum transition in GaAs/AlxGa1−xAs multiple quantum well structures
Authors: He Yunpeng , Zhu Q. S. , Zhong Z. T. , Zhang G. Z. , Xiao J. , Cao Z. P. , Sun X. H. , Yang H. Z.
Published: July 2002

We have observed an extremely narrow absorption spectrum due to bound-to-continuum transition in GaAs/AlxGa1−xAs multiple quantum wells (MQWs). Its linewidth is only about one tenth of the values reported previously. Our calculation indicates that the broadening of the excited state in the continuum has little contribution to the absorption linewidth. We have grown a sample whose MQW region contains two kinds of wells with a minor thickness inhomogeneity. Its resultant absorption linewidth is six times as large as that of homogeneous well sample, which is in good agreement with our theoretical analysis. Thus we can suggest that the wider absorption spectra reported by many authors may be due to the well width inhomogeneity.
On the nature of nanometer-scale islands formed by cadmium selenide deposition on hexagonal cadmium sulfide (0001)A
Authors: Grün M. , Funfrock F. , Schunk P. , Schimmel Th. , Hetterich M. , Klingshirn C.
Published: July 2002

Using ex situ atomic force microscopy, nanometer-scale islands were observed on CdS(0001)A surfaces to appear after deposition of small amounts of CdSe. The sizes of these islands are similar to those reported in atomic force microscopy (AFM) studies of uncapped CdSe/ZnSe structures. Investigations by force modulation microscopy and photoluminescence spectroscopy show, however, that they are difficult to assign to a CdSe Stranski–Krastanow growth. Instead, AFM points to an onset of the island formation with the exposure of the samples to atmosphere. By use of electron microprobe analysis, it is suggested that these islands consist of selenium.
Optical detection of crystallographic domains in zinc-blende crystals
Authors: Mohs G. , Shimano R. , Kise T. , Shirane M. , Kuwata-Gonokami M. , Ohkawa K. , Dhanjal S. , Svirko Y. , Zheludev N.
Published: July 2002

We introduce an experimental technique which allows the characterization of domains in nonmagnetic cubic crystals of the zinc-blende type. The technique exploits the optical anisotropy of cubic crystals near an excitonic resonance which translates to a dependence of the reflected light polarization on crystal orientation. We demonstrate sub 10 μm domains on the [110] surface of a high quality, homoepitaxially grown ZnSe film.
Microwave plasma nitridation of silicon dioxide on strained Si
Authors: Bera L. K. , Banerjee H. D. , Ray S. K. , Mukhopadhyay M. , Maiti C. K.
Published: July 2002

Growth of ultrathin (<100 Å) nitrided SiO2 on strained Si using microwave O2/N2O/NH3 plasma is reported. X-ray photoelectron spectroscopy results indicate a nitrogen-rich layer at the strained Si/SiO2 interface. The electrical properties of the nitrided oxides have been characterized using a metal–insulator–semiconductor structure. N2O plasma treatment of O2/NH3 nitrided SiO2 results in a lower insulator charge density (1.2×1011 cm−2) and a higher breakdown voltage.
The physics of negative differential resistance of an intracavity voltage-controlled absorber in a vertical-cavity surface-emitting laser
Authors: Hudgings Janice A. , Stone Robert J. , Lim Sui F. , Li Gabriel S. , Yuen Wupen , Lau Kam Y. , Chang-Hasnain Connie J.
Published: July 2002

We have constructed a vertical-cavity, surface-emitting laser with a voltage-controlled quantum well absorber in the upper mirror stack. If the lasing wavelength of this device is designed to be slightly longer than the absorber band edge, sharp negative differential resistance can be obtained in the absorber under lasing conditions. We present strong experimental evidence that this behavior arises from redshifting of the absorption excitonic peak due to the quantum confined Stark effect. Design criteria are proposed for applications including high speed modulation and self-pulsation.
Site-selective diamond growth using a platinum film and a silicon oxide mask
Authors: Sakamoto Yukihiro , Takaya Matsufumi , Sugimura Hiroyuki , Takai Osamu , Nakagiri Nobuyuki
Published: July 2002

In order to fabricate diamond microstructures, we have developed a site-selective microwave plasma chemical vapor deposition (MW-PCVD) method, which is based on the great difference, in the 106order, between the nucleation densities of diamond on Pt and SiO2 surfaces. First, we prepared a substrate consisting of a Pt film covered with a SiO2 layer on which holes of a 2 μm×2 μm square were fabricated by photolithography. Next, diamond was synthesized onto this substrate by MW-PCVD using a mixture of CH4 and H2 as a reaction gas. Under appropriate conditions, diamond crystals selectively nucleated in the holes where the Pt film surface was exposed, while the surrounding SiO2 surface remained undeposited. A microdiamond array was successfully fabricated by this method. Diamond crystals of ∼2 μm are precisely arranged at vertical and lateral intervals of 20 μm.
Strain-induced quantum confinement of electron gases: The observation of quantum dot levels
Authors: Stallard W. G. , Plaut A. S. , Thoms S. , Holland M. C. , Beaumont S. P. , Stanley C. R. , Hopkinson M.
Published: July 2002

Zero-dimensional electron gases (0DEGs) have been fabricated by the strain patterning of a GaAs/AlAs heterojunction using amorphous carbon stressors. We have observed radiative recombination of electrons with both acceptor-bound and strain-confined holes. For our smallest 0DEGs, quantum dot electron levels are clearly resolved. In finite magnetic field we have mapped the evolution of these electron states, from which we estimate the number of electrons per dot to be 15.